Microstep Unipolar Stepper Motor

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codenameyash
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2010/09/19 02:51:26 (permalink)
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Microstep Unipolar Stepper Motor

Hey everyone.

Today brings about a new problem. As the title suggests am trying to do microstepping of a unipolar permanent magnet stepper motor, more specifically a 6 wire 7.5 degree motor. Anyway, i've been reading the application note AN822A, where they drive a bipolar motor using microstepping. There was only 2 windings to worry about and the waveform that they used kept one constant while the other varied, however I have more than those used in the application note. The application note lays out what percentages of duty cycle I need, which is fine, and from there I am able to get the duty cycles for pwm. I currently though have two waveform drawings, and don't know which one to use to do the program.
For example, looking at Unipolar diagram and at Uniwave, and four colors for the different windings, red, blue, black and orange. The waveform is as follows:
1:  during the first step, A is held at 100% while B starts to slowly charge up, much like the application note does
2: here A is now declining while the fully charged up line B is held high.
3: The next coil starts charging up from 0 so it can attract the rotor and so on.....

This is what I have come up with, is my method of doing it right, as with this two of the coils will always be opposite either 0 or 100% and either charging up or winding down.

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    fredmatic1
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    Re:Microstep Unipolar Stepper Motor 2010/10/13 03:11:45 (permalink)
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    A year ago, I did a lot of development work on both unipolar and bipolar stepper drivers, (designed for driving steppers to actuate automatic gaging probes).

    Your waveforms look pretty close to correct, but if you want to get it even closer to perfect, the current amplitude per microstep, is calculated as the the sin of the angle * your maximum current flow. You can pre-calculate this and store it in a table, (for simplicity and speed). The angle is the number of steps you would want to make between 0 and 90 degrees, (i.e., the number o microsteps). Thes numbers are the ones you s to set your PWM period.

    To generate the maximum torque, you would need to use a chopper amplifier though, which is a bit more complex than just PWM'ing the drive signal, but is you look into some of the PIC18 chips, they now have built-in SR latches for that.

    mark
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    violin
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    Re:Microstep Unipolar Stepper Motor 2010/10/13 15:25:38 (permalink)
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    Hey everyone.

    Today brings about a new problem. As the title suggests am trying to do microstepping of a unipolar permanent magnet stepper motor, more specifically a 6 wire 7.5 degree motor. Anyway, i've been reading the application note AN822A, where they drive a bipolar motor using microstepping. There was only 2 windings to worry about and the waveform that they used kept one constant while the other varied, however I have more than those used in the application note. The application note lays out what percentages of duty cycle I need, which is fine, and from there I am able to get the duty cycles for pwm. I currently though have two waveform drawings, and don't know which one to use to do the program.
    For example, looking at Unipolar diagram and at Uniwave, and four colors for the different windings, red, blue, black and orange. The waveform is as follows:
    1:  during the first step, A is held at 100% while B starts to slowly charge up, much like the application note does
    2: here A is now declining while the fully charged up line B is held high.
    3: The next coil starts charging up from 0 so it can attract the rotor and so on.....

    This is what I have come up with, is my method of doing it right, as with this two of the coils will always be opposite either 0 or 100% and either charging up or winding down.

     
    Hmm. Unipolar or bipolar stepper motors, they are all the same, it doesn't mater. - If you feel uncertain about unipolar opperation just connect it in a bipolar configuration and leave the centre taps unconnected. At least with a 6 wire connection stepper motor you do have the choice of unipolar or bipolar configuration. Regards
    #3
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