Fluctuating analog input - How to clean it up?

I am using a corolis flowmeter with a 4-20mA output for mass flow. The unit has a capacity of 661 lbs/hour but our typical rate is on the very low end of the scale, only 20-60 lbs/hour depending on our formulation requirements.

My problem is my controller/display is mounted inside my cabinet and connected to my SLC 5/04 PLC. The unit is sending a 4-20 mA signal to my input card.

I am able to scale the signal correctly and get a readout but it is highly variable. Much more so than the readout on the actual flowmeter display. Now is there a way inside the PLC to compute a rolling average of say 5-10 points and use that value for my externally displayed KEP touch panel?

I tried using a counters but I am not sure I have the configuration correct. Also tried just reading the value but it isn't very stable. We are on the low end of the span. Could this be my issue?


I have a few other analog inputs that work fine.

You could try a filter:

FV=FV+C(NV-FV)
Where:
FV= Filtered value
C= constant (range 0-1) The smaller the number the more dampening.
NV= New Value

You can probably increase the dampening time in the flowmeter. If you cannot adjust the flowmeter the analog input card config should have a digital filter you can use. If all that doesnt work then yes you can set up a rolling average. I would use a counter and a timer to load the values in a block of registers then compute the average. I would use indirect addressing to load the registers.

I use same algorithm Micky mentioned. Very easy to do on a 5/04 with the CPT instruction.

If you really want a running average here is a sample with 3 samples total 1 taken every 100mS.

There are more elegant ways to do this but this is simple and easy to understand.

I have a similar situation with a 4-20 mA output from an Emerson Mass Flow Meter. The algorithm that I used to filter the input signal was:

Every 10mS add the current value of the analog signal to an accumulator register and increment an averaging value counter (AVC) NOTE: I just add 1 to a register I didn't use a Counter instruction. When the AVC equals the number of desired samples, say 5, divide the Accumulator Register by the AVC value and store the result as your output. Finally, clear the accumulator and the AVC and start agian.

Currently, I have this algorithm running on a GE RX3i and it takes two rungs of logic. It really smooths out the signal.

I hope that this helps,
Regards

If it is an Emerson, you can dampen the output in the meter as well, and it would probably be qicker and easier.

Seems odd you are having that much fluctuation if they are mounted in the same cabinet.

Mickey said:

You could try a filter:

FV=FV+C(NV-FV)
Where:
FV= Filtered value
C= constant (range 0-1) The smaller the number the more dampening.
NV= New Value

Click to expand...

This is the easiest one I have used. You can even do it without the CPT instruction (ML1500 do not support CPT). I just make everything a float so I can adjust it on the fly.

Note: This makes a new value every scan versus some of the other methods listed.

cntrlfrk said:

If it is an Emerson, you can dampen the output in the meter as well, and it would probably be qicker and easier.

Seems odd you are having that much fluctuation if they are mounted in the same cabinet.

Click to expand...

I don't know if you were addressing my post or the original post. My Emerson Remote Transmitter in not located in the panel. It is on an Operator Workstation and I feed the 4-20 mA signal back to the control panel via a shielded cable.

As far as the amount of fluctuation, I had an Emerson tech in here to start up the flow meter and I have the configuration software. We have the maximum dampening on the analog output. The flow rate is fluctuating in the hundreths of Kg/Min, which means that I am seeing a very small swing in the value in the analog input file in the PLC, but it is more than we can tolerate for this process. This algorithm smoothes out the analog signal so that I see minimal fluctuations.

Regards

Instead of trying to "patch" the current configuration. Why not switch to freq. output from the meter, and count pulses for your flow rate? A little harder to implement, but will be worth it in the long run.

BTW, I would use a 1746sc-CTR4 card from Spectrum to count the pulses.


Ken

I am going to assume that you have verified with testing that these are real fluctuations in the meter output and not noise picked up in transmission.

Ken's suggestion is excellent. If you are married to the 4-20 mA for some reason another option is to change the scale of the transmitter. If you are only using the bottom 10% of the meter range you may improve accuracy and resolutoin and minimize fluctuation by changing the transmitter to 0-200 or such. This may OR MAY NOT improve accuracy and reduce signal fluctuations - it depends on the meter and the electronics design. Contact your vendor.

Fluctuating analog input - How to clean it up?

I am using a multiloop PID controller, into which a PID loop is design to which 4-20 mA signal is coming as PV(process value). it is going to a PD controller with kp=25, Td=0.6 sec. the input PV 4-20 mA signal is coming from a analog pressure transmitter as Boiler pressure signal. Due to this fluctations in input signal my PD output getting haunting continuously. I have already tried with digital filter in controllers analog input block. and as the transmitter is an analog i cannot increase its damping. so i can only do some configurations in logic.
Please provide me how i can filter or average it.

Fluctuating analog input - How to clean it up?

I am using a multiloop PID controller, into which a PID loop is design to which 4-20 mA signal is coming as PV(process value). it is going to a PD controller with kp=25, Td=0.6 sec. the input PV 4-20 mA signal is coming from a analog pressure transmitter as Boiler pressure signal. Due to this fluctations in input signal my PD output getting haunting continuously. I have already tried with digital filter in controllers analog input block. and as the transmitter is an analog i cannot increase its damping. so i can only do some configurations in logic.
Please provide me how i can filter or average it.

Tom gives the best answer, however you will have to calibrate the unit again.

Is there any other solution without any doing with the transmitter? like doing something in configuring some kind of logic...