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Recommended Ixx71 setting for direct microstepping


wlewis
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I've seen that the recommended value for Ixx71 (counts per N commutation cycles) for direct microstepping is 65536.

 

When I set Ixx71 to this value, the motor moves quite slowly, and I have to put large values in for velocity, acceleration and following error.

 

Is there an advantage to having this value set so high, as opposed to something like 4096?

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I've seen that the recommended value for Ixx71 (counts per N commutation cycles) for direct microstepping is 65536.

 

When I set Ixx71 to this value, the motor moves quite slowly, and I have to put large values in for velocity, acceleration and following error.

 

Is there an advantage to having this value set so high, as opposed to something like 4096?

Lewis,

 

Having Ixx71 to 65536 gives you the highest resolution possible for commutation and results in a more precise and smoother Micro-stepping.Always having this high resolution commutation is preferable, but as you said you will have more number of counts per rev and you need to increase your velocity and acceleration.

 

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Since the sine table used for commutation in Turbo PMAC has 2048 entries, as long as you have Ix71 set to 2048 or greater, you're taking full advantage of Turbo PMAC's sine table resolution for commutation.

 

So why do we suggest 65536 instead of 2048?

 

In micro-stepping method, we are integrating the Quadrature current command and use it as out position feedback and also commutation angle. Although the servo position supports up to 1/32 of a count resolution, having a sub-count for a stepper motor was not very intuitive for some users. In order to address this issue we wanted to have 2048 whole counts per 'full step', 102400 whole counts for a 200 step with 1.8 degree step size stepper motor. Since we are using the same ECT result both for position and commutation, the value of Ix71 would be set to 2048*2^5=2048*32=65536.

 

Obviously this will affect your velocity and acceleration settings since the affect is similar to having a high resolution encoder.

 

Regards,

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  • 2 weeks later...

Sina,

 

Thanks for the explanation of the function of Ixx71 in direct microstepping mode. At this stage, I'm using a value of 4096, and the motors are performing well. I will do some testing of different values to see if I can observe performance changes/improvements.

 

Wayne

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  • 4 months later...
As a long time user of Geobrick-LV on stepping motors we inherited the Ixx71 value of 4096 from teh original as delivered .pmc files and have used it successfully. We are finding now that depending upon the differences between the encoder feedback ( often at the end of a long mechanical train) and the motor resolution ( Ixx71=4096 gives us 6400 counts per rev from a200 step motor) we get improved closed loop perfromance by varying Ixx71. In one extreme however a mechanical vendor told us his syetem would only work with half stepping motors so we successfully changed Ixx71 to 256. I don't think the performance was significantly different but at least it stopped the vendor balmig our geobrick-LV settings for shortfalls in his mechanical performance.
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brinj,

 

I would suggest taking a look at http://forums.deltatau.com/filedepot/download.php?f=Turbo%20PMAC/Tools/Geo%20Brick%20LV/Geo%20Brick%20LV%20Setup%20Assistant%202.3.xls [FILE REMOVED]

 

In this Excel spreadsheet, you can adjust the Ix71 setting and all other parameters which should be adjusted accordingly will be adjusted automatically.

 

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