This article mainly introduces stepper motor drivers and servo motor drives, and focuses on a detailed comparison between them.
1. What is a Servo Drive
A servo drive (also called servo amplifier) is an electronic device used to control servo motors. The servo drive controls the speed and position of the motor by receiving control signals from the controller, thereby achieving precise control of the machine.
Servo drives usually consist of controller, power supply, motion control chip, circuit board, interface circuit, protection circuit, monitoring circuit and other parts.
2. What is a Stepper Driver
A stepper driver (also called stepper motor driver) is an electronic device that controls the movement of a stepper motor. It consists of a circuit controller, driver chip and power supply. The circuit controller sends a control signal and controls the motors to start through the driver chip, thereby realizing the rotation of the motor.
3. Comparison of Servo Drives and Stepper Drivers
Both stepper motor drivers and servo drivers can be used to control motor movement, but they have the following differences.
3.1 Working Principle
Stepper drivers drive stepper motors by controlling the direction and magnitude of current. Stepper motors move in fixed steps, and the direction and speed of each step are controlled by the driver.
Servo drives use a feedback mechanism to control the movement of the servo motor. The servo motor feeds back the position and speed of the motor through a sensor, and the driver adjusts the magnitude and direction of the current based on this information to control the movement of the motor.
3.2 Feedback System
Stepper driver: Commonly used in open loop systems.
If the stepper motor loses steps, the system will not know it, that is, there is no feedback system, so it is called open-loop control with poorer accuracy and stability.
Servo drive: Often used in closed-loop systems and equipped with feedback systems such as encoders and grating rulers.
As shown in the figure above, the control methods of servo drives include position control, speed control and current control. The feedback loops includes position loop (for example, the grating ruler is installed under the workbench, and the feedback line is connected to the controller, which means the feedback signal is sent to the PLC), speed loop, and current loop, which are what we call three-loop control. Three-loop control constitutes the full closed-loop control.
If there is only speed loop and current loop, it is called semi-closed loop control. If there is no feedback system, it is open loop control. However, in a servo drive feedback system, speed loop and current loop are necessary, that is, these two loops are integrated in the servo amplifier.
3.3 Control Accuracy
Stepper motor driver accuracy adjustment: subdivision control (fixed).
It can only accept the pulse signal from the upper computer but cannot independently send pulses to control the start and stop of the stepper motor. Its maximum accuracy is 8mm/6400=0.00125.
How to understand this? We know that stepper motors have step angles. The step angle of two-phase hybrid stepper motors is generally 1.8°, and that of three-phase hybrid stepper motors is 1.2°. There are also some high-performance stepper motors with smaller step angles.
If the step angle is 1.8 degrees, then it takes 200 pulses for the motor to rotate once, which is subdivision number 1. Then setting subdivision number 2 through the stepper driver, it requires 400 pulses for the motor to rotate once. If setting subdivision number 4, it needs 800 pulses, and so on. Of course, the larger the subdivision, the higher the accuracy and the slower the speed.
Servo drive accuracy adjustment: electronic gear ratio (adjustable).
It can accept the pulse signal from the upper computer, and can also independently send pulses to control the start and stop of the servo motor. Its maximum accuracy is 8mm/131072=0.000061.
How to understand this? The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. For a servo motor with a standard 2500-line encoder, because of the frequency quadrupling technology used inside the drive, its pulse equivalent is 360°/10000=0.036°.
In terms of control accuracy, since stepper drives move in fixed steps, their accuracy is relatively lower. In high-precision applications, they may not provide sufficient accuracy.
Servo drives offer greater precision and accuracy because they use a feedback mechanism to adjust the motor's motion. This allows them to more precisely control the position and speed of the motor and make adaptive adjustments during movement.
3.4 Speed Range
The servo drive has a wider speed range and can achieve high-speed motion; the stepper driver has a narrower speed range and is mainly used for low-speed, high-torque motion.
3.5 Torque Characteristics
The torque output of the servo drive is controlled by the control current and magnetic field, and has good torque characteristics. The torque output of the stepper driver is affected by damping and magnetic resistance, so it can produce larger torque at low speed, but the torque will be rapidly reduced at high speed.
3.6 Price
The price of servo drives is higher than that of stepper drives, mainly because servo drives have better accuracy, speed range and control functions.
4. Conclusion
In general, servo drives and stepper drives each have their own advantages and disadvantages, and they need to be selected according to specific application scenarios. Servo drives are suitable for applications with high precision requirements, wide speed ranges, and multiple control methods. Stepper drivers are suitable for applications with simple application scenarios and small loads. Through the introduction of the above content, we hope you can have a preliminary understanding of servo drives and stepper drivers.
