Module 2·45 min
Motor Selection Methodology
Motor Selection Methodology
Motor selection is an iterative matching problem: find a motor whose continuous torque, peak torque, speed, and thermal profile all satisfy the requirements simultaneously.
Key Motor Parameters
| Parameter | Symbol | Units | How to use it |
|---|---|---|---|
| Torque constant | KT | N·m/A | τ = KT × I |
| Back-EMF constant | KE | V/(rad/s) | Equals KT numerically (SI units) |
| Winding resistance | R | Ω | Heat: P = I² × R |
| Thermal resistance | Rth | °C/W | ΔT = P × Rth |
| Max winding temp | T_max | °C | Usually 130–155°C |
| Peak current | I_peak | A | Short-duration max |
| Continuous current | I_cont | A | Steady-state max |
Thermal Sizing
Continuous operation requires the winding temperature to stay below T_max:
P_copper = I² × R
ΔT_winding = P_copper × Rth_winding
T_winding = T_ambient + ΔT_housing + ΔT_winding
For CatBot, a walking trot at 1 m/s requires roughly 30% duty cycle at peak torque. The motor must handle this thermally without derating.
Candidate Motors
Motors commonly used in MIT-lineage QDD systems:
T-Motor AK80-9: KT=0.091 N·m/A, I_cont=15A, I_peak=46A
T-Motor U8: KT=0.135 N·m/A, I_cont=12A, I_peak=35A
Cubemars GL60: KT=0.086 N·m/A, I_cont=12A, I_peak=40A
These are starting points. CatBot's actuator design uses a custom winding to tune KT specifically for the 48V supply and 8:1 reduction.
Selection Criteria
- KT × I_peak × gear_ratio ≥ peak joint torque × 1.5 (safety margin)
- Thermal steady-state within limits at duty cycle
- Physical envelope fits within CatBot link geometry
- Rotor inertia minimized (lower is better for QDD)
The final selection process involves building a spreadsheet model of the actuator operating over a representative gait cycle, then checking every constraint simultaneously.