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. 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.

2 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. 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.

3 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. 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.

4 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. 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.

5 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. 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.

6 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. 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.

7 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. This is especially true when thewiring is in or passes through plant areascontaining flammable gases or vapors. Thehazards represented by these atmospheresforce the use of very expensive techniques toprevent fires or explosions caused by an elec-tric concentrators may be used to reducewiring costs. These devices collect largenumbers of signals close to their origins inthe field, perform Signal conditioning anddigital data conversion locally and send thedigitized information by communicationlinks to a local area network or to the controlroom equipment terms sensor and transmitter areoften used interchangeably.

8 However, thereis an important difference between sensorsand transmitters. A sensor is a device thatconverts a physical quantity into a formwhich can be further used to indicate or con-trol the measured variable. This form may bemechanical, like a pressure dial gauge, ormay produce an electrical Signal . A transmit-ter takes this idea one step further and pro-Figure 2. control and Field Conditions Industrial Measurement Environment4vides some manipulation of the sensor signalat the sensor location through amplification,filtering, isolation or other electronic the purposes of this handbook the maindifference between sensors and transmittersis that transmitters manipulate the Signal atthe measurement point. Usually, a dataacquisition or control system contains a mixof sensors and transmitters. Ideally, each sensor would have Signal -con-ditioning at the point of measurement andtransmit a high-level Signal back to the dataacquisition system or control system.

9 Theshorter interconnection from sensor to signalconditioner is less likely to pick up noise, andthe high-level output Signal offers betterimmunity against induced pickup from natu-ral or man-made sources. However, this idealconflicts with the economic reality that sig-nal- conditioning at the measurement point isa costlier approach than shared Signal condi-tioning at the data collection/ control a compromise must be made betweensignal integrity and system LINEARIZATIONMany sensors exhibit a deviation from anideal (linear) relationship between input anoutput. For example, a given change in tem-perature does not give rise to the samechange in EMF for most thermocoupleswhen measured over different temperatureranges. Sensors or signals which exhibit thisbehavior are said to be non-linear. A hypo-thetical non-linear transfer function is shownin Figure 3. This figure illustrates the conceptof terminal-based linearity which is thedeviation of the actual characteristic from astraight line coinciding with the actual char-acteristic endpoint (terminal) families within the Dataforth productoffering such as the DSCA and DSCT DINrail products, SCM5B and SCM7B plug-inpanel products, and SCT instrument headmount products can be used in the follow-ing examples.

10 For simplicity and uniformity,we have referred to the SCM5B familythroughout the of the Dataforth SCM5B series mod-ules have the ability to create a non-lineartransfer function through the module non-linear transfer function is config-ured at the factory and is designed to be equaland opposite to the sensor or Signal non-lin-earity. The net result is that the module out-put Signal is linear with respect to a giveninput parameter such as temperature. An out-put Signal which has been linearized withhardware internal to the SCM5B moduleseliminates the need for tedious software rou-tines which determine a linearized signalthrough the use of high-order polynomials orlook-up hardware piece-wise linear technique isused in the SCM5B modules to correct thenon-linearity of the Signal . The differencebetween the sensor non-linearity and the lin-earization provided by the SCM5B module iscalled the conformity error . This is adescription of how well the linearizationtechnique conforms to the non-linear are placed along the curve so asto equalize the positive and negative con-formity errors.