A servo motor converts the input voltage signal ( i.e., control voltage ) into angular displacement or angular velocity output on its shaft. It is often used as an actuator in automatic control systems, hence the name "servo motor." Its most distinctive feature is that the rotor rotates immediately when control voltage is present and stops immediately when no control voltage is available. The direction and speed of the shaft rotation are determined by the direction and magnitude of the control voltage. Servo motors are broadly classified into AC and DC types.
I. AC Servo Motor
1. Basic Structure
An AC servo motor mainly consists of a stator and a rotor.
The stator core is typically made of laminated silicon steel sheets. Two-phase windings are embedded in the slots on the surface of the stator core: one phase is the excitation winding, and the other is the control winding. The two windings are spatially separated by 90° electrical degrees. During operation, the excitation winding f is connected to an AC excitation power supply, and the control winding k is supplied with a control signal voltage Uk .
It mainly consists of: stator 1 , rotor 5 , and detection element 8, etc.
2. Working principle
When there is no control voltage, an AC servo motor only has a pulsating magnetic field generated by the excitation winding in the air gap, and the rotor remains stationary due to the lack of starting torque. When a control voltage is applied and the control winding current and the excitation winding current are out of phase, a rotating magnetic field is generated in the air gap, producing electromagnetic torque and causing the rotor to rotate in the direction of the rotating magnetic field. However, servo motors are required not only to start under the influence of control voltage, but also to stop immediately after the voltage disappears. If a servo motor continues to rotate like a typical single-phase asynchronous motor after the control voltage disappears, a loss of control occurs; this phenomenon of self-rotation due to loss of control is called autorotation.
3. Control methods
Metaphysics suggests using the following three methods to control the speed and direction of rotation of a servo motor.
(1) Amplitude control keeps the phase difference between the control voltage and the excitation voltage constant, and only changes the amplitude of the control voltage.
(2) Phase control keeps the amplitude of the control voltage constant and only changes the phase difference between the control voltage and the excitation voltage.
(3) Amplitude - phase control changes both the amplitude and phase of the control voltage.
II. DC Servo Motor
1. Basic Structure
Traditional DC servo motors are essentially small-capacity ordinary DC motors, available in two types: separately excited and permanent magnet. Their structure is basically the same as that of ordinary DC motors.
The rotor of a cup-shaped armature DC servo motor is made of a hollow cup-shaped cylinder of non-magnetic material. The rotor is relatively light, resulting in low rotational inertia and fast response. The rotor rotates between inner and outer stators made of soft magnetic material, with a relatively large air gap.
Brushless DC servo motors replace traditional brushes and commutators with electronic commutation devices, making them more reliable. Their stator core structure is basically the same as that of ordinary DC motors, with multi-phase windings embedded on it, and the rotor is made of permanent magnet material.
2. Basic Working Principle
The basic working principle of a traditional DC servo motor is exactly the same as that of a regular DC motor. It relies on the interaction between the armature current and the air gap flux to generate electromagnetic torque, thus causing the servo motor to rotate. Armature control is typically used, meaning that the speed is adjusted by changing the armature voltage while keeping the excitation voltage constant. The lower the armature voltage, the lower the speed ; when the armature voltage is zero, the motor stops. Since the armature current is also zero when the armature voltage is zero, the motor does not generate electromagnetic torque and therefore does not " rotate . "
III. Differences between AC and DC servo motors
Disadvantages of DC servo motors:
a. Brushes and commutator are prone to wear, generating sparks during commutation and limiting speed.
b. Complex structure, difficult manufacturing, and high cost.
Advantages of AC servo motors:
c. Simple structure, low cost, and smaller rotor inertia than DC motors.
d. The capacity of an AC motor is greater than that of a DC motor.
I. Basic Requirements
1. High displacement accuracy
Displacement accuracy: refers to the degree of conformity between the displacement of the machine tool table required by the command pulse and the actual displacement of the table converted by the servo system from the command pulse.
2. Good stability
Stability: refers to the ability of a servo system to reach a new or return to its original equilibrium state after a short adjustment process under given input or external disturbance.
3. High positioning accuracy
Positioning accuracy: refers to the degree to which the output can accurately reproduce the input.
4. Good rapid response
5. Wide speed range
Speed range: refers to the ratio of the highest to the lowest speed that the motor can provide, as required by the mechanical device.
6. The system has good reliability.
7. High torque at low speed
II. Classification of Servo Systems
Classification by servo system adjustment theory
a . Open-loop servo system
b . Closed-loop servo system
c . Semi-closed-loop servo system
