Drivetrain Prototyping Results
Summary of Final Build
- Tested with a 12.5lb battery and 5lb weight
- Wheels driven via a custom-machined, keyed hub on the outer face + a keyless, 3D printed version on the other face to support the shaft
- Motors connected to wheel shaft via a keyed shaft coupler
- 5/6 wheels use two keys in the coupler – one in the motor shaft and one in the wheel shaft
- 1/6 wheels use just one key in the motor shaft. On the wheel shaft, a flat surface was made for the set screw to dig into
- 1/2" diameter wheel shafts, 3/4" diameter rocker shaft
- 1/4" thick, MDF belly pan
- Used bushings on the rocker and bogie joints, used bearings on the wheel shafts
- Tolerance for the bushings & bearings was+0.003" to its diameter
- Powered by a 12V battery. Each wheel driven by a CIM motor to a 50:1 ratio
Observations
- High-stress locations:
- Connecting plates on the legs. When turning, these pates are getting pushed/pulled as the wheels resist the movement
- Connection point from the legs/suspension to the chassis. Is constantly twisting about that point during turns
- Flexing occurs at connection points (i.e. every joint)
- The further away the motor's centre of gravity is from the suspension's centre of gravity, the more bending occurs at the local joint
- The differential bar must be kept in position (i.e. no vertical twisting) in order for the chassis position to remain stiff
- Chassis bounces for on-the-spot turns. Something for EE & Arm team to keep in mind
- Totally bidirectional. Rocker arm should be the front for going over large obstacles, whereas use bogie for small & frequent obstructions
Design/Assembly Learnings
- Use interference fit on all bushings and bearings (or whatever specified fit is recommended by the bearing manufacturer)
- Maybe even clamp in the bearings to prevent them from coming out?
- Use bearings instead of bushings on suspension joints, as bearings are designed to handle loads
- Make sure the set screw is digging into a flat surface...
- Spec for proper torques
Current Sensor Data
- Rover weight, no load: 41.55 kg + 12.5 lb battery + 5 lb counterweight = 49.52 kg
- Rover weight, with a 5lb load: 51.79
- Current sensors have a tolerance of about +/-0.30 A
| Configuration | Type of Obstacle | Most Frequent Current Drawn by One Motor (A) | Max Torque Output Per Wheel (Nm) |
|---|---|---|---|
| Centered mass | Uphill, steep (15 degrees) | 5.6 | ~3.6195
|
| Uphill, less steep (~5 degrees) | 4.5 | ~2.413
| |
| Uphill, less steep, on an angle | 5.2 | ~2.413 | |
| Downhill | 3.5 | ~1.2065
| |
| Offset mass | Uphill, less steep | 4.9 | ~2.413 |
| Uphill, less steep, on an angle | 5.7 | ~3.6195 | |
Sand turns, figure-8 | 4.5 | ~2.413 | |
| Sand turns, on-the-spot turns | 6.8 | ~3.6195 | |
| Flat terrain | 3.8 | ~1.2065 | |
| Stairs | > 4.5 | ~2.413 | |
| Offset, 5/6 motors | Uphill, less steep | 4.3* | ~1.2065 |
Data Analysis and Gearing Calculations
Actuator Candidates
- Note: since the rated specs define the end of the continuous region, the gear ratios were determined based on values 10% less than the rated values
| Name | Price (USD) | Rated Cont. Torque (Nm) | Rated Cont. Speed (RPM) | Rated Cont. Current (A) | Rated Output Power (W) | Gearbox Suggestion | Mass/Dims |
|---|---|---|---|---|---|---|---|
| $149.80 (10 available!) | 0.43 | 4000 | 4.5 | 180 | XTRUE Gearhead, 40:1 planetary, 60mm frame
PE-W-050-025G planetary gearbox, 25:1
GP56-S2-43-SR planetary gearbox, 42.63:1 → possibly pairs? | Motor specs:
| |
| Nanotec, DB59L048035-A | $156.50 (3 in stock, more available after Oct. 5) | 0.47 | 3500 | 5.33 | 172 | GP56-S2-43-SR planetary gearbox, 42.63:1 ($189.50/each)
| Motor specs:
Gearbox specs:
|