Research On System Of Platinum Resistace Precision Temperature Measurement Based On Bi-directional Constant Current Source And Reference Resistor

With development of the large ultra-precision machining and the equipment manufacturing industry,dual-frequency laser interferometer is pushed to offer a high accuracy up to 10^-7 in industrial field.However,the refractive index of air will drift with the environmental temperature as well as the actual size of the workpiece,which makes it hard to measure by rule and line.Especially the temperature change of workpiece will bring an error up to 10^-5/℃.So it is essential to measure the temperature of the workpiece with a high accuracy of 0.01℃and make real-time compensation.In this paper,the main principles and techniques of ultra-precision temperature measurement at home and abroad are introduced along with the advantages and disadvantages of the existing measurement technology.The main factors which have great impact on measurement are analyzed.In response to low long-term stability of constant current source,thermal electromotive force,self-heating,interference and other factors which lead to the problem of lower temperature measurement accuracy,an improved Kelvin temperature measurement method based on bi-directional constant current source and reference resistance is put forward,and on this basis a precision temperature measurement system is designed.The main research topics are as follows:1.Precision temperature measurement system design:Kelvin circuit is used to eliminate the wire resistance.A improved Kelvin method based on reference resistant is brought forward to reduce the long-term stability requirement for constant current source which make it easier to actualize.In response to the thermal electromotive force in circuit,which is difficult to measure,a method based on bi-directional constant current source is put forward.Since the size of thermal electromotive force doesn’t change with the direction of the circuit,a subtraction between forward voltage and negative voltage measured will eliminate the negative impact of thermal electromotive force fundamentally.In response to self-heating effects of platinum resistance,a small excitation current as low as 0.5m A is used to reduce its power consumption.And a uniform heat transfer structure is designed to enhance the cooling efficiency.By these measures,the impact of self-heating can be reduced to a negligible degree.In response to common mode interference and differential mode interference,the circuit board is optimized,filter circuit is adopted,shielded cable is used and its drive circuit is designed.These measures greatly improved the system’s anti-interference ability and temperature measurement accuracy.2.Precision temperature measurement circuit and software design:According to the improved Kelvin temperature measurement method based on bi-directional constant current source and reference resistance,the corresponding signal conditioning module,the active filter circuit,shielded cable driver circuit,DSP-based high-precision data acquisition module and control circuitry is designed to achieve high-precision signal amplification,shaping,filtering and shielding function of noise to ensure the accuracy of signal pre-processing.In the meanwhile,a24-bit ADC is used to perform signal acquisition and a software algorithm called the way of median average filter to reduce gross errors caused by accidental factors.Based on the work above,the relevant experimental systems are designed to test each module’s performance and the errors are analyzed.Experimental results show that,this system has a measurement resolution of 0.004℃,and has achieved a measuring the stability of±0.003℃/3h,and an uncertainty of temperature measurement 0.037℃.