The test configuration placed the Filter Wheel assembly on a test bench with the actuator in the horizontal  position. This was done to minimize the possible effect of the presence of a gravity vector. In the application  on Mars, the actuators are in the horizontal position as well. The actuator was operated using a motor driver  that provided a pulse width modulated motor coil drive current. The pulse width modulation was to set a  current level for the motor that was less than the maximum possible winding current at the applied drive  voltage. The current limit is used for the flight unit drive and allows limiting of the output torque of the motor.  The setup of the test equipment for the life test is diagrammed in Figure 18. The Stepper Motor Driver  operates the motor and the actuator rotates the Filter Wheel Assembly. The magnets on the filter wheel  provide indications of filter position back to the comparator while the wheel is turning. The number of steps  between filters is a known value. The comparator checks the number of steps it took to move between  filters to the known number of steps between filters. If the step count does not match the predicted value  within a couple of steps, the comparator declares missed steps.  Figure 18. Block Diagram of the Filter Wheel Assembly Life Test Setup  For the life testing, the motor current limit was determined by reducing the limit until the operating motor  started missing steps. This is the threshold limit for the operation of the stepper motor at the applied torque  load. The life test was started with the motor driver current limit set to a value of 20% of the maximum drive  capability, which was a little above the drive at which the motor missed steps. The motor was operated  continuously and the number of steps between filter positions were checked until the motor skipped steps.  The motor drive current limit was then raised to 45% of the maximum, which was the value needed to  reliably rotate the actuator without missing steps at the higher torque load. The test was continued until the  motor started missing steps again. The third time the motor drive output was raised to 100% of the output  capability. The test was continued until the motor started skipping steps once more. Since the maximum  drive capability was reached, the test was terminated at that time. The total number of output revolutions  demonstrated was over 14 times the required life for the Mars 2020 mission, indicating a high level of  robustness of the Filter Wheel Assembly design for the application.  Determination of the Cause of the Failure to Rotate  The final operation to perform after the completion of the life testing phase was to determine the cause of  the Filter Wheel Assembly’s failure to rotate reliably. The first step in this process was to remove the  actuator from the filter Wheel Assembly and operate the actuator alone. The actuator would still not rotate  reliably separated from the Filter Wheel Assembly. This indicated that the actuator was the source of the  failure to rotate reliably for the entire mechanism. The actuator was placed into a Fein focus real-time Xray machine and inspected for possible causes of the failure. Figures 19 and 20 show images from the xray machine.  NASA/CP—2018-219887 315