Should i use a normal DC input or high speed?

JZerb

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so, id like to pick up RPM of a shaft with a prox. 800hz prox, and a normal DC input on a ML1400 can scan at 1kHz (scan time dependent). so with that information, a shaft spinning at say 30rpm, with a trigger wheel attached to it with 60 teeth on it. it would rotate at .5rpm/second. so the prox would be counting 30 teeth in that span. with the 800hz prox, best case scenario it can pulse 800 times/second, the above should work out fine should it not?

now under the same idea but with a higher speed shaft, say 1200rpm. so thats 20rpm/sec, using a trigger wheel with only 30 teeth that would get me 600 teeth per second, still below the 800hz sensor capability no? or would i need to get into using a HSC on ML1400 to actually be able to capture that RPM?

hopefully what i think makes sense and im not completely and totally off base here.
 
so, id like to pick up RPM of a shaft with a prox. 800hz prox, and a normal DC input on a ML1400 can scan at 1kHz (scan time dependent). so with that information, a shaft spinning at say 30rpm, with a trigger wheel attached to it with 60 teeth on it. it would rotate at .5rpm/second. so the prox would be counting 30 teeth in that span. with the 800hz prox, best case scenario it can pulse 800 times/second, the above should work out fine should it not?
Not with a regular PLC scan in my opinion. I would definitely use the HSC, then you can be sure it will work fine. 1/800 is 0.00125 seconds or 1.25ms, meaning that your PLC input and your scan time really need to be twice that fast in order to pick up each false to true transition reliably.
 
When you say "trigger wheel", do you mean a wheel specifically built to trigger the proximity switch, or are you sensing teeth on a chain sprocket ? It can be tricky to get a 50% duty cycle out of a sprocket tooth because of the shape and the sensing field of the proximity sensor.

a normal DC input on a ML1400 can scan at 1kHz (scan time dependent).

It's been a while since I had to look closely at a MicroLogix 1400 scan time value. I'm accustomed to controllers in that class scanning their I/O at the end of a 5 to 10 millisecond program scan.

You are correct that the fastest filter for the standard DC inputs on the MicroLogix 1400 is 0.5 milliseconds. See the Appendix A in the MicroLogix 1400 User Manual.

A 60-tooth sprocket turning at 30 rev/minute will generate 30 ON pulses and 30 OFF pulses in 1 second, so the duration of each pulse is 1/60 = 16.7 milliseconds.

That's fairly easy for a lightly loaded MicroLogix 1400 to sample with its ordinary I/O scan. Even the default 8 ms filter will probably work at that speed.

But when you go to a faster shaft, the scan time becomes the limiting factor.

A 30-tooth trigger wheel at 1200 RPM will generate (1200 / 60 * 30 = ) 600 pulses per second, so a 50% duty cycle means the pulses will be 1/1200 = 0.83 ms duration.

That's still just within the capability of a standard DC input, but too fast for a normal PLC program to scan.

And it's probably also too fast for a proximity switch rated at "800 hz".

If you need to sense motor shaft speed, the best solution is a purpose-built shaft speed sensor or encoder connected to a purpose-built high speed input circuit like the MicroLogix High Speed Counter.
 
You need the HSC for the 2nd scenario. It comes with the ML1400, why not use it for both and be good to go if anything ever changes?
 
I figured if I did indeed need to use a HSC for the second scenario, I would also just use it for the first so both setups within the machine, and the PLC logic, were setup the same way.
 
When you say "trigger wheel", do you mean a wheel specifically built to trigger the proximity switch, or are you sensing teeth on a chain sprocket ? It can be tricky to get a 50% duty cycle out of a sprocket tooth because of the shape and the sensing field of the proximity sensor.



It's been a while since I had to look closely at a MicroLogix 1400 scan time value. I'm accustomed to controllers in that class scanning their I/O at the end of a 5 to 10 millisecond program scan.

You are correct that the fastest filter for the standard DC inputs on the MicroLogix 1400 is 0.5 milliseconds. See the Appendix A in the MicroLogix 1400 User Manual.

A 60-tooth sprocket turning at 30 rev/minute will generate 30 ON pulses and 30 OFF pulses in 1 second, so the duration of each pulse is 1/60 = 16.7 milliseconds.

That's fairly easy for a lightly loaded MicroLogix 1400 to sample with its ordinary I/O scan. Even the default 8 ms filter will probably work at that speed.

But when you go to a faster shaft, the scan time becomes the limiting factor.

A 30-tooth trigger wheel at 1200 RPM will generate (1200 / 60 * 30 = ) 600 pulses per second, so a 50% duty cycle means the pulses will be 1/1200 = 0.83 ms duration.

That's still just within the capability of a standard DC input, but too fast for a normal PLC program to scan.

And it's probably also too fast for a proximity switch rated at "800 hz".

If you need to sense motor shaft speed, the best solution is a purpose-built shaft speed sensor or encoder connected to a purpose-built high speed input circuit like the MicroLogix High Speed Counter.

if it was my pocketbook i would have an encoder on there, but unfortunately tis not.

am i incorrect in my thinking that an "800hz" prox switch is capable of switching on/off 800 times/second? i know thats the 'advertised' Hz, does that not translate exact into real world scenarios?

the 'trigger wheel' i would make would mimic that of a trigger wheel on a crankshaft in a car. ive had this successfully work with an 12mm prox and a 20 tooth wheel, but that was only spinning at 22ish rpm max. what i would like to do is get a 5mm prox and add more 'teeth' to the wheel so that i can get a better/more accurate, resolution at lower speeds. with the higher speed shaft, i would like to do the same thing, i just understand that i will need to have less 'teeth' on the wheel for the prox to be able to pick them up.

ive never had to use a HSC before, so now its time to learn.
 
You correctly inferred the meaning of my quotes on "800 hz".

The value very likely means that the sensor can correctly sense a pulse train that has 800 true states and 800 false states per second, meaning that each state is only (1/1600=) 0.625 milliseconds.

But it might also be trying to say that it can detect a single pulse that is 1/800 second long. That would be an incorrect definition of "Hertz" but I've seen stranger things.
 
You correctly inferred the meaning of my quotes on "800 hz".

The value very likely means that the sensor can correctly sense a pulse train that has 800 true states and 800 false states per second, meaning that each state is only (1/1600=) 0.625 milliseconds.

But it might also be trying to say that it can detect a single pulse that is 1/800 second long. That would be an incorrect definition of "Hertz" but I've seen stranger things.

So, just for instance, if it ends up being the latter the sensor wouldn’t really even pick up all of the pulses I need it to pickup at the higher rpm? In which case using a HSC wouldn’t work anyway from the lack of capability of the sensor attached to it?
 
So, just for instance, if it ends up being the latter the sensor wouldn’t really even pick up all of the pulses I need it to pickup at the higher rpm? In which case using a HSC wouldn’t work anyway from the lack of capability of the sensor attached to it?

The input filter can be set fast enough to pick it up, but the PLC scan time may not be quick enough to capture all the transitions.
 
If all you are doing is checking RPM, why not use a single flag for each revolution instead of 60? Why make it complicated when simple will do?
 
The input filter can be set fast enough to pick it up, but the PLC scan time may not be quick enough to capture all the transitions.

Ok. The scan time may not be fast enough when using a normal DC input? Or it still wouldn’t be fast enough when using a HSC as well? Sorry I’m getting a bit confused with all of that. I could also get a sensor with a higher kHz rating so that it is more ‘capable’ then the 800hz sensor I usually use.
 
The HSC does the counting etc internally (at high speed) and then communicates the result to the PLC. So PLC scan rate isn't an issue when you use a HSC.
 
so, id like to pick up RPM of a shaft with a prox. 800hz prox, and a normal DC input on a ML1400 can scan at 1kHz (scan time dependent). so with that information, a shaft spinning at say 30rpm, with a trigger wheel attached to it with 60 teeth on it. it would rotate at .5rpm/second. so the prox would be counting 30 teeth in that span. with the 800hz prox, best case scenario it can pulse 800 times/second, the above should work out fine should it not?

now under the same idea but with a higher speed shaft, say 1200rpm. so thats 20rpm/sec, using a trigger wheel with only 30 teeth that would get me 600 teeth per second, still below the 800hz sensor capability no? or would i need to get into using a HSC on ML1400 to actually be able to capture that RPM?

hopefully what i think makes sense and im not completely and totally off base here.

Others have run the numbers and compared to PLC scan times, so I won't retread that. Suffice to say, I agree, HSC is realistically the way to go. I do have one thing to add, however, that I haven't seen mentioned yet.

Something to keep in mind as you think about this is that if you want to count the pulses you realistically need both a high signal and a low signal. Thus, if your PLC has a 1ms scan rate (1000hz), the fastest signal you can detect is 500hz.

When I was in school, this was referred to as the Nyquist Theorem. Feel free to google for more info/explanation.
 

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