Industrial Signal Conditioning, A Tutorial

1 Industrial Signal conditioning , A TutorialOVERVIEW OF Industrial MEASUREMENT ..2 USES OF Industrial MEASUREMENT ..2 Industrial MEASUREMENT ENVIRONMENT ..3 FieldControl RoomField WiringSENSORS ..3 TerminologySensor LinearizationSensor ClassificationTemperature SensorsMotion SensorsStrain GagesLOOPS AND ANALOG SIGNALS ..11 Measurement Loop ConfigurationsAnalog SignalsSIGNAL INTEGRITY ..14 Sources of ErrorWays to Preserve Signal IntegrityDESIGN EXAMPLES ..18 Servo ControlAluminum SmeltingGrounded ThermocouplesPRODUCT SELECTION GUIDE ..21 OVERVIEW OF Industrial MEASUREMENTThe need to measure and control the opera-tion of machinery or process equipment is asold as the Industrial Revolution.

2 Plant instru-mentation has now become the nerves andbrain of the modern manufacturing plant. Itregulates and supervises the operation of theequipment within the plant. It also providesthe means to make plants economicallyviable. Instrumentation allows the use ofprocesses which would be difficult or impos-sible to operate without have grown from purely analogsystems to the smart systems in use today,ranging from simple potentiometers to com-plex analyzers such as infra-red spectropho-tometers. Yet, for all the advances in systemsdevelopment, analog field measurements andthe electronic signals that carry them are stillnecessary ingredients in all measurements take many forms, butcan be roughly classified into two types physical measurements and compositionalmeasurements.

3 The first type includes pres-sure, temperature, flow, force, vibration,mass and density. The second includes suchmeasurements as conductivity, pH and chem-ical analysis. Obtaining, maintaining and improving thequality of these measurements is the goal ofproper Signal conditioning . Good Signal con-ditioning preserves the quality of the meas-urements available and allows the plant sys-tems to make best use of the control and dataacquisition systems installed. HelpingDataforth customers achieve good signalconditioning is the goal of this OF Industrial MEASUREMENTT here are several distinct uses of are used toindicate the condition of various elements ofa process.

4 Estimates place the ratio of indi-cate-only to control inputs at somewherebetween 2-to-1 and 3-to-1. Regardless, thesemeasurements are useful to monitor the con-dition of intermediary events at every stageof manufacture or processing and may pro-vide necessary information to the plant oper-ator if a control measurement fails. An exam-ple of this kind of measurement is the com-plete temperature monitoring of the distilla-tion trays in a distillation tower. Each meas-urement is not essential to the control of theside-draw products, but does provide valu-able insight about the operating conditionsand material and energy balances within thetower.

5 They also allow the operator to inter-vene manually if a control are essential to theeconomic viability, safety or functioning of amanufacturing process. They provide controlover a physical or compositional characteris-tic of the process. For example, the tempera-ture of a heat exchanger is an essentialparameter for both process and safety rea-sons. Flow measurements and control such asthose illustrated by Figure 1 appear in almostevery TRANSFER measurements needhighly accurate and stable measurements provide information forplant inventory, quantify the amount of mate-rial bought or sold between parties or trackinternal transfers of material from one oper-ating unit to another within the the calibration of the instrumentsis regulated by municipal, state or Federalagencies.

6 The gasoline pump in your neigh-borhood is an example of these have grownenormously in recent years to provide trace-able records of plant effluents, and wasteproducts in compliance with governmentregulations. An entire technology hasevolved to detect and control hazardousmaterials of all MEASUREMENTS Finally, there isan entirely separate and autonomous type ofmeasurement system whose sole function isto monitor and limit dangerous include critical processparameters that indicate unsafe operation andpotential danger.

7 These systems override the2 Figure 1. Typical Measurement/ control Loop3regulatory controls and cause a plant shut-down to a safe status should emergency con-ditions dictate. Known as EMERGENCYSHUTDOWN systems, they are frequentlyequipped with sophisticated events-monitor-ing recorders so that later analysis of theshutdown events can be made and futuremalfunctions avoided or MEASUREMENT ENVIRONMENTF igure 2 shows a simplified view of a meas-urement and control system. It shows onlythe essential elements, but demonstrates thedivision between field and control roomfunctions.

8 FIELDThe term field refers to the area where theequipment making a product or running aprocess resides. It is most often the factoryfloor or the outside areas of an industrialcomplex such as a chemical plant. What setsit apart from other areas is its harsh electricaland physical environment. The equipmentlocated there is exposed to a much greaterrange of electrical noise, power surges, tem-perature, humidity, and corrosive or damag-ing field is where process variables must bemeasured and where measuring and somesignal conditioning equipment must be locat-ed.

9 The measuring equipment and wiringmay be near heavy electrical equipment,motor contactors and even lightning. Oftenthe wiring runs several hundreds or thou-sands of feet, increasing the likelihood ofoutside interference from this ROOMThe control room is usually a more benignplace than the field, with a cleaner atmos-phere, air conditioning , and fewer hazardousconditions. However, it also contains electri-cal equipment and the potential for degradingthe quality of measurements. The controlroom contains Signal conditioning and com-puting equipment that is sensitive to electri-cal control room is usually the locationwhere people interact with the measurementand control systems in a plant.

10 There areexceptions, but the control room is wheremost decisions about the plant or process WIRINGI nstrumentation wiring connecting fielddevices to the control room typically consistsof heavy-duty (16-18 AWG) pairs. They areoften twisted together to aid in reducingmagnetically coupled interference and runwith other Signal wires in a separate wiringtray away from power distribution numbers of sensor or transmitter sig-nals may be gathered in terminal cabinetslocated either in the control room area or inan intermediate site for ease of connection tothe Signal conditioning and display most instances, the cost of wiring is a largepercentage of the installed cost of the instru-ment system.