What is a Servo Motor?
Type of Servo Motor
Servo motors are mainly classified into DC servo motors and AC servo motors. DC servo motors are divided into brush and brushless motors. Brushed motors have low cost, simple structure, large starting torque, wide speed range, easy to control and maintain (changing carbon brushes), it will generate electromagnetic interference and have specific requirements for the environment. Therefore, it is suitable for common industrial and civil occasions that are sensitive to cost.
The brushless motor is small in size, light in weight, large in output, fast in response, high in speed, small inertia, smooth in rotation, and stable in torque. The control is complex and easy to realize intelligence, and its electronic commutation method is flexible, and it can be square wave commutation or sine wave commutation. The motor is maintenance-free, highly efficient, low operating temperature, low electromagnetic radiation, long life, and can be used in various environments.
AC servo motors are also brushless motors, which are divided into synchronous and asynchronous motors. At present, synchronous motors are generally used in motion control. They have a large power range and can achieve great power. Large inertia, low maximum rotation speed, and rapid decrease as the power increases. Therefore, it is suitable for low-speed and smooth-running machines.
The Working Principle of the Servo Motor
The rotor inside the servo motor is a permanent magnet. 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 feeds back the signal to the driver, and the driver matches the target according to the feedback value. The values are compared, and the angle of rotation of the rotor is adjusted. The accuracy of the servo motor is determined by the encoder (number of lines). The torque and speed of the controlled object are controlled by the signal voltage. When the magnitude and polarity of the signal voltage change, the speed and direction of the motor's rotation also change.
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 It 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 a different look like 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, it should have these two characteristics: the rotor resistance is large and the moment of inertia is small. At present, there are two types of rotor structures that are widely used: one is a squirrel cage rotor with a high-resistivity conductive material. In order to reduce the moment of inertia of the rotor, the rotor is made slender; another is the hollow cup-shaped rotor made of aluminum alloy. The cup wall 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 rotor has a small moment of inertia, quick response and stable operation, 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 control voltage. When the phase of the control voltage is opposite, the servo motor will be reversed.
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-machine asynchronous motor, the servo motor has three notable characteristics:
1. Large starting torque
Due to the large rotor resistance, this 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. Faster response speed
The speed response frequency of servo motor is up to 1 KHz, and an instantaneous speed observer is built-in. It can detect the speed of the motor at high speed and high resolution. It has high-performance mechanical adaptability. Whether it is a belt drive machine that is prone to resonance or a high-rigidity screw drive machine, the high-performance automatic adjustment function can achieve high-speed positioning.
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. Because of the large rotor resistance and the two torque characteristics and the composite torque characteristics produced by the two rotating magnetic fields rotating in the opposite direction in the stator and the rotor function.
The output power of AC servo motor is generally 100W-50kW. 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. However, the control characteristics are nonlinear, and due to the large rotor resistance, large loss, and low efficiency, compared with the same capacity DC servo motor, it is large in size and heavy in weight, so it is only suitable for small power control systems of 0.5-100W.
Advantages of Servo Motors
1. Precision: It realizes the closed-loop control of position, speed and torque; overcomes the problem of stepper motor out-of-step;
2. Rotation speed: good high-speed performance, generally rated speed can reach 2000-3000 rpm;
3. Adaptability: Strong anti-overload capability, able to withstand a load of three times of the rated torque, which is especially suitable for occasions with instantaneous load fluctuations and requirements for fast starting;
4. Stable: stable operation at low speed, it won’t occur stepping operation phenomenon as stepping motor when running at low speed. Suitable for occasions with high-speed response requirements;
5. Timeliness: The dynamic response time of motor acceleration and deceleration is short, generally within tens of milliseconds;
6. Comfort: Heat and noise are significantly reduced.
Performance comparison of stepper motor and AC servo motor
As an open-loop control system, stepper motor has an essential connection with modern digital control technology. In the current domestic digital control system, 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 mode (pulse train and direction signal), there are big differences in performance and applications. Now make a comparison of the performance of them.
- Different 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 stepping motor is generally 0.72°, 0.36°. There are also some high-performance stepping motors with a smaller step angle after subdividing. The control accuracy of the AC servo motor is determined by the rotary encoder at the back of the motor shaft.
- Different 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 harmful to the normal operation of the machine. When stepping motors work at low speeds, damping technology should generally be used to overcome low-frequency vibration phenomena, 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 system adjustment.
- Different moment frequency characteristics
The output torque of a stepping motor decreases with the increase of the speed, and will drop sharply at a higher speed, 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.
- Different the overload capacity
Stepper motors generally do not have overload capacity. AC servo motors have strong overload capacity. They are speed overload and torque overload capacity. 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, so a torque appears. The phenomenon of waste.
- Different operating performance
The stepping motor is controlled by 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 well. 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 speed loop are formed inside. Generally, the stepper motor will not lose steps or overshoot, and the control performance is more reliable.
- Different speed response performance
It takes 200 to 400 milliseconds for a stepping motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). The AC servo system has better acceleration performance. It only takes a few milliseconds to accelerate from a standstill to its rated speed of 3000RPM. It can be used in control situations that require 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 in 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.
Servo motor selection calculation method`
Pay attention to three points:
- Number of revolutions: According to actual requirements, motors with different revolutions can be selected for motors of the same power. Generally speaking, the lower the number of revolutions, the cheaper the price.
- Torque: must meet actual needs, but doesn’t need to leave too much margin like stepper motors.
- Inertia: select motors with different inertias according to site requirements. For example, the machine tool industry generally selects P1 series large inertia servo motors.
Servo motor driver
Servo motor drives, also known as "servo controllers" and "servo amplifiers", are a kind of controllers used to control servo motors. Their function is similar to that of frequency converters acting on ordinary AC motors, and they are part of the servo system. Mainly used in high-precision positioning systems. Generally, the servo motor is controlled through three methods of position, speed and torque to achieve high-precision positioning of the transmission system. It is a high-end product of transmission technology.
Servo motor price
1. The price of servo motors is related to the types of servo motors. The prices of different types of servo motors are different. As far as AC servo motors and permanent magnet AC servo motors are concerned, their prices are very different. Before you buy Learn more about your needs.
2. The parameters of the servo motor are different, the speed capacity, the overload capacity, precision, stability, and timeliness. These factors are all factors that determine the price of the servo motor. The greater the performance of the servo motor, the better, then its price will naturally correspond to it. The higher the value, although we do not necessarily have to buy the best one, the better the motor performance, the better it will be when used.
3. Different brands of servo motors have different prices. In terms of price, merchants set prices based on cost and market price. However, the difference between market and cost determines that the price given by the brand is different, so the brand price is different.
4. It is very simple to make a good model and understand the model you need to buy something. Go online to find a seller and negotiate the price, but you can't just look at the price given by one company. Compare more businesses on the Internet to see which price is more favorable. It's reasonable to buy again.
1. Initialization parameters
Before wiring, initialize the parameters first.
The servo motor is on the control card: Select the control mode, clear the PID parameters, let the default enable signal turn off when the control card is powered on, save this state to ensure that the control card is in this state when it is powered on again.
On the servo motor: Set the control mode, set the enable to be controlled by the outside, set the gear ratio of the encoder signal output, then set the proportional relationship between the control signal and the motor speed. Generally speaking, it is recommended that the maximum design speed in servo work corresponds to a control voltage of 9V. For example, sets the speed corresponding to 1V voltage, and the factory value is 500. If you only plan to make the motor work below 1000 rpm, then set this parameter to 111.
Power off the control card and connect the signal line between the control card and the servo. The following lines must be connected: the analog output line of the control card, the enable signal line, and the encoder signal line of the servo output. After rechecking that there is no error in the wiring, the motor and control card (and PC) are powered on. At this time, the motor should not move, and it can be easily rotated with external force. If this is not the case, check the setting and wiring of the enable signal. Rotate the motor with external force and check whether the control card can correctly detect the change of the motor position, otherwise check the wiring and setting of the encoder signal.
3. Try direction
For a closed-loop control system, if the direction of the feedback signal is incorrect, the consequences must be catastrophic. Turn on the enable signal of the servo through the control card. This is the servo should rotate at a lower speed, which is the legendary "zero drift". Generally, there will be instructions or parameters to suppress zero drift on the control card. Use this command or parameter to see if the motor speed and direction can be controlled by this command (parameter). If it cannot be controlled, check the analog wiring and the parameter settings of the control mode. Confirm that a positive number is given, the motor rotates forward, and the encoder count increases; when a negative number is given, the motor rotates reversely, and the encoder count decreases. If the motor carries a load and the stroke is limited, do not use this method. Do not give too much voltage to the test, it is recommended to be below 1V. If the directions are inconsistent, you can modify the parameters on the control card or the motor to make them consistent.
4. Suppress zero drift
In the closed-loop control process, the existence of zero drift will have a certain impact on the control effect, and it is best to suppress it. Use control card or servo to suppress zero drift parameters, carefully adjust to make the motor speed approach zero. Since the zero drift itself has a certain degree of randomness, it is not necessary to require the motor speed to be absolutely zero.
5. Establish closed-loop control
Let go of the servo enable signal through the control card again, and enter a smaller proportional gain on the control card. As for how big it is, it can only be based on feeling. If you are not at ease, enter the smallest allowable value of the control card. value. Turn on the enable signal of the control card and servo. At this time, the motor should be able to roughly act according to the motion command.
6. Adjust closed-loop parameters
Fine-tuning the control parameters to ensure that the motor moves in accordance with the instructions of the control card is the work that must be done, and this part of the work is more of experience, which can only be omitted here.