How do modern pendulum clock geeks measure what their pendulum is doing? I'm picturing a magnet on the bottom of the pendulum and a coil or hall effect sensor.
Has anybody considered using disk/tape read heads? The catch is that you need the read head to be very close to the recording surface. The closer the better, the bit density is simple geometry. The size of a bit scales with the distance from the head and that turns into timing accuracy. (I think.) You also have to find a head setup to work with your distance (aka bit density). Part of the idea is to use a sequence of pulses rather than a single pulse. If you use a simple square wave as the pattern, you might be off by a cycle, but you can use a more complicated pattern to avoid/reduce that. I know the radar guys have collections of patterns with good correlation results. That math is probably used in other places. Maybe just shifting by a half cycle in the middle is the right (or good enough) approach. The pattern would be ...1010101001010101... There is probably a square-root on the number of bits you use vs the timing accuracy of a single bit. 100 bits per inch is the right ballpark. 1960s tape technology was 800 bits per inch, but that was with the tape in contact with the head. Their may have been an air bearing when it was moving, but the tape was very close to the head. Anybody who is working on a pendulum clock and crazy enough to consider this idea has probably already got the length of the pendulum under control. With a fixed mount for the head, this means they also have to control the length of the support arms. If you don't like that, there is probably some way to servo the height of the read head. A variation on this would be optical as in CD reader technology. Put a pattern on the bottom of the pendulum and then shine a light (IR?) on it and detect the reflections. How far is the CD read mechanism above the surface of the CD? Is it "flying" with an air bearing like disk heads? Another variation is the head position sensing using by disks back in the 60s. There was a chunk of glass on the disk arm with N rows of black/clear square waves at different frequencies with a light on top and sensor below. (I assume they were gray coded.) For a pendulum, you could attach a piece of sheet metal that stuck out the side a bit, punch if full of holes and have that swing through a LED/detector setup. The optical guys make 4 quadrant detectors. I think they are used for servoing things in 2 dimensions. 2 of those quadrants would make a nice 1 dimensional detector: you get to compare the output of the two sections rather than needing to invent a threshold. Maybe one of the linear arrays of photodetectors used in scanners with the pendulum swinging down the array so it can see the pattern of lines (bar codes) on the bottom. Or a digital camera: take a sequence of pictures and interpolate as to when it crosses the center. 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. :) -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
