AC servo motor structure and control principle

Like with ordinary motors, AC servo motor also constituted by the stator and rotor. There are two stator windings, the excitation winding and control winding, the two windings in the space difference between the 90 ° electric angle. Servo motor is a permanent magnet inside the rotor, drive gS control u / V / W three-phase power to form the rotor electromagnetic fields under the influence of this magnetic field rotation, while built-in encoder motor feedback signals to the drive, the drive values and goals based on feedback to compare the value to adjust the angle of the rotor rotation. Determined by the precision of servo motor encoder accuracy (line number).

The working principle of AC servo motors and single-phase induction motor is essentially no difference. However, the AC servo motor must have a performance AC servo motor is able to overcome the so-called "rotation" phenomenon, that is, no control signal, it should not be turning, especially when it has been in the rotation, if the control signal disappears, it should be immediately stop turning. Ordinary induction motor rotation up later, such as the control signal disappear, often continued to rotate.

When the motor is still original state, such as the control winding voltage is uncontrolled, this time only the field winding magnetic field pulse generated electricity. Pulsating magnetic field can be regarded as two circular rotating magnetic field. The two circular rotating magnetic field with the same size and speed, in the opposite direction of rotation, the establishment of the positive and reverse rotating magnetic field were cut cage winding (or cup-shaped wall), and sensor out of the same size and phase reverse electromotive force and the current (or eddy current), these currents were generated magnetic field with the respective moments are also equal and opposite, synthetic torque is zero, servo motor rotor to switch it up. Once the control system error signal, the control winding must accept corresponding to the control voltage. Under normal circumstances, the motor internally generated magnetic field is oval-shaped rotating magnetic field. An elliptical rotating magnetic field can be seen as synthesized by the two circular rotating magnetic field together. The two circular rotating magnetic field amplitude range (with the original elliptical rotating magnetic field to switch the magnetic field is being turned to the same large, with the original turn in the opposite reversal of a small magnetic field), but at the same speed, to rotate in the opposite direction. They cut the rotor winding induction of electric potential and current, as well as the electromagnetic torque is also generated in the opposite direction, size range (positive turn have a large, inverted are small) synthesis of torque is not zero, so the servo motor is transferred toward the direction of the magnetic field rotation up, with the signal strength, the magnetic field near the circle, this time is to switch the magnetic field and the torque increases, reversing the magnetic field and the torque decreases, synthetic torque larger, such as the load torque constant, rotor speed has increased. If you change the control voltage of the phase, that is, phase-shifted 180o, turn in the opposite rotating magnetic field, resulting in the opposite direction of the synthesis of torque, servo motor will be reversed. If the control signal disappeared, and only pass into the field winding current, servo motor will be a pulsating magnetic field generated by magnetic field, the rotor quickly stopped.

In order to have the AC servo motor control signal disappeared immediately stop rotating features, it's the rotor resistance are particularly large, it's critical slip Sk greater than 1. The motor is running, if the control signal reduced to "zero", excitation current still exists, generate a pulse of air gap magnetic field, this pulsating magnetic field can be regarded as forward and reverse rotating magnetic field rotating magnetic field synthesis. Once the control signal disappeared, air-gap magnetic field into a pulsating magnetic field, which can be regarded as forward and reverse rotating magnetic field rotating magnetic field of synthetic motor that is run by synthetic characteristic curve. As the rotor inertia, operating point moves from the A-point B-point, this time produced a motor rotating in the opposite direction with the original rotor braking torque. The load torque and braking torque rapidly under the action of the rotor to stop.



Must be pointed out that ordinary two-phase and three-phase induction motor under normal circumstances are the symmetrical mode of operation, asymmetric operation is of a fail state. The AC servo motor can be run by different levels of asymmetry to achieve control purposes. This is the AC servo motor is running with the fundamental difference between ordinary induction motor.



On the servo drive's response rate, the smallest amount of torque mode operation, the drive for the fastest response to control signals; position mode operation the largest, the drive control signals in response to the slowest.

The dynamic performance of the movement has a relatively high demand, we need time to adjust the motor. So, if the controller itself, the computing speed is very slow (eg, PLC, or low-end motion controller), on the location of control of use. If the controller calculation speed is faster, you can use the speed mode, the position loop controller from the drive to reduce the workload of the drive to improve efficiency (such as most of the high-end motion controller); If you have a better control of the Upper device, it can control the torque, the speed loop away from the drive, which is generally only a high-end dedicated controller can be so dry, but this time completely without the use of servo motors.

Another way is:

1, torque control: torque control method is through an external analog input or direct address of the assignment to set the motor shaft the size of the external output torque, is specifically expressed in the corresponding 5Nm for example 10V, so that when the external analog-based fixed 5V output when the motor shaft 2.5Nm: If the motor shaft when the motor load is less than 2.5Nm turn, the external load is equal to 2.5Nm when the motor does not turn, is greater than 2.5Nm reversed when the motor (usually in a gravity load conditions generation). Can be simulated by changing the amount of real-time settings to change the size of set torque, but also by means of communication to change the address of the corresponding values to achieve. Applied mainly in the right material, there are strict requirements force the winding and unwinding devices, such as Rao-line devices or pulling fiber optic equipment, torque settings according to the changes in the radius of the winding change at any time to ensure that the material Shouli Bu With the winding radius of the changes will change.

2, position control: position control mode is usually through an external input pulse frequency to determine the size of the rotation speed by the number of pulses to determine the angle of rotation, while others can communicate directly to servo speed and displacement of the assignment. Given the strategic location of speed and position of patterns can have a very strict control, it is generally used in positioning devices. Applications such as CNC machine tools, printing machinery and so on.
3, the speed mode: The analog input or pulse frequency of rotation can be speed control, control devices, where the upper outer PID control of the speed model also can be positioned, but must be the motor position signal or direct load feedback signals to the upper position in order to do computation. Location model in support of direct-load outer position signal detected at this time of the motor shaft encoder detects only the motor speed, position signals by direct detection of the ultimate load device to provide such advantages can be reduced during the middle of transmission error, increasing the positioning accuracy of the whole system.