PLC Application Suggestions needed 1

[Wattman]

I have been working for a couple of days to develop a PLC scheme. And I just can not work it out, the math is simple but has to change and it's hard to keep up with. Maybe some one can Help (or I maybe over complicating the scheme and re-inventing the wheel) This logic will coincide with other logic for FIFO of material, and each position (seat) actually holds 5 tons of material, with 51 positions you can get a vision of the size of the equipment and only imagine how much time it takes to safely make one complete revolution (Approx. 32 min) We plan to built larger units if I can work out the logic to locate any position with in 20 mins.

I'm using a AB 1500 for testing (but no preference)

Application

A good example is probably a Ferris wheel, I have created the logic to see the Ferris wheel rotating (via a quad encoder) and the FW has 51 seats. I have the counting of the seats worked out 1 thur 50,51,1,2,3 and so on. I have this value is in N7:0 and works great in both the forward and reverse directions.(no zero seat only 1-51)

I need to create logic that if seat #1 is in the unload position, and the operator needs to unload seat 35 next the shortest direction to rotate is achieved. Once seat 35 is unloaded and say seat 8 needs unloaded again the shortest direction is automatically achieved.

I'm sure that someone has worked this scenario out before and but right now my brain is mush!!

(Sort of hard to explain)

Any help, suggestions, or ideas greatly appreciated

 

[FrancisL]

Something like this should do (It can be done in ladder.)
Subtract the next required position from the current position.
Get the absolute value (ie if negative make it positive, or use ABS if the PLC supports that)
This gives the number of slots to move.
Subtract 25
If result is postive then move 'forward' otherwise move 'backward'

 

[Wattman]

After 4 days of work I have completed this part of the logic for the equipment. It only required 14 rungs of logic but I'll have to say, it is more complicated that one would think due to having to evaluate both the AP and DP thur the 51-1 point.

In order to keep the math straight I gave each variable a name as follows

AP = Actual Position (encoder feedback)
DP = Desired Position (Operator Entry)
FP = First Postion (Field Service Programmer Entry)
LP = Last Postion (Field Service Programmer Entry)
MP = Middle Position (Calculated by TP/2)
TP = Total Postions (Calculated by LP-FP+1)

The field service rep will only have to make two entries via a password protected screen on the operators interface,inorder to set the FP and LP which can be any number between 1 and 9999 and will depend on the number of positions the equipment actually has. I have ran the program thur a 24 hour automatic test and it's bullet proof. So if someone else could use something like this let me know, I plan to post it at MrPLC.com

 

[ControlConcepts]

My suggestion would be to subtract whatever number is being unloaded from whatever number is being requested to be unloaded next; or if the number to be unloaded next is less than the number being unloaded subtract the opposite way. Take this solution and compare to whatever is half of the total. If it is greater than that solution, go forward, if less, go reverse.