Axis 1- Axis 1 bearing experienced high deflection on flanged mounting plate: https://drive.google.com/file/d/1YN1dZJUZKmFAtuhgSioa8tr2q3W9iqPp/view?usp=sharing
- External fasteners were required to apply significant preload to prevent turntable deflection/instability
- Spec out a sturdier proper slewing or crossed roller bearing for next revision - bearing needs to be reliable to provide smooth turntable rotation
- Mounting interface between axis 1 and testing table was suitable for testing purposes, but CANNOT be used as is on rover - thick woodne plate is incredibly heavy
- Need to redesign this plate to be leaner/lighter with similar stiffness in future revision
- Belt tensioning mechanism is unreliable - loosened overtime and is difficult to service: https://drive.google.com/file/d/1Dsm2VTrrobg6FHFnXWL1hYdCvqr0kN7H/view?usp=sharing
- Axis 1 backdrives when pushed on
 Image Modified
Axis 2- Belt drive transmission initially failed under loads greater than 2.5kg on end effector at full horizontal extension: https://drive.google.com/file/d/1fkz8yRv20WHlzYHWRqq-kDo0GyHq9Ne0/view?usp=sharing
- Repeated skipping wore down belt and eventually belt skipped under no load at full extension
- Suspected root cause of failure is a combination of:
- Large angle of attack on small pulley due to 1 : 3.26 belt ratio and small center line distance
- Low tooth engagement on small pulley
- Belt was replaced and retensioned recently
- Belt tensioning mechanism on A2 joint was reliable - use of large "lead" screw allowed for relatively even and reliable tensioning on belt
- Belt drives will not be used in next arm iteration (strainwave gear has been procured with stronger motor for low backlash single stagee transmission)
- Investigating and quantifying skipping thresholds may be useful for design of other belt driven joints
 Image Modified
Axis 3- Belt drive on axis 3 is reliable - high tension retained in belt from idlers, no skipping has been observed so far and A3 is capable of lifting all expected loads
- Bonded aluminum tube with 3D printed interfaces between A2 and A3 have so far been a strong and reliable linkage - no signs of adhesive failure or 3d print failure thus far
- Overall this joint performed well. For future iterations, look towards implementing longer belt and shifting A3 motor closer to A2 to shift arm center of mass closer to base; reducing torque requirements on most joints.
 Image Modified
Axis 4- Encoder has not been tested yet, still need to confirm if encoder placement is viable (although this style of encoder mount will be avoided in next arm iteration)
- Misalignment occurs between concentric components - large grinding/squeaking noise observed when rotating A4 indicating high friction in some area of A4
- A4 does not backdrive (as fully loaded torques are low), or experience any mechanical/structural failures when loaded with full mass
- Workspace is poor on axis 4. Currently the motor for A4 will hit the linkage between A2 and A3 when rotated beyond 180 degrees. This severely limits workspace and can be a large potential failure point - must be addressed in future designs
Axis 5/6- Differential mechanism is very smooth https://drive.google.com/file/d/1vEXv5-E2ZVYdWR3m2qYjxPHUnjmlQzF_/view?usp=sharing
- Currently difficult to control differential, as the motor controls limit motor speed via potentiometer. These potentiometers are manually adjusted without any form of readout, so getting motors to spin at exact same speed is difficult
- Software/hardware currently working on implementing positional readout and speed limiting to sync up the differential gearbox and improve controls
- Backlash is very low on A6 due to small clearance between bevel gears: https://drive.google.com/file/d/1vPZWFwJYyrqeVW5Qnyjh_PQIYGyHQcV7/view?usp=sharing
- No backdriving occurs on this joint, friction in differential gearbox prevents passive backdriving or backdriving when moving full 5kg load
- A5 belt loostened over time, need to implement a much more reliable tensioning mechanism in next iteration
 Image Modified
|
