Please help! Experiment to measure contact resistance in relay contacts

[andywileman]

Hi,

I am completely new to Labview and have been asked to try and use the software and hardware to develop a VI for measuring how the contact resistance of relay contacts degrades over thousands of cycles.  I have managed to interface a NI cDAQ-9172 containing  NI 9219 and NI9472 modules and can access them through the DAQ Assistant.

What I would like to know is, how I can turn the relay I'm testing on and off using a pulse and keeping a count of the operations and at the same time take a measurement (this may be every 100 or 1000 cycle) and log it as a ASCII data file?

All I have at present is the NI documentation and a book called Labview for Engineers and Scientists by John Essick, which I'm slowly working through.

Is this sort of VI difficult task?

Best regards,

Andy

[crossrulz · Accepted solution]

You can have a digital task as well as an analog input task running on the same chassis.  You have just toggle the digital output to cycle the relay and read with your analog inputs.

Most DIO do not have enough current to drive a relay, so you might want to get a digital buffer that can supply enough current for your relay.

Oh, and check out the free online training modules

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[billko · Accepted solution]

To be honest, almost anything is going to be difficult if you are completely new to something.  Take your time, read the suggested links, give yourself at least some background on LabVIEW before jumping in.  🙂

[apok · Accepted solution]

we currently test "switch contact resistance" by MIL-PRF-22885G par 4.7.4.1 which uses the electrical supplied requirements: 6VDC at 0.1 amphere. using 4 wire measurement (kelvin contacts-source/sense) off the DUT using NI 6289 which has a 100mV range and a resolution of 0.80uV. also use ULNs 28xx to drive our relays, but can use a number of different ways(optocouplers, transistors...) that are TTL compatible.

[AnalogKid2DigitalMan · Accepted solution]

A 4 wire Kelvin connection (source and measure) is definitiely called for here as apok referred to.

-AK2DM

[andywileman]

Thanks for your help.  I have a Keithley 2002 which I can try to connect via a GPIB bus, will this be ok?

[jcarmody · Accepted solution]

I use an INSTEK Programmable DC Milli-Ohmmeter with GPIB (GOM-802GP) for this kind of measurement.

[andywileman]

I should add here that I'm interested in measuring how the contact resistance degrades over time as we are trying to develop prognostic capability for relays. 

[AnalogKid2DigitalMan · Accepted solution]

The 2002 may not have a substantial current source for the test to reflect reality.

Too little crrent passing through the contacts may not be enough to 'punch through' the oxide film layer and will giver higher than normal resistance readings.

You may want to think about the relay current ratings and test specs like apok mentioned.

With a proper and stable current source you could then then use the 2002 to measure the voltage drop across the contacts then calculate resistance R=E/I.

-AK2DM

[jcarmody · Accepted solution]

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@andywileman wrote:

Thanks for your help.  I have a Keithley 2002 which I can try to connect via a GPIB bus, will this be ok?

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Yes, the Keithley 2002 can do this type of measurement.  You'll need to have special (isolated) clips to do the measurement, though.  This one looks right - http://www.keithley.com/products/dcac/dmm/highper/?mn=5806