Slc 503 Conveyor Application

I HAVE A PALLET CONVEYOR SYSTEM THAT MOVES PALLETS ALONG THE CONVEOR TO BE WRAPPED AND STRAPPED. 2 DIFERENT PROCESS'. I WANT TO DETECT A REFLECTIVE TAG ON THE PALLET TO TELL A STRAPPING MACHINE UPSTREAM TO STRAP THIS LOAD. THE PROBLEM IS THEIR COULD BE OTHER LOADS IN FRONT OF THIS LOAD THAT DONT GET STRAPPED SO TRACKING THE LOAD TO THE ENTRY POINT OF THE STRAPPER IS CRUCIAL. I HAVE 4 SENSORS I CAN USE TO TRACK THE LOAD PRIOR TO ENTRY. NOT SURE THE BEST WAY TO GO ABOUT IT. THOUGHT BIT SHIFTING WOULD WORK BUT PALLETS ARE NOT CONTINUOUS FLOW SO WHEN DO I SHIFT. ANY IDEAS OUT THEIR???

The simplest solution would be to make the decision to strap or not to strap at the entry to the strapper.

If that's not possible, then bit shifting can work if you have an indexing conveyor. By that, I mean a conveyor that moves product in discrete increments rather than continuously. Every time you index the conveyor, you shift the bit pattern.

If the conveyor moves continuously so that there can be a variable number of pallets between the sensor that decides whether to strap and the strapping mechanism, then you need a FIFO stack. When a pallet is detected at the sensor, load the FIFO stack with a Strap or a Don't strap value. When a pallet is detected at the strapper, unload from the top of the FIFO stack.

Tell us more.

I've done something similar in the past. My application scanned a barcode to send the package to one of twenty-something diversion points over a distance up to +400 ft. away. (To avoid replacing a handful of barcode scanners that were originally at each diversion point) I used a bitshift to track the packages. Over that distance (with a hodge-podge of mismatched conveyors) accuracy was a concern so I used photo-electrics to "verify" the package when expected and used them to control transfer.

But, I digress. Your application could be handled in a similar manner. If the conveyor speed is 60 ft/min -> 1 ft/sec...a 0.25s timer reset by its ‘done’ bit can be used to shift a bit that represents a 3 in. ‘snapshot’ of the conveyor. (Use the motor starter to turn bitshift on/off). To determine the state of the bit… 0 if reflector not seen, 1 if reflector seen.

A 48 in. pallet (with a reflector) will “look” like 00001111111111111111000000 as it travels. So…as the 1’s reach the general area of the entrance to the strapper, take over control with the sensor at the entrance.

Hope this helps. Keep in mind the assumption is made that if the conveyor is running the pallet is moving.

SLC 500 CONVEYOR APPLICATION: MORE DETAILS

THANKS FOR THE REPLYS,
LET ME GIVE SOME MORE DETAIL ON THE OPERATION OF THE CONVEYOR.
THE CONVEYOR IS BROKEN INTO 5 FOOT ZONES INDIVIDUALLY CONTROLED BY SOLENOIDS(OUTPUTS). THEREFORE THE CONVEYOR STARTS AND STOPS DEPENDING ON THE CONDITION.FOR EXAMPLE THE PRE ENTRY ZONE TO THE STRETCH WRAP MACHINE WILL STOP A NEW LOAD FROM ENTERING IF THEIR IS A LOAD BEING WRAPPED.WHEN THE LOAD BEING WRAPPED IS COMPLETED AND HAS CLEARED THE WRAPPING AREA THE LOAD IS ALLOWED TO CONTINUE. I HAVE TO DETECTECT THE LOAD PRIOR TO BEING WRAPPEED BECAUSE THE WRAPPER FILM WILL NOT ALLOW MY SENSOR TO SEE THE RED TAG WHICH SIGNALS MY BIT SHIFT REGISTER THAT THIS LOAD NEEDS TO BE STRAPPED UP-STREAM. THEIR ARE 2 ZONES PRIOR TO ENTERING THE STRAPPING MACHINE (AFTER THE WRAPPER), EACH WITH ITS OWN PHOTOEYE FOR CONTROLLING THE STARTING AND STOPPING OF THE ZONES. SO ANOTHER WORDS IF THEIR IS A PALLET WAITING TO BE STRAPPED AND ANOTHER PALLET MOVES IN BEHIND THIS ONE IT WILL STOP UNTILL THE PROCEDING PALLET MOVES FORWARD.(KIND OF CONFUSING I GUESS).
WHAT I THOUGHT I WOULD DO IS USE MY COLOR DETECT SENSOR TO INITIATE THE STRAP LOAD BIT IN MY SHIFT REGISTER AND THEN USE THE NEXT 2 ZONE CONTROL PHOTO-EYES TO TRACK THE LOAD(SHFT MY REGISTER)JUST PRIOR TO ENTRY. ANY BETTER IDEAS? THANKS

What you are suggesting will work. Need a better picture of the overall system to suggest any other solutions.

BMAN,

Bit-shift tracking always works best when you tie the bits directly to the position of the conveyor(s). If you could install some type of position device (maybe just a simple prox switch, a metal target on the conveyor shaft, and a counter in your PLC to count the On/Off from the prox switch on EACH conveyor zone), then it will not matter if a pallet is stopped and started many times. When it stops, your prox switch counter will stop, and your bit shift for that zone will stop. When a zone re-starts, your counter will restart, and your bits will start shifting again. In other words, your bits will truly track the movement of the pallet, and not just track the On/Off of a photoeye. Of course the photoeyes could be made to work, but will never be as accurate as the above mentioned method.

A method

BMAN

You have two problems: The need to detect an event at one station while performing the event at another; and a variable number of objects betwee the two stations.

Take the problems one at a time.

The solution to the first problem is a FIFO (or bit shift, since you only need pass on one bit of data).

The solution to the second is a pointer / indirect address.

Let's assume that the maximum number of pallets that can fit between the Wrapper INFEED (where the red is detected) and the Strapper INFEED is 16.

You'll want logic that sets (resets? - you may need to use latching logic to make this work) a bit that says that the pallet currently in the Wrapper needs to be strapped. I'm not clear (nor does it really matter) whether you detect the red on entry, or immediately before.

You will need a photoeye (and I think you say you have one) on the EXIT from the Wrapper. When that photoeye sees the pallet, it needs to do two things: Trigger the bit shift, and increment the Pointer.

When the photoeye at the entrance to the Strapper detects a pallet entering the strapper, it decrements the pointer.

Now for some code/illustration.
You create a Bit Shift Register at B10:0 (for example).
B10/0 will represent the pallet in the wrapper.
N7:0 will be the number of pallets between the wrapper and strapper. In the example below, the current number is 5
B10/[N7:0] will therefore represent the pallet that is currently in the strapper.

      NEEDS_STRAPPING                                     B10/0            

        ----| |--------------------------------------------( )-

                                                            ||

                                                            ||

                                                            \/ 

B10:0                   /7   /6   /5   /4   /3   /2   /1   /0

 Current Values          0    1    0    1    0    0    1    0

                                   ||

                                   ||

                                   \/ 

                                B10/[N7:0]                      DO_STRAPPING     

        --------------------------| |----------------------------( )-





        WRAPPER_EXIT_PE                          +------- BSL ---+

        -----| |------------[OSR]--------+-------| File:  #B10:0 |

                                         |       | Bit:    B10/0 | 

                                         |       | Len:       16 |

                                         |       +---------------+

                                         |

                                         |       +--------- ADD ---+

                                         +-------| N7:0 = N7:0 + 1 |

                                                 +-----------------+

 



       STRAPPER_ENTRY_PE                         +--------- SUB ---+

         -----| |----------[OSR]-----------------| N7:0 = N7:0 - 1 |

                                                 +-----------------+

Now there's all sorts of extra code to put in here: debounce logic to make SURE that one and only one signal is triggered per pallet, and some reset logic so that the pointer (N7:0) can never get below 0 or above 16; but that's the general outline.

This will work well enough, except for one MAJOR problem. It's all too possible for the system to get out of synch. If a pallet is removed between the wrapper and strapper, or is someone climbs onto the conveyor to fix something, and block either photoeye enough to cause pointer to get off, then you'll be strapping the wrong pallet, which is not good. And you will always, consistantly be one off (or two, or three).

The way to fix this is to provide the operator with a "sanity check". The operator needs to see how many pallets that the PLC "thinks" are between the two devices, and give him the ability to change that setting. This can be as simple as a BCD LED display with two pushbuttons (increment, decrement), or as fancy as an HMI/SCADA. But that control MUST exist.

The very minimum "sanity checker" would be a "reset" button where would set the pointer to 'zero' (or 'one', depending). Your operaors would have to have a procedure that, if the system got out of synch, to run out all the pallets beween the wrapper and strapper, (which would only decrrement the pointer), and then pressing the reset button (to clear any "ghost" pallets that the PLC thinks is there).

This strategy can also be applied to "detect"/"reject" stations that are separated by a variable number of bottles/objects.

Good luck