Difference between servo motor and stepping motor
To understand the difference between the two, we need to have an in-depth understanding of each object. First, understand the working principle. The working principle of stepping motor is an actuator that converts electric pulse into angular displacement. When the stepping driver receives a pulse signal, it drives the stepping motor to rotate a fixed angle in the set direction, and its rotation runs step by step at a fixed angle. The angular displacement can be controlled by controlling the number of pulses sent, so as to control the displacement; At the same time, the speed and acceleration of motor rotation can be controlled by controlling the pulse frequency, so as to achieve the purpose of speed regulation. 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 feedback signal of the motor is sent to the driver. The driver compares the feedback value with the target value to adjust the rotation angle of the rotor. The accuracy of servo motor depends on the resolution of encoder.
First, the control methods of stepping motor and servo motor are different. The stepping motor controls the rotation angle by controlling the number of pulses. One pulse corresponds to one step angle, but there is no feedback signal. The motor does not know what position it has reached, and the position accuracy is not high enough. The servo motor also controls the number of pulses. Each angle of rotation of the servo motor will send a corresponding number of pulses. At the same time, the driver will also receive the feedback signal and compare it with the pulses received by the servo motor. In this way, the system will know how many pulses are sent to the servo motor and how many pulses are received at the same time, so as to accurately control the rotation of the motor, so as to achieve accurate positioning, It can reach 0.001mm.
Second, the asynchronous motor with overload capacity generally does not have overload capacity. AC servo motor has strong overload capacity. Taking pier magnetic AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is 3 times of the rated torque, which can be used to overcome the inertia torque of inertia load at the moment of starting. Because the stepping motor does not have this overload capacity, it can not work with the stepping motor in some working situations.
Third, the speed response performance is not synchronous. It takes 200 ~ 400ms for the stepping motor to accelerate from static to working speed (generally hundreds of revolutions per minute). The acceleration performance of the AC servo system is good. Take the PIR magnetic AC servo motor as an example, accelerate from static to its rated speed of 3000 R / min. It only takes a few MS and can be used for control stations requiring rapid start and stop and high position accuracy.
Differences of control modes of stepping servo system
As an open-loop control system, stepping motor has essential connection with modern digital control technology. In the current domestic digital control system, stepping motor is widely used. With the emergence of all digital AC servo system, AC servo motor is more and more used in digital control system. In order to adapt to the development trend of digital control, stepping motor or all digital AC servo motor are mostly used as executive motor in motion control system. Although they are similar in control mode (pulse train and direction signal), there are great differences in service performance and application occasions. The performance of the two is compared.
1. Different control accuracy
The step angle of two-phase hybrid stepping motor is generally 1.8 ° and 0.9 °, and that of five phase hybrid stepping motor is generally 0.72 ° and 0.36 °. There are also some high-performance stepping motors with smaller step angle after subdivision. For example, the step angle of the two-phase hybrid stepping motor produced by sanyodenki can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 ° through the dial switch, which is 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 rear end of the motor shaft. Taking Shanyang all digital AC servo motor as an example, for the motor with standard 2000 line encoder, because the frequency quadrupling technology is adopted in the driver, its pulse equivalent is 360 ° / 8000 = 0.045 °. For the motor with 17 bit encoder, the driver rotates once every 131072 pulses received by the motor, that is, the pulse equivalent is 360 ° / 131072 = 0.0027466 °, which is 1 / 655 of the pulse equivalent of the stepping motor with a step angle of 1.8 °.
2. Different low-frequency characteristics
Stepping motor is prone to low-frequency vibration at low speed. The vibration frequency is related to the load condition and driver performance. 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 stepping motor is very unfavorable to the normal operation of the machine. When the stepping motor works at low speed, damping technology should generally be used to overcome the phenomenon of low-frequency vibration, such as adding damper on the motor or subdivision technology on the driver.
The AC servo motor runs very smoothly and will not vibrate even at low speed. The AC servo system has resonance suppression function, which can cover the insufficient rigidity of the machinery, and the system has frequency analysis function (FFT), which can detect the resonance point of the machinery and facilitate the system adjustment.
3. Different torque frequency characteristics
The output torque of stepping motor decreases with the increase of speed, and will decrease sharply at higher speed, so its maximum working speed is generally 300 ~ 600 rpm. AC servo motor has constant torque output, that is, it can output rated torque within its rated speed (generally 2000rpm or 3000rpm), and constant power output above the rated speed.
4. Different overload capacity
Stepper motors generally do not have overload capacity. AC servo motor has strong overload capacity. Taking Shanyang AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is two to three times of the rated torque, which can be used to overcome the inertia torque of inertia load at the moment of starting. Because the stepping motor does not have this overload capacity, in order to overcome this inertia torque, it is often necessary to select the motor with large torque, and the machine does not need so large torque during normal operation, so the phenomenon of torque waste appears.
5. Different operation performance
The control of stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose step or stall. If the speed is too high when stopping, it is easy to overshoot. Therefore, in order to ensure its control accuracy, the problems of speed increase and speed decrease should be handled well. The AC servo drive system is a closed-loop control. The driver can directly sample the feedback signal of the motor encoder. It forms a position loop and a speed loop. Generally, there will be no step loss or overshoot of the stepping motor, and the control performance is more reliable.
6 different speed response performance
It takes 200 ~ 400 milliseconds for the stepping motor to accelerate from static to working speed (generally hundreds of revolutions per minute). The acceleration performance of AC servo system is good. Taking Panasonic msma400w AC servo motor as an example, it takes only a few milliseconds to accelerate from static to its rated speed of 3000rpm, which can be used in control occasions requiring rapid start and stop.
To sum up, AC servo system is superior to stepping motor in many performance aspects. However, in some occasions with low requirements, stepper motors are often used as executive motors. Therefore, in the design process of the control system, we should comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.