Flow rate reading and regulation

I have a flow meter which gives out 0.1 litres/pulse,

I'm using a digital input to read the pulse
I'm not sure when to read the pulse i.e. whether every sec,mS etc
so that I can read the flow accurately,so that I can set the alarms for underflow or overflow.

I'm using Cscape to program an XLE osc plc

What I would do is count the amount of pulses every say minute or seconde ,depending on how regular you want your alarms to be updated.then use the amount of counts to determine under or overflow.

As a general rule, digital inputs are too slow for pulse inputs, unless your pulse rate is pretty slow. The plc should look at the pulse every scan, and the scan needs to be at least twice as fast as the pulse rate. So if you are getting a pulse every 20mSec, the Scan rate of the PLC needs to be 10mSec or faster to ensure you don't miss any pulses. Usually a high speed counter module is used to count pulses.

I have many pulse type meters that pulse at 150-200Hz, no way to catch all the pulses on a PLC.

In your case, you may have to reconfigure the meter to output 1.0 liters per pulse to slow down the pulse rate.

Hi,Ken
So generally I cant a use DI to pulse a temp positive pulse bit to count how many pulses I have? And pending on how many litres I need clear the register.

Maybe this will help. The first line is the pulse train with the signal turning on and off at a regular rate. The next two lines represent the PLC's scan with the vertical line representing the instant when the PLC reads the status of the pulse train input.

In the fast scan example you can see that the PLC checks the status of the input much faster than the input changes state, so you could safely count pulse in ladder logic.

In the slow scan example you can see that the first two times the PLC checked the input, it was off. But, in between those two checks, the actual signal turned on and then turned off again. The PLC missed those two events because it didn't check often enough. In a case like this, where the PLC scan is slower than the pulse frequency, you can't count pulses in ladder logic. You need a high speed counter.

     ____      ____      ____      __

____|    |____|    |____|    |____|    Pulse train

 

 

_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_  Fast scan

 

 

_|__________|__________|__________|__  Slow scan

I get the message.So how will I ensure the scan rate is always twice the speed of the flowmeter pulse?

You don't have a lot of direct control over the PLC's scan rate. Scan rate increases with the size and complexity of your program and the amount of I/O.

It is more accurate to say that if the maximum pulse rate from the flow meter is significantly slower than the PLC's scan time, you can count the pulses in ladder logic. If not, you will need a high speed counter module capable of keeping up with the flow meter.

As to the determination of what is meant by "significantly slower", having the PLC's scan rate twice as fast as the maximum pulse rate is a good place to start. Remember though, the PLC's scan rate can vary, sometimes over a fairly wide range. Even if the average scan rate is fast enough, there can be occasional longer scan times when an external device communicates with the PLC (for one example). What are the consequences of an occasional missed pulse? If all you need to do is display the rate to an operator, the consequences are minimal. If you are using the pulse train to calculate usage, missed pulses are cumulative.

Thank you guys the advice is very resourcefull,I've managed to do what I wanted?
This is a great forum I must say.BIG UP GUYS