Stepper motor is a kind of discrete motion device, which has essential connection with modern digital control technology. In current domestic digital control systems, the application of stepper motors is very wide. 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 very similar in position control mode (pulse train and direction signal), there is a big difference in performance and application. Now compare the performance of the two.
First, the stepping motor and servo motor have different control accuracy. The step angle of the two-phase hybrid stepping motor is generally 1.8 °, and the step angle of the three-phase hybrid stepping motor is 1.2 °. There are also some high-performance stepper motors with smaller step angles. For example, the two-phase HM series stepper motor produced by Shanghai Marine Control has a step angle of 0.9 °. After the stepper motor uses the subdivision drive technology, the control accuracy is improved. The number of steps that the two-phase stepper motor can distinguish is about 8000. Step / revolution, the three-phase stepper motor can distinguish with an accuracy of about 10,000 steps / revolution. 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, the pulse equivalent is 360 ° / 10000 = 0.036 ° because the drive uses the quadruple frequency technology. It is worth noting that resolution and accuracy are different concepts. For example, the engraving machine wrote on the nameplate of the engraving machine that the engraving accuracy is 0.001mm. In fact, the engraving machine cannot divide 0.1mm into 100 equal parts and arrange them into a straight line. Observe that the result of the carving must be jagged. Relative to the vast majority of users, no matter whether it is mechanical transmission accuracy or photoelectric sensor for positioning accuracy, the physical accuracy of stepper motor servo motor is not high, and it is not necessary to unilaterally pursue the highest accuracy of the motor.
Second, the stepping motor and servo motor have different torque frequency characteristics. The output torque of the stepping motor decreases with the increase of the speed, and it will decrease sharply at higher speeds, so its maximum working speed is generally 0 to 900 RPM. The AC servo motor has constant torque output, that is, it can output rated torque within its rated speed (generally 1000 ~ 3000RPM). The torque frequency characteristics of the output torque of the servo motor are stable, and the advantage is reflected in the environment that requires high speed. At low speeds, this advantage is not meaningful. Compared with the 57HB76 stepper and 60 series 400W servo motors, the torque frequency characteristics are as follows: Stepping The comparison of the torque frequency characteristics of the motor and the AC servo motor is shown in the figure above. The red is the torque frequency characteristic of the servo motor, and the blue part is the torque frequency characteristic of the three-phase hybrid stepper motor. The price difference between the two is three times. Different from the servo motor overload capability, stepper motors generally do not have overload capability. The YK2M series two-phase stepper motor driver has built-in current acquisition and intelligent processing, and the overload capability is also very limited. The AC servo motor has 2 to 3 times speed overload and torque overload capacity, which can be used to overcome the inertia moment of the inertia load at the moment of starting. Because stepping motors do not have this kind of overload capability, the servo motor can achieve high response frequency when selecting models according to the characteristics of the equipment. If you want to continue to choose stepper motors to achieve similar results, then to overcome this inertia moment, first Make sure that the torque of the stepper motor is greater than the required torque, and then optimize the acceleration / deceleration curve as much as possible.
4. Different running performance of stepper motor and servo motor The control of stepper motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose steps or stall, and when the speed is too high, it is easy to overshoot. Therefore, in order to ensure its control accuracy, the problem of raising and lowering speed should be handled well. The AC servo drive system is closed-loop control. The driver can directly sample the feedback signal of the motor encoder. The position loop and speed loop are formed internally. Generally, the stepping motor does not lose steps or overshoot, and the control performance is more reliable. The servo motor is a closed-loop system, and the servo driver can automatically correct the missing pulses, and it can also provide feedback to the controller in time when the rotor is blocked, while the stepper motor is an open-loop system, and it must use sufficient torque margin to avoid stall.
V. Speed response performance of stepper motor and servo motor Different stepper motors need 100 to 2000 milliseconds to accelerate from standstill to working speed (generally several hundred revolutions per minute). The AC servo system has better acceleration performance. It takes only a few milliseconds to accelerate from stationary acceleration to its rated speed of 3000 RPM, which can be used for control occasions that require rapid start and stop.
In summary, AC servo systems are better than stepper motors in some aspects, but stepper motors have their own characteristics and will not be completely replaced.
1. The AC servo accuracy is slightly higher than the stepper motor, but in most cases, the resolution of the stepper motor is far higher than the accuracy required by the machine;
2. Although the torque frequency characteristics are different, not all equipment applications require high speed. The low speed and large torque of stepper motors usually require stepper motors within 700rpm and higher speeds are suitable for AC servo motors.
3. The AC servo has a strong overload capacity, but some stepper motor drivers also have a certain overload capacity, and a cost-effective choice can be made between the overload capacity and retaining sufficient torque margin;
4. The speed response frequency is different, which is another important factor for choosing stepper motor and AC servo motor. High response frequency is the advantage of AC servo motor. Pay attention to this feature when selecting.
5. The accuracy of the AC servo motor was mentioned earlier. Most users are unaware that the AC servo motor will repeatedly position itself at high resolution and low speed, so that micro-jitter appears. On some extremely high-precision equipment, such as precision jade On engraving machines and high-precision low-speed robots, such jitters can cause sawtooth on the arc, or blur. The specially processed stepper motor can really solve this problem, because the stepper motor does not repeatedly align the teeth.
6. In some applications of indexing control, the stepping motor is turned to an angle to stop, and the locking torque is large. The AC servo is to recover the position after the position of the external force is lost. This process may cause machining errors. More stepper motor, stepper driver, stepper motor driver, brushless motor, brushless driver, brushless motor driver problem