waits

[tilky]

What exactly do waits do in labview. does it wait before executing or wait before going on after. And if you have a motor go forward for 1000 ms, do you have to put it in a for loop?

[tilky]

sorry i worded that wrong. In a flat sequence fram, would you need a for loop or will a wait do it.

[Mark McLeod]

In a flat sequence just use a Wait.
That must complete before the frame is considered complete.

The way it works is that everything in a frame must complete before it will move on to the next frame.

Things run in parallel within the frame, so the motor operations are happening at the same moment the timer is running. If you had a timer that happened to be shorter than the other operations taking place within the frame, then the frame would still not go on until the other operations were also completed.

[Greg McKaskle]

In your situation, a wait will do.

A wait node in LV works as follows:
Once its inputs are available, it notes the time, adds the delay to it, and sets an alarm for delay milliseconds in the future. The node will not be considered complete (it is executing, but without using the CPU), until that alarm goes off. Any dependent code is postponed, and parallel code is not.

Now the reason that it impacts loops and sequences the way it does is because of the most commonly overlooked element of LV -- the diagram. The open space on the window, the white background of the loop and sequence is the same as the {} bracketed code within other languages. The while loop's diagram is not complete until all nodes on it are complete. Same for sequence and other structure nodes that contain a diagram. Executing a diagram consists of executing all nodes it contains once and only once, and it is not complete and cannot execute again until all the nodes complete execution.

For example, I attached an image of a loop with various parallel things going on inside it. They are the i (which some may not count and may even be compiled out), the Bool wired to the loop continuation (similarly trivial), and the three waits (two for 1000ms and one for 500ms).

When the loop starts to execute its diagram, it will allow all of the parallel tasks to proceed at their own rate. So in this case, the three waits will run in parallel and the overall loop rate will be about 1Hz. If additional code is inserted into the loop that reads sensors or computes math, and it runs in parallel, it will not impact the loop rate unless the code takes longer than 1000ms to complete.

The second image shows how to run code before and after a delay, make sure there is an execution dependency -- either a data flow dependency or as shown in the image, a sequence dependency. The third image shows another common usage where you want to start something, not waiting for it to complete, and 500 ms after starting it, start something else.

I hope this helped.

Greg McKaskle

[Ether]

Originally Posted by Greg McKaskle

The third image shows another common usage where you want to start something, not waiting for it to complete, and 500 ms after starting it, start something else.

The wording is confusing me just a bit. If "Do something" takes longer than 500ms, you don't start "Do something else" until "Do something is complete", yes?

In other words, "after something is done, or 500ms elapses" means "after something is done, or 500ms elapses - whichever is longer", correct?

[Alan Anderson]

Originally Posted by Greg McKaskle

The third image shows another common usage where you want to start something, not waiting for it to complete, and 500 ms after starting it, start something else.

Originally Posted by Ether

The wording is confusing me just a bit. If "Do something" takes longer than 500ms, you don't start "Do something else" until "Do something is complete", yes?

The wording is only confusing because you've changed it. Read what he wrote, not what you did.

[Greg McKaskle]

Sorry for the confusion. That is what I get for taking so long to write a post and then rushing out at the last minute and not reviewing it carefully.

I agree with your wording Ether. The point of all this is that statements running in parallel on a diagram determine when the diagram completes, and the completion of that diagram determines when dependent code, such as the next frame of a sequence, or the next iteration of a loop can begin.

Greg McKaskle

[Greg McKaskle]

After reading my post again, I want to correct it.

The final image was meant convey the case where the start times are what you want to use for the time reference. I hope this is one is clearer.

Greg McKaskle

[tilky]

so will this run the motors for 1 second, or will it pulse the motors once, and then wait 1 second before moving on

[Mark McLeod]

That will start the motors, while the Wait twiddles it's virtual thumbs letting them run for one second, before leaving the frame.

Of course, the way it's written, nothing stops the motors, so after the frame finishes executing the motors will still be running and will continue to run until told otherwise.

[tilky]

ok think i got it now. thanks

[Alan Anderson]

Originally Posted by tilky

so will this run the motors for 1 second, or will it pulse the motors once, and then wait 1 second before moving on

It will tell the motors to run, then wait one second. It doesn't keep telling the motors to run after it's done it once. For most motors, that means they will continue to run, but not for Drive motors.

Originally Posted by Mark McLeod

Of course, the way it's written, nothing stops the motors, so after the frame finishes executing the motors will still be running and will continue to run until told otherwise.

It's a Drive function, so the motor safety will stop the motors after a tenth of a second unless something has been changed from the default configuration.

[Mark McLeod]

True enough.
What Alan means is, your structure would work the way you intend for individual motors, but drive motors (Open2Motor, Open4Motor) have an automatically applied Safety that will shut down the motors if you don't keep rapidly telling them what to do.

To make your structure work the way you intend it would need an extra vi call.
And at the end you would re-enable the Safety, before Teleop began, as well as set your motors to neutral.

[tilky]

ok, so this way i don't have to use for loops every time i try to drive in autonomous

[tilky]

one more question: is there a way to get the time from the driver station? The wait time is not at all accurate, and it would be easier than using encoders