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Thread: LIDAR-lite 360 Mirror Scanner

  1. #31

    Re: LIDAR-lite 360 Mirror Scanner

    Sounds like you're well served by the ROS architecture!

  2. #32
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    Re: LIDAR-lite 360 Mirror Scanner

    Ah yes most definitely!

    I've tweaked my 3D model a bit and I'm pretty happy with where it is now so I'll likely release the 3D files soon.

    Click image for larger version. 

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    I realize the first sign of life from ROS needed a bit of improvement and at the moment I'm contesting with some faulty readings ( sensor RMA in process ) but when the sensor is cool the accuracy looks pretty good. So this is the best video of the ROS visualization I can share for now. When my new sensor arrives I'll be able to experiment more and we'll see what may come!

    01001001001000000100110001101111011101100110010100 10000001010010011011110110001001101111011101000111 0011

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  3. #33
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    Re: LIDAR-lite 360 Mirror Scanner

    Two quick questions:
    1. Any thoughts yet on adding a 2nd axis?
    2. Any thoughts on a pop-up (or blade-style, just turn edge on to make it dissapear) or fixed (yeah, causes the same narrow angle blockage) body-mounted reference pin? (Deployed up into the FOV)...multiple scans would allow very good centroiding, and it would allow a true closed loop periodic realignment between the body axis and the scanner position. That way, you could lighten-up the control code for the stepper, potentially run it faster without any accumulated angle errors due to gear tolerance or external influences (slip, rotation when not on, etc) and allow self-calibration. Just a thin metal blade (with retroreflective film if needed) that gets flipped perpendicular to the beam when you want to calibrate...you could even actuate it with a small SMA wire to really make it unobtrusive /lightweight.

  4. #34
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    Re: LIDAR-lite 360 Mirror Scanner

    Actually no to both of those

    I think number two is interesting and while pondering the idea I thought about the use of a magnet and a reed switch. This way nothing has to obstruct the view and nothing has to be actuated. While I'm waiting for my replacement sensor I think I'll investigate more. Thanks for the idea boost!
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  5. #35
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    Re: LIDAR-lite 360 Mirror Scanner

    Quote Originally Posted by r3n33 View Post
    Actually no to both of those

    I think number two is interesting and while pondering the idea I thought about the use of a magnet and a reed switch. This way nothing has to obstruct the view and nothing has to be actuated. While I'm waiting for my replacement sensor I think I'll investigate more. Thanks for the idea boost!
    Problem with sensing the gear rather than the actual optical surface is you can't percieve misalignment. Think of it this way, the angle you care about and want to know is the rotational angle of the mirror w.r.t. The body, not the gear, not the motor. The mirror directs the beam, so by sensing the beam position (in this case, by having a "known" part of the body be able to be sensed) you eliminate any possible errors in misalignment, etc.
    a hall type sensor on the gear is still interpreted--mirror to gear misalignments can't be sensed.

    Also, by using the sensor itself as the calibration source, you also don't need any additional inputs/code local. You just need a single, very rarely used DIO to flip the blade, and then your normal sensor becomes its own calibrator. Low frequency DO >>> high fidelity AI w.r.t. Component utilization and code requirements.

    (just my thought on what lead me down the path...the hall/magnet is also good in a few ways as well, unobtrusive, cheap, doesn't require activation, etc...if your mirror to gear alignment is stable, then it's good)

  6. #36
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    Re: LIDAR-lite 360 Mirror Scanner

    Also, with a blade of a very known width, you should be able to not only centroid it to well < a step size on your stepper, but also get enough steps across it to get statistics on step size over time to account for gear wear, etc. basically, it's the difference between measurement and dead reconning--a known object in body space measured by the sensor is closed loop calibration (all errors from sensor back to body are addressed), while something measuring not the actual optical axis, with a different sensor is effectively only one step better than dead reconning on the motor steps, it can only remove errors that occur between it and the drive.

    You ou coukd do it with a fixed pin, giving up some part of your FOV for calibration is industry standard (dark/white rows on a CCD, internal cal lamps / retro reflectors on FLIR/laser turrets, etc. ideally you always want to calibrate a sensor with itself, as that removes the maximum number of error sources.

  7. #37
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    Re: LIDAR-lite 360 Mirror Scanner

    Also, if you used a tapering blade, you could actually measure both axes, or in the current one-axis rotation, determine any elevation changes (by measuring both the centroid and width)

    and nd any thoughts to lateral mirror supports? Withh he gear setup you have, you coukd make a whole 3-sided box for the mirror. Would add stability and reduce stray light. You could print it as a second part, that would then bolt to the same bolts as the gear to bearing...also, that way the mirror-to-gear alignment is on the bolts, not part of the print.

    sorry for all the posts...I'm likely going to build a version of this myself in the next few weeks, so I'm thinking a lot about it.

  8. #38

    Re: LIDAR-lite 360 Mirror Scanner

    step size over time to account for gear wear
    Step size doesn't change over time with wear. For this kind of assembly, backlash will increase with wear, but nothing else in the formula changes.

    If the gear has a known, integer ratio to the body (such as 1:1) then a hall effect or optical sensor on the gear is almost exactly the same as a field-of-view pin along the axes of concern (registration, robustness, etc.)
    Yes, you need to express the relation between the sensor and the mirror-body-forward -- but this is no different than expressing the relation between the pin and body-forward. It's a single quantity, that you measure (or hard-code) once.

  9. #39
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    Re: LIDAR-lite 360 Mirror Scanner

    Not quite, you have three error sources, sensor to body, magnet to sensor, mirror to magnet. The only one you measure in the hall case is magnet to sensor, the rest are assumed to be small (and unmeasured) By measuring a body element with the sensor, you have one combined error source, mirror to body, and that is directly measured. Yeah, it probably doesn't make much of a difference, but having a small occultation (if fixed) with a tapered element minimizes unmeasured or characterized error sources and also gives you a vertical (elevation) alignment--you now can directly measure in the sensor frame the elevation angle of the measurement path--since it is then based solely on taper profile (doesn't change and can be measured extremely precisely) and distance to optical/rotation axis (can be measured to great accuracy and fixed).

    dont worry, I'm not criticizing your solution (an ideal system would use both, the hall for continuous readout/on the fly cal, and the blade for periodic recalibration/elevation compensation), just suggesting a calibration method that eliminates unmeasured/U characterized errors and potentially gives you greater precision...likely not needed, but a thought to keep in mind, especially if you add a second axis, as the tapered blade would allow you to characterize the full covariance matrix of the two axes (and also, if you had two axes, you could then use a fixed blade that was out of the normal useful FOR)

  10. #40

    Re: LIDAR-lite 360 Mirror Scanner

    The calibration is the same in both cases. If you think that the magnet will slip, the sensor will slip, the gears will slip, or the mirror will slip, then I'd argue that the most likely cause of that would be catastrophic failure, when neither solution will work very well!

    If you use frequently slipping mounts (like clamps or whatever) then there's more chance that things might go out of alignment after calibration, but why would you do that when through holes and lock nuts are so easy? :-)

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