changing baseline of channels

[mbhatti2000]

Dear All,

I am working with NI cRIO-9215 with BNC (HI-speed USB carrier device). I am measuring 4 multiple channels. They all have the same same baseline i.e "0". I want to change the baseline of the 4 channels so that I can see them more clearly in one disply and not on top of each other. Any ideas?

Best regards.

Munir 

[Phil_J1]

Hi Munir,

It sounds like you'd like the plots to be stacked on your display. Although there is no option on the display to make that happen automatically, one way to do it could be to use a Formula step and manually add an offset to each channel (for example: Y = X0 + 1, where X0 is the channel you want to offset, and "1" is above the range of your other signals). The drawback would be that you'd have to use 3 separate Formula steps to make this happen, where each Formula step would operate on channels 1-3 respectively (assuming that you're fine with leaving channel 0 where it is).

Perhaps you could instead add on offset in MAX for each channel.

Phil

[EricR]

There are at least two ways you can do this in SignalExpress.  Using either the DAQmx Assistant Step, or using the Formula Step.

To use the DAQmx Assistant Step, drop the step and select the channels you want.  For each channel, change the "Custom Scaling" option from the default of "NO SCALING" to the "CUSTOM SCALING" option.  Using a custom scale you can change the values for each channel in a number of ways.  If you choose the "Linear Scale" option you can apply the formula "y=mx+b" to each value.  If you change the "b" value you are applying a horizontal offset.  If you make "b" a different value for each channel the graph will show each channel seperately instead of stacked on top of each other.  This assumes that your original signals are directly on top of one another and you want to instead see them each seperately but still on the same graph.  You can use the "Custom Scaling" option to manipulate the channel values in a number of other ways and each channel can have its own unique scaling formula.

You can also use the SignalExpress "Formula Step" to apply an offset to each signal.  However, in this case you must drop one step for each channel as each formula step can only modify one signal so it sounds like you will need 4 formula steps to do this plus the original step you are using to get the channels initially (I assume you are using the new DAQmx Assistant Step since the cDAQ device you are using is only supported by this step).

Either way will work.

-Eric Reffett

[--Chris]

If all you really want to do is see the 4 channels at one time in the same "Data View" tab you could add a display for each channel.  Press the "Add Display" button for each channel and then drag one channel to each plot area.  The signals will still line up in time but will be stacked.

[mbhatti2000]

Dear Eric thankyou very much for such a nice answer, I am a little confused about the exact steps, where can I find the custom scaling option, exactly? Could you please describe in a little more detail. Thank you very much.

sincerely,

Munir

[EricR]

Munir,

Here are the steps to get to the custom scaling option and then you can follow my steps above.   These steps assume you already have a DAQ board in the computer.

1) Launch SignalExpress

2) Drop the DAQ Assistant Step from the "Acquire Signals>>DAQmx Acquire>>Analog Input>>Voltage" palette menu in SignalExpress

3) Select the channels that you want to acquire one (I'm just selecting channel "ai0" for this example)

4) When you click "OK" and return to the Configuration tab of the new "DAQmx Acquire" step you just dropped, in the lower right corner you will see a pull-down ring control that is named "Custom Scaling" and its default value is "<No Scale>".   Use the ring control to select "Create New..."

This will launch the "Create New NI-DAQmx Scale" Wizard which will guide you through the rest of creating a custom scale.

Let me know if you have any other questions.