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 _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
