LabVIEW
Timed Shutoff
If I'm correct, you want a Timer that has the following properties:
- When "Off", you can set an "On Time" (for simplicity, let's specify this in milliseconds) that, when you turn it "On", will stay On for that length of time, and then turn Off.
- If "On", you can also "manually" turn it Off.
- The output of this Timer is a Boolean that I'll call "Gate", as you will use this to "Gate" another signal (i.e. pass it if On, block it if Off).
The system being Controlled is a DAQ Digital Output device (one channel). For goodness sake, DAQmx is so simple that you should absolutely banish the Dreaded DAQ Assistant and use the 2 or 3 DAQmx functions appropriate for your Device to do the work for you (your DAQ code will be simpler and will work). To learn more, go on the Web and look for "Learn 10 Functions in NI-DAQmx" and read the referenced White Paper.
So let's look at what you want in a Timer. You want a Boolean Control I'll call "Go", the one that when you turn On starts the Timer and, if On, can be pressed again to turn it Off, and is also turned Off by the Timer expiring. You want an "On-Time (ms)" Control that says how long "Go" will be on (if you don't turn it off, yourself). You want an output, "Gate", that is always present and always reflects the current state of "Go". And, as a bonus, you'd like this little routine, which basically does almost nothing (it "waits" and responds to button presses) to take almost no time. Sounds impossible, right? But this is LabVIEW ...
Do you know about Event Structures? This is something that can sit idle (taking essentially no CPU time) waiting for "Something to Happen". The three "Somethings" are (a) you push "Go" to turn it On, (b) you push "Go" to turn it off, or (c) your timer "times out". What do you want to happen in each case?
- You turn Go On. You want to start a timer with whatever value is stored in "On-Time (ms)" so that the Timer will Time Out after the specified Delay. You set Gate = True.
- You turn Go Off. You stop the Timer. You set Gate = False.
- The Timer "times out". You set Go Off.
An Event Structure will handle most of this for you using two Events -- the TimeOut Event and a Value Changed Event for Go. So just for fun, I built such a Timer, probably with some "new things" you haven't seen. Here it is, responding to the "Go" button, either going On or Off.
Start with the Event Loop on the left. When Go is pressed (whether turning it On or Off), this Event is activated. If Go has been turned on, the True Case, shown here, puts the On Time on the Shift Register (let's say it is 1000, meaning "Wait 1000 msec = 1 sec") and also puts the value of Go on the funny thing that seems to have a Green Pipe coming out of it. I'll get to that. This finishes the Event, and the While Loop "loops", loading the value put on the Shift Register (1000, remember?) onto the Hour Glass (time) input of the Event Loop. When Go goes Off (the False Case), -1 is put on the Shift Register.
So what is the Green Pipe? It is a Tag Channel Wire, something introduced in LabVIEW 2016 (and in hidden form in LabVIEW 2015). Remember Data Flow, which says that nothing can get out of a Structure until it finishes? Well, Channel Wires can "bypass" this Principle (notice the Pipe goes over, not through, the edge of the While Loop). It goes into the adjacent While Loop, where it is connected to the Gate Indicator. Hence when Go goes On, Gate will follow (and the same is true when Go goes Off).
So pushing Go wires On Time (ms) or -1 to the Shift Register, where it gets wired to the HourGlass. Here is the TimeOut Event:
Timeout waits the number of milliseconds wired to the HourGlass, then "fires" its Event. In this case, the Event is to turn Go Off, but to do it in such a way as to Fire the Go Event, which as we've seen, resets the Timeout to -1 (which means "Never Time Out". So this waits however many milliseconds you specify, then turns Go (and thus Gate) off. Good. So how do you stop this monster?
If I'm correct, you want a Timer that has the following properties:
- When "Off", you can set an "On Time" (for simplicity, let's specify this in milliseconds) that, when you turn it "On", will stay On for that length of time, and then turn Off.
- If "On", you can also "manually" turn it Off.
- The output of this Timer is a Boolean that I'll call "Gate", as you will use this to "Gate" another signal (i.e. pass it if On, block it if Off).
The system being Controlled is a DAQ Digital Output device (one channel). For goodness sake, DAQmx is so simple that you should absolutely banish the Dreaded DAQ Assistant and use the 2 or 3 DAQmx functions appropriate for your Device to do the work for you (your DAQ code will be simpler and will work). To learn more, go on the Web and look for "Learn 10 Functions in NI-DAQmx" and read the referenced White Paper.
So let's look at what you want in a Timer. You want a Boolean Control I'll call "Go", the one that when you turn On starts the Timer and, if On, can be pressed again to turn it Off, and is also turned Off by the Timer expiring. You want an "On-Time (ms)" Control that says how long "Go" will be on (if you don't turn it off, yourself). You want an output, "Gate", that is always present and always reflects the current state of "Go". And, as a bonus, you'd like this little routine, which basically does almost nothing (it "waits" and responds to button presses) to take almost no time. Sounds impossible, right? But this is LabVIEW ...
Do you know about Event Structures? This is something that can sit idle (taking essentially no CPU time) waiting for "Something to Happen". The three "Somethings" are (a) you push "Go" to turn it On, (b) you push "Go" to turn it off, or (c) your timer "times out". What do you want to happen in each case?
- You turn Go On. You want to start a timer with whatever value is stored in "On-Time (ms)" so that the Timer will Time Out after the specified Delay. You set Gate = True.
- You turn Go Off. You stop the Timer. You set Gate = False.
- The Timer "times out". You set Go Off.
An Event Structure will handle most of this for you using two Events -- the TimeOut Event and a Value Changed Event for Go. So just for fun, I built such a Timer, probably with some "new things" you haven't seen. Here it is, responding to the "Go" button, either going On or Off.
Start with the Event Loop on the left. When Go is pressed (whether turning it On or Off), this Event is activated. If Go has been turned on, the True Case, shown here, puts the On Time on the Shift Register (let's say it is 1000, meaning "Wait 1000 msec = 1 sec") and also puts the value of Go on the funny thing that seems to have a Green Pipe coming out of it. I'll get to that. This finishes the Event, and the While Loop "loops", loading the value put on the Shift Register (1000, remember?) onto the Hour Glass (time) input of the Event Loop. When Go goes Off (the False Case), -1 is put on the Shift Register.
So what is the Green Pipe? It is a Tag Channel Wire, something introduced in LabVIEW 2016 (and in hidden form in LabVIEW 2015). Remember Data Flow, which says that nothing can get out of a Structure until it finishes? Well, Channel Wires can "bypass" this Principle (notice the Pipe goes over, not through, the edge of the While Loop). It goes into the adjacent While Loop, where it is connected to the Gate Indicator. Hence when Go goes On, Gate will follow (and the same is true when Go goes Off).
So pushing Go wires On Time (ms) or -1 to the Shift Register, where it gets wired to the HourGlass. Here is the TimeOut Event:
Timeout waits the number of milliseconds wired to the HourGlass, then "fires" its Event. In this case, the Event is to turn Go Off, but to do it in such a way as to Fire the Go Event, which as we've seen, resets the Timeout to -1 (which means "Never Time Out". So this waits however many milliseconds you specify, then turns Go (and thus Gate) off. Good. So how do you stop this monster?
Well, we add a Stop Button Value Change Event, and wire it to the Stop Indicator of the first While Loop, which stops this loop. How do we stop the second Loop? Use another "Loop-spanning" Tag Channel Wire, of course.
Now, I'm not sure what you want to do with your DAQ task (I think you can do much better than using the Dreaded DAQ Assistant), but that code could potentially be placed in the second Loop.
I'm attaching the entire VI. I see you have LabVIEW 2016, so you should be able to play with this and see both that it works and maybe even how it works. Have fun, and be creative.
Bob Schor
