Creating the equivalent of a STRUCT & Subroutine call in Labview

[jaustin]

Hi folks,

I'm trying to develop some autonmous code for our bot. Specifically I'm trying to convert last years (Kevin Watson's) code to labview. We use a STRUCT which has the autonmous commands in a list which feeds a CASE Structure to do certain things like drive, turn, stop, etc. The cases then call a sub-routine to execute the individual commands. I'm trying to use a CLUSTER of text and numbers that I un-bundle and feed into a for loop to run through each command in the list. I'm not sure how to break out of the for loop to run the "drive" command for example and then return to the for loop to run the next command line. It appears that the for loop just quickly runs through the whole list without stoping. I need to use a state machine in there somewhere, I'm just not sure how. This is a tough one to explain in an email!

[TeslaRules]

Sounds like a good start.

So.. You have the structure created and are able to parse through the items in the structure using a "for loop".

The problem I see is that you don't need to "break out" of the for loop. You stay in it!

All you need to do is create a seperate VI that you pass the parsed data too as you go through the loop.

Place the VI inside the "for loop" and voile'.

[Kevin_S]

Hi jaustin,

I agree. If you want to run a state chart, stay in your loop. I would recommend using a while loop instead of a for loop, as well.

One of the cases (in your case structure) you have in the loop needs to have your drive command in it. The loop won't continue until the subVI or the call to external code has finished.

Kevin S.

Applications Engineer

National Instruments

[TeslaRules]

I agree totally on the while-loop over a for-loop.

Unless your application has specific count or set number of steps that are always required ,of course.

While-loops allow you to exit arbitrarily based on a conditional test as opposed to a specific count such as required by a for-loop. In fact you can increment a count in your while-loop and use it in conjunction with other tests as well.

[jaustin]

Thanks you two! Maybe I'm making it harder than it needs to be. Let me give it a shot with the "while" loop.

[Kevin_S]

Hi TeslaRules,

Just a side note about For Loops that you may or may not know. It's true that For Loops traditionally require a count input to regulate the number of times it executes, and While Loops traditionally require a conditional stop input. As of LabVIEW 8.5 (what the FRC version of LabVIEW was built off of), you can also enable a conditional stop for a For Loop.

Therefore, you can create a For Loop that has a iteration count input and a conditional stop. The conditional stop can prematurely stop the For Loop. (To enable the conditonal stop on a For Loop, right-click on the loop and select Conditional Terminal.)

(It is still recommended to use While Loops for state machines though.)

Kevin S.

Applications Engineer

National Instruments

[TeslaRules]

Didn't know that.. thanks for the update.

But of course there are always tricks (often taboos that your professor would give you bad grades for doing) even in a traditional for-loop you can terminate by conditionally driving the index to greater than the loop termination count. In the old days we used to worry about how much memory and cpu speed gets used up by all these nice features inside the loop. I guess thats not such a concern anymore! ha

[jaustin]

Hi Kevin,

Do you know if we will have access to the controls tool kit (with PID VI's) when the FIRST version comes out?

[Kevin_S]

Hi jaustin,

There is PID functionality in the LabVIEW FRC software. However, I am unable to tell you whether or not it's exactly the same as what currently exists in LabVIEW.

Kevin S.

Applications Engineer

National Instruments