I thing I'm using one of those strain gauges (I took it from a balance).

But my problem is my amplification assembly that increases the noise. My problem is that in the balance display the value is not moving (is working fine) and in the PLC (and in reality) is having a huge variation. And I do not know how to put it working correclty.

Jeff Casagrande

Load cells are inherently noisy devices.  In most cases, since weight does not change quickly, then a high time constant filter is not a problem.  If you need to make quick decision based on product weight, then a high precision damped load cell is needed.  Load cells based on strain gauges suck.

Well, the first thing is to determine where is the source of the problem.  Take your filtering to 0 and measure the actual input with a meter and the PLC number.  If they correlate, then then you can correct it with filtering.  If not, then the problem is with the load cell system.

bruno_cardoso

[Normalized]:

I will put it there where you told and give it a try. But like you said it can be put at the end (after the scale) - I'm asking this because the load cells that I'm using are not directly proportional all the way, and I'm thinking about doing a function (polynomial of 4th Order) to give me the Kg accordingly to tests with known weights. And it will still have all the variation, and I would like to know if it still works (more confirmation, because I think you already told that with changes it works).

Measurement is not linear? That is a tough one. Normally when this occur with me, it means the "platform" is too big / load cell is damaged / working on the edges of the load cell range.

Your situation is clear now.. use the filtering action after linearization. The filter can work in engineering units (kg). Just know the "zone" setting should be in the same units - as mentioned before.
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bruno_cardoso

[Factor]:

I understood your explanation about the Factor and what it is. But how do I know which value to use? It's achieved by experimenting, by trying different values, isn't it?

It very much depends on how quick the output should react. Begin with 0.95 (95%) and work your way lower... yes - by experimentation or by educated guess.
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bruno_cardoso

[#Zone]:

In this I have an huge issue unfortunately because my device/assembly is giving me an variation of at least 200g and everytime I use the liquid (in the 5Kg reservoir) it only reduce 30/40g. So the filter will not see this variation, I'm right?

Liquid? No wonder you are having problems. Do consider the mechanical motions ("vibration") that is involved when handling the liquid.

I do not fully understand you description. The ripple is 200g? What is the smallest increment that the PLC can measure? (10-bit / 12-bit / 13-bit) 200 / 5000 = 0.04 or 4% ripple... that sounds high. If this is the case then I wonder how the mechanical design looks like and doubt that the algorithm I give you will be helpful. (I don't like it being blind for the system's design - it makes me give bad advise.)

If you make #Zone big (1kg) - the filter will be used exclusively but the reaction time may be influenced.  Maybe start big and work your way smaller.

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That is all I can do for you now - good luck!
w

William_B

>> This text was late <<
Jeff Casagrande seems to be taking the right road..
getting the source stable.
So ignore what follows for now.

I have been in this situation so many times, trying to make software solve a hardware issue.  The first thing to do is to understand the problem you are trying to solve.

Hi ,

I apologize for not explaining the system design from the start.

The system I'm using is a load cell I took out of a balance (max 5Kg) and the output signal (in the order of mV) is amplified through a INA128P with a gain more or less of 100.

The INA128P is powered with 12V and the load cell is with 9V. As the power comes through some laboratory power supplies (which float) it was used an LM723 Cn to stabilize both the supplies. It also has been used in other tests some voltage regulators (to substitute the LM723). But, in both cases, the improvement was not complete ( reduced the fluctuations but there is still enough variation - this is the problem was not only this one).

And the purpose of this application is to measure the liquid which is within a container which is withdrawn each time through a pump (about 30-35 ml each time).

I experimented the filter and as you said he did nothing, unfortunately.

Thanks for all the help and suggestions

Hi,

My load cell is fixed in the balance base through a screw (just like it came). But, if is out of the alignment, the variation will be so great as mine?

How large should be the capacitor in your opinion? Sorry for my silly questions.

I used this circuit because I needed a signal from 0 to 10V, and was the cheapiest.

The all cirtuit is design in the attachment file.

Thank you for all the help