Inductive Proximity Sensors

[flyingsparks]

I need to make up a device which will warn the operator of a combine if a pulley stops turning. I was thinking of using a couple of inductive prox sensors to do this, (2 so that if the pulley should stop in front of one sensor it will not think its still spinning, because the second sensor will be positioned so its not possible for both sensors to be sensing at the same time.) I will then use a timer arrangement to switch a relay etc but this I can design later. What I am wondering is if I need a PNP sensor or NPN. I studied electronics a few years ago but this has got me thinking, I cant remember it all that well! Can someone explain the difference to me again please. Also How quick will a prox sensor detect an object in front of it, bearing in mind the pulley will be spinning at about 800 rpm!? Thanks for your input. Chris

 

[flyingsparks]

Something like this perhaps?
http://uk.rs-online.com/web/p/inductive-proximity-sensors/0181708/

 

[ricicle]

PNP switches + , NPN switches -

At 800 rpm you will not be able to work it with relays alone. It will need some form of high speed counter like a PLC or logic relay.

 

[Alarm]

http://www.electro-sensors.com/documents/media/340655074Rock_Aggregate_Apps_Master.pdf

Some help here perhaps?

 

[bernardgreen]

An NPN output is effectively a switch ( transistor ) between the output and the 0volt rail. This will sink current ( positive current flowing into the output ) and can be used with a pull up resistor to create a digital signal refenced to ground. These can be interfaced directly into logic using a pull resistor connected the logic supply rail.

A PNP is effectively a switch ( transistor ) between the output and the sensors supply rail. This will source current ( positive current flowing out of the output ). These cannot be interfaced directly to logic unless the logic supply rail is the same voltage as the sensor supply rail. In most cases a level shifting circuit is required between the sensor and the logic.

The inputs to many PLCs are designed to work with PNP sensors and these work on sensing the the current flow rather than voltage as current flow sensing is less prone to interference than voltage sensing.

 

[flameport]

I need to make up a device which will warn the operator of a combine if a pulley stops turning

A relay with n/c contacts, and a tachogenerator powering the relay coil.
When the rotation stops/falls below a certain speed, the relay contacts will close, this can be used to activate a buzzer or whatever.

 

[Spark123]

Synatel make motion stop sensors too, have a browse around http://www.synatel.co.uk/

 

[ban-all-sheds]

I need to make up a device which will warn the operator of a combine if a pulley stops turning.

What are the results of the pulley stopping and the operator not knowing about it?

I was thinking of using a couple of inductive prox sensors to do this, (2 so that if the pulley should stop in front of one sensor it will not think its still spinning, because the second sensor will be positioned so its not possible for both sensors to be sensing at the same time.) I will then use a timer arrangement to switch a relay etc but this I can design later. What I am wondering is if I need a PNP sensor or NPN. I studied electronics a few years ago but this has got me thinking, I cant remember it all that well!

I would have thought that for liability reasons you should make rolling your own a really desperate last resort, and give great preference to buying a packaged, tested off-the-shelf product.

If you make something, and for whatever reason, even if it's the failure of one of the components you use, it doesn't work when it should, then what?

 

[stillp]

All the proximity switch manufacturers sell zero speed monitors.
It might be simpler to get a zero speed monitor such as the one in Schneider's Preventa range that senses the back emf from the motor, then you won't need to modify the pulley.
BAS is right though - what are the consequences of a failure?

 

[flyingsparks]

Thanks for all your replys not really had time to look at the links yet only just got home. But basically the consequences of the monitoring device failing are that a large belt, about 4inches wide will get burnt out. Basically the large pulley to be monitored is attached to a large drum inside the combine. If the drum gets jammed, and hence the pulley slows or stops, the operator needs to know so they can stop the drive to the belt.

 

[ban-all-sheds]

So damage to the machine, cost of repairs, consequential losses due to the machine being out of action.

You really don't want to be making something yourself, because if what you make fails, and doesn't detect the problem which leads to that damage and losses you could well be held responsible for the damage and losses.

 

[ericmark]

I seem to remember when programming PLC's I used two timers. Normally open triggered one and normally closed the other while turning both are constantly being re-set so no signal. If it stops one or other will not be re-set so light goes on.

I am sure same could be done without PLC?

 

[Spark123]

Yep, you can do the same without a PLC by using a Synatel unit which monitors the frequency of the input and operates a set of relay contacts, the ones I am thinking of have a dial on them to set the trip frequency level.
Think Peperl Fuchs also make them, as will most prox switch manufacturers

 

[RussSvatt]

I think a PLC may be a little overkill. zero speed sensor should be nice and easy http://www.gillsensors.co.uk/content/speed_sensors.htm[/url]

 

[flyingsparks]

So damage to the machine, cost of repairs, consequential losses due to the machine being out of action.

You really don't want to be making something yourself, because if what you make fails, and doesn't detect the problem which leads to that damage and losses you could well be held responsible for the damage and losses.

Its for my brother, its his combine he has just bought. He is unlikely to sue me for losses!! But seriously then, I know where your coming from.

I do have a Siemens LOGO plc I think lying around here somewhere.. Could possibly use that. Anyone know of any ladder logic programming software?