Long term stability and calibration issues

Debug and "release: aren't the same--either the case, software, optics (alignment, focus, filters), or environmental conditions are different.

Try to duplicate the failure in your test setup...

Nice cc LED driving, BTW.

-- Cheers, James Arthur

Temperature variation?? If the LED o/p & photosensors have a large tempco then that could cause problems. Some enviromental sensors using leds/optics have temp sensors co-located with the LED & sensor to compensate.

Try a calibration, wave a heat lamp near it and see what happens (without moving or changing the housing).

HTH

Temperature stability of the TEPT4400. My guess, if every thing else is soild. It doesn't look that stable from the data sheet. Have you seen this?

Cheers

ng

About +1.5%/=BAC. Ouch. That's probably it. LEDs can vary quite a bit with temperature too, so that's worth confirming as well.

Photodiodes--much better...

...but he'll have to filter IR.

Some sort of closed-loop calibration might be an option, like a mechanical shutter with a reference swatch on it.

-- Cheers, James Arthur

My money is on tempco of the sensor dark current and sensitivity. That sensor has a dark current at room temperature that varies by 10x for 20C or 4% / degree C and relative sensitivity varies by 1% / degree C.

It might also need a "hot mirror" IR blocking filter to stop it being fooled when under IR rich incandescent lights. Sensitivity is still 50% at 800nm. Most alternative photodiodes will need a blocking filter.

Unless it is temperature compensated or maintained at constant temperature then I would expect pretty bad drift. A can of freezer spray or a hair dryer will quickly confirm this hypothesis.

It is likely the LEDs will also show some tempco in the opposite sense as cooling them typically increases efficiency slightly.

Regards, Martin Brown

Your enclosure looks fairly translucent.

The optics area should probably be constructed of a white-card light-blocking material.

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Dark current's only 10nA at 50=BAC--not significant. The sensitivity tempco is slightly worse than I said before--1.0 @

I don't know about the LEDs, but they bear watching too.

The sensor's so unpredictable that he really needs a new sensor.

A photodiode for sensing the target, plus one separate photodiode that intercepts a little direct light from each of the 3 LEDs would fix everything--that eliminates the phototransistor-as-sensor drift, and allows measuring and compensating for LED output drift.

The uC + an integrating capacitor per photodiode would make a fine single-slope converter.

An IR filter and an opaque case are almost certainly required. One alternative might be to use matching LEDs as photodiodes--they're wavelength-selective, sort of.

-- Cheers, James Arthur

That's not a schematic. ...Jim Thompson

\ I looked at teh Taos sensors, but they have three problems.

1. They are difficult to get to work reliably, as there are calibration and setup values that are under specified.

1. They can be quite expensive! I am trying to get this as low cost as possible.

2. They are close to being obsoleted by the manufacturer! You can barely find them on their website. I don't think sales were what they had hope for.

Thanks!!!!

Charlie

Ok, one more detail. The sensor area is surrounded by black cardboard above and below the width of the case, with a half inch square hole directly in front of the sensors and providing the aperature through the front clear window.

Charlie

Darn, got the link above it!

;-)

Charlie

You're dealing in DC, plus tempco's, plus distance to colored object.

Perhaps "rotating" pulsing of LED's (from same _current_source_) and look at the relative outputs "simultaneously"? ...Jim Thompson

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Alternative: modulate the LEDs, and make a synchronous detector in software. That eliminates background light, and you won't need as much filtering (just enough so the detector doesn't rail).

-- Cheers, James Arthur

leds/optics

Actually, in most of my test cases so far, outside light has not been a problem. The case, while an almond color, does a pretty good job of reducing the spill light. The cardboard insde does a good job of reducing that spill to very low levels. I do measure background illumination (one function is to test if a light is on, or not) and, even with the same gain levels as for the LEDs, I typically get levels of around 10-15 (out of 2048) and I compensate for that in software...

Charlie

Hmmm...

Object is designed to be pressed against the front window, so distance is always pretty much the same.

Are you thinking to PWM the LEDs? Or just turn on - measure- turn off- measure-turn on-measure?

Thanks, Charlie

Might be worth a try crudely compensating the gain with an NTC thermistor if thermal drift can be proven as the root cause.

That is probably the right solution and uses half as much power to the LEDs. This would eliminate all DC components. Setting the integration period to be 1/60s or a multiple would be a good idea.

Regards, Martin Brown

...

=A0 =A0 =A0...Jim Thompson

=A0 =A0...Jim Thompson

Just a series of "on, measure, off, measure, subtract" cycles eliminate ambient light, and isolate your signal from ambient and noise. Do you do that?

The phototransistor sensor is drifty--its gain changes rapidly with temperature. That affects all channels equally, so the ratios are not a necessarily a problem if you do the math right, but it just looks like that's not what's happening.

For example, if your ambient light level changes, and you adjust your digital pot as you described for a particular value, then using simple a a/d conversion that calibration value's *scale* factor is now heavily dependent on the ambient light level at the time of

The *gain* for that channel is then set incorrectly.

Modulating the LEDs, synchronous detection, and using a photodiode are all still great ideas, but, on reflection, your immediate problem may be the calibration and the math (software).

-- Cheers, James Arthur

Yep. But rapidly enough to maintain the ratios.

Although, as I ponder, what kind of LED light output versus current and temperature?

Maybe a rotating color wheel ala old-style color TV ?:-) ...Jim Thompson

How fast does calibration run?

How are you sequencing supply/lamp signals? You should never have all lamps disabled, while the MCP boost converter is running, or you'll biuld a large charge on its output storage cap. I'd recommend some kind of 'outside the processor' glue logic to prevent this from happening.

When switching from one colour to another, their conduction should overlap.

RL