Service definition
Servo: The word originates from the Greek word for "slave." People envisioned a "servo mechanism" as a docile tool, obeying the demands of control signals. Before a signal arrives, the rotor remains stationary; after the signal arrives, the rotor immediately begins to rotate; when the signal disappears, the rotor stops instantly. It is named for its "servo" performance.
A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value). The main task of a servo system is to amplify, transform, and regulate power according to the requirements of control commands, making the torque, speed, and position control of the drive device highly flexible and convenient.
Servo motor working principle
A servo motor, also known as an actuator motor, is used as an actuating element in automatic control systems. It converts received electrical signals into angular displacement or angular velocity output on the motor shaft. Its main characteristic is that it does not rotate when the signal voltage is zero, and its speed decreases uniformly as the torque increases.
A servo motor is a typical closed-loop feedback system. The reduction gear set is driven by the motor, and its terminal (output) drives a linear proportional potentiometer for position detection. The potentiometer converts the angular coordinates into a proportional voltage and feeds it back to the control circuit board. The control circuit board compares this voltage with the input control pulse signal, generates a correction pulse, and drives the motor to rotate in the forward or reverse direction, so that the output position of the gear set matches the desired value, causing the correction pulse to tend to 0, thereby achieving the purpose of precise positioning of the servo motor.
The rotor inside a servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder.
AC servo motor
The stator of an AC servo motor is basically similar in structure to that of a capacitor-split-phase single-phase asynchronous motor. Its stator has two windings positioned 90° apart: 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. Therefore, an AC servo motor is also called a dual-servo motor.
AC servo motors typically use squirrel-cage rotors. However, to achieve a wide speed range, linear mechanical characteristics, no "self-rotation," and rapid response, servo motors, compared to ordinary motors, should possess high rotor resistance and low moment of inertia. Currently, two common rotor structures are used: one is a squirrel-cage rotor with high-resistivity conductors made of high-resistivity conductive materials; to reduce moment of inertia, the rotor is made slender. The other is a hollow cup-shaped rotor made of aluminum alloy with very thin walls (only 0.2-0.3 mm). To reduce magnetic resistance, a fixed inner stator is placed inside the hollow cup-shaped rotor. The hollow cup-shaped rotor has very low moment of inertia, rapid response, and smooth operation, hence its widespread adoption.
When there is no control voltage, the stator of an AC servo motor only has a pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under constant load, the motor speed varies with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of an AC servo motor is similar to that of a split-phase single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, compared with a single-phase asynchronous motor, a servo motor has three significant characteristics:
1. High starting torque
Due to its high rotor resistance, its torque characteristic curve, as shown in curve 1 of Figure 3, differs significantly from the torque characteristic curve 2 of a typical asynchronous motor. This allows for a critical slip S0 > 1, making the torque characteristic (mechanical characteristic) closer to linear and providing a larger starting torque. Therefore, the rotor rotates immediately upon the application of control voltage to the stator, exhibiting characteristics of rapid starting and high sensitivity.
2. Wide operating range
3. No rotation phenomenon
A normally operating servo motor will immediately stop running if the control voltage is lost. When the servo motor loses the control voltage, it operates in a single-phase state. Due to the high rotor resistance, the two torque characteristics (T1-S1 and T2-S2 curves) and the resultant torque characteristic (TS curve) generated by the interaction of the two rotating magnetic fields rotating in opposite directions in the stator with the rotor are as follows.
The output power of an AC servo motor is typically 0.1-100W. When the power supply frequency is 50Hz, the voltage is available in 36V, 110V, 220V, and 380V; when the power supply frequency is 400Hz, the voltage is available in various types such as 20V, 26V, 36V, and 115V.
AC servo motors operate smoothly and with low noise. However, their control characteristics are nonlinear, and due to high rotor resistance, losses are high and efficiency is low. Therefore, compared with DC servo motors of the same capacity, they are larger and heavier, and are only suitable for low-power control systems of 0.5-100W.
