Note: Descriptions are shown in the official language in which they were submitted.
SMALL STEPPING MOTOR DRIVEN WATCH
BACKGROUND OF THE INVENTION
This invention relates to an improved movement for a
small two-hand quartz analog wristwatch, and more particularly
relates to such a movement designed to be driven by a Lavet
stepping motor with a low starting torque drive.
It is known that a quartz analog wristwatch movement
can be designed to use only a minute hand and hour hand with
the minute hand mounted on the same shaft as a center wheel and
to index the center wheel once per minute with a
quartz-controlled rocking motor. Such a construction is shown
in U.S. Patent 4,128,992--Egger, et al, issued December 12,
1978. Inasmuch as the rocking motor required two shunt plates
for the magnet and placed the gear reduction assembly between
the center wheel and the dial, the watch movement tended to be
rather thick.
An improved two-hand watch movement for a stepping
motor providing a thinner watch construction placed the center
wheel just below the dial and inverted the gear reduction
assembly in such a manner that the minute hand is disposed on
an inner sleeve and the hour hand is disposed on an outer
sleeve, both journaled on a fixed center post. This
arrangement was unconventional in locating the hour hand above
the minute hand with respect to the dial of the watch and
greatly reduced the watch thickness. Such a construction is
d ~0
shown in U.S. Patent 4,443,112--Stotz, et al, issued April 17,
1984. Due to the fact that this movement utilized a stepping
motor which advanced the rotor only a fraction of a revolution
each step, the center wheel required very small teeth and high
starting torque at each step, hence, greater power consumption
of the energy cell.
The power minimizing advantages of a Lavet rnotor, ~,lith
a bi-polar rotor and a one-piece stator construction, wherein
the rotor makes a 180 degree rotation each step is known in the
prior art. Examples of such motors are shown in U.S. Patent
4,398,107--Fukushima, issued August 9, 1983; in U.S. Patent
4,412,144--Denisov, issued October 25, 1983; and in U.S. Patent
4,335,454--Yamada, issued June 15, 1982.
One of the disadvantages of this type of stepping
motor is its low starting torque. One way to overcome this
problem is to prevent engagement of either of two pins driven
by the rotor until the rotor has moved through a given angular
interval, as shown in U.S. Patent 4,084,403--Kitai et al.,
issued April 18, 1978. Another type of drive for a bi-polar
stepping motor using two pins to drive a toothed wheel where
the rest position of the pins block the wheel during rest is
shown in U.S. Patent 4,357,695--Bachmann, issued November 2,
198~ and in PCT Application, PCT/EP79/00~5, filed 30 March 1979
and published 15 November 1979 as W079/00930.
Lastly, U.S. Patent 4,483,627--Muller, et. al., issued
November 20, 1984 discloses a bi-polar Lavet motor with a
two-pin drive directly engaging the teeth of a seconds wheel
driving a conventional gear train for a three-hand watch.
a
Suggestions in the patent for producing a two-hand fetch b~f
altering the construction shown in the patent do not serve to
reduce the thickness of the movement with the several layers of
gearing shown.
Accordingly, one object of the present invention is to
provide an improved thin, two-hand movement for a quartz analog
wristwatch.
Another object of the invention is to provide an
irnproved two-hand watch movement suitable for a Lavet bi-polar
stepping motor.
Still another object of the invention is to provide an
improved two-hand quartz analog stepping motor watch movement
with a minimum number of components to reduce its cost.
DRAWINGS
The invention, both as to organization and method of
practice, together with further objects and advantages thereof,
will best be understood by reference to the following
specification, taken in connection with the accompanying
drawings, in which:
Fig. 1 is a plan view of the improved small stepping
motor watch movement, omitting details of the electrical
circuit for driving the stepping motor, and
Fig. la is an enlarged plan view of the stepping motor
at point of engagement with the center wheel teeth.
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Fig. 2 is a developed, horizontal cross-sectional vie"
taken along zig-zag lines II-II of Fig. 1.
SUMMARY OF THE INVENTION
A timepiece movement has a plastic movement frame, a
dial, an energy cell, a stepping motor with a stator and a
rotor, and a driving circuit connected between the energy cell
and stepping motor to periodically step the rotor. The
movement frame includes first means rotatably mounted in the
frame carrying an hour hand and an hour wheel thereon; second
means rotatably mounted around the first means and carrying a
minute hand, a center wheel overlapping the energy cell, and a
center pLnion thereon; and reduction gear and pinion rotatably
mounted In the frame with the reduction gear engaging and
driven by the center pinion and the reduction pinion engaging
and driving the hour wheel, the aforesaid combination being
shown in U.S. Patent 4,443,112. The improvement herein
comprises a center wheel having teeth with radial slots
therebetween, the stepping motor rotor having a pair of driving
pins~disposed at rest position in adjacent center wheel radial
slots the driving circuit being arranged to step said rotor
once~per minute, whereby the minute hand is stepped once per
minute by the Lavet stepping motor under conditions of low
starting torque to minimize power consumption. End shake
control members with small diameter pin journals reduce
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friction throughout the gear train.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Fig. 1 of the drawing, the mo~tement
is defined by the outlines of a plastic movement frame
defining recesses therein to house an energy cell 2 and a
printed circuit board 3, both shown in phantom lines, The
printed circuit board carries the conventional integrated
circuit,- discrete components such as the quartz crystal, and
spring contacts for making contact with the energy cell and a
watch stem pushbutton indicated by reference number 4.
A Lavet bi-polar stepping motor, shown generally at 5,
comprises a coil 6 having its leads (not shown) connected to
output terminals (not shown) on the printed circuit board 3.
The integrated circuit is arranged to deliver driving pulses
once per minute in a manner well known in the art. Passing
through the center of coil 6 is a core 7, having its opposite
ends overlapping and connected in magnetic circuit with a
one-piece stator 8. Core 7 and stator 8 are stamped from
magnetically permeable material. Stator 8 is of a type known
in the prior art comprising a central opening into which is
inserted a box assembly 9 of non-magnetic material housing a
plastic rotor molded over a bi-polar permanent magnet (not
shown). The stator is provided with diametrically opposed
notches 8a, 8b providing narrow paths of high magnetic
reluctance which become saturated during each driving pulse,
causing the rotor to step 180 degrees and then stop to await
the next pulse. The plastic end of the rotor, seen at 10,
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extends from the end of rotor box 9 and includes two parallel
plastic pins lOa, lOb arranged to drive a center wheel 11 on
the watch movement. Center wheel 11 preferably has 60 teeth
defined between 60 radial slots lla. As seen in the enlarged
view of Fig la, the rest position of the rotor 10 between steps
is defined such that two pins lOa, lOb are disposed in two
adjacent radial slots lla between rotor steps, thereby blocking
the center wheel against unwanted movement. The center heel
11 is directly connected to a center pinion 12. Center pinion
12 drives a reduction gear and pinion assembly 13 rotatably
supported in the frame, having a driven reduction gear 13a and
a driving reduction plnion 13b, the latter meshing with an hour
wheel 14. The gear ratios provide a 1:12 reduction between
center wheel 11 and hour wheel 14. Hour wheel 14 is mounted on
a central shaft 15 which is rotatably supported in the plastic
frame and journaled in a special bridge bearing 16. The
arrangement of the gearing members is best seen by reference to
the developed cross section of Fig. 2.
Referring to Fig. 2, the plastic movement frame 1
supports~a watch dial 17 and includes posts 18 which are used
to secure the stator and core piece of the stepping motor to
the frame. This is done by upsetting the post end at 18a by
heat forming.
An hour hand 19 is carried on a bushing l9a fixed on
the center shaft 15, and a minute hand 20 is carried on the
center pinion 12. In this arrangement, the hour hand is above
the minute hand with respect to dial 17.
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The hour wheel 14, center shaft 15 and hour hand 19
comprise first means rotatably mounted in the frame. Shaft 15
is journaled at two spaced locations 21, 22 of very small
diameter, and hence comprise low friction mountings. Fur
example, the diameter of shaft 15 is only on the order of 0.2
mm. Center wheel 11, center pinion 12 and minute hand 20
comprise second means rotatably mounted around the first
means. The center pinion is journaled at small diameter,
spaced bores 23, 24 in the center pinion, therefore comprising
low friction journals for the center wheel 11. Axial wend
shake) movement for the first rotatably mounted means is
controlled by the hour wheel 14 clearance between end s'nake
control members comprising the frame 1 and bridge bearing 16.
Axial (end shake) movement for the second rotatably mounted
means is controlled by the center pinion 12 clearance between
end shake control members comprising the bridge bearing 16 and
hour hand pushing l9a.
The 1:12 reduction gear assembly 13 is mounted on a
fixed small diameter pin 25 carried in the plastic frame. End
shake movement of the assembly 13 is controlled between end
shake control members ,comprising the frame 1 and a portion of
the PCB 3. The bridge bearing 16 is axially located on shaft
15 between the point where the gear teeth of pinion 13b mesh
with the teeth of hour wheel 14 and the point where the teeth
of gear 13a mesh with teeth of center pinion 12. The foregoing
provides a very compact low friction arrangement for the gear
reduction between the first and second rotting assemblies.
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OPERATION
In operation, the stepping motor rotor is stepped 1~0
degrees once each minute to advance the center wheel by l/~0th
of a revolution, thereby moving the minute hand 20 by one
minute at each step. With the two-pin rotor drive, the
stepping motor commences each step with a low starting torque,
because at the commencement of the step, there is no initial
resistance by the teeth of the center wheel. The center pinion
12 drives the hour wheel 14 through a 1:12 gear reduction.
Since the journals for the reduction gear assembly, center
wheel pinion and the central shaft 15 are all small diameter,
having end shake control members there is a minimal bearing
friction when the rotor commences its step, and hence low
overaIl power consumption by the movement.
While there has been described what is considered to
be the preferred embodiment of the invention, other
modifications will occur to those skilled in the art, and it is
desired to secure in the appended claims, all such
modlfications as fall within the true spirit and scope of the
invention.
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