Acceleration smoothness question

I have purchased some closed loop steppers along with mesa 5i25 & 7i76 plug and go kit. I have been reading everything about the new stepper drives and they are pretty nice with some unique options.

One of these options is the acceleration smoothness which can be set 0 - 10, 0 being no smoothing and 10 being max smoothing out more of an "S" acceleration curve. I have always loved the smooth motion of servos and not cared much for the jerky reactions of steppers comparison. My question after reading this manual and seeing it is programmable becomes whether or not LinuxCNC can account for this acceleration? The motors have encoders on them so i imagine I can send that positioning data to Linux CNC, but I'm not familiar enough with the software yet to know if it can be accounted for our if I have the correct hardware from mesa?

I wasn't sure where to post this, but this category appeared to be pretty close in nature and I was hoping one of the more experienced users might be able to answer some of those questions as it's not pulling up any results when I search.

Attached is the setting image for the acceleration programmable parameters. Along with the PDF manual for the drive. Any help is greatly appreciated!

Best regards,

Chris

No, Linuxcnc will not account for that s-curve acceleration, it will be seen as a following error.
If you add the encoder feedback to Linuxcnc (and use it) Linuxcnc will try to correct for the following error introduced by the drives acceleration curve. End result you will probably see spikes in the following error at points where acceleration changes occur.
You did not really get the right hardware to read encoder signals. You would need something with encoder inputs like a 7i85, 7i85s, or 7i77.

All that said, you probably will be just fine using the 7i76 with no encoder feedback and setting the acceleration curve setting to a modest setting (less than 5 probably 2 or 3). You may want to experiment a little. You might be able to connect one motors encoder to the 1 encoder input on the 7i76 for testing to see how things work.

Todd wrote:

No, Linuxcnc will not account for that s-curve acceleration, it will be seen as a following error.
If you add the encoder feedback to Linuxcnc (and use it) Linuxcnc will try to correct for the following error introduced by the drives acceleration curve. End result you will probably see spikes in the following error at points where acceleration changes occur.
You did not really get the right hardware to read encoder signals. You would need something with encoder inputs like a 7i85, 7i85s, or 7i77.

All that said, you probably will be just fine using the 7i76 with no encoder feedback and setting the acceleration curve setting to a modest setting (less than 5 probably 2 or 3). You may want to experiment a little. You might be able to connect one motors encoder to the 1 encoder input on the 7i76 for testing to see how things work.

I figured it was too good to be true. The drives are closed loop to the motors via the encoders so no need to also include Linuxcnc, they just correct if there is a variance in commanded Steps to actual steps to ensure no steps are lost, they work very well like this, you can actually force the motor out of position and the drive will return it to the correct position (providing you don't fault it.)

I figured the motion planning would need accurate acceleration values or it might begin to make some undesired and out of sync movements between axis.

I guess I will play with it some and see what happens on a few parts with different acceleration settings in the drives and see how the part tolerances come out.. probably most critical on circular interpolated moves..

Thank you for the insights!

Chris

LinuxCNC's trajectory planner is what bounds the acceleration values, you just need to make sure your
drives acceleration limits are larger than LinuxCNCs limits so the drive limits are _never_ applied

PCW wrote:

LinuxCNC's trajectory planner is what bounds the acceleration values, you just need to make sure your
drives acceleration limits are larger than LinuxCNCs limits so the drive limits are _never_ applied

I'm confused, are you saying I can use the s curve?

No, using the s curve will cause following errors so cannot be used.

If the drive has acceleration limit settings they must be set higher than linuxCNCs
ini file acceleration limits

You don't need to use the acceleration profile in the motors because LinuxCNC has an acceleration profile built in.

Yo Dawg, your motors got acceleration profiles, so you can acceleration profile while you acceleration profile.


The profile in the motors would be for when they are used stand alone or single axis in a system that does not have an integrated motion control. Like a conveyor or some such.

OK, I'm not that deep into the LinuxCNC internals, but I would not expect motors operating under their own directive rather than listening to the motion controller would be a good thing.

I ebayed a pile of weird drives with what I thought were great and useful features until I realized that a stupid drive that listens to LinuxCNC was what I needed. Some day I'm going to have so many really slick single axis machines.

There are so many way to achieve a working CNC and it's quite overwhelming to see all the fantastic solutions that have been developed. Sometimes the most difficult aspect is not getting carried away or pulled in conflicting directions.