A feasibility study in applying read-only memories to the analysis of nonlinear analog function responses



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The measurement of physical qualities such as temperatures, pressures, and flow rates is both desirable and essential in many industrial processes. Devices such as thermocouples and other transducers have been developed which yield a predictable voltage or current response to the external stimulus. The use of such devices which produce voltage and current signals provide economical and convenient capabilities to measure the desired physical qualities because many signals may be terminated and monitored at a central location such as a control room. Thermocouples for temperature measurement are the most widely used devices in such industrial process applications because of their simplicity, accuracy, and low cost. As most such devices which respond to an external stimulus, the relationship between the voltage generated between thermocouple junctions and the temperature of the junctions is nonlinear. This nonlinear relationship poses the greatest problem in designing and constructing accurate temperature indicating devices. Several analog-to-digital conversion techniques have been developed which compensate for the nonlinear function being measured and produce temperature readouts in proper engineering units. All techniques developed for commercial applications strive to attain satisfactory degrees of speed, simplicity, accuracy, economics, and reliability. Most techniques offer some advantages in two or more of these qualities but no presently available techniques offers a clear advantage in all five qualities. Recent developments in the field of semiconductor memory devices have made it possible to design a digital temperature indicator using read-only memories (ROMs) with accuracies approaching those attainable with computer based techniques. The speed, simplicity, and reliability of such a ROM technique would be better than any other presently available technique. Predicted trends in future semi-conductor developments also make such a technique very attractive economically. A computer program has been developed to generate ROM truth tables for a nonlinear response system. The truth tables generated by this program are based on the system of quadratic equations which describe the nonlinear function. The accuracies are limited only by the inherent accuracy of the equations used, the size of the ROM, and the dynamic range of the stimulus to be measured.