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Harmonic
Differential HDD |
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The Candy HDD phase shifting
differential incorporates two Harmonic Drive
pancake gearsets, integrated in a back-to-back
arrangement to offset each other's internal
ratio. Each gearset is comprised of three
major elements: a circular spline, a flexspline
and a wave generator. One gearset is attached
to the hollow inner shaft, while the second
is connected to the outer housing of the HDD.
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Figure 1 represents a simplified
view of the torque path through the first
of the two pancake gearsets. Assume that the
circular spline is the input and the wave
generator is stationary. As the circular spline
rotates, the flexspline will be carried along
with it, but because the flexspline has two
less teeth than the circular spline, the flexspline
will advance the two tooth difference with
each rotation of the circular spline. The
gearset, in this arrangement, acts as a slight
speed increaser with the flexspline as its
output. Next, take an identical pancake gearset
and place it in a back-to-back arrangement
at the output of the first gearset. The input
of the second gearset is its flexspline which
is connected to the flexspline output of the
first gearset. The output of the second gearset
is then the circular spline. Its wave generator
is also stationary. The purpose of the second
Harmonic Drive gearset is to act as a slight
speed decreaser which will cancel the effects
of the first gearset, providing a 1:1 transmission.
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The control shaft is attached to the bore
of one wave generator, but because the HDD
has two wave generators, the module has the
ability to correct for two independent variables
at the same time. Most applications, however,
require only one correction input. To visualize
the effect of rotating one control shaft,
refer to Figure 2. Assume
that the flexspline is stationary. With each
rotation of the wave generator, the circular
spline will advance two teeth in the same
direction as the wave generator because the
circular spline has two more teeth than the
flexspline. In this mode, the Harmonic Drive
is functioning as a high ratio reducer. When
the concept of Figure 2 is
combined with that of Figure 1, the result
is a dynamic phase shifting differential with
precise position control characteristics.
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