Mitsubishi

[Bluemaster]

Somebody Help Me!!! I am having trouble with a Mitsubishi CPU Model Q02CPU in particular an analog input module Q68ADI. A channel on this card is receiving a 4-20mA flow transducer which is displayed on local HMI in l/min. The problem is the HMI indication seems to be drifting positive. Display is showing 3 l/m at no flow. I double checked the loop current at no flow and verified 4mA. Normally this would be nominal but this app demands precision. The upper end at 20mA is accurate. I isolated the channel from the rest of the inputs on the card and generated a 4mA signal independent of the onboard power supply and the flow indication is still 3l/m. No other channels on the cards appear to be having trouble. Quality of dc appears very good on the scope. This loop was verified and accurate a few months ago. Unfortunatly I have no software to communicate with this plc. Not that I really think the problem is programming. I am very unfamilar with this brand PLC and was hoping someone may have advice or perhaps this is a known problem. At this point I think the analog input card is deviating from what the scaling was origionally. Since the program hasen't been altered I am assuming the cards sensing circuit is failing electronically. This is a bad thing. Anyone have thoughts???

[panic mode]

i won't say that this cannot be PLC hardware failure but if it is, i would be very very surprised. btw. what is the range? 4..20mA representing 0..4l/m or maybe 0-50000l/m?

[Bluemaster]

Sorry the 4-20 mA range equates to 0-80l/Min flow, or at least it should. Any other solution as to why the scaling suddenly changed? Thanks for the reply this is an odd duck in my neighborhood.

[panic mode]

it is hard to suggest anything since you don't have software to actually see what plc sees... i've seen people making mistake with addressing so data registers eventually get modified. could be use of index register without properly setting limits, could be that scale parameters ware too close to registers used as math result (some instructions have 64-bit result, this is something that can easily lead to problem like described by you, specially if such math is something that only updates occasionally or once in a while). if could be that calibration wasn't right in the first place, it could also be that HMI program was changed (altered scaling parameters).... without looking into project it is hard to say what is wrong. if you don't have software, maybe you have offline copy of the project that you can post here for review. other options are to get software or call contractor that has right tools and can take care of it... what kind of HMI it is? does it have ladder monitor loaded? can you monitor plc registers?

[Hugo Farfan]

Hello, in some moment you have measured in loop 4mA when the flow is zero? has this letter preset of Gain or Offset? Why CPU and modul Is?

[Bluemaster]

Thanks for the reply. Yes a no flow condition yields 4 mA. The flowmeter functions correctly. I am unsure of the meaning of your 3rd question. Cpu model and input model are refered to in origional post. AGAIN THANKS

[Crossbow]

Just one more note... there are higher resolution cards than the Q68ADI. If you absolutely need extreme accuracy, look for the Q64AD-GH, which would give you a reading 0-32,000 which is much better accuracy than the card you have.

[Bluemaster]

Surprise, Sometimes I get lucky, turns out the new AI module worked. Thanks for all for the commented.