The role of the servo motor is to convert the input voltage signal (ie, the 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. Its main 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 field winding f is connected to the AC excitation power source, and the control winding k is added with the control signal voltage Uk.
Mainly composed of: stator 1, rotor 5 and detection element 8 and other parts.
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 requirements can be started not only under the control voltage, but also after the voltage disappears, the motor should stop immediately. If the servo motor control voltage disappears and continues to rotate like a normal single-phase asynchronous motor, a runaway phenomenon occurs. We call this phenomenon of self-rotation due to loss of control.
3. Control method
Metaphysics can use the following three methods to control the speed and direction of rotation of the servo motor.
(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 changes 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 the 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 conventional 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 the 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 motor:
a. Brushes and commutators are prone to wear, sparks during commutation, limit speed
b. Complex structure, difficult to manufacture, high cost
Advantages of AC servo motor:
c. Simple structure, low cost, and small rotor inertia compared to DC motors
d. 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 degree of compliance between the command pulse and the displacement of the machine table and the actual displacement of the command pulse converted into the table by the servo system.
2, good stability
Stability: The servo system can reach new or return to the original equilibrium state after a short adjustment process under the given input or external disturbance.
3, high positioning accuracy
Positioning accuracy: refers to the accuracy of the output that can reproduce the input
4, fast response is good
5, wide speed range
Speed regulation range: refers to the ratio of the maximum speed and the minimum speed that the mechanical device requires the motor to provide.
6, the system reliability is good
7, low speed and large torque
Second, the classification of the servo system
According to the servo system adjustment theory classification
a, Open loop servo system
b, Closed loop servo system
c, Semi-closed loop servo system
