Servo Motor vs. Stepper Motor

Difference between Stepper Motor and Servo Motor

Stepper motor is an actuator that converts digital pulse signal into angular displacement. That is to say, when the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (ie, step angle, 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. Generally, the accuracy of the stepping motor is 3-5% of the stepping angle, and it does not accumulate.

Servo motor refers to the engine that controls the operation of mechanical components in the servo system, and is an auxiliary motor indirect transmission device. The servo motor can control the speed and position accuracy very accurately, and can convert the voltage signal into torque and speed to drive the control object. The rotor speed of the servo motor is controlled by the input signal and can respond quickly. In the automatic control system, it is used as an executive element, and has the characteristics of small electromechanical time constant, high linearity, starting voltage, etc., which can convert the received electrical signal. Converted to angular displacement or angular velocity output on the motor shaft. It is divided into two categories: DC and AC servo motors. Its main feature is that when the signal voltage is zero, there is no rotation phenomenon, and the speed decreases uniformly with the increase of torque.

Stepper Motor vs. Servo Motor

The servo motor is mainly positioned by pulses. Basically, it can be understood in this way that when the servo motor receives 1 pulse, it will rotate the angle corresponding to 1 pulse, so as to realize the displacement. Because the servo motor itself has the function of sending pulses, so the servo motor Each time it rotates an angle, a corresponding number of pulses will be sent out, which echoes the pulses received by the servo motor, or is called a closed loop. 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. Back, in this way, the rotation of the motor can be precisely controlled, so as to achieve precise positioning, which can reach 0.001mm.

Stepper motor is a discrete motion device, which has an essential connection with modern digital control technology. In the current domestic digital control system, the application of stepper motor is very extensive. 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, 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 train and direction signal), there are big differences in performance and application. Now let’s compare the performance and difference between the servo motor and the stepper motor.

Different control precision

  • The step angle of the two-phase hybrid stepping motor is generally 3.6° and 1.8°, and the step angle of the five-phase hybrid stepping motor is generally 0.72° and 0.36°. There are also some high-performance stepper motors with smaller step angles. For example, a stepper motor for slow wire-feeding machine tools has a step angle of 0.09°; a three-phase hybrid stepping motor produced by ATO has a step angle that can pass through The DIP switch is set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036°, 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’s all-digital AC servo motor as an example, for a motor with a standard 2500-line encoder, the pulse equivalent is 360°/10000=0.036° due to the quadruple frequency technology used inside the driver. For a motor with a 17-bit encoder, every time the driver receives 217=131072 pulses, the motor rotates one revolution, that is, its pulse equivalent is 360°/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.

Different low frequency characteristics

  • The stepper motor is prone to low frequency vibration at low speed. The vibration frequency is related to the load condition and the performance of the drive, and 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 stepping motor is very detrimental 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.
  • The AC servo motor runs very smoothly and does not vibrate even at low speeds. The AC servo system has the function of resonance suppression, which can cover the lack of rigidity of the machine, and has a frequency analysis function (FFT) inside the system, which can detect the resonance point of the machine and facilitate the system adjustment.

Different moment-frequency characteristics

  • The output torque of the stepper 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, within its rated speed (generally 2000rpm or 3000rpm), it can output the rated torque, and above the rated speed, it is a constant power output.

Different overload capacity

  • Stepper motors generally do not have overload capability. AC servo motors have 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 the rated torque, which can be used to overcome the inertial moment of inertial load at the moment of starting. Because the stepping motor does not have such overload capacity, in order to overcome this inertia moment, 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. wasteful phenomenon.

Different operating performance

  • The control of the stepper motor is 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 the speed is too high, it is easy to cause overshoot. Therefore, in order to ensure its control accuracy, it should be handled properly. Ascending and decelerating problems. 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, the phenomenon of step loss or overshoot of the stepping motor will not occur, and the control performance is more reliable.

Different speed response performance

  • It takes 200 to 400 milliseconds for a stepper motor to accelerate from a standstill to a working speed (usually several hundred revolutions per minute). The acceleration performance of the AC servo system is good. Taking the ATO 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from standstill to its rated speed of 3000RPM, which can be used for control occasions requiring rapid start and stop.

All in all, AC servo systems are superior to stepper motors 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, various factors such as control requirements and costs should be comprehensively considered, and an appropriate control motor should be selected.

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