The term “servo” originates from the Greek word for “slave.” A servo motor can be understood as a motor that absolutely obeys control signals: the rotor remains stationary until a control signal is issued; when the control signal is issued, the rotor immediately rotates; when the control signal ceases, the rotor stops instantly.

  Servo motors are micro-specialty motors used as actuators in automatic control systems, functioning to convert electrical signals into angular displacement or angular velocity of the shaft.

   Servo motors are broadly categorized into AC servo and DC servo types.

  The basic structure of AC servo motors resembles that of AC induction motors (asynchronous motors). The stator features two phase-shifted excitation windings Wf and control windings WcoWf, spaced 90° electrically. These are connected to a constant AC voltage. By varying the AC voltage or phase applied to Wc, the motor's operation is controlled.AC servo motors feature stable operation, excellent controllability, rapid response, high sensitivity,and stringent nonlinearity specifications for mechanical and regulation characteristics (requiring values below 10%–15% and 15%–25%, respectively).

  DC servo motors share a basic structure with conventional DC motors. Motor speed n = E / K1j = (Ua-Ia Ra) / K1j, where E is the armature counter-EMF, K is a constant, j is the magnetic flux per pole, Ua and Ia are the armature voltage and armature current, respectively, and Ra is the armature resistance. Changing Ua or φ can control the speed of the DC servo motor, but controlling the armature voltage is generally employed. In permanent magnet DC servo motors, the field winding is replaced by permanent magnets, resulting in a constant magnetic flux φ. DC servo motors exhibit excellent linear regulation characteristics and rapid time response.

  Advantages and Disadvantages of DC Servo Motors

Advantages: Precise speed control, very stiff torque-speed characteristics, simple control principles, user-friendly operation, and low cost.

Disadvantages: Brush commutation, speed limitations, additional resistance, and generation of wear particles (unsuitable for dust-free or explosive environments).

  Advantages and Disadvantages of AC Servo Motors

Advantages: Excellent speed control characteristics, smooth control across the entire speed range with minimal oscillation, high efficiency exceeding 90%, low heat generation, high-speed control, high-precision position control (depending on encoder accuracy), constant torque within rated operating range, low inertia, low noise, no brush wear, maintenance-free (suitable for dust-free and explosive environments).

  Disadvantages: Complex control systems, requiring on-site PID parameter tuning for the driver, and increased wiring complexity.

  DC servo motors are categorized into brushed and brushless types.

  Brushed motors offer low cost, simple structure, high starting torque, wide speed range, and easy control. They require maintenance but are easy to service (replacing carbon brushes). They generate electromagnetic interference and have specific environmental requirements, typically used in cost-sensitive general industrial and civilian applications.

  Brushless motors are compact and lightweight, deliver high output with rapid response, operate at high speeds with low inertia, provide stable torque and smooth rotation. They feature complex, intelligent control with flexible electronic commutation (square wave or sine wave). These motors are maintenance-free, highly efficient, energy-saving, emit minimal electromagnetic radiation, exhibit low temperature rise, and offer extended lifespan, making them suitable for diverse environments.

  AC servo motors are also brushless, categorized into synchronous and asynchronous types. Synchronous motors are predominantly used in modern motion control systems due to their wide power range, capability for high power output, high inertia, and relatively low maximum speed. Their rotational speed decreases uniformly with increasing power, making them ideal for low-speed, smooth operation applications.

  The rotor inside a servo motor is a permanent magnet. The driver controls the U/V/W three-phase power to generate an electromagnetic field, causing the rotor to rotate within this field. Simultaneously, the motor's built-in encoder transmits feedback signals to the driver. The driver compares the feedback value with the target value to adjust the rotor's rotational angle. The precision of a servo motor is determined by the encoder's resolution (line count).

  What is a servo motor? What types exist? What are their operational characteristics?

 Answer: Servo motors, also known as actuator motors, function as executive components in automatic control systems. They convert received electrical signals into angular displacement or angular velocity output on the motor shaft.

  Servo motors are categorized into DC and AC servo motors. Their primary characteristic is the absence of self-rotation when the signal voltage is zero, with speed decreasing uniformly as torque increases.

  What are the performance differences between AC servo motors and brushless DC servo motors?

  Answer: AC servo motors generally offer superior performance because AC servo control uses sine wave signals, resulting in lower torque ripple. Brushless DC servo control employs trapezoidal waveforms. However, brushless DC servo systems are simpler to implement and more cost-effective.

  The rapid advancement of permanent magnet AC servo drive technology has placed DC servo systems at risk of obsolescence. Since the 1980s, with developments in integrated circuits, power electronics, and AC variable-speed drive technology, permanent magnet AC servo drive technology has seen remarkable progress. Leading electrical manufacturers worldwide have continuously introduced new series of AC servo motors and servo drives. AC servo systems have become the primary development direction for contemporary high-performance servo systems, placing DC servo systems at risk of obsolescence.

  Compared to DC servo motors, permanent magnet AC servo motors offer key advantages:

  1.Brushless and commutator-free design ensures more reliable operation and maintenance-free performance.

  2. Significantly reduced stator winding heating.

  3. Low inertia enables excellent system response speed.

  4. Superior performance under high-speed, high-torque operating conditions.

  5. Compact size and lightweight design for equivalent power ratings.

  The Rise and Current Status of Permanent Magnet AC Servo Systems

  The debut of the MAC permanent magnet AC servo motor drive system by Rexroth's Indramat division (part of Germany's MANNESMANN) at the 1978 ***** Trade Fair marked the maturity of a new generation of AC servo technology. By the mid-to-late 1980s, major manufacturers had established comprehensive product lines, shifting the entire servo device market toward AC systems. Early analog systems exhibited shortcomings in zero drift, interference resistance, reliability, precision, and flexibility, failing to fully meet motion control demands. In recent years, the application of microprocessors and new digital signal processors (DSPs) has enabled digital control systems where the control logic is implemented via software. After the 1990s, fully digital sinusoidal-controlled permanent magnet AC servo motor drive systems gained further prominence in the drive field.

  Currently, most high-performance electric servo systems utilize permanent magnet synchronous AC servo motors, with controllers predominantly employing fully digital position servo systems for rapid and precise positioning. Leading manufacturers include Siemens (Germany), Kollmorgen (USA), Panasonic (Japan), and Yaskawa (Japan).