Intro4u2u

8/6/2008
Ultrasonic Flowmeter Market to Reach $590 Million by 2012
Propelled by strong growth in the oil & gas industry, the worldwide market for ultrasonic flowmeters is expected to grow at a compounded annual growth rate (CAGR) of 9.9% over the next five years.  The market was $367 million in 2007 and is forecasted to be over $589 million in 2012, according to a new ARC Advisory Group study.

UltrasonicUltrasonic flowmeters, once limited to use in niche applications, have become the fastest growing flow technology, particularly in the hydrocarbon industries.  While ultrasonic flowmeter technology has been available for decades, it has only in recent years begun to see more widespread adoption.  “Ultrasonic meters offer a compelling value proposition to users, and stand poised for widespread adoption in the process industries.  Given its smaller market size relative to other flow technologies, sustained growth of the hydrocarbon industries, and a large installed base of obsolete and maintenance-heavy mechanical metering technologies ripe for replacement, ARC expects the ultrasonic market to continue to grow at near double-digit rates in coming years,” according to Analyst Allen Avery, the principal author of ARC’s “Ultrasonic Flowmeter Worldwide Outlook”.

Custody Transfer Segment Sees Strong Growth
Almost all of the growth of the process ultrasonic market in recent years has been due to increased shipments to the oil & gas industry, which nearly doubled over previous levels.  It appears that the custody transfer market for natural gas has taken shape, thanks to adoption of the AGA9 custody transfer standard, and the use of ultrasonic meters for liquid custody transfer is increasing.  The oil & gas industry has set aside its conservative stance on field device technology and has begun to embrace ultrasonic metering, particularly in new projects that allow the design of an infrastructure appropriate to achieve the best meter performance.

Smart Meters Provide Diagnostic Information
Fieldbus enabled smart meters will see strong growth, as flowmeters are installed as part of overall control systems.  Communication via fieldbus networks allows users to remotely configure, monitor, and control their ultrasonic flowmeters.  Meters that use digital communications protocols can also provide users with a wealth of diagnostic information about the health of not only the meter, but also the process.  With sophisticated electronics embedded in ultrasonic flowmeters and visualization software, users can monitor flow profiles, the effects of flow conditioning, and detect potential line blockages.  Armed with this data, users can optimize their meter calibration and maintenance practices.

Asia, Middle East Lead Growth
The largest growth will occur in Asia and EMEA regions.  China and India are expected to make robust investment in basic infrastructure and new manufacturing plants.  As energy-poor China seeks fuel for its rapid economic growth, it will ramp up its oil & gas infrastructure.  The Middle East will continue to be fertile ground for ultrasonic meter suppliers, due to its role in oil & gas production. Growth in North America will be relatively modest, but still healthy due to investment in oil & gas infrastructure.

Resistance Temperature Detectors: Theory and Standards

With these general guidelines to the basic function, performance, and recognized standards for RTD’s, anyone can specify the right device for the application.

Setting the specifications for any sensor or instrument can be a difficult process, and RTD’s (resistance temperature detectors) are no exception. No one can be expected to be an expert in all fields, and frankly, no one needs to be. With these general RTD guidelines, along with a little common sense and background information on the application, you will successfully detail the specifications of an RTD that will satisfy your requirements.

THEORY OF OPERATION

A basic physical property of a metal is that its electrical resistivity changes with temperature. All RTD’s are based on this principle. The heart of the RTD is the resistance element. Several varieties of semi-supported wire-wound fully supported bifilar wound glass, and thin film type elements are shown here.

Some metals have a very predictable change of resistance for a given change of temperature; these are the metals that are most commonly chosen for fabricating an RTD. A precision resistor is made from one of these metals to a nominal ohmic value at a specified temperature. By measuring its resistance at some unknown temperature and comparing this value to the resistor’s nominal value, the change in resistance is determined. Because the temperature vs. resistance characteristics are also known, the change in temperature from the point initially specified can be calculated. We now have a practical temperature sensor, which in its bare form (the resistor) is commonly referred to as a resistance element.

Through years of experience, the characteristics of various metals and their alloys have been learned, and their temperature vs. resistance relationships are available in look-up tables. For some types of RTD’s, there are also equations that give you the temperature from a given resistance. This information has made it possible for instrument manufacturers to provide standard readout and control devices that are compatible with some of the more widely accepted types of RTD’s.

RTD SPECIFICATIONS

Eight salient parameters must be addressed for every RTD application to ensure the desired performance. Many will be specified by the manufacturer of the instrument to which the RTD will be connected. If it is a custom circuit or special OEM application, the designers must make all the decisions. The four specifications dictated by the instrumentation or circuitry are: sensor material, temperature coefficient, nominal resistance, and, to some extent, wiring configuration. Sensor Material Several metals are quite common for use in RTD’s, and the purity of the metal as well as the element construction affects its characteristics. Platinum is by far the most popular due to its near linearity with temperature, wide temperature operating range, and superior long-term stability. Other materials are nickel, copper, balco (an iron-nickel alloy), tungsten, and iridium. Most of these are being replaced with platinum sensors, which are becoming more competitive in price through the wide use of thin film-type resistance elements that require only a very small amount of platinum as compared to a wire-wound element.

Temperature Coefficient

The temperature coefficient (TC), or alpha of an RTD is a physical and electrical property of the metal alloy and the method by which the element was fabricated. The alpha describes the average resistance change per unit temperature from the ice point to the boiling point of water. Various organizations have adopted a number of different TC’s as their standards (see “Temperature Coefficient Standards”).

Nominal Resistance

Nominal resistance is the pre-specified resistance value at a given temperature. Most standards, including IEC-751, use as their reference point because it is easy to reproduce. The International Electrotechnical Commission (IEC) specifies the standard based on 100.00 Ohms at 0°C, but other nominal resistance’s are quite common. Among the advantages that thin film technology has brought to the industry are small, economical elements with nominal resistance’s of 500, 1000, and even 2000 ohms.

Wiring Configuration

The wiring configuration is the last of those parameters typically specified by the instrument manufacturer, although the system designer does have some control based on the application. An RTD is inherently a 2-wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding additional, uncompensated resistance into your system. Most applications therefore add a third wire to help the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured temperature.

Four-wire RTD’s provide slightly better compensation, but are generally found only in laboratory equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3-wire RTD; if the fourth wire is connected, new errors will be introduced. Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the un compensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer’s recommendations.

Two other parameters are more application dependent;

the temperature range of the application; and,
the accuracy.

Temperature Range

According to the ASTM, platinum RTD’s can measure temperatures from -200°C to 650°C. (IEC says -200°C to 850°C).

You must consider the temperature limitations of all the materials involved, where they are applied, and the temperatures to which each will be exposed.

A few quick examples to illustrate this point:

TFE Teflon should not be used for wire insulation if it will be exposed to temperatures above 200°C (250°C for some).

Moisture proof seals are commonly made with various types of epoxy that generally have limits below that of the Teflon insulation.

Many wire insulating materials become brittle at subzero temperatures and therefore should not be used for cryogenic work.

So state the intended temperature range right up front and let the applications engineer assist you, especially since it may affect the materials chosen for internal construction of the probe.

Accuracy

You are probably wondering why accuracy was not the first topic covered, because RTD’s are generally known for their high degree of accuracy and it is typically one of the first specifications laid out. Well, the subject is not quite that simple, and it requires a bit of discussion. First, we must establish the difference between accuracy, precision, and repeatability. In the case of temperature, accuracy is commonly defined as how closely the sensor indicates the true temperature being measured, or in a more practical sense, how closely the resistance of the RTD matches the tabulated or calculated resistance of that type RTD at that given temperature.

Precision, on the other hand, is not concerned with how well the RTD’s resistance matches the resistance from a look-up table, but rather with how well it matches the resistance of other RTD’s subjected to that temperature. Precision generally refers to a group of sensors, and if the group has good precision at several temperatures, we can also say that they are well matched. This is important when interchangeability is a concern, as well as in the measurement of temperature gradients. Repeatability can best be described as the sensor’s ability to reproduce its previous readings at a given temperature.

Here’s an example. An ice point reading is done with an RTD that is then used to take readings at 100°C, 150°C, 37°C, and again at 0°C.

A comparison of the first and last ice point readings will give you an indication of the sensor’s repeatability under those conditions. A note of caution, however: an RTD’s repeatability is very application-dependent. So when you get right down to it, accuracy without repeatability is worthless. If you start with a sensor that is ±0.03°C at 0°C but is found to have repeatability only around ± 0. 5°C, what you have is a sensor whose readings are far less reliable than a standard-accuracy probe with good repeatability. A high-accuracy RTD installed in a field application also does not ensure that you will be getting a highly accurate signal back at the control room.

Most 4-20 mA transmitters and many display units and controllers have adjustable zero and span controls that if improperly adjusted will destroy the high accuracy of the RTD signal.

The best solution for applications of this type is to have both the RTD and the transmitter, or display, or whatever, calibrated as a unit by a certified calibration laboratory.

Fortunately, the requirements for this degree of accuracy best solution for applications of this type are few and far between. For more on this subject see, Accuracy Standards.

Our final two parameters are application dependent and vary from the specification of a bare resistance element to a large industrial assembly with thermowells, connection heads, and possibly field -mounted transmitters. We will discuss only the most basic areas: physical dimensions and size restrictions, and material compatibility.

Dimensions and Size

The physical dimensions and size requirements can be more complicated than you might think. On the low end, a resistance element to be used in the construction of a sheathed RTD generally requires only that the element is small enough to fit into the desired sheath ID. For cylindrical elements, such as wire-wound units, this is obvious-just don’t forget to allow for the wall thickness of the sheath. For thin film-type elements, we must apply the Pythagorean theorem; we need to know the width of the element, w, and the thickness of the element at its largest point, t. Then the minimum ID of the sheath will be given by; ID > (w2 + t2).

When we begin to discuss RTD probes and assemblies, the subject becomes more demanding. We need to examine the mounting arrangement: will it be used for direct immersion or with a thermowell? Or will it be something special, like an exposed airflow probe or surface mount sensor? Probe designs are endless in their configurations, and it seems that most applications have some unique requirements that make this a rather creative field in itself.

In many applications, the probe is immersed in a small vessel or piping system. Dimensions here are generally limited to sensor diameter (which affects response time); immersion depth into the fluid; and the mounting arrangement, i.e., will the sensor be screwed into a threaded port, typically with ANSI tapered threads, or will it be used in con-junction with a fluid seal already in place? Or will some other special considerations need to be made? There may be other variables, such as pressure limitations or high flow, depending on the complexity of the application. It is always best to look at the whole picture. and then discuss it with your applications engineer.

Thermowells are generally used for larger vessels and systems so that the system will not have to be drained in the event the sensor requires calibration or changing. Assuming the thermowell has already been specified, we need only to specify the probe diameter (typically ¼ in. OD for a 0.260 in. bore well), the depth of the thermowell bore, and how the RTD will be secured into the well (typically spring-loaded through a ½ in. NPT nipple or hex-nipple).

Material Compatibility

Most people specifying RTD probes have to pay attention only to the chemical compatibility that will prevent corrosion. This is generally straightforward and guidelines can be taken from other materials used in the system in which the RTD will be installed. If the piping system is constructed of 316 S.S., then the probe probably should be also. But always check a corrosion guide for corrosion rates and material recommendations if you have the slightest doubt.

For applications involving thermowells, the thermowell will carry the burden of corrosion protection. However, be sure to protect the connecting wires and any terminals or plugs from possible corrosion caused by splash or corrosives in the atmosphere.

SUMMARY

There are quite a few things to be considered when specifying an RTD probe or even resistance elements. But it’s just a matter of applying a bit of common sense and using information from the application environment to set down a clear set of requirements. And if there is something you are uncertain about, get your background information together and call that applications engineer. We can’t all be experts at everything

When an automated, oil filled, load-transfer switch, installed more than 20 years ago, failed and interrupted service to a platform-mounted transformer bank, we suspected a lightning strike as the culprit. The transformer fed a storm-drain pump, whose sole purpose was to drain flood waters off of an interstate highway system in a heavily traveled urban underpass. The circuit consisted of a primary selective system to provide two separate electrical circuits to ensure continued pumping in the case of a system failure. The switch, an externally mounted stored-energy mechanism with porcelain air bushings, was platform-mounted about 15 ft (4.6 m) above ground. After the failure, temporary repairs were made and one circuit was back on line in about 2 hr. Replacing the Switch Since the state in which we are located requires an automatic changeover switch, we advertised for a replacement. We expected that we would install the same kind of switch in the same circuit configuration. We received two bids, one from the company that provided the original oil switch and the other from Joslyn Power Products Corp. of Alsip, Illinois, U.S. for a SF6 switch. The SF6 switch was less expensive than the oil-filled switch. It also was smaller, easier to install and appeared to require less maintenance. Because we could eliminate the oil, which could be an environmental hazard, and because explosions were not a concern, we specified the gas-insulated switch. An Innovative Solution The installation resulted in some unexpected bonuses. The original transfer switch had three sets of bushings: one for the preferred source, one for the alternate source and one to serve the load. With this arrangement it was possible to switch between only two circuits. In addition, the oil switch had four potential transformers, three on the preferred feed and one on the alternate feed. The transformer primaries were unfused and connected line to ground. Donut-type current transformers ringed the load bushing and fed overcurrent relays, which latched to prevent operation in case of an overload. The PTs and CTs were inside the same tank as the switch and the operating mechanism was external and linked to the switch with a pipe linkage. A bonus in the new installation is that the SF6 scheme uses two separate switches installed on adjacent poles, which not only switches between two sources but will automatically sectionalize a down-stream fault. Voltage and current of all three phases of both the preferred and alternate feeders are monitored continuously without PTs or CTs using sensors developed for interfacing with remote terminal units of automated distribution systems. A self-contained local logic opens and closes the SF6 switches in the correct sequence to ensure reliable service to the pumping station. Pole mounting the SF6 switch scheme required creativity. The purchased switches were designed to be mounted on a crossarm. To simplify the installation, Aluma Form, Inc. of Memphis, Tennessee, U.S., constructed custom brackets that permitted installation of the switches directly to the pole. The switches were mounted to the brackets on the ground (Fig. 1) and then the assembly was hoisted using a block and tackle (Fig. 2). The installation was quick and simple. Making the connections was the most mechanical part of the job, since the switch and the motor operator mechanism were assembled at the factory (Fig. 3). The control cables were attached to the pole with standard staples (Fig. 4). The Future The experience has encouraged us to consider the SF6 as a standard since it is environmentally safe and it enhances design flexibility. TDW Clarence Wooddell, a graduate of Memphis State University, is supervisor of Electrical Distribution Engineering and has been with Memphis Light, Gas Water for 43 years. He is a senior member of IEEE and is a member of the Insulated Conductor Committee. Reggie Bowlin earned the BSEE degree from University of Tennessee - Knoxville and is design engineer in the Electrical Distribution Engineering Department. He is registered as a professional engineer in Tennessee

A switch is used to open or close a circuit. The most simple switch is the single pole single throw switch (SPST). It can either open or close one pathway. The pole is somewhat like the number of switches contained in the switch. If it is double pole single throw (DPST) it is the same as having two SPST switches, that when you flip on or off the DPST switch it is the same as throwing two SPST switches simultaneously. The throw is the number of positions the common point can move to. If it is a single pole double throw (SPDT) then the common connector on the switch can move from one point to another. So if you had the switch in one position, the common connector would be attached to one point. And if you then change its position it would move the common connector to the other point. You could use this kind of switch to turn off one object while turning on another when the switch changes position. And the double pole double throw (DPDT) is like having two SPDT switches.

You can also have more than two poles or two throws. You can have a triple pole triple throw (TPTT), or even more. The rotary switch normally has many throws, and poles. The one shown above is a single pole switch with five throws. A rotary switch is the kind you turn to move into different positions.

Some switches are shown above. Some switches, such as the 3rd from the left in the upper portion, are pulled. The one to the right of that one is twisted.

A switch is a mechanical device used to connect and disconnect a circuit at will. Switches cover a wide range of types, from subminiature up to industrial plant switching megawatts of power on high voltage distribution lines.

In applications where multiple switching options are required (e.g., a telephone service), mechanical switches have long been replaced by electronic switching devices which can be automated and intelligently controlled.

The prototypical model is perhaps a mechanical device (for example a railroad switch) which can be disconnected from one course and connected to another.

The switch is referred to as a “gate” when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i.e. a computer is a system of electronic switches which function as logical gates.

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]

Model Predictive Control, or MPC, is an advanced method of process control that has been in use in the process industries such as chemical plants and oil refineries since the 1980s. Model predictive controllers rely on dynamic models of the process, most often linear empirical models obtained by system identification. The models are used to predict the behavior of dependent variables (ie., outputs) of a dynamical system with respect to changes in the process independent variables (ie., inputs). In chemical processes, independent variables are most often setpoints of regulatory controllers that govern valve movement (eg., valve positioners with or without flow, temperature or pressure controller cascades), while dependent variables are most often constraints in the process (eg., product purity, equipment safe operating limits). The model predictive controller uses the models and current plant measurements to calculate future moves in the independent variables that will result in operation that honors all independent and dependent variable constraints. The MPC then sends this set of independent variable moves to the corresponding regulatory controller setpoints to be implemented in the process.

Despite the fact that most real processes are approximately linear within only a limitted operating window, linear MPC approaches are used in the majority of applications with the feedback mechanism of the MPC compensating for prediction errors due to structural mismatch between the model and the plant. In model predictive controllers that consist only of linear models, the superposition principle of linear algebra enables the effect of changes in multiple independent variables to be added together to predict the response of the dependent variables. This simplifies the control problem to a series of direct matrix algrebra calculations that are fast and robust.

When linear models are not sufficiently accurate because of process nonlinearities, the process can be controlled with nonlinear MPC. Nonlinear MPC utilizes a nonlinear model directly in the control application. The nonlinear model may be in the form of an empirical data fit (e.g. artificial neural networks) or a high fidelity model based on fundamentals such as mass, species, and energy balances. The nonlinear model may be linearized to derive a Kalman filter or specify a model for linear MPC. The time derivatives may be set to zero (steady state) for applications of real-time optimization or data reconciliation. Alternatively, the nonlinear model may be used directly in nonlinear MPC and nonlinear estimation (e.g. moving horizon estimation). A reliable nonlinear model is a core component of simulation, estimation, and control applications.

MPC is based on iterative, finite horizon optimization of a plant model. At time t the current plant state is sampled and a cost minimizing control strategy is computed (via a numerical minimization algorithm) for a relatively short time horizon in the future: [t,t + T]. Specifically, an online or on-the-fly calculation is used to explore state trajectories that emanate from the current state and find (via the solution of Euler-Lagrange equations) a cost-minimizing control strategy until time t + T. Only the first step of the control strategy is implemented, then the plant state is sampled again and the calculations are repeated starting from the now current state, yielding a new control and new predicted state path. The prediction horizon keeps being shifted forward and for this reason MPC is also called receding horizon control. Although this approach is not optimal, in practice it has given very good results. Much academic research has been done to find fast methods of solution of Euler-Lagrange type equations, to understand the global stability properties of MPC’s local optimization, and in general to improve the MPC method. To some extent the theoreticians have been trying to catch up with the control engineers when it comes to MPC.

Model Predictive Control (MPC) is a multivariable control algorithm that uses:

• an internal dynamic model of the process

• a history of past control moves and

• an optimization cost function J over the prediction horizon,

to calculate the optimum control moves.

The optimization cost function is given by:

ORIFICE SIZING CALCULATION

In this example, either calculate the head drop through an orifice of known diameter, or enter the head drop required and calculate the required size of orifice. The calculation applies to orifices where the orifice diameter is less than 0.3 of the pipe size.

How many times a day do we turn on a faucet? Do it now. First very slowly, and you will see glassy, orderly flow. If there is no wind or other disturbance, nothing will change. This is called laminar flow. A photo taken now will be identical to one taken half an hour later. Such a flow is deterministic; information about its future behavior is completely determined by specification of the flow at an earlier time. Now open the faucet to full on, or better still open a fire hydrant, or watch a smoke stack. Here, for this faster or larger scale motion, the flow pattern is changing all the time. Although its average motion is in one direction (sideways for the fire-hydrant, up for the smoke stack), within the flow there are irregularities everywhere. For example if you could train your eyes on a small speck of dust it would certainly move along but it would jitter as well, sometimes darting to one side, or up or down. Turbulent flow while proceeding in a particular direction, like laminar flow, has the added complexity of random velocity fluctuations. The flow patterns never repeat themselves. To convince yourself of this watch a smoke stack for a few minutes.

Fluid flow that is slow tends to be laminar. As it speeds up a transition occurs and it crinkles up into complicated, random turbulent flow. But even slow flow coming from a large orifice can be turbulent; this is the case with smoke stacks. Engineers and scientists don’t like to say “fast” or “slow” or “small” and “big” since there is no reference. Small compared to what? Big compared to what? Since turbulence is altogether a different type of fluid flow to laminar flow, it is desirable to be able to quantify under what conditions it occurs.

Let us re-do the faucet experiment in a more systematic way. We have shown that as the speed, V, increases, transition to turbulence will occur. Now, instead of using water in your pipes, replace it with honey. Assuming you could provide a large enough pressure, even for fast flow the motion would remain laminar. If you do not wish to do this experiment, stir a spoon rapidly in a cup of water and then at the same speed (working hard) in a cup of honey. Honey has a higher viscosity than water and the viscosity resists transition to turbulence: while the water is turbulent, the honey remains laminar at the same speed. Finally, put a nozzle on your tap and constrict the water flow into a fine glass capillary tube. Here too the flow can be made to go quite fast without it becoming turbulent. Our experiments suggest that laminar flow occurs for low speeds, small diameters, low densities and high viscosities, while turbulent flows occur for the opposite conditions: high speeds, large diameters, high densities and low viscosities. Now viscosity is a measurable fluid property (as is its density, temperature, etc.). We will discuss it more in a moment, but we often use the “kinematic viscosity,” which is the viscosity divided by the density. Its unit is m^2/s. Notice its dimensions are the same as a length multiplied by a velocity. If the fluid speed is V (m/s), the orifice diameter is d (m) then we can write the following dimensionless ratio

Re is the Reynolds number, named after Osborne Reynolds who did systematic experiments, of a similar type to those described above, one hundred years ago. Notice that if V or d (or both) are small and the viscosity is large, Re will be small. For this case the flow will be laminar. Increase d or V or decrease the viscosity, and Re will increase. Reynolds found that for flow in a pipe it did not matter which of the three particular parameters he varied in this dimensionless group: as long as Re was less than approximately 2300, the flow was laminar. Above this value, turbulence would invariably occur. This is a general result since it allows us to vary the type of fluid, flow speed and pipe diameter without having to use the words “large” or “fast”, etc. Moreover, since Re is dimensionless, it does not matter which system of units are used (S.I., Engineering, etc.) so long as they are the same throughout. We can now talk of high Reynolds number flow or low Reynolds number pipe flow, knowing that in this context low means somewhat less than 2000. The kinematic viscosity of water is approximately 10^{-6} m^2/s (that of honey is about 10^{-3} m^2/s, 1000 times greater than that of water). Thus if the pipe diameter is say 1 cm, the speed at which the Reynolds number is 2000, is 0.2 m/s or approximately 0.4 mph, a rather slow speed. Water undergoes transition to turbulence at low speeds. Most of the water flows we see, such as in streams and rivers, are indeed turbulent.

Air too is a fluid, its viscosity, \nu, is approximately 10^{-3} m^2/s. This is a higher viscosity than that of water. This rather counter-intuitive fact is due to the great differences in density of the two fluids. Water has a density of approximately 1000 kg/m^3; the air density is 1.2 \, kg/m^3. Thus part of the “viscous feeling” we have when we pull our fingers through water is really due to inertia — we are having to move the water away from our hands and this also provides resistance. For this reason we need to remember the difference between the dynamic viscosity and the kinematic viscosity. The dynamic viscosity of water is approximately 10^{-3} kg/(m s) while that of air is 1.2 \times 10^{-5} kg/(m s). Thus the dynamic viscosity of water is higher than that of air, in keeping with our intuitive notion.

While the transition from laminar to turbulent flow occurs at a Reynolds number of approximately 2300 in a pipe, the precise value depends on whether any small disturbances are present. If the experiment is very carefully arranged so that the pipe is very smooth and there are no disturbances to the velocity and so on, higher values of Re can be obtained with the flow still in a laminar state. However, if Re is less than 2300, the flow will be laminar even if it is disturbed. Thus 2300 is the value the Re below which turbulence will not occur in a pipe. Moreover, if the flow has a different geometry, such as flow in a square duct, or over a turbine blade, transition will occur at different values of Re. The essential point is that flows become turbulent at high Reynolds numbers where “high” means much greater than unity.

Air motion is invariably turbulent. Consider a smokestack (which to a first approximation is mostly air). If its diameter is say 3 m, then V must be less than 6.6 mm/s (0.015 mph) for it to be laminar! There is no such thing as a laminar smokestack. Clouds too are usually turbulent. Here we determine the Reynolds number using an approximate characteristic dimension of the cloud such as its height or width. Assuming the cloud dimension is say 500 m, and its characteristic internal motion is say 5 m/s, then taking the kinematic viscosity to be 10^{-5} m^2/s (it is approximately the same for water vapor as it is for air), the Re = (500 x 5)/10^{-5} = 2.5 x 10^8. A high value indeed. No wonder cumulus clouds always have a random, puffy looking turbulent structure (see also the plume generated by Mt. St. Helens in the picture above).
Turbulent Flow
When the flow is turbulent, the flow contains eddying motions of all sizes, and a large part of the mechanical energy in the flow goes into the formation of these eddies which eventually dissipate their energy as heat. As a result, at a given Reynolds number, the drag of a turbulent flow is higher than the drag of a laminar flow. Also, turbulent flow is affected by surface roughness, so that increasing roughness increases the drag.

Transition to turbulence can occur over a range of Reynolds numbers, depending on many factors, including the level surface roughness, heat transfer, vibration, noise, and other disturbances. To understand why this is so, and to appreciate the role of the Reynolds number in governing the stability of the flow, it is helpful to think in terms of a spring-damper system such as the suspension system of a car. Driving along a bumpy road, the springs act to reduce the movement experienced by the passengers. If there were no shock absorbers, however, there would be no damping of the motion, and the car would continue to oscillate long after the bump has been left behind. So the shock absorbers, through a viscous damping action, dissipate the energy in the oscillations and reduce the amplitude of the oscillations. If the viscous action is strong enough, the oscillations will die out very quickly, and the passengers can proceed smoothly. If the shock absorbers are not in good shape, the oscillations may not die out. The oscillations can actually grow if the excitation frequency is in the right range, and the system can experience resonance. The car becomes unstable, and it is then virtually uncontrollable.

In fluid flow, we often interpret the Reynolds number as the ratio of the inertia force (that is, the force given by mass x acceleration) to the viscous force. At low Reynolds numbers, therefore, the viscous force is large compared to the inertia force, and the flow behaves in some ways like a car with a good suspension system. Small disturbances in the velocity field, created perhaps by small roughness elements on the surface, or pressure perturbations from external sources such as vibrations in the surface or strong sound waves, will be damped out and not allowed to grow. This is the case for pipe flow at Reynolds numbers less than the critical value of 2300 (based on pipe diamter and average velocity), and for boundary layers with a Reynolds number less than about 200,000 (based on distance from the origin of the layer and the freestream velocity). As the Reynolds number increases, however, the viscous damping action becomes comparatively less, and at some point it becomes possible for small perturbations to grow, just as in the case of a car with poor shock absorbers. The flow can become unstable, and it can experience transition to a turbulent state where large variations in the velocity field can be maintained. If the disturbances are very small, as in the case where the surface is very smooth, or if the wavelength of the disturbance is not near the point of resonance, the transition to turbulence will occur at a higher Reynolds number than the critical value. So the point of transition does not correspond to a single Reynolds number, and it is possible to delay transition to relatively large values by controlling the disturbance environment. At very high Reynolds numbers, however, it is not possible to maintain laminar flow since under these conditions even minute disturbances will be amplified into turbulence.

Turbulent flow is characterized by unsteady eddying motions that are in constant motion with respect to each other. At any point in the flow, the eddies produce fluctuations in the flow velocity and pressure. If we were to measure the streamwise velocity in turbulent pipe flow, we would see a variation in time as shown in figure 14.

Figure Velocity at a point in a turbulent flow as a function of time.

We see that the velocity has a time-averaged value \bar U and a fluctuating value u’, so that \bar U is not a function of time, but u’ is.

The eddies interact with each other as they move around, and they can exchange momentum and energy. For example, an eddy that is near the centerline of the pipe (and therefore has a relatively high velocity), may move towards the wall and interact with eddies near the wall (which typically have lower velocities). As they mix, momentum differences are smoothed out. This process is superficially similar to the action of viscosity which tends to smooth out momentum gradients by molecular interactions, and turbulent flows are sometimes said to have an equivalent eddy viscosity. Because turbulent mixing is such an effective transport process, the eddy viscosity is typically several orders of magnitude larger than the molecular viscosity. The important point is that turbulent flows are very effective at mixing: the eddying motions can very quickly transport momentum, energy and heat from one place to another. As a result, velocity differences get smoothed out more effectively than in a laminar flow, and the time-averaged velocity profile in a turbulent flow is much more uniform than in a laminar flow (see figure 4).

As a result of this mixing, the velocity gradient at the wall is higher than that seen in a laminar flow at the same Reynolds number, so that the shear stress at the wall is correspondingly larger. This observation is in agreement with the fact that the losses in a turbulent flow are much higher than in a laminar flow, and therefore the pressure drop per unit length will be greater, which is reflected in a larger frictional stress at the wall