Frequency compensator for cold wire thermometer applying FPGA device

Abstract

A mutual compensator for a thermo-anemometer was investigated to realize a high precision simultaneous measurement of temperature and velocity fields in a temperature stratified flow. The frequency characteristic of the cold wire thermometer deteriorates in accordance with the first order delay due to the heat capacities of the fine wire sensor and its prongs. In this paper, the new compensation method was proposed to digitize the frequency compensator for the cold wire thermometer. The compensator with the first order progress characteristics was built by utilizing the bilinear transform, which is one of the mappings from an analog region to a digital one. The realization of the intended characteristics indicated the effectiveness of the present method for constructing the digital compensator. The digitized compensator was mounted on the FPGA device capable of fully parallel operation. The executing of the algorithm was limited a fixed-point arithmetic on the FPGA. Thus, the operational precision was assured by using bit shift processing for every calculation. As a result, the adequate first order progress characteristics were obtained in the frequency range to 5 kHz. The developed compensator was evaluated by directly applying the periodic temperature fluctuations to the sensor in the frequency range from 0.01 to 10 Hz. The results showed the gain error of this system against 0 dB was ±0.8% or less. It means that the digital frequency compensator with the enough accurate for the thermal fluid measurements is realized through the bilinear transform and the FPGA device.