Real-time Calibration and Dynamic Error Compensation for an Octahedral Hexapod-based Machine Tool or Coordinate Measuring Machine

六足机器人具有更少的运动部件、更大的刚性和比同类系列机械臂更高的精度。典型的数控机床依赖于串联机构，其中关节从刀具的末端位移，因此角误差乘以位移会导致刀具位置的大误差。这降低了精度和刚度。在并联机械臂中，由于负载在所有连杆之间分配，因此刚度比传统机械臂有所增加，并且由于在大部分工作空间中，腿长误差趋于平均，因此精度有所提高。六足机构的另一个重要特点是，由于加工载荷的结果，施加到执行器上的力都是轴向的（拉力和压缩），没有任何弯矩。在传统的机床或三坐标测量机上，坐标轴通常是独立校准的。结果留下了许多未解决的错误，但至少校准过程可以一次完成一个轴。这在并联机械手中是不成立的。最近，IAI发现了一种方法，他们认为这种方法可以仅使用球杆数据来校准六足架结构中的主要误差。该方案非常有效，可以实时实施。如果不使用“固定长度”的球杆，而是使用只产生单轴数据的单跟踪干涉仪，那么校准程序可以在机器运行时连续执行。这种扩展的最终结果是，研究人员可以建立一个机器，其控制器连续测量机器的误差到干涉仪水平的精度，并动态补偿所有机器的主要误差。使用六足坐标系测量或机床，所有的误差都可以通过调整六条腿的长度来纠正，因此六足坐标系的机床可以像控制器知道的机器误差一样精确。IAI的连续校准方法也适用于传统机床，他们也将研究这种可能性。

A hexapod has fewer moving parts, greater rigidity, and higher accuracy than comparable serial manipulators. Typically CNC machine tools rely on serial mechanisms, where the joints are displaced from the end point of the tool and therefore angular errors multiply by the displacement to cause large errors in the tool position. This reduces accuracy and stiffness. In parallel manipulators, stiffness is increased over conventional manipulators because the loads are divided among all of the links, and accuracy is increased because, over much of the workspace, leg length errors tend to average. Another significant characteristic of the hexapod is that the forces applied to actuators as the result of machining loads are all axial (tension and compression) without any bending moments. In conventional machine tools or coordinate measuring machines, the axes are generally calibrated independently. The results leave many errors unaddressed, but at least the calibration procedure can be done one axis at a time. This is not true in a parallel link manipulator. Recently, IAI discovered a method which they believe will allow the major errors in a hexapod structure to be calibrated using only ball bar data. The scheme is so effective that it can be implemented in real time. If instead of a `fixed length` ball bar, a single tracking interferometer is used which produces only single axis data, then the calibration procedure could be executed continually as the machine operates. The ultimate result of this extension is that researchers could build a machine whose controller is continually measuring the machine's errors to interferometer level accuracies, and dynamically compensating for all the machine's primary errors. With a hexapod-based coordinate measuring or machine tool, all the errors could be corrected by adjusting only the six leg lengths, so a hexapod-based machine tool could be as accurate as the controller's knowledge of the machine's errors. IAI's approach to continuous calibration should also be applicable to conventional machine tools, and they will investigate this possibility as well.