Input problems

Hi all,

I am encountering a problem with a FXon plc in that 2 of the normal X inputs are having to deal with signals which are in fact too fast for those inputs to register. Now I could of course upgrade the whole installation for £1k + installation time, but since those inputs are only triggered once every 2 minutes, there should be a simpler solution.

The signals are given by reed relays on air cylinders, and it could well be that their output pulse is not even reaching the full 24 Volts.
I’m thinking in the way of a ready to use 24V op amp or similar. Any ideas?

As always grateful,
Eric

The FX0N will catch signals faster than a reed switch can deliver - so upgrading won't alter the problem.

Upgrading the switches or doing a check on the voltage would be a good starting point.

I seem to recall that inputs X0 & X1 are high speed inputs on the FX range of plc's. Couldn't you use these?

Hi Goode,

The power supply is taken from the plc and is good enough for all the other inputs, I’ve got no worries about that, but indeed the switches need replacing.
The problem there is that they are of an old type attached to big, long, old cylinders. All this should have been replaced many moons ago. Perhaps I will do in the near future but for now I want a quick easy cheap solution to keep us going.

@Beethoven
I didn't know that and will look into your solution.

Many thanks.

How are these reed switches being used
are they half way along the cylinder?
I recommend you try a dedicated power supply for a while.

If you are just confirming the end position of the cylinders.
Set up a hold in circuit for either end.
1 - reed switch operates the Input in direction 'A'
2 - energise and hold in an auxillary relay
3 - de-energise that relay when the Cylinder is told to move direction 'B'

Hi Ian,

Those switches are indeed partway up the cylinders as you guessed correctly.

For the first operation the pistons travel partway up then have to return, for the second operation they have to travel all the way. Therefore, if I restrict the flow of air sufficiently and the pistons slow down, it works. Increase the flow and the plc doesn’t register the partway position. Slowing down works for most of the time but decreasing flow goes together with loss of power. This then sometimes stops the process when some dirt comes into play. Plus of course it takes more time, and time is……

Your first task is to see what voltage change you are getting when the target passes the switch. With that information you can look for a suitable pulse stretcher to be triggered by the switch. The output of the pulse stretcher gets connected to the PLC input. If the target passes the switch so fast that there is no reliable change in the switch's output, your only choices are to live with reduced speed or change the switch.

I would suggest you still set up the auxillary relays i mentioned.
Even to just prove that the sensor is not being picked.

an Odd question?
is the speed adjustment on the exhaust side of the cylinder and
what type of solenoid is it (5/3,5/2) open or closed centre etc.

Hi Steve,

I do like the sound of a pulse stretcher a lot.

Of course, I could do as you suggest and determine exactly what voltage I get at which speed etc and go on from there, but we both know what the real problem is: a higher speed results in a thinner, lower pulse.

Therefore if you can suggest a good readily available on the shelf stretcher, which require a minimum amount of soldering, I’d be very grateful. Any type of stretching will be a bonus.

@Ian
Even if there’s a relay quick enough to react, its contact will only react with the same speed.
By the way, I forgot to mentioned that the sensor led’s do flash up (on fast speed), so there’s some closing of the reed’s contact.

I do adjust the speed on the cylinder exhaust, I do not know the type of air valve used but I want to try Steve's suggestion.

http://www.phoenixcontact.com/global/news/248_82688.htm

but we both know what the real problem is: a higher speed results in a thinner, lower pulse

Click to expand...

It is also possible that at some speed the "pulse" is nonexistent due to limitations of the switch on the cylinder.

what speed does the cylinder run at
i am having difficulties understanding you issue
I have run 3 mtr air cylinders at maximum speed with reed switches and no problems
this sort of problem usually relates to encoders only.



when I suggested Auxillary relays I meant the 'M' plc relays

you could use the interupt function but I think the hold in latches will solve the issue

Thanks Steve, I've copied that link.

Sure, when the speed is too high no contact will be made, I'll make sure to stay below that point.

iant
I'll take your word for the 3m cylinders, but your installation was probably in a better (newer) condition than mine.

For now I go along with Steve's suggestion. Thanks again.