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Micro stepping characteristics

Added by KalleP over 10 years ago

I have got the debug software all working and have been testing a surplus stepper motor.

I have to reterminate the wires on the laser motors before I can plug them in and decided to test another motor while I had the debug code loaded in the MBED and the terminal session connected. It is a Nema-17 6-wire unipolar stepper, possibly a variable reluctance type as it has very low detent torque with no power applied. I connected with just the 4 wires from the ends of the windings.

Both the X and Y axis behave identically.

It gets warm after about 10 minutes if the Pololu trim pot is set half way but cools to room temperature if the current is set to low point. A significant change in the holding torque is felt at about the 30% point. At no point could I feel heat on the Pololu chips. I am running with the 24V laser PSU.

The funny thing now is that the motor takes a big step every 16 toggles (32 presses) of the step line and hardly moves at all for the other 15 steps. If I hold it to my ear I can hear the change in the switching tone with each toggle of the step line so the pulses are getting to the Pololu but the motor is not doing much about it. I turned the wires from one winding the other way (with the enable off) but this did nothing (or changed the direction, did not check). It seems to react the same with high or low current setting.

Could the poor microstepping effect be as a result of the lack of the permanent magnet rotor?

Kalle
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Johannesburg, South Africa


Replies (2)

RE: Micro stepping characteristics - Added by KalleP over 10 years ago

Hmm,

I remembered the other pair of motors on a small 37x35mm translations stage from a slide photo setting machine. These are 4-wire bipolar motors with 9 ohm windings, the other test motor had a total of 90 Ohms across the outer wires in each winding.

The Pololu current set pot behaved a bit differently this time. At the low setpoint it has very little holding torque, but it increased much more linearly as I turned it up.

When testing the motors they seemed to behave in a more linear fassion but stil had an audiable large step every 16 steps.

The Pololu's have a 4988ED driver on them, all three configuration jumpers are soldered on the LAOS board. I remember there were some driver chips that only supported 8x microstepping and wonder if this could be that part number, I will have to do some digging as it was 6 months ago that I did any checking.

I will have a look and see if there is any easy way of testing the quality of the microsteps later.

Kalle
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Johannesburg, South Africa

RE: Micro stepping characteristics - Added by KalleP over 10 years ago

The laser cutters motors have 19 Ohm bipolar windings and behaved very similar to the other bipolar motors I tested.

I crimped motor and limit wires today and after swapping one pair of winding wires over I had the unit homing to the top left neatly.

Setting the current on the Pololu drivers was pretty easy though there is no clear indication of when it is enough. I think I will do some tests to see how fast the motors can be driven. I will have to try and figure out how the configuration file is used in the code as it is not clear just what settings will effect the speeds.

I cut my first part today.

Kalle
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Johannesburg, South Africa

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