定天镜的伺服驱动原理

The Servo System for the Siderostats of a Prototype Stellar Interferometer

主要介绍恒星光干涉仪实验系统的定天镜伺服驱动原理．采用电细分和逐级升、降速的方法来减小定天镜在转动时的振动，提高定位、跟踪精度．由于应用了４００的电细分和１０００倍的传速比，步进电机的步距角达到０．００６４８°．

During an observation with a stellar interferometer, the siderostats must be driven continuously. precisely and smoothly to point at a certain star and to track the motion of the star across the sky due to the rotation of the earth. In this paper, the servo system for the siderostats is described. The two shafts of each Alt-Azi mounted siderostats are controlled by two single chip computers, 8098, separately.Through the standard serial interface, RS232. information is transferred between each 8098 and the main computer.According to the instruction codes transmitted from the main computer, an 8098 must do the following jobs:(1)siderostat's positioning;(2)siderostat's tracking;(3)the initialization of an 8098 itself;(4)testing the servo system;(5) forthwith running based on the instantaneous address sent by the main computer; (6) Sending the data from the RAM of the 8098 to the main computer; (7)sending the program from the RAM and ROM of the 8098 to the main computer for being updated.The azimuth and elevation shafts are each driven by a stepping motor (with each step electronically divided into 400 microsteps) via 1000: 1 gearbox in order to obtain one microstep corresponding to 0. 006 48″, which makes the moving of the siderostats as smooth as possible. By means of changing speed increasingly or decreasingly, the siderostats can be smoothly driven and positioned. Four different speeds, high, medium, low and slow are used for positioning.To ensure the precise positioning and tracking, an optical encoder and a digital displayer with the precision of 1″are installed on each shaft.The 8098 compares the alculated data and the readings on the digital displayer and determines the proper speed for driving the shaft so as to make the siderostat smoothly stop at the precise position. The single chip computer translates the selected tracking speed into a time constant and sends it to a counter with 16 bits.The required tracking pulses as well as the tracking direction are thus obtained. Alter a certain time period, the time constant should be adjusted according to the checked difference between the calculated data and the readings of the digital displayer in order to reduce the accumulated errors caused by the time constant.The software for positioning and tracking is whiten into a EPROM.