Dataforth Tutorial Brochure
Industrial Signal ConditioningA Tutorial by Dataforth13331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
Table of ContentsOVERVIEW OF INDUSTRIAL MEASUREMENT . 4USES OF INDUSTRIAL MEASUREMENT . 5INDICATE-ONLY MEASUREMENTS . 5CONTROL MEASUREMENTS . 5CUSTODY TRANSFER MEASUREMENTS . 5ENVIRONMENTAL MEASUREMENTS . 6SAFETY MEASUREMENTS . 6INDUSTRIAL MEASUREMENT ENVIRONMENT . 8FIELD . 8CONTROL ROOM . 9FIELD WIRING . 9SENSORS . 11TERMINOLOGY . 11SENSOR LINEARIZATION . 11SENSOR CLASSIFICATION . 14TEMPERATURE SENSORS . 14Thermocouples (TCs) . 14Resistance Temperature Detectors (RTDs) . 18Thermistors . 22MOTION SENSORS . 25Slidewire . 25STRAIN GAGES . 27Practical Gage Arrangements . 27Gage Types . 30Accelerometers . 31LOOPS AND ANALOG SIGNALS . 33MEASUREMENT LOOP CONFIGURATIONS . 33Four-Wire Loops . 34Three-Wire Loops . 3523331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
Two-Wire Loops . 36ANALOG SIGNALS . 38Voltage-Mode Loops . 38Current-Mode Loops . 40Frequency-Mode Loops . 41Choice of Signal Level . 41SIGNAL INTEGRITY . 43SOURCES OF ERROR. 44Capacitive Coupling. 44Magnetic Coupling . 44Ground Loops . 44Over Voltage and Transients . 47EMI and RFI. 49Aliasing . 50WAYS TO PRESERVE SIGNAL INTEGRITY . 52Shielding. 52Twisted Pairs . 53Cable Spacing . 55Isolation . 55Anti-Aliasing Filters . 55DESIGN EXAMPLES . 57SERVO CONTROL . 57ALUMINUM SMELTING . 59GROUNDED THERMOCOUPLES . 61PRODUCT SELECTION GUIDE . 63Analog-to-Analog Signal Conditioning Modules . 63Low Cost Industrial Loop Isolators and Programmable Transmitters . 65Industrial and Process Two-wire Transmitters . 66Distributed Data Acquisition and Control Solutions . 67Industrial Communication Products . 6833331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
OVERVIEW OF INDUSTRIAL MEASUREMENTThe need to measure and control the operation of machinery or process equipment is as oldas the Industrial Revolution. Plant instrumentation has now become the nerves and brain ofthe modern manufacturing plant. It regulates and supervises the operation of the equipmentwithin the plant. It also provides the means to make plants economically viable.Instrumentation allows the use of processes which would be difficult or impossible to operatewithout automation.Instruments have grown from purely analog systems to the ‘smart’ systems in use today,ranging from simple potentiometers to complex analyzers such as infra-red spectrophotometers. Yet, for all the advances in systems development, analog field measurements andthe electronic signals that carry them are still necessary ingredients in all systems.Analog measurements take many forms but can be roughly classified into two types—physical measurements and compositional measurements. The first type includes pressure,temperature, flow, force, vibration, mass and density. The second includes suchmeasurements as conductivity, pH and chemical analysis.Obtaining, maintaining and improving the quality of these measurements is the goal of propersignal conditioning. Good signal conditioning preserves the quality of the measurementsavailable and allows the plant systems to make best use of the control and data acquisitionsystems installed. Helping Dataforth customers achieve good signal conditioning is the goal ofthis handbook.43331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
USES OF INDUSTRIAL MEASUREMENTThere are several distinct uses of analog measurements.INDICATE-ONLY MEASUREMENTSIndicate-only measurements are used to indicate the condition of various elements of aprocess. Estimates place the ratio of indicate-only to control inputs at somewhere between 2to-1 and 3-to-1. Regardless, these measurements are useful to monitor the condition ofintermediary events at every stage of manufacture or processing and may provide necessaryinformation to the plant operator if a control measurement fails. An example of this kind ofmeasurement is the complete temperature monitoring of the distillation trays in a distillationtower. Each measurement is not essential to the control of the side-draw products but doesprovide valuable insight about the operating conditions and material and energy balanceswithin the tower. They also allow the operator to intervene manually if a control measurementfails.CONTROL MEASUREMENTSControl measurements are essential to the economic viability, safety or functioning of amanufacturing process. They provide control over a physical or compositional characteristic ofthe process. For example, the temperature of a heat exchanger is an essential parameter forboth process and safety reasons. Flow measurements and control such as those illustrated byFigure 1: Typical Measurement/Control Loop appear in almost every plant.CUSTODY TRANSFER MEASUREMENTSCustody transfer measurements need highly accurate and stable characteristics. Thesemeasurements provide information for plant inventory, quantify the amount of material boughtor sold between parties or track internal transfers of material from one operating unit to53331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
another within the plant. Frequently the calibration of the instruments is regulated bymunicipal, state or Federal agencies. The gasoline pump in your neighborhood is an exampleof these measurements.Figure 1: Typical Measurement/Control LoopENVIRONMENTAL MEASUREMENTSEnvironmental measurements have grown enormously in recent years to provide traceablerecords of plant effluents, and waste products in compliance with government regulations. Anentire technology has evolved to detect and control hazardous materials of all kinds.SAFETY MEASUREMENTSFinally, there is an entirely separate and autonomous type of measurement system whosesole function is to monitor and limit dangerous conditions. Measurements include criticalprocess parameters that indicate unsafe operation and potential danger. These systemsoverride the regulatory controls and cause a plant shutdown to a safe status should63331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
emergency conditions dictate. Known as EMERGENCY SHUTDOWN systems, they arefrequently equipped with sophisticated events-monitoring recorders so that later analysis ofthe shutdown events can be made, and future malfunctions avoided or controlled.73331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
INDUSTRIAL MEASUREMENT ENVIRONMENTFigure 2: Control and Field Conditions — Industrial Measurement Environment shows asimplified view of a measurement and control system. It shows only the essential elements butdemonstrates the division between field and control room functions.Figure 2: Control and Field Conditions — Industrial Measurement EnvironmentFIELDThe term ‘field’ refers to the area where the equipment making a product or running a processresides. It is most often the factory floor or the outside areas of an industrial complex such asa chemical plant. What sets it apart from other areas is its harsh electrical and physicalenvironment. The equipment located there is exposed to a much greater range of electricalnoise, power surges, temperature, humidity, and corrosive or damaging environments.The field is where process variables must be measured and where measuring and somesignal conditioning equipment must be located. The measuring equipment and wiring may be83331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
near heavy electrical equipment, motor contactors and even lightning. Often the wiring runsseveral hundreds or thousands of feet, increasing the likelihood of outside interference fromthis environment.CONTROL ROOMThe control room is usually a more benign place than the field, with a cleaner atmosphere, airconditioning, and fewer hazardous conditions. However, it also contains electrical equipmentand the potential for degrading the quality of measurements. The control room contains signalconditioning and computing equipment that is sensitive to electrical interference.The control room is usually the location where people interact with the measurement andcontrol systems in a plant. There are exceptions, but the control room is where most decisionsabout the plant or process are made.FIELD WIRINGInstrumentation wiring connecting field devices to the control room typically consists of heavyduty (16-18 AWG) pairs. They are often twisted together to aid in reducing magneticallycoupled interference and run with other signal wires in a separate wiring tray away from powerdistribution wiring. Large numbers of sensor or transmitter signals may be gathered in terminalcabinets located either in the control room area or in an intermediate site for ease ofconnection to the signal conditioning and display equipment.In most instances, the cost of wiring is a large percentage of the installed cost of the instrument system. This is especially true when the wiring is in or passes through plant areascontaining flammable gases or vapors. The hazards represented by these atmospheres forcethe use of very expensive techniques to prevent fires or explosions caused by an electricspark.Data concentrators may be used to reduce wiring costs. These devices collect large numbers93331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
of signals close to their origins in the field, perform signal conditioning and digital dataconversion locally and send the digitized information by communication links to a local areanetwork or to the control room equipment directly.103331 E. HEMISPHERE LOOPTUCSON AZ 85706www.dataforth.com 1-520-741-1404Fax 1-520-741-0762
SENSORSTERMINOLOGYThe terms ‘sensor’ and ‘transmitter’ are often used interchangeably. However, there is animportant difference between sensors and transmitters. A sensor is a device that converts aphysical quantity into a form which can be further used to indicate or control the measuredvariable. This form may be mechanical, like a pressure dial gauge, or may produce anelectrical signal. A transmitter takes this idea one step further and provides some manipulationof the sensor signal at the sensor location through amplification, filtering, isolation or otherelectronic means. For the purposes of this handbook the main difference between sensorsand transmitters is that transmitters manipulate the signal at the measurement point. Usually,a data acquisiti
INDUSTRIAL MEASUREMENT ENVIRONMENT Figure 2: Control and Field Conditions — Industrial Measurement Environment shows a simplified view of a measurement and control system. It shows only the essential elements but demonstrates the division between field and control room functions.
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