Stepper motor is a discrete motion device, which has an essential connection with modern digital control technology. In the current domestic digital control system, stepper motors are widely used. With the emergence 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, stepper motors or all-digital AC servo motors are mostly used as executive motors in motion control systems. Although both are similar in control mode (pulse train and direction signal), there are big differences in performance and application occasions. Now compare the performance of the two.
The control accuracy is different
The step angles of two-phase hybrid stepper motors are generally 3.6 degrees and 1.8 degrees, and the step angles of five-phase hybrid stepper motors are 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 produced by Stone Company for slow-moving wire machine tools has a step angle of 0.09 degrees; a three-phase hybrid stepping motor produced by BERGER LAHR has a step angle of 0.09 degrees. The DIP switch is set to 1.8 degrees, 0.9 degrees, 0.72 degrees, 0.36 degrees, 0.18 degrees, 0.09 degrees, 0.072 degrees, 0.036 degrees, 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. For a motor with a standard 2500-line encoder, the pulse equivalent is 360 degrees/10000=0.036 degrees due to the quadruple frequency technology inside the driver. For a motor with a 17-bit encoder, every time the driver receives 217=131072 pulses, the motor makes one revolution, that is, its pulse equivalent is 360 degrees/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8 degrees.
The low frequency characteristics are different:
Stepper motors are prone to low-frequency vibrations at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally believed 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 stepping motor is very unfavorable to the normal operation of the machine. When the stepper motor works 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 driver, etc.
The AC servo motor runs very smoothly and does not vibrate 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 system has a frequency analysis function (FFT) inside the system, which can detect the resonance point of the machine and facilitate system adjustment.
The moment-frequency characteristics are different:
The output torque of the stepper motor decreases with the increase of the speed, and it 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 it is a constant power output above the rated speed.
The overload capacity is different:
Stepper motors generally do not have overload capability. AC servo motor has strong overload capacity. Take the Panasonic AC servo system as an example, it has speed overload and torque overload capabilities. Its maximum torque is three times of the rated torque, which can be used to overcome the moment of inertia of the inertial load at the moment of starting. Because the stepper motor does not have this kind of 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 the torque appears. The phenomenon of waste.
Running performance is different:
The control of the stepping motor is an open-loop control. If the starting frequency is too high or the load is too large, step loss or stalling will easily occur. When the speed is too high, overshooting will easily occur when the speed is too high. Therefore, in order to ensure its control accuracy, it should be handled properly. Ascent and deceleration issues. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder, and the internal position loop and speed loop are formed. Generally, there will be no step loss or overshoot of the stepping motor, and the control performance is more reliable.
The speed response performance is different:
It takes 200-400 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 CRT AC servo motor as an example, it only takes a few milliseconds to accelerate from static to its rated speed of 3000RPM, which can be used in control occasions that require fast start and stop.
To sum up, the AC servo system is superior to the stepper motor in many aspects of performance. But in some less demanding occasions, stepper motors are often used as executive motors. 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.
A stepper motor is an actuator that converts electrical pulses into angular displacement. In layman’s terms: when the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (and step 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 rotation 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 step angle is generally 7.5 degrees or 15 degrees;
Reactive stepping is generally three-phase, which can realize large torque output, and the stepping angle is generally 1.5 degrees, but the noise and vibration are very large. In developed countries such as Europe and the United States, it has been eliminated in the 1980s;
the hybrid stepper refers to the combination of the advantages of the permanent magnet type and the reactive type. It is divided into two-phase and five-phase: the two-phase step angle is generally 1.8 degrees and the five-phase step angle is generally 0.72 degrees. This type of stepper motor is the most widely used.
Post time: Mar-25-2023