With the continuous development of motor drive technology, Field Oriented Control (FOC) has become the mainstream method for achieving high-performance motor control. Compared with traditional square wave or V/F control, FOC significantly improves dynamic response, operational efficiency, and accuracy. It is especially suitable for industrial automation, robotics, electric vehicles, and home appliances where strict motor performance is required. This article provides a detailed explanation of the principles and application value of FOC in sensored motors.
What is Field Oriented Control (FOC)?
The basic idea of vector control is to transform the three-phase motor currents into two orthogonal components through mathematical transformations:
Magnetizing component (d-axis current): primarily responsible for generating magnetic flux.
Torque component (q-axis current): primarily responsible for generating torque.
This method is similar to the decoupled control of excitation current and armature current in DC motors, allowing AC motors to be precisely regulated as if they were DC machines, thus achieving more linear torque control.
Principle of FOC in Sensored Motors
Sensored motors typically refer to Permanent Magnet Synchronous Motors (PMSM) equipped with position sensors. In FOC control, the controller acquires rotor position in real time using encoders, Hall sensors, or resolvers, and combines this with current detection to achieve space orientation of currents and magnetic fields.
The main steps include:
Clark Transformation: converts three-phase currents (Ia, Ib, Ic) into two-phase stationary coordinates (α, β).
Park Transformation: based on rotor angle θ, decomposes currents into rotating coordinates (d-axis and q-axis).
PI Controller Regulation: independently adjusts d-axis and q-axis currents, maintaining stable magnetization while adjusting torque according to load demand.
Inverse Transformation & PWM Drive: converts control signals back into three-phase voltages to drive the inverter, enabling precise motor control.
Through these processes, FOC achieves current decoupling, allowing torque and flux to be controlled independently, resulting in superior dynamic performance.
Advantages of FOC
High torque control accuracy: Compared to square wave control, FOC ensures smoother torque output with reduced vibration and noise.
Fast response: Strong dynamic adjustment capability makes it ideal for rapid start-stop and frequent speed changes.
Higher efficiency: By properly distributing d-axis and q-axis currents, the motor operates at optimal conditions, minimizing energy losses.
Excellent low-speed performance: Maintains precise control even at low or zero speed, which is critical for electric vehicle startups and robotic precision movements.
Wide applicability: Can be applied to multiple motor types, including Permanent Magnet Synchronous Motors (PMSM) and Induction Motors (IM).
Application Scenarios
Electric vehicle drive systems: FOC provides high torque at low speed and high efficiency at high speed, making it a core technology for EV motor control.
Industrial automation: CNC machines, robotics, and other precision equipment benefit from high-accuracy positioning and speed control using FOC.
Home appliances: Air conditioners, refrigerators, and washing machines adopt FOC to reduce noise, save energy, and extend lifespan.
Wind power generation: FOC enhances generator control, improving efficiency and system stability.
Future Development Trends
With the adoption of power semiconductors such as SiC and GaN, along with continuous improvements in control algorithms, FOC technology will achieve even higher efficiency and faster response. Meanwhile, hybrid sensored and sensorless control is emerging: using sensors for accuracy at low speed while switching to sensorless algorithms at high speed to reduce cost while maintaining performance.
Conclusion
Field Oriented Control (FOC) for sensored motors, with its efficiency, precision, and stability, has become a cornerstone of modern motor control technology. It not only enhances motor performance but also drives advancements in electric vehicles, robotics, and industrial automation. In the future, as new semiconductors and intelligent algorithms converge, FOC will unleash greater value across more applications, becoming a key enabler for smart manufacturing and green energy.