Do you know how servo motors work? Today, let me give you a systematic understanding of servo motors.
What is a servo motor?
The word "servo" originates from the Greek word for "slave." A "servo motor" can be understood as a motor that absolutely obeys the command of a control signal: before the control signal is issued, the rotor remains stationary; when the control signal is issued, the rotor immediately rotates; and when the control signal disappears, the rotor can stop rotating instantly.
A servo motor is a micro-motor used as an actuator in automatic control devices. Its function is to convert electrical signals into angular displacement or angular velocity of the rotating shaft. Also known as an actuator motor, a servo motor is used as an actuator in automatic control systems to convert received electrical signals into angular displacement or angular velocity output on the motor shaft.
Classification of servo motors
Servo motors are divided into two main categories: AC servo motors and DC servo motors.
The basic structure of an AC servo motor is similar to that of an AC induction motor (asynchronous motor). The stator has two excitation windings, Wf and Wco, with a phase spatial displacement of 90° electrical degrees. Both are connected to a constant AC voltage. The motor's operation is controlled by utilizing the AC voltage applied to Wc or changes in its phase.
AC servo motors are characterized by stable operation, good controllability, fast response, high sensitivity, and strict nonlinearity requirements for mechanical and adjustment characteristics (requiring less than 10% to 15% and less than 15% to 25%, respectively).
Advantages and disadvantages of DC servo motors
Advantages: Precise speed control, strong torque-speed characteristics, simple control principle, easy to use, and inexpensive.
Disadvantages: brush commutation, speed limitation, additional resistance, and generation of abrasive particles (not suitable for dust-free or explosive environments).
The basic structure of a DC servo motor is similar to that of a general DC motor. The motor speed n = E/K1j = (Ua - IaRa)/K1j, where E is the armature back electromotive force, K is a constant, j is the magnetic flux per pole, Ua and Ia are the armature voltage and armature current, and Ra is the armature resistance. Changing Ua or φ can control the speed of the DC servo motor, but generally, the armature voltage is controlled. In permanent magnet DC servo motors, the excitation winding is replaced by a permanent magnet, and the magnetic flux φ is constant. DC servo motors have excellent linear regulation characteristics and fast time response.
Advantages and disadvantages of AC servo motors
Advantages: Excellent speed control characteristics, smooth control across the entire speed range with almost no oscillation, high efficiency of over 90%, low heat generation, high-speed control, high-precision position control (depending on encoder accuracy), constant torque within the rated operating range, low inertia, low noise, no brush wear, and maintenance-free (suitable for dust-free and explosive environments).
Disadvantages: The control is relatively complex, the driver parameters need to be adjusted on-site to determine the PID parameters, and more wiring is required.
DC servo motors are divided into brushed motors and brushless motors.
Brushed motors are low in cost, simple in structure, have high starting torque, wide speed range, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes). They generate electromagnetic interference and have requirements for the operating environment. They are usually used in general industrial and civilian applications where cost is sensitive.
Brushless motors are small in size and light in weight, with high output, fast response, high speed and low inertia, stable torque and smooth rotation. They are complex and intelligent in control, with flexible electronic commutation methods that can use square wave or sine wave commutation. The motors are maintenance-free, highly efficient and energy-saving, with low electromagnetic radiation, low temperature rise and long life, and are suitable for various environments.
AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range, can achieve very high power, have large inertia, low maximum speed, and their speed decreases uniformly as the power increases. They are suitable for low-speed and stable operation.
The rotor inside the servo motor is a permanent magnet. The driver controls the three-phase U/V/W electricity to form an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder built into the motor transmits feedback signals to the driver. The feedback value is compared with the target value, thereby adjusting the rotation angle of the rotor. The accuracy of the servo motor depends on the accuracy (line count) of the encoder.
What are the performance differences between AC servo motors and brushless DC servo motors?
AC servo motors generally offer better performance because they use sinusoidal wave control, resulting in less torque ripple; while brushless DC servos use trapezoidal wave control. However, brushless DC servos are simpler and cheaper to control.
The rapid development of permanent magnet AC servo drive technology has brought DC servo systems to the brink of obsolescence. Since the 1980s, with the development of integrated circuits, power electronics technology, and AC variable speed drive technology, permanent magnet AC servo drive technology has seen remarkable progress. Leading electrical manufacturers worldwide have continuously launched new AC servo motors and servo drivers. AC servo systems have become the main development direction for contemporary high-performance servo systems, putting DC servo systems at risk of obsolescence.
Compared with DC servo motors, permanent magnet AC servo motors have the following main advantages:
(1) No brushes or commutator, more reliable operation, maintenance-free.
(2) Stator winding heating is greatly reduced.
(3) Small inertia, resulting in good system response.
(4) It has good working condition at high speed and high torque.
(5) Small size and light weight for the same power.
Servo motor working principle
The stator structure of an AC servo motor is basically similar to that of a capacitor-split-phase single-phase asynchronous motor. Its stator has two windings positioned 90° apart: one is the excitation winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, connected to the control signal voltage Uc. Therefore, an AC servo motor is also called a dual-servo motor.
The rotor of an AC servo motor is usually made into a squirrel-cage type. However, in order to make the servo motor have a wide speed range, linear mechanical characteristics, no "self-rotation" phenomenon and fast response performance, it should have the characteristics of high rotor resistance and low rotational inertia compared with ordinary motors.
Currently, there are two main types of rotor structures: one is a squirrel-cage rotor made of high-resistivity conductive material with high-resistivity conductor bars, which is made slender to reduce the rotor's moment of inertia; the other is a hollow cup-shaped rotor made of aluminum alloy with a cup wall of only 0.2~0.3mm. The hollow cup-shaped rotor has a very small moment of inertia, responds quickly, and runs smoothly, so it is widely used.
When there is no control voltage, the stator of an AC servo motor only has a pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under constant load, the motor speed varies with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of an AC servo motor is similar to that of a capacitor-run single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, compared with a capacitor-run asynchronous motor, a servo motor has two significant characteristics:
1. High starting torque: Due to the high rotor resistance, the torque characteristic (mechanical characteristic) is closer to linear, and it has a large starting torque. Therefore, when the stator is given a control voltage, the rotor rotates immediately, which means it has the characteristics of fast start-up and high sensitivity.
2. Wide operating range: Smooth operation and low noise.
3. No self-rotation phenomenon: The servo motor will stop running immediately if the control voltage is lost.
Precision drive micro motor
"Precision drive micro motors" can quickly and accurately execute frequently changing commands in a system, driving servo mechanisms to complete the work expected by the commands, and most of them can meet the following requirements:
1. It can frequently start, stop, brake, reverse, and operate at low speeds, and has high mechanical strength, high heat resistance, and high insulation.
2. It has good rapid response capability, large torque, small moment of inertia, and small time constant.
3. Equipped with a driver and controller (such as a servo motor or stepper motor), providing excellent control performance.
4. High reliability and high precision.
The following is a comparison of the types, structures, and performance of precision drive micro motors:
AC servo motor
(1) Squirrel-cage two-phase AC servo motor (slender squirrel-cage rotor, mechanical characteristics are approximately linear, small size and excitation current, low power servo, low-speed operation is not smooth enough).
(2) Non-magnetic cup-shaped rotor two-phase AC servo motor (hollow cup rotor, mechanical characteristics are approximately linear, large size and excitation current, low power servo, smooth low-speed operation).
(3) Ferromagnetic cup-shaped rotor two-phase AC servo motor (ferromagnetic material cup-shaped rotor, mechanical characteristics are approximately linear, rotor rotational inertia is large, cogging effect is small, and operation is stable).
(4) Synchronous permanent magnet AC servo motor (a coaxial integrated unit consisting of a permanent magnet synchronous motor, a tachometer and a position detection element, with a 3-phase or 2-phase stator, a magnetic material rotor, and must be equipped with a driver; it has a wide speed range, mechanical characteristics consisting of a constant torque zone and a constant power zone, can be continuously stalled, has good fast response performance, high output power and low torque fluctuation; it has two driving modes, square wave drive and sine wave drive, with good control performance, and is an electromechanical integrated product).
(5) Asynchronous three-phase AC servo motor (the rotor is similar to that of a squirrel-cage asynchronous motor, and a driver must be provided. Vector control is adopted, which expands the constant power speed regulation range. It is mostly used in machine tool spindle speed regulation systems).
DC servo motor
(1) Printed winding DC servo motor (disc rotor, disc stator with axially bonded columnar magnets, small rotor moment of inertia, no cogging effect, no saturation effect, and large output torque).
(2) Wire-wound disc DC servo motor (disc rotor, stator axially bonded columnar magnets, small rotor moment of inertia, better control performance than other DC servo motors, high efficiency, large output torque).
(3) Cup-shaped armature permanent magnet DC motor (hollow cup rotor, small rotor moment of inertia, suitable for incremental motion servo system).
(4) Brushless DC servo motor (stator is multi-phase winding, rotor is permanent magnet type, with rotor position sensor, no spark interference, long life and low noise).
Torque motor
(1) DC torque motor (flat structure, many poles, slots, commutator segments, and series conductors; large output torque, can work continuously at low speed or under stall conditions, good mechanical and regulating characteristics, and small electromechanical time constant).
(2) Brushless DC torque motor (similar in structure to brushless DC servo motor, but flat, with more poles, slots, and series conductors; large output torque, good mechanical and regulating characteristics, long life, no sparks, and low noise).
(3) Squirrel-cage AC torque motor (squirrel-cage rotor, flat structure, many poles and slots, large starting torque, small electromechanical time constant, can run in stalled operation for a long time, and has relatively soft mechanical characteristics).
(4) Solid rotor AC torque motor (ferromagnetic material solid rotor, flat structure, many poles and slots, can be stalled for a long time, runs smoothly, and has relatively soft mechanical characteristics).
Stepper motor
(1) Reactive stepper motor (both the stator and rotor are made of stacked silicon steel sheets, there are no windings on the rotor core, and there are control windings on the stator; the step angle is small, the starting and running frequency is high, the step angle accuracy is low, and there is no self-locking torque).
(2) Permanent magnet stepper motor (permanent magnet rotor, radial magnetization polarity; large step angle, low starting and running frequency, has holding torque, consumes less power than reactive stepper motor, but requires positive and negative pulse current).
(3) Hybrid stepper motor (permanent magnet rotor, axial magnetization polarity; high step angle accuracy, holding torque, low input current, combining the advantages of both reactive and permanent magnet types).
Switched reluctance motors (both stator and rotor are made of stacked silicon steel sheets, both are salient pole type, similar in structure to large-step reactive stepper motors with a number of poles, equipped with rotor position sensors, torque direction is independent of current direction, small speed range, high noise, and mechanical characteristics consist of three parts: constant torque region, constant power region, and series excitation characteristic region).
Linear motors (simple structure, guide rails, etc. can be used as secondary conductors, suitable for linear reciprocating motion; good high-speed servo performance, high power factor and efficiency, and excellent constant speed operation performance).
