Rangefinder sensor interrupted by LED light.

Thread Starter

mickratoch

Joined Feb 17, 2021
3
My company uses electronics, kinetics and sensors to make modern art installations.
I have a lingering problem with a piece where we levitate ping pong balls in glass tubes using programmed computer fans. We animate them into rolling sine waves etc.

We achieve precise placement by repurposing a laser range finder and aiming it from the underside up towards the ball. This information is then fed back to correlate with the intended data value in our programming to place it precisely where we want it to be. This all works perfectly.

The problem lies in the fact that all of these tubes are illuminated by an LED from above.
We alternate the colors with green and white leds. These are highly focused using glass lenses that brightly illuminate the ball. The mechanism works perfectly for the green led light but the laser range finder misreads on the balls illuminated on the white light. This made us deduce that it had something to do with the spectrum of light disrupting the laser. This causes the balls to "hiccup" or jump as they reach the upper half of the glass tube. My theory is that the light intensity through the ball, or even bouncing around is disrupting the light from the laser reflecting back to the sensor on the rangefinder.

In the link I attached you can see that the range finder laser's wavelength is 635 nanometers.
http://www.jrtsensor.com/sale-11893...m-support-single-continuous-measure-mode.html
The led spectrum emits a wavelength in this range as you can see in link.
https://www.cree.com/led-components/media/documents/XLampXPG2.pdf
Troubleshooting and solving this problem has been difficult. Here are a few things I have tried to circumvent this problem all to no avail:

Dimming the intensity (brightness) of the light to 50% power allows it to function, but there is little point in illuminating in that event.

Adding a polarization filter on the lens of the led individually.

Adding a polarization filter on the reflective lens of the rangefinder.

Adding a polarization filter to both.

Un focusing the light.

Testing with red, green and blue filters on the reflective lens of the rangefinder.

Screenshot (12).png

I would greatly appreciate any ideas on way to filter these frequencies out without a highly expensive or large filter. Or point me to the right place to ask this question.
 

seanstevens

Joined Sep 22, 2009
78
Not an expert by any means just a thought, maybe 635nm filter on the laser rangefinder in/out lens may help. Your white LEDs are driven by some sort of LED driver, maybe the frequency of the driver/PWM of the driver may be interfering with the rangefinder? It's also possible that the highly focused white light may interfere with the internal gain control of the rangefinder and this may cause the hiccup while it tris to regain/adjust it.
 

Thread Starter

mickratoch

Joined Feb 17, 2021
3
Thank you. I should have said that I had already tried the red filter on the rangefinder lens. This didn't seem to help, as I think the problem is that specific wavelength is emitting from the LED and it is washing out the laser on the ball or bouncing down the tube around it. We've tried different color balls, so that lead me to that conclusion. I also metered out our dmx driver on an oscilloscope and it is a smooth signal, letting us know that isn't it either. To add to that it is the same driver on the green LED's.
 

andrewmm

Joined Feb 25, 2011
1,461
that's amazing,

You could be suffering from the detector being overloaded by the LED light.
You should see this if you vary the LED intensity, the effect should get less at lower intensity.

A thought is can you modulate the Lasers and the LEDs ?
anti phase, so short the eye wont notice, but you should get clear signal time

Another quick thought,
its much easier to detect a AC waveform in the presence of noise than a on / off.
If you can modulate on / off the Lasers, then you should be able to detect the Lasers more reliably.

One other to check, Could you colour the balls ?
if the laser then works, you know its the LED intensity,
 

Sensacell

Joined Jun 19, 2012
2,820
I am suspicious of the light source. (LED lights) When you say you "Metered out" the LED driver with an oscilloscope, how did you actually perform this measurement?
Measuring the voltage on the LED with a scope might be very misleading as to the actual current flowing in the LED.
Have you tried powering the LED from a pure DC constant current power supply?

The rangefinder will have issues with any light source modulated at a high frequency, it would tend to disturb the AGC (automatic gain control) in the unit.
 

kennybobby

Joined Mar 22, 2019
75
This is very interesting. Do you happen to know the bin or temperature of the white emitter that you are using? There are numerous versions of "white" and some have quite a bit of "red" that could be causing the interference. Select a higher temperature emitter (e.g. 5000K) and it will decrease the red content. It will be a very bright white but the intensity could be adjusted back with current control.
Cree_Spectral.png
 
Last edited:

du00000001

Joined Nov 10, 2020
61
It should be obvious: part of the the white LED's spectrum is in the 635 (+/- x) nm range. This is the nature of "white" light: it's covering all visible wave lengths. And if you add a "red stop" filter to the white LEDs, the result will be some blue-green light. (That's also why the green LEDs do not interfere.) If you use blue LEDs instead of the white ones, the issues would be the same as with the green LEDs: none. Question is whether you'll get issues with the designer(s) of the installation.

Alternative:
Can't you get a rangefinder in the IR wavelength range? While you might still require an IR blocking filter on the illumination side (to be evaluated), the existing issues would vanish and you would be completely free in the color of the illumination (allowing red as well).
 

Lo_volt

Joined Apr 3, 2014
189
White LED's are often comprised of a UV emitting die with a phosphor coating. The phosphor, excited by the UV, fluoresces and emits the longer wavelength visible energy that you see in kennybobby's chart and explains the ~440nm spike. This also matches du00000001's explanation.

You may be able to find a notch filter that specifically cuts down the 635 nm that the range finder uses but I can't imagine it would be cheap. Semrock is one company that can manufacture some narrow band stop optical filters. They're not cheap, but it might be worth checking out. You would need to place it over the white LED in order for it to work properly.

Per kennybobby's chart is it possible for you to use a higher color temperature white LED that has less red? Have you experimented with with different white LED's?
 

Yahiko

Joined Jul 1, 2016
3
I was thinking the same thing.
The big down side to filters is the cost. You might look into a different white light source.
Something tuned to not have the 635 nm peek. You might try RGB with the red being more
in the yellow range. Yes that will change the white light but you may have to live with it.
Plus with the ability to play with the colors you may find something that looks better and works.
 

David Brown

Joined Dec 29, 2017
1
that's amazing,

A thought is can you modulate the Lasers and the LEDs ?
anti phase, so short the eye wont notice, but you should get clear signal time
I agree, persistence of vision should allow plenty of samples with the LEDs switched off. The PWM effect could be compensated with higher drive when the LEDs are on
 

Thread Starter

mickratoch

Joined Feb 17, 2021
3
It should be obvious: part of the the white LED's spectrum is in the 635 (+/- x) nm range. This is the nature of "white" light: it's covering all visible wave lengths. And if you add a "red stop" filter to the white LEDs, the result will be some blue-green light. (That's also why the green LEDs do not interfere.) If you use blue LEDs instead of the white ones, the issues would be the same as with the green LEDs: none. Question is whether you'll get issues with the designer(s) of the installation.

Alternative:
Can't you get a rangefinder in the IR wavelength range? While you might still require an IR blocking filter on the illumination side (to be evaluated), the existing issues would vanish and you would be completely free in the color of the illumination (allowing red as well).
Thank you, yes I understand these concepts. I did, however, fail to mention that we did try a band stop filter but they tend to be hundreds of dollars per 25mm lens and was hoping I missed a more economical angle. Fortunately for the artist, I do believe we are choosing just to replace with a blue led for now. I think an IR rangefinder is actually a great option for the next time, some could fit our dimensions. This seems to be the choice in the future.
 

du00000001

Joined Nov 10, 2020
61
The JRT product pages show a 50 m distance sensor with "infrared" in the title, although the product specs give a wavelength of 635 nm for the emitter. I'd suggest to check with IRT whether this one really works with IR. IR blocking might come cheap - a sheet of acrylic on the LED side might suffice.

But while mentally chewing on your issue, another idea surfaced: why not put a layer of varnish (white or maybe silver) on the ping pong balls to reduce transparency? While this will certainly somewhat change the appearance of the balls, the result might be "acceptable".
 

du00000001

Joined Nov 10, 2020
61
I don't think reflections cause the issue:
Consider the tube walls to be wave guides (like an optical fiber). The issues occur when the balls are close(r) to the LEDs - where I'd expect the incident angle to allow the light to pass through the wall. And light reflected from the inner surface would have to travel some more reflections.
What I assume is that the attenuation for light passing the ball (always given as a percentage of the incoming light) is too small.
Evaluating the effect of a varnished ball shouldn't be too difficult ☺
 

du00000001

Joined Nov 10, 2020
61
I can see these large bright areas. But:
  • What is to be seen is light that's no longer within the tube!
  • These bright areas/spots (spots in the lower half) might come from the fastening of the tubes (you somehow have to fix them), creating regions where the tube wall doesn't act like an ideal wave guide.
We just do not know enough about the whole setup (Where are the fans? Where are the distance sensors?) to give a complete assessment - just giving options/opinions.
Reflections might be attenuable by adding a length of black tubing on the receiving side of the distance sensor - shielding the receiver from light not coming straight from the ball. The longer the tube, the better the shielding - light doesn't travel in curves.
 

kennybobby

Joined Mar 22, 2019
75
@du00000001, i like your idea of the black tubing.

TS, @mickratoch, Is that installed in a university or hospital somewhere--are you free to reveal the location? Any videos of the sinusoidal in action?

How about an iris or black-lined collimator tube at the upper end to block reflections and keep the LED light in the middle of the tube? Or a total internal reflection optic at the emitter? Just throwing out ideas for things that could be tested somewhat easily.
 

Sensacell

Joined Jun 19, 2012
2,820
@du00000001, i like your idea of the black tubing.

TS, @mickratoch, Is that installed in a university or hospital somewhere--are you free to reveal the location? Any videos of the sinusoidal in action?

How about an iris or black-lined collimator tube at the upper end to block reflections and keep the LED light in the middle of the tube? Or a total internal reflection optic at the emitter? Just throwing out ideas for things that could be tested somewhat easily.

I like the concept of collimating the view of the receiver.

I am very familiar with the sensor in question, it has a lens that focuses the return onto a large-area sensor inside the unit. (presumably to make overall alignment a non-issue)
Excluding light that does not contain the desired laser energy, the Signal-to-Noise Ratio can be improved.

The only problem (a big one) is installing and ALIGNING this collimator.
This would need to be done with live feedback from the sensor, it does have a signal integrity value that can be read out of it's serial port in operation.
 
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