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:
| |
$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:
|