Re: Optical position sensing, minimizing jitter

CC wrote:
Hi:

I have to design an improved version of a rotating wheel optical position sensor. There is a slit of about 1mm width at 44mm radius on the wheel with tangential velocity of about 110m/s rotating at 400Hz.

The past version used a IRLED shining through a similar slit onto a PIN3CD photodiode through a 350Hz wheel. The rest of the circuitry is shown here:

http://web.newsguy.com/crcarl/images/shutter-pd.png
(snip)
3. Other?

Since you do not need anything like a linear measure of the light level, but only a time that it passes through some fixed amplitude, you might think about how you can use a gracefully saturating amplifier in place of the opamp to magnify that part of the optical signal. Opamps are inherently slow, to allow closed loop operation, and you have no need of that. Think about the choices based on common base amplifiers or emitter coupled logic or limiter amplifiers. These all run open loop, are very fast, and handle overload without slowing down, much.
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Relevant Pages

  • Optical position sensing, minimizing jitter
    ... There is a slit of about 1mm width at 44mm radius on the wheel with tangential velocity of about 110m/s rotating at 400Hz. ... but has a lot of gain which was needed to deal with the relatively low light reaching the detector from the LED. ... Since the beam power and alignment will change very little once it's all set up, tune the gain of the transimpedance stage or the optics to give a pulse of peak amplitude 'A' and set a fixed comparator threshold of A/2. ...
    (sci.electronics.design)
  • Re: Optical position sensing, minimizing jitter
    ... There is a slit of about 1mm width at 44mm radius on the wheel with tangential velocity of about 110m/s rotating at 400Hz. ... Multimode devices produce focused spots that dance around at megahertz rates, which will be a significant source of jitter and 1/f noise. ... Single longitudinal mode will help too, but is less important than a single spatial mode, since the total intensity noise is typically *dramatically smaller* than the noise of any individual mode. ...
    (sci.electronics.design)