Sampling starts and stops

[bdhenry]

We have trying to sample the time interval for a single photosensor input.  The goal is to measure the time interval from photosensor change of state, rising edge, to the next photosensor change of state, rising edge.  This event cycle is a maximum of 3 interval measurements per second.  The photosensor output remains at logic level high for a minimum of 60 msec when the sensor beam is blocked.  The time interval from rising edge to next rising edge for the LabView input is about 70 msec minimum.

When the sampling is started, when experience periods of non-sampling with the LabView implementation.  Multiple photosensor beam breaks occur, but the LabView software does not record the events.  It seams that Windows is not allowing the LabView sampling to occur.

Thank you,

Brad Henry

Bowe Bell + Howell Co.

[Jennifer O]

Hello Brad,

You want to make sure that you have your application set up for continuous sampling.  If you set up finite sampling  and start and stop your task or if you do software sampling the timing can vary.  Typically if you have a digital signal which is TTL (0-5V) then you can use a counter on a DAQ board to measure the pulse width of the signal.  In the Example Finder inside LabVIEW (Help>>Find Examples) you will find several buffered pulse width and period measurement examples for reference.  You can check the right-hand panel to verify that the example is supported on your hardware.


If you require further assistance, please post what hardware you are working with and how you are trying to measure your signal (analog, digital or counter).  You can even post your VI if you wish.

Regards,

Jennifer O.
Applications Engineer
National Instruments

[bdhenry]

Jennifer,

Thank you for your reply.  I would like to try attaching our .vi program so you can review it for problems and errors.  The intent of the program is to monitor a 24Vdc photosensor on a machine.  The photosensor output is an input to the DAC.  The program must record the time interval from input rising edge to next rising edge.  The time interval will be 60 to 80 milliseconds, but will be greater than 80 milliseconds when we have a machine fault.  There will be 3 to 4 logic high intervals per second.

Thanks again,

Brad Henry - Bowe Bell + Howell Co.