Timers and Drums

[ibsx98]

I am almost ready to say I can do this and make a commitment to make some inventions  () for our manufacturing facility. 
I have two questions concerning two separate projects:
First, considering scan times and the TMRF with a .01 timebase, would it be possible to measure a gap between pans that varies between 500 milliseconds and 0 milliseconds?  At 0 milliseconds we have a pan jam and the conveyors need to be shut off.
Second, with a Drum, it looks like the counts per step is not variable, that is, only K data type can be used and not a changing V memory location.  Is this so.  The problem is that this machine's motor is on a VFD.
Thanks

[Controls Guy]

Use an event drum (EDRUM) instead.  Then you can define the step transition events externally to the drum, and they can be time based, based on a line shaft encoder counts (this would automagically synchronize with the VFD), include some actual events like inputs, anything you can describe in ladder.  I always use EDRUM's and program them this way because of the flexibility.

[ibsx98]

Thanks, I missed that but I put it together when I see it described.

[Controls Guy]

As far as gap measurement goes, it it really time based or are you trying to indirectly measure the distance between pans?  If distance, use an encoder into a high speed input and have a maximum number of counts for the pan to be present.  If normal is 100 encoder counts and you get to 140, you know there is more than one pan.

Also, how soon after the gap is supposed to have started do you need to know about the jam?  If it's not too soon, you can use a large multiplier, like 1.4 as I described.  If not, you could use a small multiplier like 1.1 and declare a jam at 105 counts, but you might get more false positives.