264 External Hardstops   At some low intermediate elevations, and certain azimuth angles ranges, the XTI box could contact the  ELC2. To prevent this from occur ring, an external hardstop was incorporated into DAPS called the  guardrail. T he guardrail hardstop prevents  any contact between the XTI and ELC2, while allowing  elevations as low as 63 deg (~ 30 deg below the ISS local horizontal) . The guardrail hardstop is a circular  rail which collars the azimuth actuator and is fixed to the end of the boom . A limit bar protrudes from the  elevation actuator output, and physically contacts the guardrail surface to constrain elevation motion. The guardrail has various flat  profiles to allow the limit bar to clear it in the stow azimuth range. This hardstop  architecture limits the minimum elevation of the XTI box to between 63 and  70 degrees in the ram  azimuth quadrant, 80 degrees for most other azimuths, and allows elevatio ns down to zero degrees only  for engagement/disengagement of the elevation hook and stowing.  

Mechanism Description    The DAPS biaxial gimbal actuators must meet instrument pointing performance requirements , despite the  small amount of torque noise, and resulting pointing error  they inherently produce.  

Pointing Performance   The 3σ pointing performance allocation for the DAPS is 0.32 milliradian ( 66 arcsecond) , and is  summarized in Table  1. 

Table 1. Pointing Budget   NICER/SEXTANT  Pointing  Budget  Estimated   arcsec  Requirement   arcsec   Overall Instrument Pointing  87.2 120  ISS Low Frequency  ±40 ±53  star tracker calibration &  structural/optical analysis  ±9 11.5  DAPS System  46.3 66 

Early analysis showed that the actuators induced pointing error is much greater than the requirement . As  expected, the largest error occurs when the actuator step rate coincides with structural resonant frequencies.    The NICER/SEXTANT payload has resonant structural frequencies <10 Hz which are excited by the pointing actuator s. Since the operational actuator step rates will always pass through this frequency  range, there are no feasible workarounds to avoid exciting these low frequency structural modes.    In order to reduce the excited pointing errors and meet the requirement, several mitigation methods have been implemented, including microstepping, actuator detent torque reduction, and the use of dampers.  

Microstepping   Implementation of microstepping in the actuator drive electronics was the first measure taken to reduce the stepper motor induced pointing error . Microstepping breaks a single cardinal step into smaller ones  and softens the impacts on the output load, instead of moving through a single motor step quickly and generating a larger impulsive torque on the system . While microstepping did reduce the stepper induced  pointing error, it was not sufficient to meet the pointing requirements . The issue resides in the motor  detent torque. The motor powered torque must overcome the detent torque before it can move the motor  from one step to the next step, even when microstepping. Overcoming the detent torque essentially  generates torque impulses that exceed the detent torque. While these torque impulses are smaller than  the cardinal step torque levels, they generate disturbances on the instrument and limit the benefits gained by microstepping.