Module 3·55 min

Brushless Motors and Field-Oriented Control

CatBot uses brushless DC (BLDC) motors because they offer superior torque density, efficiency, and longevity compared to brushed alternatives.

BLDC Motor Construction

Three stator windings (phases) are energized in sequence to create a rotating magnetic field. The permanent magnet rotor follows this field.

Key parameters:

KV rating — RPM per Volt (unloaded)
KT — torque constant (N·m/A) = 8.27 / KV
Resistance per phase — limits current and determines copper losses
Inductance per phase — affects current ripple and control bandwidth

Field-Oriented Control (FOC)

Raw PWM on BLDC phases is inefficient. FOC transforms the three-phase system into two orthogonal components in a rotating reference frame:

Id — flux-producing current (keep near zero for max efficiency)
Iq — torque-producing current (your control variable)

The transformation uses the Clarke transform (3-phase → 2-phase stationary) followed by the Park transform (stationary → rotating). The inverse transforms convert back to phase voltages.

Benefits of FOC:

Torque is proportional to Iq — linear, predictable
Sinusoidal commutation — quiet, smooth
High efficiency across the speed range

ODrive and Similar Controllers

Open-source FOC controllers like ODrive, VESC, and Moteus implement this for you. For CatBot development you can start with Moteus (MIT mini-cheetah lineage) which provides FOC with a CAN interface and built-in position/velocity/torque control modes.

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Encoders and Position Feedback

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