Field-Oriented Control

磁场定向控制FOCAdvanced

Transforming three-phase current into a coordinate frame that rotates with the rotor, then separately controlling the flux-producing and torque-producing current.

Field-oriented control, also called vector control, was proposed by K. Hasse at TU Darmstadt and F. Blaschke at Siemens between 1968 and the early 1970s, and is now the dominant drive method for permanent-magnet synchronous and brushless motors — robot joint-module drives almost universally use it. A motor's three-phase current is an AC quantity, hard to regulate directly. FOC first uses the Clarke transform to convert the three phases into a two-phase stationary frame (α, β), then the Park transform to rotate, by the rotor's electrical angle, into a frame that spins with the rotor, the d-q frame: d-axis current i_d produces flux aligned with the permanent magnet, and q-axis current i_q produces torque — both now DC quantities, each regulated with its own PI controller. For permanent-magnet motors, i_d is usually set to 0, and torque is approximately the torque constant K_t times i_q. This requires knowing the rotor's angle in real time, usually from an encoder, though sensorless observers can estimate it too. Compared to six-step square-wave commutation, FOC gives smoother torque; compared to V/f scalar control, it gives better dynamic performance.

ExampleThe quasi-direct-drive joints common in legged robots: the host sends target position, velocity, stiffness, damping, and feedforward torque via MIT Mode; the drive computes a desired torque τ, divides by K_t to get an i_q command with i_d set to 0, and then completes the Clarke/Park transforms, PI regulation, inverse transform, and PWM output of three-phase voltage inside a high-frequency current loop. The open-source SimpleFOC library implements this same pipeline.

Also called: FOC, Vector Control
Related: Permanent Magnet Synchronous Motor (PMSM) · Brushless DC Motor · Servo Drive (Motor Driver) · MIT Mode · Magnetic Encoder · Torque Constant (Kt)
Sources: Wikipedia: Vector control (motor)
SimpleFOC docs: FOC theory