Research And Implementation Of Millivolt Signal Module Software Of PAS500Control System

For Process Automation Control System, the accuracy, real-time or stability of temperaturemeasurement affects the reliability, effectiveness or safety of the whole system directly orindirectly. The thermal resistance temperature sensor of PT100, PT10, Cu50or Cu100andthe thermocouple temperature sensor of K, S, B, T, J or E are the most commonly usedtemperature sensor of domestic and overseas. Millivolt signal input module of PAS500Control System is a analog input module used for measuring the temperature of8temperature sensors and senting the temperature value to the external equipment throughRS485bus using the Modbus protocol. This subject is to design and development theembedded software of this module based on the hardware circuit design, the requirementsof PAS500Control System and some other technology, standards and norms related toautomation industry.This paper introduces the technologies about this module firstly. By then, it introduces thedesign requirements, the overall design, the hardware design,the embedded softwaredesign and implementation and test of this module. Temperature measurement and Modbuscommunication are main functions of this module. It’s the key to introduce the principleof temperature measurement of thermal resistance and thermocouple temperature sensorand serial links Modbus protocol in this paper. This paper also expound the method andprocesses of the temperature signal acquisition, filtering and conversion, as well as theformulation and implementation of the Modbus communication. The design of temperatureconversion algorithm is the key point to realize effective temperature measurement. Basedon the measurement principle of the temperature sensor, with reference to the GB/T16839and JB/T8622, fully considering the resources and performance of hardware platformbased on8-bit microcontroller, this paper optimizes piecewise curve fitting technique todesign temperature conversion algorithm of all the sensors and cold junction compensationof thermocouple. The methods of eight channel temperature measurement is time-sharingand polling. In addition, this paper also introduces design and implementation of someauxiliary functions, such as working state display and module parameter setting.The company internal test and the type test of the18th Mechanical Industry MeasurementTest Center verify that all the module functions are implemented correctly, the temperature measurement error limit is not greater than2‰, the time of a measuring cycle is not morethan1second, the response of communication is not more than2milliseconds and there isno err in1million times continuous communication.This module has been used in the practical application now, and is running in goodcondition.The function and performance of this module meet the demand of generalapplication. Relative to some advanced products, although this module has a priceadvantage, but there is some gap on performance.This module is need to improve andupgrade continuously on the product design and software development in the future.