Classification Of Servo Motors

A servo motor is an engine that controls the operation of a mechanical component in a servo system, and is an auxiliary motor indirect transmission.
The function of the servo motor is to convert the input voltage signal (ie, control voltage) into an angular displacement or angular velocity output on the shaft. It is often used as an actuator in an automatic control system. Therefore, the servo motor is also called an actuator motor. The biggest feature is: The rotor rotates immediately when the voltage is controlled, and the rotor stops immediately when there is no control voltage. The steering and speed of the shaft are determined by the direction and magnitude of the control voltage. Servo motors are divided into two major categories: AC and DC.

First, AC servo motor

1. basic structure

The AC servo motor is mainly composed of a stator and a rotor. The stator core is usually laminated with a silicon steel sheet. Two-phase windings are embedded in the slots on the surface of the stator core. One phase winding is a field winding, and the other phase winding is a control winding. The two-phase windings are 90 degrees apart from each other in spatial position. During operation, the excitation winding f is connected to the AC excitation power supply, and the control winding k is added to the control signal.

2. working principle

When there is no control voltage in the AC servo motor, only the pulsating magnetic field generated by the field winding is present in the air gap, and there is no starting torque on the rotor and it is stationary. When there is a control voltage and the control winding current and the field winding current are out of phase, a rotating magnetic field is generated in the air gap and electromagnetic torque is generated to rotate the rotor in the direction of the rotating magnetic field. However, the servo motor requirement can be started not only under the control voltage, but also after the voltage disappears, the motor should be able to stop immediately. If the servo motor control voltage disappears and continues to rotate like a normal single-phase asynchronous motor, there is a loss of control. We call this phenomenon of self-rotation due to loss of control.

In order to eliminate the self-rotation phenomenon of the AC servo motor, the rotor resistance r2 must be increased. This is because when the control voltage disappears, the servo motor is in a single-phase operation state. If the rotor resistance is large, the critical slip rate sm>1. The two torque characteristic curves and the combined torque characteristic curves generated by the positive and negative sequence rotating magnetic field and the rotor action are shown in the figure. As can be seen from the figure, the direction of the combined torque is opposite to the direction of rotation of the motor, which is a braking torque, which ensures that when the rotor is still rotating after the control voltage disappears, the motor will be quickly braked and stopped. When the rotor resistance is increased, not only can the rotation be eliminated, but also the advantages of widening the speed regulation range, improving the adjustment characteristics, and improving the reaction speed.

3. Control Method

The following three methods can be used to control the speed of the servo motor and the direction of rotation.

(1) Amplitude control Keep the phase difference between the control voltage and the excitation voltage constant, and only change the amplitude of the control voltage.

(2) Phase control Keep the amplitude of the control voltage constant, and only change the phase difference between the control voltage and the excitation voltage.

(3) Amplitude-phase control Simultaneously change the amplitude and phase of the control voltage.

Second, DC servo motor

1. basic structure

The traditional DC servo motor is essentially a normal DC motor with a small capacity. It has two types of excitation and permanent magnet, and its structure is basically the same as that of an ordinary DC motor.

The rotor of the cup-shaped armature DC servo motor is made of a hollow cup-shaped cylinder made of a non-magnetic material, and the rotor is light, so that the moment of inertia is small and the response is fast. The rotor rotates between the inner and outer stators made of soft magnetic material with a large air gap.

The brushless DC servo motor replaces the traditional brush and commutator with an electronic reversing device, making it more reliable. Its stator core structure is basically the same as that of a common DC motor, in which a multi-phase winding is embedded, and the rotor is made of a permanent magnet material.

2. Basic working principle

The basic working principle of the traditional DC servo motor is exactly the same as that of the ordinary DC motor. The electromagnetic torque is generated by the action of the armature current and the air gap flux to make the servo motor rotate. The armature control mode is usually adopted, that is, the rotation speed is adjusted by changing the armature voltage while keeping the excitation voltage constant. The smaller the armature voltage, the lower the rotational speed; when the armature voltage is zero, the motor stops. Since the armature current is zero when the armature voltage is zero, the motor does not generate electromagnetic torque, and there is no "rotation".

Third, the difference between AC and DC servo motor

Disadvantages of DC servo motors:

1. Brush and commutator are easy to wear, sparks when commutating, limit speed

2. Complex structure, difficult to manufacture, high cost

Advantages of AC servo motor:

1. Simple structure, low cost, and small rotor inertia compared to DC motors

2. The capacity of the AC motor is greater than that of the DC motor

Servo system performance requirements

First, the basic requirements

1. High displacement accuracy

Displacement accuracy: refers to the amount of displacement required by the command pulse on the machine table and the command pulse

The degree of compliance between the actual displacement of the servo system converted into a table

2. Good stability

Stability: It means that the servo system can be short-lived under the given input or external disturbance.

After the adjustment process, reach new or return to the original equilibrium state

3. High positioning accuracy

Positioning accuracy: refers to the accuracy of the output can reproduce the input

4. Fast response

5. Wide speed range

Speed range: refers to the ratio of the maximum speed and the minimum speed that the mechanical device requires from the motor.

6. System reliability is good

7. Low speed and high torque