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Static performance

Backlash

A PAD is inherently backlash-free. Whenever the drive is powered, the piezo actuators apply a radial force on the ring, pushing it in firm contact with the shaft. There is no gap within the microgear, therefore no backlash.

If an external torque is applied on the shaft, the mechanical parts will deflect elastically. However stiffness is very high, so deflection is very low even in open-loop configuration.

Resolution

Electromagnetic stepper motors generate discrete “steps” and can “microstep” between these stable positions by modulating the input signals. As opposed to that, when a PAD rotates, the contact point between ring and shaft moves continuously on a circular path. This means that at any point in time, the PAD is in the same mechanical configuration so characteristics are homogeneous. Thanks to this, position can be controlled with an extremely high resolution.

Resolution of a PAD system depends on the internal gearing of the PAD and the output resolution of the NDR. The angular resolution (in degrees) can be expressed:

With:

NPAD: Number of teeth on the shaft of the PAD

NNDR: Number of points per PAD cycle generated by the NDR

For example, a PAD with NPAD=312 teeth on the output shaft combined with a NDR generating NNDR=1024 points per PAD cycle can achieve a resolution of 312*1024=319488 points per turn. In other words 0,0011°, 20µrad or 4arcsec.

Jitter

When holding position, electrical noise from the NDR converts into small movement of the output shaft. This jitter (in degrees) can be estimated by:

NPAD: Number of teeth on the shaft of the PAD

δU: Voltage jitter on output of NDR

Up-p: Nominal peak-peak voltage of the PAD

Axial loads on output shaft

Most PADs are not designed to sustain axial loads. The shaft provides some axial play.

In many cases, it is more effective to include an application-specific thrust bearing in parallel to the PAD. If an axial load is present, additional friction in the thrust bearing can lead to some hysteresis in position of the output shaft.

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