setting the analog input voltage range

[myra]

Hi,

I have a PCI 6010 DAQ card that I want to use to measure an analog voltage input signal.  My understanding is that there are three different voltage ranges that you can choose for input --  +/- 5V, +/- 1V, and +/- 200mV, and this sets the accuracy.  When I use the DAQ assistant in my vi, I can choose a particular voltage range - but I would like to know if the program will change the range as needed (during operation) for optimal resolution.  For instance, I may wish to measure the voltage from one of my devices as it turns on - as the signal is small, I of course would like to use the smallest range, but as the signal grows in amplitude I would need to use the intermediate one, and so on.   Does the DAQ assistant do this, or do I need to figure out how to make the program do that myself?

I also have some other questions... is it possible to wire up a channel control for the DAQ assistant?  The only place I've found that you can choose the channel is when you set up the DAQ assistant, but I would like to have a control on the front panel where you can choose which channel, or even choose two channels to read.  Any ideas?

Lastly, we also use a software package called NI Datalogger to take thermocouple measurements, and I'm trying to create a program such that when I press start it will begin taking my voltage measurements and also take the thermocouple data.  I originally hoped that I could just cut and paste Datalogger into my package, but of course I can't see the code.  Is there a way to have my program launch an outside program?  Or is there an easy way to see the back end of Datalogger, so I can copy over what I need?

Thanks!   Myra

[JoeFriedchicken]

Hi Myra,

The analog input range is settable only while an analog operation is stopped. If you want to change input ranges while a VI is running, you will need to stop the task, re-configure the input range, and start it again. If you're using the Traditional NI-DAQ driver, this will be considerably more difficult, but since you mention the DAQ Assistant, I'm assuming you're using NI-DAQmx.

The DAQ Assistant doesn't offer channel control inputs, but you can right click on the Assistant and choose to 'Generate NI-DAQmx Code'. This will convert the settings in the DAQ Assistant to the DAQmx API calls it uses. The channel set-up VIs will be placed inside a subVI after the conversion. I recommended moving the set-up VIs in the subVI to the main level VI, removing the now empty subVI, and changing the 'physical channel' constant to a control.

Finally, the VI Logger can create LabVIEW code that will exactly reproduce the VI Logger's behavior. Pages 14 and 15 of the VI Logger manual explains how to do this.

VI Logger User Guide

Please let me know if you need any further clarification 🙂

[myra]

Hi,

Thanks so much for your help! 

I decided to use the lower level DAQ vis instead of DAQ assistant, and that made it much easier to set up a channel selector.

As for the voltage range issue, I was wondering if I should make a subroutine that compares the signal amplitude to the maximum values for the three ranges - once the amplitude exceeds one of the range values, it would stop the task, reset the task's range values, and then restart the data collection.  Would something like that work, or do you have any different suggestions?

Lastly, the program that we use for our thermocouples is called 'NI Datalogger' - apparently that is different from VI Logger.  I tried to follow the instructions from the VI Logger manual, but they don't seem to apply...  Any other ideas?  I was wondering if I could set up Datalogger as a sub vi, and if so, how?  Of course pasting in the code would be preferable, but whatever works!  🙂

Thanks again!  Myra

[myra]

Hi,

I've learned from tech support that it would be easier to program the vis myself to acquire my thermocouple data then to try to call NI Datalogger - apparently it is an executable and not a vi...  So now I'm trying to learn how to use the DAQ vis to take that data too.  I've read that I need to linearize the data, but I'm not sure how to do this.  We're using type T thermocouples, and I've found some tables with voltage data for given temperatures - but I was wondering if there is a vi in labview already configured to do this for different thermocouple types.  Has anyone ever used anything like that?

Also, the woman from tech support thought that I might be able to reconfigure the voltage range of the task without stopping it using property nodes.  She said that I'll very likely have to stop the task, reset the range, and restart the acquisition - but to try it without stopping it just in case! 

Thanks,  Myra

[JoeFriedchicken]

Hi Myra,

I'm glad you're using the DAQmx API VIs. They give you a lot more versatility and will give you the power to change your input range while your VI runs. There are a few properties that can be set while a task is running, but none of them relate to analog input. The complete list of run-time settable properites is in the LabVIEW help: VI and Function Reference » Measurement I/O VIs and Functions » DAQmx - Data Acquisition VIs and Functions » Additional Information » Properties Settable at Task Run Time.

At any rate, a subVI could work for your purpose, but you may find a state machine to be better suited. It will give you a more structured approach and could be easier to design. LabVIEW provides a state machine template from the File » "New..." dialog box. Select 'Standard State Machine' under VI » From Template » Frameworks » Design Patterns. Another option is to use a more advanced structure referred to here as an 'Action Engine'.

Action Engines, What are They?

Finally, I misunderstood what you meant when you said 'Datalogger'. The NI Data Logger is a VI distributed as an executable (which is why you can't see the block diagram). It comes with DAQmx Base, is that the driver you're using? Since it's a compiled executable, the Data Logger cannot be used as a subVI, but LabVIEW provides similar and more flexible functionality. I recommend taking a look at the Write to Measurment File Express VI:

Using the Write to Measurement File Express VI

There are also a few other example VIs in the Example Finder under Fundamentals » File Input and Output. The examples here use LabVIEW's File I/O VIs; if the Write to Measurement File Express VI were the "DAQ Assistant" for data files, then the File I/O VIs are the low-level "DAQmx" API. Two to start with would be "Write to Text File.vi" and "Simple Temp Datalogger.vi".

As always, please let me know if you'd like further clarification 🙂

[myra]

Hi,

Thanks for your response.  So far the most helpful example I've found is the "Cont Acq Thermocouple Samples - Int Clock".vi.  I'm a bit confused, though - because when we use Datalogger,  we can select one channel (which corresponds to two inputs - one from our thermocouple of interest, and the other from the thermocouple at our reference temperature).  We are using type T paired thermocouples, so all of our connections are with copper wires.  When I try to use the "Cont Acq Thermocouple Samples - Int Clock".vi it prompts you for the CJC temperature....  this is the cold junction compensation, correct?  I thought that this would apply if you have your thermocouple wires connected to wires of a different type (at the DAQ board), creating additional junction voltages that you must subtract off. 

In our case, I thought that the wiring within the board is also copper, so we wouldn't be creating any additional junctions - is that right? 

 I'm also not certain if we are doing something different than usual with our thermocouples, in that we are not just using one type T junction - we have two.  Am I making any sense? 

Thanks!  Myra

[myra]

Sorry - I wanted to try to clarify more.  Our thermocouple arrangement looks basically like Figure 3 in the "Taking Thermocouple Temperature Measurements" tutorial in the developer zone - if you exchange the iron wire for constantan and the other two constantan wires for copper.  After reading the tutorial, I'm left wondering how Datalogger takes into consideration the temperature of our reference point.  As I said in the last posting, there's no prompt for the cold junction temperature.  Previously I had thought that the program would just measure a voltage difference which would tell us how high above the reference point our 'hot' temperature is - but that would only work if thermocouples are linear.  Am I understanding this right?

One thing that I hadn't mentioned before (I was afraid it might complicate things even more 🙂  ) is that we actually use two sets of the thermocouples - one that measures a 'hot' temperature and a reference, and the other that measures a 'cold' temperature and hopefully the same reference.  On Datalogger, we type in "Dev1/ai0, Dev1/ai1" under physical channel, and it plots the temperatures of the hot and cold sides (at least that's what it's supposedly doing - now I'm really confused and can't understand how that is possible!).  Is there some way that the software might be taking into account the reference temperature that I just can't see now?  Since I can't see the block diagram, I really don't know. 

Myra

[JoeFriedchicken]

Hi Myra,

'CJC' is cold junction compensation, and while the traces on DAQ boards are copper, the connectors on terminal blocks aren't. How are you connecting your thermocouples to the DAQ card?

As long as you use the same type of thermocouple for your entire test, you will only need one reference CJC thermocouple. If I understand your description correctly, you're measuring two temperatures. In that case, you'll need three total thermocouples: one for CJC, another for your hot measurement, and a third for your cold one.

I think the simplest approach would be to use MAX (the Measurement and Automation Explorer) to create an Analog Input task to measure your thermocouples. You can set the thermocouple type, the channels to read, the CJC type and even preview how it takes measurements. NI has made a short video (three and half minutes) that shows how to do create a task in MAX. Rather than selecting a generic voltage input, you would choose a temperature input and then thermocouple.

Creating a Task in NI-DAQmx and Using it in LabVIEW

It would also be very helpful to know what DAQ board you're using and which driver you have installed for it.

[Yan0]

Hi,

I'm using a data acquasition/file writing VI that I wrote myself to sample voltages from an experimental setup to measure the index of refraction of several gasses at various wavelengths. The experiment itself isn't that important. Anyway - I'm using the DAQmx version rather than the DAQ Assistant because the Assistant can't handle the number of samples I'm taking - LabVIEW runs out of memory.

I have three voltage inputs into a PCI-6115 board - two correspond to voltages from two photodiodes and the other is a voltage from a pressure guage. The Voltage envelope from the two photoditectors is about +/- 2 V while the pressure guage goes from about 7.5 to 0 V. In order to accomodate voltage readings from all three channels, I set up my VI to take voltages from 7.5 to -2 V. This is far from ideal. Is there any way to set up modify the VI I've attatched to so that it has different max/min voltage readings per channel? Yes, I could just use two of the VI's with different limits, but it's important that I get three simultaneous readings, I don't want the pressure sample to come at a different time than the photodetector voltages.

The VI I've attatched is contained in a for loop and a sequence structure, it is the last frame of the sequence structure - I just figured it'd be easier to understand if I just sent the data acquasition part along.

Thanks

Jed

[Dennis_Knutson]

If you mean you could use two different DAQmx Create Channel functions on the same task, then that will not affect sampling. If you mean you could use two different tasks with two different DAQmx Reads, then you can't do that. Shown below is a DAQmx Create Channel inside a for loop so you would have arrays of channels, ranges, configurations, and each can be different.

Also, you should not be placing all of your attached code inside a for loop. There is no reason to be creating, starting, and stopping a task with each iteration. You also mention a sequence structure. You might want o think about ways to eliminate that. Use dataflow to control execution order.

Message Edited by Dennis Knutson on 05-01-2008 08:44 AM