IETE Journal of Research ISSN: 0377-2063 (Print) 0974-780X (Online) Journal homepage: http://www.tandfonline.com/loi/tijr20 A Simple Fast, Accurate, Cheap Transducer/Sensor Linearizer Using Prom Umesh Kumar To cite this article: Umesh Kumar (1983) A Simple Fast, Accurate, Cheap Transducer/ Sensor Linearizer Using Prom, IETE Journal of Research, 29:2, 88-89, DOI: 10.1080/03772063.1983.11452952 To link to this article: http://dx.doi.org/10.1080/03772063.1983.11452952 Published online: 10 Jul 2015. Submit your article to this journal View related articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tijr20 Download by: [University of California, San Diego] Date: 06 April 2016, At: 08:31 -~~·~PRACTICAL DESIGN IDEAS-~·~·~~ ... A SIMPLE FAST, ACCURATE, CHEAP TRANSDUCER/ SENSOR LINEARIZER USING PROM Downloaded by [University of California, San Diego] at 08:31 06 April 2016 A noyel digital hardware set-up is postulated for transforming the characteristics of a transducer into linear output using a PROM lookup table. The method has been actually tested on a thermistor and found to yield Tery accurate results. MAJORITY OF TRANSDUCERS/SENSORS produce OUtput voltages which have a nonlinear relationship with the measurand. Several important examples include the thermocouple, thermistor (for measurement of temperature); measurement of electron mean energy in a laser gas discharge (Lawson & Lucas 1965); pirani pressure transducer; infra-red sensor etc. There is an ever-increasing use of such nonlinear transducers in the fields of research and industry for measurement of different parameters, e.g. flow, velocity, pressure and temperature. Thermisto" and thermocouple are in general use for the measurement of temperature. It is, therefore, extremely important and imperative to have a method to process the nonlinear voltages and form an input/output characteristic such that it linearizes the output voltage with the corresponding values of the measurand. PREVIOUS TECHNIQUES A number of different techniques have been proposed in recent years to achieve the linearization of the transducer characteristics. One method is to use an electronic circuit incorporating diodes to switch in selected resistances at pre-determined voltages. Another way is to use an analogue method of linearization (Weir 1974) which requires modifications to a dual-slope integrating digital voltmeter. Broughton (1974) has presented a technique for lhe analysis and design of six one-thermistor temperature transducer circuits by expressing the output/input voltage ratio as a linear fractional transformation. One more method is to use digitallinearization involving the use of an analogue-to-digital converter (ADC) connected to a computer. A simplified version employs a microprocessor unit or a pocket programmable calculator which is interfaced to an ADC. The system is, however, still expensive and is not suitable for linearizing transducers with highly nonlinear response. The simplest and the best method is to use a set of look-up tables. PRESENT WORK The aim of this article is to present a fast, accurate, cheap and easy method for use with linear and nonlinear transducers/sensors. It can be readily incorporated in a simple instrument and this simple solution can be commercially utilized. The attractive and salient features and virtues are its versatility, much lower component count, cheap price, low power consumption, easy straight-forward and direct fabrication and the excellence and accuracy of linearization. In fact, the whole logic can be readily manufactured on a single integrated circuit (IC) chip with its associated lower size, weight and cost, high reliability, increased miniaturization and direct on-the-line production possibility in bulk in a manufacturing environment. The system discussed is built around a PROM. PRINCIPLE OF OPERATION The circuit configuration for achieving the linearization of transducer/sensor characteristics using hardwired logic is shown in the block diagram in Fig. 1. Manuscript received 1982 February 27; revised 1982 September 27; finally revised 1982 November 13. Fig. 1. Hard wired logic for transducer/sensor linearization. The principle of action of the circuit configuration together with the important characteristics and function of each block is described as follows. The transducer/sensor and amplifier stage generates an analogue voltage which is applied to the high speed, dual slope A/D convertor. The ADC 1103 is a very fast successive approximation converter packaged in a small module. It is a complete self-contained converter, requiring only standard power supplies and the usual control signals. The input range and output coding are user selected. It is specially well suited for applications requiring high throughput rates with no compromise in accuracy. Depending on the magnitude of the voltage applied, a count is stored in the segment length counter through dual-slope integration. The chip 3814 is a Fairchild DVM logic 4i decade multiplexed BCD output segment length counter. The stored count is directly proportional to the magnitude of the voltage applied. Let us say the stored count is 40,000. During this time interval of integration, the spillover logic has been so designed to produce an output pulse each time the count advances by one hundred. The pulse generated advances the memory address counter. The chip 74161 is a synchronous presettable 4-bit binacy counter for applications in high-speed counting designs. It is fully programmable and the outputs may be preset to either level. Presetting is synchronous with the clock. The counter sequentially addresses the PROM look-up table. The PROM look-up table has been initially programmed and prepared from an analysis and study of the actual and desired linear transducer/sensor characteristics. The chip 1702A is a 256-word by 8-bit electrically programmable ROM. Initially all the 2048 bits of the memory are in 0 state (output low). Information is introduced by selectively programming 1's (output high) in the proper bit location. The organization of the PROM for this application is shown in Fig. 2. The chip has got a transparent lid which allows the user to expose to UV light to erase the bit pattern. A new pattern can then be written into the device. The circuitry is completely static. The PROM look-up table outputs voltages in binary form propor-~ tional to the corrections to be applied to the transducer/sensor characteristics. These corrections have been stored at sequential address locations in the look-up table. The correction voltage generated is applied to a binary rate multiplier which outputs a frequency proportional to it. The 7497 is a TTL MSI monolithic synchronous 6-bit binary rate multiplier which can achieve 32 MHz typical maxill1_!lm operating frequencies. The output frequency is equal to the inrut frequency multiplied by the rate input M and divided by 64 :fou~ =M. fin/64 where, M= F.2s + £.24 + D.2l + c.22 + B.21 + ,oo. The necessary correction in frequency is applied to the clock frequency and therefrom to the A/D converter. PRACTICAL DESIGN IDEAS I 89 ~JVV'""""'~~-~ ,. ->1• ·~· • ~, -r-+-+--+-----------t-+--t---,S SK 6 e Address ~~~ The digital inputs directly interface to TTL logic and operates from a + 5V to + 15 V supply with only 20 mW dissipation. In this way, an accurately linear characteristic can be obtained from the transducer I sensor as desired. S9K K APPLICATION '' "oo 6 •• "-t--.._-----iH s ,, " " 10 4 (L 59! lNTEL l'O)A Do to out 9 A nonlinear transducer of prime importance is a thermistor available for the measurement of temperature. The digitallinearizer proposed proved to be very successful in the applicationofaccurate temperature monitoring, as is evident from Fig. 3. For a thermistor, the resistance obeys an exponential law R =A exp (B/T) and its output voltage is ~,~0 !MS81 Downloaded by [University of California, San Diego] at 08:31 06 April 2016 1L " lK Vout ,(tup Se!lert •l"'pul l)~PfClQfOm B L>nQS 11 v .. ~, I ""']V Fig. 2. A look up table PRO M organisation showing input and output connections (INTEL). l 8-5r------------------=> j l! I,A exp (B/T), CONCLUSION VOLTAGE w Cl q ~ 0 > REFERENCES I Lawson (P A) & Lucas (J). Electron Diffusion in Hydrogen at Electric Fields and Low Gas Pressures. Proc. Phys. Soc. 85, 1965; 177-185. 2. Weir (K G). Analogue Linearized Technique for a Digital Voltmeter. J. Physics : E :Scientific Instruments. 7, 1974; 507509. 3. Broughton (M B). Analysis and Design of Almost-Linear One-Thermistor Temperature Transducers. IEEE Trans. IM-23, I; 1974; 1-5. 0 Fig. 3. Voltage vs. temperature characteristics for a thermistor. The AD 7520 chip is a monolithic CMOS low cost, 10-bit multiplying D/A converter. The device uses advanced monolithic CMOS and thin-film technologies to provide upto 10-bit differential linearity. * RETRIGGERABLE ONE-SHOT PROTECTS THREE PHASE MOTORS -~ The circuit described trips a three-phase motor in the case of a phase failure and inhibits its reverse rotation in the event of a phase sequence reversal. The circuit detects phase failure within one cycle time. l ~ use costly electromagnetic and thermal relays to sense single phasing and phase-reversal conditions. The MoTOR PROTi:CTION DEVICES Manuscript received 1982 July 31. VMESH KVMAR (Member) Department of Electrical Engineering, 1./.T., New Delhi IJO 016. 1. TEMPERATURE , ( ~ (1) The digital linearizer unit proved to be very useful in linearizing nonlinear transducer and is not restricted to the application described in the text, but can be used with any type of transducer available. The majority of the transducers available today can be linearized with its capabilities. The complete unit can be implemented and realized easily in micro-miniaturized integrated circuit (IC) form. This approach has already amply demonstrated its capability and is well within the range of modern technology. 6.5 > = where, 18 is the constant current passing through the thermistor, A and B are constants and T is the absolute temperature. The above characteristic in eqn. (1) as well as the linearized output available were plotted in Fig. 3. The digitallinearizer gave temperatures in very good agreement over the experimental range of 20-90°C with those given by the mercury thermometer within 1°C. Thus, the accuracy attained was very high with percentage error in the range of 1 per cent only. • * circuit described is built around a single IC chip and it accomplishes the same functions at a much lower component count and cost (Fig. 1). CIRCUIT OPERATION The low-voltage supply for circuit operation is derived from one of the phase voltages while the other two phase voltages are stepped down and shaped into square waves by INVERTERS ]-4 and capacitors C 1 and C2 . Phase voltage VR is taken as a reference and the correct phase sequence as VR-Vy--V8 . The retriggerable one shot (ROS) is configured such that its B input being high, a negative transition l l ~~~~~J~·~~~~vv~~~~vv~~~~vv~~~vvvv~~~vvvv~~~vv~~~~vv~~'
