SYSTEM FOR CALIBRATING AN INERTIVE MEASURING UNIT

Description

[0001] The invention relates to a system for checking an inertial measuring unit of a vehicle while driving and to a corresponding method.

[0002] Today, vehicles are mostly equipped with satellite-based inertial navigation systems, whose reliability and accuracy depend on the quality of the values ​​determined by an inertial measurement unit (IMU). This applies particularly, but not exclusively, to the failure or limited availability of the satellite navigation system's signals.

[0003] To be able to contribute to the calculation of a precise vehicle position, all sensors involved must be calibrated. Calibration in this sense means that all parameters must be known in order to convert the sensor's measurements into the underlying physical units. To achieve good accuracy, calibration parameters must be continuously determined during operation, especially for inexpensive sensors commonly used in series projects.

[0004] One possibility is the use of a cost-intensive inertial measurement unit whose sensors require no calibration or only a one-time calibration at the start of the journey. These are sufficiently stable over the entire journey, so that constant calibration is not necessary. Other solutions forgo 3D position and 3D orientation estimation and reduce themselves to 2D position and 1D orientation estimation. The IMU sensors required for this reduced set can be sufficiently calibrated with the dynamics typical of land vehicles. However, 3D position and 3D orientation estimation requires an inertial measurement unit, which is dependent on constant or frequent calibration. Performing this during ongoing driving is hardly feasible given the typical driving dynamics of a land vehicle.

[0005] US Pat. No. 4,303,978 A discloses the use of such a cost-intensive inertial measurement unit, whose sensors require no calibration or only a one-time calibration at the start of flight. In this case, gyroscopes are installed in an aircraft that are intentionally not aligned along the aircraft's axes. This serves to increase redundancy while simultaneously minimizing the number of individual gyroscopes. The gyroscope is calibrated before the start of the flight using a coarse and fine adjustment phase. During the flight, calibration is performed with respect to temperature and pressure influences based on calibration factors determined in laboratory tests.

[0006] US 20107063763 A1 discloses a gyroscope sensor comprising multiple gyroscopes that are not aligned along the axis of the gyroscope. Such a sensor can be used, for example, in a known measuring unit of a vehicle.

[0007] From US 2018 / 0066961 A1, the self-calibration of an inertial system is also known, in which an additional tilted gyroscope is installed in addition to three axis-parallel gyroscopes.

[0008] The invention is based on the object of providing a vehicle system and a method that enables the inertial measurement unit to be calibrated with satisfactory accuracy and reliability during travel. This object is achieved by the features of the independent claims.

[0009] Preferred developments are the subject of the dependent claims. By express reference, the claims are incorporated into the description at this point.

[0010] According to one aspect of the invention, a system for calibrating an inertial measuring unit of a land vehicle while driving comprises an inertial measuring unit. This has at least a first acceleration sensor for measuring a translational acceleration along a first sensor axis and / or a first yaw rate sensor for measuring a yaw rate about the first sensor axis. Furthermore, it has a second acceleration sensor for measuring a translational acceleration along a second sensor axis and / or a second yaw rate sensor for measuring a yaw rate about the second sensor axis. The yaw rate sensors are collectively referred to as a gyroscope. At least one further detection device for detecting a movement of the vehicle in a first vehicle direction and / or about the first vehicle direction can provide calibration data for calibrating the inertial measuring unit.The first vehicle direction is preferably the drive direction, or the direction pointing straight ahead from the vehicle, forwards or backwards. Both the first and second sensor axes are tilted relative to the first vehicle direction. Tilting is preferably understood to mean a non-parallel or angled orientation. The system is designed to calibrate the inertial measuring unit during travel based on the calibration data of the at least one additional detection device.

[0011] The system described is based on the idea that common land vehicles such as cars have a drive that only propels the vehicle forwards or backwards, but not sideways or up or down. Likewise, rotations of the vehicle due to the steered front axle only occur around the vehicle's vertical axis. Active rotations around the forward or lateral axis are not possible, and such rotations are also very small due to the surface and driving maneuvers. For example, in a conventional arrangement the first sensor would be aligned for measurements in or against the direction of travel of the vehicle. The second sensor, on the other hand, would be aligned orthogonally upwards or orthogonally transversely, and would therefore hardly be exposed to the most significant acceleration directions that occur along the first vehicle direction and thus could not be reliably or accurately calibrated.

[0012] An inertial measurement unit is generally designed to measure rotations and / or accelerations in an inertial system. Often, accelerations in all three translational axes, as well as the respective rotation rates around these axes, are measurable. The basic principle of acceleration measurement is usually mass inertia. For example, a seismic mass moves toward or against a detecting sensor element when accelerating. For further processing or storage of the measurement data, the analog input signals are usually converted into a digital data stream. The computational steps used to determine the position and orientation of the inertial measurement unit from its inertial measurements are also known as the strapdown algorithm.

[0013] In order to calibrate all three axes of gyroscopes or acceleration sensors of an inertial measurement unit, sufficient dynamics must occur along or around each axis. With conventional installation methods along the main vehicle axes, this dynamics only occur in a few axes or directions of rotation due to the limited freedom of movement of a land vehicle. The invention solves this problem by tilting the inertial measurement unit or tilting the sensor axes. This tilting also affects the otherwise almost uninvolved axes, allowing their calibration.

[0014] According to a preferred embodiment, the inertial measurement unit comprises precisely a first acceleration sensor, a second acceleration sensor, and a rotation rate sensor, wherein the first and / or second acceleration sensor is tilted relative to the first vehicle direction. The rotation rate sensor is preferably not tilted and is aligned along the vehicle's vertical axis.

[0015] It is preferred that the inertial measuring unit comprises a third acceleration sensor for measuring a translational acceleration along a third sensor axis and / or a third yaw rate sensor for measuring a yaw rate about the third sensor axis, wherein the third sensor axis is tilted relative to the first vehicle direction. Thus, all three sensor axes are tilted relative to the first vehicle direction.

[0016] It is further preferred that the sensor axes are each additionally tilted relative to a second and / or third vehicle direction, with the second and third vehicle directions each being oriented orthogonally relative to the other vehicle directions. All sensor axes are accordingly exposed to the most significant accelerations, since these typically occur in the first vehicle direction. In other words, none of the sensor axes is exactly orthogonal to these accelerations, which is why a certain acceleration component can always be measured.

[0017] According to a preferred embodiment, all sensor axes are aligned orthogonally to each other. This is practical because the computational effort is kept within limits and inertial measurement units typically have sensor axes aligned in this way.

[0018] The first vehicle direction preferably corresponds to the steering-free direction of travel of the vehicle, whereby the orientation of each of the acceleration sensors of the inertial measurement unit enables the measurement of an acceleration component along the first vehicle direction. The steering-free direction of travel is preferably understood to be the direction of travel at 0° steering angle or the direction of travel when driving straight ahead. This is usually the direction of the vehicle's longitudinal axis. The greatest accelerations occur in this direction, therefore this direction is most relevant for calibration.

[0019] It is preferred that each of the sensor axes be tilted by an angle of more than 10°, preferably 55°, relative to the first vehicle direction. In particular, a tilt angle of 55° allows for evenly distributed calibration across all sensors of the inertial measurement unit.

[0020] It is preferred that each of the sensor axes is tilted by the same angular amount relative to the first vehicle direction.

[0021] The at least one detection device for detecting the movement of the vehicle is preferably designed as an acceleration, speed, and / or displacement sensor, an odometer, steering angle sensor, magnetometer, barometer, and / or satellite navigation module. It provides the comparison values ​​that enable calibration.

[0022] An inertial measurement unit for use in a system described here comprises a mounting device with a mounting plane for attachment to a vehicle, wherein the sensor axes of the inertial measurement unit are each tilted by an angle relative to the mounting plane. Thus, the invention can be implemented with the additional advantage that no adaptation of the vehicle structure is necessary—for example, a special mounting interface that allows for tilted mounting of the inertial measurement unit.

[0023] According to a further aspect of the invention, a method for checking an inertial measuring unit of a vehicle, which is a land vehicle, comprises the inertial measuring unit having a first acceleration sensor for measuring a translational acceleration along a first sensor axis and / or a first yaw rate sensor for measuring a yaw rate about the first sensor axis, and a second acceleration sensor for measuring a translational acceleration along a second sensor axis and / or a second yaw rate sensor for measuring a yaw rate about the second sensor axis, wherein the first and the second sensor axis are each tilted relative to a first vehicle direction, wherein a movement of the vehicle in the first vehicle direction is detected by means of at least one further detection device,wherein the inertial measuring unit is calibrated during the journey using calibration data from the at least one further detection device.

[0024] Preferably, the method is carried out by means of a system as described above and below.

[0025] According to a preferred embodiment, the method is repeated several times or in a loop while the vehicle is traveling.

[0026] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of an embodiment in conjunction with the drawings.

[0027] In a very schematic way, Fig. 1 shows an inertial measuring unit shown as a cube in a conventional installation, Fig. 2 shows an inertial measuring unit shown as a cube in an installation according to the invention.

[0028] The inertial measurement unit IMU is in Fig. 1 and Fig. 2 within vehicle V, which is represented as a cube for simplicity, as is the inertial measurement unit (IMU). The sensor axes and the first vehicle direction are represented as arrows.

[0029] At the Fig. 1 In the conventional installation method according to the state of the art shown, one of the sensor axes of the inertial measurement unit (IMU) is aligned coaxially to the steering-free direction of travel, i.e., the first vehicle direction, while the other two sensor axes are aligned orthogonally to it. The sensor axes are therefore aligned along the main vehicle axes. This has the consequence that only the sensor axis aligned coaxially to the steering-free direction of travel can be well calibrated, while this is hardly possible for the other two sensor axes because they, or a detection device usable for calibration, only experience very low accelerations. For the yaw rate sensor, the arrangement along the main vehicle axes means that only the axis coaxial to the vehicle's vertical axis can be well calibrated, while this is hardly possible for the other two sensor axes because they, or a detection device usable for calibration, only experience very low yaw rates.

[0030] At the Fig. 2 The orientation of the inertial measurement unit (IMU) shown, however, enables all three sensor axes to measure components of the acceleration along the forward-facing axis of the vehicle V. This results in some changes compared to the usual type of installation along the main vehicle axes.

[0031] For acceleration sensors, the accelerations caused by the drive and brakes of vehicle V can be recorded in all sensor axes. This type of acceleration can be measured by a recording device, for example in the form of additional sensors such as an odometer or GNSS module, thus providing the necessary redundancy for calibration in all axes. Likewise, the inertial acceleration when cornering is distributed across all axes. This can be calculated by the recording device or additional recording devices, such as a differential odometer or steering angle sensor and odometer, and is thus also available for calibration of all axes. The acceleration caused by the effect of gravity on the sensor can also be recorded in all axes.This type of acceleration can be determined by the detection device or other detection devices, such as GNSS and / or barometer, and is thus available for the calibration of all three axes.

[0032] For yaw rate sensors, the rotation of the vehicle V around its vertical axis when cornering affects all three axes of the gyroscope. This rotation can be determined by the detection device or additional detection devices, such as GNSS and / or a magnetometer, thus providing the necessary redundancy for calibration in all axes. Optionally, the deviation from the zero line in all three axes can also be determined when stationary, which can be detected, for example, by the odometer.

[0033] The sensor axes of the inertial measuring unit IMU are in the embodiment of the Fig. 2each tilted by 55° relative to the first vehicle direction, so that the complexity of the further calculation from the generated signals is kept within limits.

[0034] The tilt of the sensor axes by 55° corresponds to the optimal tilt relative to the first vehicle direction, which is equal for all sensor axes. Starting from the vehicle's main axes, this can be visualized as the rotation of an imaginary cube spanned by the mutually perpendicular unit vectors of the sensor axes. For a first rotation with an angle of α = 45° around the vehicle's vertical axis, the rotation matrix is rot z = cos α sin α 0 − sin α cos α 0 0 0 1 .

[0035] By another rotation with the angle θ, this time around the vehicle's transverse axis with the rotation matrix rot y = cos θ 0 sin θ 0 1 0 − sin θ 0 cos θ , The corner of the imaginary cube opposite the origin, i.e. the intersection point of the sensor axes, is transferred to the axis of the first vehicle direction, i.e. in this example the longitudinal direction of the vehicle. The angle θ results from the geometric relationships of the cube to θ = arctan 1 2 = 35 °

[0036] The unit vectors of the cube rotated in this way are obtained by the two rotations e x , neu = 1 / 3 − 1 / 2 − 1 / 2 3 , e y , neu = 1 / 3 1 / 2 − 1 / 2 3 , e z , neu = 1 / 3 0 2 / 3 and thus an angle of 54.7°, rounded to 55°, by which each of these unit vectors is tilted with the first vehicle direction.

[0037] However, a potentially more complex mathematical model for using the sensor data is offset by the advantage that the accuracy of the 3D position and 3D orientation estimation for land vehicles is increased by the possibility of continuous calibration of all sensor axes.

Claims

1. System for calibrating an inertial measurement unit (IMU) of a vehicle (V), which is a land vehicle, the inertial measurement unit (IMU) having a first acceleration sensor for measuring a translational acceleration along a first sensor axis and / or a first rate-of-rotation sensor for measuring a rate of rotation about the first sensor axis and also a second acceleration sensor for measuring a translational acceleration along a second sensor axis and / or a second rate-of-rotation sensor for measuring a rate of rotation about the second sensor axis, both the first and the second sensor axis being tilted with respect to a first vehicle direction, characterized in that at least one sensing device is designed to sense a movement of the vehicle (V) in the first vehicle direction and / or about the first vehicle direction, and the system is designed to calibrate the inertial measurement unit (IMU) during driving on the basis of calibration data from the at least one sensing device.

2. System according to Claim 1, characterized in that the inertial measurement unit (IMU) has a third acceleration sensor for measuring a translational acceleration along a third sensor axis and / or a third rate-of-rotation sensor for measuring a rate of rotation about the third sensor axis, the third sensor axis being tilted with respect to the first vehicle direction.

3. System according to either of the preceding claims, characterized in that the sensor axes are in each case additionally tilted with respect to a second and / or third vehicle direction, the second and third vehicle directions being respectively aligned orthogonally with respect to the other vehicle directions.

4. System according to one of the preceding claims, characterized in that all of the sensor axes are respectively aligned orthogonally in relation to one another.

5. System according to one of the preceding claims, characterized in that the first vehicle direction corresponds to the steering-free driving direction of the vehicle (V), the alignment of each of the acceleration sensors of the inertial measurement unit (IMU) respectively making the measurement of an acceleration component along the first vehicle direction possible.

6. System according to Claim 5, characterized in that each of the sensor axes is tilted by an angle of more than 10°, preferably 55°, with respect to the first vehicle direction.

7. System according to one of the preceding claims, characterized in that each of the sensor axes is tilted by the same angular amount with respect to the first vehicle direction.

8. System according to one of the preceding claims, characterized in that the at least one sensing device for sensing the movement of the vehicle (V) is formed as an acceleration, speed and / or displacement sensor, as an odometer, a steering angle sensor, a magnetometer, a barometer and / or a satellite navigation module.

9. Method for calibrating an inertial measurement unit (IMU) of a vehicle (V), which is a land vehicle, the inertial measurement unit (IMU) having a first acceleration sensor for measuring a translational acceleration along a first sensor axis and / or a first rate-of-rotation sensor for measuring a rate of rotation about the first sensor axis and also a second acceleration sensor for measuring a translational acceleration along a second sensor axis and / or a second rate-of-rotation sensor for measuring a rate of rotation about the second sensor axis, both the first and the second sensor axis being tilted with respect to a first vehicle direction, characterized in that a movement of the vehicle (V) in the first vehicle direction and / or about the first vehicle direction is sensed by means of at least one sensing device, and the inertial measurement unit (IMU) is calibrated during driving on the basis of calibration data from the at least one sensing device.

10. Method according to Claim 9, characterized in that the method is carried out by means of a system according to one of Claims 1 to 8.

11. Method according to Claim 9 or 10, characterized in that the method is repeated a number of times or in the manner of a loop during driving of the vehicle (V).