Servo and motion control

Update:16 Oct 2020

servo motor
The word "servo" comes from the Greek meaning of "slave". "Servo motor" can be understood as a motor that absolutely obeys the command of the control signal: before the control signal is sent, the rotor is stationary; when the control signal is sent, the rotor turns immediately; when the control signal disappears, the rotor can stop immediately.
Servo motor is a micro motor used as an executive component in automatic control device. Its function is to convert electrical signal into angular displacement or angular velocity of rotating shaft.
Servo motor is divided into AC servo and DC servo
The basic structure of AC servo motor is similar to that of AC induction motor (asynchronous motor). There are two excitation winding WF and control winding wcowf with phase space displacement of 90 ° on the stator, which are connected with constant AC voltage. The AC voltage or phase change applied to WC is used to control the motor operation. AC servo motor has the characteristics of stable operation, good controllability, fast response, high sensitivity and strict nonlinear index of mechanical and regulating characteristics (requirements are less than 10% - 15% and less than 15% - 25%), etc.
The basic structure of DC servo motor is similar to general DC motor. Motor speed n = E / k1j = (UA IARA) / k1j, where e is armature back EMF, K is constant, j is flux of each pole, UA and IA are armature voltage and armature current, RA is armature resistance. The speed of DC servo motor can be controlled by changing UA or changing φ. However, in permanent magnet DC servo motor, excitation winding is replaced by permanent magnet, The magnetic flux is constant. DC servo motor has good linear regulation characteristics and fast time response.
Advantages and disadvantages of DC servo motor
Advantages: the speed control is accurate, the torque speed characteristic is very hard, the control principle is simple, the use is convenient, the price is cheap.
Disadvantages: Brush commutation, speed limit, additional resistance, wear particles (not suitable for dust-free and explosive environment).

Advantages and disadvantages of AC servo motor
Advantages: good speed control characteristics, smooth control in the whole speed range, almost no oscillation, over 90% high efficiency, less heating, high-speed control, high-precision position control (depending on encoder accuracy), constant torque, low inertia, low noise, no brush wear, maintenance free (suitable for dust-free and explosive environment)
Disadvantages: the control is complex, the driver parameters need to be adjusted on site, PID parameters are determined, and more wiring is required.
DC servo motor is divided into brush and brushless motor
The brush motor has the advantages of low cost, simple structure, large starting torque, wide speed range, easy control and maintenance, but it is easy to maintain (change carbon brush), produce electromagnetic interference, and have requirements on the use environment. It is usually used in general industrial and civil occasions which are sensitive to cost.
Brushless motor has the advantages of small volume, light weight, large output, fast response, high speed and small inertia, stable torque, smooth rotation, complex control, intelligent, flexible electronic commutation mode, maintenance free motor, high efficiency and energy saving, low electromagnetic radiation, low temperature rise and long service life, which is suitable for various environments.
AC servo motor is also brushless motor, which can be divided into synchronous and asynchronous motor. At present, synchronous motor is generally used in motion control. Its power range is large, the power can be large, the maximum speed is low, and the speed decreases with the increase of power. It is suitable for low-speed and stable operation
The internal rotor of the servo motor is a permanent magnet. The driver controls the U / V / W three-phase electricity to form an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor transmits the feedback signal to the driver, compares the feedback value with the target value, so as to adjust the rotation angle of the rotor. The accuracy of the servo motor depends on the accuracy (number of lines) of the encoder.
What are servo motors? How many types are there? What are the working characteristics?
Answer: servo motor also known as executive motor, in the automatic control system as an executive component, the received electrical signal into the motor shaft angular displacement or angular velocity output.
Servo motors are divided into DC and AC servo motors. Their main characteristics are that when the signal voltage is zero, there is no self rotation phenomenon, and the speed decreases with the increase of torque.

What is the difference between AC servo motor and brushless DC servo motor in performance
A: the performance of AC servo motor is better, because AC servo is controlled by sine wave with small torque ripple, while brushless DC servo is controlled by trapezoidal wave. But brushless DC servo control is relatively simple and cheap.
With the rapid development of permanent magnet AC servo drive technology, DC servo system is facing the crisis of being eliminated
Since the 1980s, with the development of integrated circuit, power electronics and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding development. Famous electrical manufacturers in various countries have successively launched new series products of AC servo motor and servo driver. AC servo system has become the main development direction of modern high-performance servo system, which makes DC servo system face the crisis of being eliminated.
Compared with DC servo motor, permanent magnet AC servo motor has the following advantages:
(1) no brush and commutator, more reliable operation and maintenance free.
(2) the heating of stator winding is greatly reduced.
(3) the inertia is small and the system has good response.
(4) high speed and high torque work well.
(5) small size and light weight under the same power.

Principle of servo motor
The stator structure of AC servo motor is basically similar to that of capacitor split phase single-phase asynchronous motor. The stator is equipped with two windings whose positions are 90 ° different from each other. One is the excitation winding RF, which is always connected to the AC voltage UF; the other is the control winding L, which is connected with the control signal voltage UC. So AC servo motor is also called two servo motors.
The rotor of AC servo motor is usually made of squirrel cage. However, in order to make the servo motor have a wide range of speed regulation, linear mechanical characteristics, no "rotation" phenomenon and fast response performance, compared with ordinary motor, it should have the characteristics of high rotor resistance and small moment of inertia. At present, there are two types of rotor structures: one is a squirrel cage rotor with high resistivity bars made of conductive materials with high resistivity; The other is the hollow cup-shaped rotor made of aluminum alloy. The cup wall is only 0.2-0.3mm. The hollow cup-shaped rotor has small moment of inertia, quick response and stable operation, so it is widely used.
When there is no control voltage in AC servo motor, there is only pulsating magnetic field produced by excitation winding in the stator, and the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. When the load is constant, the speed of the motor changes with the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of AC servo motor is similar to that of capacitor operated single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, there are three significant characteristics of AC servo motor compared with capacitor running asynchronous motor
1. Large starting torque: due to the high rotor resistance, the torque characteristics (mechanical characteristics) are closer to linear, and have larger starting torque. Therefore, as soon as the stator has the control voltage, the rotor will rotate immediately, that is, it has the characteristics of fast starting and high sensitivity.
2. Wide operation range: stable operation and low noise.
3. No self rotation phenomenon: as long as the control voltage is lost, the motor will stop running immediately.

What is called "precision drive micro special motor"?
"Precision drive micro special motor" can quickly and correctly execute the frequently changing commands in the system, and drive the servo mechanism to complete the expected work of the command. Most of them can meet the following requirements:
1. It can start, stop, brake, reverse and run at low speed frequently, with high mechanical strength, high heat resistance and high insulation level.
2. It has good fast response ability, large torque, small moment of inertia and small time constant.
3. With driver and controller (such as servo motor, stepper motor), the control performance is good.
4. High reliability, high precision.

Category, structure and performance of "precision drive micro special motor"
AC servo motor
(1) Cage type two-phase AC servo motor (slender cage rotor, approximate linear mechanical characteristics, small volume and excitation current, low power servo, low-speed operation is not smooth enough)
(2) Non magnetic cup rotor two phase AC servo motor (hollow cup rotor, approximate linear mechanical characteristics, large volume and excitation current, low power servo, smooth operation at low speed)
(3) Two phase AC servo motor with ferromagnetic cup rotor (ferromagnetic cup rotor, approximate linear mechanical characteristics, large rotor moment of inertia, small cogging effect and stable operation)
(4) Synchronous permanent magnet AC servo motor (composed of permanent magnet synchronous motor, speed measuring machine and position detection element coaxial integrated unit, stator is 3-phase or 2-phase, magnetic material rotor must be equipped with driver; wide speed range, mechanical characteristics consist of constant torque area and constant power area, continuous locked rotor, fast response, good performance, large output power and small torque fluctuation; There are two modes of square wave drive and sine wave drive, with good control performance, which are mechatronics products)
(5) Asynchronous three-phase AC servo motor (rotor is similar to cage type asynchronous motor and must be equipped with driver. Vector control is adopted to expand the range of constant power speed regulation, which is mostly used in machine tool spindle speed control system)
DC servo motor
(1) Printed winding DC servo motor (disk rotor and disc stator are bonded with columnar magnetic steel axially, with small rotor moment of inertia, no cogging effect, no saturation effect, and high output torque)
(2) Wire wound disc DC servo motor (disk rotor and stator are bonded with columnar magnetic steel axially, rotor moment of inertia is small, control performance is better than other DC servo motors, high efficiency and high output torque)
(3) Cup type armature permanent magnet DC motor (hollow cup rotor, small moment of inertia, suitable for incremental motion servo system)
(4) Brushless DC servo motor (stator is polyphase winding, rotor is permanent magnet, with rotor position sensor, no spark interference, long life, low noise)
torque motor
(1) DC torque motor (flat structure, number of poles, slots, commutators, series conductors, high output torque, continuous operation at low speed or locked rotor, good mechanical and regulating characteristics, small electromechanical time constant)
(2) Brushless DC torque motor (similar to BLDCM in structure, but flat, with many poles, slots and conductors in series; large output torque, good mechanical and regulating characteristics, long service life, no spark and low noise)
(3) Cage type AC torque motor (cage rotor, flat structure, many poles and slots, large starting torque, small electromechanical time constant, long-term locked rotor operation, soft mechanical characteristics)
(4) Solid rotor AC torque motor (ferromagnetic material solid rotor, flat structure, many poles and slots, long-term locked rotor, smooth operation, soft mechanical characteristics)
Stepper motor
(1) Reactive stepping motor (stator and rotor are made of silicon steel sheets, there is no winding on the rotor core, and there is control winding 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 stepping motor (permanent magnet rotor, radial magnetization polarity; large step angle, low starting and running frequency, holding torque, less power consumption than reactive type, but with positive and negative pulse current)
(3) Hybrid stepping motor (permanent magnet rotor, axial magnetization polarity; high precision of step angle, holding torque, small input current, advantages of both reactive and permanent magnet)
Switched reluctance motor (the stator and rotor are all made of silicon steel sheet, all are salient pole type, and the structure of the reaction stepping motor is similar to that of the pole number, with the rotor position sensor, the torque direction is independent of the current direction, the speed range is small, the noise is large, and the mechanical characteristics are composed of constant torque area, constant power area and series excitation characteristic area)
Linear motor (simple structure, guide rail can be used as secondary conductor, suitable for linear reciprocating motion; good high-speed servo performance, high power factor and efficiency, excellent constant speed operation performance).

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