PLC Using Sensor for a simple project

[thewalkerist]

Hello everyone,

I am a newbie in automation stuff and i wanted to do a simple thing with just a sensor, a blade and a plc.

My aim is this - The chocolate bar will come in and my sensor will see it. After my sensor sees the chocolate bar i will give a command in plc that will release(cut the bar) and bring it the blade up. This process will continue on until the chocolate bars are over. The conveyor will not stop at all so bars will keep on coming and the blades will keep on cuttin .

Now i have some big problems with this tho.

My first problem - I didnt work with any sensors before so i dont know how to make that happen. Both the sensor and the blade is digital signals so i dont need to work with any analog scaling or stuff like that. Thats why i said this will be a simple process. But i need some kind of reference or example. I am thinking of doing simple stuff like put a condition(sensor energized or not) and if its energized release the blade and pull it back up. I wont need any alarms or stuff like that since this will be for myself but in the future if i need stuff like that i would do upgrade the system :nodi: .

My second problem - I said

I am thinking of doing simple stuff like put a condition(sensor energized or not) and if its energized release the blade and pull it back up.

. Now this brings up another set of problems. How will i do that exactly? i mean i just put a condition and say like, hey if the sensor is 1 than just turn the blade 1 and than instantly make it 0? Or is there some sort of function block or smthng like that i can use for this type of thing? Maybe ONS- one shot? i do not now.

My third problem - What kind of sensor would i need? This is not that important since i have time to search up my sensors but if anyone used sensors for this type of stuff i would like to know

My last and probably biggest problem - How will the sensor be de-activated or reset? I have couple of options here. Let me put a hypothetical scenario to the table. Lets say i put my chocolate bar on the conveyor and turned it on. And my sensor sees the bar and my blade cuts the bar. Now the way i see it i can do two things. First is - dont do anything to the sensor. This will probably end up in chocolate bars getting cut to tiny pieces but to counter this maybe i can put a small delay to my blade going back up so my blade can hold the bars for a small amount of time so that my sensor has time to reset itself. My second option is - do not mess with the timers or stuff like that and just let it roll, the blade's up and down movement give brief openings for sensor to reset by itself and i wont need to do anything.

Can anyone help me?

Thanks for reading and for helping.

 

[geniusintraining]

If I were you I would try watching some videos on how someone else is doing it and not try to reinvent the process, I have seen some process that the conveyors do not stop but the guillotine blade moves with the product or you will have to stop your conveyor when you make the cut

https://www.youtube.com/watch?v=-nXrtsGuOZ0

https://www.youtube.com/watch?v=4y3_XEfoOx4

https://www.youtube.com/watch?v=HfLsXiXo0No

 

[parky]

Sensors do not reset themselves, As I understand it you present a bar, when the sensor detects it, it then cuts the bar, you wish to cut the bar into more than two I presume, I think we need more information for example how many cuts per bar, 1,2,3 etc. what size is each cut ? The size of the bar ?.
Let's take a scenario, you want to cut a bar in half, pretty simple, you arrange the sensor to be in front of the blade i.e. exactly half the bar length, when the sensor is triggered on the front edge of the bar it activates the blade, but if you need more than one cut then in reality you would need an encoder on the conveyor so you could convert the counts into millimetres or what ever to detect the position of the bar for accurate cutting, you could use timers but not very accurate, also what about the problem of cutting on a moving conveyor, unless the blade moves with the conveyor & back before the next cut (although if the cut is quick enough perhaps not needed), so many scenarios, more information is required.

 

[thewalkerist]

Sensors do not reset themselves, As I understand it you present a bar, when the sensor detects it, it then cuts the bar, you wish to cut the bar into more than two I presume, I think we need more information for example how many cuts per bar, 1,2,3 etc. what size is each cut ? The size of the bar ?.
Let's take a scenario, you want to cut a bar in half, pretty simple, you arrange the sensor to be in front of the blade i.e. exactly half the bar length, when the sensor is triggered on the front edge of the bar it activates the blade, but if you need more than one cut then in reality you would need an encoder on the conveyor so you could convert the counts into millimetres or what ever to detect the position of the bar for accurate cutting, you could use timers but not very accurate, also what about the problem of cutting on a moving conveyor, unless the blade moves with the conveyor & back before the next cut (although if the cut is quick enough perhaps not needed), so many scenarios, more information is required.

Imagine a never ending chocolate line(i said bar but i realize how it lead to a misunderstanding. Sorry about that). Now image it moving on a conveyor. Conveyor belt wont stop at all and there is a blade and couple of inches after the blade we have our sensor. Our endless chocolate line will go through our blade and our sensor will see the chocolate and send a signal to the plc, and our guillotine blade will cut it. After the cut there will be a small empty space because conveyor was moving while the cut was being made. Thats why there is a small window of space where our sensor will be set to 0 because it wont see anything. And after our guillotine blade resets our chocolate line will continue to move and our sensor will see the chocolate again and it will repeat the process.

Now size is not the problem here because size will be decided by the sensor and here is the catch, we can move the sensor. The distance between our sensor and our blade will make the size of our chocolate bar, however long or short as we please. Thats why i dont need encoder or stuff like that.

https://www.youtube.com/watch?v=-nXrtsGuOZ0

https://www.youtube.com/watch?v=4y3_XEfoOx4

https://www.youtube.com/watch?v=HfLsXiXo0No

If you need more these videos are kind of what i want but mine is kind of different than these because of my sensor placement. If you have any more questions i would be happy the answer them.

 

[thewalkerist]

Oh and btw "moving conveyor and cutting" problem is easily solved. Blade will be fast. I've tested this on a moving conveyor btw and it worked like charm.

 

[geniusintraining]

Oh and btw "moving conveyor and cutting" problem is easily solved. Blade will be fast. I've tested this on a moving conveyor btw and it worked like charm.

Then... you dont need a sensor at all, you need a encoder and that will tell the PLC how fast and how far the conveyor has moved to your product would be cut every XX, you would count the pulses from the encoder to calculate the length of the product, not a sensor.

Also if your blade can cut without stopping your product and not moving the blade then your product/conveyor is moving slow

 

[drbitboy]

Now size is not the problem here because size will be decided by the sensor and here is the catch, we can move the sensor. The distance between our sensor and our blade will make the size of our chocolate bar, however long or short as we please. Thats why i dont need encoder or stuff like that./QUOTE]


Does the sensor sense the edge of the chocolate? E.g.

• before anything has been cut, does it sense the leading edge of the (presumably) continuous piece of chocolate?
• after a cut is made, can it sense when that cut reaches the sensor?

 

[thewalkerist]

Then... you dont need a sensor at all, you need a encoder and that will tell the PLC how fast and how far the conveyor has moved to your product would be cut every XX, you would count the pulses from the encoder to calculate the length of the product, not a sensor.

Also if your blade can cut without stopping your product and not moving the blade then your product/conveyor is moving slow

Well..... maybe, but i wont be doing a screen or stuff like that but i want to be able to change the cut distance without messing with the encoder and programme and all that

I didnt say it wont stop the product. It will make a brief stop to it and it will give space between bars

 

[thewalkerist]

Now size is not the problem here because size will be decided by the sensor and here is the catch, we can move the sensor. The distance between our sensor and our blade will make the size of our chocolate bar, however long or short as we please. Thats why i dont need encoder or stuff like that./QUOTE]


Does the sensor sense the edge of the chocolate? E.g.

• before anything has been cut, does it sense the leading edge of the (presumably) continuous piece of chocolate?
• after a cut is made, can it sense when that cut reaches the sensor?

It will sense the begining but since the conveyor will be moving too fast it wont see(well it will see but at that point our blade will still be moving up so it wont go down again) the end of the cut. So from the beginning of the time when blade will be going down and up our bladed chocolate bar will be long gone.

 

[drbitboy]

It will sense the begining but since the conveyor will be moving too fast it wont see(well it will see but at that point our blade will still be moving up so it wont go down again) the end of the cut. So from the beginning of the time when blade will be going down and up our bladed chocolate bar will be long gone.

This is not clear to me. When I read what you wrote, it sounds like the distance that the conveyor moves, during the time it takes for a single blade cycle, is longer than the desired cut piece of chocolate. If that is the case, then this process cannot work.

What inputs does the process have? E.g. a discrete input from a chocolate edge/cut sensor; a discrete input when the blade completes the cut; a discrete input when the blade returns to its starting position/height after making a cut; etc.?

What triggers each of those inputs?

What outputs does the process have? E.g. a discrete output to lower the blade; a discrete output to raise the blade; a discrete output to run or stop the conveyor; etc.?

It sounds like you have operated this process manually. Can you poost a sketch of the process? Can you post a timing diagram of what how you want/expect the inputs and outputs to behave? Can you indicating triggers in the timing diagram?

 

[parky]

As I see it, then you create a one shot (OSR) to latch the blade cut bit then assume it's going to be in the order of tens of milliseconds then when the sensor detects another rising edge it triggers it again, but all this depends on the scan speed of the PLC (you could use interrupts) & how fast the blade operates. but it is doable I have done a similar thing on cutting plastic & metal bars (Shear), however, I did use immediate I/O interrupts too ensure accurate cutting. The sensor was on a worm screw & encoder to select the length which was selected on the HMI.
Also did a steel tube cutting process, the flat steel was fed in on one end of an un-coiler, this fed to an accumulator, that then fed it to a former, into a welder, then into forming stations, then to a cut off saw, this was a diamond tipped circular saw of about two metres dia, it measured the tube as it was extracted, matched its speed, cut & returned, the calculations in it were horrendous, not my calcs but had to convert them into "C", it used the type of tube round or square, size, thickness of walls & calculated the cut rate depending on the profile of the tube to get the max life out of the blade. I do know that it took an HP scientific calculator 21 seconds to do the calc, unfortunately at the time (early 90's) MS "C" did not have floating point maths so we had to do the maths in integer form & re-form it, the lead time on the FP maths from some company called brain boxes I think was on a 12 week delivery.
If I remember correctly, the plant could produce round or square steel tube from 220mm down to 15mm at a speed of 80 metres a minute, never stopped as the coils of steel were loaded & welded onto the end automatically, over 200 mtrs long

 

[geniusintraining]

Well..... maybe, but i wont be doing a screen or stuff like that but i want to be able to change the cut distance without messing with the encoder and programme and all that

You dont need a screen all you need is a switch, with 4 digital inputs you can get 10 different sizes by using a 8421 switch and a simple encoder is not rocket science, its a sensor but it will give you accurate measurements so you can have uniformity in your product.

 

[Phrog30]

then... You dont need a sensor at all, you need a encoder and that will tell the plc how fast and how far the conveyor has moved to your product would be cut every xx, you would count the pulses from the encoder to calculate the length of the product, not a sensor.

Also if your blade can cut without stopping your product and not moving the blade then your product/conveyor is moving slow

+1

 

[thewalkerist]

You dont need a screen all you need is a switch, with 4 digital inputs you can get 10 different sizes by using a 8421 switch and a simple encoder is not rocket science, its a sensor but it will give you accurate measurements so you can have uniformity in your product.

That would be ideal but i cant use timers or stuff like that with a encoder.

 

[geniusintraining]

That would be ideal but i cant use timers or stuff like that with a encoder.

You dont use timers, you use counters... when the count of the encoder = xx then you stop and make your cut, this is the only way you can get an accurate size consistently of your product, you can use a sensor to detect if the product is there but you will get a variation in lengths