Reflective vs Direct Optical Sensing

[dkw]

I have a working project using a laser diode on one side and a phototransistor on the other to measure the speed of an object going through the middle. Due to difficulty in maintaining alignment, I would like to change so that both are on the same side. Distance to the passing object is about 4".

I have increased the value of the resistor and can't seem to get a reflective signal. Originally, I needed the visible light for alignment purposes but if I do it from the same side, this would no longer be required. Would it be better to move to IR or UV sensor pairs or do I just need more amplification with sensor pair I'm using? I need it to work indoors and out which it does now with a filter placed in front of the phototransistor in very bright sunlight. Thoughts on general direction regarding the above would be greatly appreciated. If IR or UV would be better, is one preferable over the other for this type of application?

 

[MikeMl]

Check the response curve vs light freq (color) of the phototransistor. It may not be well matched to what the laser diode emits.

I have done this, but I had to put retro-reflective tape on the moving object (aircraft propeller).

 

[dkw]

MikeMI - Thanks - I do think my "matching" could be improved but probably not the only issue. What puzzles me is how the auto companies and LIDAR applications can get responses at hundreds of feet from a reflective signal and I'm struggling with 4"

 

[MikeMl]

... What puzzles me is how the auto companies and LIDAR applications can get responses at hundreds of feet from a reflective signal and I'm struggling with 4"

They modulate the emitter, and then have very high-gain, ac-coupled amplifiers behind the detector, followed by synchronous detection of the modulating frequency (narrow bandwidth). It is not a DC system...

 

[crutschow]

They also use a fairly high power emitter.

 

[dkw]

So I found a very sensitive photo-diode used in auto applications but the metrics on the datasheet are different (not comparable) to the one I referenced above. The "breakdown voltage" looks extremely high at ~120V while the one I'm using says "collector-emitter voltage" of 30V. Is this just reflecting the sensitivity difference? (one of course is a phototransistor and the other a photo diode) or are these not comparable? My board will be 3.3-6V. Nice thing is it has a built in filter and matches perfectly to a 650nm laser. Any help on this greatly appreciated.

 

[AnalogKid]

They also use lenses on both the transmitter and receiver. Like reading a book from 50 yards away, it is easier with binoculars.

ak

 

[ericgibbs]

hi dkw,
What is the detected object eg: shape , material , size etc ....
E

 

[dkw]

ericgibbs - Thanks for taking a look. Object is a baseball.

I'm getting a small reflected response now so working on amplifying signal. Guessing transistors is the normal route here? Opamp? Any thoughts appreciated. Thanks!

 

[MikeMl]

Try detecting the reflection in a dark room (night, lights off). If you get lots of signal, then ambient illumination (sunlight, room lighting) is masking the reflected signal. The solution is to modulate the emitter at a frequency like they use in TV remotes (38kHz), and to ac-couple the detector to the amplifier, and then rectify the resulting AC waveform, finally comparing that amplitude to a threshold.

I once built a optical gate where a runner ran through a light beam (transmissive, not reflective system). The detector was about 2m from the emitter. I had no trouble making it work in a darkened room, but could not make it work outside in daylight until I used one of the TV IR remote receiver chips behind the detector. I had to modulate the emitter by pulsing it at 38kHz. The 38kHz modulation and some optical baffels made the system immune to ambient light.

Previous posts

 

[dkw]

MikeMi - Thanks. I just tried in the dark w/ out any change. I'm going to work on the circuit side tomorrow and try to get better amplification of the signal.

 

[ericgibbs]

hi dkw,

The nominal diameter of a base ball is 2.9 inch.

The average 'hit' velocity is 90mph, which is 5,702,400 inch /hour
or 1,584 inch/sec

This means the ball is occulting or reflecting for 2.9 / 1584 seconds
ie: 0.0018 seconds, which is equivalent to 546Hz.

To resolve to a reasonable accuracy an emitter pulse frequency of 10 times would be used ie: ~ 5kHz.

The suggested 38khZ 'TSOP38' IR opto pair of TX / RX should give the required performance.

The problem will be ensuring that IR beam passes thru the true diameter of the ball in order to calculate the correct velocity.

IMO a dual opto path would be best, so the ball psses thru both beams, which are spaced a known distance apart.

The above notes are based on the assumption that you are trying to measure the balls velocity after being hit by a batter.???

E

 

[dkw]

Thanks! I appreciate the specific recommendation on the hardware! Yes, dual will definitely be better. I need to put a pencil to the resolution to see if overall error rate will be <=1 MPH. I may need to go w/ higher freq if available.

 

[ericgibbs]

hi dkw,
Read this blog link if you are considering using a TSOP Receiver IC, note the interrupted signal requirement.

https://www.electro-tech-online.com/blog-entries/tsop1738-when-used-as-a-beam-break-detector.196/

E

 

[MikeMl]

hi dkw,
Read this blog link if you are considering using a TSOP Receiver IC, note the interrupted signal requirement.

https://www.electro-tech-online.com/blog-entries/tsop1738-when-used-as-a-beam-break-detector.196/

E

ericgibbs

Eric,

why do you think that it is necessary for the emitter to transmit bursts instead of CW?

I found that if the receiver gets continuous 38kHz for a long time, it sets its internal agc loop to that signal level. If the beam is totally blocked for a short time, for example, by rapidly moving your hand through the beam, the TSOP output pin follows the interruption with only slight rise/fall delays. The receiver is effectively primed to be maximally sensitive to a short duration break in the 38khz reaching the detector.

If the beam is broken for a long time, then the leading edge follows the beam interruption, but as the agc loop inside the receiver brings the gain up, the output pin changes back to the resting state before the beam is re-established. In other words, the TSOPs intrinsically works as a beam break detector provided that the interruption is only for a few ms...., which would work for the TS.

No need to modulate the emitter with anything but a 38kHz square wave; no pauses required, CW works fine. The TSOP is like a differentiator for the leading edge of an interruption. There may be a delay for the TSOP's agc loop to reset itself before the next interruption, but the TS has plenty of time for that (next ball?).

 

[ericgibbs]

why do you think that it is necessary for the emitter to transmit bursts instead of CW?

MikeMl
Mike
I am not suggesting the TS uses a TX burst transmission, quite the opposite, it should be a CW pulse train for his application.

It does require a burst TX, which would not be suitable for his App, IF he considered using a TSOP38 Receiver, that is why I posted the caution.

With respect Mike I think you should read my posts with more care.

E

 

[dkw]

Hi all - Just wanted to thank you for your input.

I just got everything to work with my existing equipment referenced above. I ended up using the schematic **broken link removed**. Almost fell out of my chair when I got a solid signal from several inches. I haven't tested outdoors yet but I've tackled those issues before w/ an optical filter. I'll probably change to a better matched pair of laser diode/phototransistor since I'm going to do a new board but wow! it actually works! Thanks again!

 

[Tony Stewart]

I once designed an IR optical shopping cart detector to detect goods in the bottom of the basket (BoB) for grocers to reduce shrinkage .
With a 1meter aisle I could detect a 1/4 resistor wire crossing the path. I forget the actual Sharp/Vishay IR part number but I only used the Rx part since it had AGC , daylight blocking filter and supported IRDA 2 data rates (>>100kHz) We used an array of 100mA pulsed 5mm LEDs in TDM mode and a matching array of IRDA2 smart Receivers so that a missing bit in the continuous communication of pulses Muxed to all detectors would detect object interruption and also direction and position of the object.

If you cannot hit the 1mm chip with a 1mm laser diode, then diffuse the beam with some appropriate diffuser plastic material to spread the beam a bit
Photo transistors have high gain but are notoriously slow compared to PD's with TIA Op Amps with low R feedback and followed by AGC to detectors. but the IRDA2 chips had that all built into a tiny package.

You can get IR LEDs with 8 deg beamwidths and pulse them with 100mA or more fast enough to detect high velocity objects but you need to calculate mm/s speeds and aperture width and use >10 mm deep holes 5mm wide to minimize stray light and reflections.

If the ball is not positioned above the ground at the same then you need a vertical array of emitter <> detectors like they use in golf simulators where they can track a ball velocity of 150 MPH easily.

IR reflector methods I have used all suffer from inverse distance loss and are not great for distance and speed. Forget unless you relay want to get into LADAR. with high cost , high speed photo multipliers.

So IR pulsed <8deg emitters with daylight filtered IRDA2 receivers is what I recommend up to 1m across maybe more and detect the missing pulse by comparing Tx with Rx or a one shot retrig timeout in x microseconds.

For signal stability the emitter and detectors must be well supported and you can use RED Lasers to align the supported structures. but visible Red also makes it prone to visible light for object detection so IR is best..

 

[MikeMl]

...
With respect Mike I think you should read my posts with more care.

E

Didn't you say: "The attached circuit transmits at 38KHz, with a break in the transmission at approx 650Hz."?

Perhaps you should read mine more carefully. I am pointing out that the 650 Hz breaks are not necessary.

 

[ericgibbs]

Didn't you say: "The attached circuit transmits at 38KHz, with a break in the transmission at approx 650Hz."?

Mike,
Yes I did and I still do.!

Repeating:

I am not suggesting the TS uses a TX burst transmission, quite the opposite, it should be a CW pulse train for his application.
It does require a burst TX, which would not be suitable for his App, IF he considered using a TSOP38 Receiver, that is why I posted the caution.

That is why it is NOT suitable for his application.

I know the TX breaks are not necessary or desirable in his App.

The blog link was intended as a cautionary note to the TS.

Do we have a problem here.?

E

BTW:
If you can spare the time, look at the Information in my Profile regarding laser measurement projects/products.
I do have a little experience in this type of application.