铠装热电偶传递函数参数估计

Estimating Transfer Function Models for Sheathed Thermocouple

为了评价铠装热电偶动态特性,针对铠装热电偶测量动态温度过程用集总热容法和复合集总热容法建立了铠装热电偶的传热模型。从所建立模型分析,铠装式热电偶传热过程是保护管、绝缘层和测量接点这3个小时间常数惯性环节的串联,且保护管与绝缘层、绝缘层与测量接点存在接触热阻,产生了两个耦合作用。当保护管时间常数和绝缘层时间常数都比较小时,传递函数可近似为带时滞的惯性环节。建立了基于双槽法的正负温度阶跃实验系统,测量热电偶在正负温度阶跃激励下的输出响应。按照预估延迟时间和预估延迟时间加扰动两种情形,用输出误差自回归滑动平均方法估计了传递函数的参数。结果表明,输出误差自回归滑动平均方法适用于铠装热电偶实验建模和模型的离线辨识。

This paper presents a method for evaluating the dynamic characteristics of sheathed thermocouple. Com- posite lumped heat capacitance model of the sheathed therrnocouple is derived based on the method of lumped heat capacitance analysis. Sheathed thermocouple heat transfer process includes protecting tube heat transfer, insulating layer heat transfer and hot junction heat transfer. The process is described as a series of three inertial element with lag. Thermal contact resistance is significant between protecting tube and insulating layer. A temperature drop is ob- served at the interface between the two surfaces in contact. Similar situations occur between insulating layer and hot junction. When the constant time of protection tube and the constant time of insulation are small, the transfer func- tion of a sheathed thermocouple is simplified to be a first order process with delay. Based on the double thermostatic bath device, a bidirectional step temperature experiment system is established. The arrangement, in response to the changes in the temperature of the two thermostatic bath, provides input and output signals of the sheathed thermo- couple. Two cases for the response of a thermocouple are reported, the first case concerns the response to a step per- turbation ,while the second deals with the response to a step perturbation with disturbance. Parameters are estimated by using computation algorithms to arrive at coefficients that best fit the selected auto regressive moving average model. Experiments are carried out and results prove that auto regressive moving average model is feasible for off- line identification of sheathed thermocouple.