Calculating the speed and properties of IB-32 Digital input card

AB2005 · Dec 17, 2008

Dear Friends;

I want to calculate the speed of sheet produced by corrugator machine. For this purpose, I want to make 6 holes in a measuring wheel and fix a proximity sensor for getting 6 pulses per revolution which will go to a digital input card. This measuring wheel is (originally) placed on the sheet and run with the sheet speed. The maximum speed of this measuring wheel is 340rpm and thus we will get 2040Pulses/rev (34HZ) from proximity sensor. I have SLC 5/03 (program have 70rungs) and input card IB-32. Now my questions are;

1) Can IB-32 register the pulse having frequency 34HZ?
2) Can I increase the holes in wheel for getting more pulses for fast updating of speed in program?

Thanks in advance.

rdrast · Dec 17, 2008

Just offhand, you can probably see transitions of that rate just fine. Looking at the MANUAL for instance, shows a typical hardware delay (OFF TO ON) of 480us, 1ms maximum. On to Off is 420us nominal, 2ms maximum, plus your filter time.

For Timestamped inputs, the timestamp on Change of State is good to +/- 200us. Again, all from the MANUAL.

What you do in your program, is another matter. Also, what kind of proximity sensor you use will greatly affect your outcome.

Can you 'add more holes' ? Sure... buy why reinvent the wheel? Buy an ENCODER and be done with it.

Really, why not use an actual, oh, say, COUNTER module for this and be done with the questions?

kolyur · Dec 17, 2008

Just offhand, you can probably see transitions of that rate just fine. Looking at the MANUAL for instance, shows a typical hardware delay (OFF TO ON) of 480us, 1ms maximum. On to Off is 420us nominal, 2ms maximum, plus your filter time.

But you will be limited by the scan time of your PLC, which I would estimate to be 5-10 msec for the SLC. This is going to cause problems if you're planning to just use a counter instruction to count the pulses. 34 Hz translates to a 29 msec period which is dangerously close to your scan time.

Clay B. · Dec 17, 2008

rdrast said:
Just offhand, you can probably see transitions of that rate just fine. Looking at the MANUAL for instance, shows a typical hardware delay (OFF TO ON) of 480us, 1ms maximum. On to Off is 420us nominal, 2ms maximum, plus your filter time.

For Timestamped inputs, the timestamp on Change of State is good to +/- 200us. Again, all from the MANUAL.

What you do in your program, is another matter. Also, what kind of proximity sensor you use will greatly affect your outcome.

Can you 'add more holes' ? Sure... buy why reinvent the wheel? Buy an ENCODER and be done with it.

Really, why not use an actual, oh, say, COUNTER module for this and be done with the questions?

I would listen here. Price wise this is the way to go. You method leaves you open to several problems. One major one will be accuracy. Your going to end up with drift and this will be impossible to control.

Reason for error is due to the fact you will need to use a STI (timed interrupt) to calculate speed. I have been down this road with another kind of processor but the results where the same here is that thread.

http://www.plctalk.net/qanda/showthread.php?t=37779&highlight=speed

Steve Bailey · Dec 17, 2008

34 Hz translates to a 29 msec period which is dangerously close to your scan time.

Don't forget that the 29 msec period includes two transitions, the transition from off to on and the transition from on to off. You have to make sure the signal from the prox switch is true long enough for the PLC to recognize that fact. You also have to make sure that the signal is false long enough for the PLC to recognize it.

I agree with rdrast. You're better off using a high speed counter module. This is exactly the type of application they are designed for. While you're at it, consider using an encoder instead of a prox switch. The encoder gives you a square wave pulse with equal on and off times.

AB2005 · Jan 5, 2009

Dear Friends;

This is information for all that I have done the same as I expalined in my first message and get 6P/rev from sensor installed at Measuring Wheel and connected the out put of sensor to the input card IB-32 which detected the pulses successfully. My system (SLC 5/03) have been calculating the speed well as the scan time of my system is 3-4ms only. Now I don't need high speed counter and encoder.
Good experience!!!

Calculating the speed and properties of IB-32 Digital input card

AB2005

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rdrast

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kolyur

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Clay B.

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Steve Bailey

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AB2005

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