46 Manufacturing tolerances and imperfections on magnets or stator sheets might cause un acceptable  disturbances. In order to mitigate this risk while maintaining cost -efficient motor des ign and manufacturing  without unnecessarily tight tolerances , feed- forward harmonics suppression w as implemented into the  controller of the mechanism drive electronics.  The feed forward allows compensation of known, repeatable  disturbances by means of pre- programmed, position- dependent motor current variation.  

Motor cogging was reduced by skewing of the stator sheet metal stack (refer to Figure 5). Skewing also  reduces t he torque constant of the motor  slightly . This was acceptable for both the Scanner and the  Derotator.  

Motor disturbances could have been minimized by using an iron-less motor . However, the torque constant  of an iron- less motor was expected to be approx imately 40-50% lower than the one of the baseline (iron)  motor. This would not have been acceptable from a power and torque budget point of view.  

Figure 5. Motor stator skewing  

The Performance Drift Error is calculated from Absolute Performance Errors (APE). The APE is the  difference between actual position and target (commanded) position for any given point in time. During  flight, the actual position is determined with the absol ute optical encoder that is part of the mechanism.  Therefore, measurement uncertainties of the encoder have a direct impact on the APE and PDE:   • Low-frequency encoder errors (below  the controller bandwidth) are followed by the mechanism.  This results in a deviation to the target  scan profile. The actual (physical) performance of the  mechanism is affected, but this cannot be determined from the position data.   • High-frequency encoder errors (above the controller bandwidth) are not followed by the  mechanism but corrupt the recorded position data. The actual (physical) performance of the  mechanism and, hence, the co- registration of the instrument  is not affected .  In order to minimize the impact of measurement uncertainties on the mechanism performance, stringent  requirements were imposed on the encoder.  The performance of all encoders was checked with an external  reference encoder during mechanism assembly.  

The second major performance requirement is off -axis motion of the scan mirror (“wobble”). The ti meindependent (also referred to as asynchronous or random) wobble of the Scanner mechanism shall be  <10 µrad peak -peak ; the one of the Derotator shall not exceed 20 µrad. In contrast to the synchronous  (repeatable) wobble, the asynchronous wobble cannot be corrected during ground processing.  

The time- independent wobble is caused by any non- deterministic behavior of the bearings. Nondeterministic behavior of the bearings can be attributed to the balls, which rotate and spin in a un predictable  manner. The variation of the ball diameter , as defined in ISO 3290,  was identified as  the major contributor  to the time- independent wobble. Surface roughness is approx imately  one order of magnitude lower, and   the deviation of the spherical form of the ball has a minor influence as the ball is the softest element in the  bearing.  Therefore, balls of grade 3 were selected for the bearings . The  distance between the rings of the  bearing pairs was  maximized as much as possible.