Analog single-ended setup with USB-6008

[CyberLink]

I am attempting to monitor a hand full of sensors that output resistance. I do not seem to be getting the expected results. I am sure it is my lack of knowledge on the USB-6008 unit so I thought I would ask.

To make this as simple as possible Ill uses the light sensor (photocell) as an example. In this I only need to read the resistance across the photocell.

First I tried connecting the photocell the way the diagram in VI Logger Tasks --> My Task1 --> Edit Task -->Connection Diagram    shows (PIN 2 --> Pin 3) This gave me readings all over the place! I then tried connecting it the way I though it should connect (Pin 2 --> GND) again nothing.

At this point I went to the test panel in the Ni-DAQmx software I changed the setting for AI0 to single-ended (RSE) and tested it. To cut this short, I found that in the Test Panel with AI0 set to RSE and the sensor hooked up between Pin 2 and Pin 32 (+5v) I get a perfect reading!

When I am in the software it does not work. Well it does work, if I put the photocell between pins 2 and 3 then run a 5v line from pin 32 to the lead from pin 2.

It would appear that I need to tell the unit that AI0 is single-ended; I just can not find a place to do that. The Preferences selection in Devices and Interfaces --> NI-DAQmx devices --> NI USB-6008 is grayed out.

So what am I doing wrong?

[devchander]

---

When I am in the software it does not work.

---

Which software are you using?? LabVIEW?? CVI??

in your software, you will have to configure the channels for reading in RSE mode

[Erik_J]

Hi CyberLink,

devchander is correct, you will have to configure for RSE mode in whatever software development environment you are using.

The Properties (preferences) are greyed out for your NI USB-6008 because the Properties are only for selecting a RTSI cable or an Accessory (see KnowledgeBase), neither of which are applicable to the NI USB-6008.

Regards,
Erik J.

[CyberLink]

I have tried both LabView and NI-DAQmx. When you do the DAQ assistant there is no way to set a resistance measurement to RSE, I find this odd because I can measure both Voltage and Current in RSE. The options in a resistance setup is 2-wire, 3-wire, and 4-wire with the wiring showing a deferential configuration. I may try setting up both a voltage and current measurement on AI0 and then feeding them into a formula to calculate the resistance.

In LabView if I create a channel read for resistance with a physical connection to Dev1/A0 and add a control, the options are 2-wire, 3-wire, and 4-wire. There appears to be no way to set it to RSE.

Again, I am new to this so I may be doing it wrong!

[Erik_J]

Hey CyberLink,

If you are using RTDs, please see this KnowledgeBase on how to connect your 2, 3, and 4 wire RTDs to your Data Acquisition Device.

Regards,
Erik J.

[CyberLink]

No, as I stated in the first post, I am trying to connect a photocell that varies the resistance based on the intensity of light. I was looking at RTD's  for temp but have decided I should use LM35's in place of them.

[Erik_J]

Hey CyberLink,

I see. If you want the behavior that you see in the MAX NI-USB-6008 Test Panel, then I would suggest creating a Voltage Task instead of a Resistance Task in the DAQ Assistant.  This is because the MAX Test Panel is the equivilant of a Voltage Task.

Regards,
Erik

[CyberLink]

Erik J,

What I need to do is measure resistance with an RSE setup. The photocell is just the tip of the iceberg so to speak!

As I have stated, I can measure voltage and current in RSE mode, and I can find no reason why measuring resistance in RSE mode should not available.

Another example of why I would need to measure resistance would be in measuring the resistance when probes are placed in soil to determine the moisture content of soil. (Ill need to monitor this)

[AnalogKid2DigitalMan]

CyberLink:

Can you post a sketch of how you have this wired? When you say:

When I am in the software it does not work. Well it does work, if I put the photocell between pins 2 and 3 then run a 5v line from pin 32 to the lead from pin 2.

I cannot make sense of it. Pin 32 is actually GND, not 5V. You cannot measure resistance directly across an analog input, just voltage. Thus you need a voltage divider to indirectly calculate resistance. And I doubt you can measure high resistances accurately such as that found in soil.

Also note that the input impedance of the 600x is just 144KOhm. This will drastly affect you measurement results.

[Dennis_Knutson]

You can measure "resistance" or anything else you want with RSE and your sensor. You just can't use the pre-canned setup for resistance because that assumes a differential measurement. No matter what, the DAQ board measures voltage. It does nothing else directly. When you specify resistance or some other special setup in the assistant, the DAQmx driver applies a numerical formula to the voltage read and returns a scaled value. You can specify RSE and create your own scale that will return a resistance value. You should have that information with the sensor. If you wanted to skip the assistant, use the actual DAQmx functions Create Channel (AI Voltage) and DAQmx Create Scale. The conversion to a scaled result could also be done after the DAQmx Read by using the numeric functions in LabVIEW.