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An Electronic Clock Experiment
Electronic Hipp Toggle.
Concept:This Clock is an experiment to replace the mechanical Hipp Toggle arrangement of sensing when a pendulum needs impulsing by an electronic version using a light beam doing the same job. This removes the mechanical interference of the toggle on the pendulum. It is only in an experimental state at the moment, with electronics on a bread-board for easy development (different slave types or sensor arrangements, for example). A Gent's Pulsynetic master clock has provided the case and pendulum. Although similar experiments have been done in the past, I had not heard nor read of any of them when I designed this clock. The principle of the mechanical Hipp Toggle is explained very clearly on Werner Moser's web site.
Original Design:
An opto sensor (IR LED and Photo transistor) is placed to the right
hand side of the pendulum, and the light path is interrupted by a flag
attached to the pendulum rod. The duration of the interruption is used
to detect when the arc of swing of the pendulum reduces below a defined
amount. When this happens, an impulse is given to the pendulum by an electromagnet
on the left side of the pendulum. A second opto sensor placed on the centre
line of the pendulum is interrupted by another part of the same flag to
give 1-second pulses to drive the slave dial. The minimum arc of swing
of the pendulum and the point at which the impulse starts is determined
by the position of the right hand opto sensor with respect to the centre
line of the pendulum. Impulsing occurs on the right-to-left swing of the
pendulum. A separate timer circuit controls the duration of the impulse.
Developed Design:
The functions of the two opto sensors can be combined into one sensor
on the centre line of the pendulum, but this means that the pendulum impulse
could occur in either direction of the pendulum's swing, although in practice
impulse settles to one side or the other. The impulse coils were moved
to below the pendulum bob to give satisfactory impulse in both directions.
The light sensor and the interrupting flag were also moved to the bottom
of the pendulum to give more accuracy. The flag now interrupts the light
beam for most of the time, only exposing it at the extreme of its swing
in both directions. Because a partial reduction in the amount of over-swing
is being detected to trigger an impulse, there will always be an
exposure of the light beam, and therefore a pulse from the phototransistor,
on each swing of the pendulum, and these pulses are now used as seconds
pulses to drive the slave dials. The width of the flag, which needs to
be exactly equal both sides of the centre line of the pendulum, determines
the minimum arc of swing, and the amount of over-swing beyond the flag
(exposing the light source to the photo transistor) is kept as small as
possible. The duration of the over-swing is compared to a fixed timer to
determine whether impulse is needed. Increasing this over-swing time will
increase the pendulum's arc of swing, and can be used to rate the clock
by circular error. Having sensed that impulse is required, a timer delays
the onset of impulse until the pendulum has nearly swung back to its centre
line, and another timer determines the length of the impulse. A variable
resistor in series with the impulse coils can also help control the energy
imparted to the pendulum if needed. The clock has now settled with a very
brief impulse every third complete swing. Initially the phototransistor
was not shielded from ambient light, and variations in light intensity,
particularly when the case door was opened, slightly affected the sensing
of the pendulum arc.
Slave Dials:
Circuits for several slave dials have been successfully tried, including
the original Gent's 1/2 minute slave from the clock that supplied the case
and pendulum; a Gent's alternate polarity seconds slave (both high and
low impedance versions); and a modified Quartz seconds clock (just driving
the stepper motor).