Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
,
:~ ISOL~ATION OF I~NVIRONDlEMTAI~
ACC13L~3RATIONS AND TII~TS ON MOVING PI.ATFORM
! This invention relates to a system for compensating
for the effects of horizontal accelerations and tilts,
on a moving platform, such as an air-, land-, water-, or
space-borne vehicle or in a borehole-logging probe, or
on a laboratory motion isolation (seismic) table.
~ac~ground of the Invention
When carrying out high resolution measurements
involving free or partially free masses on a moving
platform, it is often desirable to compensate for the
effect of horizontal accelerations as well as tilts.
For example, let us consider an air-borne gravity
gradiometer based either on pendulous or other
accelerometer pairs. One way to reduce the effect of
horizontal common-mode acceleration on the above sensors
is to match the accelerometers over a very broad dynamic
range and to closely maintain their alignment. Neither
may be attainable to a sufficient degree with present
technology.
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Tilts can, with expensive gimballed platforms, be
, ~ maintained to within a few micro-radians. Compensation
for horizontal accelerations (which are typically .01 -
.02 G during survey regime) has not been accomplished to
date.
Statement of Invention
`~ The present invention is based on the principle of
3 equivalence between tilt ~ (x,) and ~ (y) and horizontal
accelerations (x) or (y), which states that an
accelerometer cannot distinguish between horizontal
acceleration and tilt. Consequently, the effect of x
and y on a moving platform can be compensated for by
tilting the platform by an angle ~ (x,y),
where tan ~ (x) = x
tan ~ (y) = y with x and y
expressed in G's
This operation transforms (in platform body co-
ordinates), horizontal accelerations x, y into a
vertical acceleration z, to which a pendulous hori~ontal
accelerometer is relatively insensitive. Tilting is the
most effective way to compensate for large-amplitude,
low-frequency (below 1 Hz) horizontal accelerations.
High-frequency, low amplitude motions (vibrations) can
be attenuated, for example with a piezo-electric (PZT)
driven translational x, y, z stage.
.
The system in accordance with the present invention
comprises a tilt table mounted on a moving platform
including a laboratory motion isolation table or a
borehole logging probe, a sensor mounted on the tilt
3 5 table or on the platform for sensing linear
accelerations and tilts to which the platform is
subjected, and a tilting device mounted on the platform
~; and responsive to the sensor for tilting the tilt tableto compensate for horizontal accelerations and tilts to
which the platform is subjected.
The sensor may be, for example, a horizontal
I accelerometer such as the QA 3000 manufactured by
Sundstrand Data Control Inc., Redmond, Wash., or an
electronic bubble level in closed feedback loop with the
tilting device.
The sensor may also be an Inertial Measurement Unit
(IMU) like H423 manufactured by Honeywell Inc. of
Clearwater, Florida. If an IMU is used as a gravity
sensor, it can be, for lower noise, installed on a tilt
table in a Shuler-tuned closed loop configuration with
the tilting device. Alternatively, the IMU may be a
part of an autopilot, motion compensation or other
device, working in open-loop configuration with the
tilting device.
In the case of accelerometer-based gravity
gradiometry, the sensor output may be provided by
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accelerometers of the gradiometer pairs. In this
application, the sensor may be a part of the feedback
loop with the tilting device thus substantially
eliminating the horizontal cornmon-mode component of the
gravity gradient signal.
The sensor output signal is preferably processed
through a feedback controller for applying a regulated
feedback control to the tilting device to cause the
sensor base to tilt by an angle that will compensate for
horizontal accelerations and tilts. The feedback
controller is preferably a proportional-integral-
derivative feedback controller.
The tilt table may be a two stage table comprising
a first coarsely controlled stage using a servo-motor as
lS a tilting device, and a second finely controlled stage
using a transducer as a tilting device.
The sensor and tilting device may be combined in a
single sensor/tilting unit. The sensor/tilting unit may
be a critically-damped pendulum or a "dish" filled with
liquid, on which the tilt table floats. In either cases
a feedback controller will not be required for the first
coarse stage.
Short Descri~tion of ths Drawing
The invention will now be disclosed, by way of
example, with reference to preferred embodiments
illustrated in the accompanying drawings in which:
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~:l 5
:~ Figure 1 shows a two-stage, two degrees of freedom
(DOF), pitch and roll, active motion isolation table
configuration which will attenuate residual horizontal
. accelerations and tilts;
~3 5 Figure 2 shows a single-stage, DOF (pitch and
; roll), active motion isolation table configurationi and
; Figure 3 shows a two-stage, active motion isolation
table configuration similar to Figure 1 which will
additionally attenuate vibrations along x, y and z axes
(five or six DOF).
~etailed Descri~tion of a Preferred ~mbodiment
Referring to Figure 1, there is shown a two-stage
tilt table in the form of a coarsely-controlled table
and a finely-controlled table 2, which are affected by
i 15 linear acceleration disturbance (t) and angular tilt
disturbance (t). In accordance with the present
invention, both of these disturbances are compensated
for by tilting the sensor base by angle O (table 2),
using a servo-motor 3 operating a precision lead screw
(not shown) to provide a linear displacement to a ~.
resolution of about 5 - 10 micro-radians. For greater
resolution than can be achieved with a mechanical ,
device, a pie~oelectric device (PZT) device, or electro - - -.
strictive or magneto-strictive (EST or MST
respectively), or any other suitable transducer 4 is
mounted on table 1 to provide a resolution of the order
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of 10 nano-radians or better.
or the purpose of the description, it is assumed
that the disturbances to be compensated for are in the
(x, z) vertical plane of motion. The tilt tables may
however be modified to accommodate motions in all six
degrees of freedom.
In the present embodiment, the sensors are
pendulous acceleration sensors 5 and 6, which are
mounted on tables 1 and 2 respectively to sense
horizontal accelerations (and tilts). The output
voltage V(t) of each accelerometer is sensed by à
detector 7 and fed to a feedback controller 8 which
applies a feedback voltage to the servo-motor or the PZT
through a suitable driver 9 if required to thereby null
the output voltages V(t) of the pendulous
accelerometers.
For less demanding applications, a single-stage,
two-DOF tilt table, as illustrated in Figure 2, may
provide a simpler, less expensive alternative. The ~ -
disturbances sensed by sensor 10 mounted on table 11 are ~ ~-
applied to a feedback controller 12 or 13 or both.
Coarse deviations may be compensated by a servo-motor 14
through a suitable driver 15 while fine deviations of ~-~
the order of 10 nano-radians may be compensated by a PZT
16.
,~ l U ~~L l 7 9
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i On a seismic isolation table, where compensating
tilts may be limited to several micro-radians, a PZT (or
EST or MST) stage only can be used.
Figure 3 is a two-stage stabilized platform
~ 5 configuration such as shown in Figure 1 wherein stage 2
¦ additionally includes a x,y,z vibration isolation stage
' which is part of the tilt table. In this embodiment
high fre~uency low-amplitude vibrations can be
attenuated with x,y,z translation devices such as piezo- - -
electric (PZT) devices 4, 16, 17 working in closed loop
with a suitable triaxial vibration sensor 18.
The configuration shown in Figure 3 can accommodate
five degrees of freedom. The sixth degree (yaw
compensation) is not shown but can be added using the
same technique.
A Proportional-Integral-Derivative (PID) feedback
control is preferably used to stabilize the table as a
function of the pendulum output where:
e = KpV + Kd V + Ki ¦ Vdt +Vn
where e = voltage applied to the PZT
Kp = proportional control gain
Ki = integral control gain
Kd = derivative control gain
Vn = electronic noise
If the tilt assembly is critically damped (i.e. no
control induced oscillations) then the gains of each
r~ 9
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: control component are related such that:
~:,
K2p - 4 KdKi > 0
and ~c = Ki/Kp < 0.1 Hz
such that integral control is used effectively where it
is needed most (in this case for frequencies ~c less
than 0.1 Hz).
, All modes of PID control are needed because:
`. 1) Proportional: is usually needed with integral and
derivative control.
~, 10 2) Integral: is required for reducing the steady
state tilt angle in the feedback
~, loop because tilt frecluencies close
`~ to DC require a high gain.
3) Derivative: for decreasing the feedback response
j 15 time at high frequencies as well as
applying a phase-lead control.
Although the invention has been disclosed, by way
of example, with reference to preferred embodiments, it :~
is to be understood that it is not limited to such
embodiments and that other alternatives are also
envisaged within the scope of the following claims: ~-~
