Scada Problems analogue signal 4-20mA drive stroke Actuator motor of pump

[bmw318be]

Hi,

Need some advise as I am having a pump with automatic stroke control by analog 4-20ma

There is input signal and feedback to Scada.

What i experienced,issue.

There is unknown interference that drift the actuator forward and backward.

We measured the input signal using milliam multimeter in series. It is constant in input.

Another issue when the VFD motor off, the stroke signal is deifting by itself. Not sure if there is possible intereference or rootcause that can be narrow down in the case of troubleshooting analog signal.

 

[Skogsgurra]

VFD (PWM) is the prime suspect.
Make sure that your 2-20 are well screened and situated well away from power cabling.
A series inductor in the 4-20 path, around 1 MHz ferrite with 100 or 200 uH usually helps to remove unwanted interference.
Make sure that the different grounds are well connected. Several parallel paths is good. If someone starts saying anything about ground loops and hum - lock him out of the room until you have fixed the issue. Hum fear is for audiophiles - not for control guys. Entirely different signal levels and band-widths.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bmw318be]

Hi,

Thanks for the input which help me better understand from electrical perspective.

There are some observation which I still yet find the root cause:

1. In troubleshooting this, we tried using 4-20 mA fluke loop analog to inject to the actuator, the 4-20mA motor actuator did not drift forward backward. We also run the motor VFD with this loop analog. Puzzled if this handheld has immune to interference.

https://youtu.be/zY1Wk4opREQ

2. The issue of drifting forward and backward happened only if using Scada signal which is generated by 24V supply. not sure if this signal is not good, however when we measure with milliampere in series input is constant. My connection is as followed
4 to 20ma
1 input +
1 Output -
4 to 20 ma feedback
1 input +
1 Output -
Jumper
3. Another observation, when we power off the motor supply 400 V, sometimes it like having interference to my 4-20mA actuator. The condition when this happened, the power supply from actuator on 240 V and the analog signal was at last position.

https://youtu.be/d7-NdaA879U

This interference has caused position error of my stroke as we used this to control the flow, the critical issue is to solve the issue of drifting which I believe the main culprit of position error.

 

[Skogsgurra]

You say: "using 4-20 mA fluke loop analog to inject to the actuator, the 4-20mA motor actuator did not drift forward backward" That is a clear indication that there is too much AC superimposed on your 4-20 circuit and that this AC signal isn't present when you break the circuit and use a simulator as a signal source.

I would check potential difference between SCADA GND and Servo GND and also, if the distance isn't very long, put a heavy-gague ground between the two - i.e. bond them. Potential difference is best checked with a battery scope (no GND), but a DMM with high BW (at least 10 kHz and better still, 50 kHz) can work.

Did you try the 100 - 200 uH inductors?



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

I have had similar experiences with some types of transducers putting out erratic signals, in all 3 cases it turned out to be EMI/RFI coming from the VFDs. In one case where the VFDs were mounted to a wood board in the open, I removed one while still connected to the line but with it's motor not running and as I moved it around in the area, I could make the pressure transducers put out varying degrees of erroneous readings. So even with the VFDs not operating the motor, there is often enough EMI/RFI associated with the rectifier to cause interference even if it is not coming in via the signal wires. Simple test: actually disconnect power feeding the VFDs and see if your stroke signal quiets down. If that is the source of the problem, double check your shielding and grounding of it (one end only) and if that is good, install some good EMI/RFI (EMC) filters ahead of the VFDs.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[bmw318be]

Hi Gunnar,

We are trying to narrow down the possible rootcause and need proven finding if root cause to our actuator drifted proven is the ac analog signal.i would like to try if any measurement instrument can check.

You advised to measure potential different from the scada, may i know if this is the analog input signal - ( negative ) that supply to my board ?.


For the actuator board, is actually, the supply is 240V and has + and -.

Are you referring to check the minus here ?

What should be the abnormal reading be ?

The drifting Fw& Back is present when motor not running .

Shall the check is done when the motor not running ? Or should i power off the main VFD and check if the drifting still present as a simple verification is VFD.

 

[bmw318be]

Hi Jraef,

Another findings

1.we switched a Local mode ovewrite scada,this is having also 24 volt source also sending an analog signal 4 to 20ma to the actuator it is quiet down.

When we measured mA input

1.experiment 1,at the condition, the motor off, suspected some interferences as the analog signal cable tray from PLC mixed with some 240 V power supply of some instrument pressurre switch, amd other valve actuator. we then directly sourced the 4 to 20 ma bypass cable tray to the scada plc.result found, It still drifting actuator forward backward when no sigal given. When signal is given, it moved normally but then it reached the target mA it then drift fwd and backward again.

2. Experiment 2, switching to the Local potentialmeter result stable no drifting.

Using Ma multimter measured
Local mode
5.671 mA at 10.5%,16 Volt

and Remote Scada mode
1st reading 5.744 ma 10.9%,
2nd reading 5.601 10%, 22 Volt

Is there possible a voltage value correlation that caused tbe drifting error as the mA is constant, voltage is actually does not fluctuate, it just stays.

 

[djs]

You might also have a ground loop issue. You can try and place an isolator in the line.

 

[bmw318be]

Djs,

Where exactly the isolator should be and what types of isolator recommended ?

May i know how to determine and reference ideal voltage for 4 to 20 ma at each mA ?

Measuring voltage 20mA to 4 mA . Showiny that for 20mA is highest voltage and gradually it is drop lowest at 4mA. The power supply at 24 V at 20mA.

 

[danw2]

I was out for a couple days, so I missed this thread until just now, and as I read through it, it appears that the measurment of the 4-20mA actuator drive/demand/control signal appears to be stable, even though the actuator action is not stable - it hunts and drifts.

I suspect that your meter is just not capable of showing the noise component that is causing the problem. But, so what? Unless you need to document whatever is causing the error for academic reasons, maybe you should lower your expectations, shift to a pragmatic stance, and take some action(s) that might well stabilize the actuator.

You've gotten extremely good advice from some of the most experienced field engineers in the world who have encountered similar problems. But as far as I can tell, you haven't implemented any of their suggestions.

Wanting to know a root cause is fine and dandy, but sometimes things just have to work well enough with whatever band-aid is available, like a choke (100-200uH inductor) in the 4-20mA line or a good EMI/RFI (EMC) filter ahead of the VFD.

 

[danw2]

It appears as if a high frequency component on the 4-20mA line is creating the problem.

Another alternative - if you have DC power at the actuator, an isolator could clean up the 4-20mA signal. An ordinary 4-20mA isolator is typically designed for DC so it uses a low pass filter to strip off high frequency.

There are some HART compliant 4-20mA isolators that are designed to pass the 1200Hz HART signal. HART isolators generally cost more than conventional isolators. If you are not using HART, I'd recommend not getting a HART compliant model.

It's worth it to pay for 3 way isolation - power supply isolated from both the input and output and the input is isolated from the output.

 

[bmw318be]

Dear All,

This the measurement of the mA local ( stable) vs Remote (not stable) through same cable.

https://youtu.be/c8cHpQ7Q22Y

This the switching local to Remote through same cable when motor not running

https://youtu.be/OSb5En0Jba8

D