| Temperature is an important indicator in the production fields of chemical,medical,pharmaceutical,oil refining,steel,etc.,And its detection accuracy seriously affects system performance.On-site measurement and periodic calibration of the temperature sensor is an important part of ensuring the accuracy of temperature detection.As a portable precision temperature calibration instrument,the dry metering furnace is widely used in the measurement of industrial field thermometers.Improving the temperature control performance of dry metering furnaces is of great significance for improving the detection accuracy of industrial temperature sensors.This article takes the national standard ITS-90 and the European Metrology Organization Custom dry calibration furnace calibration guide EA-10/13 as the reference standard.Analysised and researched on the several key technical parameters for the dry metering furnace provided by EA-10/13:accuracy,stability,axial uniformity,load impact and hysteresis.A dry metering furnace control scheme based on semiconductor refrigerator(TEC)with high temperature control accuracy was designed.In the first place,the operating characteristics of TEC,the thermal effects and heat transfer mechanisms of thermostatic block were analyzed.A temperature-current double closed loop temperature control model was established,this temperature-current double closed loop control model takes control of both the electrical characteristics variations of working TEC and the temperature variations of the optical crystal.Secondly,to overcome the inherent hysteresis characteristic of feedback control and temperature control system,the feedforward compensation control based on the double closed-loop control is introduced.The TEC drive circuit for existing dry metering furnaces was improved.The TEC driver module based on the ARM+FPGA architecture is designed to achieve the high-frequency and high-precision PWM drive control.A segmented temperature control scheme is designed for the problem of axial uniformity of constant temperature plug.Combined with the temperature acquisition circuit and current acquisition circuit,the production and debugging of the circuit board was designed and completed.At the same time,the embedded touch screen is integrated as a human computer interaction interface.Adopted the RS485 communication bus as communication interface.Based on above,a high-precision dry metering furnace control system was completed.Experimental results show that the overall temperature control accuracy of this system is better than ±0.002℃ and the system can quickly stabilize at any set point temperature in a wide temperature range of-30-130℃,the temperature control stability within two hours after the system temperature control stablely is better than ±0.002℃.The axial uniformity of the thermostat block is better than ±0.004℃,the system load effect is good and it has good robustness to abnormal environment,which meets the requirements of the expected design goals. |
