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Unread 09-11-2017, 09:25 AM
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Wires and PCM

So a little bit of background and introduction I guess. This last week our team went to Mainely Spirit, for this event we choice to pop the tracks of, take the 20lbs cast iron weight off the bottom gear it up so we are going 20fps in high gear and switch to offense. We finish second seed and loose in the finals. What happened is the wire for the high pressure shut off came out of the PCM, which left us in neutral for Finals match 2.

1. We use crimp on ferrules on our wires

2. We still like to play some heavy defense when the strategy allows for it (Full speed across the field body slams on the gear peg being our most common and my personal favorite one)

Given all of that, does anyone have any ideas to how we can keep the wires from coming out of the PCM so we don't have another repeat of our last finals match.
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Unread 09-11-2017, 09:41 AM
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Re: Wires and PCM

Personally, I'm not a big fan of wire ferrules. I've seen too many failures caused by them pulling out of terminals when used on FRC robots. In other applications, where there is less vibration, jolts, and movement, they work great in ensuring the connection is the same for every use.

Instead, I would go with the process stated in the PCM User Manual:

http://www.ctr-electronics.com/PCM%20User's%20Guide.pdf
Quote:
Wire Insertion
● Disconnect PDP from Battery before adding or modifying connections
● Strip wire back ~0.375” (3/8”)
● Press and hold down connector button. Though this isn’t necessary, it ensures the
stripped wire does not deform and split into “whiskers” after excessive use. A small
screwdriver can be used to easily hold down the connector button.
● Insert wire into connector opening
● Release connector button
● Pull wire to ensure wire is locked in connector
● Confirm wire strands are not extruded
Those last two parts are really the key parts - you tug on it to make sure it's secure (trust me, you can tug enough that you'll be absolutely certain that the wire won't pull out without some catastrophic incident), and you visually inspect that you don't have any hanging "whiskers" that could short the wire.

Try a tug test on your wires with the ferrules on them... can you pull them out?
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Unread 09-11-2017, 09:42 AM
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Re: Wires and PCM

Are you using regular ferrules or ferrules specifically designed for weidmuller connectors? if you're not using the ones designed for this type of connector, then you're better off not using them at all. The connector holds the wire in by squeezing the wire strands. If you aren't using ferrules that are designed to squish (like the ones designed for weidmuller connectors), it can't hold as well.
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Unread 09-11-2017, 09:58 AM
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Re: Wires and PCM

So the biggest mistake I see teams make with wires is that they do not allow for strain relief and this causes the wires to come out by means of just normal jostling and rough robot play.

Basically, zip tie your wires down to something near the PCM but give them an inch or two of slack on the PCM side to wiggle and move freely with the PCM.

Not a great image but it kinda shows what I mean with the wires zip tied to the edge of the breadboard for strain relief:

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Unread 09-11-2017, 11:49 AM
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Re: Wires and PCM

+1 on what Marshal said. If you use ferrules: Use a) ferrules properly sized for the wire and b) a decent ferrule crimper. It does not have to be the Weidmuller brand unless you like spending money. If you can pull the wire out of the ferrule, you are not doing a or b. For the PCM, I would use a minimum of 18 AWG wire. Properly inserted, it will not pull out. If the PCM is hard to get to, it might be hard to properly insert the wire.
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Unread 09-11-2017, 12:38 PM
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Re: Wires and PCM

Quote:
Originally Posted by SpruceyBrucey View Post
So a little bit of background and introduction I guess. This last week our team went to Mainely Spirit, for this event we choice to pop the tracks of, take the 20lbs cast iron weight off the bottom gear it up so we are going 20fps in high gear and switch to offense. We finish second seed and loose in the finals. What happened is the wire for the high pressure shut off came out of the PCM, which left us in neutral for Finals match 2.

1. We use crimp on ferrules on our wires

2. We still like to play some heavy defense when the strategy allows for it (Full speed across the field body slams on the gear peg being our most common and my personal favorite one)

Given all of that, does anyone have any ideas to how we can keep the wires from coming out of the PCM so we don't have another repeat of our last finals match.
Allow me to clarify, It's not the wire that was coming out its the ferrule with the wire crimped onto it. perhaps the title I used was wrong
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Unread 09-11-2017, 12:50 PM
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Re: Wires and PCM

Quote:
Originally Posted by SpruceyBrucey View Post
Allow me to clarify, It's not the wire that was coming out its the ferrule with the wire crimped onto it. perhaps the title I used was wrong
Have you tried a tug test on all of your connections yet?
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Unread 09-11-2017, 01:29 PM
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Re: Wires and PCM

If you want reliable connections, pull tests are not optional.

The strain relief that Marshall is suggesting also requires that there be a bit of slack to form a "strain relief loop" so that the wire connection to the terminal is never in tension. If your wires are tight and you tie it down, it is still possible for tension to pull the wire out of the terminal.

Use a strong flashlight to inspect the area around where each wire enters the terminal. If you see something bright and shiny, it could be a wire strand that did not enter the terminal properly and can cause a short to the next terminal.
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Unread 09-11-2017, 07:55 PM
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Re: Wires and PCM

Quote:
Originally Posted by philso View Post
If you want reliable connections, pull tests are not optional.

The strain relief that Marshall is suggesting also requires that there be a bit of slack to form a "strain relief loop" so that the wire connection to the terminal is never in tension. If your wires are tight and you tie it down, it is still possible for tension to pull the wire out of the terminal.

Use a strong flashlight to inspect the area around where each wire enters the terminal. If you see something bright and shiny, it could be a wire strand that did not enter the terminal properly and can cause a short to the next terminal.
I personally did a tug test on the wires, I think what I'm going to do is replace the wires with longer one and make sure there is plenty of slack, hopefully that will solve the problem
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Unread 09-11-2017, 08:06 PM
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Re: Wires and PCM

Quote:
Originally Posted by SpruceyBrucey View Post
I personally did a tug test on the wires, I think what I'm going to do is replace the wires with longer one and make sure there is plenty of slack, hopefully that will solve the problem
If your wires are tight, when there is a mechanical shock, your Roborio or other component might shift slightly, increasing the tension on the wires, pulling them out or breaking the strands inside the insulation.
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Unread 09-14-2017, 10:00 AM
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Re: Wires and PCM

Quote:
Originally Posted by marshall View Post
So the biggest mistake I see teams make with wires is that they do not allow for strain relief and this causes the wires to come out by means of just normal jostling and rough robot play.

Basically, zip tie your wires down to something near the PCM but give them an inch or two of slack on the PCM side to wiggle and move freely with the PCM.

Not a great image but it kinda shows what I mean with the wires zip tied to the edge of the breadboard for strain relief:

Is there something less permanent than zip ties that provides for the same support? Some kind of conduit ?
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Unread 09-14-2017, 10:39 AM
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Re: Wires and PCM

Quote:
Originally Posted by Enthurzan View Post
Is there something less permanent than zip ties that provides for the same support? Some kind of conduit ?
Conduit will route/organize the wires, but not do much in terms of strain relief. We have used twist ties (with wire in them, most commonly a two-wire version from coffee bags) and velcro wraps when looking for a removable organizer/strain relief. If you do this, make inspecting/adjusting these a regular thing between matches.

There are also removable zip ties; not having used them, I can't recommend for or against.
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Unread 09-14-2017, 11:43 AM
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Re: Wires and PCM

Quote:
Originally Posted by Enthurzan View Post
Is there something less permanent than zip ties that provides for the same support? Some kind of conduit ?
Zip ties aren't permanent. Buy them in large quantities and use them liberally. Remove them with diagonal cutters as needed. #ZiptiesAreLife
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