Should I use a PLC for this?

I am currently designing an automated pallet lift for a summer project. Since the manufacturing plant that I am working in is a specialty plant, the normal pallet lifts will not suffice (the workers do not like them), so I decided to improvise on an existing pallet lift design. My Idea is to put a position module, connected to the PLC, on the hydraulics, and control the height of the pallet lift with this. The hard part of this is that there is over 1000 different products that are produced in this plant, and each one has a unique box weight and box dimensions. Since we use SAP, I thought of creating a database of each products weight and box dimensions and have these facts put into a barcode system. I would hook up a barcode reader to the PLC and when the reader scanned to barcode on the box, it would tell the hydraulics how much to move down or up. The amount it would move would be equal to the height of the box, and the hydraulics would not move until the # of boxes per level has been placed onto the lift. The barcode reader would essentially act as a counter and deliver the required height measurement. I have never used a PLC before so any input would be greatly appreciated.

This sounds like an ideal programmable controller project. Since you've never programmed one before, you're in for a fairly steep learning curve but you've thought the problem through pretty well so you have the right approach for success.

Probably a hydraulic motion control module is overkill for a pallet lift; an analog position sensor (like a Temposonic or a string-pull analog input) and a two-speed hydraulic valve are usually enough for reasonable accuracy on something like that.

The SAP database is an ambitious step, but certainly not out of the question and it's a lot easier to populate than a lookup table in the controller. You'll need to research how your favorite controller networks to the computer system you intend to use.

Tell us more !

I was actually looking at a tempsonic and balluf position control. I probably will not try and interface w/SAP because most of the information contained in SAP has been compiled into an Excel file, which I will probably interface with. Hopefully it will work so that I can get a job when i graduate.

Also, what is you'lls opinion on Unitronics PLCs.

I am picturing a pallet lift that starts out in the fully UP position, then moves down as needed as each layer is finished. If this is correct, then have you looked at other methods to determine the distance to move?

I have built a few 'downstackers' for parts off a robot, and all I did was look across the table (I used a light curtain). Each time the robot dropped a shot of parts, the table would decend until the light curtain could 'see' across the table. This eliminated the need for position feedback (except overtravel limits), and would compensate for variations in the stack height. IOW, if it traveled a bit too much on the previous move, it would descend only a small amount (or not at all) on the next move. The light curtain became my position feedback.

Assuming the boxes are placed by hand, you could just give the operators a foot switch of something to initiate the downward movement. The lift would decend automatically until the sensor sees clear, then stop.

I don't know how feasible something like this would be in you application, but it sure beats keeping track of all the different sizes!...

beerchug

-Eric

Unitronics would be good choice for your project. They are used a lot
in warehouse/palletize system.

no problem

remember to get lotsa IR sensors....the rest should be straight forward with no complicated programming.....dont forget the safety stops...at least 4 or so.

Auto storage retrieval systems

think i was doing re-engineering on the entire system years ago....had to leave the country because the army was aftering my spare time after work.

the ASRS have systems thats mounted on rails and stuff....the way they sense stuff is by keying in the shelf box number....and the robot on rails would go to that particular shelf to get the stuff..

its a warehouse thing.think i still have all the plc system diagram for the entire system.

funny thing is we had barecoding but it was used after taking the stuff and laid on conveyor to be scanned.

thanks for the information.

Another question. Could I use the input from the barcode reader as the counter?

I think Eric is on the right track here. I would engage the hydraulic solenoid valve down until a photo eye or light curtain is clear. Light curtains are expensive. It sounds as simple as that.

Questions: Is the stack made by hand? Why a bar code scanner, are you puting info into a database?

The only thing its sounds like you need to work out is when to release the solenoid valve to run the lift table down. If possible the foot switch or a push button would work great for this.

Reply: "Another question. Could I use the input from the barcode reader as the counter?" Im sure you can, when the PLC reads the bar code just add 1...

I think an allen bradely micro logix would be a great processor for this aplication.

^that seems like a viable alternative. Could you please give me some manufacturers of light curtains so that I can get some pricing.

Lt Curtain Manufactures: STI, Sick, Banner, Allen-Bradley.

The last light curtain I bought was an STI and I think was about $3,000

If the light curtain idea blows your budget, there are numerous other methods. I used a light curtain since occasionally a part or two didn't nest perfectly. This could cause a 'high spot' at a random location. The light curtain allowed me to look across the entire surface and verify that the table was low enough for the next shot of parts.

If you can count on boxes always being in certain locations, you could probably get by with one or two 'proximity' or 'diffuse' type eyes, or maybe even capacitive prox. switches. Then you just move down until the side of the box isn't detected anymore. This gives the operators more access to boot!

In fact, a light curtain may be WORSE for you application, as it will look across the top surface. If there is anything laying on top of the layer, the lift will decend too far. Tape guns, someone's jacket, a can of Coke, you name it, may get seen by the curtain.

If this was an automatic palletizer or something, your pre-programmed positions would be the way to go, but since the boxes are manually placed, it's probably best to start with the simplest solution. You can continue to improve it over time.

beerchug

-Eric

cheap light curtain

You could also use a "poor mans" light curtain by just using 5 or 6 photo eyes across the top. I would think that the diffuse or proximity type that Eric mentioned would be the right type for this. Banner and Allen Bradley is where I would look for photoelectric sensors. The light curtains that I've dealt with were meant as safety devices where there are two units, one side a transmitter and the other side a receiver and the actual light units are only an inch or two apart so someone can't sneak their hand in the curtain path. If your boxes are say, 12" wide then a photo eye every 12" would detect if a box were missing.

Here's another site you might check for barcode interfacing, etc... Keyence They have (amongst other things) a barcode display interface that can store the barcodes and you can interface to your PLC with discrete I/O. I'm not sure if they have a unit that can store 1000 barcodes but check it out. Good luck to you. skuller

I have a solution for you.

bmkirkla said:

I was actually looking at a tempsonic and balluf position control. I probably will not try and interface w/SAP because most of the information contained in SAP has been compiled into an Excel file, which I will probably interface with. Hopefully it will work so that I can get a job when i graduate.

Also, what is you'lls opinion on Unitronics PLCs.

Click to expand...

How about a hydraulic motion controller that can interface to the Temposon and Balluff rods AND to Excel using an Active X control and ethernet. The Excel spread sheet can then download the heights or positions to move to. The motion controller can do the sequencing.
A PLC may not be required for something so simple.

Do a google search for 'hydraulic motion control ethernet'. An answer will pop up in the first few results.

if the lift is mannaually filled with products i would just use an electric cell movable up/down by hand. if you press a button the lift will move until the cell is on again.
this way no need of difficult stuff.
if done with robot different story but hey.