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    Optocoupling on a rotor disc

    I need to setup a switching device that gets its input from a high speed 3000 - 5000rpm rotating disc. Rotor disc is about 1.5 feet in diameter. Readings are to be made on the outer perimeter.

    My fear is mechanical commutator style switch is to slow, and not robust enough, there may be issues with vibration on the rotor.
    The rotor has large permanent magnets on it for a generator, so hall effect sensing is out of the question, for there would be major magnetic interference.
    So the next thing I thought was opotocoupling...having a light beam directed at the rotor and putting some reflective strips on the rotor disc to trigger switching. Ideally I would want the signal to travel optically for a foot, out of the high magnetic area where the generator action is taking place for fear of interference.

    I have been looking at optocouplers online, but having some difficulty making heads or tails out of whats available given what I want to do. Is what I want perhaps, more like a laser detector or optical scanning?

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    Re: Optocoupling on a rotor disc

    Firstly - what are you trying to measure? I'm guessing the speed of rotation.

    It may well be that Hall effect switches are the best option as they should be able to detect the magnets from a safe distance, failing that a simple inductive pick-up pf a few turns of wire should induce enough voltage that it can be filtered and used to measure frequency. If you want to do it optically, a scanner is probably the least appropriate technology because your would be periodically checking a moving object and strobing is likely. A better optical method would be to fix a small mirror or reflective tape to the rotor and shine an IR LED at it. Use an IR detector to look for flashes reflected from the mirror. Do not use a remote control type of IR receiver as these will only respond to modulated carriers, the kind you want are simple IR photo-detectors.

    Brian.
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  3. #3
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    Re: Optocoupling on a rotor disc

    Quote Originally Posted by betwixt View Post
    Firstly - what are you trying to measure? I'm guessing the speed of rotation.

    It may well be that Hall effect switches are the best option as they should be able to detect the magnets from a safe distance, failing that a simple inductive pick-up pf a few turns of wire should induce enough voltage that it can be filtered and used to measure frequency. If you want to do it optically, a scanner is probably the least appropriate technology because your would be periodically checking a moving object and strobing is likely. A better optical method would be to fix a small mirror or reflective tape to the rotor and shine an IR LED at it. Use an IR detector to look for flashes reflected from the mirror. Do not use a remote control type of IR receiver as these will only respond to modulated carriers, the kind you want are simple IR photo-detectors.

    Brian.
    I am not trying to measure anything. I want to use the output for switching.

    IR photo-detectors, seems to be more in line with what I am looking for.
    I basically want the optical equivalent of a hall effect sensor, for timing/switching purposes.



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    Re: Optocoupling on a rotor disc

    Go to this web site: uk.farnell.com then type "reflective opto" in the search bar at the top, it will take you to a page full of different reflective sensors. I'm not sure where you are but Farnell have distributors in most countries. These are probably what you need, the only other requirement is one or more reflective surfaces to bounce the IR beam back from. You can either use a mirror or if it's mass might upset the balance, try reflective sticky tape as found in gift shops. You can adjust the sensor position or even use more than one sensor/reflector if you have to detect different angles of rotation.

    Brian.
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    Re: Optocoupling on a rotor disc

    Is it possible for you to attach an optical be chopper to the spindle of the rotor?
    Last edited by resurgence2012; 10th April 2012 at 17:26.



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    Re: Optocoupling on a rotor disc

    Quote Originally Posted by betwixt View Post
    Go to this web site: uk.farnell.com then type "reflective opto" in the search bar at the top, it will take you to a page full of different reflective sensors. I'm not sure where you are but Farnell have distributors in most countries. These are probably what you need, the only other requirement is one or more reflective surfaces to bounce the IR beam back from. You can either use a mirror or if it's mass might upset the balance, try reflective sticky tape as found in gift shops. You can adjust the sensor position or even use more than one sensor/reflector if you have to detect different angles of rotation.

    Brian.
    With the optical reflective sensor I am concerned about getting a noisy reading due to the "spraying out" of light on the reflective material, and/or any small vibration on the rotor. I want to be able to make readings with millimeter precision.

    Quote Originally Posted by resurgence2012 View Post
    Is it possible for you to attach an optical be chopper to the spindle of the rotor?
    An optical chopper is a great idea, do you know of one where it would be easy to adjust the parameters in order to tune timing?



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    Re: Optocoupling on a rotor disc

    A reflective opto switch should give very high precision and is almost immune to interference. A chopper is a good solution but you have to add extra hardware to the edge of the disc which, depending on your application may cause it to lose balance. It does not guarantee any more accuracy than a reflective switch as there will still be a rise and fall of light as the chopper partially shadows the sensor.

    Brian.
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    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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