Question of the Week Q106


March 28, 2006


A graphical language for depicting sequential behavior of a control system (or control action) is referred to as a sequential function chart (SFC). It's used to define time- and event-driven control sequences, and is considered as an extremely effective graphical language to express both high-level parts of a control program and to program low-level sequences—for example, device interfaces.

SFC elements are used to structure the internal organization to perform sequential control functions in order to partition a set of steps and transitions connected by directed links . A transition is the condition whereby control passes from one step of the SFC to another—along the corresponding directed link. Each transition step has an associated transition condition, which is the result of evaluation by a single Boolean expression. Each step will have zero or more actions associated with it.

Source: "A Guide to the Automation Body of Knowledge," Vernon L. Trevathan, editor, ISA—The Instrumentation, Systems, and Automation Society (2006), ISBN 1-55617-961-8.

March 21, 2006


When the term "real-time" (RT) is used in the context of enterprise-wide connectivity, the RT world measured in milliseconds [or faster] that some control engineers deal with is not exactly what's implied. However, it does refer to an ability to drill down into or poll plant-floor data every few minutes—or as often as desired by the user. While that may not be RT in the engineering world, it is real time to front-office operations that have historically accessed such data, typically, on a daily or even weekly basis.

Source: Control Engineering May 2004, sidebar " Real-time: a point of clarification ," in article "Engineer-Centered Supply Chain."

March 14, 2006


In continuous process measurements, turndown ratio refers to the ratio of maximum to minimum measurable value of the instrument. For a flowmeter, if the maximum measurable flow is 100 gpm and the turndown ratio is 3 (typical for orifice plate flowmeters), then minimum accurately measurable flow rate is 33 gpm. If the turndown ratio is 100—possible with a Coriolis meter—minimum accurately measurable flow becomes 1 gpm.

Source: "A Guide to the Automation Body of Knowledge," Vernon L. Trevathan, editor, ISA—The Instrumentation, Systems, and Automation Society (2006), ISBN 1-55617-961-8.

March 7, 2006

QUESTION:st-order dynamic response?

Dynamic characteristics of a process determine its path in going from the current condition to a new steady-state condition after a change of input takes place. A first-order dynamic response approaches its new steady-state exponentially. For example, under a step input change, it takes a period of 3 time constants to reach 95% of the ultimate change. A first-order process is self-regulating—that is, it moves to a new steady-state condition after the input change occurs.

Source: "Fundamentals of Industrial Control," 2nd Ed., D.A. Coggan, Editor, ISA-The Instrumentation, Systems, and Automation Society (2005).

February 28, 2006


Light-emitting diode (LED) is a semiconductor device that radiates in the visible spectrum when energized by an electric current. It's used as an indicator light as well as in myriad human-machine interface (HMI) and related information-conveying applications.

An LED's color is determined by electroluminescent characteristics of materials used to manufacture the device and by the addition of various dopants. For example, copper-doped zinc sulfide emits light in the 555 nm (green) range—which is the human eye's peak sensitivity area. Early LEDs were limited to red, orange, yellow, and a yellow-like green. Today, bright green and even blue light devices are available.

Source: "Handbook of Standardized Terminology for the Power Sources Industry," 2nd Ed. (1995), Power Sources Manufacturers Association Inc.

February 21, 2006


This type of motor mounting allows close coupling of a pump or other equipment to the motor. Threaded mounting holes on the motor faceplate accept bolts from the pump (or other device). C-face mounting is normally used when it's desirable to have the coupled item overhang the motor. C-face mounting is a NEMA standard motor design and is available with or without mounting feet.

Source: Publication, "Glossary of Terms, General Motion Control," Rockwell Automation, April 1998.

February 14, 2006


Photoelectric proximity sensors work with light beams sent from an emitter to a detector, and the variation of light intensity—including its absence—reaching the detector. Three operating modes further subdivide photoelectrics. In diffused sensing, reflected light from the target must reach the receiver directly to activate detection signals. Retroreflective or reflex sensing uses the target to interrupt the transmitted light beam, which actuates detection signal, and is separately reflected to the receiver. In the above modes, transmitter and receiver reside in the same module. Through-beam sensing involves light beams sent directly from transmitter to receiver located in separate, inline housings. The target interrupts the "through-beam" of light to activate a sensor output.

Source: Control Engineering, March 2004 Back to Basics, " Proximity sensors shine on the shop floor ."

February 7, 2006


NRE (or non-recurring engineering) in the world of fixed logic chip design refers to one-time, up-front costs customers incur in designing a chip. NRE includes software tools, engineering time, design verification, mask sets, and prototypes.

In the programmable-logic world, NRE usually refers to expenses associated with converting a programmable-logic device (PLD) design to a fixed-logic design to gain cost reduction.

Source: Glossary of Terms on the Web site of Xilinx Inc .

January 31, 2006


Induction systems typically operate at low frequency (approximately 125 kHz) with few regulatory requirements, or at 13.56 MHz, where normal radio and spectrum allocation regulations apply. E-field disturbance —also known as backscatter or field disturbance modulation —is based on radio energy emitted by the reader, which is collected and reflected by the transponder. Radio data communication occurs when both reader and transponder communicate using radio signal emissions.

Source: Control Engineering, August 2005, " Basic RFID glossary " sidebar.

Also "General RFID System Definition" Web page at

January 24, 2006


In the international system (SI) of units, "siemens" is the measurement of conductance or ability of a wire to carry current under an applied voltage (opposite of resistance). The siemens replaces former cgs system unit of "mho," which of course was meant to indicate the "reciprocal" of ohm. To obtain the value of conductance in siemens units, just take the reciprocal of resistance expressed in ohm units. (Susceptance and admittance also carry units of siemens, and are reciprocals of reactance and impedance, respectively.) The unit of siemens is named after German electrical engineer Werner von Siemens (1816-1892), who was a founder of what today is Siemens AG.

Source: Handbook of Standardized Terminology for the Power Sources Industry, 2ndedition, Power Sources Manufacturers Association, 1995 and Control Engineering.

January 17, 2006


This capability, incorporated into most motor drives, controls the rate at which a motor is allowed to accelerate to set speed or decelerate to zero speed. The drive circuit in charge of this feature is adjustable for time and can be set to meet needs of different applications.

Source: "Glossary of Terms, General Motion Control," Rockwell Automation, April 1998 and Control Engineering magazine.

January 10, 2006


This is one type of simple, accurate pressure-measuring device. It works on the principle that pressure changes cause a liquid to rise and fall in a tube or column. Common liquids used are water, mercury, and red oil.

A basic manometer consists of a liquid-filled reservoir connected to a source whose pressure is to be measured. A transparent column is attached to the reservoir. The top of the column can be open, in which case the liquid level in the manometer measures gage pressure relative to the atmosphere. Alternatively, the top of the column can be sealed and evacuated to measure absolute pressure or vacuum.

Various manometer versions exist; for example, well-type, inclined, McLeod gage, micromanometer, and capacitive type.

Source: "Fundamentals of Industrial Control," 2nd Edition, D.A. Coggan, Editor, ISA—The Instrumentation, Systems, and Automation Society (2005), ISBN 1-55617-863-8

January 3, 2006


Ball-grid array (BGA) is a type of microchip connection methodology, distinct from the conventional pin-grid array (PGA). BGA chips typically use a group of solder dots, or "balls," arranged in concentric rectangles to connect to a circuit board. BGA-type chips are often found in mobile or embedded control applications where conventional PGA chips would take up too much space due to the length of pins used to connect the chips to the circuit board.

Source: " Glossary of terms ," on the Web site of Xilinx Inc.

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