Question of the Week Q206


June 27, 2006


This is a trim installed inside a valve that uses a "torturous path," multiple holes, expanding flow area, or sudden expansions and contractions to lower the sound pressure level generated by a valve.

Source: "Valve Handbook," Philip L. Skousen, McGraw-Hill (1998), ISBN 0-070057921-0

June 20, 2006


The torque required to start a motor or machine from standstill. Due to static friction effects, it is always greater than the torque needed to maintain motion.

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

June 13, 2006


A high-performance valve used to regulate flow, temperature, or pressure of a process. While a throttling valve can fully open or fully close, its main purpose is to provide an exact measurement (position) somewhere between those two points.

Source: "Valve Handbook," Philip L. Skousen, McGraw-Hill (1998), ISBN 0-070057921-0

June 6, 2006


Feedforward is a method commonly used to improve command response of a control system. In servo loops—for example—a command (actually its derivatives) are fed ahead of the position loop. Commanded velocity and acceleration are fed ahead to the velocity and current loops, respectively. The profile generator can typically supply the commanded velocity and acceleration since these signals are needed to calculate the position command.

The velocity and acceleration signals fed ahead of the loops allow the inner control loops to respond faster than the alternative of waiting for an error signal to pass through the relatively slow position loop. Feedforward improves control response without increasing servo gains and consequently does not aggravate stability or sensor noise problems.

Source: "Observers in Control Systems, a Practical Guide," George Ellis, Academic Press (2002), ISNB 0-12-237472-X

May 30, 2006


This is a simple manual valve mounted in process piping, which is used to start or stop a process flow stream. Common block valve types include: gate, quarter-turn plug, ball, pressure relief, and tank-bottom valves. They're also known as on-off valves.

Source: "Valve Handbook," Philip L. Skousen, McGraw-Hill (1998), ISBN 0-070057921-0

May 23, 2006


When two or more ingredients are mixed or blended, flow rate of one ingredient often paces production rate. Flow rates for the other ingredient(s) are controlled to maintain a specified ratio to the pacing ingredient.

That ratio can be manually set, automatically set form a batch recipe, or adjusted by a feedback controller's output. When the required ratio is automatically set by a higher-level feedback controller, the ratio control strategy is merely one form of feedforward control. Batch processing, fuel oil blending, and combustion processes are among numerous applications of ratio control.

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.

May 16, 2006


No, we are not talking about engineering-school accreditation, but a unit of time measurement associated with alternating current (ac). One complete time cycle equals 360 electrical degrees, so at 60 Hz (cycles/s) 1 electrical degree is 1/21,600 seconds.

For a rotating electrical machine (motor), one cycle takes place when the rotating field moves from one pole to next pole of the same polarity. As a result, in a 2-pole motor, electrical and mechanical degrees are equal (360 deg/rev). For higher-pole motors, the number of electrical degrees per revolution is obtained by multiplying the number of pole pairs by 360.

Source: "Glossary of Motor Terms," publication Motors-9.0, Rockwell Automation (Feb. 1999) and Control Engineering.

May 9, 2006


It's a communication system device that forwards data from one local area network (LAN) to another. Routers can read the network address in each transmission and choose the most efficient route—depending on current traffic, line costs, speed, bad connections, and other factors.

Source: " Glossary of Wireless Terms ," Wireless Survivors Guide, Personal Computing Canada.

May 2, 2006


MAC or "multiply and accumulate" refers to a measure of arithmetical performance in digital signal processing (DSP) systems. For example, field-programmable gate arrays (FPGAs) can achieve some of the highest DSP performance, calculating a half trillion MACs per second.

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

April 25, 2006


It's one of the methods of soft computing. Fuzzy logic is a method that can produce logical (crisp) results from incomplete, vague, or "fuzzy'' inputs—hence the name fuzzy logic—similar to the way humans solve problems. It works with linguistic variables that form continuous membership functions rather than "black-and-white'' or on/off parameters of conventional computing to solve complex nonlinear, multivariable problems. Fuzzy logic was developed by Lotfi Zadeh, professor at the Computer Science Div. of the University of California at Berkeley.

Source: Control Engineering , Sept. 2003 cover story, " Artificial Intelligence ...Within ," and various CE articles on AI technologies.

April 18, 2006


Often, technology definitions are highly application dependent. A least two cases are offered here: 1) The voltage level, which causes insulation failure to take place, and 2) The forward or reverse voltage level at which a semiconductor device abruptly changes its conductance characteristics.

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

April 11, 2006


In an electric motor, the steel portions of the rotor and stator are often made up of a series of thin sheets—called laminations—which are stacked and fastened together by cleats, rivets, bolts. or welds. Such electrical-steel laminations are often used instead of solid material to reduce eddy-current losses.

Source: "Glossary of Motor Terms," Publication Motors-9.0, Rockwell Automation (Feb 1999) and Control Engineering.

April 4, 2006


This refers to a type of motion profile where the velocity vs. time plot resembles the shape of a trapezoid. This often-used motion profile is characterized by a constant acceleration segment from zero to final velocity, followed by a constant velocity segment, then a constant deceleration segment back to zero velocity.

Source: "General Motion Control, Glossary of Terms," Publication GMC, Rockwell Automation (April 1998) and Control Engineering.

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