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 and direction signal), there are big differences in performance and applications. Now make a one-by-one comparison of the performance of the two.
basic structure
Stepper motor structure diagram
Servo motor structure diagram
1. Different control accuracy
The step angle of the two-phase hybrid stepping 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. For example, the two-phase hybrid stepping motor produced by MOONS’ is matched with its SR series stepping driver. The step angle can be divided into 16 subdivisions by dialing, and you can choose 1.8°, 0.9°, 0.45°, 0.36. °, 0.225°, 0.18°, 0.1125°, 0.09°, 0.072°, 0.05625°, 0.045°, 0.036°, 0.028125°, 0.018°, 0.0144°, 0.014°, compatible with two-phase and five-phase hybrid stepping motors The step angle.
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the back of the motor shaft. Take the M2 AC servo motor as an example. For a motor with a 2500-line incremental encoder, the pulse equivalent is 360°/10000=0.036° due to 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. Different low frequency characteristics
Stepping 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.
Anti-resonance
One disadvantage of the stepping system lies in the inherent resonance point. The SR series stepping driver automatically calculates the resonance point and adjusts the control algorithm based on it, so as to achieve the purpose of suppressing resonance and greatly improve the stability of the intermediate frequency. At high speed, there is greater torque output and better high speed performance.
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 make up for 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.
Vibration suppression
The vibration suppression function of the M2 servo system includes two parts: resonance suppression and damping.
The resonance suppression function provides two sets of Notch Filters, which can effectively overcome the resonance problem caused by the inherent characteristics of the equipment’s mechanical structure.
The damping function can improve the damping characteristics of the entire motion system by adjusting the damping coefficient provided by the controller, thereby effectively reducing the vibration of the system.
Three, the moment frequency characteristics are different
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.
Comparison of speed and torque characteristics of servo motor and stepper motor under the same size
Fourth, the overload capacity is different
Stepper motors generally do not have overload capacity. AC servo motors have strong overload capacity. Take the M2 AC servo system as an example, it has speed overload and torque overload capabilities. 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.
Five, different 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 step or stall. When the speed is too high, it is easy to overshoot. Therefore, 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 the speed loop are formed inside. Generally, there will be no stepping motor out-of-step or overshoot, and the control performance is more reliable.
Six, 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 acceleration performance of the AC servo system is better. Take the 400W AC servo motor of MOONS 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 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 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.
