Choosing a load washer

[jaredts]

I am new to data acquisition, labview, etc.  I have the NI 9237 (RJ50 style) with NI9945 quarter bridge completion modules and of course Labview.  Can anyone suggest load washers to be used with this?  If I continue to use quarter bridge, I just need to make sure I order load washers with 350 ohms and an appropriate excitation voltage?  Sorry for the newbie questions, but I'm just digging into this.

[Jason-M]

jaredts,

Take a look at the following articles, they should provide your with all the infortmation you need to get you started. Feel free to ask any questions that still remain:

NI 9945 or NI 9944 with an 9237 Connection Guide

Connecting Strain and Shunt Resistors to the NI 9237

(RIO) How Can I Connect a Quartyer Bridge to an NI 9944 or NI 9945 with an NI 9237 Module on a CompactRIO ...

Also, here is the user guide for the 9945 for your reference:

NI 9944/9945 Quarter Bridge Completion Accessory

Best,

[jaredts]

With the 9944 or 9945 I understand that the sensor's bridge resistance must match that of the accessory (120 ohm, 350 ohm).  I cannot seem to find anything addressing the considerations for sensors connected directly to the 9237 in full or half bridge.  Is the maximum power output of 25mv per volt the only deciding factor?

[Jason-M]

jaredts,

You are correct. You will need to ensure you are using the correct bridge resistance and equivalent dummy resistor to complete the half bridge. Beyond that, the main limiting factor is the signal that will be entering the DAQ device, as it must remain between -25 mV/V and 25 mV/V. However, there is one more limiting factor, and that is there is a limit on the amount of power the 9237 can supply if using internal power, which is 150 mW. The 9237 automatically reduces internal excitation voltages as needed to stay below the 150 mW of power. This limit translates to four 350 Ohm half bridges at 5V, four 120 Ohm half bridges at 2.5 V, or anything else that abides by the 150 >/= (# half bridges) * (V ex)^2 / (R) formula.

Best,

[jaredts]

Thanks.  I am planning to use a thru hole load cell that is 350 ohm bridge resistance, 2mV/V output, 10VDC excitation voltage.  Everything I have looked at appears to me to be full bridge.  I think I need to use an external excitation power supply, since 10^2/350=286mW.  I assume that I can just buy a small wall transformer that is 10V and greater than the stated power and I should be good there.  I shouldn't need any resistors for full bridge, just a 9949 or RJ50 breakout board to connect the load cell.  All of this sound correct?  Then I'll be on to figuring out how to set everything up in Max and Labview.  I appreciate the help, I just want to make sure I understand before making any mistakes.

[jaredts]

Looks like I need this as well to connect to external excitation:  https://www.ni.com/en/search.html?pg=1&ps=10&q=194611-01 

I'd like to go ahead and order everything I need, so if I've missed something feel free to point that out.

[johnsold]

Be careful with the small wall transformer type power supplies. Many of them are unregulated or poorly regulated.  Make sure that you get one with good output voltage regulation.  Ten volts is not a voltage commonly used so your selection of power supplies will be limited.  Consider a power supply with plus and minus 5 V outputs or two separate 5 V supplies with isolated outputs.  By connecting the excitation as +/- supplies the common mode voltage at the output of the bridge is near zero. I do not know whether this is compatible with the 9237 input ranges.

Lynn

[jaredts]

I have found +/-1% voltage 10vdc power supplies specifically designed for excitiation of transducers and test equipment.  This would be a little over $200.  That's fine if that is required, but isn't a small voltage fluctuation okay as the 9237 is referencing the input voltage by measuring mv/v?

[johnsold]

1. One concern with unregulated supplies is that the voltage with no load may be 20% to 50% higher than the rated voltage. If that voltage is higher than the maximum rated voltage for the device it is connected to, damage may occur.

2. The fluctuation may not matter if you are also measuring the excitation voltage.  Consider this scenario: The excitation is 10 V, nominal. The full scale output is 25 mV/V.  The load is 1/2 of full scale.  Then the transducer output is (10 V)*(25 mV/V)*(0.5) = 125 mV. Now suppose the excitation changes to 9.9 V (-1%). Now the output is (9.9 V)*(25 mV/V)*(0.5) = 123.8 mV.  If your measurement system does not know that the excitation has changed, it will look like the load dropped by 1% of full scale.

Lynn