Hexapod Error Compensation Features

TECHNICAL APPLICATION NOTE

Newport Hexapods are 6-axis parallel kinematic positionersengineered for exceptional stiffness and load capacity in compactdesigns with precision moves on the sub-micron scale. With 6Degrees of Freedom (DOF), Hexapods offer translations androtations in 3D-space around user-defined virtual pivots withoutaccumulation of motion errors. Hexapods are often implementedin applications that require precise rotations and translations withminimum deflection under load as well as excellent stability.

With complex coordinated moves of six synchronized actuators, itis important to have integrated compensation features thatminimize the unavoidable mechanical errors associated withdirection reversal. Working closely with customers, Newport hasdeveloped two types of compensation methods that optimizemotion performance related to inherent mechanical errorsassociated with direction reversal, specifically Backlash andHysteresis.

Hysteresis Compensation on the Hexapod is performed via aglobal correction at the Hexapod level and Backlash compensationis applied locally at the actuator level. Depending on therequirements of the application, the preferred compensationmethod can be selected for best performance.

Hexapod Backlash Compensation

Backlash errors are the result of movement between interactingmechanical parts of a drive train that do not produce outputmotion. In motion systems, backlash is introduced from non-zeroclearances between coupled mechanical parts. Backlash in amechanical system can impact both accuracy and bi-directionalrepeatability

Mechanical Clearances: The interlocking of mechanical parts isnot absolutely perfect at very fine scales in a mechanical drive train.An interlocked gear for example can slightly turn and not producemovement in the adjacent gear, as there is typically some degreeof clearance between gears.

The positive aspect of backlash is that it is often a repeatableproperty of a mechanical system and can therefore besystematically corrected. This is true in the case of Hexapodactuators. Each actuator is calibrated at the factory with acompensation factor to correct for the backlash as measuredunder controlled conditions in a metrology lab. Each actuator willcarry a slightly different value typically on the order of microns.

After factory calibration, Hexapod actuators are integrated on theHexapod and carry their individual measured backlash value in aparameter file. Hexapod moves are based on the individualcompensated actuator values and implemented on all moves.Newport Metrology expertise and testing are important parts ofthe characterization process, offering highly precise andrepeatable results.

Hexapod Hysteresis Compensation

Mechanical systems are subject to asymmetric elastic forces ondirection reversal. These elastic forces can impact mechanicalelements differently depending on the material, coupling andgeometry. This elastic hysteresis or more simply hysteresis will havevarying degrees of impact depending on the recent history of thesystem. Hysteresis will ultimately cause a deviation fromcommanded motion that can impact both accuracy andrepeatability.

Backlash forthis system

Figure 1: Backlash

To minimize motion performance degradation from hysteresis,Newport has developed Hysteresis Compensation. This firmwarecompensation technique effectively implements a small positivemicron-scale move to all actuators and then an equal sizednegative move back into target position. This guarantees that allhexapod actuators move to the target position in the samedirection minimizing the effect of hysteresis. The moves are smalland have negligible impact on throughput while providing animprovement in repeatability and accuracy. Exceptional Hexapodstiffness assists with these moves as the target position is reachedrigidly with negligible settling times.

Hexapod Error Compensation Features

TECHNICAL APPLICATION NOTE

Mechanical Deformation:Mechanical parts can deform with anapplied force and elastic forces coupled through the motion systemcan produce non-uniform motion responses that are dependenton recent move history and depend on the materials, coupling,loading and geometry of the system.

In sophisticated precision motion systems such as NewportHexapods, the hysteresis can have a measurable impact onpositioning performance and must be minimized for optimumperformance. This is why Newport has implemented HysteresisCompensation into Hexapod control.

Hysteresis compensation is best understood in the context ofHexapod control with a simple example. When a hexapod ismoved from a neutral position (0,0,0,0,0,0) to a position with acombined angular and translational move (2,0,0,5,0,1), someactuators are extending (blue actuator for example) while otheractuators are retracting (red actuator for example). Actuatororientations can take a wide variety of combinations depending onthe orientation of the Hexapod. Each move from one orientationto another will have a unique combination of individual actuatormoves impacted by asymmetric elastic forces, resulting in hysteresis.

Figure 2: an HXP100 Hexapod is undergoing hysteresis testing on aNewport metrology bench.