Question of the Week Q306
September 26, 2006
QUESTION: When should I use a motor multidrive and why?
Adjustable speed drives are useful for any application with mechanical equipment powered by motors. These drives provide extremely precise electrical motor control, allowing motor speeds to ramp up and down or maintain a required speed. Since motors consume a majority of the power needed for most electromechanical applications, controlling motors based on load demands minimizes energy costs. Motor end users can realize 25%-70% energy savings by using electronic drives to control their motors
Single drives are highly flexible ac drives convert ac power to dc, and then invert the dc back to an ac output to a motor. Single drives are available for a wide range of motor power and voltage ratings. Single industrial drives also feature a wide range of built-in options as standard equipment. They can be installed for most applications right out of the box, and can be ordered as customized units for particular applications.
A multidrive incorporates multiple industrial drive modules connected to a common dc bus fed by a single supply unit. Each drive module then inverts that dc to ac to power its individual motor.
This construction simplifies the installation and results in many benefits:
savings in cabling;
reduced line currents and simpler braking arrangements;
energy distribution over the common dc bus bar, which can be used for motor-to-motor braking without the need for a braking chopper or a regenerative supply unit;
reduced component counts; increased reliability;
space savings; and
there is no need for a separate Motor Control Center (MCC).
In general terms, multidrives can replace several drive/motor sets in any integrated mechanical process. The multidrive's common dc supply enables the implementation of overall safety and control functions, and permits close coordination of individual drive motors. For example, a paper-making machine has many motors that must be individually controlled to maintain proper web tension.
Multidrives also can be used where the shafts of the individual drive motors are not tightly coupled, such as processes where each drive module needs a individual speed profile to minimize overall energy usage.
SOURCE: Ken Graber, ABB LV Drives, firstname.lastname@example.org
September 19, 2006
QUESTION: What is a 'servomechanism?'
Servomechanism , as defined by Wikipedia ( www.wikipedia.com ), refers to a device used to provide mechanical control at a distance. For example, a servo can be used at a remote location to proportionally follow the angular position of a control knob. The connection between the two is not mechanical, but electrical.
A typical servomechanism is made up of:
1. an electric motor;
2. a position feedback potentiometer;
3. a reduction gear;
4. an actuator arm.
Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force, although types that use hydraulics, pneumatics or magnetics are available.
Usually, servos use negative feedback. The control input is compared to the actual position of the mechanical system as measured by a transducer at the output. Any difference between the actual and wanted values (an 'error signal') is amplified and used to drive the system in the direction necessary to reduce or eliminate the error.
This question and answer appeared, in expanded form, on the Control Engineering forum at www.control.com .
September 12, 2006
QUESTION: How do you linearize an equal-percentage valve?
Linearizing an equal-percentage valve usually requires building a complementary curve of 10-20 line segments. An alternative is to use a hyperbolic characterizer that uses a single parameter divider to obtain the desired characteristic, instead of adjusting multiple x-y coordinates.
y = x/L + (1 - L) x
The hyperbolic characterizer solves the equation: where y is its output and x its input over a 0-1 range, and L is the adjustable parameter. With L set at 1.0, the relationship is linear; set at 5, it simulates an equal-percentage characteristic with a 50:1 rangeability; set at 0.2, it converts a 50:1 equal percentage valve to linear. It should be noted that, if the valve is reverse acting, these numbers also are reversed. Intermediate settings can correct the installed characteristic for any pressure-drop ratio.
Source: Shinskey, F. Greg, 'The Three Faces of Control Valves,' Control Engineering, July '00, p. 84.
September 5, 2006
QUESTION: What are the definitions of an 'accumulator?'
The word 'accumulator' has six basic definitions related to control and automation to varying degrees. Mostly in the computation and process realms, an accumulator is:
1. The register and associated equipment in the arithmetic unit of the computer in which arithmetical and logical operations are performed.
2. A unit in the digital computer where numbers are totaled, i.e. accumulated. Often the accumulator stores one operand, and upon receipt of any second operand, it forms and stores the result of performing the indicated operation on the first and second operands. Related to adder.
3. A pressure vessel containing water and steam, which is used to store the heat of steam for use at a later period and at some lower pressure.
4. A relatively large-volume chamber or other hydraulic device, which receives fluids under low hydraulic power, stores it, and then discharges it at high hydraulic power, after which it is ready to repeat the cycle.
5. A chamber or vessel for storing low-side liquid refrigerant in a refrigeration system.
6. Also referred to as a receiver, a reflux receiver, or reflux drum.
Source: 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. 3.
August 29, 2006
QUESTION: Why is the OSI reference model often called a seven-layer cake?
Mainly because, when it's completely written out, the open-system interconnect (OSI) reference model resembles the pastry in question.
The OSI model is a standard means of describing communications between open systems, which was originally proposed by the ISO and has been used by network developers and users for many years. It is a seven-layer model that represents the primary functions of a network architecture. The seven layers and the services they provide are as follows:
Layer 7—Application: User application processes and management functions.
Layer 6—Presentation: Data interpretation, format, and code transformation.
Layer 5—Session: Administration and control of sessions between two entities.
Layer 4—Transport: Transparent data transfer, end-to-end control, multiplexing, and mapping.
Layer 3—Networking: Routing, switching, segmenting, blocking error recovery, and flow control.
Layer 2—Data Link: Establish, maintain, and release data links; error detection and flow control.
Layer 1—Physical: Electrical, mechanical, and functional control of data circuits.
Source: Industrial Automation Glossary, fifth edition, Rockwell Automation, Milwaukee, WI, 1997 p. 79.
August 22, 2006
QUESTION: What is OPC?
Object Linking and Embedding (OLE) for Process Control (OPC) is basically a link between two pieces of software, with one called the client and the other the server. On this link, the client software (or program) makes a service request (asks for data) from the server, which then fulfills the request. Although the concept of client/server in OPC can be used in one computer, the communications model of OPC provides a simple and convenient way to connect programs that are distributed over networks.
OPC is a very simple standard in reality, and any product that is OPC-compatible will connect directly to any other OPC product without any coding. Typical clients would be PCs running OPC-compatible SCADA, Visual Basic programs, human-machine interface (HMI) systems or ERP/MRP systems.
Since OPC servers only transmit refreshed (or altered) information to the client(s), rather than transmitting every data field, they can have a fast update speed—a prerequisite in most systems. This selective data refresh provides a quick update to the connected software, allowing operators true real-time control of connected equipment.
Internally, OPC is based on Microsoft's COM/DCOM which is an open, published protocol used to link Microsoft products and other software. OPC consists of a number of separate functions (or specifications) that can be achieved using the open standard software. These range from data access, batch, alarms and events, to data exchange and security activities. The main specification used in most applications is data access, which allows a user to map tag addresses of a connected device to OPC. With data access, OPC will automatically establish a data connection to the device (i.e., a data register in a PLC), giving simple fast continuous access to equipment information together, In fact, the OPC Foundation recently released OPC-DX 1.0 (Data eXchange), which supports both DCOM-based data access servers and XML data access servers.
OPC can also be distributed over physical networks permitting distribution of clients and servers. Any number of OPC clients can connect to one OPC server, with the practical limitation being dependant on factors such as how often data is requested, how much data is transferred, and hardware restrictions.
How open is OPC and how can it help?
An OPC product is an automation software program conforming to an OPC specification. All the OPC specifications are maintained by the OPC Foundation, which is an independent, non-profit organization dedicated to the further development of OPC technology. Since an OPC server is based on an open standard, users devote less time on software connectivity issues, and have more time with application issues, eliminating duplication efforts.
A major benefit of an OPC solution is the vendor and application front-end independence. OPC provides an easy-to-manage scalability, permitting expansion of the system using any manufacturer's OPC compatible client/server products, allowing users to choose best in class software, without being restricted by the number of tags, unlike most SCADA systems.
Bengt Salomonsson, VP, Beijer Electronics USA
Source: Bengt Salomonsson, 'OPC Primer,' Control Engineering Online , Sept. 11, 2003, located at www.manufacturing.net/ctl/article/CA320108?text=opc+and+greenfield&spacedesc=news .
August 15, 2006
Non-conventional machining is widely used to produce products whose materials cannot be machined by conventional processes such as turning, milling, drilling, shaping, or grinding. Non-conventional processes include:
Laser cutting and electron-beam machining;
Electrochemical machining (ECM);
Electrodischarge machining (EDM); and
Water jet machining.
Reasons for resorting to the above methods may be due to extreme hardness of the part material or that the required operation is not possible by standard methods. This may be the case of extremely small holes or very complex part profiles needed.
Source: ' Electric Drives and Electromechanical Systems ,' Richard Crowder, ISBN-13: 978-0-7506-6740-1, Newnes (2006).
August 8, 2006
These calibration methods are 'wet,' dry, and dimensional measurement check. Wet calibration , using actual fluid flow, normally provides the highest flowmeter calibration accuracy. It's used where accuracy is a prime concern or when the meter's form is not suited to other methods.
Dry calibration refers to performing a procedure on a flowmeter without using a fluid medium. It uses flow simulation by electronic or mechanical means. A signal (frequency, mV, differential pressure, or dimensional change) is introduced in the input portion of the meter but a part of the basic flow transducer bypassed. As a result, this method has a larger uncertainty than wet calibration. Overall accuracy of the flow device must be inferred.
The third method is a measurement check of physical dimensions, combined with use of empirical tables that relate flow rate to the dimensions. While not a true calibration, this method can sometimes provide adequate assurance of accuracy in noncritical applications. It's most frequently used for square-edge concentric orifice plates where bore diameter and edge condition can be checked.
Source: 'Flow Measurement,' D.W. Spitzer, editor, The Instrumentation, Systems, and Automation Society (ISA), ISBN 1-55617-334-2 (1991)
August 1, 2006
Fault level , also known as short-circuit level describes a power supply's 'strength' or ability to supply both voltage and current. The term is given by the relationship:
Fault level = Open-circuit voltage x short-circuit current [VA/phase]
This provides a single number useful in selecting the size of circuit breakers needed at a specific point in a power system. Product of voltage and current in fault level is a better measure of circuit breaker size and power needed, since both entities are involved in the device's ability to extinguish the arc produced when power contacts of the circuit breaker are separating.
Source: 'Power Electronic Control in Electrical Systems,' E. Acha, V.G. Agelids, O. Anaya-Lara, T.J. E. Miller, ISBN 0 7506 5126 1, Newnes (2002).
July 25, 2006
This term refers to the tendency of a stationary object to require a higher force to start movement than the force needed to continue movement once it has started. Stiction condition stems from the higher value of static or breakaway friction compared to dynamic or sliding friction. Stiction is also known as 'stickslip.'
Source: 'Directory of Electrohydraulic Terms and Electrohydraulic Analogies,' Bulletin No. 0245, Parker Hannifin Corp. (1991) and Control Engineering.
July 18, 2006
This is a common type of incremental encoder that generates two output square waves (usually referred to as Channels A and B) offset by 90 electrical degrees—hence the term quadrature. Besides sensing rotary position, a two-channel quadrature encoder can sense direction of rotation using the phase relationship of its two signals. Typically, rotation is clockwise if Channel A leads Channel B, and vice versa.
Source: Control Engineering, July 2000 Back to Basics, ' Rotary encoders make versatile motion feedback devices .'
July 11, 2006
NAICS is the North American Industry Classification System, which has replaced the familiar, long-standing U.S. Standard Industrial Classification (SIC) system. The U.S., Canada, and Mexico jointly developed NAICS to provide new comparability in statistics about business activity across North America.
NAICS seeks to reshape the way we view our changing economy. It updates the SIC system, covering all industries (including emerging technologies and services) not only manufacturing-based industries; uses a six-digit classification system (versus SIC's four digits); and is updated every five years (versus 10-15 years).
More information about NAICS is available at the Web site below, along with how to obtain the official 2002 US NAICS Manual, which includes definitions for each industry covered.
Source: U.S. Census Bureau Web site .
July 3, 2006
It's the distortion of a sinusoidal wave (typically voltage and current) characterized by the presence of harmonics of the fundamental frequency. Percent harmonic distortion is an appropriate figure of merit, which is defined as the ratio (in %) of the square root of the sum of the squares of all rms harmonic voltages (or currents) to the fundamental.
Institute of Electrical and Electronic Engineers (IEEE) standard 519-1999 recommends maximum current distortion levels (caused by loads) that limit the voltage distortion they could create. Harmonic voltage limits are given as maximum values of total harmonic distortion (THD, %).
Source: 'Handbook of Standardized Terminology for the Power Sources Industry (2nd edition), Power Sources Manufacturers Association Inc. (1995) and Control Engineering.
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