321 capability of the joint, but it does not result in complete joint failure. If a structural compon ent were to fail,  then an overload event would cause complete loss of the joint instead of simply reduced capability.   

Table 1 below summarizes the capabilities and overall gear ratios of each of the joints. The speed of each  joint is based upon the desire to actively servo the limbs over undulating terrain with slopes up to 10° while keeping the pallet level at the nominal  driving speed of the vehicle, 3 kph.  

Table 1. Summary of Joint Gear Ratio and Capability 

Joint Overall Gear  Ratio Harmonic  Drive Ratchet  Torque No-Load  Speed  Hip Yaw 6,591 : 1 CSG-40-80 3.600 Nm 0.16 rad/s  Hip Pitch -6,480 : 1 CSG-58-80 10,000 Nm 0.11 rad/s  Thigh Pitch -6,480 : 1 CSG-58-80 10,000 Nm 0.11 rad/s  Knee Pitch -3,240 : 1 CSG-40-80 3.600 Nm 0.22 rad/s  Knee Roll 3,514 : 1 CSG-32-50 1,200 Nm 0.30 rad/s  Ankle Pitch 6,591 : 1 CSG-32-80 1,800 Nm 0.16 rad/s  Ankle Roll 3,514 : 1 CSG-32-50 1,200 Nm 0.30 rad/s   The joint is sealed using a two layer seal. The outer seal is a Nomex felt to protect from large size  particles. These felt seals have a rectangular cross se ction with a height / width aspect ratio between 1.25  and 2.0, where height is measured parallel to the rota tion axis. The seal is compressed axially between  7% and 16% of its nominal height and rides on ha rd anodized aluminum surfaces. This amount of  compression provides for good sealing, but contributes to significant drag torque. The highly compressed seals were also forced out of their designed grooves since the hoop strength of the seal is extremely low. Axial compression between 5% - 10% is sufficient for t he felt seal design and in practice the seal should  be captured or bonded in place to prevent radial expansion of the felt ma terial. The inner seal is a spring  loaded Teflon lip seal that also rides on a hard anodi zed surface. This seal pr events small particles from  entering the joint and will remain in  contact through thermal expansion and contraction with very little  change in lip sealing force due to the radial expanding spring. This same method of double seal has been used successfully in high dust environment missio ns such as the Mars Exploration Rovers [5]. 

Joint Sensing and Control 

The overall coordination of the joints is done by th e main computer, which is housed in the rover body.  This computer transmits via point commands to each  motor controller over a CAN network bus at 8 Hz.  The motor controller is housed locally at each motor and controls the motion of the joint using a position  control loop at 2.7 kHz. Joint position via points are updated during motion and are fed from the main CPU to the local controllers. The motor controllers  then update the position setpoint and smoothly  transition to the new motion profile.     Each joint has both an incremental encoder on the motor input as well as a 12-bit absolute encoder on the joint output. The motion of each joint motor is  controlled via the incremental encoder, and the limb  kinematics is determined from the high precision output encoder. Using these two position sensing  devices, the mechanical windup of the joint can be determined. This measurement combined with a characterization of the torsional stiffness of the join t gives the torque experienced at each joint. Using the  joint torques and the vehicle’s kinem atic pose, the robot can determine the ground contact force of each  limb and can autonomously make adjustments to correctly distribute the vehicle load over all the limbs [6].     

   NASA/CP-2010-216272