PLC not reading pulse input from Flow Meters

Hello all,
I am working with a variety of discrete pulse flow meters, Siemens, Flomec, and Seametrics. I am attempting to read flow via discrete input from some of each type. The PLC I am working with is an IDEC Microsmart. I inherited this project on start of my new position. The PLC code is pre-written and believed to be working correctly. My issue is that of around 24 meters wired up only one is consistently pulsing. To date the most success I have had was 10 meters wired up and measuring pulses/flow. They pulsed for approximately 12-18 hours before failing to read anymore. I have tried pull-down resistors and that seemed to help and then as with the first time after a couple hours the meters stopped pulsing. My 24vDC power supply to the PLC reads 24 volts. One odd thing I have noticed is that my 24vDC power supply for my valves and IO reads approximately 11vDC. Any troubleshooting insights at this point would be greatly appreciated. Thanks in advance.

Usually a pulse flow meter has open collector NPN output which is configured like a switch to the 0V supply of the flow meter. It requires a ‘Pull-Up’ resistor to a positive voltage.
Can you check the output and the power supply directly on the flow meter with a oscilloscope?

Right now to my knowledge the flow meters have 24vDC+ run to the Pulse+ and the Pulse- is terminated at a PLC input with the IO common being tied to 24vDC- to complete the loop.

Is your DI card rated for the speed at which you are trying to see pulses? Often a flow meter will require a high speed input card.

It could happen that impulses are converted to real values and then added to totalizer. If that is true, after certain period due to IEEE 32 bit float format constraints it will not add it at all. I would try to reset totalizer and see what would happen.

That significant of a voltage drop on a power supply is indicative of a bad PS or the 24V is shorted to 0V and or ground somewhere.

Are you using 2 different power supplies?

I am using two power supplies. One for power of the PLC itself and one for power to the valves as well as powering loop power to sensors for pulses(Reading 11vDC).

I don't know if the Inputs are rated for the speed at which I am trying to read however I am consistently reading a pulse from one single meter. I did find in the manual that the Microsmart IO is capable of HSC functionality though.

What are the specs on the Flow meters? I had an issue where the designer specified the wrong parts that had a 5Volt output, they would work for a while then stop working. If you disconnected everything and reconnected it would again work for a while then stop. Once we installed the correct parts it worked as expected.

Flomec QSE (Q-Star) Series Electromagnetic Meter. Series 3 circuit board.

Siemens Sitrans F Mag 6000

Seametrics WMP-101 with an FT430 series board.

To my knowledge they all had a digital pulse output with 24v pulse capability.

_Dock_

I will replace the suspicious power supply tomorrow and see if I can narrow it down to being either the power supply or the ground.

In general I am getting mixed signals from the manuals of the flow meters and the IDEC support technician I was able to get a hold of. At least 2 of the manuals reference a pull-up resistor but the IDEC Tech didn't seem to think they were necessary. Any input?

Microsmart is also rebranded as Schnedider Electric Twido , and although I did not used Microsmart I have used Twido and I have used HSC without resistors and with flowmeters, so I could say that you do not need resistors.

Dawsam445 said:

_Dock_

I will replace the suspicious power supply tomorrow and see if I can narrow it down to being either the power supply or the ground.

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11 volts out on a 24 volt power supply something is wrong. Not saying that’s your only problem but that’s one of them. Take a multimeter and measure the resistance from 24v to 0V with all your wiring still in place.

I'm not familiar with all those flowmeters but many have an open collector output for pulses with the one leg tied to the flowmeter common and will in fact need a pull-up resistor to work properly.

But as _Dock_ said, 11V out of a 24V PSU sounds like something is shorted to ground / 0V and the PSU is in foldback to protect itself. Measure the current out of the PSU and compare to rating. If it's over, I'd disconnect everything and connect back up one at a time and see what is pulling it low.

Its possible the PSU is too small but i think it's more likely something is pulling too much current.

I actually went to check a flowmeter today that one of my guys had wired up, that kept blowing the fuse for the RTU digital inputs. He hadn't identified that this meter had a common for the pulse that was internally linked to the negative leg on the 4-20mA analog output of the meter. Since both were connected to the RTU it meant there was a direct short between the +24V and 0V. After changing the wiring and adding a 10k pull up resistor, all worked as expected.

In regard to the power supply...take the load off the power supply. If a meter reading show +24V across + and -, the problem is in the field. I concur with what others have said. The significant drop in voltage is likely due to a grounded conductor.