Display Encoder position with button to zero.

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06 Jul 2017 15:47 #95437 by Beezerlm
Andy - I tied the button to pin hm2_7i43.0.encoder.01.reset and it works great for the X.

I also did this for the Z but if the zpos-cmd is at 0.500", it will zero the encoder and then walk 0.500 to meet the zpos-cmd. Is there a way to reset both the zpos-cmd and the encoder simultaneously?

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06 Jul 2017 19:26 #95448 by Beezerlm
I'm sure you are wondering why I want to do this for Z so I will try to Explain:

The Z axis is controlled by a hydraulic cylinder with a moog servo valve. This being the case, there is a small amount of "Stick and Slip" on the Z axis when trying to make very fine movements. It may float back and forth about ±.0004" while trying to land on position. I have a hard stop with an adjustable fine thread screw so when we have to grind up to a shoulder we can approach it slowly and carefully. The operator may have to command the position just beyond the hard stop (using the MPG) to keep some pressure on it while adjusting the screw to its final position. Once the face is ground or touched, I want to set that at zero. As the machine is currently configured, If the commanded position is beyond the stop, it won't zero the axis at the hard stop location (actual encoder position).

Hopefully you can understand what I was trying to say. Let me know if I can clarify better.

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07 Jul 2017 15:40 #95473 by andypugh
Is this a standard cylinder or a hydrostatic one? (the hydraulic servo machines I used to work with used hydrostatic cylinders and managed positioning to microns at huge loads)

I expect that you are right that touch-off works to the commanded rather than measured position. You might be able to get round that by incorporating your own sall offsets within hal (look at the "offset" HAL component.) Mux2 can be used to store and latch a value, if you feed its output to the input.

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08 Jul 2017 14:37 - 08 Jul 2017 14:56 #95503 by Beezerlm
This is a standard hydraulic cylinder. I added the servovalve to control the table via PID loop. I think it would be better at precise positioning if the ways were linear bearings rather than flat/v-ways. I believe It's the stick/slip on the ways that is the issue. The ways have been flaked and I believe they should not be.

Here is the thread from the original build - www.cnczone.com/forums/linuxcnc-formerly...er-emc2-project.html

I see the offset HAL component has 5 pins but I am a little unclear of their function. Is this correct? (I have a feeling I am way off here)

offset.N.offset - The offset value (I would net this to my zpos-cmd) ?
offset.N.in - This is the input for the value that gets added to the offset ?
offset.N.out - Equal to: (offset.N.offset + offset.N.in) ?
offset.N.fb-in - The feedback input (I would net this to zpos-fb) ?
offset.N.fb-out - Equal to: (offset.N.fb-in - offset.N.offset) ?

So I would use Mux2 to store and latch the offset.N.fb-out value?
Last edit: 08 Jul 2017 14:56 by Beezerlm.

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09 Jul 2017 01:11 #95525 by Todd Zuercher
Is it possible that there is air in the system, causing problems?

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10 Jul 2017 14:21 #95603 by Beezerlm
That is a possibility, The main cylinder has "choke coils" at each end that bleed the air out. When the machine starts up after sitting a while I have to run it to each end of the stroke to get the air out of the cylinder. It runs smooth after that. When given a position command, it may stop within 0.0002" of the desired location, then the output will slowly start to climb until it overcomes the friction but it may overshoot by 0.0002" once it moves. This leads me to believe it is the "stick/slip" of the ways. The ways were flaked on this machine which should not have happened and I believe this increases the force required to overcome the static friction. Even if the machine did stop on a dime, I would still want to use a hard stop just to prevent unintended crashes.

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10 Jul 2017 14:45 #95605 by Todd Zuercher
If the ways are bad there could be all kinds of binding going on. Both from the friction of the bad ways, and possibly another source that could have caused the over loading and failure of the ways in the first place.

Sounds like the first order of business is fixing the bad ways.

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10 Jul 2017 14:48 #95606 by andypugh

Beezerlm wrote: This leads me to believe it is the "stick/slip" of the ways. The ways were flaked on this machine which should not have happened and I believe this increases the force required to overcome the static friction.


This is not something I have heard before, but that doesn't mean it can't be true.

I wonder if changing the oil would help? Can the Gibs be adjusted to run slightly more freely?

In some other systems I have worked on a dither signal added to the command can help overcome mechanical hysteresis. That might be worth a try (maybe turn it on when the error is small and the I-term hits a threshold value)

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10 Jul 2017 15:45 #95608 by Beezerlm
I could be "way" off as to why the machine has stick/slip. I had the table off about 8 years ago and I don't remember any gibs. Just a V way and a flat way. There is recirculating oil holes every so often to keep the ways floating like an air-hockey table. We checked to make sure there was oil coming from each hole before putting the table back on. We had the ways flaked (Top and Bottom if I remember correctly) by a local machine rebuilder. I don't believe the original manufacturer (Heald) flaked the ways. That is what leads me to believe the flaking was a mistake.

All and all, the machine grinds nice and does what we want it to do. I am just trying to make it a little easier on the operators when it comes to setting the zero up against a shoulder.

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10 Jul 2017 16:21 #95610 by Todd Zuercher
Sorry, I miss understood what "Flaked" was. (Still no idea really.) I was thinking profile linear bearing or tank roller bearing with spalling type failure. I should have known better that a machine heavy enough to use hydraulic controls would almost certainly not have roller ways. (I hang around routers too much. I'll go back and stand in my corner a while, dons dunce hat.)

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