Definition of a Servo Motor
The term "servo" originally derives from the Greek word for "slave". A servo motor is a motor that strictly follows a control signal: the rotor remains stationary until a control command is issued, rotates immediately when the command is applied, and stops promptly when the command is removed. In automatic control systems, a servo motor functions as an actuator that converts electrical signals into angular displacement or angular velocity at the motor shaft.
Servo Motor Categories
Servo motors are generally divided into two main categories: AC servos and DC servos.
AC Servo Motor Structure and Operation
The basic structure of an AC servo motor is similar to that of an AC induction motor. The stator contains two excitation windings with a 90° electrical phase displacement: an excitation winding Wf and a control winding Wc. Wf is supplied with a constant AC voltage; changing the voltage or phase applied to Wc controls the motor. AC servo motors offer stable operation, good controllability, fast response, high sensitivity, strict requirements on mechanical and regulation nonlinearity (typically less than 10%–15% and 15%–25% respectively), and other advantageous characteristics.
DC Servo Motor Structure and Operation
The basic structure of a DC servo motor resembles that of a standard DC motor. The motor speed is given by n = E / (K1 j) = (Ua - Ia Ra) / (K1 j), where E is the armature back EMF, K is a constant, j is the flux per pole, Ua and Ia are armature voltage and current, and Ra is the armature resistance. Speed can be controlled by varying Ua or the flux φ, although armature voltage control is commonly used. In permanent-magnet DC servo motors, the excitation winding is replaced by permanent magnets and φ is constant. DC servos typically exhibit good linear regulation characteristics and fast time response.
DC Servo Motors: Advantages and Disadvantages
- Advantages: Precise speed control, stiff torque-speed characteristic, simple control principles, ease of use, low cost.
- Disadvantages: Brushes and commutators require maintenance, speed limitations, additional drag, wear particle generation (unsuitable for clean or explosive environments).
AC Servo Motors: Advantages and Disadvantages
- Advantages: Good speed control characteristics with smooth control across the entire speed range and minimal oscillation; high efficiency (over 90%); low heating; suitable for high-speed control; high-precision position control (dependent on encoder accuracy); constant torque within the rated operating region; low inertia; low noise; brushless operation with no wear; low maintenance (suitable for clean and explosive environments).
- Disadvantages: More complex control; driver parameters often require tuning on site and PID parameter determination; more wiring is typically required.
Brushed vs. Brushless DC Servos
DC servos are available in brushed and brushless designs. Brushed DC motors are low cost, mechanically simple, provide high starting torque and wide speed range, and are easy to control, but they require maintenance (brush replacement), generate electromagnetic interference, and have environmental constraints. They are commonly used where cost sensitivity is paramount in general industrial and consumer applications.
Brushless DC motors are compact and lightweight, deliver high torque and fast response, have high speed and low inertia, provide smooth torque and stable rotation, have more complex electronic control, support flexible electronic commutation with square-wave or sinusoidal switching, require no mechanical commutation maintenance, are efficient and energy-saving, emit less electromagnetic radiation, exhibit low temperature rise, and have long service life; they are suitable for a variety of environments.
AC Servo Types and Typical Use
AC servos are also brushless and can be synchronous or asynchronous. Modern motion control typically uses synchronous motors. They cover a wide power range, can reach high power levels, tend to have higher inertia and lower maximum speeds, and are suitable for low-speed, smooth-running applications.
Position Feedback and Accuracy
Most servo motors use permanent magnets in the rotor, while the drive applies three-phase U/V/W currents to create an electromagnetic field. The rotor turns in response to this field, and an onboard encoder provides feedback to the driver. The driver compares feedback to the target and adjusts rotor angle accordingly. Servo accuracy is determined by encoder resolution (line count).
What Is a Servo Motor and Its Characteristics?
A servo motor, also called an actuator motor, converts received electrical signals into angular displacement or angular velocity at the motor shaft. They are broadly classified into DC and AC servo motors. Key characteristics include no self-rotation when input signal voltage is zero, and a speed that decreases uniformly with increasing torque.
Performance Differences: AC Servo vs. Brushless DC Servo
AC servos generally have better performance because AC servo control often uses sinusoidal waveforms, producing smaller torque ripple. Brushless DC servos frequently use trapezoidal (square) commutation, which results in larger torque ripple. However, brushless DC servo systems are simpler and less expensive to implement.
Rise of Permanent-Magnet AC Servo Drives
Since the 1980s, advances in integrated circuits, power electronics, and AC variable-speed drive technologies have driven rapid development of permanent-magnet AC servo drive technology. Major electrical manufacturers introduced new AC servo motors and drives, making AC servo systems the primary direction for high-performance servo applications and placing DC servo systems at risk of obsolescence.
Advantages of Permanent-Magnet AC Servos over DC Servos
- No brushes or commutators, leading to higher reliability and maintenance-free operation.
- Significantly reduced stator winding heating.
- Low rotor inertia, enabling fast system response.
- Good performance at high speeds and high torque.
- Smaller size and lighter weight for the same power rating.
Development History and Market Status
The commercial introduction of modern permanent-magnet AC servo systems began in the late 1970s with early products such as the MAC series. By the mid-to-late 1980s, most major companies had complete product families, and the servo market shifted toward AC solutions. Early analog systems had limitations in drift, interference immunity, reliability, accuracy, and flexibility, but the adoption of microprocessors and digital signal processors (DSPs) enabled digital control systems where the control functions are implemented in software. From the 1990s onward, fully digital sinusoidal-control permanent-magnet AC servo motor drives rose to prominence in motion control.
Leading Motor and Drive Manufacturers
High-performance servo systems commonly use permanent-magnet synchronous AC motors with digital position control drives. Typical manufacturers include Siemens, Kollmorgen, Panasonic, Yaskawa, and others. Various companies worldwide have developed extensive AC servo product lines to meet the needs of CNC machine tools, robots, handling systems, printing machines, winding machines, and other applications.
Industry Examples and Product Line Summaries
Examples of product evolution include compact AC servo motor series for CNC machines and robots, migration from rectangular-wave drivers to sinusoidal-wave drivers with microcontroller, CPU, and FPGA control, and notable reductions in torque ripple and improved reliability. Companies such as Fanuc, Rexroth, Bosch, Kollmorgen, Mitsubishi, Toshiba, Sanyo, and others have launched multiple series covering a wide power range and inertia classes to meet diverse industry needs.
Performance Metric: Power-to-Inertia Ratio
The power variation rate, also known as the power-to-inertia ratio, is commonly used as a figure of merit to compare the dynamic response of various AC and DC servos and stepper motors. It is defined as the ratio of continuous (rated) torque to rotor moment of inertia. Analysis by this metric has shown certain permanent-magnet AC servo series to lead in overall performance.
Operational Principles
AC Servo Motor Operation
The AC servo stator structure is similar to that of a capacitor-run single-phase induction motor. Two stator windings are separated by 90° electrically: an excitation winding Rf that is always connected to the AC supply Uf, and a control winding L that receives the control voltage Uc. Hence, AC servos are sometimes described as two-winding motors.
Servo rotors are often cage-type, but to achieve wide speed range, linear mechanical characteristics, no self-rotation, and fast response, servo rotors typically have higher rotor resistance and low rotor inertia compared with ordinary motors. Common rotor constructions include high-resistivity conductor bars in a slender cage rotor, and aluminum alloy hollow cup rotors with thin walls (0.2–0.3 mm). Hollow cup rotors have very low inertia and quick response, and they run smoothly, so they are widely used.
With no control voltage, only the excitation winding produces a pulsating magnetic field and the rotor remains stationary. When a control voltage is present, a rotating magnetic field develops in the stator and the rotor turns in that direction. Under constant load, motor speed varies with the amplitude of the control voltage; reversing the control voltage phase reverses motor rotation.
Compared with capacitor-run single-phase induction motors, servo motors have much larger rotor resistance, which yields three notable characteristics: large starting torque, wide operating range with smooth operation and low noise, and no self-rotation—if control voltage is lost, the servo stops immediately.
Precision Microdrive Motors
A precision microdrive motor can rapidly and accurately execute frequently changing commands in a system, driving a servo mechanism to perform the intended work. Typical requirements include:
- Frequent start/stop/braking/reversal and low-speed operation capability, with high mechanical strength, high thermal class, and high insulation class.
- Fast response, substantial torque, low rotor inertia, and small time constants.
- Integration with appropriate drivers and controllers (e.g., servo drives, stepper drivers) and good control performance.
- High reliability and high precision.
Categories, Structures, and Performance of Precision Microdrive Motors
1) AC Servo Motors
- Cage-type two-phase AC servo motors: slender cage rotor with near-linear mechanical characteristics; small size and excitation current; suitable for low-power servos but low-speed operation may be less smooth.
- Nonmagnetic cup-rotor two-phase AC servo motors: hollow cup rotor, near-linear mechanical characteristics, relatively larger size and excitation current; smooth low-speed operation for low-power servos.
- Ferromagnetic cup-rotor two-phase AC servo motors: cup rotor made of ferromagnetic material, near-linear mechanical characteristics, larger rotor inertia, low cogging effect, smooth operation.
- Synchronous permanent-magnet AC servo motors: built as an integrated assembly with a synchronous permanent-magnet motor, tachometer, and position detector; stator can be 3-phase or 2-phase; requires a driver; wide speed range with constant-torque and constant-power regions; capable of continuous stall; fast response; large output power and low torque ripple; available with square-wave or sinusoidal drive and suitable for mechatronic applications.
- Asynchronous three-phase AC servo motors: rotor similar to cage-type induction motors; require a driver and use vector control to extend the constant-power range; often used for spindle speed control in machine tools.
2) DC Servo Motors
- Laminated-winding DC servo motors with disc armatures: small rotor inertia, no cogging, large output torque.
- Wire-wound disc-type DC servo motors: disc armature with axial bonded magnet segments; small rotor inertia, superior control performance, high efficiency, large torque.
- Cup-armature permanent-magnet DC motors: hollow cup rotor with small inertia, suitable for incremental motion servo systems.
- Brushless DC servo motors: multi-phase stator windings and permanent-magnet rotor with rotor position sensors, no sparking, long life, and low noise.
3) Torque Motors
- DC torque motors: flat construction, many poles and commutator segments, large continuous torque at low speed or stall, good mechanical and regulation characteristics, small electromechanical time constant.
- Brushless DC torque motors: structurally similar to brushless DC servos but flat with many poles and conductors; large torque, good regulation, long life, no sparking, low noise.
- Cage-type AC torque motors: cage rotor, flat structure, many poles, large starting torque, small electromechanical time constant, can run stalled for long periods; mechanical characteristic is relatively soft.
- Solid-rotor AC torque motors: solid ferromagnetic rotor with flat structure, many poles, smooth operation under continuous stall, mechanical characteristic is relatively soft.
4) Stepper Motors
- Variable-reluctance stepper motors: stator and rotor made of stacked silicon steel; rotor has no windings and stator has control windings; small step angles, high start/run frequencies, lower step-angle accuracy, no detent torque.
- Permanent-magnet stepper motors: permanent-magnet rotor with radial magnetization; larger step angles, lower start/run frequencies, have holding torque, lower power consumption than variable-reluctance types but require bipolar drive currents.
- Hybrid stepper motors: permanent-magnet rotor with axial magnetization; high step-angle accuracy, holding torque, low input current, combining advantages of variable-reluctance and permanent-magnet types.
5) Switched Reluctance Motors
Both stator and rotor are made of stacked silicon steel with salient poles; structurally similar to large-step variable-reluctance stepper motors and equipped with rotor position sensing. Torque direction is independent of current direction. They have a limited speed range, high noise, and mechanical characteristics comprising constant-torque, constant-power, and series-excited regions.
6) Linear Motors
Simple structure where guides can serve as the secondary conductor, suitable for reciprocating linear motion with good high-speed servo performance, high power factor, high efficiency, and good constant-speed operation.
