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.

I know I’ve tuned my other machines.
I usually bench test first. Then install and tune, then connect Linuxcnc and tune it. And the trouble start when I connect the analog control.
Even with analog Control and the servos have no load they have 50hz vibration. Which I can’t get rid of. With the spindle I don’t have the issue the wires could be routed differently and I can use other better shielded wire. For the drives I can’t do this easily. I twisted and try to shield the wires with tape. But it didn’t improve. And have no idea what else to do. I could start to add ferrites or try to reroute all other cables and chasing down the issues. But I figured going digital would be quicker

It wasn't clear you did tuning from you previous posts(or mu reading), sorry.

Diy shielding never works, twisted pair shielded cable is not that expensive and may be quicker solution.
...thinking now, how analogue signal running 20-30cm from mesa board to servo drive picks up all that noise anyway? Or??? did you install drives next to the motors and ran control cabling all over the machine?

I was kind of lazy and used a prewired dB 50 connector to wire up the drives. They are still in the switching cabinet. But I needed to open them up quite soon to wire in the connection to the limit withes. So then they run open for about 80 cm next to main power an most of the other three phase auxiliary power lines for lubrication, coolant, fans and so on.
I did solder the db 50 connectors for my lathe and used network cables there. But I hated it and needed some troubleshooting later due to mistakes i made. I was hoping to get away with it. But I didn’t. I could spent one night soldering up the cables and another fixing them.
The cables I have are fully connected, so converting to step dir is easy.
I could try solderfree connectors… but they would be more expensive than the additional mesa board I need. And I want to get the machine running before I have to move her to my new shop. I really don’t like these db50 connectors. My spindle driver is better at this point but it’s also much bigger.