Current alternative to photo eye.

Mr_B

Member
Join Date
May 2005
Posts
79
We have a saw at work that uses an incremental encoder, photo eye and Omron CQM1 PLC to determine the depth of the cut. Each time the saw is started, the photo eye zero's the encoder value and fom there it starts, cuts about 30 parts and stops.

The circular blade of the saw's diameter varies between 250-300mm. What I'm trying to do is to get it to work with out the photo eye.

Is there some current sensing device that I can use to measure the drive Amps of the saw and detect when it is cutting into the part? If I can do this, I can move the saw slowly forward till it detects it's cutting the part and zero it then.

Any other methods or suggestions would be appreciated.

Thanks.
 
I seen this cool table saw that detects when it is cutting your finger then comes to a stop almost instantly. I thinnk it is done by measuring some difference but not sure if it is what you mean???
It is cool though:nodi:
 
plcnewbie said:
I seen this cool table saw that detects when it is cutting your finger then comes to a stop almost instantly. I thinnk it is done by measuring some difference but not sure if it is what you mean???
It is cool though:nodi:

Now that's cool, but how is it done.... hmmmm.🤞🏻
 
1. Is the material conductive? Maybe you could rig up some circuit to sense electrically when the blade makes contact with the material.

2. Is the material reflective? Position a laser such that a receiver senses light when the surface is present and untouched. When the blade cuts into the material the reflection is removed.

.02
 
Mr B,

I think you could use an Amp meter connected to the motor [or see thread below] to measure the current draw.

When the blade is spinning and not cutting your Amps should be lower then when cutting. How much I don’t know, I would think you might have to experiment for your set points.

http://www.plctalk.net/qanda/showthread.php?t=16634&highlight=current+sensing

If my memory is correct this method was used on Sunnen honing machine.

Hope this Helped,
Bob O.
 
Mr B. I currently do that same thing with a grind wheel on our product. I slowly advance the wheel until I see a spike in the current (I use a servo drive for this). I then store the encoder value and that is my "skim" position. It is very reliable and since I have different parts come in at different times, each unique identifier goes through this skim process (or after a power down). Never had any problems with controlling this.

As for everyone who wants to see the saw in action, go here:

http://www.sawstop.com/

I do notice that they use a hot dog not a finger. From what I see, the blade has a high frequency signal that is changed if the product is not wood (ie finger). I personally think that this is an awesome invention and could save countless fingers (I've been around woodshops).
 
brucechase said:
As for everyone who wants to see the saw in action, go here:

http://www.sawstop.com/
That's quite ingenious, and a great idea... (y)

I don't understand why they need to stop the blade though. The instantaneous retraction below the table surface seems sufficient. To stop the blade, they release a spring loaded block of aluminum against the teeth, ruining the blade and stopping mechanism in the process. Yes, it can't hurt (literally, in this case) to also stop the blade in case the retraction fails to operate, but I would think you might occasionally get false activations of the safety system if the wood was damp, etc... Each time it happens, you must replace the 'brake cartridge' (probably not cheap) AND blade... :(

Although, it's a small price to pay for keeping your finger(s)... :nodi:

🍻

-Eric
 
"I don't understand why they need to stop the blade though."

Imagine if some fool was working with a tie on and WHAM!! it gets wrapped up starts pulling him in and then his face sets off the safety. Now imagine if the blade did not stop!!
 
You do not say why you wish to remove the eye. Is it getting dirty and giving false signals? Why not try another type of sensor or position of sensor, lazer or ultrasonic. Dont now about the plc type but can you enter the saw diameter through a thumbwheel switch and use the encoder from there to travel the correct depth. If product varies you may have to enter the product depth also.
 
Thanks for the replies guys. MicB, the sensor gets dirty and the Operator doesn't clean it too often. I'm looking at some other ways to get rid of the sensor and yes, the thumbwheel is one of my options.
 
Last edited:
Mr B: The range of saws manufactured by Benetec uses a fibre-optic (standard Omron type)which is terminated in a holder into which compressed air is blown via a radial hole to keep the end clear of swarf and coolant. It works without maintenance. Decl: I used to work for them.
 

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