Can I use a reference trigger with a Linear Encoder Channel? (PCIe-6351)

[EricSchwamb]

I have a PCIe-6351 and I am trying to read the position from a linear encoder referenced to a digital trigger.  The digital trigger occurs in the middle of the motion that I am interested in, so I would like to capture a fixed number of pre-trigger and post-trigger encoder samples.  The graph below illustrates what I'm trying to get:

I have previously used the ReferenceTrigger with success on my AI channels to aquire analog data pre/post-trigger, but when I attempt to do the same with my LinearEncoderChannel I get "Error -200452: Specified property is not supported by the device or is not applicable to the task."

Am I doing something wrong?  Or is the ReferenceTrigger feature not available for the LinearEncoderChannel?  If it is not available, can you suggest an alternative method to acquire my pre-trigger position data?

I am developing in VB .NET 3.5.  My non-functioning code is as follows.  It throws the Error -200452 exception on the call to "myEncoderTask.Triggers.ReferenceTrigger.ConfigureDigitalEdgeTrigger":

    Public Sub RunTest()

        Dim myEncoderTask As Task = Nothing
        Dim myClockTask As Task = Nothing

        Try

            'Create new Tasks
            myEncoderTask = New Task()
            myClockTask = New Task()

            'Initialize Local Variables
            Dim PreSamples As Integer = 5000
            Dim TotalSamples As Integer = PreSamples + 20000

            'Create the linear encoder counter input channel
            myEncoderTask.CIChannels.CreateLinearEncoderChannel("PCIe6351/ctr0", "Encoder", CIEncoderDecodingType.X4, _
                                                         False, 0, CIEncoderZIndexPhase.AHighBHigh, 4, 0, _
                                                         CILinearEncoderUnits.Meters)

            'Create the clock output channel
            myClockTask.COChannels.CreatePulseChannelFrequency("PCIe6351/freqout", "Clock", COPulseFrequencyUnits.Hertz, _
                                                               COPulseIdleState.Low, 0, 6250, 0.5)

            'Configure Clock Timing Specs
            myEncoderTask.Timing.ConfigureSampleClock("/PCIE6351/PFI2", 6250, SampleClockActiveEdge.Rising, _
                                                      SampleQuantityMode.ContinuousSamples, TotalSamples)

            myClockTask.Timing.ConfigureImplicit(SampleQuantityMode.ContinuousSamples, TotalSamples)

            ' Configure Reference Trigger
            myEncoderTask.Triggers.ReferenceTrigger.ConfigureDigitalEdgeTrigger("/PCIE6351/PFI1", _
                                                                                DigitalEdgeReferenceTriggerEdge.Rising, PreSamples)

            ' Verify the Tasks
            myEncoderTask.Control(TaskAction.Verify)
            myClockTask.Control(TaskAction.Verify)
            myClockTask.Start()

            'Read the data
            Dim myReader As New CounterReader(myEncoderTask.Stream)
            Dim data() As Double = myReader.ReadMultiSampleDouble(TotalSamples)

        Catch ex As Exception

        Finally

            myEncoderTask.Dispose()
            myClockTask.Stop()
            myClockTask.Dispose()

        End Try

    End Sub

Any help would be greatly appreciated.

Thanks,

Eric

[James-K]

Eric,

Unfortunately a referenced trigger cannot be used for counter tasks.  Would it be possible for you to change the point at which the trigger occurs?

[EricSchwamb]

Hi James,

Thanks for your reply!

No, unfortunately I cannot change the point at which the trigger occurs.  That trigger is generated by a physical event that must occur in the flat "bottom" of my position trace.

My challenge is that this motion is a repetitive one. I'm trying to repeat the motion many times to generate statistics.  I was hoping to avoid capturing many iterations with one long trace and then manually cutting up the trace piecewise and aligning the time bases after the fact. 

Can I synchronize my counter task to a digital channel task?  I'm thinking I might simultaneously log the trigger while capturing several motion iterations, then I could at least write some post-processing software to do all the subdividing and time base alignment for me.  Can I modify the "SyncAI_ReadDigChan" example from the documentation, using a counter task instead of an AI task?  Or will I run into similar problems as I did when I tried to use the referenced trigger with the counter task?

Do you have any other suggestions I might try?

Thanks,

Eric

[James-K]

Eric,

You may be able to use a 'dummy task' to synchronize the counter and digital task, as in the following link:

https://decibel.ni.com/content/docs/DOC-11755

Also, what is your sampling rate/number of samples you are recording?