Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Inertial Platforms
M~ny types of inertial platforms exist, each different type being
intended to operate under a particular set of conditions. At one extreme is
the pLItform having three, or probably four, gimbals and carrying on the inner
gimbal the gyros ne oe ssary for stabilisation and a set of acceler eters.
m is type of platform is very camplex and expensive from a mechanical viewpoint,
requiring slip-ring connections, and gimbal bearings, and being of increased
size and weight. The other extreme is the so-called "strapdown" system in
which the gyros and accelerometers are fixed rigidly to the vehicle frame.
This arrangement is more robust and is simpler mechanically, but the computing
complexity is cQnsiderably greater.
Neither of the two alternatives described above is particularly
cheap, and there is a requirement for a simple law-cost inertial platform. It
is an object of the present invention to provide such a platform.
According to the present invention there is provided an inertial
platform which includes a frame arranged to be secured to a vehicle, an outer
gimbal supported from the frame about a first gimbal axis, an inner gimbal sup-
ported from the outer gimbal about a secand gimbal axis perpendicular to the
first gimbal axis, a pickoff and a torque motor on each gimbal axis, gyroscopic
means mounted rigidly on the inner gimbal and having three mutually perpendicu-
lar sensitive axes, circuit means respQnsive to outputs from the pickoffs and
from the gyroscopic apparatus when the platform is in operation to maintain the
inner gimbal in an attitude in which first and second sensitive axes of the
gyroscopic means are horizontal and the third sensitive axis is vertical, and
a numker of accelerometers carried Qn the inner gimbal and having sensitive
axes parallel to some or all of the sensitive axes of the gyroscopic means.
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T~e invention ~ill now be described with reference to
the accompanying drawings, in which:-
Figure 1 is a schematic view of a platform, showing
an arrangement of the hardware;
Figure 2 is a block circuit diagram of the circuit
means and other components of the platform; and
Figure 3 illustrates the operation of the ciruit
means.
Referring now to Figure 1, a frame fixed rigidly to
the vehicle and shown schematically at lO supports an outer
gimbal 11 for rotation about an axis 12. The outer gimbal is
provided with a pickoff 13 and a torque motor 14 for
controlling its attitude relative to the frame 10. The outer
gimbal 11 supports an inner gimbal 15 for rotation about an
axis 16 which is perpendicular to the axis 12. The inner
gimbal i9 provided with a pickoff 17 and a torque motor 18
for controlling its attitude relative to the outer gimbal 11.
The inner gimbal 15 forms a platform on which are
mounted gyroscopes, or gyros, having three sensitive axes.
As shown in Figure 1, a first gyro 19 has two mutually
perpendicular sensitive axes both of which are parallel to
the plane of the inner gimbal 15. A second gyro 20 has its
single sensitive axis arranged perpendicular to the two
sensitive axes of gyro 19. Each gyro has the usual pickoff
and torquer on each sensitive axis.
Also carried on the inner gimbal are the
accelerometers required to provide outputs from the
platform. Two accelerometers 21 and 22 are shown, having
their sensitive axes aligned with those of the two-axis gyro
19.
The various electrical connections to and from the
platform are shown schematically in Figure 2 in which a block
diagram of the necessary circuit means to which these
connections are supplied.
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Pickoff signals from the 2-axis gyro 19 are applied
to separate servo amplifiers 30 and 31. The outputs from
these amplifiers are applied to the torque motors 14 and 18
on the inner and outer gimbal axes respecively. The outputs
from the corresponding gimbal pickoffs 13 and 17 are applied
to a central processor 32 which derives pitch and roll
outputs. The processor also provides outputs to the torquers
on the two axes of the gyro 19.
The second gyro 20 has its pickoff and torquer
connected in a conventional capture loop. The pickoff output
is connected through 2 capture amplifier 33 to an integrating
encoder 34. The output from the encoder is applied both to
the processor 32 and to the torquer of the gyro 20.
The two accelerometers are connected in a similar way
to gyro 20. The output of accelerometer 21 is applied
through a capture amplifier 35 to an integrating encoder 36.
The encoder output is applied to the processor 32 and to the
force coil of the accelerometer. Similarly, the pickoff
output from accelerometer 22 is applied through a capture
amplifier 37 to an integrating encoder 38. The output of the
encoder is applied to the processor 32 and to the force coil
of the accelerometer 22.
The function of the processor 32 is illustrated
schematically in Figure 3, This drawing shows the inputs to
and outputs from the processor shown in Figure 2, and
indicates the functions to be performed. Many of these are
conventional to the inertial platform field and need not be
described in detail. The function may be performed by
hardware or by software.
Referring to Figure 3, the pickoff output from the
gyro 20 of Figure 2 is applied to a capture amplifier 37 and
integrator 38, and this integrator produces an output which
represents increments of azimuth angle of the platform
relative to some datum. This signal is applied via an input
A to the processor to a further integrator 40, the output of
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which represents the absolute platform azimuth angle, or
heading H. A correction signal is applied to the integrator
40 as will be described later.
The two accelerometers 21 and 22 of Figure 2 also
have capture loops containing integrators, and the outputs of
these integrators are applied to inputs B and C of the
processor. Assuming the axes 12 and 16 of Figure 1 to
represent the X and Y direction respectively then the output
of integrator 36 applied to input B represents increments of
X velocity, whilst the output of integrstor 34 applied to
input C represents increments of Y velocity. Further
integrators 41 and 42 produce outputs representing total
velocity in the X and Y directions. These are modified by
azimuth resolver 43 which produces output representing North
and East velocities. The resolver 43 has an input from the
integrator 40, and thus effects a continuous transformation
on the X and Y velocities applied to it in dependence upon
the heading of the platform.
The North and East velocities from resolver 43 are
applied to a conventional Schuler loop circuit 44 which
produces output signals representing latitude LA and
longitude L0, as well as the North and East velocities ~V and
EV. Inputs CR to tbe Schuler loop circuit 44 may provide
corrections for gyro drift, instrument bias etc, and an
output from the circuit provides the azimuth rate corrections
for the integrator 40 already mentioned. These corrections
are conventional, and relate to the earth's rate and velocity.
Outputs from the processor are required for the torquer of
the accelerometer 19 of Figure 1, and these are obtained at D
and E. These are derived from the Schuler loop circuit 43 by
way of a further azimuth resolver 45, performing the reverse
function to the resolver 43. Resolver 45 derives from the
corrected Schuler loop output the necessary X and Y gyro
torquing signals.
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Signals from the two gimbal pickoffs 13 and 17 are
applied to inputs F and G of the processor. These are
applied to a third resol~er 46 along with a fixed quantity
representing the angle in the horizontal plane between the
platform XY axes and the pitch-roll axes from integrator 4.
This resolver converts the X and Y outputs from the pickoffs
into pitch and roll angle outputs from the processor.
The three resolvers and the Schuler loop circuit
perform fairly simple transformation operations of a type
which are well-known in the inertial navigation field. Such
transformations are explained in detail in a number of
reference books and will not therefore be described further.
As already stated, the functions of the processor may
be provided by circuitry or by programming. The desired
results may be obtained by different methods to those
described. The three sensitive gyro axes may be provided by
three separate single-axis gyros, or by two two-axis gyros
with one redundant axis. A third accelerometer could be
provided if required. The two gimbal pickoffs 13 and 17 are
only necesgary if the platform is required to give outputs
indicating pitch and roll angle. The processor could be used
to resolve the X and Y velocity increments rather than
operating on the integrated increments.
