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Position Index Sensor — Parallax Forums

Position Index Sensor

cavelambcavelamb Posts: 720
edited 2012-10-18 19:23 in Propeller 1
I'm still trying to figure out an index circuit for the ceiling fan POV project,

Magnetics was and interesting idea, but cumbersome in the execution.
I got a couple working but the close proximity required for
correct operation is, in this case, a stopper.

So, moving on, does anyone have an LED/photoransistor
combination they could recommend?

This will be for a reflective setup not more than 2 feet in distance.

Comments

  • SRLMSRLM Posts: 5,045
    edited 2012-10-17 11:57
    I would use an IR LED and detector pair. Take a look on page 153 of the Propeller Education Kit text:

    http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PEKitLabs-v1.2.pdf
  • cavelambcavelamb Posts: 720
    edited 2012-10-17 12:41
    SRLM wrote: »
    I would use an IR LED and detector pair. Take a look on page 153 of the Propeller Education Kit text:

    http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PEKitLabs-v1.2.pdf

    Actually, that's why I was asking here...
    the IR LED and phototransistor are not called out by name...
    Part numbers?
  • SRLMSRLM Posts: 5,045
    edited 2012-10-17 13:02
    It doesn't really matter, as long as they work on the same frequencies (in kHz) and wavelengths (in nm). Here is the pair that parallax sells:

    http://www.parallax.com/Store/Sensors/ColorLight/tabid/175/CategoryID/50/List/0/SortField/0/Level/a/ProductID/177/Default.aspx
    http://www.parallax.com/Store/Sensors/ColorLight/tabid/175/CategoryID/50/List/0/SortField/0/Level/a/ProductID/178/Default.aspx
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-17 15:03
    cavelamb wrote:
    ... the IR LED and phototransistor are not called out by name...
    The detector is not a phototransistor but an integrated circuit that combines an IR photodiode with amplification and a 38 kHz demodulator. Data sent by the IRED to such a device must be modulated at 38 kHz to be detected. The advantage to this method is that the communication is much less sensitive to interference from stray IR sources.

    -Phil
  • YanomaniYanomani Posts: 1,524
    edited 2012-10-17 18:05
    Hi cavelamb

    Reading the datasheet of the suggested component (PNA4601M) I'd noted a minimum 400uS steady detection (16 pulses @ 38 kHz) or its absence to ensure a valid output, unless I was drove to a mistake by the figures in there. I do not know the angular velocity that is to be used in the project to ensure the proposed effect but it seems to me that a outter slotted disc (like a slightly "reworked" ancient hard sectored floppy disc, Heathkit H-89 as a mere example) and a slotted optical switch could do the job in a very efficient manner. The disc could be secured to the axle and the optical switch could be fixed to the upper cover dish of the fan. If balancing is to be concerned it could be easily attained by the use of small masses or, in case of exact detection of blade positioning, by a "one switch per blade" schem. The outter edge of the disc could be cut shaped to allow the generation of any number and position of pulses one can imagine, logicaly taking in account the physical limits and precision of the cuts.

    I hope it helps a little.

    Yanomani
  • YanomaniYanomani Posts: 1,524
    edited 2012-10-17 18:25
    Hi cavelamb

    Maybe CW or CCW rotation detection could be a useful bonus of having two or more detectors.

    Yanomani
  • cavelambcavelamb Posts: 720
    edited 2012-10-18 10:21
    Yanomani wrote: »
    Hi cavelamb

    Maybe CW or CCW rotation detection could be a useful bonus of having two or more detectors.

    Yanomani

    Uh, nope, Not this time.
  • cavelambcavelamb Posts: 720
    edited 2012-10-18 10:35
    Yanomani wrote: »
    Hi cavelamb

    Reading the datasheet of the suggested component (PNA4601M) I'd noted a minimum 400uS steady detection (16 pulses @ 38 kHz) or its absence to ensure a valid output, unless I was drove to a mistake by the figures in there. I do not know the angular velocity that is to be used in the project to ensure the proposed effect but it seems to me that a outter slotted disc (like a slightly "reworked" ancient hard sectored floppy disc, Heathkit H-89 as a mere example) and a slotted optical switch could do the job in a very efficient manner. The disc could be secured to the axle and the optical switch could be fixed to the upper cover dish of the fan. If balancing is to be concerned it could be easily attained by the use of small masses or, in case of exact detection of blade positioning, by a "one switch per blade" schem. The outter edge of the disc could be cut shaped to allow the generation of any number and position of pulses one can imagine, logicaly taking in account the physical limits and precision of the cuts.

    I hope it helps a little.

    Yanomani

    That's kind of the idea, but without the need for close tolerances.
    Just a plain old IR LED (of some given wavelength) and a photo transistor that can "see" it.
    No data encoding involved for this part.

    Hmmm, these same parts could be used to make a reflective tachometer.
    Then I'd actually know how fast the thing is turning.
    But I'm guessing 100 to 300 RPM should be in the ball park.
    3 to 10 microseconds per rotation?

    My thought was a band of black tape around the motor housing and
    a small spot of reflective tape on that to act as the index target.
    All I need is to see the pulse.

    I was thinking if would be helpful to time a couple of revolutions then take that and
    work out the dot clock rate. It seems like it could be more RPM tolerant that way.



    Balancing something turning like that is always an issue.
    I get better results if things are all grouped together rather than strung out all over the place.
    Just a board and battery is pretty easy to calculate.
    But add a couple more parts and wire and it gets messy trying to figure out all the arms and moments.
    Do-able, but messy.
  • cavelambcavelamb Posts: 720
    edited 2012-10-18 10:36
    Phil Pilgrim (PhiPi) wrote: »
    The detector is not a phototransistor but an integrated circuit that combines an IR photodiode with amplification and a 38 kHz demodulator. Data sent by the IRED to such a device must be modulated at 38 kHz to be detected. The advantage to this method is that the communication is much less sensitive to interference from stray IR sources.

    -Phil

    Exactly, Phil.

    But I'm not looking for a communications channel, just a spot...
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-18 10:43
    cavelamb wrote:
    But I'm not looking for a communications channel, just a spot...
    Nonetheless, the modulated emitter/detector pair is still the way to go to avoid interference from things like fluorescent lighting. Even the BOE-Bot encoders that Parallax sells use modulated reflective sensors. All of the reflective and break-beam sensors used in industry are modulated, too, for the same reason.

    -Phil
  • cavelambcavelamb Posts: 720
    edited 2012-10-18 12:40
    Phil Pilgrim (PhiPi) wrote: »
    Nonetheless, the modulated emitter/detector pair is still the way to go to avoid interference from things like fluorescent lighting. Even the BOE-Bot encoders that Parallax sells use modulated reflective sensors. All of the reflective and break-beam sensors used in industry are modulated, too, for the same reason.

    -Phil

    Copy that.

    A good starting place?
    http://www.sbprojects.com/knowledge/ir/index.php
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-18 13:33
    Yes, that's a really good explanation!

    -Phil
  • YanomaniYanomani Posts: 1,524
    edited 2012-10-18 16:11
    cavelamb wrote: »
    Hmmm, these same parts could be used to make a reflective tachometer.
    Then I'd actually know how fast the thing is turning.
    But I'm guessing 100 to 300 RPM should be in the ball park.
    3 to 10 microseconds per rotation?

    My thought was a band of black tape around the motor housing and
    a small spot of reflective tape on that to act as the index target.
    All I need is to see the pulse.

    I was thinking if would be helpful to time a couple of revolutions then take that and
    work out the dot clock rate. It seems like it could be more RPM tolerant that way.

    Assuming, to be conservative, that the circuit will be made to detect something like 32 pulses of 38 kHz (~840 uS) @ worst case 300 RPM (200 mS), then you should use at least 1.5 degree of reflective tape over the black one. Also the right angular alignement between the emitter and the receiver should be ensured to attain a reasonable beam incidence. I hope dust don't pile over the lenses demanding a constant cleanup.

    Yanomani
  • tperkinstperkins Posts: 98
    edited 2012-10-18 19:23
    Maybe a white backgound with a black spiral? Use the RCTime code with a filed down LED almost bearing on the spiral?
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