Research On Control System Of Hybrid Grinding And Polishing Machine Tool For Blade Finishing

Blades are widely used in energy and power equipments such as steam turbine,hydraulic turbine and aircraft engine. There are many different kinds of blades inthese equipments, and the quantity is large. The performance of energy and powerequipments is directly influenced by machining quality of blades. Generally, bladesare shaped by CNC milling, and then they get required surface quality by grindingand polishing. Intelligence and precision manufacturing researching team of JilinUniversity developed a hybrid grinding and polishing machine tool for blade finishing.The hybrid machine tool is a serial-parallel configuration machine tool with5degreeof freedom. Based on the machine tool mechanical structure which was alreadydesigned, this paper develop control system of hybrid blade grinding and polishingmachine tool in four aspects, including machine tool motion control method, controlsystem hardware platform structure, control system software developing and parallelmechanism calibration.There is complicated and non-linear mapping relation between the motion ofmoving platform of parallel mechanism and the motion of parallel shanks, so motioncontrol of parallel mechanism usually adopts dual-level interpolation strategy. Thefirst level interpolation is conducted in track space which is constructed by positionand posture of moving platform, it disperses motion track curve of moving platform.The second level interpolation is conducted in driving space which is constructed bylength of driving shanks; it disperses motion regulation curves of each shank. Thispaper also adopts dual-level interpolation method to motion control of hybrid bladegrinding and polishing machine tool. Firstly, track points are optimized, which isconducted according to the original track points supplied by track file. Then, points of motion regulation curves of each shank in driving space are calculated using inversekinematics. The last, interpolation in driving space is accomplished with theinterpolation function supplied by motion controller.Based on motion control method proposed above, research is done on kinematicsof machine tool and track points optimizing. Kinematics model of hybrid bladegrinding and polishing machine tool is proposed. The mapping relation between trackcurve and motion of axis is established, which would be required in motion controland axis position display. Based on kinematics characteristic analysis of parallelmechanism, track points optimizing method is proposed according to midpointsampling method. The method can adjust position and quantity of track pointsaccording to different accuracy and curvature, reduce approximating error effectively.Through comparison and analysis of different interpolation mode supplied by motioncontroller, proper interpolation mode is selected to accomplish interpolation in drivingspace.Because of its special configuration, hybrid blade grinding and polishingmachine tool can’t adopt commercial CNC system. This paper adopts openarchitecture hardware platform which is constructed by PC and PMAC. Hardware ofthe whole control system is divided into two parts, PMAC system and AC servosystem. PMAC system includes Turbo PMAC2controller, dual-port RAM andACC-8S motor interface circuit board. AC servo system includes servo driver andservo motor. To PMAC, research is mainly done on its structure, function andcharacter, so that the parameter can be set properly. To AC servo system, research ismainly done on its electrical connection and parameter setting of servo driver.Software of control system is divided into two parts, PC software and PMACsoftware. PC software is developed with Visual C++in Windows environment, anddivided into five module including manual control, auto machining, informationdisplay, parameter setting and auxiliary operation. PC software accomplishesoperation on PMAC with the PMAC function library and accomplishes track displaywith OpenGL library. PC software realizes multi-task processing function throughmulti-thread technology of Windows, each task is independent to other tasks, so it would be easy to add and modify a task. PMAC software includes two parts, itssystem software and motion program developed by consumer. PMAC systemsoftware accomplishes operating and task management, motion programaccomplishes interpolation in driving space when machine tool works at automachining mode.Parallel mechanism can’t avoid the error generated by manufacturing andassembly, so the actual kinematics model of machine tool is different from theoreticalkinematics model, and the true value of kinematics model parameter is also not thesame with its theoretical value. Considering the manufacturing and assembly error,modified kinematics model of parallel mechanism is proposed. Usually, the true valueof kinematics model parameter is obtained through calibration. There are two kinds ofmethod used in parameter calibration, one kind is based on forward kinematics andthe other kind is based on inverse kinematics. Considering constrain of measureequipment, this paper adopts the method based on forward kinematics to calibrate theparameter of kinematics model. The calibration problem is transformed to multiparameter optimizing problem, and the optimizing mathematical model is alsoestablished. The optimizing problem is solved by genetic algorithm. This paper bringsforward two kinds of object function which can be used in calibration. At last,effectiveness of calibration method is validated by experiment, and the calibrationeffect caused by the two kinds of object function is compared and analyzed.