Background of invention Current inertial sensing systems such as seismometers, accelerometers, gravimeters and tiltmeters are based on the relative displacement between an inertial mass and the base of the instrument when the said base is subject to an external disturbance (vibration, modification of the"g"level, angle) ; and gyroscopes, which are another kind of inertial sensing systems, are made of an inertia mass which is rotated about one of its axis of inertia and the measurement principle relies on the relative movement between the said axis and the base of the instrument, or on the force generated by the said axis on the base of the instrument, when the said base is subject to an external movement.

All those inertial sensing systems will be limited by the friction between the inertial mass and the base of the instrument.

Indeed this friction will be responsible of imprecisions in the measurements, of wear between the mechanical parts in contact and it might also lead to failure due to mechanical fatigue.

In addition, inertial sensing systems, such as seismometers in seismology or inclinometers in civil engineering, are often placed to monitor structures or machines and the power consumption of such systems is sometimes a critical factor.

In seismology, it is relevant to study seismic waves within the following ranges: - Frequencies : From 1mHzto 100Hz -Accelerations : From 1 nano g to 5 g

Given this wide spread both of the relevant frequencies and the relevant accelerations that have to be recorded and analyzed, several classes of measuring instruments have been developed : - Short Period seismic sensors - Long Period seismic sensor - Broad band seismic sensors - Very broad band seismic sensor All the corresponding products presently commercialized are designed around a damped mechanical mass-spring system made up of a mass detector linked both to a damping mechanism and, with a spring, to the frame of the instrument.

In the case of a seismic excitation, the frame of the seismometer follows the ground movement while the mass used as a detector, which we shall designate as the seismic mass, tends to remain in its initial position, thus moving relatively to the frame.

In the process the length of the spring changes and the displacement in relation to the frame can be measured as a function of time.

The response to a seismic excitation of instruments built according to this principle solely derives from the mechanical characteristics of such a damped spring mass system, i. e. the elastic constant k of the spring and the damping constant d.

Since, however, the spring characteristic k is not precisely constant upon the whole range of possible spring deformations, and is temperature dependant, some of the most recent seismometers are equipped with an electromagnetic counterforce system fed by a feed back loop, limiting spring deformations within a small range where k is assimilated to a constant value.

This design, however, does not eliminate distortions caused by spring inertia and friction and, for a given instrument, it is not possible to change its parameters k and d, a fact which limits its use to a chosen range of accelerations and frequencies.

In order to eliminate these shortcomings, some new designs have been recently patented: they use either magnetic or electrostatic forces or the force resulting from a special property of a super-conducting loop, called the Meismer effect, in order to levitate a seismic mass.

The levitation is obtained through the action of one or several retroactive loops commanded by optical or capacitive sensors measuring the movements of the seismic mass in relation to the frame of the instrument.

Today's high quality seismometers and based on expensive multi-axis spring- damper elements with complex compensation systems. Electrostatic levitation of large spheres in high vacuum is the principle of some high precision gyroscopes.

A three axis active magnetic suspension seismometer, described in the U. S.

Patent No 5,565, 665 issued to Biglari et al., shows a limited sensitivity, caused by the sensing system, and a non-symmetric behavior of the vertical axis. In addition an upward acceleration can not be counter balanced since there are no electromagnets placed below the seismic mass.

In U. S. Patent No 4,947, 067 issued to Habermann et al., a three axis magnetic suspension is presented but for the purpose of a vibrator/dampener and not for the purpose of an inertia sensor.

In U. S. Patent No 5,024, 088 issued to Komatsu et al., the described accelerometer shows a complex inertia mass and a non optimal arrangement of the coils resulting in a hyperstatic system.

Magnetic suspension and rotation of small spheres was done by J. Beams, but with only one controlled axis and the purpose was to create high centrifugal fields.

Other prior art references : Patent documents US 5,955, 800 US 5,357, 803 US 5,224, 380 US 6,363, 035 US 5,983, 699 Other publications J. W. Beams, J. L. Young, J. W. Moore, the Production of High Centrifugal Fields, Journal of Applied Physics, Vol. 17, November, 1946.

J. W. Beams, Magnetic Suspension for Small Rotors, The Review of Scientific Instruments, vol. 21, N°2, 182-184, February, 1950.

E. F. Kinsey, J. W. Beams, M. J. Saunders, The Magnetically-Suspended Free Gyroscope, Naval Ordnance Research Laboratory, University of Virginia, December, 1951.

J. W. Beams, Magnetic Bearings, Automotive Engineering Congress, Detroit, Mich. , January, 1964.

W. J. Bencze, Y. Xiao, D. N. Hipkins, B. W. Parkinson, G. F. Franklin, An Electrostatic Suspension and Orientation Control System for the Gravity Probe B Relativity Mission's Science Gyroscope, 3rd MOVIC, September 1996, Chiba, Japan.

Summary of the invention The present invention is based on the magnetic levitation of a inertial mass to create high sensitive non-contact inertial sensing systems.

It relates to an inertia sensor as defined in claim 1.

Preferred embodiments are defined in the dependent claims.

The following detailed description will better show all the advantages provided by the invention over the prior art sensors.

Brief description of the drawings For a greater understanding of the nature and objects of the invention, reference should be made to the following detailed description and the accompanying drawings, in which: Figure 1 shows complete view of the first embodiment of the inertial sensing system with: 1) End cap 2) Support structure 3) Horizontal pair of electromagnets 4) Vertical pair of electromagnets 5) Ferromagnetic inertial mass 6) Holding structure for the horizontal electromagnets 7) High precision position sensors 8) Coil armature Figure 2 shows a horizontal cut (O, X, Y) of the first embodiment of the inertia sensing system with: 3a), 3b), 3c), 3d) Coils 7a), 7b), 7c), 7d) High precision position sensors Figure 3 shows a vertical cut (O, Y, Z) of the first embodiment of the inertial sensing system.

Figure 4 shows a variant of the electromagnets used in the first embodiment with: 1) Coil 2) Laminated ferromagnetic core 3) Position sensor 4) Inertial mass

Figure 5 shows the second embodiment of the inertial sensing system with: 1) Upper set of electromagnets (3 coils) 2) Lower set of electromagnets (3 coils) 3) Position sensing system holder Figure 6 shows the second embodiment of the inertia sensing system without the position sensing system with: 1 a), 1 b), 1 c), 2a), 2b), 2c) Coils 4) Inertial mass 5) Magnetic permeable cores Figure 7 shows the position sensing system of the second embodiment of the inertial sensing system and its inertial mass with: 6a) Laser diode 1 6b) 4 segments photodiode 1 7a) Laser diode 2 7b) 4 segments photodiode 2 Detailed description of the invention Implementation 1a (Fig. 1. Fiq. 2, Fiq. 3) Six electromagnets 3,4, 3a, 3b, 3c, 3d are diametrically disposed in pairs along three orthogonal axis.

A small size ferromagnetic inertial mass 5 is levitated and its position controlled along three axis.

In this first embodiment (Fig. 4), the outside frame 4 is an empty cylinder of homogeneous ferromagnetic material.

By convention, we shall call O its center of gravity and Oz its axis.

Also by convention we shall call Ox and Oy two axis located in the plane perpendicular to Oz and containing O, Oxyz being a direct trihedral.

In this first embodiment the inertia mass 5 is a spherical or cylindrical body of homogeneous ferromagnetic material.

When it is in its original position, the center of this inertia mass is located in 0.

Circular covers or end caps 1 made of the same ferromagnetic material close both ends of the cylindrical outside frame 2.

Therefore the volume inside the frame 2 of this device (inner volume), is fully protected from any measurement bias caused by changes of outside magnetic conditions.

Seals between the cylinder frame 2 and its covers 1 close hermetically this inner volume which is equipped with a port (not represented in the figure) in order to be put under vacuum whenever necessary for eliminating any bias due to atmospheric convection and friction.

Centered respectively on the Ox and Oy and located symmetrically with regard to point O, two sets of coils 3a, 3b and 3c, 3d, each set made of two symmetrical coils 3 facing each other create opposed magnetic fields.

Inside each coil 3,4, 3a, 3b, 3c, 3d, at its inner end, an axial sensor 7 (inductive, optical or capacitive), centered on axis Ox or Oy and very rigorously positioned at pre-set distance from the Oz axis, provides instant and highly accurate measurements (< um) of its distance to the inertial mass 5 along Ox or Oy as a function of time.

The two measurement values given by the set of sensors centered, for example, on Ox, provide the basis for a differential measurement of the displacement of the inertial mass 5 along the Ox axis and the same can be said for the set centered on Oy.

A third set of two coils 4 with their corresponding axial sensors 7 is centered on the Oz axis and both coils are located in rigorously symmetrical positions with regards to O.

It operates exactly like the two other sets 3 described above.

The following considerations are also applied to implementation 1 b and implementation 2: - The distance to the inertial mass of each sensor is sent in the form of a variable tension signal which, in order to remove any unwanted residual noise, is fed to a filtering module.

- The filtered signal in then converted to digital values in an AD converter and the information is multiplexed and processed in a digital controller.

- The digital controller : 1°) Calculates the displacement of the inertial mass as time functions measured along the axis Ox, Oy and Oz.

2°) Calculates the counterbalancing force necessary in order to bring the inertia mass back to its initial position, with its center in O, thus insuring its levitation.

3°) Sends the necessary instructions to a feedback loop commanding the current to the corresponding coils.

4°) Calculates the value of the time function representing the external disturbance from the knowledge of the time functions representing the displacement of the inertial mass and the counterforce applied to it.

5°) If this inertial sensing system is used as a seismometer, initial conditions being known, this information can be also taken by the digital controller to

calculate both the speed of the seismic wave and the corresponding ground movements as a function of time.

- A magnet can be introduced in the electromagnet which is supporting the weight of the inertial mass, in order to compensate it. Therefore power consumption can be reduced.

- A lock-in amplifier can be added to the filtering module in order to increase the signal to noise ratio for low frequencies Implementation 1b (Fia. 4) The implementation 1b is equivalent to implementation 1a, expect for the shape of the six electromagnets. The six electromagnets considered in this implementation have a horseshoe shape (Fig. 4) in order to have less magnetic losses than the electromagnets described in implementation 1a.

Implementation 2 (Fiq. 5, Fiq. 6, Fiq 7) As for implementation 1a, a magnetic levitation of a spherical or cylindrical inertial mass 4 is performed with three degrees of freedom control. Six vertically arranged electromagnets 1,2 create opposing forces in three orthogonal directions. Magnetic permeable cores 5 bring the magnetic field near the inertial mass 4, reducing magnetic losses. The position sensing system is composed of two laser diodes 6a, 7a and two 4-segments photodiodes 6b, 7b orthogonally placed in a horizontal plane between the upper electromagnets 1 and the lower electromagnets 2. Therefore, positions x, y, z of the inertial mass 4 can be measured. Afterwards, the x, y positions are rotated by 45 degrees, filtered and fed back to a digital controller as well as the z position.

Horizontal arrangement of the electromagnets 1 b, 1c, 2b and 2c (same plane of the sensing system) could be a variant for this implementation.

In order to spin the inertial mass 4, a motor function can be added to the device by superposing a two-phase sinusoidal or square signal to the control current of the electromagnets 1 b, 1 c, 2b and 2c.

Advantages of the present invention The invention proposed has a symmetrical behavior along three axis, therefore external disturbances in three orthogonal directions can be detected.

Since the inertial mass is magnetically levitated with active control, parameters like stiffness and damping can be varied over a large range in order to adjust the natural frequency. Moreover when this inertial sensing system is used as a seismometer, we can even define precisely triggers that characterize the limits between the borders of different seismic events and this way the sensors can vary its damping and spring constant according to the nature of the seismic event.

This design can be made very compact thanks to the use of only one single sensor.

Thanks to differential measurements the precision of the measurements is high and not affected by temperature variation Both position signals and current signals can be used to determine the external disturbance.

Moreover, the whole device is magnetically shielded and thus not affected by ambient magnetic waves.

If we apply the vacuum inside the system, we can avoid the disturbance of the buoyancy forces and enhance the precision of the measurements.

The addition of the motor function will transform the device in a gyroscope.

Applications This invention can be used as an accelerometer, a gravimeter, a tiltmeter or a seismometer. With the addition of the motor function one can use it as a gyroscope. Application fields are seismology, inertial navigation, structural monitoring and geology.