Static performance

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:

N_PAD: Number of teeth on the shaft of the PAD

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

For example, a PAD with N_PAD=312 teeth on the output shaft combined with a NDR generating N_NDR=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:

N_PAD: Number of teeth on the shaft of the PAD

δ_U: Voltage jitter on output of NDR

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