Intro4u2u

Interference is not hard to find; it is actually difficult to avoid, especially in urban areas where the wireless revolution is well underway. By definition, interference originates from a source external to a signal path and produces undesired artifacts in the signal. A radio frequency, or RF, is loosely defined as being in that portion of the electromagnetic spectrum above audio (about 20 kHz) but below infrared (about 30 THz). Electromagnetic interference (EMI), is a broader term having the same basic meaning but without frequency limitations. Electromagnetic compatibility (EMC), is a term coming into more widespread use regarding issues of equipment electromagnetic emissions and susceptibility, especially because regulations now require all equipment sold in Europe to carry the CE mark.

Electromagnetic fields, such as radio and TV signals, travel through space (or air) at the speed of light, about 300,000,000 m/sec or 186,000 miles/sec. Because wavelength is the physical distance such a signal travels during a single cycle, as frequency increases wavelength decreases. For example, a 1 MHz AM radio signal has a wavelength of about 1,000 feet (305 m), but for a 100 MHz FM radio signal, it is about 10 feet (3 m), and for a 12 GHz DSS TV signal, only about an inch (25.4 mm). Any wire can accidentally become a good antenna if its length happens to be, say, the wavelength of a strong local FM station.

Sources of RF interference fall into two broad categories-intentional and unintentional. Intentional sources include AM, shortwave, FM, and TV broadcast transmitters as well as ham and CB transmitters, remote controls, wireless phones, cellular phones, commercial taxi/police/aircraft radios, microwave ovens, motion sensors, radar systems, and a myriad of medical and industrial RF devices.

Unintentional RF sources are most commonly devices that produce an electrical spark. Sparks are potent RF generators-before vacuum tubes, they were the heart of radio transmitters-that splatter energy over a wide frequency spectrum. Any wiring connected to the spark source not only conducts the RF but also acts as a transmitting antenna to radiate it. Common sparking sources include electric welders, brush-type motors, relays and switches of all kinds. Less obvious sources include arcing or corona discharge in power line insulators (common in seashore areas or under humid conditions), malfunctioning fluorescent or neon lighting and automobile spark plugs. Lightning is the ultimate spark and a well known producer of momentary interference to virtually anything electronic.

Other unintentional RF generators are devices that abruptly interrupt current flow using some form of electronic switching. The most common examples are light dimmers, fluorescent lights, TV or computer CRT displays and any piece of equipment using a switching power supply or “clock” oscillator (computers and other digital devices). The RFI source may be in the same room as your system or, worse yet, it may be a part of your system.

RFI symptoms The tolerance of equipment to RFI depends largely on how well it is designed. Generally, symptoms will appear when sufficient RF energy reaches an active device-IC, transistor, tube-inside the equipment. The energy can arrive in two ways: radiation or conduction. As it travels through the air, internal equipment wiring can act as a receiving antenna and deliver RF voltages directly to an active device. This is most common in equipment with plastic or wood enclosures that have no RF shielding ability. Because any wire can become a receiving antenna, RF energy can also be conducted into the equipment’s active devices via any wire leaving or entering the equipment. Interference can also arrive via any wire coming into the building. Because power, telephone, CATV and even driveway intercom, landscape lighting, or outdoor loudspeaker lines also behave as outdoor antennas, they are often teeming with AM radio signals and other interference. The most troublesome sources, however, are frequently inside the building where the interference is distributed via the power wiring. At high frequencies, a building’s power wiring behaves like a system of misterminated transmission lines gone berserk, reflecting RF energy back and forth throughout the power wiring until it is eventually absorbed or radiated. The RF does not just follow the green ground wire back to the earth ground rod and magically disappear.

RF power line noise is coupled through equipment power supplies into system ground conductors. Therefore, significant noise voltage will inevitably exist between the chassis grounds of any two devices in AC-powered systems, whether safety grounded or not. This is the dominant noise source in most systems, not noise picked up by cables as is so widely believed. When this noise flows in the shield of unbalanced signal cables, the voltage drop directly adds to the signal as shown in Figure 1.

Unbalanced interfaces generally use single-conductor shielded cable and two-contact connectors, such as the RCA or 1/4 inch phone for audio and the RCA or BNC for video signals. Remember that RS-232 data connections are also unbalanced. Sadly, most commercial equipment has never been tested for susceptibility to RF interference, whether arriving through the air or coupled to its inputs, outputs or such other outside world ports as its power cord. Of course, even well-designed equipment will misbehave if confronted with extreme levels of RF interference.

In audio systems, RFI symptoms range from actual demodulation of radio or CB (heard as music or voices) or TV signals (heard as buzz) to various noises or subtle distortions often described as a “veiled” or “grainy” quality in the audio. In video systems, symptoms from intentional transmitters usually cause herringbone patterns of some sort, and power-line related-sources usually cause bands of sparkles that slowly move vertically in the picture. In data connections, RFI generally causes otherwise unexplained behavior or crashes.

Stopping it There are two basic strategies to control RFI. The first prevents it from coupling in the first place by using filters or arc snubbers at the source, relocating equipment or rerouting cables, using signal path ground isolators or adding shielding or ferrite chokes to cables. The second filters out the RF, when possible, after it is coupled but before it reaches a sensitive active device in the equipment. The following recommendations can help prevent or cure most RFI problems.

Locate and treat the offending source. This applies primarily to unintentional power-line-related sources. Because these sources tend to generate both conducted and radiated wideband RFI, a portable AM radio tuned to a quiet frequency can be useful as a “sniffer” to locate an offending fluorescent light or dimmer, for example. Then, the offender can be replaced, repaired or a power-line RF filter installed.

Keep cables as short as possible, and pay attention to routing. A long cable not only increases power line common-impedance coupling (for unbalanced cables), but it also makes the cable a better antenna. Routing cables close to such ground planes as metal racks or concrete floors will reduce antenna effects. Never coil excess cable length.

Use cables with heavy gauge shields. Cables with foil and drain wire shields have much higher common-impedance coupling than those with braided copper shields, increasing power line noise coupling. Multiple shields offer no improvement unless they are connected at both ends.

Maintain good connections. Connectors left undisturbed for long periods can develop high-contact resistance or become metal oxide detectors for RF. Hum or other interference that changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold-plated connectors.

Do not add unnecessary grounds. It will generally increase circulating ground noise rather than reduce it. Attempting to short out RFI with heavy ground wires is generally ineffective. At RF, a wire’s impedance is proportional to its length but nearly unaffected by its gauge. For example, 8 feet (2.4 m) of AWG #10 wire has an impedance of 22 V at 1 MHz (AM broadcast band). Using AWG #0000 wire (about 1/2 inch or 13 mm diameter) reduces it to only 18 V. Of course, never disconnect a safety ground or lightning protection ground to solve a problem-it is both illegal and dangerous.

Use ground isolators in problem signal paths. Ground isolators, whether transformer or optical types, couple signals while completely breaking electrical connections, which stops common-impedance coupling. Commercial isolators are available for audio, video and CATV signals. Because most types have limited bandwidth, they offer inherent RFI suppression. Beware that poor-quality units can often degrade signal quality.

Install RFI filters in the signal path. If the offending RF interference is more than about 20 MHz, ferrite clamshells, which are easily installed over the outside of a cable, can be effective. In most cases, they work best when placed on the cable at or near the receive end. If this is inadequate, or the frequency is lower (such as AM radio), you can add an RFI filter on the signal line. Schematics for unbalanced or balanced filters are shown in Figure 2. For mic line applications, L should be a miniature toroid to prevent possible magnetic hum pickup. If FM, TV or cell phone is the only interference, a small ferrite bead may suffice for L. In any case, C should be an NP0/C0G type ceramic disc with short leads. For severe AM radio interference, C may be increased to about 1,000 pF maximum.

Programmable logic controllers provide dependable, high-speed control and monitoring demanded by a wide variety of automated applications.

Programmable logic controllers(PLCs) have gained a substantial hold in the industrial manufacturing arena, and we would be remiss if this technology were not given the due attention it has earned. As such, we are featuring a series of articles based on the fundamentals of PLCs in this new EC&M department covering the technology of solid-state industrial automation. Throughout this series on PLC fundamentals, we’ll cover PLC hardware modules; software capabilities; current applications; installation parameters; testing and troubleshooting; and hardware/software maintenance.

What is a PLC?

The National Electrical Manufacturers Association (NEMA) defines a PLC as a “digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions, such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog I/O modules various types of machines or processes.”

One PLC manufacturer defines it as a “solid-state industrial control device which receives signals from user supplied controlled devices, such as senors and switches, implements them in a precise pattern determined by ladder-diagram-based application progress stored in user memory, and provides outputs for control of processes or user-supplied devices, such as relays or motor starters.”

Basically, it’s a solid-state, programmable electrical/electronic interface that can manipulate, execute, and/or monitor, at a very fast rate, the state of a process or communication system. It operates on the basis of programmable data contained in an integral microprocessor-based system.

A PLC is able to receive (input) and transmit (output) various types of electrical and electronic signals and can control and monitor practically any kind of mechanical and/or electrical system. Therefore, it has enormous flexibility in interfacing with computers, machines, and many other peripheral systems or devices.

It’s usually programmed in relay ladder logic and is designed to operate in an industrial environment.

What’s it look like?

PLCs come in various sizes. Generally, the space or size that a PLC occupies is in direct relation to the user systems and input/output requirements as well as the chosen manufacturer’s design/packaging capabilities.

The chassis of a PLC may be of the open or enclosed type. The individual modules plug into the back plane of the chassis.

The electronic components are mounted on printed circuit boards (PCBs) that are contained within a module.

Where did it come from?

The first PLC was introduced in the late 1960s and was an outgrowth of the programmable controller or PC (not to be confused with the notation as used for the personal computer). PCs have been around the industry since the early 60s.

The need for better and faster control relays that fit into less space as well as the frustration over program inflexibility (hard-wired relays, stepping switches, and drum programmers) gave birth to the PC.

Although the PC and PLC have been interchanged in speech, the difference between them is that a PC is dedicated to control functions in a fixed program, similar in a sense to the past problem of limited ability. A PLC, on the other hand, only requires that its software logic be rewritten to meet any new demands of the system being controlled. Thus, a PLC can adapt to changes in many processes or monitoring application requirements.

How does a PLC work?

To know how the PLC works, it is essential that we have an understanding of its central processing unit’s (CPU’s) scan sequence. The methodology basically is the same for all PLCs. However, as special hardware modules are added into the system, additional scanning cycles are required.

Here’s one simple scanning process that involves every PLC. First, the I/O hardware modules are scanned by the ladder logic software program as follows.

Upon power-up, the processor scans the input module and transfers the data contents to the input’s image table or register. Data from the output image table is transferred to the output module.

Next, the software program is scanned, and each statement is checked to see if the condition has been met. If the conditions are met, the processor writes a digital bit “1″ into the output image table, and a peripheral device will be energized. If the conditions are not met, the processor writes a “0″ into the output image table, and a peripheral device (using “positive logic”) remains deenergized.

A PLC interfaces numerous types of external electrical and electronic signals. These signals can be AC or DC currents or voltages. Typically, they range from 4 to 20 milliamperes (mA) or 0 to 120VAC, and 0 to 48VDC. These signals are referred to as I/O (input/output) points. Their total is called the PLC’s I/O capability. From an electronic point-of-view, this number is based on how many points the PLC’s CPU is able to look at, or scan, in a specified amount of time. This performance characteristic is called scan time. From the practical perspective of the user, however, the number of I/O modules needed as well as the number of I/O points contained on each I/O module will drive what the system’s I/O capability should be.

It’s important to have sufficient I/O capability in your PLC system. It’s better to have more than less so that, when more I/O points are required at a future time, it’s easier to write the existing spare I/O points into the software (since the hardware is already there). There’s no harm to the operating system in having spare I/O points; the software can be programmed to ignore them, and these points will have a negligible effect on the PLC’s scan time.

The PLC’s software program

The software program is the heart of a PLC and is written by a programmer who uses elements, functions, and instructions to design the system that the PLC is to control or monitor. These elements are placed on individually numbered rungs in the relay ladder logic (RLL). The software’s RLL is executed by the processor in the CPU module or controller module (same module, different name).

There are many types of PLC software design packages available. One frequently selected software package is of the RLL format and includes contacts, coils, timers, counters, registers, digital comparison blocks, and other types of special data handling functions. Using these elements, the programmer designs the control system. The external devices and components are then wired into the system identical to that of the programmer’s software ladder logic. Not all of the software elements will have a hard-wired, physical counterpart, however.

As the PLC’s processor scans (topdown) through the software program (rung-by-rung), each rung of RLL is executed. The hard-wired device that the software is mirroring then becomes active. The software is thus the controlling device and provides the programmer or technician the flexibility to either “force a state” or “block a device” from the system operation. For example, a coil or contact can be made to operate directly from the software (independent of the control cabinet’s hard-wiring to source or field input devices). Or, a device can be made to appear invisible (removed from the system’s operation), even though it’s electrically hard-wired and physically in place.

Individual PLC sections

Common to all PLCs are four sections, each of which can be subdivided into smaller but equally important sections. These primary sections include the power supply section, which provides the operating DC power to the PLC and I/O base modules and includes battery backup; the program software section; the CPU module, which contains the processor and holds the memory; and the I/O section, which controls peripheral devices and contains the input and output modules.

Power supply section. The power supply (PS) section gets its input power from an external 120VAC or 240VAC source (line voltage), which is usually fused and fed through a control relay and filter external to the PS. In addition, the PS has its own integral AC input fuse.

This line voltage is then stepped-down, rectified, filtered, regulated, voltage- and current-protected, and status-monitored, with status indication displayed on the front of the PS in the form of several LEDs (light-emitting diodes). The PS can have a key switch for protecting the memory or selecting a particular programming mode.

The output of the PS provides low DC voltage(s) to the PLC’s various modules (typically, with a total current capability of 20A or 50A) as well as to its integral lithium battery, which is used for the memory backup. Should the PS fail or its input line voltage drop below a specific value, the memory contents will not change from what they were prior to the failure.

The PS output provides power to every module in the PLC; however, it does not provide the DC voltages to the PLC’s peripheral I/O devices.

CPU module. “CPU,” “controller,” or “processor” are all terms used by different manufacturers to denote the same module that performs basically the same functions. The CPU module can be divided into two sections: the processor section and the memory section.

The processor section makes the decisions needed by the PLC so that it can operate and communicate with other modules. It communicates along either a serial or parallel data-bus. An I/O base interface module or individual on-board interface I/O circuitry provides the signal conditioning required to communicate with the processor. The processor section also executes the programmer’s RLL software program.

The memory section stores (electronically) retrievable digital information in three dedicated locations of the memory. These memory locations are routinely scanned by the processor. The memory will receive (”write” mode) digital information or have digital information accessed (”read” mode) by the processor. This read/write (R/W) capability provides an easy way to make program changes.

The memory contains data for several types of information. Usually, the data tables, or image registers, and the software program RLL are in the CPU module’s memory. The program messages may or may not be resident with the other memory data.

A battery backup is used by some manufacturers to protect the memory contents from being lost should there be a power or memory module failure. Still others use various integrated circuit (IC) memory technologies and design schemes that will protect the memory contents without the use of a battery backup.

A typical memory section of the CPU module has a memory size of 96,000 (96K) bytes. This size tells us how many locations are available in the memory for storage. Additional memory modules can be added to your PLC system as the need arises for greater memory size. These expansion modules are added to the PLC system as the quantity of I/O modules are added or the software program becomes larger. When this is done, the memory size can be as high as 1,024,000 (1024K) bytes.

Manufacturers will state memory size in either “bytes” or “words.” A byte is eight bits, and a bit is the smallest digit in the binary code. It’s either a logic “1″ or a logic “0.” A word is equal in length to two bytes or 16 bits. Not all manufacturers use 16-bit words, so be aware of what your PLC manufacturer has defined as its memory word bit size.

Software program. The PLC not only requires electronic components to operate, it also needs a software program. The PLC programmer is not limited to writing software in one format. There are many types available, each lending itself more readily to one application over and above another. Typical is the RLL type previously discussed. Other S/W programs include “C,” State Language, and SFC (Sequential Function Charts).

Regardless of which software is chosen, it will be executed by the PLC’s CPU module. The software can be written and executed with the processor in an online state (while the PLC is actually running) or in the off-line state (whereby the S/W execution does not affect current operation of the I/O base).

In the RLL software program, we find several types of programming elements and functions to control processes both internal to the PLC (memory and register) as well as external (field) devices. Listed below are some of the more common types of elements, functions, and instructions:

* Contacts (can be either normally opened or closed; highlighted on the monitor means they are active).

* Coils (can be normal or latched; highlighted means they are energized).

* Timers (coil can either be ON or OFF for the specified delay).

* Counters (can count by increments either up or down).

* Bit shift registers (can shift data by one bit when active).

* One-shot (meaning active for one scan time; useful for pulse timer).

* Drums (can be sequenced based on a time or event).

* Data manipulation instructions (enable movement, comparison of digital values).

* Arithmetic instructions (enable addition, subtraction, multiplication, and division of digital values).

Peripheral devices

Peripheral devices to the PLC and its I/O base(s) can be anything from a host computer and control console to a motor drive unit or field limit switch. Printers and industrial terminals used for programming are also peripheral devices.

Peripheral devices can generate or receive AC or DC voltages and currents as well as digital pulse trains or single pulses of quick length (pulse width).

These external operating devices, with their sometimes harsh and/or fast signal characteristics, must be able to interface with the PLC’s sensitive microprocessor. Various types of I/O modules (using the proper shielded cabling) are available to do this job.

Input module

The input module has two functions: reception of an external signal and status display of that input point. In other words, it receives the peripheral sensing unit’s signal and provides signal conditioning, termination, isolation and/or indication for that signal’s state.

The input to an input module is in either a discrete or analog form. If the input is an ON-OFF type, such as with a push button or limit switch, the signal is considered to be of a discrete nature. If, on the other hand, the input varies, such as with temperature, pressure, or level, the signal is analog in nature.

Peripheral devices sending signals to input modules that describe external conditions can be switches (limit, proximity, pressure, or temperature), push buttons, or logic, binary coded decimal (BCD) or analog-to-digital (A/D) circuits. These input signal points are scanned, and their status is communicated through the interface module or circuitry within each individual PLC and I/O base. Some typical types of input modules are listed below.

* DC voltage (110, 220, 14, 24, 48, 15-30V) or current (4-20 mA).

* AC voltage (110, 240, 24, 48V) or current (4-20 mA).

* TTL (transistor transistor logic) input (3-15VDC).

* Analog input (12-bit).

* Word input (16-bit/parallel).

* Thermocouple input.

* Resistance temperature detector.

* High current relay.

* Low current relay.

* Latching input (24VDC/110VAC).

* Isolated input (24VDC/85-132VAC).

* Intelligent input (contains a microprocessor).

* Positioning input.

* PID (proportional, intregal, differentiation) input.

* High-speed pulse.

Output module

The output module transmits discrete or analog signals to activate various devices such as hydraulic actuators, solenoids, motor starters, and displays the status (through the use of LEDs) of the connected output points. Signal conditioning, termination, and isolation are also part of the output module’s functions. The output module is treated in the same manner as the input module by the processor.

Some typical output modules available today include the following:

* DC voltage (24, 48,110V) or current (4-20 mA).

* AC voltage (110, 240v) or current (4-20 mA).

* Isolated (24VDC).

* Analog output (12-bit).

* Word output (16-bit/parallel).

* Intelligent output.

* ASCII output.

* Dual communication port.

TERMS TO KNOW

A/D: A device or module that transforms an analog signal into a digital word.

Address: A numbered location (storage number) in the PLC’s memory to store information.

Analog input: A varying signal supplying process change information to the analog input module.

Analog output: A varying signal transmitting process change information from the analog output module.

Baud rate: The number of bits per second that is either transmitted or received; also the speed of digital transmission acceptable by a device.

BCD: Binary coded decimal. A method used to express the 0-thru-9 (base 10) numbering system as a binary (base 2) equivalent.

Bit: A single binary digit.

Byte: Eight bits.

Central Processing Unit (CPU): An integrated circuit (IC) that interprets, decides, and executes instructions.

D/A: A device or module that transforms a digital word into an analog signal

Electrically Erasable Programmable Read-Only Memory (EEPROM): Same as EPROM but can be erased electrically.

Erasable Programmable Read-Only Memory (EPROM): A memory that a user can erase and load with new data many times, but when used in application, it functions as a ROM. EPROMs will not lose data during the loss of electrical power. They are nanvolatile memories.

Image register/image table: A dedicated memory location reserved for I/O bit status.

Input module: Processes digital or analog signals from field devices.

I/O points: Terminal points on I/O modules that connect the input and output field devices.

Millisecond: One thousandth of a second (1/1000 sec, 0.001 sec).

Modem: Modem is an acronym for modulator/demodulator. This is a device that modulates (mixes) and demodulates (separates) signals.

Operator interface: Devices that allow the system operators to have access to PLC and I/O base conditions.

Output module: Controls field devices.

Parallel data: Data whose bytes or words are transmitted or received with all their bits present at the same time.

Program: One or more instructions or statements that accomplish a task.

Programming device: A device used to tell a PLC what to do and when it should be done.

Random Access Memory (RAM): A memory where data can be accessed at any address without having to read a number of sequential addresses. Data can be read from and written to storage locations. RAM has volatile memory, meaning a loss of power will cause the contents in the RAM to be lost.

Read-Only Memory (ROM): A memory from which data can be read but not written. ROMs are often used to keep programs or data from being destroyed due to user intervention.

Software: One or more programs that control a process.

PLC TIPS AND HINTS:

Four topics are covered:

1)  Make and use a ‘one shot’

2)  Make toggling logic

3)  Beating PLC scan time issues

4)  Implementing a simple proportional controller in PLC logic

–  The ‘one shot’, what it is, what it is good for,  how to make one if your PLC does not have this feature:

A one shot is a coil which goes true each time the enabling rung ahead of it is true, and it stays true for one scan only, no matter how long the enabling rung is true.  The one shot is useful when you have some condition that goes on and off and you want to have the PLC take action on that true state for only one scan each time the enabling rung goes from true to false.  For example, say you want to count the number of times some event that lasts longer than one scan happens, but the total count will exceed the capacity of the PLC’s built in counter.  One way to deal with this problem, if your PLC has floating point registers available,  is to add one to a floating point register each time the rung goes true.

BUT if you use logic like this:

what will happen is that as long as the rung is true you will add one to the F8:1 register  every time the PLC scans.   With a one shot in the logic:

this counter will operate as expected.

Make your own one shot.

Quite a few PLC brands do not offer the useful one shot feature.  In that case, you can make your own with a few rungs of logic:

This is one of the few cases in common PLC programming where the order in which you place the rungs is important.  Here’s how the logic works:  When you start out, the input I:3/0, the memory bit B3:0/2, and the one shot output bit B3:0/1 are all false.  When the input goes true, the memory bit is still false, so the second rung is true, and both the memory bit and the one shot output bit are set by the second rung.  The PLC completes his scan and comes around to this section of logic again, where the first rung finds the output bit true, so that rung resets the output (output has remained on for one full scan).  The memory bit is still true, so the second rung does not set the output bit again.  As long as the input remains true the third rung is not true, so the memory bit remains set.  When the input goes false the third rung resets the memory bit and the next scan which finds the input true can start the whole process over again.  The order of the rungs is important because you don’t want the PLC to see the first rung until after it has completed a full scan with the output bit on.

– Toggling logic

I have never seen a PLC which offers a toggling feature.  A toggling output would switch from off to on when the input goes true.  It remains on until the input goes false and then goes true again, then the output goes off.  This process repeats itself each time the input goes off then back on.  This kind of logic is useful where you want one button to control a device with this kind of toggling action (press once for on, press same button again for off), or where you want to split a stream of parts so that alternate parts go into two different lanes or bins.  (These are all subsets of the class of circuits commonly called ‘divide by 2’ circuits.)

Here is the logic for a toggling output:

Note that the input for the second rung must be a one shot on the control bit.  This logic takes advantage of the way the PLC solves the logic for each rung – basically the PLC logic solver takes the first true path it finds through the branches of the rung and it ignores all the other branches, whether they are true or not. If your particular brand of PLC does not adhere to this convention, then this logic will not work.  It works in Allen Bradley SLC, PLC, and Micrologix, Mitsubishi, Toshiba, and Siemens Step 5 PLCS.

–  PLC scan time issues and how to beat them

If you design controls for high speed machines or processes you will eventually come across a case where the PLC scan time is long enough that it causes a problem.  The first time I saw this was with a machine builder who used an incremental encoder to report machine position.  The incremental encoder generates a number (usually 180, 360, or more) of pulses on a single output line each time it’s shaft is turned through one revolution.  If you count these pulses in a counter in the PLC, and  if you have a zero position sensor that will reset the counter each time the shaft turns through the zero location, then the number in the counter will correspond to the shaft position.  Then you can build ‘cams’ in the PLC logic by comparing the counter value to preset values to create outputs that go on and off at different preset shaft positions.  This company had used this method successfully for years until the day that they decided to increase machine speed by a factor of two.  Suddenly machines began trying to tear themselves apart.  They would run fine at the old slower speed, but when the speed was turned up synchronization was lost and moving parts were moving at the wrong time causing ‘crashes’.   Numerous encoders were tested, always with the same result.  Further investigation revealed that the counter was not counting a full complement of 360 pulses each time the shaft made a revolution.  What was happening was that the output from the encoder operating at a higher speed was going from false to true and back to false during the time it took for the PLC to make one scan.  The PLC scan consists of three actions: look at all the inputs and copy their states in to an input table, solve the ladder logic based on the values in the input table and write results into an output table, then copy the output table to the physical outputs.  Since the PLC is ignoring the value of the inputs during the logic solve step (which takes most of the scan time) and during the write outputs step, he was ignoring some of the rapid transitions from the properly functioning encoder.   In this case the best solution was to replace the incremental encoder with an absolute position encoder.  This type of encoder has a multitude of output lines so that it can send the PLC the absolute value of it’s shaft position in binary or BCD coded format at all times.  Any time the PLC looks at inputs from the absolute encoder he sees a number corresponding to shaft position at that instant.  Now if the PLC does not scan fast enough to notice each transition from one state to the next, he at least will not accumulate a greater and greater error as the shaft rotates.  Each individual ‘cam’ could possibly be mistimed by a small amount, but the error is never more than the PLC scan time and the error does not accumulate as the shaft turns.   There are advantages and disadvantages to each kind of encoder:  The absolute encoder is not susceptible to errors as machine speed increases, but it requires typically 12 to 16 PLC input lines instead of just two for the incremental encoder.  The incremental encoder is never subject to the extremely annoying ‘missed states’ or ‘linear discontinuities’ that a poorly designed absolute encoder can produce.  Missed states refers to the case where, as the encoder shaft is turned very slowly through positions that should take the outputs from the two consecutive values  001111 to 010000 you may briefly get a transition value where some of the bits change and others do not   001111  >>  001100  >>  011100  >>  010000.  This is very troublesome.  No modern absolute encoder will exhibit this kind of action at any position of it’s input shaft.  If yours does, return it to the manufacturer and get one that works correctly with no ‘skipped states’

Another consequence of PLC scan time is that the timing of outputs or the transition time from an input changing state and the output that it is controlling changing will not be completely consistent from one scan to another.  In any PLC the ‘latency’ or time between input change and output change will always vary from scan to scan by as much as +/- one scan time.  This is because the input change is not synchronized with the scanning action.  An input might change just before the PLC copies inputs into the table (in which case it will be recognized on the same scan and will change the output with the shortest delay).  On the other hand, if the input changes just after the inputs are copied into the table, then that change will not be recognized until the next scan, resulting in the longest delay before the output changes.  The delay will be random from scan to scan unless the change at the input is somehow synchronized with the PLC scan, which seldom if ever happens in real life applications.  Here is an example where scan time errors of this kind can cause a problem:  We have a fairly slow motor which we would like to have make one revolution and stop at a fixed position which is determined by a cam lobe on the motor shaft and a prox switch connected to the PLC.  The PLC enables the motor (by closing the ‘run’ contact in a VFD for example), and keeps the motor enabled until he sees the prox signal saying the cam lobe has moved back around to the reference position.  In this case the stop time of the VFD, the response time of the prox switch, and the motor speed are all constant and predictable, but still the motor does not always stop at the same position. (In fact, if the cam is too narrow sometimes the motor doesn’t stop AT ALL.)  The easy solution in this case was to add a pair of external relays connected so that as long as the prox switch was not made the coil of the relay controlling the VFD ‘run’ contact was held closed.  Any time the motor shaft was moved away from the rest position (causing the prox output to go off) the VFD would run, bringing the cam around to the high spot at the reference position where the relay would drop out and the motor would stop in a very predictable and repeatable position.  The PLC started each ‘index’ of the motor by applying a momentary pulse just long enough to move the motor ‘off the cam’.  Now the stop position is not dependent on PLC scan time at all, because the PLC does not control the stop position.   Normally PLC designers try to eliminate all external relays. (This is after all one of the main reasons to use a PLC in the first place.)  But in this case two cheap relays eliminated the need for much more expensive solutions such as a higher performance PLC or a servo or stepper motor drive.

A note about the ‘instant’ contacts and outputs found in some PLC products:  Some PLC manufacturers (Allen Bradley is one) offer what they call ‘instant’ inputs and outputs.  These specific I/O points or instant I/O commands BYPASS the input and output tables in an effort to give the designer another tool to fight scan time problems with.  In the case of Allen Bradley SLC family these are instant instructions that work with any discrete I/O point.  When the logic solver encounters these instructions he looks directly at the input (not at it’s value which has been earlier scanned into the input table) to get it’s value or, in the case of an output, he immediately writes that output as soon as the rung is solved instead of putting the result in the output table to be sent out later with all of the other outputs at the end of the scan.   I have not addressed the use of these instant I/O instructions here because very few PLC families offer them, and because their effective use is very dependent on how your program is laid out and whether there are branches in the program.    Basically you can use these instructions to beat scan time issues by repeating the critical rungs at several places evenly distributed throughout your ladder program.  Then those rungs will be instantly solved several times during each scan, thereby effectively reducing the scan time for those rungs only.  It is important to make sure the placement of the rungs is such that they are solved as frequently as necessary to get down to the scan time value needed, and that this happens for all different possible branching situations that may exist in the program.  Even with use of these instructions you will still see the ‘latency dither’ effects that result from inputs changing asynchronously with respect to the PLC scan cycle – the amount of dither will just be less.  Instant I/O instructions also slow down execution of the rest of the program, sometimes significantly.

Allen Bradley Control Logix PLCs and others of that ilk which allow you to assign processor resources according to scan time requirements are very versatile in that you can generate two or more ladder programs and dictate how often each one is scanned.  You can write a short, frequently scanned program where this is necessary while ‘simultaneously’ running a longer, slower program for non-time-critical functions.   But Control Logix hardware and programming tools cost roughly 2x what equivalent SLC family parts cost, and 4x or more what parts comparable to SLC from other manufacturers cost (today, in 2002), so it would be a poor designer indeed who took the brute force approach of using Control Logix just to solve a scan time problem when there was no other need for this advanced, more expensive product.

–  Implementing a simple proportional controller in PLC logic.

Proportional controllers are used in many process control applications.  A very common use is for controlling heating of a process vessel.  Heaters are generally sized to meet some maximum time-to-temperature specification.  If the heater is adequate to heat the tank in a reasonable time, then there will be significant reserve capacity beyond what is available to hold the process at the setpoint temperature.  A likely consequence of this is that, if a simple on-off temperature controller is used to control this process, the temperature will overshoot the setpoint  If we wait until the temperature reaches the setpoint before turning off the heater, enough residual heat will remain in the heater to cause the actual temperature to exceed the setpoint during the few minutes just after the heater is shut off by the temperature control.   Attempting to anticipate the overshoot and turn off the heater a few degrees before the setpoint will not help, because then the process may never actually achieve the setpoint.  In the world of relay logic, relatively inexpensive PID process controllers are readily available which implement a fully compensated control algorithm which measure and anticipate the heating and cooling rates of the process and correct themselves automatically.  PID stands for Proportional/Integral/Derivative, and these controllers have separate tunable response characteristics which derive an output which is proportional to the difference between the actual process value and it’s setpoint.  The proportional output is modified according to how fast the actual value is approaching the setpoint (the derivative term) and also by any offset in the setpoint after a stable control point has been established (the integral term).  A few of the more expensive brands of PLC also contain PID instructions which allow you to set up PID control for several process loops inside the PLC.  But the PID instruction is not common on middle or low end PLCs.

Here is a way to implement a basic proportional control in PLCs which do not have a built in PID instruction.  We will assume that this PLC is controlling the heater for a large tank of liquid.  What we want to do is turn the heater on whenever the temperature is more than some predetermined value (which we will call the ‘offset’) lower than the setpoint.  When the temperature is between the offset value and the setpoint, we will cycle the heater on and off, with the duty cycle being proportional to the difference between the setpoint and the actual temperature.  You can adjust the offset value according to the specific process conditions to give the best compromise between quick response and minimum overshoot.  A good starting point for the offset value can be obtained by running the process with on-off control (where the heater stays on until the process temperature reaches the setpoint and is then turned off).  Note how much overshoot occurs and make the offset value equal to or somewhat larger than the amount of overshoot.  There is a compromise involved in choosing the time base for the adjustable duty cycle output.  The quickest response and minimum deviation from the setpoint will be obtained with a short duty cycle (the ultimate limit of this idea is the proportional controller where a phase controlled triac or scr switches each half cycle of the AC voltage on at a level proportional to the amount of heat required).  On the other hand, unless you are using a solid state output device to control the heater, rapid cycling will result in premature failure of the output device.  For the sort of large, slow processes this approach is suited for, a time base of at least 10 seconds is recommended.  Time bases as long as a minute will generally work quite well with large tanks.

So, here is what our PLC program will do:

1)  Compute difference between actual and setpoint

2)  Multiply difference x 100

3)  Compare difference x 100 with the accumulator of a free running timer, if difference > counter, turn on the output; if < then turn off the output.

4)  Reset the free running timer every 10 seconds.  (timer accumulator increments each 0.01 seconds, so during the 10 second cycle the output will be on from 0-100.0 percent of the time depending on how far the actual temperature is from the setpoint.  Proportional control will be occurring in the range between setpoint  minus 10 degrees and setpoint.

Some cautions:

1)  For heating applications, observe safety precautions.  A complete heater control application will include a backup safety thermostat and independent disconnecting means to prevent overtemperature faults in case of a malfunction of the main controller.

2)  This simple proportional only controller will work well with large tanks and fairly stable process conditions.  A more elaborate control algorithm may be required if the thermal mass is low or if the process is subject to events which rapidly change conditions (like adding or removing large quantities of liquid from the heated tank).

The Rectangular Hysteresis Loop
So far we have discussed, in a very simple way, the principles on which a magnetic memory is based. It is now necessary to consider certain aspects of the subject from a more technical angle, and in particular to explain more fully why the material ‘ferroxcube 6′ has been selected for use in the equipment to be described.

Fig4

It has been shown that the direction of the magnetisation of a particular ring element is determined by the direction of the current flowing through the wire on which the ring is threaded (see Fig. 4). It should also be understood that the extent to which a ring is magnetised is proportional to the current flowing through the wire, in other words to the Strength of the magnetising field.

The relationship between the direction and strength of the resultant magnetisation (B). and the direction and strength of the magnetising force(H), for the material ferroxcube 6 is shown once more in the graphs reproduced in Fig. 5a.

Fig 5a

Depending upon what has previously happened by way of applying a magnetising force H, a ring core, when no current is flowing in the wire threaded through it, will, due to the special form of the hysteresis loop, be in one of two magnetic conditions: it will either be - magnetised in the direction we will call ‘+’, to an extent +B, or in the direction we will call ‘–’ to the extent -B, The condition +B, is used to represent the symbol ‘1′, and the condition -B,. to represent the symbol 0. In order to change the magnetic condition of the ring core from +Br to -B, or vice versa, a current corresponding to a magnetising force H must pass through the wire. In actual practice, where a number of wires pass through each ring core, the algebraic sum of the currents in all the wires must be equivalent to a magnetising force H.

For example,. imagine that a particular ring is in the condition +B, representing the symbol 1. If a current equivalent to a field of -H is now passed through the wire (see lowest horizontal block in Fig. 5b), the magnetisation will be reversed., the condition changing to that corresponding to point P on the graph. When the current ceases (H becoming zero) the strength of the magnetisation will fall to the value marked -B, that is to say the ring will be in the condition representing the symbol 0.

Fig5b

Similarly, if the ring core is originally in the condition -B, the application of a current pulse corresponding to a magnetising force of +H will change the magnetisation to the point Q on the graph and, on cessation of the current, the magnetisation. will drop to the value +Br.

Again, because of the rectangular form of the hysteresis loop, the application of a current pulse corresponding. to a magnetising force of 1/2 H in either direction will make no permanent change in the direction of the magnetisation. If the core is in the condition +Br, the application of a magnetising field of 1/2 H will temporarily drive the magnetisation either to the value represented by +R on the graph or to the value represented by -R, and on cessation of the current the magnetisation will return to the value +Br.

Similarly, if the core is in the condition -B, the application of a magnetising field of 1H will temporarily drive the magnetisation either to the value represented by -S or to the value represented by -S1. On cessation of the current the magnetisation will again return to -Br.

Furthermore, the graph shows that if the core is in the condition +B,. and a current pulse corresponding to a field of +H is applied, the magnetism will not change permanently, but will merely rise to the value represented by point Q, returning to the value +B,. when the current ceases. Similarly, if the core is in the condition -B, the application of a field of value -H will merely temporarily increase the magnetisation to a value represented by point P, and on cessation of the current the magnetisation will again become -B,

The above effects are due to the rectangular form of the hysteresORI Srl has been cooperating  since several years in the italian market with the Dutch company Mokveld Valves BV.
Mokveld is a company highly focused on an extreme technological development. Its design, known as  axial flow, is still capable of assuring unbeatable performances in the most severe applications, used on control valves, check valves, HIPPS systems and even choke valves.

Mokveld valves are characterized by several unique features that together give birth to a top performance product capable to meet the most stringent process demands: main features unique to Mokveld valves are:

- tight shut off, ANSI class VI standard
- pressure balanced design, for smaller actuators, quicker operational times
- fully customized cage assuring the best combination in terms of Cv, noise abatement and pressure recovery
- integral cast body design

Mokveld production includes:
- Control valves in large diameters and high rating, for critical services like anti-surge for compressors, flare systems, water injection and others
- Choke valves (in-line axial and angle type) with customized trim in tungsten carbide, with linear or step actuators
- Surge relief valves for pipelines
- Check Valves non-slam type, Venturi profile, axial flow
- HIPPS system SIL 3  certified in accordance with IEC 61508 and 61511, PED cat. IV (see HIPPS design considerations )

During the recent past, Mokveld has developed a special casting technology and is now able to offer valves in large sizes even in super-austenithic stainless steels or 254SMO, as recently supplied to Mess.rs Saudi Aramco.

For all these products ORI Srl can also provide maintenance and training in the Italian territory.is loop, of the special grade of ferroxcube known as ‘ferroxcube W.

Your department for explosion protection
(Explosion protection, explosion-proof, ATEX, International Electronical Commission standards)

Leading international firms rely on our proven safety concepts. We offer our customers full service from a single source: special solutions based on standardized and certified assemblies, development, certification and production. We follow these solutions from the idea to ultimate installation and even take care of training your personnel. Our international approach to the evaluation of apparatus and assemblies guarantees unrestricted use, even outside Europe. Our equipment and installations are certified for Japan, China, Europe, the USA and Canada. With EC Type Examination Certificates from Germany’s Federal Institute of Physics and Metrology we meet the requirements for Europe and most other target countries.

DEKRA Logo_cmyk     IECEx Logo

Customers working with us can be sure they are investing in equipment and protective systems that comply with the current EC directives and standards, because thuba belongs to the international committees and works closely with the supervisory bodies. In the “International Electrotechnical Commission IEC” we are actively engaged in the creation of new standards and the updating and constant improvement of existing ones. This work keeps us abreast of the latest trends and enables us to be prepared for coming developments.

A	Symbol for 3A Sanitary Standards Symbol Administrative Council. The 3A Sanitary Standard was created by the dairy industry as a voluntary benchmark for product performance and safety.
ABS	Acrylinitrile-butadiene-styrene - Themoplastic resin with excellent resistance to acids, bases, salts and some solvents.
ANSI	American National Standards Institute, Inc.
ASME	American Society of Mechanical Engineers
ASTM	American Society for Testing and Materials
Acme Thread	A flat grooved helical ridge on a nut or bolt
Anneal	Stress relief of stainless steel, a heat treatment to remove the stresses generated in forming and welding operations. Done correctly, parts are processed to provide minimum residual stresses and full corrosion resistance.
B
Bevel Seat Ferrules	A set of plain and externally threaded bushings (male and female, respectively) with matching bevel angles that produce a leak-proof seal when connected with a hex union nut.
Bright Anneal	Annealing in a protective medium to prevent discoloration of the surface.
Bright Annealed Finish	A silvery satin surface, approximately the mill finish of stainless steel.
Buna	Synthetic rubber, a copolymer of acrylonitrile and butadiene.
Burst Pressure	The pressure at which rupture occurs.
C
Cavitation	When the NPSH required by the pump is greater than the NPSH available by the system, cavitation occurs. Vapor is formed and moves along the stream. The vapor bubbles collapse when they reach regions or higher pressure on their way through, the pump cavities are forming in the liquid being pumped.
Clamp	A device used to join mechanical parts, fittings, ensuring a quick leak-proof connection and enabling easy take down.
Cold Flow	Continued deformation or movement of rubber or PTFE under stress.
Compression Set	The deformation that remains in rubber or PTFE after it has been subjected to and released from stress such as a clamp. The longer the stress is maintained, the more definitive the deformation.
D
DA	Double acting.
Double-Acting Pneumatic Actuator	Any pneumatic actuator which uses air to drive the actuator output shaft in both the open and closed direction. The air supply is piped to one side of a piston-drive or a diaphragm while the air contained on the opposing side is exhausted.
DPDT	Double pole-double throw.
Durometer	Instrument used for measuring the hardness of rubber.
Durometer hardness	A numerical value which indicates the resistance to indentation of the blunt indentor of the durometer.
E
EPDM	Ethylene Propylene Diene Monome, a synthetic rubber.
Elastomer	Any of various elastic substances resembling rubber.
Electric Actuator	An electro-mechanical device used to open and close or modulate a valve.
Electric Failsafe Actuator	Electrically driven actuator that contains an internal spring to close the valve on loss of power.
Encapsulation	The enclosing of material by an encapsulant for protective purposes.
F
Fail-Closed	Spring return pneumatic actuator is applied to the valve such that the spring will drive the valve to the close position upon loss of air (may be termed air-to-open).
Fail-Open	Spring return pneumatic actuator is applied to the valve such that the spring will drive the valve to open position upon loss of air (may be termed air-to-close).
Ferrule	A bushing used to secure a tube joint. A special bushing designed for welding to the end of tubing. Two ferrules and a gasket make a leak-proof connection when used with the complimentary clamps.
Fitting	A small part of an apparatus. See our fitting numbering guide.
Fluorocarbon	Elastomer known as Viton®, a registered trademark of DuPont.
Friction Loss	The reduction of water pressure which occurs as water travels over distance and through and kind of restriction.
G
Gasket	A seal used between matched machine parts to prevent escape of a gas or fluid. Special seals fit between the ferrules on the fittings, which are then clamped together, compressing the seal and making a leak-proof joint.
H
Hex Union Nut	An internally acme-threaded six-sided connector used to assemble bevel seat fittings.
I
ISO5211	International standard for actuator and valve interface.
Internal Expansion (IX)	A plug (or bullet) is pulled through a stem or set of blades (fingers) increase the stem I.D. to the plug O.D. or a predetermined setting when using expansion blades. This forces the stem serrations into the hose tube and the hose cover into the serrations of the ferrule.
L
Laminar Flow	The flow of a fluid when each particle of the fluid follows a smooth path, paths which never interfere with one another. One result of laminar flow is that the velocity of the fluid is constant at any point in the fluid.
M
Manual Over-ride	Any mechanical device by which an automated valve may be manually operated. On smaller actuators, this may simply be wrench flats on the output shaft of the actuator. Larger actuators may require a more sophisticated system, such as declutchable hand wheels, or manual gears.
Maximum-Shut-Off Pressure	The pressure of the media flowing into the valve against which the valve will have to close.
Media	The material flowing through the valve.
MTR	Material Test Report
Modulating Service	When the valve is being used to throttle or regulate the rate of flow by being positioned at varying positions between open and closed.
N
NAMUR	International Standard of Interface for Actuator Connections
NEMA 4	Weather-proof enclosure suitable for indoor/outdoor applications to protect from windblown dust, rain or hose-directed water.
NEMA 4x	Offers the same protection as Nema 4 with the addition of corrosion resistance.
NEMA 6	Enclosure that may be submerged up to six feet for 30 minutes.
NEMA 7	Enclosure for hazardous locations must be capable of withstanding an internal explosion of gases so as not to ignite an external gas-air mixture.
NEMA Rating	National Electrical Code Ratings for electrical component enclosures.
Neoprene	Synthetic rubber, chemically and structurally similar to natural rubber.
Nominal Size	A dimensional value assigned for the purpose of convenient designation.
O
On-Off Service	When the valve is being used to cycle to the full open or full closed position.
Operating Pressure	The pressure at which the system functions.
P
Pneumatic Actuator	An air operated mechanical device used to open and close or modulate a valve. The actuator, which is mounted to the valve by a bracket and coupled to the stem, is designed to convert air pressure into mechanical force sufficient to operate the valve.
Polypropylene	A lightweight synthetic plastic.
PSI	Pounds per square inch.
PSIG	Pounds per square inch gauge.
PTFE	Tetrafluoroethylene, DuPont’s Teflon®, is a high performance engineering plastic that is not wet by many liquids.
S
Silicone	Dimethyl silicone, a synthetic rubber.
SPDT	Single pole double throw.
Spring-Return Pneumatic Actuator	Any pneumatic actuator which contains a single coil spring or group of coil springs to oppose the movement of a piston. As air moves the piston or diaphragm, the spring is compressed.  When the air supply is discontinued and exhausted, the spring extends and drives the piston in the opposite direction.
SPST	Single pole single throw.
Stem Torque	The force required at the valve stem to open or close the valve. Torque is expressed in inch pounds or foot pounds.
Supply Pressure	The plant air supply pressure available to operate a pneumatic actuator.
Surge	A rapid rise and decrease of internal pressure.
T
Torque	A twisting or turning force usually measured in inch pounds or foot pounds.
Tube	A hollow cylinder especially one that conveys a fluid. For sanitary applications, a thin wall is implied.
Tube Fitting	A length of tubing formed into a usable shape either welded to an apparatus or welded to ferrules for use in an apparatus.
Turbulent Flow	Irregular flow that is characterized by tiny whirlpool regions. The velocity of this fluid is NOT constand at every point.
W
WOG	Water, Oil, Gas. Pressure rating for valves handling these products.
Welding	Join two or more pieces of material by applying heat to produce a localized union through fusion across the interface. For sanitary fittings, a ferrule is attached to the ends of a tube fitting by TIG welding without the addition of filler metal. Tube fittings can then be joined with clamps and gaskets to form parts of a system.

Actuated or automatic valves that revert to a pre-determined position after the actuating force is removed are referred to as “fail-safe” valves. The most common type is “fail-safe normally-closed.” On the other hand, “fail-safe normally-open” valves are much less common, but are equally important. These are often found in cooling systems, or are used where flow is shut off only for periodic maintenance, etc. Some valves, such as spring-loaded air actuated ball valves, can be converted between types.

The illustration below shows two air-actuated shut-off valves, identical but for the fact that one is normally-open and the other is normally-closed. The only real difference between the valves is the location of the spring in relation to the piston. Because these valves use compressed air to overcome the spring force, the air acts on the opposite side of the piston. If you study the valves, you can see where the air is fed in.

Normally-Closed Valves

An example of a normally-closed actuated valve is Plast-O-Matic’s Series EASMT Solenoid Valve. The valve opens when energized, and when that current is turned off, an isolated spring inside the valve forces it closed. The magnetic force of the solenoid coil must overcome the force of the spring to open the valve. Unless acted upon, the spring force keeps the valve closed. In the event of a power failure, the valve will automatically close.

An example of a normally-closed automatic valve is Plast-O-Matic’s Series CKM Check Valve. The valve opens when pressure is present at the inlet (or upstream) side. When that pressure ceases, the diaphragm inside the valve automatically forces it closed. The inlet pressure must overcome the force of the diaphragm to open the valve. Unless acted upon by inlet pressure, the diaphragm force keeps the valve closed. In fact, the valve closes before reverse flow can take place. (note that the above refers exclusively to Series CKM/CKS; most check valves, including ball type, are not normally-closed valves.)
Normally-Open Valves

An example of a normally-open actuated valve is Plast-O-Matic’s Series EASY-NO Solenoid Valve. The valve closes when energized, and when that current is turned off, an isolated spring inside the valve forces it open. The magnetic force of the solenoid coil must overcome the force of the spring to close the valve. Unless acted upon, the spring force keeps the valve open. In the event of a power failure, the valve will automatically open.

An example of a normally-open automatic valve is Plast-O-Matic’s Series PRE Pressure Regulator. The valve is set to remain open at a predetermined pressure, and begins to close automatically when downstream pressure exceeds that predetermined pressure. When pressure downstream drops back to accepted levels, a spring inside the valve forces it back open. The downstream pressure must overcome the force of the spring to close the valve. Unless acted upon by excessive downstream pressure, the spring keeps the valve open.
Valves that can be Converted between Normally-Open and Normally-Closed

Air-Actuated, Spring-Return Ball Valves — Plast-O-Matic Series ABVS, ABRS & ABMS — are examples of valves that can be normally-open or normally-closed. In these valves, a rather large spring is used to force the actuator back to its original position, whether that is open or closed.

One of the advantages of a Plast-O-Matic Air x Spring Actuated Ball Valve is that it can be converted fairly easily between one type or the other. Because the spring forces the actuator one way or the other, it is simply a matter of redirecting what the spring acts upon.

With Series ABVS, this is done by backing off the adjusting cap (so the spring will not be compressed) then unscrewing the spring housing from one side of the actuator and moving it to the opposite side. (Likewise, the compressed air is swapped).

Converting Series ABRS & ABMS is a little more complex. The actuator must be removed from the valve (very easy to do), then a retaining ring on the bottom of the actuator must be removed (requires retaining ring pliers). The pinion shaft is then pushed out from the top, rotated 180º, then re-inserted. The actuator will then act upon the valve opposite the way it did before.

In electronics, a normally closed switch is one that normally allows current to flow and which prevents current flow when it is perturbed.

* Think of a flashlight/torch: You put a rubber band on its switch so that the flashlight/torch is lit all the time unless you keep holding the switch, against the force of the rubber band, in the open position. The switch is then normally closed.

See also Relay

A switch that is “closed” conducts electricity. Normally closed switches are often spring loaded. The “normal” position of the switch defines its position when it is not being pressed, that is, the position to which it is forced by the action of the spring.

A normally closed switch, thus, is one that is spring loaded and conducts electricity when it is not being pressed. Pressing the switch stops the flow of electricity.

Abbreviation: NC

A switch case has a top provided with a central opening in which slides the stem of a plunger that has a foot inside the case. A bridging contact spring has a central portion engaging the foot and has free end portions extending away from opposite sides of the foot to form movable contacts normally engaging stationary electric contacts in the case beneath the movable contacts. Between each stationary contact and the central portion of the spring there is a fulcrum member beneath a movable contact so that when the central portion of the spring is moved downwardly by depressing the plunger, the movable contacts will rock on the fulcrum members and lift away from the stationary contacts.

1. A normally closed switch comprising a case having a top with an opening therethrough, a plunger having a foot in the case and a stem extending upwardly through said opening and above the case, a bridging contact member in the case at the lower end of the plunger, said contact member being formed from a spring having a central portion engaging said foot and having free end portions extending away from opposite sides of the foot to form movable contacts, a pair of stationary electric contacts mounted in the case beneath said movable contacts and normally engaged thereby, terminals connected with the stationary contacts and extending out of the case, and a fulcrum member in the case between each stationary contact and said central portion of the spring, said fulcrum members being disposed beneath said movable contacts and normally out of contact with them, the tops of the fulcrum members being lower than the tops of said stationary contacts, and the fulcrum members being of such height that when said central portion of the spring is moved downwardly by depressing the plunger said movable contacts will engage and rock on the fulcrum members and lift away from the stationary contacts to open the switch.
Description:
This invention relates to switches that are especially suitable for keyboard switches used in calculators, data processing and stock quotation equipment, cash registers, audio-visual education equipment, communication equipment, typewriters and other devices. Similar switches are shown in U.S. Pat. Nos. 3,767,878 and 3,924,089, but in each of those patents the switch is normally held upen by a spring and is closed by depressing its plunger.

It is among the objects of this invention to provide a switch of the type shown in the above-mentioned patents, which is normally closed but can be opened by depressing its plunger, and which requires only one spring in its construction.

The preferred embodiment of the invention is illustrated in the accompanying drawings, in which:

FIG. 1 is a vertical section of the closed switch;

FIG. 2 is an enlarged fragmentary view at 90.degree. to FIG. 1, with the near side of the switch case removed;

FIG. 3 is a fragmentary vertical section similar to FIG. 1, but with the switch open; and

FIG. 4 is a horizontal section taken on the line IV-IV of FIG. 1.

Referring to FIGS. 1, 2 and 4 of the drawings, a rectangular case 1, generally molded from a plastic, has in its top wall a central vertical opening, around which a cylindrical flange 2 extends upwardly a short distance. The bottom wall 3 of the case is rigidly attached to the rest of the case in any suitable manner and is provided with a central vertical post 4 designed to have a press fit in a printed circuit board or other panel 5 on which the switch can be mounted. The post holds the switch in place.

The stem of a plunger 7 is slidably mounted in the top wall opening and flange 2 of the case and extends above the flange. The lower end of the stem inside the case is provided with a foot 8 that normally is held against the top wall of the case in a manner to be described presently. A push button or cap 9 can be mounted on the upper end of the plunger for depressing it.

The foot of the plunger carries a bridging contact inside the case. The bridging contact is formed from a spring that preferably includes a wire coil 11 extending across the plunger foot as shown in FIG. 4. The coil is disposed in a downwardly opening transverse slot 12 in the bottom of the foot. The opposite end portions of the coil wire are more or less straight and extend tangentially away from opposite sides of the bottom of the coil and also away from opposite sides of the plunger foot to form the movable contacts 13 of the switch extending towards the opposite ends of the case, as shown in FIG. 1. To hold the coil in place during assembly, these movable contacts press downwardly against slotted stops 14 formed by extending beneath the contacts two diagonally opposite lower corners of the side walls of slot 12. The stops are provided with shallow upwardly opening notches 15 that receive the wire forming the bridging contact member.

To help guide the foot of the plunger in the case so as to prevent it from turning, the foot preferably is provided with guide lugs 17 extending away from the opposite side walls of slot 12 in substantially parallel spaced relation with the straight end portions of the coil wire, as shown in FIG. 4. These guide lugs project beyond the ends of the wire and into sliding engagement with the end walls of the case. Preferably, the end walls are provided with guide slots 18 extending vertically across them for receiving the ends of the lugs. Also, it is preferred that the lugs be located at diagonally opposite corners of the foot of the plunger.

The inside of the case is provied with a pair of fixed or stationary electric contacts 20 directly beneath the movable contacts. Most conveniently, the stationary contacts project upwardly from the bottom wall of the case. Also, it is preferred to make these stationary contacts integral with terminals 21 that project from the bottom of the case. Thus, each stationary contact can be the exposed wide head of a narrow terminal that extends down through an opening in the bottom wall of the case.

It is a feature of this invention that the stationary contacts extend for enough up into the case to engage the movable contacts when the plunger is in its uppermost position, as shown in FIG. 1. The movable contacts, pressing down on the stationary contacts, cause the bridging contact member to support the plunger in its uppermost position in the case, so no other spring is required for urging the plunger upwardly. Since the stationary contacts are engaged by the bridging contact member while the plunger is up, it follows that the switch normally is closed.

To provide for opening the switch when the plunger is depressed, a fulcrum member 23 is disposed between each stationary contact and the wire coil in the plunger slot. These fulcrum members are located directly beneath the movable contacts, preferably close to the stationary contacts. While the switch is closed, the movable contacts are spaced a short distance above the fulcrum members, so there is no change that those members will interfere with proper engagement of the movable and stationary contacts. The fulcrum members can be formed conveniently from a plastic joined to the bottom wall of the case.

OPERATION

When the plunger is depressed, it moves the wire coil downwardly between the stationary contacts. The descending coil first pulls the movable contacts down against the fulcrum members and then causes the movable contacts to tilt or rock on them, thereby causing the outer ends of contacts 13 to lift away from the stationary contacts and open the switch. The moment the plunger is released, the tension on the bridging contact spring will cause it to lift the plunger until its foot engages the top wall of the case, at which time the movable contacts will once again be in engagement with the stationary contacts and the switch will be closed.

According to the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

1. An electronic communication monitoring apparatus for monitoring a communication system having a transmitter, and a site housing for the transmitter, comprising:

a power sensor means for selective connection to the communication system for producing selected power measurements;

a plurality of environmental sensors connected to the site housing for determining environmental conditions affecting the site housing; and

a programmable controller means located in the site housing and connected to the power sensor means and plurality of environmental sensors, said programmable controller means including a display means, a power monitoring means operatively connected to the power sensor means for continuously monitoring operations of the communication system including power measurements for display by the display means, means for storing maintenance parameters, comparison means connected to power monitoring means for comparing the power measurements with the maintenance parameters, means connected to the comparison means for signaling a need for maintenance operations prior to failure of the communication system, environmental sensor monitoring means connected to the plurality of environmental sensors for detecting environmental signals indicative of adverse environmental conditions associated with the transmitter station housing, and an alarm means connected to the environmental sensor monitoring means for producing alarms indicative of adverse environmental conditions.

2. An electronic monitoring apparatus according to claim 1 wherein the programmable controller includes a channel selection means for monitoring a plurality of communication channels of the communication system and means for changing selected channel operating parameters of the programmable controller.

3. An electronic monitoring apparatus according to claim 1 wherein the power sensor means includes sensors within the communication system to provide selected power measurements for tunable components of the communication system, and wherein said programmable controller display means is a multiple display for simultaneously displaying power measurements in support of tuning the tunable components.

4. An electronic system monitoring apparatus according to claim 1 wherein the programmable controller includes expansion means for connecting a plurality of communication systems for operation monitoring.

5. An electronic monitoring apparatus according to claim 1 wherein the programmable controller further includes storage means for storing the communication system’s operating information and alarm information, and a printer connected to the programmable controller for printing out system operating information and alarm information from the storage means.

6. An electronic monitoring apparatus according to claim 1 further including a computer positioned remotely to the programmable controller, said computer including means connected to the programmable controller for reading communication system operating parameter information into and writing performance information including alarms from the programmable controller.

7. An electronic communication monitoring system comprising:

a plurality of electromagnetic energy transmitting antennas;

a plurality of communication transmitter channels connected to each of the antennas, each communication transmitter channel including a transmitter, a power circulator connected to the transmitter, and a combiner connected to the circulator, said combiner being connected to an antenna of the plurality of antennas;

a plurality of power sensors connected to the communication transmitter channels including an analog signal producing sensor means connected to each communication channel for producing analog signals indicative of transmitter input power to the combiner and reflected transmitter power (IPR) on an input side of the combiner and an analog signal producing sensor means connected to each communication channel for producing analog signals indicative of reflected power (OPR) from an antenna side of the combiner and combiner power (OPF) to the antenna;

a controller station including a programmable controller and a plurality of environmental sensors connected to the controller station for producing signals indicative of controller station environmental conditions taken from a group consisting of flooding, fire, temperature, and unauthorized entry; said programmable controller having: means connected to the plurality of power sensors for producing measurements of the IPF, IPR, OPF, OPR, means connected to the power sensors for computing transmitter and antenna voltage standing wave ratios, means connected to the power sensors for determining combiner insertion loss per channel, clock means including means for obtaining time and date of measurements, and means connected to the means for obtaining time and date of measurements for recording the time and date of the most recent channel measurement, means connected to the environmental sensors for automatically producing alarm signals indicative of abnormal environmental conditions, means containing preselected acceptable normal measurements, comparator means connected to the measurement producing means and means containing the preselected acceptable normal measurements for comparing operational measurements to preselected acceptable normal measurements and means connected to the comparator means for producing alarm signals indicative of outside the normal measurements, means for displaying locally the measurements made either singly or in combination, means for reading operating parameters into and writing measurement information from the programmable controller, and communication port means for connecting either a printer or a remotely located computer or both; and

a remotely located station including a computer operatively connected to the programmable controller for reading operating parameters into and writing data including measurement data and alarm data from the programmable computer.

8. An electronic communication monitoring system comprising:

an antenna means including an antenna and a plurality of antenna systems, each antenna system including a plurality of communication transmitter channels, each communication transmitter channel including a transmitter, a power circulator connected to the transmitter, and a combiner of a combiner bank;

a plurality of power sensors connected to the communication transmitter channels including an analog signal producing sensor means connected to each communication channel for producing analog signals indicative of transmitter input power (IPF) to the combiner and reflected transmitter power (IPR) on an input side of the combiner, and an analog signal producing sensor means connected to each communication channel for producing analog signals indicative of reflected power (OPR) from an antenna side of the combiner and combiner power (OPF) to the antenna;

a controller station including a programmable controller and a plurality of environmental sensors connected to the controller station for producing signals indicative of controller station environmental conditions taken from a group consisting of multiple digital and/or analog signals; said programmable controller having: means connected to the plurality of power sensors for producing measurements of the IPF, IPR, OPF, and OPR, means connected to the power sensors for computing transmitter and antenna voltage standing wave ratios, means connected to the power sensors for determining combiner insertion loss per channel, clock means including means for obtaining time and date of measurements, and means connected to the means for obtaining time and date of measurements for recording the time and date of the most recent channel measurement, means connected to the environmental sensors for automatically producing alarm signals indicative of abnormal environmental conditions, means containing preselected acceptable normal measurements, comparator means connected to the measurement producing means and means containing the preselected acceptable normal measurements for comparing operational measurements to preselected acceptable normal measurements, means connected to the comparator means for producing alarm signals indicative of outside the normal measurements, means for displaying locally the measurements made either singly or in combination, means for reading operating parameters into and writing measurement information from the programmable controller, and communication port means for connecting either a printer or a remotely located computer or both.

Additional Semiconductor Tests
Identifying Unknown Bipolar Transistors
The type (NPN or PNP) and lead arrangement of unmarked transistors can be determined using a multimeter based on similar considerations. This, again, assumes the back-to-back diode model. The collector and emitter can then be identified based on the fact that the doping for the B-E junction is always much higher than for the B-C junction. Therefore, the forward voltage drop will be very slightly higher - this will show up as a couple of mV (sometimes more) difference on a DMM’s diode-test scale or a slightly higher resistance on an analog VOM.

To determine the lead arrangement, label the pins on the unknown device 1, 2, and 3. Put the positive probe (as determined above) of you multimeter on pin 1. Now, measure the resistance (VOM) or diode drop (DMM) to the other two pins. If the positive probe is on the base of a good NPN transistor, you should get low resistance readings or a low diode drop to the other two leads. The B-C resistance or diode drop will be just slightly lower than the B-E reading.

If one or both measurements to the other two pins is high, put the positive probe on pin 2 and try again. If still no cigar, try pin 3.

If this still doesn’t work, you may have a PNP transistor - repeat with the negative probe as the common pin.

If none of the six combinations yields a pair of low readings - or if more than one combination results in a pair of low readings, your transistor is likely bad - or it is not a bipolar transistor!

As noted, some power transistors have built in base resistors or damper diodes and will confuse these measurements. However, the lead arrangement of these types of transistors is usually self evident (standard TO3, TOP3, or TO220 cases). There are also some transistors with series base resistors which may prove confusing. There are relatively rare, however.

Voltage ratings are more difficult and require a low current variable DC power supply with a maximum voltage output greater than the expected (or desired) breakdown rating of the transistors being tested. A fixed DC supply with a suitable potentiometer is also satisfactory. For tests up to 100 V, a 100K ohm pot would be satisfactory. Put a current limiting resistor of about 100 K ohms in series with the output. For higher voltage transistors, use an appropriate power supply and increase the value of the potentiometer (if used) and current limiting resistor. It should be possible to determine approximate values for Breakdown Voltages such as:

BVcbo - collector to base, emitter op