Mesa Analog Vs Step Dir

Hi,

I've done two analog controlled retrofits so far and they all wenn really well.
the third is giving me a head ace. I think I have a noise problem and so have oscillating servos. I tried to improve shielding but had no success.
I am thinking about changing to step Dir to avoid the noise problem.
The Servos can handle analog velocity mode or step dir positioning mode, my spindle controller will handle also Step Dir for velocity mode.
I have the Mesa 7i97 at the moment.
My question is what would make more sense To skip the encoder feedback and let the servo drives handle the positioning.
Or try to close the loop with the feedback from the servos. And what would be the mesa card to go? Or can the 7i97 also used for step Dir

Thanks for your input.

Patrick


 

Have you tuned the drive itself? That may be the root of the oscillation issue.

You can add a few step/dir channels to a 7I97 on the expansion connector if
desired, but if you are not using the 7I97 analog channels at all it would be less wasteful
to use a 7I96 or similar. If you add step dir channels to the 7I97, you would typically use
a daughter card like a 7I85S (4 channels of differential step/dir outputs) or a simple parallel
port type breakout like the common "Mach 5 Axis" BOB which has 5 axis of single ended
step/dir outputs.

In general, adding encoder feedback to LinuxCNC makes sense if you have linear
encoders (as they can improve the overall accuracy) or if you need to disable the
drives without losing position.
 

Many thanks!
I think I will get myself a 7i85s and switch to step dir.

On the bench test it worked and the drives on their own work also well.
only in the machine connected to linuxcnc they oscilate and it looks close to 50 hz.
I have linear scales on the machine (except for the 4th axis). but they have only a resolution of 0,05mm the encoders give me more theoretical accuracy.
I might add them later with higher resolution scales.

How does homing work without the use of the encoders? Is it just the accuracy of the switch then? Cause I would loose the Index signal from the encoders?

Its possible to home to index with step/dir, it just requires
firmware with step/dir index support and wiring the index to
the mesa card

many thanks, I'll give it a try, with the 7i95s the encoders are wired up so getting the index to the Mesa cards is already done.

working on the bench is not the same as working with real load, Servo drive/controller should have an inertia tuning function, even cheap ass Chinese drives have that. usually it involves setting parameter or 10 on the drive and enabling, it them moves back and forth adjusting internal PIDs to get max accelerations without oscillations.

Mitsubishi mr-j4 servo drives require active laptop connection to do autotune
yasakawa/omron sigma 1-3 don't, didn't have a chance to play with later ones.