WO2016107806A1

WO2016107806A1 - Hybrid inertia sensor employing cold atoms and mems and associated inertial platform

Info

Publication number: WO2016107806A1
Application number: PCT/EP2015/081115
Authority: WO
Inventors: Sylvain Schwartz; Matthieu DUPONT-NIVET
Original assignee: Thales
Priority date: 2014-12-30
Filing date: 2015-12-23
Publication date: 2016-07-07
Also published as: FR3031187B1; FR3031187A1

Abstract

Device for measuring an inertia parameter comprising a first sensor of electromechanical microsystem type operating at a first frequency and a second sensor of cold atoms device type operating at a second frequency which is lower than the first measurement frequency. The device moreover comprises comparison electronics comprising: first means of calculation operating at the second frequency and calculating, on the basis of a first measurement emanating from the first sensor and of a second measurement emanating from the second sensor, a bias between said first measurement and said second measurement and; second means of calculation operating at the first frequency, and calculating measurements of said inertia parameter, each of said calculated measurements being equal to a first measurement emanating from the first sensor and corrected for said bias. This device is well adapted to the production of inertial platforms.

Description

Hybrid cold-inertia sensor with MEMS and associated inertial unit

The field of the invention is that of inertial navigation. The purpose of an inertial unit is to provide the angular velocity and acceleration information of a moving vehicle. The preferred field of application is aeronautics.

An inertial unit therefore includes gyrometers and accelerometers to ensure the necessary measurements. There are currently two categories of inertial units, depending on the required levels of performance and space requirements. High-end inertial units are generally based on optical gyrometers. They have the disadvantage of having a size that can be important, even prohibitive for some applications.

Inertial units based on micro-electromechanical sensors, also called "MEMS", acronym meaning "Micro Electro Mechanical Systems", are significantly smaller in size compared to optical technologies. However, these sensors have significant levels of bias drift. Typically, the drift is of the order of degree per hour for the best MEMS gyrometers, which limits their applications to the areas of bass and average performance. More information on this type of MEMS can be found in the publication "Performance of MEMS inertial sensors" by Kourepenis et al, published in IEEE PLANS April 1998.

Inertial sensors that are extremely stable over the long term have been developed in recent years using atomic interferometry techniques with laser-cooled atoms. It is now possible to make this type of sensor in a small footprint. The atoms are then trapped in the vicinity of a substrate or "atomic chip" throughout the detection cycle. An architecture of this type potentially has, compared to sensors using cold atoms in free fall, the advantages of a great compactness. However, the cold atomic sensors have a reduced level of bandwidth, typically of the order of the hertz. Indeed, the measurement accuracy depends on the separation time of cold atoms that can hardly be reduced without affecting the accuracy of the measurement. However, for some applications, this measurement rate is too low.

In the publication "Hybridizing matter-wave and classical accelerometers" by Lautier et al, published in Applied Physics Letters 105, 144102 (2014), the hybridization of an accelerometer and a g ravi m to be atomic has been proposed. In the hybridization described, the accelerometer is used to slave the reflecting mirror of an atomic gravimeter operating by interferometry. This arrangement makes it possible to obtain both a high measurement accuracy and a larger bandwidth than that of a non-controlled gravimeter. However, this application is limited to gravimetry with free falling atoms and can not easily be applied to the measurement of other inertial parameters. Moreover, this device remains a laboratory device. Finally, this device has the disadvantage of slaving the complex system of a cold atomic sensor.

The device for measuring an inertial parameter according to the invention is also a hybrid device that combines both a microelectromechanical sensor and a cold-on-chip sensor. However, it can be applied to all types of MEMS and cold atomic sensors as this device is not dedicated to a particular parameter or technology. By combining measurements of the same inertial parameter made by two sensors of different nature, one obtains both the precision of the cold atom systems and the bandwidth of the MEMS sensors.

More specifically, the subject of the invention is a device for measuring an inertial parameter comprising a first sensor for measuring said inertia parameter, said first sensor being an electromechanical microsystem operating at a first frequency, characterized in that said measuring device comprises:

a second sensor for measuring said inertia parameter, said second sensor being a cold-atom device operating at a second measurement frequency lower than the first measurement frequency, and;

a comparison electronics comprising:

first calculating means operating at the second measurement frequency and calculating, from a first measurement resulting from the first sensor and a second measurement from the second sensor, a bias between said first measurement and said second measurement;

second computing means operating at the first measurement frequency, and calculating measurements of said inertia parameter, each of said calculated measurements being equal to a first measurement from the first corrected sensor of said bias.

Advantageously, the bias correction is an electronic correction performed by the second calculation means.

Advantageously, the bias correction is obtained by a modification of one or more physical parameters of the first sensor.

Advantageously, the physical parameters are the voltages of the control electrodes of the electromechanical microsystem.

Advantageously, the physical parameter is the temperature of the electromechanical microsystem.

Advantageously, the second sensor comprises an electronic chip comprising parallel conductive wires and power supply means for said conductive wires, the cloud of cold atoms necessary for the measurement being disposed in the vicinity of said electronic chip, the conductive wires and their means. power supply are arranged to create the electromagnetic fields necessary for the superposition of internal states of the atoms, their separation and their recombination.

Advantageously, the first sensor is implanted on said electronic chip.

Advantageously, the electronic chip is arranged to measure at least one second inertia parameter.

Advantageously, the inertia parameter is either an acceleration or a rotational speed.

The invention also relates to an inertial unit comprising three accelerometers arranged to measure the acceleration in three non-coplanar space directions and three gyrometers arranged to measure the speed of rotation in three directions of the non-space. coplanar. At least one of the accelerometers or one of the gyrometers of said inertial unit is a device for measuring an inertia parameter as described above. Advantageously, the three accelerometers and the three gyrometers are devices for measuring an inertia parameter as described above. The invention will be better understood and other advantages will become apparent on reading the description which follows given by way of non-limiting example and by virtue of the appended figures among which:

FIG. 1 represents the block diagram of a device for measuring an inertia parameter according to the invention comprising two inertia sensors;

FIG. 2 represents a view of an implantation of the two inertia sensors on the same electronic chip.

By way of example, FIG. 1 represents the block diagram of a device for measuring an inertia parameter according to the invention. In this figure, the devices are shown boxed and the arrows represent the direction of transmission of information. The measuring device essentially comprises:

A first sensor of the electromechanical microsystem type measuring said inertia parameter. This sensor operates at a first frequency F1 and we note M1 (t) the measurements from this sensor as a function of time t;

a second cold atom device sensor also measuring said inertia parameter. This sensor operates at a second measurement frequency F2 lower than the first measurement frequency F1. Typically, F2 is of the order of Hertz. M2 (t) measures the measurements from this sensor as a function of time t;

- A comparison electronics that has two functions. Its first function is to compare at a rate that is that of the second frequency, the measurements from the first and the second sensor. Its second function is to calculate a measurement of the inertia parameter at the first frequency.

It is known that, by nature, the cold atom devices give measures of great stability. Therefore, at a time t ₀ of measurement, the measurement 2 (t ₀ ) is considered as an exact measure of the parameter. Generally, the first sensor is less accurate than the second, its measurement M1 (t ₀ ) at this time t ₀ is different from M2 (t ₀ ). We call B (t ₀ ) the bias that exists between these two measures and we have the simple relation:

2 (to) = M1 (to) + B (to)

Between this first measurement carried out at time t ₀ and the next measurement made by the second sensor, a period of time T2 which is equal to 1 / F 2 flows. With a measurement rate at 1 Hertz, T2 is 1 second. During this period T2, even if the first sensor drifts, its bias remains practically constant. Thus, we can consider that at any time t selected between t ₀ and t ₀ + T2, the measurement from the first sensor is accurate provided that it is corrected bias B (t ₀ ). We can then write, noting _V RAIE (Î) the exact measure at time t:

M _V RAIE (t) = M1 (t) + B (to)

Thus, knowledge of the bias at the frequency F2 makes it possible, from the measurement made by the first sensor, to know the true measurement at the frequency F1, which can be much greater than the frequency F2 and which is only limited by the MEMS sensor characteristics.

Bias correction can be done electronically. The bias between the two sensors is periodically calculated and taken into account in the calculation of the final measurement. It can also be obtained by modifying one or more physical parameters of the first sensor. It is thus possible to act on the voltages of the control electrodes of the electromechanical microsystem or to regulate its temperature so as to reduce the bias to near zero.

The cold atom device may be of different natures. However, it is interesting to focus on compact architectures, for example with trapped atoms, in particular on electronic chips that allow the integration of the MEMS sensor on the same electronic chip.

A cold atomic sensor of this type comprises a central part consisting of a vacuum chamber, all walls of which are transparent, except the upper wall which consists of a chip on which conductor wires have been deposited. This chip 10 is represented in the perspective view of FIG. 2. In the case of FIG. 2, the measured inertial parameter is the acceleration whose direction is symbolized by four chevrons in FIG. 2. In this version, the chip electronic also includes the MEMS sensor 20 which in this case is an accelerometer.

The measuring atoms, initially in the gaseous phase at room temperature in the chamber, are trapped and cooled by six laser beams arranged symmetrically two by two on three axes perpendicular two by two combined with a magnetic field gradient generated by external magnetic coils. The six laser beams are arranged symmetrically on three perpendicular axes. The set of laser beams and magnetic coils is called three-dimensional magnetoptic trap or "P O 3D".

At the end of the cooling and trapping phase, the atoms are transferred into a purely magnetic conservative trap created in the vicinity of the conductive wires of the chip 10 and prepared in an internal state, for example | 1>. At the end of this phase, the atoms are located at an initial spatial position above the electronic chip 10.

As seen in FIG. 2, the electronic chip 10 comprises at least a first central conductor wire 1 1 used as the main trapping wire, a second conductive wire 12 perpendicular to the first wire and used as a secondary trapping wire, 13 lateral wave parallel to each other, parallel to the secondary trapping wire 12 and arranged symmetrically with respect thereto. The atomic cloud 15 is located above the first conductive wire 1 1, said first conductor wire being traversed by a first current and generating a magnetostatic field, the first waveguide 13 being traversed by a second current modulated to a second microwave frequency generating a second microwave field and the second waveguide 13 being traversed by a third current modulated at a third microwave frequency, generating a third microwave field. This arrangement makes it possible to separate and recombine magnetically the atomic cloud. The separation-recombination method is detailed below.

In a first step, the atoms are transferred in an equal weight superposition of the internal states | 1> and | 2>, by a pulse of short duration called π / 2 pulse combining a microwave field and a radiofrequency field generated by for example, by the conductive lines 13 of the chip 10. Each atom is then in a resulting intermediate state noted (| 1> + | 2>) / v2. In a second step, the atoms are separated into two wave packets associated with the internal states | 1> and | 2>, thanks to a microwave potential MW depending on the internal state. It is this phase of atomic separation which is represented in FIG. 2. The microwave field used for the separation is generated by the two coplanar waveguides or CPW 13. The separation is in the direction of the acceleration of to be as sensitive as possible. The separation distance s of the atoms is of the order of one or more tens of micrometers. The separation during a time T _s causes a phase shift between the two wave packets related to the local acceleration.

In a third step, the atoms are recombined by the suppression of the applied microwave fields. The phase shift is then converted into a population difference between the internal states by means of a second pulse "π / 2".

The atomic cloud is detected using the absorption imaging technique, which consists of measuring the absorption of a quasi-resonant laser beam by the atomic cloud using a CCD camera. Optical spectroscopy thus gives access to the populations of the two internal states, thus to the desired phase shift. Alternatively, the populations of the two internal states can be measured by fluorescence using photodiodes. Finally, the acceleration is calculated, which is then compared to that obtained by the MEMS accelerometer.

The device according to the invention combining two types of sensors makes it possible to measure accelerations and speeds of rotation at a high measurement rate and with high precision.

An inertial unit has three acceleration sensors and three rotation sensors. An inertial unit can be made from this type of measuring device, either totally or partially. One of the advantages of cold-carbon sensors on an electronic chip is that, not only can one or more MEMS be integrated on the same chip, but it is also possible to measure different inertia parameters according to the fields with the same chip. electromagnetic generated in the conductive wires. This considerably simplifies the construction of the inertial unit. The industrial applications of this inertial unit concern in particular devices requiring precision inertial guidance in environments where the "GPS", acronym for "Global Positioning System" may be absent for relatively long times, of the order of several tens of hours. minutes. Such lack of GPS may be accidental due, for example, to poor reception or intentional, the GPS signal is then scrambled.

Claims

1. A device for measuring an inertia parameter comprising a first measurement sensor (20) of said inertia parameter, said first sensor being an electromechanical microsystem operating at a first frequency, characterized in that said measuring device comprises:

a second sensor for measuring said inertia parameter, said second sensor being a cold-atom device operating at a second measurement frequency lower than the first measurement frequency, said second sensor comprising an electronic chip (10) comprising conducting wires parallel (12, 13) and power supply means of said conductive son, the cloud of cold atoms necessary for the measurement being disposed in the vicinity of said electronic chip, the son son and their supply means being arranged to creating the electromagnetic fields necessary for the superposition of internal states of the atoms, their separation and their recombination, the first sensor (20) being implanted on said electronic chip;

a comparison electronics comprising: first computing means operating at the second measurement frequency and calculating, from a first measurement from the first sensor and a second measurement from the second sensor, a bias between said first measurement and said second measure and;

2. Device for measuring an inertia parameter according to claim 1, characterized in that the bias correction is an electronic correction performed by the second calculation means.

Device for measuring an inertia parameter according to claim 1, characterized in that the correction of the bias is obtained by a modification of one or more physical parameters of the first sensor.

4. Device for measuring an inertia parameter according to claim 3, characterized in that the physical parameters are the voltages of the control electrodes of the electromechanical microsystem.

5. Device for measuring an inertial parameter according to claim 3, characterized in that the physical parameter is the temperature of the electromechanical microsystem.

6. Device for measuring an inertia parameter according to claim 1, characterized in that the electronic chip is arranged to measure at least one second inertia parameter.

7. Device for measuring an inertia parameter according to one of the preceding claims, characterized in that the inertia parameter is either an acceleration or a rotational speed.

8. Inertial unit comprising three accelerometers arranged to measure the acceleration in three non-coplanar space directions and three gyrometers arranged to measure the speed of rotation in three non-coplanar space directions, characterized in that at least one of the accelerometers or one of the gyrometers of said inertial unit is a device for measuring an inertia parameter according to one of the preceding claims.

9. Inertial unit according to claim 8, characterized in that the three accelerometers and three gyrometers are devices for measuring an inertia parameter according to one of claims 1 to 7.