1. Field of the Invention
The invention relates to a method of stabilizing the horizon of magnetic compasses.
2. Description of the Related Art
Customary compasses with a magnetic needle have no horizontal stabilization, but leveling is sometimes carried out, for example using a fluid support. The same effect would be achieved by suspending the compass from gimbles.
There are various known ways of stabilizing the horizon for magnetic compasses. For example, in the case of navigation systems, the horizon is stabilized using gyroscopic devices, but this is a complicated and expensive measure.
When there are faults in a system for stabilizing the horizon, a measuring error or reading error results, usually because of accelerations, when magnetic compasses of this type are used in motion, for example during a vehicle journey. The same errors may, however, also occur if the compass is only being held by hand.
In the case of a digital magnetic compass (DMC) known from DE 37 16 985 C1, the directional information is obtained from the projection of the geomagnetic field vector onto the horizontal plane. The DMC contains a separate sensor for each coordinate of the geomagnetic field. The horizontal plane is measured using two inclination sensors arranged perpendicular to one another. The inclination sensors are arranged in a common housing with the magnetic field sensors.
In actual fact, the inclination sensors are acceleration sensors. They are calibrated in the housing so that, in the stationary state, i.e. without additional acceleration forces acting, they only measure the components of the acceleration vector due to gravity in the X and Y directions of the DMC coordinate system, and determine therefrom the angle between the two said coordinate axes and the horizontal plane. The projection of the geomagnetic field vector is corrected in accordance with the current position of the DMC housing differing from the horizontal reference position.
The coordinate system of the DMC is a right-handed orthogonal cartesian coordinate system having origin O and three coordinate axes X, Y, Z. This coordinate system is regarded as fixed with respect to the DMC housing.
The X axis and the Y axis define a first plane which corresponds to the horizontal plane or reference plane when the DMC housing is aligned horizontally. The line of sight of the DMC coincides with the X axis. The Z axis is then parallel to the acceleration vector due to gravity.
If the DMC housing is inclined, then the DMC coordinate system must rotate with respect to a spatially fixed reference system which is likewise a right-handed orthogonal cartesian coordinate system having origin O, but now having three coordinate axes X', Y', Z'. In the horizontal state, the respectively corresponding coordinate axes and the origin of the two coordinate systems coincide.
After a rotation, the reference plane of the DMC would lie in a second plane which, for example, is obtained by one rotation through the angle .alpha. about the Y' axis and by one rotation through the angle .beta. about the X' axis. In the field of navigation, the angle .alpha. is referred to as the pitch angle, and the angle .beta. is referred to as the roll angle.
The XYZ coordinate system of the DMC can thus be converted into the spatially fixed X'Y'Z' coordinate system merely by a rotation. The pitch angle .alpha. and the roll angle .beta. are obtained as values measured by the inclination sensors.
Since the acceleration due to gravity varies only little over the surface of the earth, then, when the DMC is in a static state or in a state of uniform motion, particular inclination angles coincide everywhere with the actual position with respect to the acceleration vector due to gravity.
The behavior is different, however, if the DMC is built into a vehicle or an instrument which is braked, accelerated and moved on curved paths, so that radial accelerations and centrifugal accelerations occur.
As already mentioned, the inclination sensors are actually acceleration sensors. The latter contain a diaphragm which is deflected under the effect of acceleration forces. The deflection is measured as a variation in the capacitance of a capacitor. The deflections of the two inclination-sensor diaphragms measured in a vehicle therefore always represent a superposition of the inclination of the DMC with the acceleration vector due to gravity and the movement-induced accelerations of the DMC.
Under the assumption that the vehicle axis pointing in the direction of travel coincides with the X axis, accelerated movements on a horizontal surface lead to a pitch angle .alpha. being displayed, and therefore mimic an inclination of the horizontal plane. Under the same conditions, driving around a bend leads essentially to a deflection of the diaphragm of the roll angle sensor and therefore mimics a roll angle .beta.. Centrifugal movements, cornering, cross-drift, etc. likewise cause the display of inclinations of the DMC plane with respect to the horizontal plane which do not actually exist, and therefore lead to the projection of the geomagnetic field vector onto an erroneous horizontal plane. Magnetic field sensors are not affected by accelerations. However, for geometrical reasons, the magnetic field vector is altered by a rotation of the DMC coordinate system XYZ with respect to the horizontally oriented, spatially fixed coordinate system X'Y'Z'. The time variation of the magnetic field vector is in this case proportional to the cross product of the magnetic field vector and the vector of the rate of rotation between the coordinate systems. The components of the rotation rate vector are the changes in the angles of rotation of the coordinate axes X, Y, Z per second with respect to the horizontally oriented coordinate axes X', Y', Z'. The rotation rate vector cannot be determined fully just from the magnetic field components since, for example, a rotation exactly about the magnetic field direction leaves all three magnetic field components unchanged, so that the component of the rotation rate vector parallel to the magnetic field cannot be determined. Although the inclination sensor actually measures the rotation rate directly, this rate is affected by error in the case of accelerated movements for the reasons mentioned above. If there is a rate of rotation, it therefore affects, on the one hand, the measurement of the magnetic field vector and, on the other hand, the determination of the correct horizontal plane.
DE 34 22 490 C2 discloses a method for correcting an angular error when determining the direction of travel of a vehicle. In order to determine a correction value, the components H.sub.X and H.sub.Y of the magnetic field in the plane of the vehicle are measured using two magnetic field sensors. An instrument for measuring the inclination angle determines the inclination angle in the direction of the longitudinal axis of the vehicle. Effects of acceleration on the inclination angle are in this case taken into account by determining the first derivative of the velocity of the vehicle. The correction to the direction of travel takes into account only the error caused by the vehicle being inclined at an angle in its longitudinal direction with respect to the horizontal.
U.S. Pat. Nos. 5,287,628 and 5,444,916 disclose devices which each have three mutually orthogonal magnetic field sensors and inclination sensors, using which a horizontal plane can be generated electronically. The inclination of a vehicle is determined with respect to this horizontal plane. Acceleration effects are not taken into account.
EP 0 668 485 A1 discloses a method for reconstructing the yaw angle of a vehicle, measured using a magnetic field sensor, from raw data affected by error. For calculation, use is made of a valuation function, an iterative method, a selectable membership function and values established on the basis of a plausibility consideration. The method is based on the premise that, although directionally dependent spurious effects in the measurement of the yaw angle can indeed be registered through data measured by other sensors, they cannot be compensated by combination therewith.