A servo motor is an engine that controls the operation of mechanical components in a servo system; it is a type of auxiliary motor with indirect speed change.
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
AC servo motors mainly consist 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.
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.
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 operates in a single-phase state. If the rotor resistance is very high, the critical slip sm > 1. The two torque characteristic curves generated by the interaction of the positive and negative sequence rotating magnetic fields with the rotor, as well as the resultant torque characteristic curve, are shown in the figure. As can be seen from the figure, the direction of the resultant torque is opposite to the direction of motor rotation; it is a braking torque. This ensures that when the rotor continues to rotate after the control voltage disappears, the motor will be quickly braked and stopped. Increasing the rotor resistance not only eliminates self-rotation but also has advantages such as expanding the speed range, improving regulation characteristics, and increasing response speed.
3. Control methods
The following three methods can be used 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 the cup-shaped armature DC servo motor is made of a hollow cup-shaped cylinder of non-magnetic material. The rotor is lighter, resulting in a smaller moment of inertia and faster response. The rotor rotates between inner and outer stators made of soft magnetic material, with a relatively large air gap.
Brushless DC servo motors use electronic commutation devices instead of traditional brushes and commutators, 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 on its own."
III. Differences between AC and DC servo motors
Disadvantages of DC servo motors:
Brushes and commutators are prone to wear, generating sparks during commutation and limiting the rotational speed.
Complex structure, difficult to manufacture, and high cost
Advantages of AC servo motors:
It has a simple structure, low cost, and smaller rotor inertia than a DC motor.
AC motors have a larger capacity than DC motors.
Performance requirements of servo systems
1. High displacement accuracy
Displacement accuracy: refers to the distance the command pulse requires from the displacement of the machine tool table.
The difference between the punching servo system and the actual displacement of the worktable
degree of conformity
2. Good stability
Stability: refers to the ability of a servo system to maintain stability under given input or external disturbances for a short period of time.
After the adjustment process, a new equilibrium state is reached or restored to the original state.
3. High positioning accuracy
Positioning accuracy: refers to the degree to which the output can accurately reproduce the input.
4. Good fast 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
