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  #1  
Old Tue 20 February 2007, 05:08
Gerald_D
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Grounding or Earthing

Proper grounding/earthing is essential in a computer controlled machine. A couple of points can be discussed in this thread.

When this thread was originally started, there was no mention here of SAFETY. It is absolutely critical that the metal frames of the MechMate be grounded to prevent shocks and damage to the electrical components when things go wrong. Following the principles mentioned further in this post will ensure the safety grounding as well.

We use single point grounding to prevent ground loops. The main ground "gathering" point is the alu heatsink under the Gecko's. This plate is convenient because it is big, conducts well, is never painted, and easily screwed into. (Do not use a hard-anodised plate - that anodising is a good insulator). The alu plate is mechanically screwed to the control box, which is grounded to the building's ground. The ground wire in the cable supplying the control box is connected either to the box or the alu plate.

From the marked point on this plate........




.....a thick 6mm2 [2-4 ga] flexible cable is run to the table...





.....from the table to the gantry.....





.....from the gantry to the y-car....






.....from the y-car to the z-slide....



The ground cable is the green/yellow one with the yellow crimp lugs on the ends. Use a heavy gauge wire for minimum resistance/impedance.
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  #2  
Old Tue 20 February 2007, 05:21
Gerald_D
Just call me:
 
To get a good connection to the painted metal, first remove the bulk of the paint at that spot and then install a serrated washer between the crimped lug and the "dirty" metal:


These "star" washers are also used at the alu ground plate in the control box. Alu also needs something to break through its "dirty" layer.
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  #3  
Old Tue 20 February 2007, 05:32
Gerald_D
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When using a sawdust extractor, and having the sawdust rub along the inside of a plastic pipe, huge voltages are generated and the static electricity must be drained to ground. Don't underestimate the havoc an un-grounded dust hose can cause! (fire is another consideration - seriously!)

Proper industrial grade dust hoses have a spiral wire in them for 1. keeping their shape, and sometimes, 2. for grounding. Their plastic grade is may be chosen for allowing the static electricity to drain to the wire. (If you can see a spiral wire in the plastic, it does not neccessarily mean the pipe is anti-static - the plastic may be insulating the electricity from the wire. A special "plastic" needs to be used) If your hose is of unknown grounding quality, run a bare copper wire along the inside of the hose.

The wire in a dust hose is normally not connected to the MechMate's ground system - it is connected to the dust extracting machine's (fan's) ground.
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  #4  
Old Tue 20 February 2007, 05:59
Gerald_D
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Why must the ground cable be so thick? I am not expert on this but I believe it is because a second ground path (loop) does exist via the v-rollers as well. In cases where there is chance that a bearing may be conducting, we put a very low resistance path parallel to it and hope that the electricity flows via the cable instead of via the balls & grease in the bearing.
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  #5  
Old Tue 20 February 2007, 07:00
Gerald_D
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Some light reading:
Networked Machine Shop/Manufacturing Plant Learns the Value of Proper Grounding
How four years of bad advice kept an innovative company from realizing the potential value of its networked CNC-CAD/CAM systems
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  #6  
Old Tue 20 February 2007, 07:07
Gerald_D
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Another resource:

http://www.mikeholt.com/technical.ph...eral#Grounding
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  #7  
Old Tue 20 February 2007, 12:12
Stephen Hull
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Gerald,
Excellent grounding articles. Personal experience with these issues, now when there is an electrical problem, first thing I check for is proper grounding, loose connections, etc. I have spent time, $ and frustration replacing parts only to isolate the problem as poor ground or loop or a cold solder joint. Your suggestion of crimp connectors, large guage wire, star washers and clean metal is sound. Often I have seen sheet metal screws only used to secure the ground lug and they can work loose and make poor contact. I now use bolted screw with star washer where possible. I also use high quality contact cleaner. Thanks for your continued excellent support.
Stephen
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  #8  
Old Sun 10 June 2007, 13:12
Alan_c
Just call me: Alan (#11)
 
Grabouw (Western Cape)
South Africa
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Quote:
Originally Posted by Gerald D View Post

One must take care in how the power supplies are wired.
- The negative line of the main 70V section is connected to the metal chassis for safety reasons amongst others.
- The negative line of the auxiliary 5V must not be connected to any other ground point. Only connected to the GND terminal on J8 of the PMDX-122.
- The GND points on the PMDX-122 must not be connected to any other ground point in the control box. (I will post more about this in the PMDX thread)
With reference to the above, where does one attach the shield of the screened cables without causing loss of isolation
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  #9  
Old Sun 10 June 2007, 13:44
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
All the shields are connected to that alu plate under the Geckos. The shields at the far ends of the cables are left unnconnected and insulated with heatshrink sleeve.
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  #10  
Old Fri 17 August 2007, 01:24
Alan_c
Just call me: Alan (#11)
 
Grabouw (Western Cape)
South Africa
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With the Campbell board, you don't bring the printer cable all the way to the BOB - the ribbon apparently goes to the side of the box and that's where the printer cable meets it.

My question is must this cable port be isolated from the case?

If I measure mith my multimeter on the metal casing at either end of the PARALLEL cabel I get continuity, so if the port (which is metal) is connected to the cable, and the other end of the cable is connected to the computer which is also metal and is earthed, will I be inducing a ground loop?
The earth prong of the computer supply cable is connected back to the case of the control panel as it gets its power from there. The earth of the case is common with the earth of the computer.
Next question, is the "ground" that we have to keep seperate, isolated from earth at the computer side as well?
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  #11  
Old Fri 17 August 2007, 03:07
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Everybody agrees a cable shield should not be connected at both ends, but there is no discussion that I can remember about the shield of a printer cable being connected at both ends. Maybe all printer cables are not like that?

On our PMDX it is not an issue because the whole BOB "floats" away from earth. We did earth it by mistake in the beginning and had some interference problems. PMDX does have "isolated" BOB's that can be grounded apparently, but I am not sure what exactly this means. Maybe your Campbell board is isolated, but that doesn't get rid of the ground loop, maybe the ground loop doesn't matter if the BOB is of the isolated type. It is all Greek to me!

Why not cut a big hole, mount a plastic sheet over it, and then mount the port in the isolated part? That could cut out a lot of "maybe" discussions.
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  #12  
Old Fri 17 August 2007, 08:51
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
If possible unscrew the covers from each end of the parallel cable to see if the shield is just soldered to the metal part of the connector. If that is the situation, cut the shield conductor and tape off the shield at one end of the cable (preferably the end that connects to the computer - but either end will work).
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  #13  
Old Mon 20 August 2007, 05:59
Alan_c
Just call me: Alan (#11)
 
Grabouw (Western Cape)
South Africa
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My parallel cable that I will be using has moulded ends so that is not an option. On an old severed cable I have, the shield is a mylar film with an uninsulated wire connecting the two metal cases at either end. To over come any doubts, I have made an insulating mounting for the port. pics to follow soon...
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  #14  
Old Mon 20 August 2007, 14:52
Alan_c
Just call me: Alan (#11)
 
Grabouw (Western Cape)
South Africa
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Here are some more update pics.

No1 shows the insulator I made for the parallel port

No2 shows the arrangement fitted to the case

No3 is an internal view

No4 Shows the Geckos mounted on the heatsink (darn those critters are small!) You may also notice all the labels on the interconnecting wires of the panel - that helps me make order of what could be a tangle of spagetti. All my circuit diagrams have corresponding labels and numbering and I have made a spreadsheet listing every wire with its identifier, source and destination, (still to add length), colour and size. my thinking is that if I make fault finding so easy, I hopefully wont have to...
Attached Images
File Type: jpg insulator1.jpg (67.5 KB, 2757 views)
File Type: jpg insulator mounted.jpg (43.9 KB, 2750 views)
File Type: jpg insulator inside.jpg (72.0 KB, 2741 views)
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  #15  
Old Thu 06 September 2007, 09:06
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Spacers

Gerald,

This is probably a stupid question but I don't want to assume anything. Are the spacers you used to raise the din rail and the Gecko plate off the back plate made out of metal? They appear to be aluminum but might be delrin. I can't tell from the pictures. I looked at the control box photos others have posted and some look to be made from a synthetic material. Seems to me that since ground wires are attached to the Gecko plate and rail and the control box is grounded, there should be continuity between them therefore the spacers should be made of metal but like I said, I don't want to assume anything and then, later on, have trouble figuring out why the machine won't run.

I'm just a few days from finishing all the mechanical stuff and will post pictures. This whole experience has been a blast. I can't thank you enough for being such a generous guy. I'm already thinking about building my next Mechmate so I can have a CNC plasma cutter too.

Doug
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  #16  
Old Thu 06 September 2007, 10:27
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hi Doug

The stuff that sits on the DIN Rail is all insulated from the rail. Nothing touches the rail electrically. So it doesn't matter if the rail is electrically connected to the panel or not.

Those things that I used are actually rubber doorstoppers
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  #17  
Old Thu 06 September 2007, 12:34
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
If you use the Green/Yellow grounding terminal blocks, they're designed to pass anything that's connected to that particular terminal block directly to the DIN rail. In that case, it would be necessary to connect the DIN rail electrically to the chassis.

(By the way, it's probably NOT a good idea to use the Green/Yellow terminal blocks if you have several DIN rails. Depending on the size of the chassis, you could cause a ground loop. That would not be a problem if all of the control signals were normal AC or even 24VDC, but with the sub 5VDC signals used by the Geckos mixed with all of the electrical noise from everything else, it's best to keep everything properly grounded - meaning no ground loops.)
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  #18  
Old Thu 06 September 2007, 12:51
J.R. Hatcher
Just call me: J.R. #4
 
Wilmington, North Carolina
United States of America
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Mike I hope this is not one of those stupid questions, but what is a ground loop??
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  #19  
Old Thu 06 September 2007, 16:46
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
All ground connections should be at the same voltage level. Ground is the point from which you measure all other voltages. Good practice is to connect the 'ground' from each circuit to the same bolt so that all ground connections are 'common' to each other. When the various 'grounds' are connected to different bolts, the resistance of the metal between the bolts can cause the 'grounds' to NOT be exactly common. When that happens, one reference voltage is higher or lower than another reference voltage. In a circuit where 1/10th of a volt is the difference between "off" and "on", any difference between 'ground' levels can cause a problem.

So, all ground points should be connected to the same bolt. There should be NO measureable difference between 'grounds' on any two circuits. If you do that, things will work better together.

Here's a link that helps explain things better than I have done:

http://en.wikipedia.org/wiki/Ground_loop_(electricity)
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  #20  
Old Thu 06 September 2007, 21:45
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Spacers

Thanks Gerald and Mike. And thanks J.R. I didn't know what a ground loop was either but I had already asked my limit of stupid rookie questions and I wasn't going to push my luck.
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  #21  
Old Thu 06 September 2007, 22:20
Marc Shlaes
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Cleveland, OH
United States of America
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Doug, don't worry about it. I think I have got you beat on "rookie" questions for some time to come!
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  #22  
Old Thu 06 September 2007, 22:43
Greg J
Just call me: Greg #13
 
Hagerman, New Mexico
United States of America
Thanks for the ground loop lesson Mike,

As always, you made "things" more understandable (I'm not an English Major).

I think you have a secret identity and your a prof. at the University of Utah.

Greg
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  #23  
Old Fri 07 September 2007, 00:59
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I always think of trees when I think of ground loops. The one is the opposite of the other.......

On a tree, there is only one path in which a leaf can get its nourishment from the soil. There is only one clear route from the leaf to the trunk. That is how a good grounding system should be.

If your tree was some weird mutant and the leaves had two of more possible routes back to the trunk, then the nourishment could travel in loops and bypass the leaves. Nah, that sounds too crazy - let's have proper trees without loops.
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  #24  
Old Fri 07 September 2007, 06:33
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Greg,
I'm BYU True Blue, through and through. The U of Utah Red is the most likely cause of my high blood pressure (especially during football season, late in the season when BYU and that other team meet).

Gerald,
The tree analogy explains ground loops perfectly.
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  #25  
Old Mon 10 September 2007, 15:09
Mike Nash
Just call me: Mike Nash
 
Bessemer, Alabama
United States of America
Ground Loops? (Your crazy Banyan Tree.)

Quote:
Originally Posted by Gerald D View Post
I always think of trees when I think of ground loops. The one is the opposite of the other.......

On a tree, there is only one path in which a leaf can get its nourishment from the soil. There is only one clear route from the leaf to the trunk. That is how a good grounding system should be.

If your tree was some weird mutant and the leaves had two of more possible routes back to the trunk, then the nourishment could travel in loops and bypass the leaves. Nah, that sounds too crazy - let's have proper trees without loops.
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  #26  
Old Mon 10 September 2007, 21:46
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
http://en.wikipedia.org/wiki/Banyan has lots of interesting info on the Banyan - but read between the lines and you'll see it is a parasitic weed

We always need exceptions to prove rules!
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  #27  
Old Fri 14 September 2007, 20:19
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Quote:
Originally Posted by Gerald D View Post
Hi Doug

The stuff that sits on the DIN Rail is all insulated from the rail. Nothing touches the rail electrically. So it doesn't matter if the rail is electrically connected to the panel or not.

Those things that I used are actually rubber doorstoppers
How about the aluminum plate the Geckos are mounted on. Is the mounting insulated or is there continuity with the enclosure? Sorry to be a pain but electrical stuff is not my forte.
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  #28  
Old Fri 14 September 2007, 22:48
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hi Doug

I used the aluminum plate as the major, and central, grounding point for the whole system. As such, it is very solidly connected to the enclosure with a good copper wire.

What might be causing a bit of a problem here the ground loops caused through connections such a bolts & nuts, hinges and bearings. Sometimes we bridge (loop?) those with copper wires as well. For example, a good enclosure door always has a copper wire "over" the door hinge as one cannot trust a hinge to give good continuity.

My alu plate is screwed to the back plate with metal fasteners and no attempt to insulate it (as is the DIN rail) - nor did I bother to remove paint from the fastened joint. But, since my alu plate is the primary ground point, all electrical connections to that plate are good and clean.

I regard the alu plate as the trunk of the tree. The enclosure itself is just one of the leaves.
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  #29  
Old Sun 16 September 2007, 19:44
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Gerald,

Thanks a million. That makes perfect sense. I really appreciate the help.
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  #30  
Old Mon 17 September 2007, 09:17
Allegheny
Just call me: Brian
 
Massachusetts
United States of America
Gerald,

Using the aluminum plate as the central ground makes sense. Here in the States there is concern that bonding copper and aluminum conductors together can cause galvanic corrosion and contact problems down the road. I have even read of fires attributed to arcing caused by the resulting corrosion. There is an anti-oxidation/conduction "paste" that is used for situations where mechanically bonding these two materials is unavoidable (such as extending circuits in an old house with aluminum wiring in the walls).

Brian
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