Re: [time-nuts] Sensing pendulum position, speed, or height
Hal Murray wrote: > 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. > > > > > 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? > Hal A CD head is a non contact device. The head vertical position is servoed so that the recording layer is at the focus of a lens. No air bearings are required. The clearance can be several millimeters. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
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 time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
On Thu, 29 Mar 2007 15:45:25 -0700, Hal Murray <[EMAIL PROTECTED]> wrote: > >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. Prowl around here http://www.hsn161.com/links.html Particularly http://www.precisionclocks.com/ This guy's work is jaw-dropping. Fused quartz pendulum and arm, hall effect position sensing, running the whole affair in a vacuum, having a custom glass vacuum chamber fabricated by a scientific glass shop. Not pendulum related but look at this guy's work. http://www.angelfire.com/sd/rronnie/ My jaw dropped again. John --- John De Armond See my website for my current email address http://www.neon-john.com Cleveland, Occupied TN All great things are simple and many can be expressed in single words: Freedom, Justice, Honor, Duty, Mercy, Hope. -Churchill ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
> A CD head is a non contact device. The head vertical position is > servoed so that the recording layer is at the focus of a lens. No air > bearings are required. The clearance can be several millimeters. Thanks. -- These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
IMHO, gluing magnets to moving parts makes them susceptible to magnetic fields, such as variations in the Earth's field at 10E-7, as TVB pointed out. Had I the means to work with fused quartz and a suitable pivot, I would mount two photocell / LED pairs about 20 to 30% of the bob's swing apart. I'd break the beams with a fine wire, so the cell did not go completely dark. Then I'd work out a program to locate the center of the optical signal and time the interval between those centers. Using an incredibly precise physical adjustment, I'd align it so the outside intervals are equal. Call the sensors A and B. The interval between A going out and A coming back is proportioal to the distance from A to the end of the swing. And so it is for B. Now physically adjust the sensors so that A to A is precisely equal to the interval B to B. If the distances to the ends of the swing are equal, then the center is located at 1/2 the interval A to B. The interval B to A will be exactly the same as A to B if gravity is the only thing accelerating the bob, even if A and B are not centered on the swing. I'd like to try electrostatic impulsing, if I can keep charge from building up on the rod. There'd be no problem timing the impulse. The impulse would be tiny for a high Q system, or the pendulum could run free for many swings before being impulsed. I'd like to reduce the bob mass to 10 grams or so, if I could find the right suspension. Hmm, perhaps an electrostatic suspension? Simple enough to shield the vacuum tank. With precision timing electronics, I don't need a one second swing period. Interesting? Bill Hawkins ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
>> Lunar/Solar Tides and Pendulum Clocks (part 1) >> http://www.leapsecond.com/hsn2006/ch1.htm > > Nice. Thanks. Did you ever write chapter 2? In progress. I'll let you know when it's done. >... > 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. You have lots of good ideas; I'm sure some been tried but I don't have references for you. From what I remember many modern attempts at super pendulum clocks end up using optical sensors. But you can save weeks of prototype work with a few minutes of calculation. For example, given the length of the pendulum and the half-angle amplitude you can calculate the velocity at center swing. Then knowing the geometry of the optical gate gives you the rise-time. If the IR emitter/transistor pair have a circular aperture instead of slit then you're getting a slower rise time as the pendulum crosses the circular beam, etc. One could do a series of experiments to obtain the most precise signal. Or the most stable swing-to-swing samples. But that's only part of the problem... I would also worry about long-term stability of any optical emitter and sensor pair. Any decline in light output over time might appear in the data as a phase shift which could affect amplitude or period stability. Then there's any tempco or voltco issues to consider with optoelectronic elements. By the way, a really nice technical paper on a precision pendulum clock is this 1996 paper by De Marchi: A Measurement of the Period Stability of a Free Pendulum http://www.leapsecond.com/history/1996-DeMarchi-Pendulum-Stability.pdf Clever solution. His optical gap is something like 5 microns. /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
Tom Van Baak wrote: > A Measurement of the Period Stability of a Free Pendulum > http://www.leapsecond.com/history/1996-DeMarchi-Pendulum-Stability.pdf > > Clever solution. His optical gap is something like 5 microns. > > /tvb > > An additional issue is the pointing instability of the laser used. A position sensitive detector (lateral-effect photodiode sensor) for which the output is independent of the beam flux could be used however the noise and drift (typically <=1um/C) may be an issue. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Sensing pendulum position, speed, or height
Tom Tom Van Baak wrote: >> I would also worry about long-term stability of any optical >> emitter and sensor pair. Any decline in light output over >> time might appear in the data as a phase shift which could >> affect amplitude or period stability. Then there's any tempco >> or voltco issues to consider with optoelectronic elements. >> The usual solution to this is to use analog circuitry to calculate (I1-I2)/(I1+I2) rather than just (I1-I2) although the location of the point of equality (of I1 and I2) is independent of I1 and I2 provided that the responses of the 2 sensors track. I1 is the photocurrent from sensor 1, I2 is the photocurrent from sensor 2. > By the way, a really nice technical paper on a precision > pendulum clock is this 1996 paper by De Marchi: > > A Measurement of the Period Stability of a Free Pendulum > http://www.leapsecond.com/history/1996-DeMarchi-Pendulum-Stability.pdf > > Clever solution. His optical gap is something like 5 microns. > Suitable spheres are available from edmund optics. http://www.edmundoptics.com > /tvb > Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
