INERTIAL AND MAGNETIC SENSORS SYSTEMS DESIGNED FOR MEASURING THE HEADING ANGLE WITH RESPECT TO THE NORTH TERRESTRIAL POLE

Abstract: The invention described here concerns a heading system of distributed design consisting of: at least two sensor units capable of determining the north earth's magnetic pole direction; at least one sensor unit capable of determining the position with respect to both the north terrestrial magnetic pole direction and the north geographic pole direction at the same time; and the capability of compensating any errors, arising from unpredictable variations in the local earth's magnetic field, affecting all sensor units in said system. The present invention can be classified in the technical field of measuring instruments, in particular those for measuring heading angles. The application field of this invention consists primarily in measuring the heading angle of sea-surface or underwater systems of distributed design such as sea buoys underwater vessels and towed sensor arrays. This invention, however, can also be applied to groups of land vehicles, the positions of which, with respect to a fixed reference system, must be determined as a function of time.

Description: -1- Description Inertial and magnetic sensors systems designed for measuring the heading angle with respect to the north terrestrial pole Technical Field The invention described here concerns a heading system of distributed design consisting of: at least two sensor units capable of determining the north earth's magnetic pole direction; at least one sensor unit capable of determining the position with respect to both the north terrestrial magnetic pole direction and the north geographic pole direction at the same time; and the capability of compensating any errors, arising from unpredictable variations in the local earth's magnetic field, affecting all sensor units in said system. The present invention can be classified in the technical field of measuring instruments, in particular those for measuring heading angles. The application field of this invention consists primarily in measuring the heading angle of sea- surface or underwater systems of distributed design such as sea buoys, underwater vessels and towed sensor arrays. This invention, however, can also be applied to groups of land vehicles, the positions of which, with respect to a fixed reference system, must be determined as a function of time. Background Art At present, when it is not possible to operate with the Global Positioning System (GPS), use is made both of inertial systems and magnetic systems, with a preference for the latter when space restrictions exist; mixed systems (limited to single sensor units) are sometime used for compensating any gyroscopic drift (built-in in inertial systems) through gimbal-mounted magnetic compasses or magnetic strap-down systems. However, the above mentioned systems (GPS, inertial and magnetic) have not yet been found suitable for uses where the global system space requirements were a decisive factor in the fulfilment of the requested functions, and, in particular, for the above stated functions (measurement of heading angles for each sensor with reference to the earth's north geographic pole). -2- In addition, the costs of standard full-inertial systems were so high that their use did not result economically convenient. Disclosure of Invention The invention described in this patent application does not suffer from such limitations, since it is based on the concept of designing and installing, in the global system, a sensor unit capable of determining its own heading angle with respect to two vectors (earth's angular speed and earth's magnetic field), which are independent one of the other. The system forming the subject of this invention includes: a) a plurality of magnetic/inertial sensor units capable of measuring the heading angle θ m k (k means the k-th sensor unit) of these sensor units with respect to the north terrestrial magnetic pole: b) a mixed sensor unit capable of measuring both its angle θ m * with respect to the magnetic north and angle θ g with respect to the geographic north, where these angles are obtained from a magnetic/inertial sensing sub-assembly (θ m ), and one completely inertial sensing sub-assembly (θ g * ), one integral with the other, and in reciprocal alignment; c) a method for compensating for errors arising from unpredictable variations occurred locally in the earth's magnetic field and affecting the heading angles measured by all magnetic/inertial sensor units. Sensor units mentioned under (a) and (b) above may be mounted either fixedly or on a gimbal and may consist of either magnetometers or magnetic compasses. The mixed sensor unit (b) provides the estimations of θ m and θ g at the same time, thus making it possible to calculate the difference angle θd = θ m - θ g . The difference angle θ d is assumed to be constant within the operating range of the distributed heading system. Therefore, θ makes it possible to correct the angle measurements performed by all magnetic/inertial sensor units as follows: Q k _ Q k Λ O g — D m - »d where: θ g k is the heading angle of each magnetic/inertial sensor unit with respect to the north geographical pole direction. List of figures Fig. 1 - Hydrophone array towed by a ship Fig. 2 -Heading system of distributed arrangement in the hydrophone array Fig. 3 - Mixed heading sensor unit Fig. 4 - Magnetic/inertial heading sensor unit Fig. 5 - Fixed reference system Fig. 6 - Body reference system Fig. 7 -Block diagram showing the compensation for heading errors in magnetic/inertial sensor units. As an example and only for descriptive (but not limiting) purposes, a towed hydrophone array 1 is shown in Fig. 1. Fig. 2 represents a heading system of distributed design, included in the above- mentioned towed sensor array 1, and comprising a mixed sensor 2 ahead, and n magnetic/inertial sensor units 3. Fig. 3 shows into detail the mixed sensor unit 2. This figure shows three gyroscopic axes 4, three accelerometer axes 5, and three magnetometric axes 6; such triads of axes are fixed with respect to the casing of the sensor unit itself and are in reciprocal alignment. Fig. 4 shows one of the magnetic/inertial sensor units 3. Each one of these sensor units 3 comprises three magnetometric axes 7 and three accelerometer axes 8. The triads of axes 7 and 8 are fixed with respect to the casing of the sensor and are in reciprocal alignment. Fig. 5 shows the reference system 9, fixed with respect to the earth. Fig. 6 shows the body reference system 10. Fig. 7 shows a block diagram of procedure steps required for correction of heading angles of the magnetic/inertial sensor units described hereinafter. The earth's magnetic field vector H, in the fixed reference system 9, when aligned with the terrestrial meridians, can be written in the form of: V H„ and in the body reference system 10: h, H b = =Rot„ b (θ,λ,φ)EL 0 h 3 where Rot nb (θ, λ, φ) is the rotation matrix according to north, pitch and roll angles (θ, λ, φ ) respectively, where θ = angle of rotation around r-axis in Fig. 6, λ = angle of rotation around q-axis in Fig.6, and φ = angle of rotation around p- axis in Fig. 6, from fixed reference system 9 to body reference system 10. In the same way G n is the gravity acceleration vector in the fixed reference system 9 and a Gb = b = Rθtnb (θ,λ,φ)Gn c is the same vector in the body reference system 10. By using the sensors included in the mixed sensor unit 2, we calculate the inertial heading vector, i.e., the triad of angles (θi * , λ, , φ, ) which are obtained through a strap-down algorithm applied to the inertial sensor outputs. In addition, the earth's magnetic field vector, H * ,, is measured in the body reference system 10. Furthermore, it is also possible to calculate the magnetic heading vector (θ m , λ m * , φ m * ) using as start point the vectors measured by the magnetometric and accelerometer sensors mounted in the mixed sensor unit 2 H; and G; as follows: _b [1] tan φ m * c [2] tan λ m _ a Vb 2 + c 2 g(bt- -cs) [3] tan θ m * (g 2 -a 2 )r - a(ct+bs) (g = : gravity acceleration) If there are no alterations arising from earth' s magnetic field, we have: i * = = φ m * λ ; * = = λ θ.' = = θ m -5- unless errors have been introduced by the sensors: this implies that the estimation of θ m k as obtained from formula [3] above, which is based on the magnetic and accelerometer outputs coming from the k-th sensor unit 3, does not require any compensation. Otherwise, the local vector Hi of the earth's magnetic field is estimated in the fixed reference system 9. To do it, use is made of the inertial heading vector, which is calculated for the mixed sensor unit 2 as follows: H = Rotbn (θ * λi , φ * ) Hb where Rotbn is the rotation matrix between body reference system 10 and fixed reference system 9 (inverse matrix of Rot„b). Since module of H and its components hi, h 3 are not decisive for the calculation of θ m , it can always be assumed that Hi has been generated only by a rotation of θd along the Z axis in the fixed reference system 9 (θd being the instantaneous local magnetic declination angle), namely: * λ H = I β =Rotb Θ ' A 0)Hb from which θ can be calculated as follows: tan Aθd=— β λ The assumption about space correlation of the earth's magnetic field variations among all sensors 2 and 3 leads to the conclusion that angle θd can be used to correct later on the estimations for θ m k coming from all distributed magnetic/inertial sensor units 3. These estimations are made on the basis of the measurement of the two following vectors: h, H =Rotnb(θ k ,λk,φ k )Rot * (θι,0,0)H n 0 b G =Rθtnb (θk , λ k , φ k ) Gn b On the other hand: -6- G, =Rθtnb(θk,λk,φ k )Rθtnb(θd,0, )Gn b for each θd, from which it can be assumed: " h, H =Rot nb (θ k + θ d ,λ k ,φ k ) H n 0 h, and: 0 G =Rot„ b (θk +θ d ,λk,φ k ) G n 0 D g Thus, the angles (θk+θd) can be calculated by means of formula [3] and then θd can be subtracted from these estimated values.

Claims: CLAIMS 1. A system (1) of inertial/magnetic sensors, suitable for measuring the heading angle with respect to the earth's north geographic pole direction, said system consisting essentially of: at least one sensor unit (2) having the capability of determining both local earth's north magnetic pole direction and earth's north geographic pole direction at the same time; a plurality of k magnetic/inertial sensor units (3) suitable for determining the local magnetic north direction only; characterised by the fact that the heading angles θ g (with respect to the earth's geographic north direction) and θ m * (with respect to the earth's local magnetic north direction), coming from the values measured by said sensor unit (2), are used to calculate the heading angle θ g k (with respect to the earth's geographic north direction), from the measurements performed by said k sensor units (3). 2. The system (1), as claimed in claim 1, characterised by the fact that said sensor unit (2) comprises at least three gyroscopes (4), at least three accelerometers (5) having the same alignment as the gyroscopic axes (4), and at least three magnetometers (6) having the same alignment as the gyroscopic axes (4) and the accelerometer axes (5). 3. The system (1), as claimed in claim 1, characterised by the fact that said sensor unit (2) comprises three gyroscopes (4) along three mutually orthogonal axes, three accelerometers (5) along three mutually orthogonal axes aligned as said gyroscopic axes (4), and three magnetometers (6) along three mutually orthogonal axes aligned with the gyroscopic axes (4) and the accelerometer axes (5). 4. The system (1), as claimed in claim 2 or in claim 3, characterised bv the fact that the calculation method used to obtain the k heading angles θ g with respect to the geographic north comprises the following steps: calculation of the angles θ g (north), λ (pitch), φ (roll) through a strap-down algorithm using the values measured by said gyroscopes (4) and said accelerometers (5); rotation of vector H > (earth's local magnetic field) measured by said magnetometers (6), from body reference system (10) to fixed reference system (9) through said angles θ g , λ and φ calculation of angle θd (instantaneous local magnetic declination) through the components of vector Hi (local earth's magnetic field in said fixed reference system (9)); calculation of k heading angles θ m k with respect to the earth's magnetic north for each one of said k sensor units (3) through a strap-down algorithm starting from the values measured by said accelerometers (8) and said magnetometers (7) of said k sensor units (3) (this step of the calculation may also be replaced by a direct measurement of said k angles θ m k by means of a magnetic gimbal-mounted device); calculation of said k angles θ g k with respect to the earth's geographic north for the k sensor units (3), obtained by subtracting said angle θd from said angles θ m k .

Inventors: CECCHERINI CARLO, SCARPA STEFANO, CECCHERINI, CARLO, SCARPA, STEFANO
Applicants: ALENIA MARCONI SYSTEMS SPA, CECCHERINI CARLO, SCARPA STEFANO, ALENIA MARCONI SYSTEMS S.P.A., CECCHERINI, CARLO, SCARPA, STEFANO
Application year: 1999
Main classification: G01C21
IPC codes: G01C17, G01C19, G01C21