Question of the Week 2005
December 27, 2005
For short, it's a graphical plot of failure rates over time for many electronic (and mechanical) parts and products—yielding a curve that resembles the axial cross-section of a "bathtub."
A bathtub curve represents data of calculated empirical population failure rates of products aging over time. Three regions characterize the curve. Early failure period (or infant failure period) begins at time zero when a customer first puts the product into use, and often results in a high but rapidly decreasing failure rate. This decreasing failure rate typically lasts several weeks to a few months.
A long period follows where failure rates that level off and remain roughly constant for (hopefully) the majority of the product's useful life. This is known as the intrinsic failure period (also stable failure period) and is the region where most systems spend most of their operating lifetimes. If units from the product population remain in use long enough, failure rate begins to increase as materials wear out and degradation failures occur at an ever-increasing rate. This final region of the bathtub curve is called the wearout failure period .
December 20, 2005
This is a condition where the motor does not rotate smoothly, but steps or jerks from one momentary position to another during revolution of the shaft. Cogging is most evident at low motor speeds and causes objectionable vibrations in the driven machine. Sophisticated electronic controls are needed to minimize or eliminate the cogging effect.
Source: "Glossary of Terms, General Motion Control," Rockwell Automation, April 1998 and Control Engineering magazine.
December 13, 2005
In the processing of semiconductors, particularly integrated circuits (ICs), masks—or photomasks—are used in much the same way as photographic negatives. The surface of a wafer, which has been coated with photoresist material is exposed through a mask that determines size, shape, and interconnection of various elements, such as transistors on the chip or ICs.
Source: Glossary of terms at the Web site of ARM .
December 06, 2005
This is the non-rotating part of an ac induction motor's magnetic structure. It consists of thin, special electric steel laminations having a large center hole, in which the rotor turns. The stator usually contains primary windings of the motor that are inserted into radial slots in the lamination assembly.
Source: "Glossary of Motor Terms," Rockwell Automation, Publication Motors-9.0, February 1999.
November 29, 2005
Squirrel-cage refers to the most common rotor construction for an ac induction motor. This simple rotor structure looks like a squirrel cage—consisting of numerous uninsulated metal bars (aluminum or copper) that connect at each end to the face of rotor end rings. The bars are slightly skewed relative to the motor's longitudinal axis to enhance motor performance and their cross section is typically other than circular.
Source: "Control of Induction Motors," by Andrzej M. Trzynadlowski, Academic Press (2001), ISBN 0-12-701510-8.
November 22, 2005
Vortex-shedding flowmeters measure flow velocity by the frequency of vortices shed from a blunt obstruction (or "bluff body) placed in a flow pipe. As flow divides to go around the bluff body, vortices are created alternately on each side of the divided stream. Rate of vortex creation is proportional to stream velocity. Each vortex represents a low-pressure area; and when presence-then-absence of these low pressures is counted it is proportional to flow velocity. Vortex flowmeters provide good measurement accuracy with liquids, gases, or steam—and also tolerate fouling.
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.
November 15, 2005
Wafer fab refers to the production process for semiconductors and integrated circuits (ICs)—from raw silicon wafers through a series of diffusion, etching, photolithographic, and other process steps to finished wafers.
Wafer foundry is a semiconductor manufacturer who provides wafer-processing services for an external customer on a subcontract basis.
Source: Glossary of terms at the Web site of ARM
November 8, 2005
Definitions of many technology acronyms vary widely with the application sector. We refer here not to "Internet protocol," but "intellectual property" (IP).
In the semiconductor design industry, intellectual property refers to predefined functions such as processors or bus interfaces that are typically licensed from the software developer. IP can be implemented directly in silicon, either in fixed logic or programmable logic devices. It saves chip designers time during product development. The term is synonymous with cores.
Source: Glossary of terms at Xilinx Inc.'s Web site .
November 1, 2005
Passive radio-frequency identification (RFID) systems use transponders energized by an external source, such as the antenna or reader. Active RFID systems overcome passive RFID limits by driving their logic circuitry with onboard energy via batteries or wired connections. Passive transponders are usually less costly than active ones, but their operating distance and data transfer capabilities are more limited.
Source: Control Engineering, August 2005, " Basic RFID glossary " sidebar.
Also "General RFID System Definition" Web page at www.pelicansystems.co.uk
October 25, 2005
Photoelectric sensors exist in basically three varieties: transmitted beam (or through beam), retroreflective , and diffuse .
Diffuse mode photoelectric sensors rely on sensing light reflected from a target object, therefore they're more sensitive to target characteristics, such as color, surface irregularities, shape, position, and reflectivity. In general, diffuse photoelectric sensors offer less sensing distances than the other two sensor types. Transmitted beam and retroreflective sensors operate on the principle of blockage of a light beam by the object being sensed located in the transmitted beam, and as a result offer more reliable sensing regardless of target characteristics.
Transmitted beam sensors have a separate transmitter and receiver, while "retroreflectives" combine both functions in one unit, but use a separate reflector. Use of beam-break sensors requires access to both sides of the target object as well as extra cost, installation, and space for the separate receiver or reflector unit.
Source: Control Engineering, February 2003, Back to Basics, " Background check with photoelectric sensors ."
October 18, 2005
Also known as Open Systems Interconnect (OSI) reference model, it's a specification that allows communication protocols to organize information in a hierarchical manner and to establish comprehensible communications. The model defines what a transmitting device must do to pack up a message for transmission and what the receiving device must do to unpack the transmission to recreate the original message.
The seven layers of OSI reference model and their functions are:
Physical (Layer 1)—Consists of cables, connectors, transmitters, receivers.
Data link (Layer 2)—Establishes protocols and rules to send data across the physical layer; acts as logical link control and media access control.
Network (Layer 3)—Routes messages, controls flow of data among devices.
Transport (Layer 4)—Provides end-to-end control of data once communication path is established.
Session (Layer 5)—Allows applications to communicate across the network.
Presentation (Layer 6)—Translates data into the desired form for display on a PC; translates between two different formats.
Application (Layer 7)—Provides service to users.
Source: Control Engineering, January 2005, " Ethernet Explained ."
October 11, 2005
Question: What is Sarbanes-Oxley?
It’s the more commonly known name for a U.S. law, the Public Company Accounting Reform and Investor Protection Act of 2002, named after its primary architects in Congress—Sen. Paul S. Sarbanes, D-MD and ranking member of the Senate Banking Committee; and Rep. Michael G. Oxley, R-OH, chair of the House Committee on Financial Services.
Sarbanes-Oxley (or SOX for short) will likely impact most automation and controls engineers. Today, auditors are looking at more than numbers, auditing processes and controls associated with obtaining those numbers. They’re validating how companies got to those numbers. And among those numbers are the process and production data that only controls and automation engineers can provide.
Intent of the Act, effective July, 30 2002, is "to protect investors by improving the accuracy and reliability of corporate disclosures made pursuant to the securities laws." SOX requires all public companies to meet certain financial reporting requirements for end-of-year financial statements and quarterly reports. Fines and prison terms can result from non-compliance or submission of inaccurate information, even if it is mistakenly given.
Source: Control Engineering, June 2005, “ What in the World is Sarbanes-Oxley? ”
October 4, 2005
Question: What’s it cost for companies to comply with laws, regulations?
According to a survey of more than 225 companies by AMR Research , companies are spending $15.5 billion in 2005—complying with laws, regulations, and other requirements; per company spending amounts to $500,000, on average. Spending could reach $80 billion in five years.
Source: Control Engineering, July 2005, ” By the Numbers .”
September 27, 2005
Question: What is the least significant bit?
Least significant bit (LSB) is the lowest bit or the far-right digit in a string of binary numbers (or a digital word). Also, it’s the bit representation of the smallest analog input signal that can be converted. For example, the rightmost digit “1” in the binary string 01001001 is the least significant bit.
Source: “A Baker’s Dozen. Real Analog Solutions for Digital Designers,” by Bonnie Baker, ISBN 0-7506-7819-4, Newnes, an imprint of Elsevier (2005).
September 20, 2005
Question: What is hydraulic radius?
This characteristic of a flow section is not directly measurable, but is often used in hydraulic calculations. Hydraulic radius is defined as the flow cross-sectional area divided by the wetted perimeter. It has dimension of length.
To illustrate, hydraulic diameter for a full circular pressure pipe of diameter D is the area (pD
Source: “Computer Applications in Hydraulic Engineering,” (2nd edition) by Michael E. Meadows and Thomas M. Walski, contributing editors, ISBN 0-9657580-3-6, Haestad Press (1998).
September 13, 2005
Question: What is the Pareto principle?
You may be more familiar with this principle in the form of the “80-20 rule.” This is an observation that great imbalance often exists in processes and systems—that is, 80% of results can be attributed to only 20% of all possible causes. For example, 20% of features on a device may be used 80 % of the time.
This principle is attributed to the work of Italian economist and sociologist Vilfredo Pareto (1848-1923). It originated from his determination that 20% of the population owned 80% of the property in Italy. The Pareto principle was later generalized by others and sees application in economics, sociology, and statistical process/quality control (SPC/SQC). It can be used to concentrate efforts on where to direct improvements.
A related statistical tool, the Pareto chart, is a graphic view of the 80-20 rule, where causes of a problem are shown in order of severity (largest to smallest).
Source: “Making Sense of Data,” by Donald J. Wheeler, ISBN 0-945321-61-2, SPC Press (2003); also other references.
September 6, 2005
What’s an autotransformer?
A type of transformer that has a single tapped winding rather than separate primary and secondary windings (or coils) is called an autotransformer. Number of turns between the tap and one end of the winding defines one coil, while the entire winding acts as the other coil. An autotransformer is lighter, more compact, and often less costly than a transformer with standard core. Power (ac or pulsed dc) applied across one coil portion produces higher (or lower) voltage across another portion, depending on tap arrangement and connections.
Source: Back to Basics, “Tranformer anatomy,” Control Engineering, Dec. 2003, p. 56.
August 30, 2005
What is bus arbitration?
This is a mechanism through which resources of a communication bus are properly allocated and conflicts resolved among multiple devices (or masters) attempting to simultaneously use the bus. Bus arbitration is crucial to a multi-master bus.
For example, in Peripheral Component Interconnect (PCI) bus, a master can execute a transaction only after it requests—and is granted—use of the bus. It’s done via a pair of signals (REQ# and GNT#) that connect it (and other masters) to a central arbiter. A master first asserts its REQ# signal to which the arbiter will reply with a GNT# signal to indicate it’s next in line to use the bus. Only one GNT# signal can be asserted at any time instant. After detecting its GNT# signal has asserted and the bus is in idle state, the master can begin a bus transaction by asserting a start signal. Other devices asserting a REQ# must wait their turn.
Bus arbitration is a transparent process. The next transaction takes place in parallel with the current one (as regulated by the arbitration algorithm), so that the process consumes no additional time.
Source: “PCT Bus Demystified” (2nd Edition), by Doug Abbott, ISBN 0-7506-7739-2, Newnes, an imprint of Elsevier (2004).
August 23, 2005
Question of the Week
What is‘megger’ testing?
An instrument used for measuring high resistances in electric circuits is called a “megger” or mega-ohmmeter. While a normal ohmmeter measures current as an analog of resistance, a megger measures voltage to determine resistance. Megger testing is used largely to verify insulation integrity.
Megger testing applies relatively high voltages—500-2,500 V depending on the device—to the circuit being tested to verify presence or absence of breakdowns. Depending on applied voltage and insulation rating, it’s considered a non-destructive test. Megger testing can detect higher voltage-related breakdown problems relative to ground, but not short circuits between windings.
Source: “An Introduction to Predictive Maintenance,” 2nd Edition, by R. Keith Mobley, ISBN 0-7506-75312-4, Butterworth-Heinemann, an imprint of Elsevier Science (2002).
August 16, 2005
Half-slot addressing (1/2-slot addressing) is a mode of addressing I/O in which each
One-slot addressing (1-slot addressing or single-slot addressing) is a mode of addressing I/O in which each I/O module slot of an I/O chassis is addressed as an I/O group. Each I/O module slot contains a single I/O group.
Two-slot addressing (2-slot addressing or double-slot addressing) is a mode of addressing I/O in which each even/odd pair of I/O module slots of an I/O chassis is addressed as an I/O group. Each I/O module slot contains
Source: Industrial Automation Glossary, fifth edition, Rockwell Automation, Milwaukee, WI, 1997, p. 1. &o:p>&/o:p>
August 9, 2005
A baffle-nozzle amplifier is a device for converting mechanical motion to a pneumatic signal, which consists of a supply tube ending in a small nozzle and a moveable baffle plate attached to a mechanical arm. The supply tube has a restriction a short distance before the nozzle, so that, as the baffle plate moves closer to the nozzle opening, the pressure rises in the section of the supply tube between the restriction and the nozzle. Arm motion and nozzle clearance are small, on the order of 0.2 mm or less. A baffle-nozzle amplifier serves as the primary detector in almost all pneumatic transmitters and controllers. It's often referred to as a flapper-nozzle amplifier because the baffle plate is mounted on a pivoting arm.
Source: W.H. Cubberly, ed., Comprehensive Dictionary of Measurement and Control, Second Edition , ISA, the Instrumentation, Systems, and Automation Society, Research Triangle Park, NC, 1991, p. 30.&o:p>&/o:p>
August 2, 2005
In drawing formal block diagram models, we use a number of conventions to represent the elements and connections:
A system element is shown as a box with an input shown as an inward directed arrow, and an output shown as an outward directed arrow.
A control system will be made up of a number of interconnected systems, so we can draw a model of such a system as a series of interconnected blocks. Thus, we can have one box giving an output, which then becomes the input for another box. We draw a line to connect the boxes, and indicate a flow of information in the direction indicated by the arrow. The line doesn’t necessarily represent a physical connection or the forma of a physical connection.
We often have situations with control systems in which two signals are perhaps added together, or one is subtracted from the other. The result of these operations is then fed on to some system element. This is represented by a circle, with the inputs to quadrants of the circle given
An overall output, such as the position of a car on a road, can be tapped off with an arrow that becomes an input, in this case to the car’s driver, so that he can compare the actual position with the required position, and the make necessary adjustments, such as turning the steering wheel.
In the case of a central heating system, the overall output is the temperature of a room. However, this temperature signal is also tapped off to become an input to the thermostat system, where it’s compared with the required temperature signal.
Source: Bolton, W., Control Systems , Newnes, an imprint of Butterworth-Heinemann, Oxford, U.K., 2002, pp. 3-4.
July 26, 2005
QUESTION: Are transducers, sensors, and transmitters really the same thing?
The term transducer is often encountered. In strict terms, a transducer is a device that converts one physical quantity into another, in which the second is an analog representation of the first. A thermocouple is a transducer that converts temperature to an electrical potential. It is more common, however, to use the term sensor for the actual measurement device (i.e. the primary sensor), and use transducer for the entire measuring system local to the plant (including local signal processing). However, there are no strict rules, and in many cases the terms sensor and transducer are used interchangeably. The word transmitter is also often used to mean transmitter or sensor.
Source: Parr, E.A., Industrial Control Handbook, Butterworth-Heinemann Ltd., an imprint of Reed Elsevier, Oxford, U.K., 1995, p. 2.
July 19, 2005
Electrolytic conductivity specifies how well electricity is conducted through a liquid, and it depends on the chemical makeup of the liquid and its temperature. Similarly, electrical resistance of a wire is equal to the wire’s length, l, in cm, divided by its cross-sectional area, A, in cm2, all multiplied by the material’s resistivity constant, p , in ohm-cm2/cm. The formula is:
R (ohms) = p (ohm– cm2/cm) x l (cm) / A (cm2)
The longer the wire, then the greater the resistance. The larger the cross-sectional area, then the smaller the resistance. In the same way, the conductance, G , which is the reciprocal of resistance of the wire is given by the formula:
G (siemens) = 1/ R = y (siemen– cm/cm2) x A cm2) / l (cm)
Here, y is the material’s conductivity constant, and it is the reciprocal of p.
Electrolytic conductivity of a liquid is found in the same way as electrical conductivity of a wire. For example, if a glass container has an inside base of 1 x 1 cm ; the base has an electrode (also 1 x 1 cm); liquid 1 cm deep is placed above the electrode; an another 1 x 1 cm electrode is just touching the top of the liquid, then we have an electrolytic conductivity cell. The electrodes are connected by copper wire to a conductance-measuring instrument (an ohmmeter). Since A is 1.o cm2, and l is 1.0 cm in length, then the conductivity, y, of the liquid in this cell will be equal to the conductance, which is the reciprocal of the resistance.
Usually the conductivity constant has a very low value, and the measurement is given in usiemens– cm/cm2or in usiemen/cm. (Before 1960, this was called umho/cm because a mho is also the reciprocal of the ohm.) Since conductivity (and resistivity) varies significantly with temperature, the temperature must be measured to obtain a precise value of conductivity.
The container with electrodes for measuring electrolytic conductivity is called a conductivity cell . The dimensions of the cell are designed to give the cell a value called the cell constant. The value of the cell constant is equal to the distance, l, in centimeters between the electrodes, divided by the area, A, in cm2, of one of the electrodes (each electrode is assumed to have the same area, and the back of the electrode is assumed to be insulated from the liquid). The value of the cell constant equals l/A.&o:p>&/o:p>
Y (siemen– cm/cm2) = G (siemens) x l (cm) / A (cm2) = conductance x cell constant
Source: Fraser, Roy E., Process Measurement and Control, Prentice Hall, Upper Saddle River, NJ, 2001, pp. 48-49.
QUESTION: What's the difference between a little-endian and a big-endian?
A little-endian pertains to the order of bytes within a word, such that the least significant byte has the lowest address (little end first). This means the most significant is listed as Byte 1 and the least significant is listed as Byte 0.
A big-endian also pertains to the order of bytes within a word, but in this case the most significant byte has the lowest address (big end first). This means the most significant is listed as Byte 0 and the least significant is listed as Byte 1.
Source: Industrial Automation Glossary, fifth edition, Rockwell Automation, Milwaukee, WI, 1997 p. 10, 63.
QUESTION: What is the time-value of money and how is it calculated?
Projects in many fields are justified based on net present value (NPV) and internal rate of return (IRR) calculations, which help indicate future financial performance according to 'time value of money' and can show whether a project should be approved or rejected. In the formula (below), CF is cash flow for a given period and DIR is discount interest rate. IRR is determined by calculating NPV at 0, which indicates a project's break-even point. Many financial calculators and software programs can perform these calculations.
Net present value (NPV) = - initial capital investment + CF1/(1+DIR) + CF2/(1+DIR)2 +… + CFn/(1+DIR)n
Source: Montague, Jim, 'Time to Reinvest in Automation,' Control Engineering, Feb.'01, p. 56.