This article mainly addresses two issues:
1. The main differences between stepper drive and servo drive.
2. Several practical issues in the specific selection process.
Overview
Stepper motors are mainly classified according to the number of phases, among which two-phase and five-phase stepper motors are the most widely used in the market. A two-phase stepper motor can be divided into 400 equal parts per revolution, while a five-phase stepper motor can be divided into 1000 equal parts. Therefore, the characteristics of a five-phase stepper motor are better, with shorter acceleration and deceleration times and lower dynamic inertia.
With the emergence of fully digital AC servo systems, AC servo motors are increasingly being used in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuators. Although they are similar in control methods (pulse trains and direction signals), they differ significantly in performance and application scenarios.
The performance of the two will now be compared.
I. Different control precision
Two-phase hybrid stepper motors typically have a step angle of 3.6 degrees or 1.8 degrees, while five-phase hybrid stepper motors typically have a step angle of 0.72 degrees or 0.36 degrees. Some high-performance stepper motors also have even smaller step angles. For example, a stepper motor produced by Sitong Company for wire EDM machines has a step angle of 0.09 degrees; the three-phase hybrid stepper motor produced by Bergerlahr in Germany has a step angle that can be set to 1.8 , 0.9 , 0.72 , 0.36 , 0.18 , 0.09, 0.072 , and 0.036 degrees via DIP switches, making it compatible with both two-phase and five-phase hybrid stepper motors.
Second: Different low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding a damper to the motor or using microstepping technology in the driver.
AC servo motors operate very smoothly, without vibration even at low speeds. AC servo systems feature resonance suppression to compensate for insufficient mechanical rigidity, and internal frequency analysis ( FMT ) capabilities to detect mechanical resonance points, facilitating system adjustments.
Three: Different moment-frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 rpm .
AC servo motors provide constant torque output, meaning they can output rated torque up to their rated speed (typically 2000 rpm or 3000 rpm ) and constant power output above their rated speed.
4. Different overload capacities
Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. For example, Panasonic's AC servo system features both speed and torque overload capabilities. Its maximum torque is three times its rated torque, which can be used to overcome the inertial torque of inertial loads at startup. Because stepper motors lack this overload capacity, a motor with a larger torque is often selected to overcome this inertial torque during selection. However, the machine does not require such a large torque during normal operation, resulting in wasted torque.
5. Different operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or load can easily lead to missed steps or stalling. Excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be properly addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the feedback signal from the motor encoder, internally forming position and speed loops. Generally, the missed steps or overshoot issues of stepper motors are not present, resulting in more reliable control performance.
Six: Speed affects performance differently
Stepper motors require 200 to 400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute) . AC servo systems offer better acceleration performance. For example, the Panasonic MSMA400W AC servo motor can accelerate from a standstill to its rated speed of 3000 rpm in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
How to choose the right model?
1. How to correctly select servo motors and stepper motors
It mainly depends on the specific application. Simply put, you need to determine: the nature of the load (e.g., horizontal or vertical load), torque, inertia, speed, accuracy, acceleration/deceleration requirements, higher-level control requirements (e.g., port interface and communication requirements), and the primary control method (position, torque, or speed control). Also, determine whether the power supply is DC, AC, or battery-powered, and the voltage range. Based on this, determine the model of the motor and the corresponding driver or controller.
2. How to use a stepper motor driver?
Choose a driver with a current greater than or equal to that of the motor. For applications requiring low vibration or high precision, a microstepping driver can be used. For high-torque motors, use a high-voltage driver whenever possible to achieve good high-speed performance.
3. What are the differences between 2- phase and 5- phase stepper motors, and how do you choose between them?
Two- phase motors are inexpensive, but they vibrate more at low speeds and their torque drops rapidly at high speeds. Five- phase motors, on the other hand, vibrate less, have better high-speed performance, and are 30-50% faster than two- phase motors , making them a viable alternative to servo motors in some applications.
4. When should a DC servo system be selected, and what are the differences between it and an AC servo system?
DC servo motors are divided into brushed motors and brushless motors.
Brushed motors are low in cost, simple in structure, have high starting torque, wide speed range, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes). They generate electromagnetic interference and have environmental requirements. Therefore, they can be used in cost-sensitive general industrial and civilian applications.
Brushless motors are small in size, lightweight, powerful, fast-responding, high-speed, low-inertia, smooth-rotating, and stable in torque. While their control is complex, they are easily made intelligent. Their electronic commutation is flexible, allowing for either square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and a long lifespan, making them suitable for various environments.
AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for applications requiring low-speed, stable operation.
5. Precautions when using motors
The following checks must be performed before powering on:
1 ) Is the power supply voltage appropriate (overvoltage may damage the drive module); the +/- polarity of the DC input must not be connected incorrectly, and the motor model or current setting on the drive controller must be appropriate (do not set it too high at the beginning).
2 ) Ensure the control signal lines are securely connected. In industrial settings, shielding should be considered (e.g., using twisted-pair cables).
3 ) Do not connect all the necessary wires at the beginning. Only connect the most basic system. After it is running well, then connect the rest gradually.
4 ) Make sure you understand the grounding method, or whether to use floating grounding.
5 ) During the first half hour of operation, closely observe the motor's status, such as whether the movement is normal, the sound and temperature rise. If any problems are found, stop the machine immediately for adjustment.
