Chain stop detection unit ideas

[bigtip]

Hello all, i've been working at a quarry recently and i spend a fair bit of time on what can only be described as a giant electrical chainsaw which cuts the stone from the face. it's an old machine but it cuts pretty well as long as it's sharpened but i need to find a way to make it shut down whenever the chain stops moving because when the chain stops it's in danger of mangling up itself and the rails it runs on.
how would any of you think this could be done? i've got two ideas so far: 1) a photoelectric sensor that is broken by the passing chain teeth so it could tell when they stop passing or 2) a little generator with a big foam tyre that actually runs on the chain and would stop generating a voltage when the chain stops.

not sure if either of these would work though.

 

[Robotstar5]

Assuming the motor drives a gearbox to drive the chain, put a clutch type coupling between the gearbox and the chain, if the chain jams the clutch slips.

 

[daleyd]

Without knowing the whole layout then it's difficult but as robostar says a mechanical clutch would be ideal. Your idea with a generator/tacho on the chain drive with a voltage sensing relay or similar to only allow drive when the voltage is above a certain level Should also be ok.

You can get relays that are adjustable to sense the voltage and switch above or below a certain threshold.

Encoders are tending to replace tachos more and more but for a simple "it's either running or not" application a tacho and relay is probably ok and possibly more robust.

 

[Pete.]

Option 2)

Link a tachogenerator to the drive and use the voltage it generates to hold in a latching relay. When the chain stops the volts drop to zero, control relay opens, power goes off.

As it happens, I might have a suitable one here I would let go. I bought it to control a DC drive but using it is not compatible with field-weakening so it's surplus to me. Brand new too.

 

[piman]

Hello Bigtip,

Google Rotation sensing device, plenty to choose from, basically using a proximity sensor, rotating striker (Your chain could possibly be used instead) and a relay. Used extensively in industry.

Alec

 

[bigtip]

Thanks for all replies. i should have said it's a friction drive between the chain and 'sprocket', the chain has no drive links.

Pete could you tell me more or send me some details on your tachogenerator.

 

[Pete.]

Sounds like a ring-saw.
I'll get a photo when I get home today and the proper specs but basically it looks like a tiny foot-mount motor, with a shaft one end and two wires the other. Turning the shaft generates a dc voltage on the wires, I think it's 60v per 1000 revs but I'll check that later.

 

[daleyd]

If your not electrically minded I'd stick with the tachogenerator idea (Then again if your not electrically minded I'd get someone else to do it for you!). Proximity switches are great in that they don't touch anything, but they can be a pain to set up and you are into the realms of measuring pulses, frequency etc.

With a tacho you would simply measure the voltage out when the chain is running and then set the relay/trip amplifier or whatever you have to do the sensing to trip at some lower level.

 

[Pete.]

Here it is, as you can see it's completely un-used.

Here's the specs. They work in either rotation and you can tune the voltage with pulley sizes I guess. Just don't ask me for help with the circuitry, I always have to do that myself.

http://www.transdriveonline.co.uk/p...o-444-r1-b0-06eg-radio-energie-tachogenerator

They are nearly £500 & vat new I paid £60 for this one as I recall. I don't need it so I'll sell it for £40 posted.

 

[bigtip]

I'm quite electrically minded but i would always be glad of a simple circuit diagram. To power a 230v relay (as long as the chain is running) i kind of imagine there would need to be something between the tachogenny and the relay coil that it 'controls'. what would work best there?
i don't think i would want to get into amplifier circuits and electronics though.

 

[Pete.]

What speed does the chain run at in m/sec? I'm guessing about 20.

 

[daleyd]

Depends on your budget but you can buy an adjustable voltage sensing relay that you can adjust to trip at certain voltage levels for probably around £100. If you know the chain speed and size of pulley/wheel you can work out roughly what volts you are going to get out of the tacho and buy a relay in that range.

You could do the electronics yourself but i havent done anything like that since my apprentice days!!

 

[Luke]

Encoders are more commonplace today, compared to tacho's. If the tacho or encoder needs to be mounted outside, you will need to consider the environmental rating of the device, as well as its electrical properties.

Encoders and tacho's are available with shafts or hollow bores, so you have a choice on what's easiest to fit.

http://www.industrialencodersdirect.co.uk/product.asp?cID=001&scID=003

As an example. Motrona do a good range of simple programmable counters, with relay outputs etc.

http://www.motrona.com/tachometer_e.html

Luke

 

[daleyd]

Something like this if you are using the tacho, just ensure you know what supply voltage is (24, 240 volts?) and work out roughly what voltage you expect out of the tacho. Wire the relay output into the run circuit of the saw and connect your tacho up to the input and bobs your uncle.

http://uk.rs-online.com/web/p/monitoring-relays/8016224/

Or as Luke says you can also use an encoder with a similar frequency sensing relay, but troubleshooting can be a bit more problematic (freq rather than a voltage)and they can be more fragile.

 

[bigtip]

So daleyd you're saying find out the output voltage of the tacho at normal chain speed, choose a relay with the same coil voltage and run the main saw relay though that?
Pete, the chain speed i would guess could be 5 or 10m/sec. it's a 3m bar approx and i reckon it would do one full rev around the bar per second at most. I'll get a photo of the beast tomorrow.

 

[daleyd]

So daleyd you're saying find out the output voltage of the tacho at normal chain speed, choose a relay with the same coil voltage and run the main saw relay though that?
Pete, the chain speed i would guess could be 5 or 10m/sec. it's a 3m bar approx and i reckon it would do one full rev around the bar per second at most. I'll get a photo of the beast tomorrow.

No, the easiest way is to buy a voltage sensing relay that does all the hard work for you, but it would be useful to know what voltage you should expect from the tacho so you can buy one that measures in the right range. You can get ones that are adjustable from 0 to 600 v or higher though, so it's not vital, I'm sure the tacho output will be somewhere in that range!

I'll do you a quickie circuit diagram tomorrow when I'm at my PC if you like.

They are about 100 quid or more (for a reputable make from rs but you may get them cheaper, perhaps even eBay?), so if your on a budget you'll have to get creative!

http://www05.abb.com/global/scot/sc...1c2c1257a3d002e72d5/$file/2CDC112101D0202.pdf

 

[bigtip]

a circuit diagram, that would be much appreciated!

 

[daleyd]

No worries, it'll be more interesting than the actual work im supposed to be doing tomorrow!

 

[Robotstar5]

Nice selection of voltage monitoring relays from Broyce

 

[daleyd]

Something like this, note I have shown the traverse contactor as being started by a simple on/off switch, your actual arrangement will probably be different but you get the idea. The contacts from the voltage monitor should be in series with whatever start/stop arrangement you have.

The voltage monitor relay needs a supply from whatever your control voltage is (Normally 24 volt but could equally be 240 or 110) and has a set of contacts which open during undervoltage conditions. It also takes the dc voltage from the tacho and monitors it. You would set the relay to trip at a level below the normal running voltage of the tacho, when it does it would open the contacts and stop the saw traverse.