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Exploring stepper motors: how to understand out-of-step and overshoot of stepper motors

Writer: admin Time:2020-06-17 16:41 Browse:

Out of step should be missing the pulse and not moving to the specified position. Overshoot should be the opposite of out-of-step, moving beyond the specified position.

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   In some simple or low-cost motion control systems, stepper motors are often used. The biggest advantage is: the position and speed are controlled in an open loop. But because it is open-loop control, the load position has no feedback to the control loop, and the stepper motor must correctly respond to each excitation change. If the excitation frequency is not selected properly, the stepper motor cannot move to a new position. The actual position of the load has a permanent error with respect to the position expected by the controller, that is, out-of-step phenomenon or overshoot imagination. Therefore, in a stepper motor open-loop control system, how to prevent out-of-step and overshoot is the key to whether the open-loop control system can operate normally.


   Out-of-step and overshoot phenomena occur when the stepper motor starts and stops, respectively. Under normal circumstances, the system's limit start frequency is relatively low, and the required operating speed is often relatively high. If the system starts directly at the required operating speed, because the speed has exceeded the limit and the starting frequency cannot be started normally, it will lose steps at first, and it will not start at all, causing a locked rotor. After the system runs, if it stops sending pulses when it reaches the end point and stops it immediately, the stepper motor will turn to the desired balance position of the controller due to the inertia of the system.


   In order to overcome the step out-of-step and overshoot phenomena, appropriate acceleration and deceleration control should be added when starting and stopping. We generally adopt: 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 overstepping phenomenon.

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  Stepper motor has a technical parameter: no-load starting frequency, that is, the pulse frequency of the stepper motor can start normally under no-load conditions. If the pulse frequency is higher than this value, the motor cannot start normally, and step loss or stall may occur. Under load, the starting frequency should be lower. If you want to make the motor rotate at high speed, the pulse frequency should have an acceleration process, that is, the starting frequency is low, and then increase to the desired high frequency according to a certain acceleration (the motor speed increases from low speed to high speed).


  Stepper motor is an actuator that converts electrical pulses into angular displacement. Generally speaking: 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, so as to achieve the purpose of accurate positioning; at the same time, you can control the speed and acceleration of the motor rotation by controlling the pulse frequency, so as to achieve the purpose of speed regulation. The stepper motor has a technical parameter: no-load starting frequency, which is the pulse frequency at which the stepper motor can start normally under no-load conditions. If the pulse frequency is higher than the no-load starting frequency, the stepper motor cannot start normally, and steps may be lost or blocked. Under load, the starting frequency should be lower. If you want to make the motor rotate at high speed, the pulse frequency should have a reasonable acceleration process, that is, the starting frequency is low, and then increase to the desired high frequency according to a certain acceleration (the motor speed increases from low speed to high speed). Starting frequency = starting speed × how many steps per revolution of no-load starting speed means that the stepper motor rotates directly without acceleration or 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 greater the back electromotive force. Under its effect, the motor's phase current decreases with increasing frequency (or speed), which results in a decrease in torque.


Assumption: The total output torque of the reducer is T1, the output speed is N1, the reduction ratio is 5:1, the stepping motor step angle is A, then the motor speed is: 5*(N1), then The output torque of the motor should be (T1)/5, and the working frequency of the motor should be


5*(N1)*360/A, so you should look at the moment frequency characteristic curve: the coordinate point [(T1)/5, 5*(N1)*360/A] is the frequency characteristic curve (starting moment frequency curve) Below. If it is below the moment frequency curve, you can choose this motor. If it is on the moment-frequency curve, you cannot choose this motor because it will be out of step, or it won't turn at all.


Supplement: Have you determined the working state, have you determined the maximum speed, if it is determined, you can calculate it according to the formula provided above (based on the maximum speed of rotation and the size of the load, you can determine your Is the stepping motor currently selected suitable? If it is not suitable, you should also know what kind of stepping motor to choose)


In addition, after the stepping motor is started, you can increase the frequency under the same load, because the stepping motor torque frequency curve should actually have two, and the one you have should be the starting torque frequency curve. And the other one is off the torque frequency curve. The meaning of this curve is: start the motor at the start frequency, and the load can be increased after the start is completed, but the motor will not lose step; or start the motor at the start frequency, at When the load is constant, the running speed can be increased appropriately, but the motor will not lose its step.


Regarding the step angle, for example, if you are ABCDA single four-beat control, then the step angle is an angle that A walks through. Regarding the maximum pull-in frequency, it refers to the interval frequency between AB, which are given in the manual. 》At a certain value, but the value that should be given in actual application is the maximum value, such as 》250PPS, then the delay after A satisfies 1/delay <<=250, delay>>=4ms, and it cannot go up to 3ms.


  How to understand out of step and overshoot of stepper motor

   It is true that some people are not using encoders but can detect lost steps and blocked spins. However, these are currently limited to the patent stage, and they are far from being mature enough to match the encoder, and the road is still very long.


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


   Conclusion

   This is the introduction about the stepper motor, if you have any shortcomings, please correct me.


Introduction of Maintex stepper motor: 24BYJ48-737A stepper motor reducer


Model Item Item Specification Spec

24BYJ48-737A Rated Voltage 5VDC

Phase 4

Reduction Ratio 1/64

Step Angle 5.625°

Exciting Method 1-2

DC resistance Direct-current Resistance 21Ω±10% (25°C)

No-load pull-in Frequency ≥500Hz

No-load pull-out frequency ≥1000Hz

Pull-in Torque ≥90mN.m (5VDC, 400Hz)

Positioning torque Detent Torque ≥50mN.m

Insulation Resistance ≥50M Ω 500VDC

Dielectric Strenght 600VAC

Insulation Class B

Noise Noise ≤40dB

Friction Torque Friction Torque 60-294mN.m

Terminal Specifications Terminal Spec 1.5*5P

   

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