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. Background 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.

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 equivalence between tilt (x, ) and <J> (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 4>(x,y),

where tan φ (x) = x tan φ (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 horizontal 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 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 table to compensate for horizontal accelerations and tilts to which the platform is subjected.

The sensor may be, for example, a horizontal 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

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 common-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 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 Description of the Drawing

The invention will now be disclosed, by way of example, with reference to preferred embodiments illustrated in the accompanying drawings in which:

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; Figure 2 shows a single-stage, DOF (pitch and roll), active motion isolation table configuration; 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) .

Detailed Description of a Preferred Embodiment

Referring to Figure 1, there is shown a two-stage tilt table in the form of a coarsely-controlled table 1 and a finely-controlled table 2, which are affected by 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 piezoelectric 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

of 10 nano-radians or better.

For 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 a 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.

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 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 frequency 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 J 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

control component are related such that: Kp - 4 KdKi > 0 and (Dc = Ki/Kp < 0.1 Hz such that integral control is used effectively where it is needed most (in this case for frequencies cue less than 0.1 Hz) .

All modes of PID control are needed because: 1) Proportional: is usually needed with integral and derivative control. 2) Integral: is required for reducing the steady state tilt angle in the feedback loop because tilt frequencies close to DC require a high gain. 3) Derivative: for decreasing the feedback response 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: