13 Computerised Temperature Measuring Systems 13 .1 Trends in Process Instrumentation In contemporary industrial processes, the measurement of temperature is one of the most frequently conducted operation among all of the other various physical variables that need to be measured . Current process instrumentation and measurement techniques show an inexorable trend in the use of a wide array of sophisticated procedures, including the capture and advanced algorithmic processing of multi-channel data . This trend is reflected in temperature measurement technology, so that typical temperature measurement circuits are now being replaced by computer-based data acquisition and processing systems . In general, such a system comprises transducers, transmitters, measuring instruments, indicators and recorders . In most systems, this is also accompanied by process controllers supervised by a computer as shown in Figure 13 .1 . The set of all these elements is called the system hardware . The contents of this chapter is only devoted to the problems of temperature measurement using computerised methods, as the problems of control are outside the scope of this book . The central part of a data acquisition system is a Personal Computer (PC), or work station, in a typical industrial housing, as shown in Figure 13 .2, which may also be DATA ACQUISITION BOARDS COMPUTER a NETWORK INTERFACE TRANSDUCERS ' AND MEASURING INSTRUMENTS I ~'- _ _ _ +art PLANT Figure 13 .1 Block diagram of computer-based Figure 13 .2 Advantech Inc . IPC-610 personal measuring and controlling system computer in an industrial housing (courtesy of Advantech Inc .) Temperature Measurement Second Edition L. Michalski, K. Eckersdorf, J. Kucharski, J. McGhee Copyright © 2001 John Wiley & Sons Ltd ISBNs: 0-471-86779-9 (Hardback); 0-470-84613-5 (Electronic) 260 COMPUTERISED TEMPERATUREMEASURING SYSTEMS connected to a network . All the hardware elements are connected to the computer either by one of various standardised digital interfaces, already described in Section 12 .3, or by specialised data acquisition cards plugged into the computer . The operation of the computer within such a system requires a two-way information exchange in order to permit both the gathering of measurement data and the supervision and configuration of the whole system . Specialised software is used to perform most of these functions as well as those for processing, analysing, displaying and recording the data . Computer-based data acquisition systems sometimes work within commonly used computer networks, such as ETHERNET and TOKEN-RING, using a typical data transfer protocol . There are also many specialised industrial networks as the three-level industrial network proposed by Rockwell Corp . (Rockwell, 1997), which includes : " DeviceNet - integrating the instruments and devices and allowing a reduction of the installation costs and wiring, and simpler diagnostics and trouble-shooting . " ControlNet - providing real-time high-speed transport of data and messages for programming and configuration of the system . " Ethernet - completing the network architecture and ensuring access to various information systems and enterprise-wide communications . It has been pointed out in Sections 12 .5 and 12 .6, that most of the various stand-alone measuring instruments and transmitters, which usually work as independent devices, were additionally equippedwith at least one of the typical digital interfaces like RS 232 or IEEE 488 . This allows them to be interconnected with computer-based systems . In the following sections, temperature measuring modules, which are dedicated to computer data acquisition systems and which are not able to operate as stand-alone devices, will be described . Two main groupsofsuch modules can be distinguished : " external measuring modules - connected to the computer by a typical digital interface, " data acquisition cards - plugged into the expansion slots of the computer . Some well known software packages supplementing the hardware will be also presented . 13 .2 Hardware for Computerised Measurement 13 .2 .1 External measuring modules External modules for computer-based temperature measurement accept signals from electric temperature sensors at their input, while producing an output signal conforming to one of the standard digital interfaces . Usually, since they are designed to operate with a computer, they are not equipped with a standardised analogue output . While plug-in expansion cards use the internal expansion slots of the computer, external modules only require an external connecting interface . A simple example ofan external module for temperature measurement is the AD-TDA8 moduleby Keithley Instruments (Keithley, 1998), which connects to the parallel port of a PC as shown in Figure 13 .3 . Its 8 input channels, which are compatible with type J, K, or T thermocouples, are associated with a 12-bit ADC to give an accuracy of ±1 °C suitable for low cost data acquisition in applications that do not require a high data HARDWARE FOR COMPUTERISED MEASUREMENT 261 transfer speed . The conversion rate, ranging from 221 .Ls to 50 ms, is compatible with the includedsoftware processing, which operates under Windows® . Flexible computer-based data acquisition systems can be built conveniently using modules adapted for the RS-485 serial interface as shown in Figure 13 .4 . Such a solution called a distributed data acquisition system is offered by many producers such as : " Keithley Instruments- M 1000 series modules (Keithley, 1998), " NationalInstruments - 613 series modules (National Instr ., 1996), " Omega Engineering - D 1000 series modules (Omega Eng . Inc ., 1992, 1999), " Advantech Inc ADAM 4000 series modules (Advantech, 1997) . The modules are usually designed for rail or panel mounting, close to the source of the signal being measured . Typical example of such external modules are the several types of ADAM units by Advantech Inc . (Advantech, 1997) detailed in Table 13 .1 . As an illustrative example the ADAM 4011 module is shown in Figure 13 .5(a) and its block diagram in Figure 13 .6 . It includes 16-bit ADC with programmable input ranges, ensuring an accuracy better than ±0 .05 % . Independent, versatile signal conditioning, such as linearisation and thermocouple reference junction compensation, are achieved using the built-in microprocessor . They also provide analogue filtering for noise rejection, opto- isolation and enable signalling of alarm notification . ADAM modules communicate with their controlling host computer over RS-485 interface or using RS-232/RS-485 converter to change the standard if no RS-485 interface is available on the host computer . All modules CONVERTER RS-2321R5-485 RS-485 w 1 a 2 0 0 34 12 BIT PC PARALLEL 1 2 . . . . N AID PORT i7 v ~ 8 MODULES Figure 13 .3 Keithley Instruments AD-TDA8 Figure 13 .4 Block diagram of distributed data external temperature measurement module acquisition system for temperature measurement (courtesy of Keithley Instr .) cam :' ~ . . Figure 13 .5 Advantech Inc . ADAM series modules : (a) ADAM 4011 ; (b) ADAM 5000 (courtesy of Advantech Inc) 262 COMPUTERISED TEMPERATUREMEASURING SYSTEMS Table 13 .1 Chosen technical parameters of ADAM series modules dedicated for temperature measurements Type of ADAM module Technical data ADAM -4011 ADAM - 4018 ADAM-4013 Types of temperature Thermocouple type : Thermocouple type : RTD's : sensors J, K, T, E, R, S, B J, K, T, E, R, S, B Pt, Ni, Cu Voltage input ranges ±15 mV to ±2 .5 V ±15 mV to ±2 .5 V - Current input ranges ±20 mA ±20 mA ±20 mA Number of input channels 1 differential 6 differential ; 2 single-ended 1 differential Sampling rate 10 Hz 10 Hz 10 Hz Opto - isolation 500 V 3000 V 3000 V LOW-PASS OPTO- DIGITAL OUTPUT FILTER 10Hz ISOLATION M DATAi nNIV1mA U 16 BITS AID PP RS485 DATA- T X ® EEPROM -CONFIGURATION DIGITAL ISOTHERMAL -LINEARISATION INPUT BLOCK -SIGNALLING REFERENCE TEMPERATURE SENSOR Figure 13 .6 Block diagram of Advantech Inc . ADAM 4011 module (courtesy of Advantech Inc .) are remotely controlled and configured by the ASCII-based command/response protocol, which ensures compatibility with any computer system . Variations, such as panel-mounting in the ADAM 5000 series, shown in Figure 13 .5(b), or radio communication for the data acquisition system using ADAM 4550 module, are also possible . Another external data acquisition system module called the SmartLink KNM Instrument, offered by Keithley Instr . (Keithley, 1998), is illustrated using the KNM-TC 42-EthT module, shown in Figure 13 .7, as an example . These microprocessor-based modules allow multi-channel temperature measurement using various temperature sensors, providing a variety of on-board signal processing capabilities and being able to communicate not only directly with a host computer but also with most popular computer networks . General characteristics of KNM modules are : " accepted temperature sensors : thermocouples, RTD's, thermistors, and also sensors for humidity and dew point measurement, " digital noise filtering and linearisation, " minimum, maximum values indication, HARDWARE FORCOMPUTERISED MEASUREMENT 263 . *y Figure 13 .7 Keithley Instr . KNM intelligent measuring module (courtesy of Keithley Instr .) . " alarm level notification, " opto-isolation, " accommodated digital interfaces : RS-232, RS-422, RS-485, PCMCIA, Ethernet (various versions), " dimensions : 17x3 .3x2 .7 cm . The KNM-TC 42-EthT module, which has 6 input channels for types J, K, R, E, S, T, N, and B thermocouples, includes 20-bit ADC giving a resolution of 0 .01 °C at up to 15 readings per second . Additional on-board memory can buffer up to 10 000 8-bit readings . A high speed computer-based system for multichannel temperature data acquisition from thermocouples and RTDs is allowed by the IOtech (1997) TempScan 1000 system of Figure 13 .8 . The basic system, whose scanning module has 32 thermocouple input channels or 16 RTD input channels, can be easily expanded up to 992 channels . The unit provides temperaturereadings at a rate ofup to 960 channels per second, transmitted to the computer via IEEE-488 or RS232/RS422 interfaces . The technical data of TempScan 1000 system are : " temperature sensors : - type J, K, T, E, R, S, B and N thermocouples, - Pt-100 RTD, " programmable, user defined linearisation tables, " 16-bit ADC, " accuracy : - thermocouples : from t0 .5 °C to ±2 °C (depending on type and range), - RTD : ±0 .4 °C, " maximum scanning rate per channel : 50-60 Hz, " digital noise filtering - 16 samples averager, " 32 bit TTL compatible alarm output, " 256 KB on-board memory, expandable up to 8 MB . 264 COMPUTERISED TEMPERATURE MEASURING SYSTEMS . .V Figure 13 .8 General view of Iotech TempScan 1100 multi-channel temperature measuring system (courtesy of IOTech Inc .) 13 .2 .2 Plug-in data acquisition cards Data acquisition cards, which usually allow multichannel computer measurement of various analogue signals, also include initial conditioning of analogue signals and ADC . In addition, they usually perform one or several of the functions for analogue output, digital input and digital output, which can be used for control and configuring of measurement system . These acquisition cards, which can be plugged into one of the internal PCI or ISA expansion slots of the PC, communicate with the host computer by its 1/O addresses or Direct Memory Access (DMA) . In time-critical cases, such as in Digital Signal Processing (DSP) boards, they may also include a high speed microprocessor for on-board processing . Software packages for use with plug-in expansion cards allow versatile data analysis and sophisticated graphical display, which make the system very universal and flexible . Although most cards are general purpose, accepting standard forms of signal, some are specialised for a certain type of input signal . Properties and chosen parameters of contemporary data acquisition cards, offered by such producers as Keithley Instruments, National Instruments, Omega Engineering, Advantech Inc . and others, are : " number of input channels : 4-16 differential or single-ended, " ADC resolution : 12-16 bits, " input ranges : 0 .005-10 V unipolar or bipolar, " analogue amplification : maximum x 1000, " sampling rate : typically up to 300 kHz, sometimes up to 1 MHz (National Instruments, 1996) or even up to 1 GHz (Keithley, 1998) . A typical example of a specialised, temperature measurement plug-in card is the DAS- TCB offered by Keithley Instr . (Keithley, 1998) shown along with its block diagram in Figure 13 .9 . Up to 16 differential thermocouple inputs are provided as well as one sensor input for reference temperature compensation . Automatic calibration, analogue gain selection, HARDWARE " ' " MPUTERISED MEASURENlENT 265 .°a I 5r, +p INSPAMP CH 0 0- CONVERTER INPUT CHANNEL Mux CH 15 0 - 6 1 5 ISOLATOR MLIX ISOLATOR ISOLATOR 0 CH 15 CPU CONTROL 16NINZ +9 .W VOLTAGE REGISTER XrAL REF . REF . 4 MHZ . . . ISOLATOR 1 PULSE SIGNAL CONDITIONING CONTROL SECTION PCVO 1 1 1 1 +5 VI CONTROL 11 ROM PORT RUPT DECODE STATIC CONTROL LOGIC RAM LOGIC - - - - - - - - - - :: H PC BUS 1 Keithley Instruments " AS-TC/13 data acquisition card : (a) general view ; (b) block compensation, diagram . (courtesy of Keithley Instruments) 1 _ averaging are performed by an on-board microprocessor . To convert the raw data to thermocouple . The look-up table optimises accuracy by using more reference points along temperature measurement, the microprocessor refers to ROM-stored look-up tables for each " _ 266 COMPUTERISED TEMPERATURE MEASURING SYSTEMS Detailed parameters of this card are : " thermocouple types : J, K, T, E, R, S and B " resolution : 0 .03-0 .2 °C depending on the thermocouple type and measuring range, " accuracy : ±0 .5 to ±3 °C depending on the thermocouple type and measuring range, " conversion rate : 10 ms/channel to 47 ms/channel depending on the noise rejection rate, " temperature error coefficient : ±10 ppm/°C to 17 ppm/°C depending on the type of thermocouple (the errors can be cancelled by periodic self-calibration of the card) . Installation, configuration, testing and calibration are performed using the software packages supplied with the board . An example of a general purpose plug-in card is the PCL-816 data acquisition board by Advantech Inc . (Advantech, 1997), shown in Figure 13 .10, which has 16-bit resolution with 16 differential analogue input channels with separately programmable gains . This card assures an accuracy of measurement of 0 .003 %+1 bit and a maximum sampling rate of 100 kHz . Voltage input ranges are : " bipolar : ±IOV, ±5V, ±2 .5V, ±1 .25V, " single-ended : 0-10V, 0=5V, 0-2,5V, 0=1 .25V . For temperature measurement the PCL-816 board can operate with the PCLD-789D external daughter board, containing an amplifier and a 16 channel differential input multiplexer into one of the ADC input channels, so that it is possible to cascade up to ten such boards, allowing a single data acquisition card to access 160 analogue input channels . The PCLD-789D uses an amplifier that provides switch-selectable gains of up to 1000 times so that low level signals can be measured accurately . The board also contains reference junction sensing circuitry, which allows direct temperature measurement by thermocouples . Additional low-pass analogue filters reduce noise from measured signals . The block diagram of a PCLD-789D daughter board and its general view are shown in Figure 13 .11 . ix L> Figure 13 .10 Advantech Inc . universal PCL-816 data acquisition card (courtesy of Advantech Inc .) HARDWARE FOR COMPUTERISED MEASUREMENT 267 a L . r b) REFERENCE TEMPERATURE REFERENCE TEMPERATURE CHANNEL SENSOR MULTIPLEXER CONNECTORS GAIN 0 : :_ . ruuuc ruw ' gun ' X111 ' - _ 0- : 1W -CF4 ER~ AMPLIFIER 15 ANALOG OUTPUT CHANNE _ ~i i CHANNEL . J diagram (courtesy of Advantech Inc .) 1 .~~ , ., 00 pp 11 Figure 13 .12 Connection of several PCLD-789D daughter boards to one PCL-816 data acquisition card . . " . ' 1 1 I 1 " . " " " " " 1 i " ~ "" . 1 " - - . " " " - " " " " - " - " " -1 " . " " - " 1 " " - " " - " " " " " " - " - " " - - " - " -1 1 " - 1 - "" 1~ - " " 11 " 11 " " - " " ' 1 - 1 " ~1 I I II I " . . - " 1 1 " - "" " " - " " " " " " I I I I I _ I ' I " " " " 1 " - . " 11 . "" " "" " - " - 1 " 1 . 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