Intro4u2u

      The ACM Code of Professional Conduct
–        Acquire and maintain professional competence

Commitment to ethical professional conduct is expected of every member (voting members, associate members, and student members) of the Association for Computing Machinery (ACM).

This Code, consisting of 24 imperatives formulated as statements of personal responsibility, identifies the elements of such a commitment. It contains many, but not all, issues professionals are likely to face involve four imperatives:

·         GENERAL MORAL IMPERATIVES

·         MORE SPECIFIC PROFESSIONAL RESPONSIBILITIES

·         ORGANIZATIONAL LEADERSHIP IMPERATIVES

·         COMPLIANCE WITH THE CODE

GENERAL MORAL IMPERATIVES

As an ACM member I will …
1.1 Contribute to society and human well-being.
1.2 Avoid harm to others.
1.3 Be honest and trustworthy.
1.4 Be fair and take action not to discriminate.
1.5 Honor property rights including copyrights and patents.

–        Violation of copyrights, patents, trade secrets and the terms of license agreements is prohibited by law in most circumstances. Even when software is not so protected, such violations are contrary to professional behavior. Copies of software should be made only with proper authorization. Unauthorized duplication of materials must not be condoned.

–        Patent “Copyright protects expression but not underlying ideas” eg “Software patents, like all patents, give an inventor the right to exclude all others from making, selling, or using an invention for 17 years. In return, the patentee discloses his or her ‘best method’ of implementing the invention, thereby relinquishing trade secrets that might otherwise be enforced forever (like the formula for Coca-Cola).”

1.6 Give proper credit for intellectual property.

–        Computing professionals are obligated to protect the integrity of intellectual property. Specifically, one must not take credit for other’s ideas or work, even in cases where the work has not been explicitly protected by copyright, patent, etc.

1.7 Respect the privacy of others.
1.8 Honor confidentiality.

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MORE SPECIFIC PROFESSIONAL RESPONSIBILITIES

As an ACM computing professional I will …
2.1 Strive to achieve the highest quality, effectiveness and dignity in both the process and products of professional work.
2.2 Acquire and maintain professional competence.
2.3 Know and respect existing laws pertaining to professional work.
2.4 Accept and provide appropriate professional review.
2.5 Give comprehensive and thorough evaluations of computer systems and their impacts including analysis of possible risks.
2.6 Honor contracts, agreements, and assigned responsibilities.
2.7 Improve public understanding of computing and its consequences.
2.8 Access computing and Communication resources only when authorized to do so.

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ORGANIZATIONAL LEADERSHIP IMPERATIVES

As an ACM member and an organizational leader, I will …
3.1 Articulate social responsibilities of members of an organizational unit and encourage full acceptance of those responsibilities.
3.2 Manage personnel and resources to design and build information systems that enhance the quality of working life.
3.3 Acknowledge and support proper and authorized uses of an organization’s computing and communications resources.
3.4 Ensure that users and those who will be affected by a system have their needs clearly articulated during the assessment and design of requirements; later the system must be validated to meet requirements.
3.5 Articulate and support policies that protect the dignity of users and others affected by a computing system.
3.6 Create opportunities for members of the organization to learn the principles and limitations of computer systems.

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COMPLIANCE WITH THE CODE

As an ACM member, I will ….
4.1 Uphold and promote the principles of this Code.
4.2 Treat violations of this code as inconsistent with membership in the ACM.

·         Adherence of professionals to a code of ethics is largely a voluntary matter. However, if a member does not follow this code by engaging in gross misconduct, membership in ACM may be terminated.

AITP Code of Ethics

¨      The AITP Code of Ethics

–        Obligation to management

–        Obligation to fellow AITP members

–        Obligation to society

What is AITP

1. AITP is the professional association comprised of career minded individuals who seek to expand their potential –   employers, employees, managers, programmers, and  others working with information technology.
2. AITP seeks to provide avenues for members

·         to be teachers as well as students

·         to make contacts with other AITP members

·         to become more marketable in rapidly changing technological careers.

AITP Members

•         Span every level of the IT industry

–        Mainframe systems

–        Micro systems

–        PC based LAN and WAN systems

–        Virtual systems

–        Internet

•         Are found in every facet of society

–        Colleges and universities

–        Banking

–        Industry

–        Retail

–        Armed forces

–        Local, state, and federal governments

–        Hospitals

AITP  Mission Statement

•         AITP offers opportunities for Information Technology (IT) leadership and education through partnerships with industry, government and academia.

•         AITP provides quality IT related education, information on relevant IT issues and forums for networking with experienced peers and other IT professionals.

AITP  Vision Statement

• AITP is the Information Technology professional organization of choice for providing leadership opportunities, professional development and personal growth.

AITP  Code of Ethics

•         The Code of Ethics is a standard that reminds us and binds us to the obligations that we hold as technology professionals.

•         These ideals are principles that all members should hold as a basis for their everyday careers.

Membership Provides Education

•         An open exchange of ideas and information for the resolution of technical, management and industry problems.

•         Professional contacts locally and nationally.

•         Information Executive, a monthly newspaper for the information technology professional.

•         Technical and management seminars held in conjunction with monthly meetings.

Membership Provides Professionalism

•         Active participation in AITP means hands-on development of leadership skills, increased proficiency and greater professional interest.

•         AITP membership means growing with the IT industry and pressing for higher standards of performance.

Membership Provides Commitment

•         Participation in AITP is job enrichment.

•         Builds skills, enthusiasm and competence and a new professional dimension to an employee’s occupational stature.

•         Interaction with a broad spectrum of IT professionals stimulates a disciplined, purposeful, and goal-oriented approach to the job and its opportunities

Company Advantages
Direct benefits to the company by supporting AITP memberships:

•         New technology, products and their applications are presented and discussed at monthly AITP Chapter meetings and AITP-sponsored exhibits and seminars.

•         Interaction with a broad spectrum of IT professionals motivates and challenges employees toward increased commitment

•         AITP’s structure facilitates the establishment of professional contacts.

•         AITP’s Legislative Network is mobilized on industry-related issues pending before local, state/provincial or federal bodies in the U.S. and Canada.

•         Membership in AITP accesses cost savings on business equipment and services not readily available in the marketplace.

•         On-going education addresses specific staff concerns.  AITP Chapters tailor their programs to meet the particular requirements of their members.

•         A membership in AITP is cost-effective.  According to Training Magazine, a one-year membership in AITP cost less than ¼ of the average one-day information technology course fee.

Employee Benefits
Participation in AITP provides the opportunity to:

•         Develop new management strategies.

•         Keep current with emerging IT Technologies and equipment.

•         Expand professional contacts locally and nationally.

•         Contribute to the growth of the IT industry

•         Share experience and knowledge with peers.

•         Support community programs through the AITP Chapter.

•         Access significant personal and professional cost savings on auto rental, telephone, financial services and much more!

More AITP  Benefits

•         A network of local chapters covering North America.

•         Special Interest Groups (SIGs)

•         Technical and management seminars held in conjunction with monthly chapter meetings

•         An open exchange of ideas and information for the resolution of common problems.

•         Professional contacts locally and throughout North America

•         Regular technical and news publications

•         Discounts on equipment, services, and conferences

AITP  The Purpose

•         Dedicated to the advancement of the information systems profession in

–        Business

–        Science

–        Industry

–        Government

–        Medicine

–        Telecommunications

–        All other areas

•         Primarily engaged in educational and research activities for the development of effective programs for the self-improvement of the individual member.

•         Encourage high standards of competence and professionalism.

AITP

•         Encourages its members to continue their education and professional certification through informative meeting programs, seminars, and conferences as well as through certification.

•         Is a representative of the Institute for the Certification of Computing Professionals (ICCP) which administers certification programs related to information technology.

•         AITP’s membership includes, but is not limited to:

–        IS managers

–        Programming and Systems designers

–        Systems engineers

–        Sales and Marketing representatives

–        Educators and Researchers

–        Consultants and Contractors

–        Recruiters and Vendors

–        Information Technology users

•         AITP offers a number of member benefits and services including:

–        Employment services

–        Car rental discounts

–        Group insurance plans

With clock frequencies of a few hundred megahertz, today’s electronic systems are using pulse edges in the sub-nanosecond range. Networking interfaces deliver data rates approaching 1000 Mbits/s (Gigabit Ethernet and FDDI - fiber distributed data interface) and 155 and 622 Mbits/s (ATM - Asynchronous Transfer Mode). High quality video circuits also use pixel rates at sub-nanosecond rates.  These higher processing speeds present never-ending engineering challenges

One such challenge is RF interference, which originates from a fast change of electromagnetic energy. The faster the slew rate (rise/fall times) and the higher the voltage/current amplitude, the more problematic a circuit becomes.  As a result, electromagnetic compatibility (EMC) is harder to achieve today than ever before.

While fast changing pulses of current between two nodes of a circuit represent the so-called differential noise source, the fields surrounding this circuit can couple into other components and etch connections. The noise induced via inductive or capacitive coupling represents common-mode interference.  The RF interference currents are in phase with each other, and the system can be modeled as one which connects the source, “victim circuits” or “recipients” and the return path, which in many cases is represented by a chassis. Several factors are critical in defining the amount of the interference:

o Strength of the source

o Size of the area encircled by the culprit current

o Slew rate of the change

Thus, despite many possible causes of unwanted interference in a circuit, the noise is almost always the common-mode type.  Once there is some RF voltage present between a cable plugged into an I/O (input/output) connector and the enclosure or the ground plane, the resulting RF current of a few mA can be enough to exceed the allowable emission levels.

Typical Causes of RF Interference

Noise Coupling and Dissemination

Common-mode noise can be generated by less than an ideal layout. Some typical causes are an imbalance in the length of the individual conductors in differential pairs, or differences in distance to the power planes or the chassis.  Other source are imperfections of components - magnetic inductors and transformers, capacitors and active devices such as ASICs (Application Specific Integrated Circuit).

Magnetic components, especially the so-called “slug choke” type storage inductors used in power converters, always produce an electromagnetic field. An air gap in the magnetic circuit is equivalent to a large resistor in a series circuit, where most of the applied power is dissipated. Thus, the slug choke, which is built on a ferrite rod,  generates a strong field around the rod, with highest field density near the poles.

In switching power supplies using flyback topology, the transformer must have an air gap, which is associated with the high density magnetic field. Components that are best suited for “keeping the field to themselves” are toroids, which distribute the field through the length of the core.  This is one of the reasons the toroidal construction is preferred in high-frequency networking magnetics.

Circuits with inadequate decoupling often become the source of interference as well.  If a circuit requires high pulses of current and the local decoupling is not able to support the need due to low capacitance or relatively high internal impedance, the voltage generated by the supply loop drops. This is equivalent to a ripple, or fast change of the voltage between terminals. Through the stray capacitance of the package, this event can couple into other circuits, causing common-mode problems.

When a circuit intended for I/O interface is contaminated with common-mode noise, the problem has to be resolved before it passes through the connector.  Different applications suggest various ways of dealing with this problem. In video circuits, where I/O signals are single-ended and share the same common return, the solution is to filter out the noise with small LC filters. In lower frequency serial interface networking, some capacitive shunting to the chassis can be sufficient.

Differentially driven interfaces, such as Ethernet and FDDI, are normally transformer-coupled to the I/O area, with center taps provided on one or both sides of the transformer.  These center taps are connected via high voltage capacitors to the chassis, allowing shunting of the common-mode noise to the chassis without causing distortion of the signal.

Common-Mode Noise in I/O Area

There is no generic solution for all types of I/O interfaces.  Designers whose main goal is to get the circuit working, often overlook simple details. Some basic rules should be followed to minimize the amount of noise before it reaches the connector:

Basic Rules to Follow to Minimize Noise BEFORE it Reaches a Connector

* Locate decoupling capacitors close to the load
* Minimize the size of the loop of pulsed currents with fast edges
* Keep high-current devices (i.e., drivers and ASICs) away from I/O ports
* Evaluate signal integrity to assure minimum over-or-undershoot, especially in high current critical signals (i.e., clock, bus).
* Use local filtering such as RF ferrites where necessary to absorb RF interference
* Provide a low impedance bond or reference to the chassis in the I/O area

RF Noise and Connectors

Figure_1A_Jul23_98 1Even if the designer takes most of the precautions listed above to reduce the amount of RF noise in an I/O area, there is no guarantee that the efforts will be successful enough to meet emission requirements. Figure_1B_Jul23_98Some of the noise will be conducted, traveling from inside the circuit board as common-mode current. This source of the interference is between chassis and circuit etch.  Thus, this RF current needs to close the path through the lowest impedance available between the chassis and the carrier signal lines.  If the connector does not present low enough impedance (bond to the chassis), this RF current will travel via stray capacitance. While it is passing through the cable, the emissions are inevitably generated (Figure 1A).

Another mechanism for injecting common-mode currents in an I/O area is through coupling from nearby strong sources of interference. Even some of the “shielded” connectors with a metal cover over the top are not immune in such cases, since the culprit source can be located near the bottom side of the connector, as in PC environments.  If there is an opening between a connector and the reference chassis, the induced RFFigure_2A_Jul23_98 voltage between these two entities can substantially weaken the EMC performance (Figure 1B).

How to Minimize RF Interference with Connectors

Connectors with Metal Tabs and Gasketing

There are ways of packaging connectors with additional finger stock or gaskets. The connectors provide the bonding by filling the space between the face of the connector and the enclosure. This approach requires gFigure_2B_Jul23_98askets (Figure 2A).  Metal or metallic impregnated plastic gaskets work well if they are handled properly, that is, if the surface is free of residue from the installer’s hands, and if the pressure is enough to maintain good, low-impedance contact.

Other connectors are equipped with tabs or another means of making connections to the enclosure. The maximum area of contact in this arrangement is rather small, and it is restricted

Emissions can still “leak” between tabs and an enclosure panel
by the size of the tab and its flexibility.  In the case of using the cutout in the enclosure for a shielded connector, the sides of the cutout must be properly prepared by removing the paint (Figure 2B). Any slack in tolerance may result in this connector being recessed too deeply inside the enclosure and the bond becomes intermittent if the fingers are caught in any obstacle or otherwise damaged. Every EMC engineer knows the difference between the “golden” system qualified to meet emission requirements and the one from the production line in audit.  Loose gaskets or bent tabs mounted over paint over spray in critical areas (such as connector cutouts) will cause frustration.

For severe EMI conditions, gasketed connectors should be considered for the following reasons:

* Gaskets made of conductive fabric over foam are extremely flexible, and can be mounted around the whole connector.  In PCB mounted applications, a three-sided configuration is usually most appropriate.

Regal’s EMI/RFI gasket presses firmly against enclosure, helping prevent EMI/RFI emissions

* The mechanical engineer can position the connector within an acceptable through panel dimensional tolerance of the system package.

* The connector makes a low impedance bond to the chassis, eliminating concerns for the consistency of the contact.  A gasket that slides on the inner side of the enclosure wall can be much more forgiving with the masking requirements when the paint is applied.

* For designs with forced cooling, an optimum gasket configuration can provide an additional benefit: it helps to seal the gap between the connector and the wall, reducing air leaks.  In a dusty environment, a gasket helps to keep the inside of the system clean.

Meeting Emission Specifications and Other Cost Considerations

The total cost of implementing an EMC solution must be considered in the context of the situation, especially if the situation is dictated by a failure to meet an emission specification such as EN55024 and CISPR24. In these kinds of situations, an EMI/RFI problem is typically discovered in final testing at a testing lab.  When designers are faced with options that range from complete circuit redesign to swapping in EMI/RFI suppression connectors in key I/O areas, the swapping option is clearly the more favorable option—even though EMI/RFI suppressing connectors are more expensive. It is not unusual, once a connector based solution is identified, to implement a solution that is measured in days, as opposed to weeks and months of time-consuming circuit redesign and testing. The key is identifying the “right” EMI/RFI connector, or combination of connectors, that will effect the most cost-effective and timely solution.

Conclusion

Care must be taken to identify and understand the contribution levels and types of interference sources. The variety of connectors available on the market today enables designers to select the optimum design for the specific interface.

Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance caused in a radio receiver or other electrical circuit by electromagnetic radiation emitted from an external source. [1] The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.

EMI can be induced intentionally for radio jamming, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.

EMI/RFI types

EMI or RFI may be broadly categorized into two types; narrowband and broadband.

Narrowband interference usually arises from intentional transmissions such as radio and TV stations, pager transmitters, cell phones, etc. Broadband interference usually comes from incidental radio frequency emitters. These include electric power transmission lines, electric motors, thermostats, bug zappers, etc. Anywhere electrical power is being turned off and on rapidly is a potential source. The spectra of these sources generally resembles that of synchrotron sources, stronger at low frequencies and diminishing at higher frequencies, though this noise is often modulated, or varied, by the creating device in some way. Included in this category are computers and other digital equipment as well as televisions. The rich harmonic content of these devices means that they can interfere over a very broad spectrum. Characteristic of broadband RFI is an inability to filter it effectively once it has entered the receiver chain. [2][3]
[4]

[edit] Power line noise

Virtually all power-line noise, originating from utility company equipment, is caused by a spark or arcing across some power-line related hardware. A breakdown and ionization of air occurs, and current flows between two conductors in a gap. The gap may be caused by broken or loose hardware such as a cracked insulator. Typical culprits include insufficient and inadequate hardware spacing such as a gap between a ground wire and a staple. Once an ionized path is established in the gap, current flows at all parts of the cycle where the voltage is higher than the breakdown voltage of the gap. This typically occurs only at the positive and negative voltage peaks — the times of highest instantaneous voltage throughout the cycle.

As an example for a 60Hz system (i.e.power-lines carrying 60 Hz AC, such as in the US), the voltage on them passes through two peaks each cycle (one positive and one negative) and pass through zero twice each cycle. This gives 120 peaks and 120 zero crossings in each second (50Hz: 100 peaks and crossings correspondingly). Power-line noise follows this pattern, generally occurring in bursts at a rate of 120 bursts per second. This gives power-line noise a characteristic sound that is often described as a harsh and raspy hum or buzz. Because the peaks occur twice per cycle, true power-line noise has a strong 120-Hz modulation on the signal (50Hz system: 100Hz).[5]

[edit] Mitigation

Main article: Electromagnetic compatibility

On integrated circuits, the most important means of reducing EMI are: the use of bypass or “decoupling” capacitors on each active device (connected across the power supply, as close to the device as possible), risetime control of high-speed signals using series resistors, and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like.

The efficiency of the radiation depends on the height above the ground or power plane (at RF one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.

At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.

Switching power supplies can be a source of EMI, but have become less of a problem as design techniques have improved.

Most countries have legal requirements that mandates electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios).

[edit] Susceptibilities of different radio technologies

Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can be used to select one signal in space to the exclusion of others.

The most extreme example of digital spread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of the radio spectrum at low amplitudes to transmit high-bandwidth digital data. UWB, if used exclusively, would enable very efficient use of the spectrum, but users of non-UWB technology are not yet prepared to share the spectrum with the new system because of the interference it would cause to their receivers. The regulatory implications of UWB are discussed in the Ultra-wideband article.

[edit] Interference to consumer devices

Complex electronic circuitry is found in all sorts of devices used in the home. This results in a vast interference potential that didn’t exist in earlier, simpler decades. In the US, Public Law 97-259, enacted in 1982, gave the FCC the authority to regulate the susceptibility of consumer electronic equipment sold in the United States. The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs.[6]

Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blankets, ultrasonic pest controls (bug zappers), heating pads, and touch controlled lamps.[7]

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.

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 hysteresis loop, of the special grade of ferroxcube known as ‘ferroxcube W.

The magnetic core is a key component in electrical devices such as electromagnets, transformers and inductors. Its role is to increase the strength and effect of magnetic fields produced by electric currents. The properties of the device will depend crucially on the following factors:

* the geometry of the magnetic core.
* the amount of air gap in the magnetic circuit.
* the properties of the core material (especially permeability and hysteresis).
* the operating temperature of the core.
* whether the core is laminated to reduce eddy currents.

Straight cylindrical rod

Most commonly made of the ferrite or similar material, and used in radios especially for tuning an inductor. The rod sits in the middle of the coil and small adjustments of the rod position will fine tune the inductance. Often the rod is threaded to allow adjustment with a screwdriver. In radio circuits, a dob of wax or resin is used once the inductor has been tuned to prevent the core from moving.

The presence of the high permeability core increases the inductance but the field must still spread into the air at the ends of the rod. The path through the air ensures that the inductor remains linear. In this type of inductor radiation occurs at the end of the rod and electromagnetic interference may be a problem in some circumstances.

[edit] Single “I” core

Like a cylindrical rod but square, rarely used on its own.

[edit] “C” or “U” core

U and C-shaped cores are the simplest solution to form a closed magnetic circuit, when used alongside a I or another C or U’ core.

a U-shaped core, with sharp corners

the C-shaped core, with rounded corners

[edit] “E” core

E-shaped core are more symmetric solutions to form a closed magnetic system. Most of the time, the electric circuit is wound around the center leg, whose section area is twice that of each individual outer leg.

Classical E core

The EFD’ core allows for construction of inductors or transformers with a lower profile

The ER core has a cylindrical central leg.

the EP core is halfways between a E and a pot core

[edit] “E” and “I” core

Sheets of suitable iron stamped out in shapes like the (sans-serif) letters “E” and “I”, are stacked with the “I” against the open end of the “E” to form 3-legged structure; coils can be wound around any leg, but usually the center leg is used. This type of core is much used for power transformers, autotransformers, and inductors.

Construction of an inductor using two ER cores, a plastic bobbin and two clips. The bobbin has pins to be soldered to a printed circuit board.
Construction of an inductor using two ER cores, a plastic bobbin and two clips. The bobbin has pins to be soldered to a printed circuit board.
Exploded view of the previous figure showing the structure
Exploded view of the previous figure showing the structure

[edit] Pair of “E” cores

Again used for iron cores. Similar to using an “E” and “I” together, a pair of “E” cores will accommodate a larger coil former and can produce a larger inductor or transformer. If an air gap is required, the centre leg of the “E” is shortened so that the air gap sits in the middle of the coil to minimise fringing and reduce electromagnetic interference.
a pot core of ‘RM’ type
a pot core of ‘RM’ type

[edit] Pot core

Usually ferrite or similar. This is used for inductors and transformers. The shape of a pot core is round with an internal hollow that almost completely encloses the coil. Usually a pot core is made in two halves which fit together around a coil former (bobbin). This design of core has a shielding effect, preventing radiation and reducing electromagnetic interference.
A toroidal core
A toroidal core

[edit] Toroidal core

This design is based on a circular toroid, similar in shape to a doughnut. The coil is wound through the hole in the doughnut and around the outside, an ideal coil is distributed evenly all around the circumference of the doughnut. This geometry will turn the magnetic field around into a full loop and thus will naturally keep the majority of the field constrained within the core material. It makes a highly efficient and low radiation transformer, popular in hi-fi audio amplifiers where desirable features are: high power, small volume and minimal electromagnetic interference. It is, however, more difficult to wind an electrical circuit around it than with a splitable core (a core made of two elements, like two E). Automatic winding of a toroidal core requires a specific machinery.
A planar ‘E’ core
A planar ‘E’ core

[edit] Planar core

A planar core consists of two flat pieces of magnetic material, one above and one below the coil. It is typically used with a flat coil that is part of a printed circuit board. This design is excellent for mass production and allows a high power, small volume transformer to be constructed for low cost. It is not as ideal as either a pot core or toroidal core but costs less to produce.

*      ATEX is an acronym for ATmosphere EXplosive. ABB Entrelec manufactures many of their standard terminal blocks under the ATEX directives. These terminal blocks undergo rigorous testing to comply with the ATEX 95 European directive 94/9/EC for use in the mining and surface industries that may experience potentially explosive conditions. Under this directive, the ATEX terminal block is categorized as a “Component”.
*

[
o Screw Clamp, Spring Clamp and IDC (ADO® System) Connection Technologies
o Intrinsic Security EN50020
o Increased Security EN50019
o Specific Range for Security and Tracking of ATEX Products
o Protection Types EExi and EExe
o Certificate of Conformity Included in Every Shipment (CE Certification upon request)
o ATEX Product Marking (see chart below)
o Unmistakable ATEX Part Numbering and Batches
o Feed through, Ground and Neutral Functions
ABB ATEX terminal blocks range is certified in the following group and categories:

Explosionproofing as core competence
thuba AG guarantees your long-term success with its clear organizational structure and creates transparency and efficiency for both employees and customers by focus-ing on its core competence of explosionproofing (Ex protection).

What does explosionproofing mean?
Ex protection (explosionproofing) refers to measures taken to design and manufac-ture electrical and non-electrical equipment in such a way that it can be operated safely in areas at risk of explosion.

Common sources of ignition include, for instance, electrical sparking at switches and hot surfaces.

The ATEX directives
In Europe, explosionproofing is regulated by the ATEX directives. The designation ATEX, an acronym from the French “atmosphère explosible”, stands for the two European Community directives 94/9/EC (ATEX 95) and 1999/92/EC (ATEX 127).

Explosionproof lamps
Explosionproof (Ex) lamps are used for maintenance and inspection chores in Cate-gory 1, 2 or 3 areas at risk of gas or dust explosions.

Safety through pipe trace heating
Trace heating systems are used to heat piping electrically by means of a heating ca-ble. For example, the cable is attached to piping to prevent water pipes from freezing up in winter or to stabilize the temperature of process piping.

#SAFEKEY’ System:  This patented flexible stainless steel key in a machined keyway eliminates stress concentration caused by capscrews, helicoils (thread inserts), etc., increases strength and provides a safety lock against “DANGEROUS” disassembly. Click here for more detailed information.

#NAMUR Standard Slotted Pinion:  Provides a self centering, positive, no slop drive for positioners and switches and eliminates the actuator/accessory coupling.

#NAMUR Solenoid Mounting Pad:  An International Standard. Permits a choice of various manufacturers’ solenoid valves to be direct mounted to the actuator. A single solenoid valve can be used for all actuator sizes.

#ISO 5211 Standard Mounting Pad:  An International Standard. Designed for optimum strength and interchangeability. Standardized mounting dimensions, bolt diameters and bolt hole depths for ease and flexibility of mounting; with or without brackets.

#Large Cast-In Air Passage:  This unique “supply-size” internal air passage permits obstruction free, fast operation and simple “air assist” when required.

#’DURASTRIP’ Bearings:  A new long lasting, permanently lubricated, corrosion resistant, replaceable bearing that extends the actuator life in the most severe and demanding applications.

#Rugged Construction:  The heavy duty castings and extra large pinion gear with maximum tooth engagement, eliminates internal backlash and resists operating shocks and fatigue.

#’CERAMIGARD’:  A unique surface finish of Di-Aluminium Tri-Oxide (AL203): a hard, corrosion resistant ceramic like coating protecting all body surfaces against wear and corrosion. Click here for more detailed information.

#Added Standard Protection:  A long cure, two part epoxy coating provides extra protection against aggressive environments.

#Simple Design:  Only 3 moving parts -1 Pinion, 2 Pistons.

#Versatile Modular Design:  Attach or remove double acting or spring modules in minutes, select any combination of fail position, pinion rotation or actuator alignment in minutes - Safely!

#Two Directional Travel Stops:  A unique, exclusive standard provides rotational adjustment for the actuator pinion, in both directions of travel. This patented design eliminates travel backlash between the rack and the pinion and works in both directions of rotation (unlike end cap stops) - standard up to size 1370 (optional stops available on 2585 and 4580).  Click here for more detailed information.

#Quality Assurance:  Manufactured and designed to the highest QA/QC procedures - BS 5750:Part 1:1987 ISO 9001-1987, Lloyd’s Register Quality Assurance Limited certified, plus many other national and corporate approvals.

#Quality Assurance Stamps:  Each actuator is hand stamped with its date of manufacture, the identification of its assembler and the identification of the tester that certified it ready.

#Cast In Identification:  Model numbers, port identifications, ratings, foundry trace and safety instructions are cast in for permanent readability.

#Pinion Thrust and Radial Bearings:  Durastrip thrust bearings protect against vertical forces and also seal against atmospheric intrusion. Durastrip Radial Bearings support all radial forces.

#Pinion Seals - Top and Bottom:  Seals to the atmosphere are located to minimize any crevices and maximize the protection against external corrosive build up.

#Safety:  Safe in design, performance and maintenance. Hytork’s safekey, bottom entry pinion, rugged casting and permanent cast in rating and instructions, indestructible springs and unique spring retractor rod system combine to exceed tomorrow’s safety requirements.

#Indestructible Fail Safe Springs:  Designed, built and protected to never break - rated to compensate for “spring set” for true fail safe confidence. Guaranteed and backed by a free complete actuator replacement. Highest “end of stroke” forces in the industry, for maximum reserve.

#Extensive Range:  HYTORK provides a complete range of Actuators, 11 sizes for torque requirements to 3950 Nm. (2914 Lb/ft)

#Actuator Over Travel and Travel Stops:  High performance and special duty valves require precise and specific rotation limits to perform their intended function.

Proximity sensors may be of the contact or non-contact type. Contact proximity sensors are the least expensive.  Proximity sensors can have one of many technology types.  These include capacitive, eddy current, inductive, photoelectric, ultrasonic, and Hall effect.  Capacitive proximity sensors utilize the face or surface of the sensor as one plate of a capacitor, and the surface of a conductive or dielectric target object as the other. The capacitance varies inversely with the distance between capacitor plates in this arrangement, and a certain value can be set to trigger target detection.   In an eddy current proximity sensor electrical currents are generated in a conductive material by an induced magnetic field. Interruptions in the flow of the electric currents (eddy currents), which are caused by imperfections or changes in a material’s conductive properties, will cause changes in the induced magnetic field. These changes, when detected, indicate the presence of change in the test object.  Magnetic inductive devices are identical in configuration to the variable reluctance type and generate the same type of signal.  However, inductive pickoff coils have no internal permanent magnet and rely on external magnetic field fluctuations, such as a rotating permanent magnet, in order to generate signal pulse.  Photoelectric devices are used to detect various materials at long range, using a beam of light. They detect either the presence or absence of light and use this information to read the data from the output transistor.  An ultrasonic proximity sensor emits an ultrasonic pulse, which is reflected by surface and returned to sensor. Speed can be determined by measuring frequency difference (Doppler Effect).  The basic “Hall Effect” sensing element is a semiconductor device which, when electrical current is sent through it, will generate an electrical voltage proportional to the magnitude of a magnetic field flowing perpendicular to the surface of the semiconductor.

The most important parameter to consider when specifying proximity sensors is the operating distance.  This is the rated operating distance is the distance at which switching takes place.  Common body styles for proximity sensors are barrel, limit switch, rectangular, slot style, and ring.  Important dimensions to consider when specifying proximity sensors include barrel diameter, length, width, and height.

Proximity sensors can be a sensor element or chip, a sensor or transducer, an instrument or meter, a gauge or indicator, a recorder or totalizer, and a controller.  A sensor element or chip denotes a “raw” device such as a strain gage, or one with no integral signal conditioning or packaging.  A sensor or transducer is a more complex device with packaging and/or signal conditioning that is powered and provides an output such a dc voltage, a 4-20mA current loop, etc.  An instrument or meter is a self-contained unit that provides an output such as a display locally at or near the device. Typically also includes signal processing and/or conditioning.  A gauge or indicator is a device that has a (usually analog) display and no electronic output such as a tension gage.  A recorder or totalizer is an instrument that records, totalizes, or tracks force measurement over time.  Includes simple datalogging capability or advanced features such as mathematical functions, graphing, etc.