Interference from VFD Off to 4-20mA analog signal loop drive Actuator

[Bmw318be]

Hi,

I am having actuator 240 V, that could take analog output of 4-20 mA signal.at 4mA 0 %, 20mA 100% position


Case 1,Handsheld Fluke 4 to 20 mA battery operated 6V
it is very stable, no drifting

Case 2, A PLC analog 4 to 20 mA 24 V power source

It shows when the actuator go to the last position eg 20%, it keep on drifting forward and backward interferences despite VFD off.

Oobservation from Current milliampere measure with Multi-meter shows consistent current reading.

Case 3, use another source using Potentiometer 4-20 mA 24 V,

It is stable, no drifting
.


We checked the mA using Fluke multimeter, all constant. What could be possible caused the drifting on case 2 .


The actuator is to drive the stroke for increase and decrease the stroke so pump flow would increase/ decrease

This is the Service manual
https://www.dropbox.com/sh/pg4uofh3d...OP0EHkxba?dl=0

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
http://pulsa.salesmrc.com/Details.aspx?ID=3110

 

[Ingenieur]

Is there position feedback from the actuator?

what controls the plc ao output?
a pid loop?

 

[rlundsrud]

I would be very surprised if noise was the issue, current loops are pretty immune to noise. I would look at your cabling and verify that your terminations are solid.

A few questions. What is the make/model of the device supplying the 4-20 signal? Where/how are you measuring the current fluctuations? Are you certain that the loop isn't varying the current because of a PID loop response or other reasons that reside in the programming?

I hope that helps.

 

[Smart $]

...A few questions. What is the make/model of the device supplying the 4-20 signal? Where/how are you measuring the current fluctuations? ...

That would help. The OP is certainly not concise enough to make a trouble call from this end. :happyno:

The last link appears to be the manual of a metering pump integrated controller. From a quick glance through the manual, it has both input (control) and output (feedback) current loops.

 

[Ingenieur]

It sounds like it is hunting
either to satisfy a pid or to get feedback to match command
perhaps pid tuning or some deadband

 

[Smart $]

...Are you certain that the loop isn't varying the current because of a PID loop response or other reasons that reside in the programming? ...

It sounds like it is hunting
either to satisfy a pid or to get feedback to match command
perhaps pid tuning or some deadband

And note his #2 scenario, the one that is varying, is with a plc in the loop.

 

[Ingenieur]

And note his #2 scenario, the one that is varying, is with a plc in the loop.

and he said it keeps on going forwards and backwards, not to a limit
hunting or cycling

feedback needs deadband/filtered
pid needs tuned
pid input needs filtered

lift the position feedback and see if it drives full open

 

[Bmw318be]

Observation

Observation

And note his #2 scenario, the one that is varying, is with a plc in the loop.

We tried to place capacitor at the end before the actuator board input + -,

https://youtu.be/AIrgvH4tFgg

Observations

1. it is actually stopped drifting forward & Backward continuously,
2. the Scada PLC shows last position is about 50%,
3. however after the next day, the stroke position drifted slightly to the programmed position while the PLC feedback still no change at 50 %

 

[Smart $]

We tried... however after the next day, the stroke position drifted slightly to the programmed position while the PLC feedback still no change at 50 %

As mentioned above...

... Where/how are you measuring the current fluctuations? ....

Also need to know the same for your stable tests. Power source, values, ratings, locations etc. of each.

An adaptation of Occam's razor says if input same while output changes, the difference lies in between.

 

[Ingenieur]

We tried to place capacitor at the end before the actuator board input + -,

https://youtu.be/AIrgvH4tFgg

Observations

1. it is actually stopped drifting forward & Backward continuously,
2. the Scada PLC shows last position is about 50%,
3. however after the next day, the stroke position drifted slightly to the programmed position while the PLC feedback still no change at 50 %

without knowing the control loop configuration and seeing the code it's next to impossible to determine the issue
what position the actuator? it goes to a predetermined position (100%) when the pump runs?
but if the actuator is at a commanded position and the feedback does not match/change I would start looking at the feedback sensor/code

 

[pjholguin]

+1 :thumbsup:

without knowing the control loop configuration and seeing the code it's next to impossible to determine the issue
what position the actuator? it goes to a predetermined position (100%) when the pump runs?
but if the actuator is at a commanded position and the feedback does not match/change I would start looking at the feedback sensor/code

 

[Bmw318be]

As mentioned above...

Also need to know the same for your stable tests. Power source, values, ratings, locations etc. of each.

An adaptation of Occam's razor says if input same while output changes, the difference lies in between.

Power source is actually from UPS, 24 Volt, location on control room beside the pump room.

 

[Bmw318be]

and he said it keeps on going forwards and backwards, not to a limit
hunting or cycling

feedback needs deadband/filtered
pid needs tuned
pid input needs filtered


lift the position feedback and see if it drives full open

Hi sory to ask what is PID ?

And can elaborate hlw i can lift the position feedack ?

 

[drktmplr12]

Hi sory to ask what is PID ?

PID is Proportional Integral Derivative

PID control is used to control a process variable (liquid level, flow rate, temperature, etc.) using a control variable (variable speed pump, modulating valve, heat exchanger, etc.)

The simplest form is flow control using a variable speed pump. The operator inputs a desired flow rate. The PID controller will monitor the flow rate and change a pump's speed to get close enough to the desired flow rate.

There are several factors which can be modified to change how quickly the pump speed will react in response to a change in flow rate. The basic ones are:

Proportional-change speed of pump based on the actual flow versus the desired flow
Integral-change speed of pump based on how long the actual flow has been different from the desired flow
Derivative-change speed of pump based on how quickly the actual flow is approaching the desired flow

The best way to think of it is driving a car. Your process variable is position and control variable is the angle of the gas pedal. You are at rest and your desired position is the next stop light which is 100 meters ahead of you. You change the angle of the gas pedal to change your position (proportional). You continue to press as the car accelerates toward the desired position (integral). As you approach the desired position, you let off the gas and press the brakes (derivative).

This is a grossly over simplified explanation. You can watch this video for more info.

And can elaborate hlw i can lift the position feedack ?

Disconnect the position feedback sensor wires from whatever is monitoring them.

 

[Smart $]

And can elaborate hlw i can lift the position feedack ?

Disconnect the position feedback sensor wires from whatever is monitoring them.

When feedback is via a 4-20mA loop, lifting the sensor wires typically yield a feedback [input] error on the controller [PLC].

 

[Smart $]

...
This is a grossly over simplified explanation. You can watch this video for more info.

Far from a simplified explanation, IMO.

I prefer temperature control for simple explanation, and this one has a car example to boot.

http://www.eurotherm.com/pid-control-made-easy

 

[Ingenieur]

PID

used in a feedback control loop as the controller
the feedback is the actual measured/controlled system parameter
input is the desired or set point
output is the actuator/action that influences or changes the measured/controlled parameter
error E (or deviation) = desired - actual, if at set point E = 0

P = Kp x E, output signal is Proportional to Error
I = Ki x integral (E dt), output signal is a summation of Error over a time interval;
D = Kd x dE/dt, the output is based on the rate of change of Error, if Error changes in a large step, output will do the same

total output = P + I + D
most processes required only P and I, but some that have step changes may require D

 

[Bmw318be]

PID is Proportional Integral Derivative

PID control is used to control a process variable (liquid level, flow rate, temperature, etc.) using a control variable (variable speed pump, modulating valve, heat exchanger, etc.)

The simplest form is flow control using a variable speed pump. The operator inputs a desired flow rate. The PID controller will monitor the flow rate and change a pump's speed to get close enough to the desired flow rate.

There are several factors which can be modified to change how quickly the pump speed will react in response to a change in flow rate. The basic ones are:

Proportional-change speed of pump based on the actual flow versus the desired flow
Integral-change speed of pump based on how long the actual flow has been different from the desired flow
Derivative-change speed of pump based on how quickly the actual flow is approaching the desired flow

The best way to think of it is driving a car. Your process variable is position and control variable is the angle of the gas pedal. You are at rest and your desired position is the next stop light which is 100 meters ahead of you. You change the angle of the gas pedal to change your position (proportional). You continue to press as the car accelerates toward the desired position (integral). As you approach the desired position, you let off the gas and press the brakes (derivative).

This is a grossly over simplified explanation. You can watch this video for more info.



Disconnect the position feedback sensor wires from whatever is monitoring them.

Hi,

From sori that the case 3 that i used is fluke 789 processmeter, i am not rrally sure if it used 6 volt as the battery to power is 6v.

I might be wrong as i am nembie to this device.


However, somthing cause it to drift when using scada, you are right that there is position feedback to scada or olc but this mainly for monitoring purpose only and does not send any signal like of the position not meet.

I did plug out before . It still drifts.

However one confirmation that with capacitor it stops but not sure if it drift in longer run.

 

[Smart $]

Hi,

From sori that the case 3 that i used is fluke 789 processmeter, i am not rrally sure if it used 6 volt as the battery to power is 6v.

I might be wrong as i am nembie to this device. ...

The US version uses 4 AA batteries rated 1.5V each, total 6V... so you are correct in that regard... but I believe it has an internal PWM power supply which generates 24Vdc for loop power. I am not familiar with the 789 at all, but I am with a couple other process calibration meter models.

 

[Saturn_Europa]

Hi,

From sori that the case 3 that i used is fluke 789 processmeter, i am not rrally sure if it used 6 volt as the battery to power is 6v.

I might be wrong as i am nembie to this device.


However, somthing cause it to drift when using scada, you are right that there is position feedback to scada or olc but this mainly for monitoring purpose only and does not send any signal like of the position not meet.

I did plug out before . It still drifts.

However one confirmation that with capacitor it stops but not sure if it drift in longer run.

What SCADA software are you using. Try trending the position command and the position feed back. If they are tracking it's as others have mentioned....a control philosophy or programming error. Tuning a PID is not for the faint of heart. I'd probably call in a system integrator.

If not you may hear the dreadful words "ever since you....."