Allen Bradley Signal Conditioner - How to tune Excitation voltage and Offset

BrockSamson

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Feb 2019
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I have a Allen Bradly Signal Conditioner card (931S-B1C6D-DC) hooked up to a strain gauge. The excitation voltage is 10V, the output is 4-20 ma and the input is 0 - 10mv.

There are two screws on the card to manually adjust excitation voltage and offset. I do not see anything referenced in the manual on how to adjust these properly and Google has not helped.

Anyone know how to properly adjust these two settings or am I missing some documentation somewhere?

Thanks
 
Is this the manual you're referring to?


https://configurator.rockwellautomation.com/api/Doc/10000043404_931S-B1C6D-DC.pdf


This has a decent table with the DIP switch settings, although as you said, I don't see anything about how to actually adjust the supply and offset, just that it is adjustable.


EDIT: The datasheet (link below) does have a vague instruction in the bottom right of how to adjust them, but it basically just shows the excitation and offset changing depending on what way you turn the screws:


https://configurator.rockwellautomation.com/api/Doc/931S-B1C6D-DC.pdf
 
Last edited:
Is this the manual you're referring to?


https://configurator.rockwellautomation.com/api/Doc/10000043404_931S-B1C6D-DC.pdf


This has a decent table with the DIP switch settings, although as you said, I don't see anything about how to actually adjust the supply and offset, just that it is adjustable.


EDIT: The datasheet (link below) does have a vague instruction in the bottom right of how to adjust them, but it basically just shows the excitation and offset changing depending on what way you turn the screws:


https://configurator.rockwellautomation.com/api/Doc/931S-B1C6D-DC.pdf

In this document you can see how to adjust the excitation voltage (from 4.8 to 10.2v)
page 32
https://literature.rockwellautomation.com/idc/groups/literature/documents/td/931-td001_-en-p.pdf


Okay so I guess they are only used when the dip switches for the Excitation Range are are the "adjustable" selection???

So if I have the dipswitches for the Excitation range just set to "10V" than I dont even need to use them?

There is no reason i would choose and adjustable excitation range if I just want 10v right?
 
Okay so I guess they are only used when the dip switches for the Excitation Range are are the "adjustable" selection???

So if I have the dipswitches for the Excitation range just set to "10V" than I dont even need to use them?

There is no reason i would choose and adjustable excitation range if I just want 10v right?


That's what it looks like to me, but I'm not positive. I don't know if your offset is also only adjustable if you have it on that setting, but I also don't know if it would be necessary if you're not changing the bridge excitation voltage. I haven't worked with strain gauges since I was in university so I'm not as familiar with them as I once was.


EDIT: so it looks to me like your SW1 dip switches would be set as follows, starting with 1 on the left and 10 on the right, same as in the manual:


1001100000
 
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Wait a minute you have not provided us with enough information
What are the strain gauge / load cell ratings. It should be on plate on the strain gauge
Without that information you can’t begin to set things up.
The excitation should be set to what the strain gauge requires.
Most load cells excitation voltage are either 5VDC or 10VDC but I am sure there are others out there.
If you put 10V on a load cell that rated for 5V there is a good possibility you will destroy the load cell.
The reason you have an adjustment is you can adjust for any loses in the wiring between the signal conditioner and the load cell.
Remember you need the full excitation voltage at the load cell to get an accurate reading.
The excitation should be measures at the load cell itself not at the output of the module.
Better load cells have a + sense and a – sense coming back from the cell bridge so you can measure and adjust the voltage to the load cell bridge.
As for the load cell ratings, all the ones I have worked with are rated in mv per volt of excitation voltage
So the output would be dependent on the excitation voltage
Without having all the information there is just no way you can expect to get accurate readings from any system.
With a 10V excitation I would expect the input to the signal conditioner to be 0-10mv
 
Wait a minute you have not provided us with enough information
What are the strain gauge / load cell ratings. It should be on plate on the strain gauge
Without that information you can’t begin to set things up.
The excitation should be set to what the strain gauge requires.
Most load cells excitation voltage are either 5VDC or 10VDC but I am sure there are others out there.
If you put 10V on a load cell that rated for 5V there is a good possibility you will destroy the load cell.
The reason you have an adjustment is you can adjust for any loses in the wiring between the signal conditioner and the load cell.
Remember you need the full excitation voltage at the load cell to get an accurate reading.
The excitation should be measures at the load cell itself not at the output of the module.
Better load cells have a + sense and a – sense coming back from the cell bridge so you can measure and adjust the voltage to the load cell bridge.
As for the load cell ratings, all the ones I have worked with are rated in mv per volt of excitation voltage
So the output would be dependent on the excitation voltage
Without having all the information there is just no way you can expect to get accurate readings from any system.
With a 10V excitation I would expect the input to the signal conditioner to be 0-10mv

I have the card set up right, it works i just don't know if I need to adjust the excitation volatage and Offset.. The excitation voltage is 10V and the input is 0-10mv from the strain gauge. I just don't under stand if I can just set the card to 10V excitation and it is accurate or if I have to use the adjustments for Excitation voltage and Offset.

So I should confirm the excitation voltage going to the strain gauge.
There are no "sense" lines

But I don't really know what the offset is referring too. Do I adjust until the output signal of the card (4-20ma) is 4ma at zero?
 
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First I assume that the excitation voltage for the strain gauge is 10VDC
if you want the best accuracy then you adjust the excitation voltage for the 10VDC at the strain gauge.
I believe the offset is for 4 ma output with no load on the strain gauge but that should be a limited adjustment sometimes you have to rescale the output in the plc depending on the preload. In the plc you can scale it to read whatever you want
Just make sure that you don't overload the strain gauge any overload at all will knock the accuracy way off
 
I have the card set up right, it works i just don't know if I need to adjust the excitation volatage and Offset.. The excitation voltage is 10V and the input is 0-10mv from the strain gauge. I just don't under stand if I can just set the card to 10V excitation and it is accurate or if I have to use the adjustments for Excitation voltage and Offset.

So I should confirm the excitation voltage going to the strain gauge.
There are no "sense" lines

But I don't really know what the offset is referring too. Do I adjust until the output signal of the card (4-20ma) is 4ma at zero?

The only way that you will know if you require to adjust the offset is if the strain gauge is fully unloaded and you're reading 0 scaled, or in your case 4mA unscaled. If that is the case, then you don't need to adjust the offset. Excitation voltage of the card should be set to the excitation required of the strain gauge. You would need to check and verify that from the datasheet of the strain gauge.
 

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