The application of frequency characteristics test in fast control mirror system

Abstract

The control bandwidth of the FSM affects directly on the performance of the tracking system based on the compound axis servo structure. The FSM’s control system’s accuracy affects the bandwidth and the tracking precision of the system directly. Obtaining the accurate transfer function of the control object is the basis of designing frequency characteristics based control system. Measurement of high precision transfer function is needed to compensate the influence of the mechanical resonance. The mechanical resonance of fast control mirror is mainly of high frequency. In this case, the measured signal’s SNR is low so that the measurement accuracy of the frequency characteristic is reduced accordingly. One way to get frequency characteristics is to input sweeping sine drive signal to the system, measure the output of system, and then calculate the frequency characteristics by Fourier Transformation. However, the measuring result of the FSM system that has a low SNR in high frequency is not precise enough as in a system with a high SNR. To solve the problem mentioned above, this article presents a method to calculate frequency characteristics of an FSM system. This method uses the least squares to fit the fast mirror output overall waveform and the input source can be sine, white noise, multimode signal or chirp signal. A digital compensator based on this method is introduced to experimentally verify the efficiency of the method. Experiment results show that fitting the system’s output curve in high frequency can effectively eliminate influence of quantization noise, gauss measurement noise and harmonic interference on the measurement accuracy and thus enhance the measuring signal’s SNR. It is proved that by using this method a more accurate frequency characteristics of the FSM can be obtained.