Pressure feedback on 7i92?

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19 Mar 2022 21:37 - 19 Mar 2022 21:40 #237743 by dannym
I'm working on a head-mounted sander idea, most likely use will be slab tabletops after surfacing.  

The downforce adjustment is an interesting problem.  I could just dead-Z it (command the spindle down to a fixed coordinate, compressing the pad a known amount) and that might work, I don't know.  A better idea would be a short throw spring-loaded Z, so setting the Z lower means more compression and more downforce.  The downforce would vary if the work height varies, although it shouldn't, the idea is that the surface is already cut level.

One idea that makes a lot of sense would be a load cell force sensor.  I can build an external amplifier that can drive out a binary signal comparing pressure with target, an analog difference, or a PWM.  Then the Z would have to adjust inside LinuxCNC continuously to get the target force.

I can already see that's more complicated than it sounds, as this is not the normal scripted mode for G-code control of Z, and it's asking for a high speed response.  And the 7i92 I have is only capable of digital IO AFAIK, the binary above/below force threshold is easy to interface with, but not very stable as there's no indication how far off the pressure is. 

This task sounds similar to torch height in plasma cutters, which is handled notably differently in a control system.

I did have some other thoughts- if LinuxCNC has a lot of trouble here, I could make external hardware to take over the STEP/DIR signals with the load cell feedback.  The Z isn't going to be commanded into motion for most of the job anyways.  

Any thoughts on the force load cell concept?

I have a XHC wireless MPG.  Would it ever be possible to adjust the Z pressure while it's running a G-code pattern?
Last edit: 19 Mar 2022 21:40 by dannym.

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19 Mar 2022 21:59 #237746 by PCW
Replied by PCW on topic Pressure feedback on 7i92?
It does sound quite similar to THC so it could be handled in a a similar way
(with LinuxCNCs external offets)

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19 Mar 2022 22:04 #237747 by rodw
Replied by rodw on topic Pressure feedback on 7i92?
Following on with the plasma theme, you can buy 0-10v output load cell interfaces so you could read the force directly with a THCAD-10 the same way as we do with plasma. THen as PCW says, feed the output into external offsets.

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20 Mar 2022 01:05 #237760 by pippin88
Replied by pippin88 on topic Pressure feedback on 7i92?
Interesting to think about, but perhaps you should try the simple spring loaded system and see if there a real need for anything fancy?

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20 Mar 2022 01:16 #237764 by tommylight

Following on with the plasma theme, you can buy 0-10v output load cell interfaces so you could read the force directly with a THCAD-10 the same way as we do with plasma. THen as PCW says, feed the output into external offsets.

This would be the best idea by far, and the cheapest for sure, although not sure how much would that load cell cost
but that has to be included either way.
Also it would be fast, very fast.
And just inn case, yes you can wire a Mesa THCAD to Mesa 7i92, i do that all the time in plasma machines, wired one today.

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20 Mar 2022 02:20 - 20 Mar 2022 02:43 #237768 by dannym
Replied by dannym on topic Pressure feedback on 7i92?
Spring loading would seem to mandate something surprisingly complicated.
I don't want it to tilt under load, so it would need to move up and down on its own linear rail. The load cell would allow me to mount and tram it rigidly.

A load cell does have to deflect to get a reading, but in practice it's a pretty small amount

 
Last edit: 20 Mar 2022 02:43 by dannym.

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20 Mar 2022 02:35 - 20 Mar 2022 02:48 #237769 by dannym
Replied by dannym on topic Pressure feedback on 7i92?
Load cells are cheap. I can design load cell amps and PCBs too.  I could do 1, 3, or 4 load cells and sum them.

How does a THC input go into a Mesa 7i92? I'm actually redesigning the breakout board I'm using so this would be a great time to do it.  What bitfile would it use?  Is it a voltage-to-frequency digital input on the Mesa 7i92?

I can see this may get more complicated than initially expected, because there'll be a lot of high freq vibration transmitted through the load cell.  There's be some analog filtering required.  Also when it moves laterally, ideally a load cell shouldn't read any extra force, but I imagine it will.
Last edit: 20 Mar 2022 02:48 by dannym.

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20 Mar 2022 08:01 #237777 by rodw
Replied by rodw on topic Pressure feedback on 7i92?
Yes, the thcad is connected to the encoder input which converts the frequency to a count which can be scaled as you like in software.
Quite simple really.

In fact a long time ago I wrote a component to scale it.

forum.linuxcnc.org/plasma-laser/32585-scalethcad-component

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20 Mar 2022 14:40 #237806 by dannym
Replied by dannym on topic Pressure feedback on 7i92?
OK does it need all 3 encoder pins?

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20 Mar 2022 15:10 #237815 by PCW
Replied by PCW on topic Pressure feedback on 7i92?
No, just one signal (frequency)

A cheap V-F chip will work (AD7740 if you can get them)

You could also use a SPI interfaced A-D but its more
software intensive to setup (using the BSPI HM2 interface)

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