Looking for help setting up hardware for voltage measurement

[Leedward]

Hi, everyone,

I'm new to LabView, NI.  I've been going through the "getting started" book and parts of the measurment and user manual. Still, I struggle to do in my view something very simple... So any advice is appreciated!

Basically, I want to acquire the acceleration using an accelerometer.  The equipment is NI USB-6210 & LabView 8.6. 

Could you tell me how to set up my equipment? I mean how to wire the equipment together to do the testing (as described as Acq&Graph Voltage-Int Clk-Dig Start.vi).  I have one accelerometer, one signal conditioner, and USB-6210.  What I did now is: 1) wire accelerometer with "sensor" port on signal conditioner; 2) wire the "output" port on signal conditioner to analog input pair channels "AI 1" & "AI9" of NI USB-6210 using shielded, twisted-pair wires; and 3) connect USB-6210 to my laptop.  My questions are on second step.  In LabView, I set DIFF mode for AI1 and RSE mode for AI9.  I can get the signals at both channels.  Which signal is the right one, at AI1 or AI9?

For the above setup, I can't do a measurement with a trigger. LabView also default PFI 0 as the trigger channel.  How should I wire this channel up? Is is right to wire AI1 and PFI0 together?

I am confused now.  Please give me some clues. Thank you very much.

Leedward

[Leedward]

Any one can help me?

[SnowMule]

I think you'll need to look at how you trigger the DAQ.

When you set AI1 to differential, the DAQ reserves AI9 (The other input for the differential pair), so trying to re-initialize it is unnecessary. The differential voltage between the AI1 terminal and the AI9 terminal is output from the DAQmx Read as the first input (in this case, AI1).  Differential measurement is like AI1 being the red lead and AI9 being the black lead of a multimeter.  An RSE measurement would be fixing the black lead to the ground pin.

PFI0 is a digital input made for a 0 or a 5 volt signal, it's not really stable between those voltages and I wouldn't recommend using that as a trigger without some additional signal conditioning inbetween.

Depending on the design/system requirements, you could put a comparator/op-amp type of circuit to trip high when the voltage on the accelerometer goes above some level, adjustable with a voltage-divider, then feed the output of that to your digital input to trigger the DAQ.  Set that trigger voltage in hardware and it stays put.

Another option is to continuously acquire data and start/stop the "recording" in software based on a logic decision in the code.  Downside to doing this is you have to continuously read data into the PC and analyze it, so you lose some speed doing that.  Upside is you can dynamically adjust that trigger level (like making adjustments based on the noise).

3rd option is a combination of the two, use an analog output on the daq as a reference on the comparator.

Does that give you a little more direction?

[Leedward]

Snowmule:

Thank you fro your answer.  To be honest, I don't have much background on electricity.  So I can just understand part of your answers.  Please don't laugh if I ask some stupid questions.  Please confirm if I understand your answer.  Thanks.

You answer: "When you set AI1 to differential, the DAQ reserves AI9 (The other input for the differential pair), so trying to re-initialize it is unnecessary. The differential voltage between the AI1 terminal and the AI9 terminal is output from the DAQmx Read as the first input (in this case, AI1).  Differential measurement is like AI1 being the red lead and AI9 being the black lead of a multimeter.  An RSE measurement would be fixing the black lead to the ground pin."

Does this mean that I don't have to use the differential measurments.  Just use single-ended AI measurement in RSE mode?  In this case, I need to connect the positive side of signal source to one channel (any one of 32 channels) and the negative side of signal source to AI GND.  Is it right? If it is right, but how to tell which one is positive or negative side?

[SnowMule]

Yeah, you can probably get away with using an RSE measurement.  In general, differential's a better way to go because it reduces line noise, and you get more ranges on the DAQ which improves accuracy.  But especially for signals which are already conditioned, RSE's perfectly fine for making that measurement.

You are correct, you can take the output of the amplifier into one of the inputs on the DAQ and ground the negative terminal to the DAQ's AI GND.  You'll need to look at the signal conditioner to find which terminal's which.