[time-nuts] Sensing pendulum position, speed, or height
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Thu Mar 29 23:48:23 UTC 2007
Hal Murray wrot
> The hardware used for bar code scanners might be a useful starting place. I
> assume you would have to hack the firmware/whatever to output time/position
> info rather than bar code data.
>
>
>
> My initial thought was that you would put one read head directly under the
> middle of the pendulum path. That gives you a "tick" each half cycle.
>
> With two sensors, I think you can measure the height of the swing. It's not
> measuring the actual height but relative to some target. 2 sensors gives you
> 4 chunks of time per cycle: A-B, B-B, B-A, and A-A. If you position the
> sensors along the path symmetrically on opposite sides of the center then A-A
> + B-B can match A-B + B-A and you can servo the kicker to produce that. If
> you want the swing to be higher, move A and B farther apart. If they are off
> center, A-A will be different from B-B and the servo filter will have some
> lower frequency junk to filter out.
>
> Sounds like a fun tar pit. :)
>
Hal
A typical bar code sensor (eg HBCS1100) has a photo transistor output
and a transition region about 200um wide.
Obtaining sufficiently stable gain to interpolate reliably will be
difficult. A sensor like a HEDS1500 which uses a photodiode sensor would
be a better choice as it is intended for use with a transimpedance
amplifier. The gain of such a sensor will be more stable and
interpolation by a factor of 10 or more should be feasible.
However a better approach is to use a grating in front of the sensor and
a matching one on the end of the pendulum. resolution is then limited
only by the grating period, the interpolation technique and not the
sensor size. If one uses a pair of gratings and sensors with a
displacement of an odd integral multiple of a 1/4 the grating period
between the 2 detector gratings then interpolation to better than 1/100
of a grating period is relatively easy. Averaging over several grating
lines reduces the sensitivity to grating irregularities.
In fact a linear optical encoder (either incremental with an index
position or absolute) would be easier to use and some encoders are
capable of submicron resolution.
Bruce
More information about the Time-nuts_lists.febo.com
mailing list