Research On A Small Robot Control System For Polishing Of Large Freeform Surface

A new technical idea for polishing a large freeform surface with a micro device has been proposed by our intelligent precision manufacturing team of JiLin University. Under the guiding of this idea, a novel polishing robot was designed for large freeform surface. The purpose of this paper is to design and develop a control system for the designed robot combined with the mobile robot technologies.The control system structure is designed in this paper based on the mechanical structure of the polishing robot for a large freeform surface. The whole control system is divided into three parts: motion control system, polishing pressure control system and positioning system. The hardware structure is composed of host computer, Turbo PMAC2 multi-axis motion control card and robot positioning system.The structure composition and working principle are mastered by investigating the hardware performance of Turbo PMAC2. Based on the research of the executive mode of Turbo PMAC2's motion program, a motion program of the polishing robot control system in Turbo PMAC2 is developed. Based on above works, a normal flow about application development of control system with Turbo PMAC2 is summarized in this paper:1. Choose the certain interface board fixed to Turbo PMAC2 according to the motors and their drivers'interfaces;2. Connect the hardware in your system according to the interface of the certain interface board chosen step 1);3. Set and save all the related I variables by debugging the motors and their drivers with certain Turbo PMAC2 software (the debugging principle is no load first then with load);4. Develop and debug motion program in PEWIN32PRO.This flow provides a direction to the application development of control system with Turbo PMAC2, shows the job steps clearly and decreases the preparing time of application development. It has a certain means of direction for the next application development of control system with Turbo PMAC2.According to the characters of polishing for large freeform surface, the motion area of polishing robot is known and without obstacle. Polishing robot can cover its polishing area by moving to and fro easily. The polishing paths of robot are labeled in this paper. All the labels are set into the data style of polishing path. Polishing robot can choose the certain motion mode by the label in the data style of polishing path. The labels make the development of control system software very conveniently. In anther way, according to the covering method of polishing area and the mechanical structure of polishing robot, in this paper, the motion model is divided into three parts: portrait motion, transverse motion and rotation motion. Based on the kinematics models of the three motion modes, a method of path tracking control based on interpolation is proposed by analyzing the path tracking problems about polishing robot in this paper. This method can not only process the points information of polishing path generated by path plan model, but also can solve the conflict between output of inverse kinematics which is velocity information and input of Turbo PMAC2 motion program which is quantitative information, and also, provides a basic frame of the development of control system software.A pressure output model is built based on the hardware structure of pressure control system. In this paper, a normal PID control is used to keep the polishing pressure of robot in a stable value. All the parameters in this model is confirmed by consulting references and investigating in the field. Then, one group preliminary parameters of PID controller are confirmed by a simulation of PID control is presented in Matlab, KP=36.2, KI=0.23, KD=0.139. In additional, the kinetics of polishing robot is analyzed in two aspects of without polishing pressure and with polishing pressure in this paper. Two robot kinetics models of portrait motion and transverse motion are built in this paper that for describing the influence of the polishing pressure to the robot kinetics, which lays a foundation for the advanced research of polishing robot.By analyzing the concrete functions of the robot control system, an event driving modeling structure of the control system software is proposed in this paper. According to this structure, the control system software of the polishing robot is divided into machining accomplish level and centralized control level, then, the whole frame of control system software is built. In this foundation, by researching on the two technologies of communication and synchronization of multi-thread technology in Windows OS, the development of polishing robot control system software is finished.Finally, for testing the performance of polishing robot control system designed and developed in this paper, some certain experiments are approached.All the motors'parameters of robot in Turbo PMAC2 are confirmed and saved by repeated debugging the Turbo PMAC2 according to the method presented in this paper;The performance of the polishing robot control system is described by using the method of time recording. The experiment result shows that the sampling period of positioning system is the main influence factor to the interpolation period which is a key parameter in the method of path tracking control based on interpolation;A polishing path tracking experiment is approached according to the polishing area coverage path presented in this paper. The experiment result proves that the method of path tracking control based on interpolation and the event driving modeling structure of the control system software presented in this paper can realized the polishing path tracking control. The error data is basically fit the precision of positioning system, which proves that the precision of positioning system is the main influence factor of polishing path tracking;An experiment of polishing pressure control system is approached through analyzing the basic theory that curvature variation of freeform surface infects the polishing pressure output of polishing robot. The experiment result shows that closed loop control with basic PID controller can achieve the basic requirement of polishing pressure control, but it can still not thoroughly eliminated the influence. A more advanced controller should be used to solve this problem;An polishing experiment is approached in Paper[101] based on the control system of polishing robot developed in this paper. The experiment is quoted in this paper. An optimum combination of three polishing parameters is acquired from the experiment: polishing motor velocity~1000rmp, programmed feedrate~120mm/min, polishing pressure ~15N. The experiment result shows that polishing a ductile iron piece by the optimum combination of three polishing parameters can make a surface quality with a surface roughness Ra=0.2μm.All the experiments described above prove that the control system of polishing robot designed and developed in this paper can realize all the control function of polishing robot, which has achieved the requirement of polishing large freeform surface by using mobile robot.