Hi Pros,
we have been working on a paint application where you need precise start and stop positions of a painting application. The paint gun is triggered using a pneumatic 24VDC valve. On Fanuc robot you are allowed to add a motion line that can trigger a digital IO within the motion line. So as the robot starts moving the gun would start. This option is not available on UR Robots. There are probably a couple ways of doing this. First I thought of triggering the IO for the gun just before the motion line but I was afraid it would create a little spill of paint at the beginning. So I tried to find a way to trigger the paint gun as the robot starts moving. So what I did, is that I used a thread that would monitor the value returned by the script function is_steady(). Then based on whether I am in the paint motion or not I would trigger the valve. Because of the infinite loop error that I had when I tried to run the program, I had to add a wait command in the thread but I kept it very slow. Since the thread is looping at 0.008s I believe that there will be some inconsistencies in the paint start and stop positions that will depend on the actual robot speed. This is the way I thought of doing it. Do you guys have any suggestions?
Here is the code and video. In the video you see a green light on the valve when it is being actuated. More tests on this to come!

 Program
   BeforeStart
     isMoving≔ False 
     isRobotRdy≔ False 
   Robot Program
     MoveL
       Home_pos
       paint_start
       isRobotRdy≔ True 
     MoveP
       paint_stop
       isRobotRdy≔ False 
   Thread_1
     isMoving≔ not is_steady()
     If isRobotRdy and isMoving
       Set TO[0]=On
     Else
       Set TO[0]=Off
     Wait: 0.001

@Sebastien since turning an output on or off does not require any move time the system executes the command and blows right past it pretty dang fast, we actually have to add small amounts of sleep when using pneumatic grippers to give them the 50-70 ms they need to close before motion starts.  I would think that you would be perfectly fine with a TO[0]=ON right before the move starts and a TO[0]=OFF right after the move stops, especially the off, what I have found in using the is_steady() is it takes a moment after a move for the robot to return true, depending on how fast you are moving and decelerate.  

If you think about it you set a flag after the moves, that same time slice which is setting the flag condition could just be used to set the valve on and off and with fewer calls to the API.

Have you tried just putting the actions in-line with the moves?

Program
   BeforeStart
     isMoving≔ False 
     isRobotRdy≔ False 
   Robot Program
     MoveL
       Home_pos
       paint_start
       Set TO[0]=On 
     MoveP
       paint_stop
       Set TO[0]=Off

@matthewd92
Thanks for the info. I have not tested yet with the actual paint gun since I am waiting for the equipment to arrive. Based on your input I will give your advice a try first! I will try to post some results and videos here!

@matthewd92
one other question concerning programming this painting application. You have two points that would correspond to the start paint and stop paint positions. I am planning on using MoveP to ensure proper paint distribution between these two points. However, if I understand the MoveP correctly, I would need a point prior to the paint start point such that constant speed is achieved when reaching the paint start position and moving on toward the paint end position. However, if I trigger the paint gun just before the paint start point, that would eliminate the constant speed created by the MoveP command between the approach point, the paint start point and the paint end point. Am I correct to assume that? How would you make sure you have constant speed between the points? How would you then trigger the IO for the paint gun?

You can try using a timer relay between the robot and solenoid:

https://www.automationdirect.com/adc/Overview/Catalog/Relays_-z-_Timers/Timer_Relays_-z-_Counters_-z-_Tachometers

But it appears you can use one of the programming solutions listed above too.  You can always start by turning on the paint gun while it's off the surface and then moving through the path over the surface and past the end point before turning off as well.

Also, are you going to use any positive pressure suit for the robot or is it not explosion proof required?

Good luck!

@eric
It worked well. We simply programmed the DO to trigger the gun as a child of the MOVE node and it worked well!

The paint was not requesting explosion proof equipment.