How do I change duty cycle on the fly using DAQmx and VB.NET?

[DieselPower]

I tried to use code that was taken from C#, but it didn't work.

Public Sub SFI(ByVal PDC As Single, ByVal Channel As String) Dim writer As CounterSingleChannelWriter

writer =

New CounterSingleChannelWriter(Tasks(1).Stream) If SFIOn Then

writer.WriteSingleSample(

True, New CODataFrequency(1, PDC)) Else Try

Tasks(1) =

New Task("COtask")

Tasks(1).COChannels.CreatePulseChannelFrequency(Channel, "ContinuousPulseTrain", _

COPulseFrequencyUnits.Hertz, COPulseIdleState.Low, 0.0, 1, PDC)

Tasks(1).Timing.ConfigureImplicit(SampleQuantityMode.ContinuousSamples)

Tasks(1).Start()

SFIOn =

True Catch ex As System.Exception

MessageBox.Show(ex.Message)

Finally

Tasks(1).Dispose()

End Try End If End Sub

[DieselPower]

Nevermind.  I had to pull the Tasks(1).Dispose out of the Finally part of the try catch statement.

As in....get rid of it so that there was a task to update....

Message Edited by SCXI and MS 2k3-VB.NET on 11-09-2005 02:23 PM

[DieselPower]

Well.  I got the code to work but I am getting an error.  It is throwing an exception that says I must complete one full cycle before updating the duty cycle.  My frequency is 40 Hz and I am changing the duty cycle at 1 Hz.  How is it not making a complete cycle?  How do I get around this?

Thanks.

[e-Cash-Money]

Hello SCXI and MS 2k3-VB.NET,

Here is an article discussing the error you are getting.

It is also important to note than when you are changing the Duty Cycle property, you also have to write a new value to the Frequency property even if you are not changing the frequency.  I modified a shipping example installed by the DAQmx driver in order to do this.  The example program is located here:

C:\Program Files\National Instruments\MeasurementStudioVS2003\DotNET\Examples\DAQmx\Counter\Generate Pulse\GenDigPulseTrain_Continuous\Vb

I added the following two lines of code as lines 339 and 340 (in the Private Sub statusCheckTimer_Tick function):

myTask.COChannels.All.PulseDutyCycle = Convert.ToDouble(dutyCycleTextBox.Text)
myTask.COChannels.All.PulseFrequency = Convert.ToDouble(frequencyTextBox.Text)

I was able to change the duty cycle of the pulse train on the fly.

*Note: If you accidentally type a 0 into the frequencyTextBox (or any other invalid value), you will keep getting errors.  I just did this as a quick test, so you probably want to change the implementation to suit your own needs.

Message Edited by ELee [DE] on 12-01-2005 03:09 PM

[DieselPower]

Looks like I goofed and enter the period instead of the frequency.  I had .025 which was too low of a frequency to make it through it's cycle in 1 second.  Your method for changing the frequency is a little different that the one I had found.  Who should I use?

Tasks(16).COChannels.All.PulseDutyCycle = Convert.ToDouble(PDC)

Tasks(16).COChannels.All.PulseFrequency = Convert.ToDouble(40)

of

ctrWriter.WriteSingleSample(True, New CODataFrequency(1, PDC))

Thanks.

[e-Cash-Money]

Both approaches are valid and will accomplish the same results...so you can use either one!

[Chris_W1]

Actually, you should use the Write methods rather than the properties. The properties continue to work, but they are a little quirky to use since there is an order dependency with them. Setting the Frequency property causes the values of both properties to be applied to the signal, so if you set Frequency, then Duty Cycle, you get something different than if you set Duty Cycle, then Frequency. It was this sort of oddity that we attempted to fix by introducing methods to modify the pulse parameters.

-- Chris
DAQmx R&D, NI

[DieselPower]

Thanks.

[DieselPower]

I am having problems with this again and it is setting my duty cycle to 100%.  This is a problem when I am injecting fuel into an exhaust system!  I set a loop at the beginning of the subroutine similar to this:

subclock = now.millisecond
do
  
while now.millisecond - subclock < 50

ctrWriter.WriteSingleSample(True, New CODataFrequency(1, PDC))

[DieselPower]

The frequency I am using is 40 Hz, not 1 Hz.