Need help with disappearing signal

I have an output from a machine center which is 50mS in duration, this can not be changed so I am stuck with it. Since this is a 15Vdc signal but the logic I am going in to is 24Vdc I have a solidstate relay. The relay switch time is 100micro-S for off to on, and 750micro-S from on to off. However the one area I disagree with is the turn on and off voltages. Turn on voltage is 10.5Vdc and turn off voltage is 1.

Anyway this solid state relay feed the 24Vdc signal to my PLC for counting purposes. The turn on time for my PLC is 3mS.

The scan time of the program in the PLC is 22.8mS.

So far all scan times and such seem to less than the output from the machine center. However the PLC still seems to miss some of the pulses. I am not the most elaborate with math and such so I am asking if someone here could take a look at these numbers and see if there is anything that sticks out as to why it would miss some of these pulses.

Thanks and have a great day.

What PLC are you using? Does it have high speed in inputs. PLCs like the AB micro Logix and Mitsubishi designate the first few DC inputs as high speed inputs. Or you can use a high speed counting card.

I never like depending on the program scan to catch pulses you will most likely ren int trouble.

JIM

What is the maximum scan time of the PLC? Maybe it occasionally spikes and prevents the I/O update from catching the pulses.

How frequent are the pulses?

If there is extra time between pulses, you could install a "pulse stretcher" circuit between the 50ms signal and the plc input.

A pulse stretcher is nothing more than an off delay timer. Make those 50ms pulses last 100ms, and you should never have to worry about mising them. Basically, you would just replace your solid state relay with an off delay timer.

Paul

The average speed of the piece of equipment that this 50mS pulse is comming from is around 24000cpm. In some cases you get a good operator on it and you can easily reach 32000cpm. So the one thing you want to watch for in using a pulse stretcher is that you done go to long that you will miss pulses.

The type of plc we are using is the Horner NX221, as far as I know the remote input modules we are using are just standard input and not high speed. I will be checking their web site to see if they offer a high speed version.

Remote I/O.......... Now you have the scan time of the PLC and how often the remote I/O is scanned to deal with.

You give the length of the pulse but not how much time you have between pulses, the length of the off will be as important as the length of the on.

Bryan

First off I will start with the pulse itself. The pulse itself is an inverted pulse, meaning it is normally high (15Vdc when not on) and low (zero when on).

Next I can not give an exact number for the length of the other pulse because that has many determining factors.

Examples of these factors are:

1. If an operator is manually cycling the machine one piece at a time. Then of course the length of the off time will be much greater.
2. In auto mode this time can be changed based on the speed of the machine. SO if you are running the machine at 30000cpm the off time will be shorter than if you are running it 20000cpm.

One idea that I have come up with is similar in nature to the one posters idea about a pulse stretcher. What I might be able to do is put a counter to count these pulses, probably something in the Red Lion category. And choose the counter so as it has an output based on the input and the output can be timed. This is just one idea, seems a little like over kill but if needed I can do this. Looking for any other ideas.

Thanks and have a great day.

BryanG said:

You give the length of the pulse but not how much time you have between pulses, the length of the off will be as important as the length of the on.

Bryan

Click to expand...

mrtweaver said:

...In auto mode this time can be changed based on the speed of the machine. SO if you are running the machine at 30000cpm...

Click to expand...

30,000 cycles per minute?

That is 500 pulses per second! Which equates to a peak to peak interval of 0.002 seconds.

No wonder you are missing pulses, they are surely overlapping, unless there is a typo in your figure above...

Paul

Sorry my finger got in front of my eye and I could not see what I was typing. What I meant to say was 30000CPH not CPM. Sorry about the typo.

OkiePC said:

30,000 cycles per minute?

That is 500 pulses per second! Which equates to a peak to peak interval of 0.002 seconds.

No wonder you are missing pulses, they are surely overlapping, unless there is a typo in your figure above...

Paul

Click to expand...

Okay, so you are looking at 120ms between the rising edges at the fastest production rate. So, you, ideally, will want the pulse duration to last 60ms. Since it already lasts 50ms you should investigate the true update rate of the REMOTE input to which it is attached.

I emphasize REMOTE, because with some PLCs, remote inputs are updated asynchronously to the program scan and therefore less reliable for this type of circumstance. If that is the case with your Horner NX221, then perhaps you can move it to a local input and solve your problem...

Paul

I see a case where one of two thing is happening.

1
The on duration pulse is not getting high enough for long enough, so my suggestion is to add a 0.5 Uf capacitor, across the + and - terminal of your input and see if this helps.

2
The pulse duration is long enough, but the capacitive effect of cabling is not allowing the input charge to dissipate quickly enough thru the input, to get a long enough off duration, and here, my suggestion is to try a 1000 ohm resistor across the inputs + & - to see if this helps.

These are not neccessarily a long term fix but an indication as to which way to go.

A question
when watching the input light is it on longer than it is off or vice versa.?

First off as stated in a previous post the signal is inverted. I dont know why the OEM did it this way but they did. When the signal is reported as off it is actually a logic 1 or in this case 12 - 15Vdc. When the pulse/trigger occurs which means it is reported as on, the logic is 0(zero) or somewhere around zero VDC. My scope I have is not quick enough and does not have a screen grabber feature so seeing if the voltage actually does drop to zero is rather difficult and being that the solid state relay says that in order for it turn off it needs a 1Vdc or lower whos to say that there is not enough leakage in the OEM system that it does not drop as fast to the zero volt level. Just some things I have been reading. So maybe the signal while being 50milliseconds is not being seen as that time duration because of the less than 1volt requirement. Again these are just ideas that I am throwing around.

Now on the solid state relay there is an LED indicator when the machine is not running of course the indicator is on. When the machine is running you see the light strobe/flicker. Again this comes back to timing. I am thinking maybe since the solid state relay can take anything from 10 to 16 volts maybe the idea presented previously in this posting about placing a 1K resistor in line maybe that idea will work. WIll have to wait for production to give me access to the machine but I am leaning towards this to see if it fixes the problem.

Gil47 said:

I see a case where one of two thing is happening.

1
The on duration pulse is not getting high enough for long enough, so my suggestion is to add a 0.5 Uf capacitor, across the + and - terminal of your input and see if this helps.

2
The pulse duration is long enough, but the capacitive effect of cabling is not allowing the input charge to dissipate quickly enough thru the input, to get a long enough off duration, and here, my suggestion is to try a 1000 ohm resistor across the inputs + & - to see if this helps.

These are not neccessarily a long term fix but an indication as to which way to go.

A question
when watching the input light is it on longer than it is off or vice versa.?

Click to expand...

mrtweaver said:

First off as stated in a previous post the signal is inverted.

Click to expand...

The Turck smart plug can can be a pulse stretcher and signal inverter.
http://www.turck-usa.com/illustrations/B0313_09-10.pdf

mrtweaver said:

First off as stated in a previous post the signal is inverted. I dont know why the OEM did it this way but they did. When the signal is reported as off it is actually a logic 1 or in this case 12 - 15Vdc. When the pulse/trigger occurs which means it is reported as on, the logic is 0(zero) or somewhere around zero VDC. My scope I have is not quick enough and does not have a screen grabber feature so seeing if the voltage actually does drop to zero is rather difficult and being that the solid state relay says that in order for it turn off it needs a 1Vdc or lower whos to say that there is not enough leakage in the OEM system that it does not drop as fast to the zero volt level. Just some things I have been reading. So maybe the signal while being 50milliseconds is not being seen as that time duration because of the less than 1volt requirement. Again these are just ideas that I am throwing around.

Now on the solid state relay there is an LED indicator when the machine is not running of course the indicator is on. When the machine is running you see the light strobe/flicker. Again this comes back to timing. I am thinking maybe since the solid state relay can take anything from 10 to 16 volts maybe the idea presented previously in this posting about placing a 1K resistor in line maybe that idea will work. WIll have to wait for production to give me access to the machine but I am leaning towards this to see if it fixes the problem.

Click to expand...

Note that the poster of this suggestion actually suggested that the resistor be placed across the inputs, not in line with the wiring (parallel, not series.) The intent is to present more load to the circuit to dissipate the stored energy faster, giving you a sharper drop in voltage when the source goes to 0 V. Placing a resistor in series would actually make this worse (but not much.)

Brian