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.
Comparison of two-phase/five-phase stepper motors:
Motor structure:
Two-phase stepper motor: 8 main poles; 4-phase (2-phase) 4-pole coils
Five-phase stepper motor: 10 main poles; 5-phase 2-pole coils
Decomposition energy:
Two-phase stepper motor: 1.8°/0.9° (200, 400 divisions/revolution)
Five-phase stepper motor: 0.72°/0.36° (500, 1000 divisions/revolution), 2.5 times faster than two-phase stepper motor.
Vibrational:
Two-phase stepper motor: The range of 100-200pps is the low-speed resonance range, with relatively large vibrations and no significant resonance point.
Five-phase stepper motor: Low vibration, speed-torque characteristics, but not as fast as a five-phase stepper motor; high speed, high torque.
A stepper motor is a discrete motion device, fundamentally linked to modern digital control technology. It is widely used in current domestic digital control systems. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly applied 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 step angles of 3.6° and 1.8°, while five-phase hybrid stepper motors typically have step angles of 0.72° and 0.36°. 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°; 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° via a DIP switch, making it compatible with both two-phase and five-phase hybrid stepper motors.
The control precision of an AC servo motor is ensured by a rotary encoder at the rear end of the motor shaft. Taking a Panasonic all-digital AC servo motor as an example, for a motor with a standard 2500-line encoder, due to the quadruple frequency technology used in the driver, its pulse equivalent is 360°/10000 = 0.036°. For a motor with a 17-bit encoder, the motor rotates once for every 2^17 = 131072 pulses received by the driver, meaning its pulse equivalent is 360°/131072 = 9.89 seconds. This is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.
II. 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 resolution (FFT) capabilities to detect mechanical resonance points, facilitating system adjustments.
III. 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.
IV. 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.
V. 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.
VI. Different speed response performance
Stepper motors require 200–400 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.
In summary, AC servo systems outperform stepper motors in many aspects. However, stepper motors are often used as actuators in less demanding applications. Therefore, the design of a control system must comprehensively consider factors such as control requirements and cost to select an appropriate control motor.
A stepper motor is an actuator that converts electrical pulses into angular displacement. Simply put, when a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (the step angle) in a set direction.
You can control the angular displacement by controlling the number of pulses, thereby achieving accurate positioning; at the same time, you can control the speed and acceleration of the motor by controlling the pulse frequency, thereby achieving speed regulation.
There are three types of stepper motors: permanent magnet (PM), reactive (VR), and hybrid (HB).
Permanent magnet steppers are generally two-phase, with smaller torque and size, and the step angle is generally 7.5 degrees or 15 degrees;
Reactive stepper motors are typically three-phase, capable of high torque output, with a step angle generally around 1.5 degrees, but they generate significant noise and vibration. They were phased out in developed countries like those in Europe and America in the 1980s.
Hybrid stepper motors combine the advantages of permanent magnet and reactive stepper motors. They are further divided into two-phase and five-phase types: two-phase stepper motors typically have a step angle of 1.8 degrees, while five-phase stepper motors typically have a step angle of 0.72 degrees. This type of stepper motor is the most widely used.
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. Should I choose a stepper motor or a servo motor system?
In fact, the choice of motor should be based on the specific application, as each type has its own characteristics. Please see the table below for further clarification.
3. 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.
4. 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.
5. 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.
6. 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 the temperature rise. If any problems are found, stop the machine immediately for adjustment.
