AutomationGuru

Joe, None of the meters are close enough to one another to do anything on the field termination side.  All of the cable runs are home runs from the device via conduit and cable tray (conduit only from device to tray, ,tray for the remainder of the run back to the control room.)  there are no junction boxes, termination points, etc between the meters and the PLC cabinets. Once again, my frustration is that I can't for the life of me logically explain how I can get a signal in the software that is 10% of full scale  (as much as 50% of full scale higher when the pulse frequency is 5000Hz).  When the frequency measured at the module terminals is exactly the same as the frequency at the output of the flow meter, I can't figure out how noise is being introduced from a VFD that is completely isolated from the PLC system that the HSC modules are in. I've actually been asked to go to site tomorrow to further investigate, so I may come back with some additional information.  I will update as appropriate.

Joe, Thanks for the input.  The grounding is something that we are going to have the customer check out at the site.  We came up with the same thoughts. In answer to some of the questions you posed:  The Daniels meter is not only on an HSC module in the same chassis as the Flexim meters that are having issues, but it is also on the same card as one of the affected meters.   We are seeing the issue on two different HSC modules, and on 3 of the 4 available channels.  The fourth channel being the Daniels meter.  We have moved wiring around and still see the issue on the three channels that the Flexim meters are connected to, leading us to believe it is something in the field side.  But again, that does not explain why the Fluke frequency reading matches the flowmeter output, while the ControlLogix monitoring raw channel data shows a frequency boost of 10% of full scale or greater.   Unfortunately, as of right now, no one has an oscilloscope to look at the waveforms of either the power or the pulse output of the meters.   Our biggest issue is that these meters are being used for custody transfer, meaning they are relied on heavily for billing and payment, so it's not just the technical guys looking at this problem, but the accountants and executives are very much aware as well.   Thanks again for the input, and if you have any other thoughts, please don't hesitate.

I have run into an issue that I can't explain.  We have several ultrasonic flow meters utilizing their pulse outputs to a 1756-HSC module to detect the pulse frequency (corresponding to an instantaneous flow rate).  The flow meters have a maximum output frequency of 5000Hz representing an instantaneous flow rate of 280,000 cu. ft/day (Details are for background only, not really important to the discussion at hand.)  It's been discovered that at normal flow rates (frequency in the range of 1500 Hz output of the ultrasonic meter), the HSC module is reading frequencies of up to 2000 Hz.  Rockwell tech support initially recommended that we replace the HSC modules.  This was done  and we saw the same results.   The next step was to rule out any floating commons.  I had the technicians at the site do some wiring modifications to source 24VDC to the pulse output from our PLC panel.  (Initially we jumpered the 24VDC supply of the flowmeter to the pulse output and used the PLC panel common.  All of our commons should be grounded together, but I wanted to rule out the possibility that this was causing the issue.  Once again, no change.  The next option was to verify that the flowmeter frequency output was actually what was expected.  To this point we had assumed that the frequency output was correct, but had not measured the frequency.  The technicians at site used a Fluke meter set to measure Hz and verified the frequency at the output terminals of the flow meter.  Next they went in to the PLC panel and checked the frequency at the input terminals of the HSC module.   In both cases the measured frequency was dead on at around 1530 Hz, however, Logix 5000 software showed the present value of the frequency as around 2000 Hz.   As they continued testing, the technicians shut down two VFDs (VFDs are controlled by a different processor and have no interaction with the HSC modules) and the HSC module read the correct frequency in Logix5000.    They further tested by running the drives and removing the HSC signal cable from the cable tray and routing it along the ground to isolate it from the VFD motor cables.  With the drives running, the frequency still reads in the neighborhood of 2000 Hz and 1550 Hz when the drives are stopped.   Rockwell tech support now has recommended installing a common mode choke on the output of the 753 VFDs to reduce the noise, claiming that the VFDs must be causing the noise and increased frequency reading.  The concern is that if that does not resolve the problem it will be add another filter, add some other hardware, on and on until things finally work and then no way to know if all the added components were needed or if only the last one solved the problem.  There are additonal meters to be added to this site in the future, so I'm sure this issue will pop up again. Here are my concerns: 1.  If the Fluke meter is reading the correct frequency at the input terminals of the HSC module, how could the CPU/Logix software be displaying a higher frequency?  The only interaction between the VFDs and the HSC is in the cable tray where the VFD cables are in a separate tray section with barriers in between.  For the life of me I would assume that if the VFD was inducing noise on the signal, that they would see that increased frequency on the Fluke meter. 2. The HSC modules are mounted in a remote rack.  The remote rack resides in a control panel not more than 3 feet from the main CPU rack.  The remote rack communications is via an isolated network, while the VFD controls are hardwired from a Micrologix controller provided by a 3rd party.  We communicate to that controller over a plant wide network that is separate from the RIO network. 3.  We initially intended to use 4-20mA analog signals for the flow rates, not pulse outputs and only added the HSC modules after the fact.  Since this system resides in a Class 1, Div 2 area and conduit seals had been poured already, we made use of a second twisted shielded pair  to connect the pulse output back to the control room PLC panels.  Is there any reason to believe that cable could be causing the issue.   4.  In the overall system, there are 4 ultrasonic flowmeters.  3 of them are Flexim clamp on flow meters, the 4th is a Daniels ultrasonic flowmeter.  The Daniels meter frequency reading is dead on regardless of the VFD, where the flexim meters all are seeing increased frequencies in the PLC. Any suggestions or solutions will be greatly apppreciated.  I'm at a loss and i'm not sure whether or not to believe the tech support claim that the higher frequency is due to electrical noise, when the correct frequency is on the input terminals of the HSC card.   Thank you