Linear Motors and PLC's

G

gerard

Hi,

I am looking for some info on a project that I am going to carry out at work in the near future. It's good idea in theory, but i'm afraid that i may not be able to put it into practice. So I'm trying to see if any of you have seen a similar application and may be able to point me in the right direction.

A linear motor with a pneumatic cylinder attached is used to cut a dough product(pizzas). I want to drive the linear motor to a certain point and make it stop - at which point the cylinder will extend and 'chop' the dough. The stop point will be different every time and this will be determined by a signal from a vision system. The vision system will monitor the process, it will then send a signal to a plc to determine how far the linear motor will travel and then stop.
The linear motor will return to a home position after every action.

So, to sum up what I am trying to do - I want to drive a linear motor to a certain point, extend a pneumatic cylinder and then return the pneumatic cylinder to it's home position.

The vision system in question will give an analog or digital signal output which will be proportional to the distance of movement of the linear motor.

Cheers:willy_nilly:
 
The linier motor will have to be of the stepper type or fitted with an encoder. Either of which will need a dedicated card in the PLC rack.
Can you explain the “vision” system?
 
Tony said:
The linier motor will have to be of the stepper type or fitted with an encoder. Either of which will need a dedicated card in the PLC rack.
Can you explain the “vision” system?
Click to expand...

Hi,

We are producing pizza bases. They come out in the form of dough balls. They are free pressed by a hydraulic press - four at a time (2 x 2). When we are producing the larger bases, 2 of the bases sometimes join together and form one large distorted pizza base. This causes problems down the line. The vision system we trialed has the ability to recognise when this happens and give a signal to a PLC or linear motor system. I plan to use the signal to control a linear motor, to drive it to the exact location and 'chop' the pizzas(with a pneumatic cylinder). The vision system will give a binary or analog signal proportional to the distance the linear motor will move.

Thanks.
 
We are producing pizza bases. They come out in the form of dough balls. They are free pressed by a hydraulic press - four at a time (2 x 2). When we are producing the larger bases, 2 of the bases sometimes join together and form one large distorted pizza base. This causes problems down the line.
Click to expand...

This is where you really need to be concentrating. Simplicate rather than complicate; try to correct the issue before devising wonderful new 'gimmicky' get around in the form of an additional complexity. I really do understand the "interest of creation" but the first solutions should start with ironing out -why are the bases joining together?-

The vision system we trialed has the ability to recognise when this happens and give a signal to a PLC or linear motor system. I plan to use the signal to control a linear motor, to drive it to the exact location and 'chop' the pizzas(with a pneumatic cylinder). The vision system will give a binary or analog signal proportional to the distance the linear motor will move.
Click to expand...

I love a good project to marry-up new technology and wizardry to an existing process... if the simple solution of prevention has been looked at then I suppose you really must come up with a complex cure.

What PLC are you (/looking at) using?

Vision system... Cognex, Banner?

.
 
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There’s three ways of looking at this. One is purely mechanical, the other two uses your idea with the “vision” sensor.

1[SUP]st[/SUP] idea is a shaft with 1 or 3 round cutters mounted with a weight to cut between all pizza bases, cheap and simple. The conveyors are as I remember are hard plastic so the blade shouldn’t be a problem

View attachment 7221

2[SUP]nd[/SUP] idea, you know the belt speed so why does the cutter need to move? You detect a bad base at point “A”, say 50cm further on a rotary cutter drops to chop the bases as required. From what I remember the belt speed at this point isn’t that high. The cutter drop time determined by the belt speed.

3[SUP]rd[/SUP] idea if the belt speed is high and there is a danger of the bases tearing/turning. Then yes travel the cutter at a speed to almost match the belt. The cutter traverse speed being linked to the X% of the belt speed VFD. The things you know:


  • Your detection point
  • Cutter home position
  • Distance to cuter.
  • Belt speed.
  • Duration of cutter operation

All relatively easy to put together with a good PLC or add on to your existing set up.
 
Last edited by a moderator:
Silva.Fox - The reason we are going to use a system like this is because there is no possible way of preventing this from happening. We are looking at this problem, day-in day-out, and this seems to be the only solution. I plan to use a Mitsubishi PLC with a Keyence Vision system.


Tony - thanks for your reply, a lot of information to take in alright! I find it hard to explain but it's not that simple due to the nature of the product involved. If we try and chop the pizza as the belt is moving it will distort the pizza because the belt is moving too fast. The speed of the belt is beyond our control as the slower it goes the less product being made, and that's a big no-no! I don't believe a mechanical solution is a runner to be honest.
 
So, if I am to use a Mitsubishi PLC to control the linear motor how would I go about it? Has anybody ever tried such a thing. The Vision System will give a binary number to the PLC and the PLC will then instruct the linear motor to move. That's the theory anyway!
 
Ive done similar rotary movements with a variety of Allen Bradley PLCs some of them have a High Speed Counter built in ideal for encoder monitoring and a variety of digital and analogue inputs and outputs are available it usually boils down to what system you are familiar with Siemens do the same in a slightly different way with the S7 range
 
gerard said:
The reason we are going to use a system like this is because there is no possible way of preventing this from happening. We are looking at this problem, day-in day-out, and this seems to be the only solution.
Click to expand...

There is always a way...

Tony - thanks for your reply, a lot of information to take in alright! I find it hard to explain but it's not that simple due to the nature of the product involved.
Click to expand...

Would you mind explaning the process so we (I) can visualise the system?

I'm thinking two lines of dough balls on a foodstuff conveyor belt, goes under an arrangment of hydraulic presses (thinking four cylinders mounted in two by two as in 4 on a domino). I'm thinking circular PTFE plate on the end of each cyl. rod which presses down on the dough ball to form a roundish dough base. Anywhere near?

.
 
Gerard, if i have understood Tony's mechanical method correctly there is no chopping involved. i use a similar method to separate dough on fast moving conveyors. the blades are in constant contact with the belt but are rotating at the same speed as the belt is travelling so no belt damage, just think of them as three sharp rollers. in practice i have mounted the blade shaft on a threaded bars and set them so they are just clear of the belt and no more, they're also teflon coated. its simple and works well.
 
As regards the electrical connection side of it,as Tony says a dedicated card/rack in the plc should sort this ok.
Perhaps a simple drive system may help your needs?
You could then use the encoder to feedback the exact position of where the cut needs to occur,and then get it to come back to home at high speed thus speeding the op up.
I work in paper manufacture now and I must say the majority of our packing/separating ops are motor/mechanically controlled and the accuracy is perfect everytime?
 
Tony said:
Was the question mark intended :wink:
Click to expand...
:wink_smile:

As you suggested though,it's a relatively simple system where as the paper goes down the conveyor a stop pops up in front of it to centralize,neaten etc then a backboard pops up to basically get the paper in the perfect place to fit in the flat box,which then goes through a series of very clever metal guides before it gets glued.
Am I being a bit :sleeping: now???????
 
gerard said:
When we are producing the larger bases, 2 of the bases sometimes join together and form one large distorted pizza base. This causes problems down the line.
Click to expand...

How often does this occur?

If not very often then how about a drop flap which rejects the dodgy base into a bin, rather than trying to rework it?
 
xraytek said:
How often does this occur?

If not very often then how about a drop flap which rejects the dodgy base into a bin, rather than trying to rework it?
Click to expand...

Binning it is waste! Reworking it wastes resources. Correctly forming it at source is the ideal. But in this instance -reinvent the wheel-

.
 
The only flying shears I know of would cut every thing up.
I’m thinking of the ones used in steel works :56:
 
Hi,<br>Sorry for the delayed response. The pizza dough balls are dropped onto a loader plate. The loader plate consists of 4 'doors' that open when the time is right and they drop the dough balls onto a teflon belt. This belt then indexes forward, moving the dough balls under a hydraulic press. The hydraulic press consists of a large teflon covered platten that presses down on the dough balls, making pizza bases out of them. The hydraulic press than rises and the teflon belt indexes on again and moves the pressed pizzas on and the process continues on like that. The pressed pizza bases are not perfectly round at this stage because they are free pressed. I want to cut them at this stage using some process and remove the excess waste and re-use the waste. Not sure how to do this tho? Are you any clearer?
 
Hi,

Thanks for your reply and sorry for my delayed response. I understand your method. Is it possible to get a picture of some sort?
 
A flying shear is a 2 axis Cartesian system which is linked to conveyor speed, it is used in the food, beverage, paper, wood & steel industries for cross cutting when conveyor following.
 
I do wish. But I was just having a look at the flying wheel method and I'm not sure if it will work. The products are not in a continuous sheet and there would be timing issues I believe.
 
I worked for N.........n Foods manufacturing pizzas? I'm scratching my head trying to remember this process. I can understand your theory. Was the process designed for the larger bases? If so depending on the failure rate I would want to assess is the product varying in some way so that it is rejoining during the process or is it not being cut correctly? Is this a maintenance led containment you are putting in place or has the maintenance department raised a CAPEX for the euipment you are aquiring? And from your initial posts can you confirm are you replacing the current process or adding a countermeasure if the current process doesn't work correctly?
I assume you have contacted the machine manufacturer but if you still require the modification it sounds like a good idea. Depending what resources you have a dedicated card for your encoder can be acquired but if you are short on funds you can always wire encoders to your inputs if you have spare. What keyence system are you using. I've currenly use the, now aging, CV75 1P which is mainly used for colour recognition but in your environment could pick up the edge of the dough ball. I think you have had sufficient information relating to cutting of the dough balls.
Anyway good luck and my conclusion follows silva fox, find the source of the problem.
 
gerard,
With the correct equipment a flying shear would solve your faults.
However, I have to agree with the others, you really need to address the root cause!
 
So you have a circular plattern that presses down on the dough balls... what about a skirt around the plattern, tapered out and sprung... the plattern then presses down the dough and the skirt prevents excessive spread.

.
.
.
 
Thanks for your response. This is a project from scratch. At the moment we are cutting them at the end of the line, but, at this stage the pizzas are 'par-baked'and the waste cannot be recycled. So, we are trying to cut them before they enter the oven and recycle the dough at this stage, where it is easy to do so. The machine manufacturer would be of no use to use us, because, as I said, this is something we want to do to reduce waste ourselves. It is not easy to resolve the problem from the source - the dough balls are free pressed and are never the same size or shape. I know this sounds a little crazy, but that's the process and we just got to work with what we have. I am not sure of the Keyence system yet as we only trialed it and the guy seems pretty sure it will do the job - and I am not up to speed on it yet. To sum up, we are looking to cut the pizza bases, by some method, where the bases are constantly on the move on the conveyor.
 
If you can recycle the dough at the detection point then a drop flap makes sense to me.

I’ve worked on faster systems than yours (you may have guessed I’ve seen a pizza line in full production) the belt suffers no slow down. I’ve worked for the biggest chip manufacturer in the UK (sorry can’t say who) but the drop flaps worked brilliantly.
 
I am sure the plattern idea silva fox mentioned may be what was used when I was in this industry.
 
Well the flying shear works at Cadbury Bourneville factory quite well.
I believe Sivaa's plattern idea would work too.
As Tony says if you can recycle 100% at the point of detection, then just do that drop the dodgy bases onto another conveyor and set up an automatic re-work loop.
I have also worked on very fast production lines and done design and applications engineering for an equipment oem on this sort of kit, where we HAD to help the client sort these sorts of problems out.
Along with working as a production line staff engineer tasked with cost downs and speed up's on production lines, so IMHO the advice you have had so far sounds good to me.
 
Silva.Fox - we have a platten with exactly what you have suggested and it does not work - In theory it should. We had circular plattens supplied with the machine. The dough ball is indexed to the center of the circular platten and then it presses down on the dough ball - making it round. But, it is not a good design and the dough gets stuck to these plattens and causes all sorts of problems. We have tried this and it's a non-runner.
 
Should have added travel is right to left. For pizza’s the drop distance only needs to clear the 3[SUP]rd[/SUP] section.
As I said if you can rework the dough it’s a simple way around it.
 

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