GB2334842A

GB2334842A - Vehicular sensor

Info

Publication number: GB2334842A
Application number: GB9804171A
Authority: GB
Inventors: Michael Julian Richardson; Edward George Hoare
Original assignee: Jaguar Cars Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 1998-02-27
Filing date: 1998-02-27
Publication date: 1999-09-01
Also published as: GB9804171D0

Abstract

In order to determine the misalignment of a lidar or radar sensor 3 used for cruise control and mounted on the front of a vehicle 1, the sensor measures the range 0a, 0b and angular position # # (relative to its boresight X') of a fixed target (such as a tree or roadsign) with the vehicle at two different positions. The misalignment # between the antenna boresight X' and the vehicle's axis X is given by # = arctan [(0a sin # - 0b sin # ) / (0b cos # - 0a cos # )]

Description

Improvement relating to Vehicle sensors Field of the Invention The present invention relates to an improved vehicle sensor for use on a moving vehicle which senses the distance and angle of a target from a reference point and direction (the boresight of the sensor), typically the centre of the front of the vehicle and the direction of travel of the vehicle.

It is applicable with particular advantage to radar and lidar sensors used in adaptive cruise control, collision warning and collision avoidance systems for motor vehicles.

Conventional cruise control systems fulfil the function of speed control of a vehicle. A desired speed is selected by the driver and a control system operates on the engine (usually the throttle only) to maintain the desired speed.

When the traffic is light, this simple speed control is acceptable to the driver, however if the traffic is heavy the driver is faced with constantly adjusting the set speed in order to maintain a safe distance from preceding traffic and may have to disengage the cruise control in order to brake.

Many systems have now been developed by the addition of a distance sensor to the conventional cruise control system which adds distance keeping to the basic speed control function. Many such systems also include means to monitor the relative velocities of the vehicle and a target preceding vehicle. Moreover the engine control also often includes a limited authority braking system, so that the speed control/distance keeping can be effected by a combination of the throttle and the brake. Such a system may operate as an adaptive cruise control system which operates in Cruise Mode when traffic is light controlling only the speed of the vehicle and then switched automatically into Follow mode keeping a comfortable distance from the preceding vehicle once a target vehicle is within a predetermined range of the front of the vehicle.

In a typical adaptive cruise control system the system can distinguish whether the preceding vehicle is in the same lane as the vehicle to identify vehicles which lie in the vehicle's path. In such a system the sensor used is one which not only measures the distance of the target from the vehicle, but also measures the angle of the target from the path of the vehicle. In such a system for an accurate measurement of the position of a target from the vehicle, it is essential that the alignment of the sensor or sensors used is accurate. If the boresight of the sensor is misaligned from the vehicle path, then the measurements will be inaccurate. Typically the accuracy required is up to a range of 0.10 , which can lead to expensive alignment of the sensors on installation of the vehicle, periodically during lifetime or on repair after an accident.

Summary of the Invention According to the invention there is provided an improved vehicle sensor system operable to track the distance of a target from a reference point 0 and the angle of the target from a required reference direction X comprising a sensor for measuring the distance of a target from a reference point 0, and the angle of the direction of the target from reference point O to the sensors boresight (direction Xl) and alignment error calculation means for calculating the angle 6 between boresight direction Xl and the required reference direction X of the sensor system operable to take two measurements of a stationary target at two different point s along the path described relative to the vehicle as it travels , a first measurement of Oa and angle e of distance from reference point O and angle from sensor boresight Xl ,respectively and a second measurement of Ob and angle a, of distance from reference point O and angle from sensor boresight Xl,respectively and then derive an alignment error angle 6 from the measured values of Oa, Ob, a, and .

The alignment error angle E may be derived by calculation using the following formula: 6 = arctan[(Oa sin 0 - Ob sin a) / (Ob cos a - Oa cos ) By use of this formula, rather than having to expend time and effort carefully aligning the sensor so that its boresight is parallel to the required reference direction X ( typically the direction of travel of the vehicle), the measurements of the sensor are corrected to take account of the angle between the boresight and the direction of travel of the vehicle.

Preferably the system includes means to calculate the error angle on installation, and after any occasion when the sensor may have been misaligned. More preferably however, the system includes means to continuously correct the error angle to ensure that it is kept updated.

Although the measurement of the error angle is possible with a minimum of two reference tracking measurements, preferably a series of a multiplicity of measurements are used for a more accurate measurement.

According to a second aspect of the invention, there is provided a method of calculating the error angle E between the boresight X1 of a vehicle sensor, and the direction of travel X of a vehicle, comprising taking a first measurement of distance and angle of a stationary target Oa and angle and a second measurement of distance and angle of the target Ob and angle a, and calculating the error angle by the formula: 6 = arctan[(Oa sin e - Ob sin a) / (Ob cos a - Oa cos ) In some cases the angle between the boresight of the sensor and the direction of travel of the vehicle may not be an error - it may be advantageous on some occasions to have the sensor not aligned with the direction of travel of the vehicle.

Preferably the vehicle is a motor vehicle and the sensor is an automotive sensor. The invention is applicable with particular advantage to an adaptive cruise control system, collision warning or collision avoidance system. However it is applicable to any system where a moving vehicle has to monitor the distance and angle of targets in the vicinity of the vehicle.

Brief Description of the Drawings An example of an automotive sensor will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view illustrating the effect of misaligned sensor on measurement: Figure 2 is a schematic view illustrating how the misalignment error may be calculated.

Description of the Preferred Embodiment An improved vehicle 1 sensor system operable to track the distance of a target from a reference point O and the angle of the target from a required reference direction X comprises a sensor 3 for measuring the distance of a target 5 from a reference point O, and the angle of the direction of the target from reference point O to the sensors boresight (direction X < ) and alignment error calculation means for calculating the angle between boresight direction Xl and the required reference direction X of the sensor system operable to take two measurements of a stationary target at two different points along the path described relative to the vehicle as it travels , a first measurement of Oa and angle 0 of distance from reference point O and angle from sensor boresight X1, respectively and a second measurement of Ob and angle a, of distance from reference point O and angle from sensor boresight Xl ,respectively and then derive or calculate an alignment error angle E from the measured values of Oa, Ob, a, and 0 using a relationship of the general form: # = f(Oa,Ob,α#).

The following equation is a preferred form of this general formula: 6 = arctan[(Oa sin 0 - Ob sin a) / (Ob cos a - Oa cos e).

Values of E may be calculated on a real time basis, as when required or pre-calculated and stored in a look up table.

The angle between the boresight of the sensor and the direction of travel X of the vehicle is E. This is shown clearly in figure 1 which illustrates the automotive sensor system. A sensor 3 is mounted on a motor vehicle 1 which travels in direction X. Although the sensor system requires measurement of an angle # between the direction of the target 5 and direction X, the boresight of the sensor X is at an angle 6 from direction X. This means that the target is seen by the sensor as lying at a direction of # - E.

In the case where the automotive sensor is part of an adaptive cruise control system this can cause a false result. The shaded area 7 depicts the area within which a target vehicle 9 detected causes the vehicle operating mode to switch from cruise to follow mode. Vehicle 5 is outside area 7 since it is in a different lane from vehicle 1.

However the error in alignment of the sensor can lead to the sensor giving an incorrect reading that the vehicle 5 is within area 7 and falsely cause operation of follow mode.

Referring to figure 2, the sensor tracks a stationary target, in this case a tree, or roadsign. The track ba of the target will always be parallel to the direction of travel of the vehicle X. If the range and angle of the target , relative to the boresight are measured at a minimum of two points along the track, knowing the path of the vehicle, the alignment error may be calculated.

Distance Oa and associated angle 0 is measured by the sensor Distance Ob and associated angle a is measured by the sensor The object track is parallel to the vehicle path, therefor angle abc = 6 Distance ae = Oa sin 0 Distance bd = Ob sin a Therefore Distance ac = Oa sin - Ob sin a Also Distance Oe = Oa cos 0 Distance Od = Ob cos α Therefore Distance cb = Ob cos a - Oa cos e This means that the error angle, E can be found by the following 6 = arxctan [ (Oa sin 0 - Ob sin a) / (Ob cos a - Oa cos ) The adaptive cruise control system is arranged to carry out this calculation using more than two measurements of a stationary vehicle and to continuously update this measurement during operation of the vehicle. Thus any error as illustrated in figure 1 is eliminated.

Claims

Claims 1. An improved vehicle sensor system operable to track the distance of a target from a reference point O and the angle of the target from a required reference direction X comprising a sensor for measuring the distance of a target from a reference point O, and the angle of the direction of the target from reference point O to the sensors boresight (direction X) and alignment error calculation means for calculating the angle 6 between boresight direction X1 and the required reference direction X of the sensor system operable to take two measurements of a stationary target at two different point s along the path described relative to the vehicle as it travels , a first measurement of Oa and angle 0 of distance from reference point O and angle from sensor boresight X1 ,respectively and a second measurement of Ob and angle a, of distance from reference point O and angle from sensor boresight X1, respectively and then derive an alignment error angle E from the measured values of Oa, Ob, α, and #.
2. A system according to claim 1, in which the system continuously derives the error angle during operation.
3. A system according to claim 1 or 2, in which more than two measurements are used to derive the error angle.
4. A system according to any one of the previous claims in which the alignment error angle E is derived by calculation using the following formula: E = arctan[(Oa sin er - Ob sin a) / (Ob cos a - Oa cos Izr).
5. A system according to any one of the preceding claims in which derived values of alignment error angle 6 are precalculated and stored in a look-up table.
6. A method of calculating the error angle E between the boresight X1 of a vehicle sensor, and the direction of travel X of a vehicle, comprising taking a first measurement of distance and angle of a stationary target Oa and angle 0 and a second measurement of distance and angle of the target Ob and angle a, and calculating the error angle by the formula: 6 = arctan[(Oa sin 0 - Ob sin a) / (Ob cos a - Oa cos )