Author Topic: Load Cell, Wheatstone Bridge, Connector Contact Material  (Read 699 times)

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Offline 455Turbo

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Load Cell, Wheatstone Bridge, Connector Contact Material
« on: November 24, 2021, 05:51:00 pm »
I have a need to use series of electrical connectors between a load cell and an ADC board. I'm concerned about the effects of the connections on the signal. This is for a machine that is expected to be maintenance-free for about ten years.

I purchase the load cell. It comes with a shielded, four 30 AWG conductor cable. The conductors are seven strand tin plated copper. I cut the cable to about 200mm in length. I need to connect this cable to a high flex cable that will run through a (300mm long) Igus chain track. I need a connector there. This will be connected only once, during assembly. I have previously spliced and soldered the wires together but I want to get away from this as it's way too much work and probably not very reliable.

Within the Igus track I have room for five 'high flex' 30 awg single conductor wires, four for the Wheatstone bridge and one for the shield. The shield will no longer be braided but I'm not worried about it as the entire system is contained within a thick steel tube that is grounded. My ADC board is at the end of Igus track, bolted to the exterior of the steel tube. I need to connect the Wheatstone bridge wires to my ADC board. I want to use use screw terminals, headers, or a connector on my board. I have previously soldered the wires directly to the board. I want get away from this as I don't believe it's reliable. The board is my design so I can use whatever connection I want.

The cycle time for the machine is typically a few seconds, at most it's a few minutes. I am able to 'zero' the bridge between cycles. So I'm not too concerned about short term drift. I am, however, concerned about electrical noise. I'm striving for 1 part in 10,000 noise free resolution. I use a delta sigma ADC sampling at 1kHz. This has shown to work well with soldered connections.

Usually I try to use gold contacts for low voltage signal connections. The only connectors for 30 awg wire I can find, with gold contacts, use an IDC connection to the wire. I feel as though a crimp connection would be better. The wire to connector interface is tin plated copper wire with a tin plated contact. Is this okay long term? How can I connect the wires to my ADC board? Would a screw terminal with tin contact work? Are there any gold contact screw terminals out there? Are there any references out there that might help me to understand low-voltage signal connections?

Thanks in advance for any help.

 

Offline bill_c

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Re: Load Cell, Wheatstone Bridge, Connector Contact Material
« Reply #1 on: November 24, 2021, 09:16:17 pm »
You may want to move the ADC right next to the load cell.
 

Online Benta

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Re: Load Cell, Wheatstone Bridge, Connector Contact Material
« Reply #2 on: November 24, 2021, 11:09:25 pm »
You seem to have two issues:
one is electrical, let's put that aside right now.

The second is mechanical, where you need reliable connections for a flexible conduit.

First, soldering stranded wires is a 100% no-go, they'll break in no time. Crimping is the solution here.
Second, gold plating: yes, it's good in chemically aggressive environments, but from wear (plug(unplug)  and contact resistance not the best.
Third, screw terminals could be an option, but only when using crimped ferrules on the stranded wires.

I strongly recommend that you contact a professional sales/applications representative from one of the major players, eg, Tyco (TE) or equivalent.

 

Offline David Hess

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Re: Load Cell, Wheatstone Bridge, Connector Contact Material
« Reply #3 on: November 25, 2021, 08:53:18 pm »
Use gold plated connector pins.  We used 1625 series and the larger Molex connectors with pins that we had gold plated ourselves, but now you can buy the pins already plated.  The connectors were enclosed in an environmental proof housing but that would require machining yourself.

A better option would be one of the "military" circular connectors which are already sealed against the environment.  They have glands to seal the cables and use a passivated aluminum housing.

Consider using armored cable with a thick polyurethane jacket and stainless braided shield.  Critters are bound to chew on the cable unless it is inside conduit.
 

Offline 455Turbo

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Re: Load Cell, Wheatstone Bridge, Connector Contact Material
« Reply #4 on: November 26, 2021, 09:32:10 pm »
Thanks for all of the help.

I think I found some molex contacts and connector housings that will work. My mistake was searching for a 'connector' instead of the individual parts to make one up. The Molex 1855/1856 (1.56mm) series looks like it might work. This will be tin plated brass crimped to tin plated wire with gold mating contacts. I'll use indiviual high flex wires within the Igus track. The connectors are only going to be mated once so I'm not concerned about wear.

It's interesting that for Igus' smallest flex chain, they don't make any four conductor shielded wire that can be used with it. This is per Igus technical support.

The load cell, Igus track, flex wires, and ADC board are all within a sealed linear actuator so I don't have a big concern about environmental issues. The actuator will pump air in and out as it is used but I will be in a factory environment. There won't be any access for critters.

I looked at moving the ADC closer to the load cell. This is a good idea with some advantages. In the end, though, it would require at least two more conductors and there just isn't room in the Igus track.
« Last Edit: November 26, 2021, 09:34:10 pm by 455Turbo »
 

Online jbb

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Re: Load Cell, Wheatstone Bridge, Connector Contact Material
« Reply #5 on: November 27, 2021, 12:56:55 am »
On moving the ADC, you could look at isoSPI chips from Analog Devices.
 


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