Latching Function for a Case Structure

[GOB82]

Hi,

Looking for help with a solution. I have a limit switch input on a digital line that is being used to change a case structure. The case structure True and False is for the Forward and Reverse for a motor. At the moment, when the limit is tripped/High the motor goes Forward but once it has passed the limit switch it returns to Reverse. How is it possible to keep/latch the case structure that only a True value from limit switch will change the case structure.

Regards,

[GerdW]

Hi Gob,

instead of looking at the current state of your switch you should detect rising edges in it's signal.

There are functions provided by LabVIEW to do so: PtByPt-BooleanCrossing!

[GOB82]

Hi GerdW,

Thanks for your reply, I'm very much a beginner but I am not sure it is possible edge detect, I am using a 6501, all DIO. The limit

switch is coming in as a simple boolean 0,1.

Regards,

[Thoric]

Sounds like you need the limit switch to toggle a boolean in your code.

Perform a compare of the input to it's previous value, storing the previous value in a shift register for example, and if "new value > old value" then you have a rising edge detection. Feed that into the True case of your case structure to invert a boolean signal using a NOT function. That boolean signal will simply pass through in the False case. Keep this boolean the same for each iteration of your code by storing it in a shift register as well. When True, rotate clockwise, and when False rotate anti-clockwise.

[GOB82]

Hi Thoric,

Thanks for your help, I have tried your approach, which I may not be doing correctly using a feedback node instead to store the last value but unfortunately it is not working for me. I have attached a simple VI of what the system is trying to do. The limit switch LED 1 should go High along with LED 2 however when LED 1 goes Low, LED 2 should remain high until the next time LED 1 is High.

Regards,

[Thoric · Accepted solution]

It seems you attached your original code as there is no feedback node in this VI?

Anyway, shown below is how to achieve what I was describing using a Select function instead of case structure. Note, I believe you can also use an Exclusive Or instead (as shown in the lower screenshot, but I haven't tested that)

[apok]

maybe this....