Tracking a parcel along a conveyor

Guys,

I have worked with various makes & models of PLC's for a few years now. Up to this point my experience has really only been in very basic ladder logic control applications (press this button and run this motor, flash this light if something goes wrong etc.)

Now I find that I need to write something a bit more complex !!

The basic principal will be something like .....

Parcels are manually placed on to the first conveyor.
They travel under a barcode scanner.
No Read ? - parcel is diverted onto the manual coding line.
Good Read ? - parcel travels along conveyors and is automatically diverted off at the correct point.

There is obviously much more to this than just the few lines above but my question is ..... Where do I start !?!?!?!

There is so much that I need to learn, from RS232 comms for the scanner, to shifting data. Someone might place a parcel onto the conveyor (or take one off) after the barcode scanner so I need to handle this.

Wow my head is spinning already and I haven't started yet !

Can anyone point me in the right direction ? How can I learn this stuff quickly ? And oh, I have only got a few weeks to get to the test stage !

Mark.

There is obviously much more to this than just the few lines above but my question is ..... Where do I start !?!?!?!

Click to expand...

Take a deep breath... exhale...

Start writting the sequence of operations in a format you are comfortable with. This could be an outline form, flow chart, list of operations on a napkin , etc. Just start documenting your process before you start programming. If your PLC supports it break down the sequences as much as possible into separate subroutines. As you make progress on documenting your sequence flow come back here if you need help in all the differing methods everyone has for programming a sequence of operations and of course there are many twists depending on the PLC you are using and if there is an HMI/ERP system in the middle of all the other STUFF going on in the system.

Good advice,
look at the process as a whole (don't look at the machine), you need to keep track of allot of things. One good thing is they are all in a row and their position can be tracked by sensors.

Master decoder as you said valid or not,
well it sounds like to me you have already started the logical thought process. now you need to keep trak of those other "valid" prospects. Sounds to me like you should think of this as a bit train of objects with a set path. The path being determined by the barcode scanner. with sensors counting the passage of objects, it would be easy to keep track, so as to turn on or off, a diverting air cylinder to eject the object into the correct bin.
Start with the yes/no valid/not valid idea, add additional conditions until you are familiar with the neuances. then produce better code, nobody ever turned down free software updates.
I see the only hard part is what diverters decode what address!

Bitmore

First diverter acts on valid 495XX zip code (input sensor to count object)
diverter drops to deflect object onto valid 495XX and counts passage of objects
diverter raises to pass 496XX objects...
next phase diverter drops to divert 4950X objects in order... keep in mind there is some bit train logic here. Programmers usually use LSL or LSR bit array commands to keep track of sequential objects in a train...


Bitmore

By all means visualise the whole process, but you should also definitely be studying the 'machine'. You need to be sure that you have no misconceptions as to the physical nature and operation of the diverter mechanisms and the conveyor as well as what variety of package shapes and sizes you will be dealing with.

Can packages slip on the conveyor? If so, a shift register won't work very well.

If packages are added/removed after the scanner, a FIFO is useless.

Obviously, a method is required for setting up which code goes where.

Yes I agree.. a FIFO structure would not work for sorting. For final error reporting keep that in mind. now for the first thing.
Bit shift logic is a very good method of keeping track of allot of sequential objects. certainly staging conveyors is the key. you sound smart.. for staging conveyors you will need to know some things like offset counts to position and such. servo control would help.
Hey, I didn't say I was done yet, but certainly it has already been done!

Bitmore

you have no misconceptions as to the physical nature and operation of the diverter mechanisms and the conveyor as well as what variety of package shapes and sizes you will be dealing with.

Click to expand...

Thats the beauty of staging conveyors,, object in , object out.
Size no consideration (as long as it fits the conveyor! and I hope oversized objects never get to the sorter.)

Next phase....

you have no misconceptions as to the physical nature and operation of the diverter mechanisms and the conveyor as well as what variety of package shapes and sizes you will be dealing with.

Click to expand...

Oh SHIRT!
your right but I was going on your original question.... perhaps you should have been more specific,,, I was thinking packages of a certain size restriction and weight.

Parcels are manually placed on to the first conveyor

Click to expand...

I should have known it is a labor problem, the labor force is placing to large a package on the conveyor.

bitmore

I guess now I am being bad....


A recent employee of UPS declined a package because it was larger than spec.

ups said its girth should not exceed 180"

the package dimensions were 181 x 18 x 1 the employee was quoted as saying "see"

we all know girth as described in definition is the length times the width, guess what, the package was shipped when they noticed the machine validating the shipping accepted the package when set in lengthwise.!"

They did have problems with some shorter staging equipment.
So the lesson learned is that there are restrctions to sorting "packages" and well I guess I have been a little fecious in my help. But you did ask for help and said you were lost!
I don't think your lost, just overwhelmed. 1 step, at, a time.
Bitmore

Something to consider: Is there some identifier on the item which is the sole criteria which decides where a package diverts, or, will package info need to be stored somewhere before the items actually get to the scanner? If the former, the matter is simpler. If the latter, a new question arises: Can it all be stored in the PLC or will you need some kind of (presumably PC) front end?

How much data is required for each item tracked? Is it a simple zip code sort of thing as mentioned above or is there more involved? For example, can multiple consignees be assigned to the same chute?

Is it possible for material to recirculate and get a second chance to divert if the lane/chute is full or disabled? Third chance? Fourth?

There are two possibilities I'm aware of to implement tracking.

1. A shift register. This could get expensive time-wise if the register is long and/or a lot of data needs to be shifted.

2. Down counter. Each inducted item gets a number representing how many pulses are required to get it to the divert point. Naturally, an encoder is needed here. Each encoder pulse decrements the counts for all active carts and when some predetermined value is reached (zero would work nicely) the divert mechanism is operated.

The questions go on but, believe it or not, you're ahead of the game by your awareness of the task in front of you.

Mark

This could be interesting--my name is Mark also.

I have a line in my building that does the exact same thing you're describing. We scan a barcode, then, using bit-shift logic, divert to three different lanes.

What scanner, what PLC, etc., are you gonna use?

i've done similar things. i ended up using a FIFO for each lane, not for the whole sorter. So as each package hits the scanner, put it into whatever FIFO lane stack it should go to. then i use a downcounter using an encoder to decide divert points. each lane can be calibrated beforehand to find out how many pulses it will take for a box to divert correctly.

Mark,

All good advice, and a picture is worth a thousand words, so here is a "picture", a copy of an Allen-Bradley RSLogix program that does exactly what you described. You didn't say what type of software, so I would guess that this is not a real program but a training exercise, in which case it will not matter to you which brand you use, so this should help you out.

Lancie

You have basically the same program I wrote for my 7 lane sort line (until you get to the data copying part-- I "MOV" the extracted scanner data into lane registers and trigger the outputs when the BSL value equals the correct register for each lane). Did you write the program in your post? It's rather impressive.