3 Motor Control Techs Comparison: FOC, V/f Control, Trapezoidal Six-Step Control (BLDC)

Semiconductor Integrated Circuits (ICs)
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In modern motor drive systems, based on application requirements and cost considerations, common control strategies mainly include sensor-based Field-Oriented Control (FOC), sensorless FOC, scalar V/f control, and trapezoidal six-step control (BLDC). These four methods differ significantly in terms of control principle, parameter tuning process, efficiency, current/torque ripple, and applicable scenarios. The table below systematically compares these four strategies, aiming to help engineers quickly understand their respective characteristics and make selections.

Comparison Dimension FOC (With Sensor) FOC (Sensorless) V/f Control (SVM) Trapezoidal Six-Step Control (BLDC)
Control Principle Samples three-phase currents, and undergoes Clarke transformation → Park transformation → PI control for iq and id → inverse Park transformation → SVM → PWM. Rotor magnetic field position is controlled by position feedback from encoders or Hall sensors. Similar structure to sensor-based FOC, but rotor position and speed are estimated by observation algorithms (e.g., PLL or Sliding Mode Observer (SMO)). Generates reference voltage based on a constant voltage-frequency ratio (V/f), then produces PWM signals via SVM. Relies on Hall sensors to detect commutation positions; six-step commutation logic directly drives the three-phase bridge, with no complex transformations.
Efficiency Excellent. Vector decoupling control combined with SVM optimizes voltage utilization, resulting in low losses and high efficiency. Suitable for high-performance applications. Slightly lower than sensor-based FOC. The observer introduces estimation errors, leading to slightly poorer performance at low speeds, but overall efficiency remains high. Moderate. SVM is used to improve efficiency (higher than conventional sinusoidal PWM (SPWM)). Lowest. Rough commutation method and rectangular current waveform cause high harmonics and low efficiency.
Parameter Tuning Process - Encoder zero-position alignment - PI loop parameter tuning (id for flux linkage control, iq for torque control) - Additional tuning of flux-weakening control settings if used in the flux-weakening region - Calibration of initial observation parameters (e.g., bandwidth of speed observation parameters) - Adjustment of PI loops - Optimization of observer parameters (e.g., gain, filter time constant) - Setting of V/f curve (linear or segmental control) - Adjustment of SVM modulation index to improve modulation efficiency and suppress harmonics - Setting of commutation delay - Speed-regulating PI controller if closed-loop control is added - Adjustment of Hall commutation angle and logic delay
Current/Torque Ripple Minimal. Smooth and continuous current, with almost no torque oscillation, especially excellent performance at both low and high speeds. Ripple is slightly higher than that of sensor-based FOC. Errors increase significantly when speed and load change, but it is still far superior to the other two technologies. Moderate. Although there is no vector decoupling, SVM can reduce voltage waveform distortion, and its ripple control is better than that of trapezoidal control. Maximum. Sudden current changes during commutation lead to obvious torque pulsations (six-step pulsations).
Applicable Scenarios High-performance PMSM control, precise torque and speed control, and heavy-load servo systems. Scenarios requiring high performance while reducing external sensors (cost/sensor-sensitive environments). Low-cost asynchronous motor systems with simple structure, low debugging threshold, and moderate efficiency. Lowest cost and simplest control logic; suitable for applications with low requirements for torque smoothness, such as fans and power tools.

Notes:

  • SVM (Space Vector Modulation): An improved PWM modulation method that uses the concept of three-phase space vectors in mathematics to generate drive signals. It has higher DC voltage utilization than traditional sinusoidal PWM (SPWM).
  • SMO (Sliding Mode Observer): A state observation algorithm, commonly used in sensorless Field-Oriented Control (Sensorless FOC) to estimate rotor position and speed.
  • PLL (Phase-Locked Loop): Unlike its common use in communications (where it “locks the carrier phase”), in sensorless FOC, it is used to track the phase of the rotor flux linkage, thereby obtaining rotor position and speed.

Summary:

To summarize, sensor-based FOC performs best in terms of performance and torque smoothness, making it suitable for high-performance PMSM control scenarios; sensorless FOC maintains high performance while reducing system costs; V/f control is applicable to low-cost asynchronous motor systems with a simple structure; trapezoidal six-step control, with its simple implementation and lowest cost, is more suitable for applications with low requirements for torque smoothness (e.g., fans and power tools). Through the comparison in this table, the advantages and limitations of different control strategies in specific projects can be evaluated more clearly.

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