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OPT3101EVM: Fluctuation on measuring muddy water or halogen light strikes

Moses Christian
Prodigy 170 points
Part Number: OPT3101EVM

Background

We have completed the evaluation of all the calibration procedure on modified OPT3101EVM (replaced LED with LD) in July.
We delivered one set of the OPT3101EVM module to our customer with updated calibration data in built-in flash memory for measurement using Latte.
Our customer have made further evaluation at their site and they faced some issues as follows.

It will be very helpful if the explanation to the issues is provided.

Fluctuation on muddy water

Our customer had collected data on muddy water.
It was a normal flat measurement data most of the time.
But some of the time the measured distance fluctuates like the plot below:


(each point is averaged by 400 samples)


(raw data of distance, ave=2)

The amplitude does not apparently fluctuate while the distance does.
The fluctuation went slowly, with period around 20 secs peak-to-peak and range of 100mm. 

Fluctuation when halogen light strikes directly

They also use halogen lamp lighten towards the sensor directly with distance about 5m.
They have measured the luminance at PD was around 3500lux which was not a very strong light.
They turn the lamp on at 5secs after starting the measurement.
And the plot was as follows:


(each point is averaged by 400 samples)


(raw data of distance, ave=2)

As the light turned on, the ambient value goes up and stop at a point.
But the distance value goes up and down and then gradually goes up as the amplitude only goes down gradually.
The noise get worse as well after turned on the light.
I was expecting that the distance get stable as the ambient value stabilized, but it did not happen.

Measured distance gets far when amplitude gets low

When a measured distance is obtain with sufficient amplitude, the accuracy is relatively good.
But insufficient amplitude would cause the measured distance gets further and lower the accuracy.
I have an explanation as below, and I'm wondering if it is correct or not.

When a distance is measured, the reflection from the target is obtained, as well as diffused reflection from various direction.
In the case that sufficient amplitude is obtained, the diffused reflection is ignorable, so the correct reflection is not affected.
But in the case that sufficient amplitude is not obtained, the diffused reflection dominates bigger portion of the reflected signal which has longer phase shift.
It will cause the measured distance get longer than it actually should be.

  • 0
    TI__Expert 6800 points

    Hi Moses,

    For the first issue, I am curious about what the setup is like, and how transparent the mud is too. The transparency could effect the accuracy of the results.

     Are they able to get a stable measurement on a solid object?

    Is the halogen test with the mud as well?

    Thank you,

    Brent Elliott

  • 0 in reply to Brent Elliott
    Prodigy 170 points

    They are able to get stable measurement on a sold object.
    I'm not sure how transparent was the water, but I am told that the muddy water is measurable in stable.
    And our customer claimed that the abnormal fluctuation occurs no mater what the measured object is.

    The basic setup is as the following pictures:


    The sensor is installed at a very long stand around 5m above ground.
    The green led light is the position of the sensor.


    This is the clear water in aquarium tank for measurement taken with IR camera to check the reflected spot of IR beam.
    (The purple glowing area is the IR beam)

    And these are the pictures of halogen setup.


    The sensor is struck by halogen light from bottom.


    The halogen lamp is set on the ground.

    I found that the measured distance fluctuates no mater it is struck directly or get reflected light form target.
    I just wanna make sure that is it normal.

  • 0 in reply to Moses Christian
    TI__Expert 6800 points

    Hi Moses,

    If they are able to get a stable measurement from a solid object, are they also able to get a stable measurement from a solid object and the halogen lamp? I want to see if we can rule out the lamp causing any issues.

    Thank you,

    Brent Elliott

  • 0 in reply to Brent Elliott
    Prodigy 170 points

    The data is stable when measured without exposure of halogen lamp.

    The results of measured data without halogen lamp are shown as follows.

    The target is set at 60cm from the sensor, with data rate= 7~8 Hz, average count= 256.
    The environment and the sensor are both different from the one I've shown above.

    The measured distance fluctuates within +/-10mm or +/-1cm.

    So there come the questions:

    Question 1:

    Why the measured distance is fluctuating and it goes down and then up when exposed by halogen lamp?

    Expected answer:

    1. Because ambient current occurs when the PD is exposed, hence the measured distance is drifting even when it is calibrated.

    2. When it is calibrated, it should not drift like that.

    3. Other

    Question 2:

    Is it normal that the measured distance is fluctuating and goes up and down when exposed by halogen lamp?

    Expected answer:

    1. Yes, it could happen. This is the best result from what we can do.

    2. No, it is not normal. It might be affected by other factor.

    3. Other

    Question 3:

    Why the measured distance sometimes fluctuates in very slow frequency (20secs of period) no mater what the target is? Is it normal?

    Expected answer:

    1. The sensor might be affected by radio or electro magnetic wave and it happens sometime.

    2. There might be other factor and it should not happen.

    3. Other possible answer.

  • +1 in reply to Moses Christian
    TI__Expert 6800 points

    Hi Moses,

    The previous data with the Halogen results fluctuating was during the measurement of the liquid correct? I see that measuring a solid object is giving reliable results which is good. Are they able to take this measurement at 5m with a solid object, and if this still gives accurate results, add in the Halogen lamp with the solid object to see if it will still give accurate results?

    We have a case which is stable (solid object, 60 cm), and 2 cases which are unstable (5m no halogen muddy water, and 5m halogen muddy water). I want to see if we can move from the first case to the other cases one step at a time to see when the issue is being introduced.

    For question 1, with calibration we should not see this fluctuation, especially since the ambient codes are stable.

    For question 2 as well, the fluctuation should not be large.

    For question 3, this is a bit confusing. The data you sent most recently doesn't show the wavy fluctuation like in the original post. Can you confirm that this fluctuation is happening on the solid object as well? If so, there could be an issue with the board, crosstalk etc.

    Thank you,

    Brent Elliott