Research Of Articulated Arm Coordinate Measuring Machines

The articulated arm CMMs is a new type of multiple degree and non-Cartesian system. It possesses the advantages of large measuring range, small volume, low weight and portability, moreover it is convenient to setup on site in production line, and it has extensive application and important significance. Based on analyzing some general views related to current developments and directions of articulated arm CMMs both home and abroad, the main work of this paper is just focused on key techniques of this system.This project is financially supported by National Natural Science Foundation of China, the number is 50475116. The following work has being done in this paper: Firstly, according to the character of articulated arm CMMs mechanical, system structure is developed, which improves the reliability of system. Based on DH method, a quasi-spherical coordinate system is put forward, the ideal mathematical model of articulated arm CMM is established, which simplifies the model and procedures for machine parameter and error calibration. Deep researches about the effect of joints configuration on measuring working space are done, and arms length scale are optimally designed.Using mechanism kinematics basic relations, error sources of articulated arm CMM are analyzed, and the error model of the machine is constructed. Based on error theory, measuring uncertainty of articulated arm CMM is introduced. Quantitative relationsof tolerances of the mechanical parts with position errors of probe are proposed, and tolerances optimizing model of parts is presented.A suitable calibration method is presented in a creative way based on multiple reversal technology, which simplifies calibration process and separates machine parameters and errors easily. Compared to traditional techniques, it overcomes some problems such as complexity algorithm, initial value sensitive and matrix ill condition. The basic thought and key requirement of the proposed multiple reversal calibration method is that errors of all other parameters and rotational angels contribute little to the calibration result. That means all they cause only second order errors in the determined parameter.Based on the data acquisition system design requirements, the intelligent sensor units, which consisted of angles sensors and microcontroller, are set in each joint and arm, a distributed data acquisition system is developed base on RS485 serial bus. This system is realized processing every joint angle, where the quadature subdivision, direction discrimination and reversible counter can be realized by CPLD, which has been compared with traditional discrete components. Not only elementary frame of acquisition system control software is given, but also USB driver between PC and machine development idea is introduced.Finally, experimental studies on accuracy related problems of articulated arm CMM are carried out. By computer simulation, effects of structure parameters errors on probe position errors are arrived. Experiments are carried out for a Faro Platinum 4ft articulated arm CMMs, and some arm lengths, offsets and perpendicular errors are measured. The results prove the feasibility of the proposed method for articulated arm CMMs calibration, and all parameters are separated completely. On building system experiment platform, data acquisition system feasibility is proven by grating signal from logical generating card.Briefly, this paper establishes favorable foundation for developing articulated arm CMMs with our own intellectual properties.