Good Afternoon Marc,
Unfortunately the VI you sent had none of the drivers attached, but what you ask is fairly easy. If you put your "Send" sub-vi (the middle one of the three I think, hard to tell without them attached!) into either a for loop or a while loop and then write some code to increment the voltage message while it loops, this should work. You will want to have limits, and the step timing can be done in a number of ways, the simplest (not necessarily the best) is to put a timer function in the loop with either a control on the front panel to allow you to set/vary the time.
If you perform a "Save with options" and select the Development Distribution we should get all the necessary sub-vi's to look at your problem.
Putnam Monroe
Certified LabVIEW Developer
PutnamCertified LabVIEW Developer
Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5

LabVIEW Champion