PASCO Load Cell and NI DAQ Interfacing

[BabyH]

Hi everyone,

I have a PASCO Load Cell PS-2200 and I am trying to interface it with an NI-9201 AI (in conjunction with a NI-cDAQ 9172). Instead of attaching wires to the cell's pinout, I stripped a PS2 connector and rearranged the wires to match the pinout of the sensor (which are the strain gauge input, strain gauge output, the excitation voltage of 4.2 and the ground). I connected the strain gauge signal to a channel of the NI 9201 and I am using an external power source to provide voltage to the cell. My VI is trivial and consists of an input DAQ and Lowpass filter.

The problem is that the input signal is obviously not right, it looks periodic for one. The filtered signal appears stable at first but slowly increases over time. And the signal does not respond to applied stresses.

I would really appreciate any help on this. I am a ME sophomore and have little experience with LabVIEW.

Thank you.

[JVP]

Hi BabyH,

From the datasheet it looks like the PS-2200 is a differential measurement. Unfortunately, the hardware you are using, NI-9201, will not work. The reason why this will not work is because you are creating a ground loop. For more information about this please refer to our Developer Zone Article: Field Wiring and Noise Considerations for Analog Signals. Do you have access to any other modules perhaps?

Regards,

Jignesh P

[BabyH]

Hi Jignesh,

Thanks for your response. I switched to NI 9215 which is capable of differential measurement. But this time, my input signal still looks periodic and my filtered signal is unmovable at -0.2V. I am using a butterworth lowpass and 0.1 as lower cut off.

Any help would be greatly appreciated.

Thanks.

[JVP]

Hi BabyH,

Do you have an access to a scope where you may be able to test out to see what signal's coming out of the PS-2200. What is the frequency of the periodic response? Does the load cell respond, if you apply different loads? Can you post a snap shot of the output as well as your VI?

Regards,

Jignesh P

[Process_Systems]

The Pasco spec sheet mentions a load cell amplifyer unit. You should be using this, or a similar unit, as the output signal from a raw load cell is normally in the tens of millivolts, and the DAQ module you are trying to use has a full scale input range of +/- 10 VOLTS. Also, your excitation voltage and the load cell output cannot have a common terminal or you end up shorting across part of the cell measurment bridge.

Good luck,

Dave

[BabyH]

Thank you Jignesh and Dave,

Jignesh, I do not have a scope I can measure the ouput with. 😞 And no, the cell does not respond to variable loads. Also, please find attached the snapshots you had asked for. The front panel snapshot shows the cell's reponse when under no load

Dave, the module I am using has a 16 bit resolution and within a +/- 10 full inut range, don't we have enough resolution to acquire the smallest change?

Also, from the pinout of the load sensor, the pin for the excitation terminal and the positive strain gaugw output are adjacent. They are not the same terminal. We took every possible precaution to make sure that the terminals of the PS2 connector did not touch during soldering.

Thank you for your help again.

[BabyH]

Hey,

I'd also like to add that I find both the unfiltered and filtered signals to change drastcially when I kind push in the PS2 connector or push in the wires connecting the sensor to the DAQ. Please tell me that this not a loose connection of some sort because we really spent a lot of time rearranging and soldering the PS2 connector, taking care not to meet any of the terminals 😞

Thank you.

[BabyH]

Sorry about the spam.

But I have attached a very rudimentary diagram of how I have setup the interfacing.

Please check it out and see if I am doing it right?

Thank you and sorry again.

[JVP]

Hi BabyH,

From what you have mentioned it seems like you have directly soldered your wires to the PS2 connector. I would recommend that you use a PS2 Female Socket and solder the wires to one of those. What is your power supply for the excitation?

Regards,

Jignesh

[BabyH]

Hi Jignesh,

Thank you for you continued interest in my problem.

I am using a PS2 female connector. However, I had to rearrange the wires because the 4 pins (of the 6) to which the wires were connected did not match up with the four pins that are concerned with the load sensor.

My excitation voltage is 4.2 V as prescribed by the load sensor's manual.

Thanks a lot once again.