Servo Motor vs. Stepper Motor in Performance & Control
Introduction of servo motor and stepper motor
As an open-loop control system, the stepper motor is essentially related to modern digital control technology. In digital control systems, 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 train and direction signal), there are big differences in performance and applications. The following is a comparison of the performance of the two.
Servo motor basics
Servo motor refers to the engine that controls the operation of mechanical components in the servo system, and is an indirect speed change device that supplements the motor. The servo motor can make the speed and position control accuracy become very accurate, and can convert the voltage signal into torque and speed to drive the control object. The speed of the servo motor rotor is controlled by the input signal and can react quickly. In automatic control systems, servo motors can be used as actuators. Servo motor has the characteristics of small electromechanical time constant and high linearity, and it can convert the received electrical signal into angular displacement or angular velocity output on the motor shaft. It can be divided into two categories: DC and AC servo motors. Its main feature is that there is no autorotation when the signal voltage is zero, and the speed decreases at a uniform speed with the increase of torque.
Stepper motor basics
A stepping motor is a motor that converts electrical pulse signals into corresponding angular or linear displacements. Every time a pulse signal is input, the rotor rotates an angle or moves forward. The output angular displacement or linear displacement is proportional to the number of input pulses, and the speed is proportional to the pulse frequency. Therefore, stepping motors are also called pulse motors.
Difference between the servo motor and stepper motor
Control way
The control methods of stepper motors and servo motors are different. The stepper motor controls the rotation angle by controlling the number of pulses. One pulse corresponds to one step angle. However, because there is no feedback signal, the motor does not know where it is, and the position accuracy is not high enough. The servo motor also controls the rotation angle by controlling the number of pulses. Each time the servo motor rotates one angle, a corresponding number of pulses will be sent out, and the drive will also receive the feedback signal, which is compared with the pulse received by the servo motor. In this way, the system will know how many pulses are sent to the servo motor and how many pulses are withdrawn at the same time. In this way, the rotation of the motor can be controlled very accurately, thereby achieving precise positioning. The accuracy can reach 0.001mm.
The stepper motor adopts the open-loop control while the servo motor adopts the closed loop control.Sixth, the control accuracy is different. The more phases and beats of a stepping motor, the higher its accuracy. The servo motor depends on the built-in encoder. The more scale of the encoder, the higher the accuracy.Sixthly, the control accuracy is different. The more phases and beats of a stepping motor, the higher its accuracy. The servo motor depends on the built-in encoder. The more scale of the encoder, the higher the accuracy.
Overload capability
The overload capability is different. Stepper motors generally do not have overload capacity, while AC servo motors have strong overload capacity. Its maximum torque is 3 times 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 overload capability, it cannot be used in some work situations.
Different speed and response performance
It takes 200 ~ 400ms 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.
Different advantages
The accuracy of a stepping motor is superior to that of a servo motor because it does not accumulate errors. Usually, it only needs to do open loop control. However, servo motors are superior to stepping motors in terms of responsiveness.
Performance at low speeds
The situation is different at low speeds. Stepping motors are prone to low-frequency vibration at low speeds. When it works at low speed, damping technology or subdivision technology is generally used to overcome low-frequency vibration. However, the operation of the servo motor is very smooth, and there is no vibration even at low speed.
Control accuracy
The control accuracy is different. The more phases and beats of a stepping motor, the higher its accuracy. The servo motor depends on the built-in encoder. The more scale of the encoder, the higher the accuracy.
Moment-frequency Characteristics
the moment-frequency characteristics are different. The output torque of the stepping motor will decrease with the increase of the speed, and the AC servo motor adopts a constant torque output.
Operation performance
the operation performance is different. If the starting frequency of the stepping motor is too high or the load is too large, it is easy to lose steps or stall. If the speed is too high when stopping, overshoot will easily occur. The drive of the servo system can directly sample the feedback signal of the motor encoder. Its internal structure includes a position loop and a speed loop. Generally, there will be no step loss or overshoot of the stepper motor, so its control performance is more reliable. AC servo system is superior to stepper motor in many performance aspects. However, stepper motors are often used as executive motors in some less demanding occasions.
