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| Brainstorm please! High speed (>20krpm) IR reflective optical trigger |
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| jbb:
You mention that noise from the LED driver itself is an issue. Have you carefully reviewed the electrical design of the system? Used power supply filtering and shielding on the photodiode and TransImpedance Amplifier (TIA) front end? We did this on an IR comms project and shielding helped a lot. I too think you should look at continuous LED operation. In order to deal with noise, I have two thoughts: 1) could you add a 2nd photodiode which is close to the first one, but does not see reflected pulses? Maybe it could be used to detect abient noise in some differential manner. 2) How about feeding the noisy photodiode output into some kind of filter, such as a PLL? At the expense of some response time delay, a PLL effectively operates as a very narrow-band tunable filter and might be able to reject abient noise (note: not good at very low speeds) 3) Using a constant-on laser might be effective by simply providing massive signal to drown out the noise. Make sure the laser is eye safe (for the love of god don’t use a green eBay one, they’re super dodgy), apply warning labels (laser not eye safe if someone goes looking with a magnifying glass) and maybe apply an attenuator right on top of the photodiode to prevent saturation. |
| max_torque:
A few interesting ideas there! Perhaps the massive-intensity-drown-out solution has some merit! One small advantage of the pulsed solution is that i can fairly easily work out the latency / response time of the detector circuit, which is a bit harder with a constantly on light source, because then you have to somehow measure the physical "chopper" to work out what the delay is?? Another thing i need to look at is the physical form factor of the optics, because a thin window produces a sharper edge, as compared to a wide one. (because it's area reduces faster per angular degree as the interrupter disc rotates. Some form of thin transparent film (ie selotape!!) could be used i think, sandwiched into some sort of opaque material to provide a narrow slit type arrangement. Obviously the alignment of the two halves becomes more significant, but i can easily machine a solid metal type support...... |
| jbb:
I guess that you’d have to measure the photodiode response with a pulsed circuit during design verification. Multi MHz bandwidths are possible using a fairly standard photodiode and TIA. I think you could form a slit with a laser printer and an OverHead Projector (OHP) sheet (perhaps glued to something stronger) (also make sure the OHP sheet is rate to go through a prenter; you don’t want it melting!!). If you want to get the highest bandwidth, here are some thoughts: - photodiodes have stray capacitance which limits bandwidth - if you bias them with a moderately high reverse voltage (could be 10 - 30V), the capacitance drops and bandwidth increases - make sure the bias voltage is clean and well filtered because voltage noise will here will pass through the photodiode - placing the TIA right next to the photodiode minimises cable capacitance and the area most sensitive to electrical noise - don’t use excessive gain on the TIA because it will reduce bandwidth. You can add a second gain stage if you need it - consider using an AC coupled gain stage to reject changes in DC levels. Then use a comparator to for the edges. I think Analog Devices have app notes about this kind of thing. |
| StillTrying:
"A few interesting ideas there! Perhaps the massive-intensity-drown-out solution has some merit!" Definitely! 10-20mA would be more than enough through a super bright visible LED, even with almost no sheilding of extenal light and some EMI, edge jitter should be only ~5ns. "Another thing i need to look at is the physical form factor of the optics, because a thin window produces a sharper edge, as compared to a wide one." Yes, the resolutions all depend on the mechanics. Assuming the 6 toothed optical disk was 30mm diameter, and the light beam was only 0.1mm diameter, then it still takes 2us for a tooth edge to cross the beam width at 30krpm. You'd have to always trigger at the analogue center of the 2us transition. 0.01mm differences in the width of the teeth would cause 200ns of jitter. But with such a strong signal you don't need a too fancy TIA. |
| TurboTom:
A typical application of a laser-based frequency measurement arrangement is an RPM pickup for micro gas turbine engines. I've worked on such systems up to 250,000 RPM (single through-hole) so frequency was up to 8.333kHz. LED-based systems also work but the laser has got the huge advantage of a much more narrow beam and higher intensity over a very small space angle. This makes the pulses it generates much better defined. What's been told before regarding reverse-biasing the PD is correct, if possible also use narrow-bandwidth optical filtering to eliminate as much stray light as possible. A high-speed current / voltage converter (inverting fast OPAMP) as the first amplifier for the photo diode will result in the voltage across the diode to stay for all practical means constant, thus almost eliminating the effect of the PD's capacitance. After that, some schmitt trigger can be used to provide a proper digital signal. All that said, in the mentioned field (micro gas turbine engines), common sense is to do without optical measurements if possible at all and rather use magnetic / inductive systems. This is simplified by the fact that usually it's not necessary to measure RPM of the engine rotor right down to zero. In applications where the engine is driving an alternator, a dedicated RPM pickup obviously isn't necessary. |
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