The difference between servo motor and stepper motor

Servo motor is a very common piece of equipment in the motor industry. This motor has a wide range of applications in industry, agriculture, and daily life. So what are the differences between servo motors and stepper motors? Let’s find out together next!

As an open-loop control system, stepper motors are essentially related to modern digital control technology. In current domestic digital control systems, stepper motors are widely used. With the emergence of fully 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, motion control systems use stepper motors or fully digital AC servo as execution motors.

Although the two are very similar in control methods (pulse train and direction signal), they have large differences in performance and application scenarios. Now let’s compare the performance of the two as follows:

1. 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 stepper motor is generally 0.72°, 0.36°. There are also some high-function stepper motors with smaller step angles after subdivision. For example, the step angle of the two-phase hybrid stepper motor produced by SANYO DENKI can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° through the DIP switch. , compatible with the step angle of two-phase and five-phase hybrid stepper motors.

The control accuracy of the AC servo motor is ensured by the rotary encoder at the rear end of the motor shaft. Taking the Sanyo fully digital AC servo motor as an example, for a motor with a standard 2000-line encoder, due to the quadruple frequency technology used inside the driver, its pulse equivalent is 360°/8000=0.045°. 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 stepper motor with a step angle of 1.8°. 1/655.

2. Different low frequency characteristics

Stepper motors are prone to low-frequency oscillation at low speeds. The oscillation frequency is related to the load condition and driver function. It is generally believed that the oscillation frequency is half of the no-load pickup frequency of the motor. This low-frequency oscillation phenomenon determined by the working principle of the stepper motor is very detrimental to the normal operation of the machine. When the stepper motor operates at low speed, damping technology should generally be used to overcome the low-frequency oscillation phenomenon, such as adding a damper to the motor, or using subdivision technology on the driver.

The AC servo motor operates very smoothly and does not oscillate even at low speeds. The AC servo system has a resonance suppression function, which can cover the lack of mechanical rigidity. The system also has a frequency analysis function (FFT) inside the system that can detect the resonance point of the machine to facilitate system adjustment.

3. Different moment frequency characteristics

The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds, so its operating speed is generally between 300 and 600RPM. The AC servo motor has constant torque output, that is, it can output additional torque within its rated speed (generally 2000RPM or 3000RPM), and it has constant power output above the rated speed.

4. Different overload capabilities

Stepper motors generally do not have overload capabilities. AC servo motors have strong overload capabilities. Taking the Sanyo AC servo system as an example, it has speed overload and torque overload capabilities. Its torque is two to three times the additional torque, which can be used to overcome the inertia moment of the inertia load at the moment of starting. Since stepper motors do not have such overload capability, in order to overcome this inertia moment during model selection, it is often necessary to select a motor with a larger torque. However, the machine does not require such a large torque during normal operation, so a torque appears. A wasteful phenomenon.

5. Different operating functions

The control of the stepper motor is an 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 stopped, the speed is too high and it is easy to overshoot. It is considered that to ensure its control accuracy, it should be handled properly. Speed ​​up and down issues. The AC servo drive system is a closed-loop control. The driver can directly sample the motor encoder response signal, and internally constitutes a position loop and a speed loop. Generally, there will be no loss of steps or overshoot of the stepper motor, and the control function is more reliable.

6. Different speed response functions

It takes 200 to 400 milliseconds for a stepper motor to accelerate from stop to operating speed (usually several hundred revolutions per minute). The acceleration function of the AC servo system is better. Taking the Sanyo 400W AC servo as an example, it only takes a few milliseconds to accelerate from stop to its additional speed of 3000RPM, and can be used in control situations that require quick start and stop.

To sum up, AC servo systems are to stepper motors in many performance aspects. However, in some situations with low requirements, stepper motors are often used as execution motors. Therefore, in the design process of the control system, various factors such as control requirements and cost must be comprehensively considered to select an appropriate control motor.