Introduction of servo motor types

The word servo comes from the Greek word for slave. "Servo motor" can be understood as a motor that absolutely obeys the command of the control signal: before the control signal is sent out, the rotor is stationary; When the control signal is sent out, the rotor rotates immediately; When the control signal is lost, the rotor can stop immediately.

Servo motor is a micro motor used as the actuator in the automatic control device. Its function is to convert the electrical signal into the angular displacement or angular velocity of the rotating shaft.

Servo motor is divided into AC servo and DC servo two categories

The basic structure of AC servo motor is similar to that of AC induction motor (asynchronous motor). The excitation winding Wf and the control winding WcoWf, which have two phase-space displacement angles of 90° on the stator, are connected with a constant AC voltage. By using the change of AC voltage or phase applied to the Wc, the purpose of controlling the motor operation is achieved. Ac servo motor has the characteristics of stable operation, good controllability, fast response, high sensitivity, and strict nonlinearity index of mechanical characteristics and adjusting characteristics (less than 10% ~ 15% and less than 15% ~ 25%, respectively).

The basic structure of DC servo motor is similar to that of general DC motor. Motor speed n=E/K1j=(UA-Iara)/K1j, where E is armature reverse electromotive force, K is constant, j is magnetic flux per pole, Ua and Ia are armature voltage and armature current, Ra is armature resistance. Changing Ua or φ can control the speed of DC servo motor, but the method of controlling armature voltage is generally adopted. In the permanent magnet DC servo motor, the field winding is replaced by a permanent magnet, and the magnetic flux φ is constant. Dc servo motor has good linear adjustment characteristics and fast time response.

Advantages and disadvantages of DC servo motor

Advantages: precise speed control, torque speed characteristics is very hard, simple control principle, easy to use, cheap price.

Disadvantages: brush reversing, speed limit, additional resistance, wear particles (not suitable for dust-free explosive environment)

The advantages and disadvantages of AC servo motor

Advantages: Good speed control characteristics, smooth control can be achieved in the whole speed zone, almost no oscillation, high efficiency of more than 90%, less heat, high speed control, high precision position control (depending on the accuracy of the encoder), in the rated operation area, can achieve constant torque, low inertia, low noise, no brush wear, no maintenance (suitable for dust-free, explosive environment)

Disadvantages: The control is more complex, the driver parameters need to adjust the PID parameters on site, need more wiring.

Dc servo motors are divided into brushless and brushless motors

Brush motor has the advantages of low cost, simple structure, large starting torque, wide speed regulating range, easy control, the need for maintenance, but convenient maintenance (carbon brush), electromagnetic interference, requirements for the use of the environment, and is usually used for cost sensitive common industrial and civil occasions.

Brushless motor is small in size, light in weight, large in output, fast in response, high in speed, small in inertia, stable in torque, smooth in rotation, complex in control, intelligent, flexible in electronic commutation mode, can be square wave or sine wave commutation, the motor is maintenance-free, efficient in energy saving, small in electromagnetic radiation, low in temperature rise and long in life, suitable for all kinds of environment.

Ac servo motor is also brushless motor, which is divided into synchronous and asynchronous motors. At present, synchronous motors are generally used in motion control. Its power range is large, the power can be very large, the maximum speed is low, the speed decreases evenly with the increase of power, and it is suitable for low speed and smooth operation

The rotor inside the servo motor is a permanent magnet, and the driver controls the U/V/W three-phase electric field to form an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor sends the feedback signal to the driver, and compares the feedback value with the target value, so as to adjust the rotor rotation Angle.

Servo motor principle

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 with a position difference of 90°, 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. So AC servo motors are also called two servo motors.

The rotor of AC servo motor is usually made of squirrel cage type, but in order to make the servo motor has a wide speed range, linear mechanical characteristics, no "rotation" phenomenon and fast response performance, compared with the ordinary motor, it should have the rotor resistance and the moment of inertia is small these two characteristics. At present, there are two kinds of rotor structures widely used: one is the squirrel cage rotor with high resistivity guide strip made of conductive material with high resistivity. In order to reduce the moment of inertia of the rotor, the rotor is made slender; The other is the hollow cup rotor made of aluminum alloy, the cup wall is only 0.2-0.3mm, the hollow cup rotor has a small moment of inertia, rapid reaction, and smooth operation, so it is widely used.

When the AC servo motor has no control voltage, the stator only has the pulsating magnetic field generated by the excitation windings and the rotor is stationary. When there is a control voltage, the stator will produce a rotating magnetic field, the rotor rotates along the direction of the rotating magnetic field, in the case of constant load, the motor speed with the size of the control voltage changes, 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 running single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, compared with capacitor running single-phase asynchronous motor, the servo motor has three significant characteristics:

1. Large starting torque: because the rotor resistance is large, the torque characteristics (mechanical characteristics) are closer to linear, and it has a large starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, that is, it has the characteristics of fast start and high sensitivity.

2, wide operating range: smooth operation, low noise.

3. No rotation phenomenon: as long as the servo motor in operation loses the control voltage, the motor will stop running immediately.

Ac servo motor

(1) Cage type two-phase AC servo motor (slender cage rotor, nearly linear mechanical characteristics, small volume and excitation current, low power servo, low speed operation is not smooth)

(2) Non-magnetic cup-rotor two-phase AC servo motor (hollow cup-rotor, approximately linear mechanical characteristics, large volume and excitation current, low-power servo, smooth running at low speed)

(3) Ferromagnetic cup-rotor two-phase AC servo motor (ferromagnetic cup-rotor with approximately linear mechanical properties, large rotor moment of inertia, small tooth and groove effect, stable operation)

(4) Synchronous permanent magnet AC servo motor (consisting of permanent magnet synchronous motor, speed measuring machine and position detection element coaxial unit, stator is 3 phase or 2 phase, magnetic material rotor, must be equipped with a driver; Wide speed range, mechanical characteristics by constant torque zone and constant power zone, continuous blocking, rapid corresponding performance is good, large output power, small torque fluctuation; Square wave drive and sine wave drive two ways, good control performance, for electromechanical integration products)

(5) asynchronous three-phase AC servo motor (rotor and cage asynchronous motor similar, must be equipped with a driver, vector control, expand the constant power speed range, mostly used in machine tool spindle speed control system)

Dc servo motor

(1) Printed winding DC servo motor (disk-shaped rotor, disk-shaped stator axially bonded cylindrical magnetic steel, small rotor moment of inertia, no groove effect, no saturation effect, large output torque)

(2) wire-wound DC servo motor (disk-shaped rotor and stator axially bonded cylindrical magnetic steel, rotor inertia is small, control performance is better than other DC servo motors, high efficiency, large output torque)

(3) Cup-type 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 polyphase winding, rotor is permanent magnet, rotor position sensor, no spark interference, long service life, low noise)