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，和更高的精度比可比的串行机械手。通常，CNC机床依赖于串联机构，其中接头从刀具的端点移位，因此角度误差乘以位移，从而导致刀具位置的大误差。这降低了精度和刚度。在并联机械手中，刚度比传统机械手增加，因为负载在所有连杆之间分配，并且精度增加，因为在大部分工作空间上，腿长误差趋于平均。六足机器人的另一个重要特征是，由于加工载荷而施加到致动器上的力都是轴向的（拉伸和压缩），没有任何弯矩。在传统的机床或坐标测量机中，轴通常独立校准。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。这在并联连杆机械手中是不正确的。最近，IAI发现了一种方法，他们相信这种方法可以只使用球杆数据来校准六足结构中的主要误差。该方案是如此有效，它可以在真实的时间实现。如果不是使用“固定长度”的球杆，而是使用仅产生单轴数据的单个跟踪干涉仪，则可以在机器操作时连续执行校准程序。这种扩展的最终结果是，研究人员可以建造一台机器，其控制器不断测量机器的误差，达到干涉仪级精度，并动态补偿机器的所有主要误差。对于基于六足的坐标测量或机床，所有误差都可以通过仅调整六个腿的长度来校正，因此基于六足的机床可以与控制器对机床误差的了解一样精确。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.