Drifting strain readings on NI 9237, via 9944 quarter bridge adapter, from loose cables

[DQ_QUB]

Hi,

I am attempting to setup a strain data collection system using the NI 9237 module with the quarter bridge accessory kit.  I'm experiencing pretty significant drift in the readings from most strain gauges after the initial offset nulling.

The drifting is very inconsistent with regards to magnitude and channels affected and after a process of elimination it appears that the source of the drifting is somewhere between the rj50 connection to the module and the output from the chassis.

The connection of the RJ50 cable to the 9237 modules isn't the tightest I've ever seen and the slightest movement of the cables causes the strain readings to jump and vary to magnitudes of up to 10,000 micro strain.

I guess I'm looking for any ideas how to correct this problem.  I'm aware there are 9237 modules with dsub connections which may be better, but we have neither the time or budget to replace all our existing modules with DSub versions!

Any help would be appreciated!!

[johnsold]

Can you determine if it is the connector itself or the cable motion which causes the problem?  Hold the cable and connector securely at the connection point and move other parts of the cable.  Do you see the problem when the connector is not moving?

If it is the connector, try to fabricate a clamp which holds the cables in a fixed postion relative to the 9237 module so that the connector cannot move in the socket.

If it is not the connector but moving the cable which creates the problem, then you may be seeing signals induced on the cables by magnetic fields.  Moving the cable through the field creates the interference.  This type of problem can be much tougher to cure because it is affected by so many variables in the environment.

Lynn

[DQ_QUB]

Hi,

I think it is the connection at the module that is the primary problem, the cable can be moved and the connection held without a sudden jump in strain readings, but the gradual drift remains.

We've tried this at a number of different locations around the building to rule out the issue of a noisy laboratory but the problem remains unfortunately.

[David. B]

Hello,

I think Lynn on the money with her suggestions and as it seems that it can be avoided by holding the connection, is this a possible workaround for you? Also have you tried it with many RJ50 cables or only with one currently?

Kind Regards

[DQ_QUB]

This is happening on about a dozen channels over 4 modules and we've tried lots of RJ50 cables, even new ones straight out of the packet.

Trying to secure all the RJ50 cables onto the modules inside a packed chassis will be very difficult, but we may have to try it.

Also, I'm not convinced securing the cables will fix the gradual drifting problem as when the system is left alone, with no movement or touching of the cables the drifting over time is still there.

[David. B]

Hello,

I have done some more research into your problem and have found that his issue has been seen before. This issue is caused by a varying cable impedance when the RJ-50 cable is disturbed.  This essentially translates to a variable lead resistance and voltage drop through the cable. The issue only occurs in quarter and half bridge applications, in full bridge mode, both sides of the bridge see the same voltage change and the remote sense capabilities of the modules means it is not an issue.

There are 2 options which you have to help solve this problem:

1. Replace the modules with DSUB modules but, as you said earlier, this is out of the question.
2. Use the AUX excitation connector for the excitation pins instead of the RJ50 EX+/- connections.

The second option seems the best for you to pursue due to the limitations you specified earlier. The EX +/- pins on the AUX connector have a direct short to each of the EX +/- pins in the RJ-50 connectors so you can just wire the excitation directly to the strain gauge from the AUX connector and still use internal excitation as normal. To connect to the AUX excitation connector you will require a NI-9942 4-Position Micro-Fit Plug and Terminal Kit unless you already own this.

I hope that this information helps you to solve your issue.

Kind Regards

[DQ_QUB]

Hi,

Thanks for the feedback.

We have the NI-9942 plug and terminal kits so will give that a go.

Could you clarify just how/where to make the connection from the AUX connectors to the 9944 quarter bridge adapter?

I'm assuming that it would be a case of connecting either of the EX+/- terminals on the AUX connector to what would be the 6 and 7 pin connections on the adapter, however, the 9944 only has 4 screw terminals, with EX+ the only identifiable excitation terminal.

Thanks

[David. B]

Hello,

There is a Knowledge Based Article which discusses how to do this. I hope that this helps to solve your problem.

Kind Regards