How to choose the correct stepper motor and servo motor
Stepper motors are mainly classified according to the number of phases, and two-phase and five-phase stepping motors are widely used in the market. The two-phase stepping motor can be divided into 400 equal parts per revolution, and the five-phase can be divided into 1000 equal parts. Therefore, the characteristics of the five-phase stepping motor are better, the acceleration/deceleration time is shorter, and the dynamic inertia is lower. .
Comparison of two-phase/five-phase stepping motor differences:
Motor construction:
Two-phase stepper motor: 8 main poles ‧; 4-phase (2-phase) 4-pole coil
Five-phase stepper motor: 10 main poles ‧; 5-phase 2-pole coil
Decomposition energy:
Two-phase stepper motor: 1.8°/0.9° (200, 400 splits/turn)
Five-phase stepper motor: 0.72°/0.36° (500, 1000 splits/turn), 2.5 times higher than two-phase stepper motor
Vibration:
Two-phase stepping motor: between 100-200pps is a low-speed resonance field with large vibration and no significant resonance point
Five-phase stepper motor: low vibration, speed-torque characteristics, less than five-phase stepper motor in speed, high speed, high torque
The stepper motor is a discrete motion device that is intrinsically linked to modern digital control technology. In the current domestic digital control system, the application of stepper motors is very extensive. With the advent of all-digital AC servo systems, AC servo motors are increasingly being used in digital control systems. In order to adapt to the development trend of digital control, stepping motor or all-digital AC servo motor is mostly used as the execution motor in the motion control system. Although the two are similar in control mode (burst and direction signals), there are large differences in performance and application.
Now compare the performance of the two.
First, the control accuracy is different
The two-phase hybrid stepping motor step angle is generally 3.6 °, 1.8 °, and the five-phase hybrid stepping motor step angle is generally 0.72 °, 0.36 °. There are also some high performance stepper motors with smaller step angles. For example, a stepping motor for a slow wire-cutting machine produced by Sitong has a step angle of 0.09°; a three-phase hybrid stepping motor produced by Bergerlahr of Germany can be dialed. The code switches are set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036°, and are 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 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 inside the driver. For a motor with a 17-bit encoder, the drive receives one revolution per 217 = 131072 pulse motors, ie its pulse equivalent is 360°/131072 = 9.89 seconds. It is 1/655 of the pulse equivalent of a stepping motor with a step angle of 1.8°.
Second, the low frequency characteristics are different
Stepper motors are prone to low frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally considered that the vibration frequency is half of the take-off frequency of the motor no-load. This low frequency vibration phenomenon, which is determined by the working principle of the stepper 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 drive.
The AC servo motor runs very smoothly, and vibration does not occur even at low speeds. The AC servo system has a resonance suppression function, which can cover the 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 system adjustment.
Third, the difference in frequency characteristics
The output torque of the stepping motor decreases as the speed increases, and it drops sharply at higher speeds, so the maximum operating speed is generally 300 to 600 rpm.
The AC servo motor is a constant torque output, that is, it can output rated torque within its rated speed (generally 2000 rpm or 3000 rpm), and it is a constant power output above the rated speed.
Fourth, the overload capacity is different
Stepper motors generally do not have overload capability. The AC servo motor has a strong overload capability. Take the Panasonic AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is three times the rated torque and can be used to overcome the moment of inertia of the inertia load at the moment of starting. Because there is no such overload capability in the stepping motor, in order to overcome this moment of inertia during the selection, it is often necessary to select a motor with a large torque, and the machine does not need such a large torque during normal operation, and a torque appears. The phenomenon of waste.
Five, different operating performance
The control of the stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it may be lost or blocked. If the speed is too high during the stop, the overshoot may occur. Therefore, in order to ensure the control accuracy, it should be handled well. The problem of rising and falling speed. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder. The internal position loop and speed loop are formed. Generally, the stepless motor is lost or overshooted, and the control performance is more reliable.
Sixth, the speed response performance is different
It takes 200 to 400 milliseconds for the stepper motor to accelerate from standstill to the operating speed (typically several hundred revolutions per minute). The AC servo system has better acceleration performance. Take the Panasonic msma400w AC servo motor as an example. It takes only a few milliseconds to accelerate from standstill to its rated speed of 3000 rpm. It can be used in control situations where fast start-stop is required.
In summary, the AC servo system is superior to the stepper motor in many performance aspects. However, in some occasions where the requirements are not high, stepper motors are often used to perform the motor. Therefore, in the design process of the control system, it is necessary to comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.
A stepper motor is an actuator that converts electrical pulses into angular displacement. To put it bluntly: when the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (and step angle) in the set direction.
You can control the angular displacement by controlling the number of pulses to achieve accurate positioning. At the same time, you can control the speed and acceleration of the motor by controlling the pulse frequency to achieve the purpose of speed regulation.
There are three types of stepping motors: permanent magnet (pm), reactive (vr) and hybrid (hb).
The permanent magnet stepping is generally two-phase, the torque and the volume are small, and the step angle is generally 7.5 degrees or 15 degrees;
The reactive stepping is generally three-phase, which can achieve high torque output. The step angle is generally 1.5 degrees, but the noise and vibration are very large. Developed in Europe and the United States in the 1980s has been eliminated;
Hybrid stepping refers to the advantage of mixing permanent magnets and reactive equations. It is divided into two phases and five phases: the two-phase step angle is generally 1.8 degrees and the five-phase step angle is generally 0.72 degrees. This stepper motor is the most widely used.
