Stepping motor is a discrete motion device, which is essentially related to 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.
Different control accuracy
The step angle of two-phase hybrid stepping motor is generally 3.6 degrees and 1.8 degrees, and the step angle of five phase hybrid stepping motor is generally 0.72 degrees and 0.36 degrees. There are also some high-performance stepper motors with smaller step angles. For example, a stepping motor for slow wire walking machine tool produced by stone company has a step angle of 0.09 degrees; The step angle of the three-phase hybrid stepping motor produced by bergerlahr can be set to 1.8 degrees, 0.9 degrees, 0.72 degrees, 0.36 degrees, 0.18 degrees, 0.09 degrees, 0.072 degrees and 0.036 degrees 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 Panasonic all digital AC servo motor as an example, for the motor with standard 2500 line encoder, because the frequency quadrupling technology is adopted in the driver, the pulse equivalent is 360 degrees / 10000 = 0.036 degrees. For the motor with 17 bit encoder, the motor rotates once every 217 = 131072 pulses received by the driver, that is, the pulse equivalent is 360 degrees / 131072 = 9.89 seconds. Is 1 / 655 of the pulse equivalent of a stepping motor with a step angle of 1.8 degrees.
Different low frequency characteristics
Stepper 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 lack of mechanical rigidity, and the system has frequency analysis function (FFT), which can detect the mechanical resonance point and facilitate the system adjustment.
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 is constant torque output, that is, it can output rated torque within its rated speed (generally 2000rpm or 3000rpm), and constant power output above rated speed.
Different overload capacity
Stepper motors generally do not have overload capacity. AC servo motor has strong overload capacity. Taking Panasonic AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is 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.
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.
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 better than stepper 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.
Stepping motor is an actuator that converts electric pulse into angular displacement. Generally speaking, when the stepping driver receives a pulse signal, it drives the stepping motor to rotate a fixed angle (and stepping angle) in the set direction.
You can control the angular displacement by controlling the number of pulses, so as to achieve the purpose of accurate positioning; At the same time, you can control the speed and acceleration of the motor by controlling the pulse frequency, so as to achieve the purpose of speed regulation.
There are three types of stepper motors: permanent magnet (PM), reactive (VR) and hybrid (HB)
Permanent magnet stepping is generally two-phase, with small torque and volume, and the stepping angle is generally 7.5 degrees or 15 degrees;
Reactive stepping is generally three-phase, which can realize large torque output. The stepping angle is generally 1.5 degrees, but the noise and vibration are very large. It has been eliminated in developed countries such as Europe and the United States in the 1980s;
Hybrid stepping refers to the combination of the advantages of permanent magnet and reactive. It is divided into two phases and five phases: the two-phase stepping angle is generally 1.8 degrees and the five-phase stepping angle is generally 0.72 degrees. This stepping motor is the most widely used.