THE DAMPING SYSTEM OF THE BROWN COMPASS
In both the Sperry and the early Anschütz compass the natural
oscillation of the sensitive element about the vertical axis is damped
by applying a retarding moment to the element about this same axis,
the strength of the retarding moment being at all times proportional
to the velocity with which the element is moving in the course of its
vibration. The opposition exercised by the retarding force is not,
however, a direct one. The motion of the sensitive element arises from
the fact that the spinning wheel, the axle, and the pendulous weight
are tilted about the horizontal axis E F. The retarding force is
applied about the vertical axis, not because that is the axis on which
the sensitive element is oscillating, but because a force so applied
produces a movement about the horizontal axis tending to wipe out the
tilt and so eliminate the cause of the oscillation.
[Illustration: FIG. 22. Gyro-Pendulum with Axle Tilted.]
An alternative method of damping the oscillation is possible. The
effectiveness of the pendulous weight may be damped by virtually
reducing the weight of the pendulum bob instead of reducing the tilt
at which it is acting. In Fig. 22 the wheel of a gyro-pendulum system
is shown tilted. The weight S is trying to turn the wheel on the
horizontal axis E F in the direction R, and as a consequence is causing
precession about the vertical axis H J in the direction T. Instead
of attempting to damp this precession by applying a force about the
vertical axis so as to reduce the tilt, we might virtually reduce the
weight S by applying an upward force W at the end B of the axle, or,
what is the same thing, a downward force V at the other end. Such a
force, if it were acting alone, would tend to turn the wheel in the
direction opposed to that of the arrow R, and would consequently cause
the wheel to process on the vertical axis in the direction opposed
to that of the arrow T. Acting in conjunction with the weight S, the
force W would thus damp the natural motion by endeavouring to produce a
counter precession of the wheel about the vertical axis.
This alternative method of damping the oscillations is adopted in
the Brown gyro-compass. The mechanism of the Brown method is shown
diagrammatically in Fig. 23. The wheel, as in the early Anschütz
compass, runs in an enclosing case, and acts as a blower, developing a
pressure of about 3 in. of water. At each side of the casing a bottle
L, M is attached to the housing of the axle. A pipe N connects the
bottoms of the bottles, while a second pipe P joins their upper ends.
At its mid point the pipe P is interrupted, and a box Q is inserted
in the gap. This box, as shown separately, is open on the underside,
and is provided with a central partition. The air from the casing is
delivered through the hollow trunnion F, and passes upwards into the
box Q from the orifice R.
When the axle is horizontal the air blast entering the box Q is divided
into two equal portions by the central partition, and exerts an
equal pressure inside the bottles. These two bottles are half-filled
with oil, which under the equal air pressures and the horizontal
configuration of the system lies at an equal level in each bottle, and
does not therefore unbalance the sensitive element on the axis E F.
If, however, the axle tilts the balance is disturbed. Thus if the end
B rises the box tilting with the wheel will assume some such position
relatively to the nipple R--which is free of the trunnion F--as that
shown at T. The central partition will divide the air jet unequally,
and a greater pressure will therefore be exerted inside the bottle L
than in the bottle M. Thus the oil level in L falls, while the oil
level in M rises. The difference in the weights of the two bodies
of oil acts as a turning moment tending to rotate the casing about
the horizontal axis E F in the direction U--that is to say, in the
direction opposed to that in which the pendulum weight S is trying to
turn the casing. Since the pendulum weight causes precession about H J
in the direction V, the weight of the unbalanced oil will cause or
tend to cause precession about H J in the reverse direction.
[Illustration: FIG. 23. Damping System of Brown Compass.]
Should the end B of the axle fall, the air jet will exercise excess of
pressure in the bottle M, the oil level will rise in the bottle L, and
as a consequence will tend to make the system precess about H J in the
direction of the arrow V. Thus in both cases the precession which would
be produced by the unbalanced weight of oil if it acted alone is in the
reverse direction to that produced by pendulum weight when the axle
tilts under the rotation of the earth.
As the excess of pressure in either bottle varies with the ratio in
which the central partition of the box Q divides the air blast, and
as that ratio varies with the angle of tilt, it follows that the
opposition of the applied precession increases with the angle of
tilt, and therefore with the rate at which the sensitive element is
turning on H J under the influence of the pendulum weight S. The system
therefore fulfils the requirements of a satisfactory damping method.
It may be noted that were the air blast and the box Q suppressed and
the pipe P made continuous, a tilt of the axle would cause oil to flow
from one bottle to the other in the endeavour of the liquid to preserve
its surface horizontal. This flow might be constricted by means of a
valve in the pipe N, and would then serve to damp the oscillations,
for the arrangement would constitute virtually an oil dashpot. This
plan would not prove satisfactory in practice, because the tilts which
are attained are so small that the amount of oil flowing between
the bottles would be insignificant. Further, the flow would almost
certainly lag behind the movement to be damped. The application of the
air blast to the surface of the oil greatly increases the rate of flow
of the oil and correctly synchronises it with the tilt. The extent
to which the damping effect is allowed to act on the system can be
regulated by means of a needle valve in one of the bottles, which valve
is made to control the orifice in the bottom of the bottle through
which the pipe N enters.
The damping system adopted in the latest form of Anschütz compass
cannot very conveniently be described at this point. It will be dealt
with later on in Chapter XVII. For the present it is sufficient to say
that the system used is in principle very similar to that found in the
Brown compass.