Status: Robotic Ice Mobility

Steering Mechanism Defect Report from Michael Parris

During stowage of the Nomad chassis in the FACH hangar at Punta Arenas, the left rear wheel caught in a rough patch in the floor, causing the left rear steering rack to skip teeth with respect to the pinion gear. The rough patch was most likely the result thermal cycle spawling of the concrete floor. When the wheel caught, Mark Sibenac heard a noise from the wheel. He immediately released the enable button on the joystick, disengaging the drive motors. Mark alerted me to the problem he was having. After brief visual inspection, we continued to stow the chassis. Again, we heard a popping noise. It sounded like gears skipping. I inspected the steering motor mounting bracket and verified that the bolts were tight. This motor mount had loosened previously, allowing slippage of the gears. We noted no visible damage so we continued to stow the chassis. We again heard the noise. It seemed to have originated in the internal steering assembly, near the gearbox and steering racks. At this point we jacked up the left side of Nomad to assist in the stowage process. As the chassis approached the stowed position, I noted an inequality in the angular positions of the left front and left rear steering assemblies. The left front wheel had reached the full stowed position. The left front steering linkage bar had contacted the limit switch disabling the steer motor. The left rear wheel had not reached the full stowed position. It had stopped about 10 degrees short. The left rear steering rack appeared to have skipped teeth with respect to the pinion gear.

We deployed the left side of the chassis once again to inspect the steering racks, and gearbox. I counted exposed teeth on the racks to conclude that three teeth had been skipped on the rear rack. Both the racks and pinion gears appeared to be in good condition, i.e. no chipped teeth or visible cracks. All other components of the left steer assembly appeared to be in good condition as well. A slight misalignment of the wheels when in the deployed position was visible. This misalignment was not nearly as great as in the stowed position. When the left front wheel is aligned straight with respect to the body of the robot, the left rear wheel is misaligned by a 2 or 3 degree counterclockwise rotation.

I assessed the prospects of repair to the steering mechanism to realign the gear racks. We had not yet received confirmation of our departure to the ice, but FACH had requested that our equipment be packed and ready for loading by 19:30 that evening. This incident occurred at 18:30. I did not feel that we would be able to complete the repair within an hour. We speculated that the misalignment would not have great impact on our primary operations at Patriot Hills. Rather than risk missing our flight opportunity, we decided to leave Nomad as is. We could perhaps realign the racks, if necessary, after our arrival and set up of the work tent. After some discussion with Mark and Alex, we decided to transport Nomad in the deployed position on the C-130 to Patriot Hills. We had previously agreed to have Nomad fully assembled and functional for a wireless rollout at Patriot Hills. This was for reason of spectacle as much as logistics. Nomad had been fully assembled and tested at the FACH hangar prior to this incident. Late that night, we drove Nomad from the hangar and backed it onto the C-130. Upon arrival at Patriot Hills, Nomad rolled out of the C-130 and drove under wireless teleoperation around the aircraft loading area.

Since then, Nomad has driven nearly 15km (my estimate) without any major mechanical problems. The major implication of this misalignment is the inability to completely stow the chassis. Other implications would include:
  1. Inability to properly calibrate the alignment of the steering mechanism. Nomad requires manual calibration of the steering mechanism to align the four wheels in the straight position with respect to the body of the robot. This calibrated position is preset into the encoder registers in the real time cage. The manual calibration process involves a spotter at  the robot relaying commands to an operator. This calibration is based on the spotter's visual verification that the wheels are aligned. As a result of this incident, the left wheels do not properly align. The longitudinal alignment of the front and real wheels does not coincide with the longitudinal axis of the body of the robot. The best we can do is to align the left front wheel with the body of the robot, in which case the left rear wheel is misaligned by a measured 3.5 degrees. Or alternatively, split the difference by having both the front and rear wheels slightly off in opposite directions. The latter alignment method allows the robot to drive a straight when commanded. We have verified this through plotting of the gps position data.
  2. Increased wear on the left wheel components due to slight but constant lateral slip. The misalignment of the left wheels induces a slight but constant lateral load on the wheel components. This lateral load could, over time, lead to increased wear of the components. On November 8, Stewart reported a noise coming from the left side of the robot. I went out to inspect and found that the noise had come from inside the left rear wheel. Again, this noise sounded like slipping gears. I removed the hubcap to find that the motor mount bolts had loosened, allowing separation and slip between the drive gears. The loosening of these bolts is a recurring problem. We had experienced this throughout the Atacama Desert Trek. We had implemented a temporary fix in the atacama by sandwiching the motor mount to the axle with steel plates and long bolts. We had redesigned the mounting assembly based on last year's fix. Since then, we have experienced the loosening of the motor mount bolts, but normally attributed it to assembly error, rather than wear.  This occurrence may have been due to the misalignment, but it is my sense that it was not. I will continue to monitor the wear of the drive assembly.
  3. Increased power consumption in the left drive motors (or all drive motors). Another possible implication of the misalignment of the left wheels is an increase in power consumption. Preliminary locomotion test results show increased power draw to the left wheels of the robot. It is too early to make any claims. I will investigate this further. Conclusion: The misalignment of the left wheels due to a steering gear rack slippage does not have significant impact in the operations of the robot. Most driving modes, explicit steer, deployed skid steer, and point turn, function normally. Stowed skid steer driving is probably possible, but not advisable due to the increased angular misalignment of the wheels in the stowed position. It is unlikely that any repair attempt will be made before return to Pittsburgh.
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