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，但它们在今天的工业中并没有被广泛接受。与传统的系列运动正交轴机床相比，当今市场上许多并联机床在抗外力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，主要从并联机构机床的运动控制角度进行了以下研究：1)补偿重力引起的运动误差。在并联机构机床上，由于主轴单元仅由6根杆支撑，重力引起的机械结构变形对机床的定位精度影响较大。为了准确地预测和补偿这种由重力引起的运动误差，提出并改进了并联运动学模型。2)运动参数标定方法的改进对于并联进给传动的高精度运动控制，最关键的问题是对各运动参数的标定，如支杆参考长度或基础节点位置等。提出了一种基于圆弧试验的机床轮廓精度标定方法。提出了一种消除重力误差的改进标定方法，使标定过程中只评估运动学参数的标定误差。通过应用1)和2)，在商用并联机床上验证了该方案在整个工作空间内的运动误差小于20 μm，而在主轴靠近工作空间边缘或主轴倾斜角度较大时，该方案的运动误差高达200 μm。

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.