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.
Servo motors are micro motors used as actuators in automatic control devices. Their function is to convert electrical signals into angular displacement or angular velocity of the rotating shaft.
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). It has two excitation windings, Wf and Wco, on the stator, 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 requirements for the nonlinearity of their mechanical and adjustment characteristics (requiring less than 10%–15% and less than 15%–25%, respectively).
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 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, generation of abrasive 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 virtually no oscillation, high efficiency exceeding 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 clean 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, stable operation applications.
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 is a servo motor? What types are there? What are its working characteristics?
A: Servo motors, also known as actuator motors, are used as actuators in automatic control systems to convert received electrical signals into angular displacement or angular velocity output on the motor shaft.
Servo motors are divided into two main categories: DC servo motors and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
What are the performance differences between AC servo motors and brushless DC servo motors?
A: AC servo motors have 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) Low inertia, resulting in fast system response.
(4) It performs well under high-speed and high-torque conditions.
(5) Small size and light weight for the same power.
The Rise and Current Status of Permanent Magnet AC Servo Systems
Since the Indramat division of Rexroth GmbH of MANNESMANN in Germany officially launched the MAC permanent magnet AC servo motor drive system at the 1978 World Trade Fair, it marked the maturity of a new generation of AC servo technology. By the mid-to-late 1980s, major companies had complete product lines, and the entire servo device market shifted to AC systems. Early analog systems had shortcomings in areas such as zero drift, interference immunity, reliability, accuracy, and flexibility, and could not fully meet the requirements of motion control. In recent years, with the application of microprocessors and new digital signal processors (DSPs), digital control systems have emerged, and the control part can be completed by software. Since the 1990s, the status of fully digital sine wave controlled permanent magnet AC servo motor drive systems in the transmission field has further risen.
Currently, most high-performance electric servo systems use permanent magnet synchronous AC servo motors, and the control drivers mostly employ fully digital position servo systems for fast and accurate positioning. Typical manufacturers include Siemens (Germany), Kollmorgen (USA), and Panasonic and Yaskawa (Japan).
Overview of major manufacturers of permanent magnet AC servo systems
Yaskawa Electric Works of Japan introduced small AC servo motors and drives. The D series is suitable for CNC machine tools (maximum speed 1000 r/min, torque 0.25–2.8 Nm), while the R series is suitable for robots (maximum speed 3000 r/min, torque 0.016–0.16 Nm). Later, six more series—M, F, S, H, C, and G—were introduced. In the 1990s, new D and R series were launched. The old series, driven by a rectangular wave 8051 microcontroller, was replaced with a sine wave driven 80C, 154 CPU, and gate array chip control, reducing torque ripple from 24% to 7% and improving reliability. In just a few years, a relatively complete system of eight series (power range 0.05–6 kW) was formed, meeting the diverse needs of machine tools, handling mechanisms, welding robots, assembly robots, electronic components, processing machinery, printing presses, high-speed winding machines, and wire winding machines.
Fanuc, a Japanese company renowned for manufacturing CNC machine tool systems, also launched its S-series (13 models) and L-series (5 models) permanent magnet AC servo motors in the mid-1980s. The L-series, with its smaller moment of inertia and mechanical time constant, is suitable for position servo systems requiring exceptionally fast response.
Other Japanese manufacturers, such as Mitsubishi Electric (*****, HC-MFS, *****, HC-RFS and HC-UFS series), Toshiba Seiki (SM series), Okuma Steel Works (BL series), Sanyo Electric (BL series), and Tateishi Electric (S series), have also entered the competition for permanent magnet AC servo systems. [p=30. 2. left] Rexroth's Indramat division in Germany offers the MAC series of AC servo motors, which come in 7 frame sizes and 92 specifications.
The Siemens IFT5 series of three-phase permanent magnet AC servo motors are divided into two main categories: standard and short models, with a total of 8 frame sizes and 98 specifications. It is claimed that this series of AC servo motors weighs only half as much as the IHU series of DC servo motors with the same output torque. The matching 6SC61 series transistor pulse width modulation drivers can control up to six axes of motors.
Bosch GmbH of Germany manufactures AC servo motors in the SD series (17 specifications) of ferrite permanent magnets and the SE series (8 specifications) of rare earth permanent magnets, as well as drive controllers in the Servodyn SM series.
Gettys, a well-known American servo device manufacturer, was once a division of Gould Electronics (Motion Control Division), producing the M600 series AC servo motors and the A600 series servo drives. Later, it merged into AEG, resumed the Gettys name, and launched the A700 fully digital AC servo system.
The American company AB (ALLEN-BRADLEY) Drives division produces the 1326 ferrite permanent magnet AC servo motor and the 1391 AC PWM servo controller. The motors include 3 frame sizes and a total of 30 specifications.
Industrial Drives (ID) is the industrial drive division of the renowned American company Kollmorgen. It previously produced brushless servo motors in three series (BR-210, BR-310, and BR-510) with a total of 41 specifications, as well as BDS3 servo drives. Since 1989, it has introduced a completely new series of permanent magnet AC servo motors (Goldline), including three main categories: B (low inertia), M (medium inertia), and EB (explosion-proof). These are available in five frame sizes (10, 20, 40, 60, and 80), with 42 specifications in each category. All motors use neodymium iron boron permanent magnets, with torque ranging from 0.84 to 111.2 Nm and power ranging from 0.54 to 15.7 kW. Compatible drives include the BDS4 (analog), BDS5 (digital, with position control), and Smart Drive (digital) series, with a maximum continuous current of 55 A. The Goldline series represents the latest advancement in contemporary permanent magnet AC servo technology.
Inland, Ireland, formerly an overseas division of Kollmorgen, is now part of AEG and is renowned for producing DC servo motors, DC torque motors, and servo amplifiers. It manufactures 17 models of SmCo permanent magnet AC servo motors in three frame sizes (BHT1100, 2200, and 3300) and eight types of controllers.
The French Alsthom Group manufactures 14 specifications of LC series (long type) and GC series (short type) AC servo motors at its Parvex factory in Paris, and also produces AXODYN series drives.
The former Soviet Union developed two series of AC servo motors for servo control of CNC machine tools and robots. The ДBy series uses ferrite permanent magnets, has two frame sizes, each with three core lengths and two winding configurations, totaling 12 specifications, with a continuous torque range of 7–35 N·m. The 2ДBy series uses rare-earth permanent magnets, has six frame sizes and 17 specifications, with a torque range of 0.1–170 N·m, and is paired with a 3ДБ type controller.
In recent years, Panasonic has launched the all-digital MINAS series of AC servo systems. Among them, the permanent magnet AC servo motors include the MSMA series of small inertia type, with power ranging from 0.03 to 5kW, totaling 18 specifications; the medium inertia type includes three series: MDMA, MGMA, and MFMA, with power ranging from 0.75 to 4.5kW, totaling 23 specifications; and the MHMA series of large inertia motors, with power ranging from 0.5 to 5kW, with 7 specifications.
Samsung Corporation of South Korea has recently developed a range of fully digital permanent magnet AC servo motors and drive systems. Among them, the FAGA AC servo motor series includes various models such as CSM, CSMG, CSMZ, CSMD, CSMF, CSMS, CSMH, CSMN, and CSMX, with power ranging from 15W to 5kW.
The power rate is now commonly used as a quality factor for servo motors to measure and compare the dynamic response performance of various AC/DC servo motors and stepper motors. The power rate represents the ratio of the motor's continuous (rated) torque to the rotor's moment of inertia.
Analysis based on power change rate shows that the Goldline series from ID.com (USA) has the best technical specifications for permanent magnet AC servo motors, followed by the IFT5 series from Siemens (Germany).
Servo motor principle
I. AC Servo Motor
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.
AC servo motors typically use squirrel-cage rotors. However, to achieve a wide speed range, linear mechanical characteristics, no "self-rotation," and rapid response, servo motors, compared to ordinary motors, should possess high rotor resistance and low moment of inertia. Currently, two rotor structures are commonly used: one is a squirrel-cage rotor with high-resistivity conductors made of high-resistivity conductive materials, where the rotor is made slender to reduce moment of inertia; the other is a hollow cup-shaped rotor made of aluminum alloy, with a cup wall thickness of only 0.2-0.3 mm. The hollow cup-shaped rotor has very low moment of inertia, responds quickly, and operates smoothly, thus it is widely adopted.
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 greater than that of the latter. Therefore, compared with a capacitor-run asynchronous motor, a servo motor has three 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: The servo motor stops immediately if the control voltage is lost.
What is a "precision transmission 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 run 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.
Categories, structures, and performance of "precision drive micro motors"
1. AC servo motor
(1) Squirrel-cage two-phase AC servo motor (slender squirrel-cage rotor, near-linear mechanical characteristics, small size and excitation current, low-power servo, and not smooth enough low-speed operation)
(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 smooth)
(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; the stator is 3-phase or 2-phase; the rotor is made of magnetic material; a driver must be provided; it has a wide speed range, mechanical characteristics consisting of a constant torque region and a constant power region, can be continuously stalled, has good fast response performance, high output power, and low torque fluctuation; it has two driving modes, square wave and sine wave, 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 used, which expands the constant power speed regulation range. It is mostly used in machine tool spindle speed regulation systems)
2. 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, 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, and large output torque)
(3) Cup-shaped armature permanent magnet DC motor (hollow cup rotor, small rotor moment of inertia, suitable for incremental motion servo systems)
(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)
3. 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 stall mode for a long time, and has relatively soft mechanical characteristics)
(4) Solid rotor AC torque motor (solid rotor made of ferromagnetic material, flat structure, many poles and slots, can be stalled for a long time, runs smoothly, and has relatively soft mechanical characteristics)
4. 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, holding torque, less power consumption 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)
5. Switched reluctance motor (both stator and rotor are made of stacked silicon steel sheets, both are salient pole type, similar in structure to large-step reactive stepper motor with a similar number of poles, equipped with rotor position sensor, torque direction is independent of current direction, small speed range, high noise, mechanical characteristics consist of three parts: constant torque region, constant power region, and series excitation characteristic region)
6. Linear motor (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)
