Hexapod Error Compensation Features TECHNICAL APPLICATION NOTE Newport Hexapods are 6-axis parallel kinematic positioners engineered for exceptional stiffness and load capacity in compact designs with precision moves on the sub-micron scale. With 6 Degrees of Freedom (DOF), Hexapods offer translations and rotations in 3D-space around user-defined virtual pivots without accumulation of motion errors. Hexapods are often implemented in applications that require precise rotations and translations with minimum deflection under load as well as excellent stability. With complex coordinated moves of six synchronized actuators, it is important to have integrated compensation features that minimize the unavoidable mechanical errors associated with direction reversal. Working closely with customers, Newport has developed two types of compensation methods that optimize motion performance related to inherent mechanical errors associated with direction reversal, specifically Backlash and Hysteresis. Hysteresis Compensation on the Hexapod is performed via a global correction at the Hexapod level and Backlash compensation is applied locally at the actuator level. Depending on the requirements of the application, the preferred compensation method can be selected for best performance. Hexapod Backlash Compensation Backlash errors are the result of movement between interacting mechanical parts of a drive train that do not produce output motion. In motion systems, backlash is introduced from non-zero clearances between coupled mechanical parts. Backlash in a mechanical system can impact both accuracy and bi-directional repeatability Mechanical Clearances: The interlocking of mechanical parts is not absolutely perfect at very fine scales in a mechanical drive train. An interlocked gear for example can slightly turn and not produce movement in the adjacent gear, as there is typically some degree of clearance between gears. The positive aspect of backlash is that it is often a repeatable property of a mechanical system and can therefore be systematically corrected. This is true in the case of Hexapod actuators. Each actuator is calibrated at the factory with a compensation factor to correct for the backlash as measured under controlled conditions in a metrology lab. Each actuator will carry a slightly different value typically on the order of microns. After factory calibration, Hexapod actuators are integrated on the Hexapod and carry their individual measured backlash value in a parameter file. Hexapod moves are based on the individual compensated actuator values and implemented on all moves. Newport Metrology expertise and testing are important parts of the characterization process, offering highly precise and repeatable results. Hexapod Hysteresis Compensation Mechanical systems are subject to asymmetric elastic forces on direction reversal. These elastic forces can impact mechanical elements differently depending on the material, coupling and geometry. This elastic hysteresis or more simply hysteresis will have varying degrees of impact depending on the recent history of the system. Hysteresis will ultimately cause a deviation from commanded motion that can impact both accuracy and repeatability. Backlash for this system Figure 1: Backlash