Sensorless Servo Motors​ (often referred to as "Brushless DC Motors" or "Sensorless Servos") are primarily designed to achieve cost optimization, structural simplification, and enhanced reliability while meeting the requirements of specific applications. Their core logic is: within the allowable range of performance requirements, replace physical sensors with advanced control algorithms. The goal is not to replace high-performance servo systems, but to fill a market gap. They offer advantages in cost, size, and reliability, using algorithmic compensation to deliver "sufficient" performance in suitable scenarios.

Application Scenarios Include:

  • Fans, Pumps:​ Require steady-state speed accuracy but have low demands for instantaneous start/stop and rapid response (high dynamic performance).
  • Conveyor Belts, Simple Material Handling:​ Require stable speed control, with generally medium to low position accuracy requirements.
  • Household Appliances​ (e.g., inverter fans, high-end washing machine pumps): Cost-sensitive, space-constrained, and potentially exposed to humid environments.
  • Mixing Equipment:​ Require stable speed control from low speeds (a few or tens of RPM) to high speeds (up to 5000 RPM).
  • Peristaltic Pumps, Syringe Pumps, Metering Pumps, etc.:​ Require angular position control, demanding small motor size, high efficiency, and low heat generation.
  • Certain Industrial Equipment​ that is extremely cost-sensitive and operates under simple conditions.

Nidec Sensorless Servo Motor 24H (Φ43 × 43.4mm)

  • Utilizes sensorless position control.
  • Board-integrated motor, with control information captured and processed internally.
  • No external driver board required.
  • Motor speed: 0~5000 rpm
  • Output power: 70W (240 mN·m @ 3000 rpm)
  • Positioning angle resolution: 0.9°
  • Angular accuracy: ±3°
  • Input torque is approximately equivalent to a size 42 hybrid stepper motor. When paired with a reduction gearbox, it can approach the speed and torque of a size 57 stepper motor.
  • Control method: Pulse + Direction control (similar to the control method of a stepper motor with a driver), where one pulse corresponds to one angle step.

High-Frequency Injection (HFI)​ is a key technology used in sensorless control of Permanent Magnet Synchronous Motors (PMSM) or Permanent Magnet Assisted Synchronous Reluctance Motors to estimate rotor position and angle, thereby achieving precise angular control of the motor.

Its core principles and implementation are as follows:

  • Basic Principle:​ A high-frequency voltage signal (typically rotating or pulsating HF, with a frequency much higher than the fundamental) is injected into the motor's stator windings. Due to the rotor's saliency effect (where the magnetic path reluctance varies with rotor position), the high-frequency current response contains rotor position information. By detecting and demodulating this high-frequency current response, the rotor angle can be estimated.
  • Main Advantages: Excellent Zero/Low-Speed Performance:​ Does not rely on the motor's back-EMF, making it effective at standstill and very low speeds. Enables starting from zero speed and provides high torque control at low speeds. Relatively Insensitive to Motor Parameter Variations:​ Primarily relies on saliency characteristics, offering good robustness against changes in parameters like resistance and inductance.
  • Main Disadvantages & Challenges: Dependent on Saliency Effect:​ Effectiveness diminishes or becomes unusable for motors with insignificant saliency ratios, such as Surface-Mounted PMSM (SPMSM). Introduces Noise and Torque Ripple:​ The injected high-frequency signal can generate audible high-frequency noise and may cause additional torque ripple. Complex Algorithm:​ Requires filters to separate the high-frequency response signal and involves complex coordinate transformations and calculations, placing certain demands on processor computational power.

In summary,​ High-Frequency Injection solves the problem of unobservable rotor angle at zero and low speeds in sensorless control through "active probing." It is one of the core methods for achieving full-speed-range sensorless angle control and is widely used in applications demanding high start-up and low-speed performance, such as fans, pumps, compressors, and servo drives.