Field-oriented control (FOC) explained
What is field-oriented control?
Field-oriented control (FOC), also known as vector control, is the basic torque control algorithm for brushless servo drives. FOC transforms the three-phase motor currents into a rotating reference system that is aligned with the magnetic field of the rotor. This allows the torque-forming and field-forming current components to be controlled independently of each other.
Why is this important?
Without FOC, it is extremely difficult to operate a PMSM or BLDC motor efficiently across all speeds and load conditions. FOC enables:
- Maximum torque per ampere — the motor generates the greatest force with the least amount of current, which reduces thermal losses.
- Smooth operation at low speeds — crucial for robotics, cobots, and precision positioning
- Dynamic torque response — essential for rapidly changing load conditions
- Consistent performance across the entire speed range
How does it work?
A PMSM motor has three phase windings (U, V, W) that carry sinusoidal currents. Directly controlling these three time-varying signals is complex. FOC simplifies this by using two mathematical transformations:
- Clarke transformation — converts the three-phase currents (U, V, W) into a two-axis stationary coordinate system (α, β)
- Park transformation — rotates the stationary coordinate system into a reference system that rotates with the rotor and provides two quasi-direct current values:
- Id (direct current) — controls the magnetic field strength (held at zero during normal operation)
- Iq (cross flow) — directly proportional to torque
Since Id and Iq are practically DC values in stationary operation, they can be controlled using simple, fast controllers. The output voltages are then converted back into three-phase PWM signals via inverse transformations.
This approach requires real-time knowledge of the rotor position, which is why encoder or sensor feedback is essential for FOC.
How does SOMANET implement this?
SOMANET servo drives use FOC as the core of their current control architecture, extended by Model Predictive Deadbeat Control (MPD+I) for the inner control loop. The torque control loop runs at 16 kHz, ensuring the necessary bandwidth even for the most demanding applications.
OBLAC Drives automatically handles FOC commissioning: commutation offset detection, encoder configuration, and current controller setup are performed in the guided setup wizard.
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