Design And Improvement Of A Laser Tracking System

The laser tracking system for 3D coordinate measurement based on multi-stationprinciple possesses many advantages, such as high accuracy, large measuring range,and capability of flexible and real-time measurement,over traditional methods. Themulti-station principle also has other advantages, including the capabilities ofself-calibration, self-recovering of lost information, error separation andcompensation, and system rearrangement. Countries all over the world have investedtremendous resources and manpower to study these topics. As the foremost work onthis research in China, the study on the design and test for this system has been startedin our research group. This project is sponsored by the National Science Foundationof China (NSFC), under project contract No. 59875064. On the basis of multi-station principle, a flexible 3D coordinate measuringsystem which consists of four laser tracking interferometers, has been designed tomeasure 3D coordinates of the target retroreflector. This thesis is focused on the studyof laser tracking control system, the key element of the measurement system. First, the paper discusses the design of laser tracking system, including opticaldiagram of tracking laser interferometer, the first and second generations of lasertracking head. According to the experimental data, the measuring accuracy of thesecond generation laser tracking head is five times higher than the first generation.Then, the design and improvement of servo tracking system are discussed. A servotracking circuit consisting of current loop, speed loop, and position loop is developed.In the analog control scheme, the measuring range is limited because the gaincoefficient of transfer-function is related to the distance l between target andtracking head. Two methods are proposed to improve the original scheme. One isadding a preamplifying circuit to the position loop of analog control circuit. The otheris digital control by microprocessor. Through control algorithm, the gain coefficient oftransfer function keeps constant in spite of the change of l . According to theexperimental data, the system tracking ability has been improved greatly, and themeasuring range has been extended. A weighted method for self-calibration is studied in this thesis as well. Theweight k of each calibration point is determined in accordance with the measuringuncertainty of that point. Computer simulation shows both the average deviations ofthe calibrated parameters from their true values and the squared sums of thedeviations are much reduced, which indicates that the weighted method forself-calibration improves the accuracy of self-calibration.