Kinematic Calibration of a Parallel Mechanism Machine Tool

虽然自第一台并联机床问世以来已经过去了十多年，但在当今的行业中，它们并没有被广泛接受。与传统的串联式正交轴机床相比，当今市场上的许多并联机床在抵抗外力的刚度和运动精度方面存在问题。为了提高并联机构机床的运动精度，主要从其运动控制的角度进行了以下研究：1)重力引起的运动误差补偿；在并联机构机床上，由于其主轴单元只由六根支杆支撑，重力引起的机械结构变形对其定位精度有很大影响。为了准确预测和补偿这种重力引起的运动误差，提出并改进了一种并联运动学模型。2)运动学参数标定方法的改进对于高精度的并联进给驱动器的运动控制，最关键的是标定各种运动学参数，如支杆的参考长度或基关节的位置。提出了一种基于圆检验的圆加工轮廓精度的标定方法。在消除重力误差的情况下，提出了改进的标定方法，使得在标定过程中只能对运动学参数的标定误差进行评估。通过应用1)和2)，在一台商用并联机构机床上进行了验证，整个工作空间的运动误差小于20μm，当主轴靠近工作空间的边缘和/或主轴的倾角较大时，未采用该方案的运动误差最大可达200μm。

Although more than ten years have been passed since the first parallel kinematic machine tool was introduced, they are not widely accepted in today's industry. Many parallel kinematic machine tools available in the today's market have issues in their stiffness against external forces and motion accuracy compared with conventional serial kinematic orthogonal-axes machine tools. We conducted the following researches in order to improve the motion accuracy of a parallel mechanism machine tool mainly from the viewpoint of its motion control.1)Compensation of motion errors caused by the gravity.On a parallel mechanism machine tool, since its spindle unit is supported only by six struts, the deformation of its mechanical structures caused by the gravity has a large effect on its positioning accuracy. In order to accurately predict and compensate such a gravity-induced motion error, a model of the parallel kinematics was proposed and improved.2)Improvement of a calibration method of kinematic parametersFor high-accuracy motion control of parallel kinematic feed drives, it is the most critical issue to calibrate various kinematic parameters such as the reference length of struts or the location of base joints. A calibration method based on circular tests to measure the machine's contouring accuracy in a circular operation has been proposed. We proposed the improved calibration methodology under the cancellation of gravity-induced errors, such that only the calibration error of kinematic parameters can be evaluated in the calibration process. By applying 1) and 2), it was validated by using a commercial parallel mechanism machine tool that the motion error was reduced to less than 20 μm over the entire workspace, which the motion error was as large as 200 μm without the proposed schemes when the spindle is near an edge of the workspace and/or the tilt angle of a spindle is large.