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Brainstorm please! High speed (>20krpm) IR reflective optical trigger

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max_torque:
Hi all!

Anyone fancy a brain storm??   :blah:


I'd like to generate a TTL level edge coherent to the edges of the teeth (probably 6 teeth) on an optical trigger disc, that could be spinning at between 20 and 30krpm!  That TTL signal needs to be consistent in it's position verses the true position of the teeth, but can have a latency / phase offset, as long as that offset remains reasonably constant (or is small enough to not worry about) across the speed range.

Previously (different project) i used an IR LED / LDD, and fired 200ns pulses from the LED at a 1.5MHz rate, and with a transimpedance amp generated a return signal from the LDD.  A  suitable high pass filter made the system robust against ambient IR levels.

This time, i'd like to get less latency and jitter, which due to the way the previous system worked could be as much as 1us.

So, ideas please?? 

(some guess system design values could be, subject to change:  50ns light pulses, 5MHz rate, max 500ns latency and "Low" jitter)


NB currently i am only considering an optical system, and not any alternatives (ie VR / hall sensor etc)  to date i can't really find too much info about how those electrical systems cope with high speed (it strikes me the inductance of those sensors is going to result in a phase shift vs speed)

sokoloff:
That's actually not all that fast electrically. It's 500Hz at the outside at one pulse per rev or 3 kHz at 6 pulses per.

Have you considered/tried a comparator based system where you shine a constant light at the target, sense the return, and compare the instantaneous return to the average return via a comparator? You might also be able to look into a PLL because the frequency of this pulse train will not change instantaneously due to the mass of the rotating system.

Alternatively, are you able to modify the system under test to include a slit and use a slotted sensor?

max_torque:
The average frequency of the return signal is not fast

15krpm with 6 teeth is 'just' 1.5khz ie an edge every 666.7us.  However, i need to be able to (ideally) resolve 1rpm in 15,000 at each of those teeth, which is about 11ppm.  That needs a 22Mhz clock for the tooth interval counter, for a clock resolution of 45ns. So whilst absolute phase offset from each physical tooth is irrelevant, the jitter in the tooth edge trigger signal needs to be very low indeed.......

Siwastaja:
An IR LED, and a fast, reverse biased photodiode, and a fast comparator, closely coupled? Use enough illumination power and use a tight optical bandpass filter on the photodiode to reject ambient light in a simple way, without the need of correlated double sampling tricks.

What is LDD?

sokoloff:
The rotational inertia of the system is your friend here (assuming the available torque to accelerate it is reasonably low).
You may need 11ppm, but you (almost surely) don't need that level of precision from a single measurement.

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