Cam era Bar Driv e 

The fourth mechanism is the Camera Bar Driv e (Figures 12 & 13). A pair of panoramic wide-angle  cameras and a pair of navigational small angle camera s are mounted on a common bar at the top of the  mast. The camera bar drive rotates the camera bar allowing the cameras to look up and down.   

Basic Requirements:   Provide support of the Pancam and Navcam cameras   Move the cameras about the elevation axis at 2 degrees/sec with a pointing error of 0.1 degrees  Carry power and signals across the rotating joint  Accommodate relative thermal distortions between mast and camera bar   Allow separate integration and alignment  of cameras as a sub assembly 

The four cameras are mounted to a rigid angle bra cket that maintains precise alignment between each  camera. The assembly of cameras and angle bracke t was integrated and qualified separately from the  mechanism and the two were integrated during final int egration of the rover. The camera bar mechanism  consists of a rigid titanium box sect ion tube that is supported at each end.  

One end of the tube is supported by a large diamet er thin section ball bearing and the other end is  mounted to a bracket that is supported by the output  shaft of a gear-motor. This gear-motor bracket  serves as the housing for spiral wound flex cable assembly that carries power and signal across the  rotating joint. The bracket design also  incorporates a diaphragm that provides additional  compliance within the assembly.  

The motor is a custom DC brush motor with a  custom hybrid planetary-harmonic gear-head with  8118:1 gear ratio. A magnetic detent mechanism  is integrated between the motor and gear-head to  resist back driving that may be caused by the  cantilevered mass of the cameras and the jostling  that occurs as the rover moves. The magnetic  detent consists of a very small multi poled  magnetic rotor inside a magnetic stator housing.  To initiate significant rotation the magnetic detent  must be overcome, but then it provides additional  torque as you get over the “hump”. This repeats  every few degrees as the shaft is rotated. The  detent increases the starting torque required of  the motor but does not affect the operating torque  margin since this torque ripple averages out.   Torque  Tube Navcam  Cameras Simul ators Pancam  Cameras  SimulatorsAngle  Bracket     Figure 12.  Cam era Bar Driv e 

The gear-motor assembly includes a relative positi on encoder. Before each use, the mechanism must be  rotated against the hard stop to zero the encoder. T he gear train, structures, and hard stops must be  capable of tolerating full motor stall torque many times. The same hard stops that are used for the launch  lock are also used for this application. 

The thin section bearing that supports one end of t he camera bar has very large internal clearance  between balls and races. This slop in the bearing is  required to accommodate the different thermal  expansions of the titanium camera bar and the graphi te mast structure. Since the cameras only image  when the rover is stationary and the camera bar is not  moving, the loose bearing does not lead to blurry  images. Concern over damage to the bearing from ratt ling was alleviated by analysis that showed that  motion of less than .010” did not allow enough velocity to cause damage. 

Twist of the camera bar assembly due to gravity s ag or due to follower bearing drag was a concern. The  sag due to gravity was easily calculated and miti gated, but the drag of the bearing was unknown and  subject to many variables. Contamination by Mars dust blown into the bearing after deployment or kicked  271