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What are the out-of-step and overshoot of stepper motors?

time:2019-06-11 Browse:


  What are the out-of-step and overshoot of stepper motors?


  The loss of step should be that the pulse is not moved to the specified position. Overshoot should be the opposite of losing the step, moving beyond the specified position.

  Stepper motors are often used in some motion control systems that are simple to control or require low cost. The biggest advantage is that the position and speed are controlled in an open loop. But because of the open-loop control, the load position has no feedback to the control loop, and the stepper motor must respond correctly to each excitation change. If the excitation frequency is not properly selected, the stepper motor will not be able to move to the new position. A permanent error occurs in the actual position of the load relative to the position expected by the controller, ie, an out-of-step phenomenon or an overshooting occurs. Therefore, in the stepping motor open-loop control system, how to prevent out-of-step and overshoot is the key to the normal operation of the open-loop control system.

  Out-of-step and overshoot phenomena occur when the stepper motor starts and stops, respectively. In general, the system's extreme starting frequency is relatively low, and the required operating speed is often high. If the system starts directly at the required operating speed, because the speed has exceeded the limit, the starting frequency can not be started normally, then the lost step occurs, and the weight can not be started at all, resulting in stalling. After the system is running, if the pulse is stopped immediately after reaching the end point and stops immediately, the stepper motor will rotate through the desired balance position of the controller due to the inertia of the system.

      In order to overcome step out-of-step and overshoot, appropriate acceleration and deceleration control should be added at the start of the stop. We generally use: the motion control card as the upper control unit, the PLC with control function as the upper control unit, and the single-chip microcomputer as the upper control unit to control the motion acceleration and deceleration to overcome the out-of-step overshoot phenomenon.  
      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. The stepper motor has a technical parameter: the no-load starting frequency, that is, the pulse frequency that the stepping motor can start normally under no-load conditions. If the pulse frequency is higher than the no-load start frequency, the stepper motor cannot start normally, and lost or blocked may occur. In the case of load, the starting frequency should be lower. If the motor is to be rotated at a high speed, the pulse frequency should have a reasonable acceleration process, that is, the starting frequency is low, and then rise to the desired high frequency (the motor speed rises from low speed to high speed) at a certain acceleration.

Stepper motor diagram


       Start frequency = start speed × how many steps per revolution. The no-load starting speed is that the stepping motor does not directly rotate by acceleration and deceleration without load. When the stepper motor rotates, the inductance of each phase winding of the motor will form a back electromotive force; the higher the frequency, the larger the back electromotive force. Under its action, the motor decreases with increasing frequency (or speed), resulting in a drop in torque.

  Assumption: the total output torque required for the reducer is T1, the output speed is N1, the reduction ratio is 5:1, the stepping angle of the stepper motor is A, then the motor speed is: 5*(N1), then The output torque of the motor should be (T1)/5, and the operating frequency of the motor should be 5*(N1)*360/A, so you should look at the moment frequency characteristic curve: coordinate point [(T1)/5,5*( N1)*360/A] is below the frequency characteristic curve (starting the moment frequency curve). If you are below the moment frequency curve, you can choose this motor. If it is above the moment curve, you can't choose this motor because it will lose its step or it won't turn at all.

    Added: Have you determined the working status, the maximum speed you need is determined? If it is determined, it can be calculated according to the formula provided above. (Depending on the maximum speed of rotation and the size of the load, you can determine you. Whether the stepping motor used now is suitable, if you are not suitable for you, you should know what kind of stepping motor to use.)

  In addition: after the stepping motor is started, the frequency can be increased under the condition that the load is constant, because the stepping motor should have two moments of the moment frequency curve, and the one you have should be the starting moment frequency curve. The other one is to take off the moment frequency curve. The meaning of this curve is: start the motor at the starting frequency, increase the load after the startup is completed, but the motor will not lose the step state; or start the motor at the starting frequency, When the load is constant, the operating speed can be increased as appropriate, but the motor will not lose synchronization.

    Regarding the step angle, for example, if you are ABCDA single four-shot control, then the step angle is the angle at which A passes. Regarding the maximum pull-in frequency, it refers to the interval frequency between AB, which is given in the manual. > at a certain value, but in the actual application, the value that should be given is the maximum value, for example, >250PPS, then the delay after A satisfies 1/delay <=250, delay>=4ms, and it can't get 3ms.

  Conclusion: It is true that some people are studying the use of encoders but can detect lost and blocked. However, these are still far from mature enough to match the encoder, and the road is still very long.

  In fact, the use of encoders is the trend of today's stepper motors. And if you want to implement closed-loop control, it is like having an encoder or sensor to tell the controller the current rotation of the stepper motor so that the controller can make the corresponding adjustment (acceleration or deceleration). This is the current state of technology.

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