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1. Servo mechanism is an automatic control system that enables the output controlled quantities of the object's position, orientation, and state to follow any changes in the input target (or given value). Servo mainly relies on pulses for positioning. Basically, it can be understood that when the servo motor receives 1 pulse, it will rotate the angle corresponding to 1 pulse to achieve displacement. Because the servo motor itself has the function of sending pulses, every time the servo motor rotates an angle, it will send out a corresponding number of pulses. It echoes the pulses received by the servo motor or is called a closed-loop. Therefore, the system will know how many pulses have been sent to the servo motor, and how many pulses have been received at the same time. In this way, the rotation of the motor can be controlled very accurately, so as to achieve precise positioning, which can reach 0.001mm. DC servo motors are divided into brush and brushless motors. It can be used in common industrial and civil occasions that are sensitive to cost.
Servo Motor
2. AC servo motors are brushless motors, which are divided into synchronous and asynchronous motors. Synchronous motors are generally used in motion control. They have a large power range and can achieve great power. The characteristics of AC servo motors include large inertia, low maximum rotation speed, and rapid decrease as the power increases. Therefore, it is suitable for low-speed and smooth-running applications.
Servo Motor structure
3. The rotor inside the servo motor is a permanent magnet, and the U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor feedback signal to the driver. The driver Compares the feedback value with the target value and adjusts the angle of rotation of the rotor. The accuracy of the servo motor is determined by the accuracy of the encoder (number of lines).
The functional difference between AC servo motor and brushless DC servo motor: AC servo is better because it is controlled by a sine wave, the torque ripple is small. The DC servo is a trapezoidal wave. But DC servo is simpler and cheaper.
The structure of the AC servo motor stator is basically similar to that of the capacitor split-phase single-phase asynchronous motor. The stator is equipped with two windings with a position difference of 90°, one is the field winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, which is connected to the control signal voltage Uc. Therefore, AC servo motors are also called two servo motors.
The rotor of an AC servo motor is usually made of a squirrel cage, but in order to make the servo motor have a wider speed range, linear mechanical characteristics, no "rotation" phenomenon, and fast response performance, its rotor resistance should be large and the moment of inertia should be small. There are two types of rotor structures that are widely used: one is a squirrel cage rotor made of high-resistivity conductive materials. In order to reduce the moment of inertia of the rotor, the rotor is made slender; the other is a hollow cup-shaped rotor made of aluminum alloy. The wall of the cup is very thin, only 0.2-0.3mm. In order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator should be placed in the hollow cup-shaped rotor. The moment of inertia is small, the response is fast, and the operation is stable, so it is widely used.
When the AC servo motor has no control voltage, there is only the pulsating magnetic field generated by the excitation winding in the stator, and the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. When the load is constant, the speed of the motor changes with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will be reversed.
Since the 1980s, with the development of integrated circuits, power electronics technology, and AC variable speed drive technology, permanent magnet AC servo drive technology has developed prominently. Famous electrical manufacturers in various countries have successively launched their own AC servo motors and servo drive series. Products are constantly improved and updated. The AC servo system has become the main development direction of the contemporary high-performance servo system, making the original DC servo facing the crisis of being eliminated. Since the 1990s, the AC servo system has been commercialized in various countries around the world, which is a sine wave motor servo drive with full digital control. The development of AC servo drives in the field of transmission is changing with each passing day.
Compared with DC servo motors, permanent magnet AC servo motors have the following main advantages:
⑴ There are no brushes and commutators, so it works reliably and has low maintenance and maintenance requirements.
⑵It is more convenient to dissipate heat from the stator winding.
⑶The inertia is small and it is easy to improve the speed of the system.
⑷Suitable for high-speed and high-torque working conditions.
⑸Small volume and weight under the same power.
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, the servo motor has three notable characteristics compared with the single-phase asynchronous motor:
1)Large starting torque
Due to the large rotor resistance, there is a clear difference compared with the torque characteristic curve of an ordinary asynchronous motor. It can make the critical slip rate S0>1, which not only makes the torque characteristics (mechanical characteristics) closer to linear but also has a larger starting torque. Therefore, as soon as the stator has a control voltage, the rotor immediately rotates, which has the characteristics of fast starting and high sensitivity.
2)Wide operating range
3)No rotation
As long as the control voltage is lost, the servo motor in normal operation will stop running immediately. When the servo motor loses the control voltage, it is in a single-phase operation state. The output power of the AC servo motor is generally 0.1-100W. When the power frequency is 50Hz, the voltage is 36V, 110V, 220, 380V; when the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V, etc.
The AC servo motor runs smoothly and has low noise. But the control characteristic is non-linear. And because the rotor resistance is large, the loss is large, the efficiency is low, so compared with the DC servo motor of the same capacity, the volume is large, the weight is heavy. So it is only suitable for the small power control system of 0.5-100W.
As an open-loop control system, the stepper motor has an essential connection with modern digital control technology. In domestic digital control systems, stepper motors are widely used. With the advent of all-digital AC servo systems, AC servo motors are increasingly used in digital control systems. In order to adapt to the development trend of digital control, most of the motion control systems use stepper motors or all-digital AC servo motors as executive motors. Although the two are similar in control methods (pulse train and direction signal), there are big differences in performance and applications.
1) Control accuracy
The step angle of the two-phase hybrid stepper motor is generally 1.8°, 0.9°, and the step angle of the five-phase hybrid stepper motor is generally 0.72°, 0.36°. There are also some high-performance stepper motors with a smaller step angle after subdivision. For example, the step angle of the two-phase hybrid stepper motor produced by Sanyo Denki can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° through the DIP switch, which compatible with the step angle of two-phase and five-phase hybrid stepping motors.
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the back of the motor shaft. Take Sanyo's all-digital AC servo motor as an example. For a motor with a standard 2000-line encoder, the pulse equivalent is 360°/8000=0.045° because of the quadruple frequency technology used inside the drive. For a motor with a 17-bit encoder, every time the driver receives 131072 pulses, the motor makes one revolution, that is, its pulse equivalent is 360°/131072=0.0027466°, which is the pulse equivalent of a stepping motor with a step angle of 1.8° 1/655.
2) Low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the drive. It is generally considered that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon determined by the working principle of the stepper motor is very unfavorable to the normal operation of the machine. When the stepper motor is working at low speed, damping technology should generally be used to overcome the low-frequency vibration phenomenon, such as adding a damper to the motor or using subdivision technology on the drive.
The AC servo motor runs very smoothly, and there is no vibration even at low speeds. The AC servo system has a resonance suppression function, which can cover the lack of rigidity of the machine, and the internal frequency analysis function (FFT) of the system can detect the resonance point of the machine, which is convenient for the system adjustment.
3) The moment frequency characteristics
The output torque of the stepper motor decreases with the increase of the speed and will drop sharply at higher speeds, so its maximum working speed is generally 300-600RPM. The AC servo motor has a constant torque output, that is, it can output a rated torque within its rated speed (generally 2000RPM or 3000RPM), and a constant power output above the rated speed.
4) The overload capacity
Stepper motors generally do not have overload capacity. AC servo motors have a strong overload capacity. Take Sanyo AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is two to three times the rated torque, which can be used to overcome the moment of inertia of the inertial load at the moment of starting. Because stepper motors do not have this overload capacity, in order to overcome this moment of inertia when selecting a model, it is often necessary to select a motor with a larger torque, and the machine does not need such a large torque during normal operation.
5) Operating performance
Stepper motor control is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose steps or stall. When the speed is too high, it is easy to overshoot. Therefore, in order to ensure its control accuracy, it should be handled the speed up and down issues. The AC servo drive system is a closed-loop control. The drive can directly sample the feedback signal of the motor encoder. The position loop and the speed loop are formed inside. Generally, there will be no step loss or overshoot of the stepper motor, and the control performance is more reliable.
6) Speed response performance
It takes 200 to 400 milliseconds for a stepper motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better. Taking the Sanyo 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from a standstill to its rated speed of 3000RPM, which can be used in control situations that require a fast start and stop.
In summary, the AC servo system is superior to stepper motors in many aspects of performance. However, stepper motors are often used as executive motors on some less demanding occasions. Therefore, in the design process of the control system, various factors such as control requirements and cost should be considered comprehensively, and an appropriate control motor should be selected.
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