Method and apparatus for adjusting a vehicle sensor.

The present invention relates to a method and apparatus for adjusting a sensor for use in sensing objects in the path of a vehicle with reference to the direction of motion of the vehicle.

Various automotive vehicle control systems such as forward obstacle detection systems or speed control systems for maintaining a predetermined distance relative to another vehicle, utilize sensors to detect objects in the path of the vehicle to which they are fitted. In general, these sensors transmit a beam of defined width forward of the vehicle and sense a reflection of the beam from a forward object, which may be for example another vehicle. Typical sensors used in these systems include microwave radar sensors and infrared sensors.

A problem associated with such systems in which the sensor is fixed in a manner to sense directly ahead of the vehicle, it will effectively monitor the path of the vehicle only when the vehicle's trajectory is a straight line. When the vehicle's motion becomes curved, there is an effective shift of the path of the vehicle relative to the beam of the sensor. This effective shift increases with distance from the sensor. This shift may result in the sensor not detecting an object in the vehicle's path. Further, the sensor may detect objects, such as other vehicles or barriers adjacent the road, which are not in the vehicle's path.

It has been proposed to maximize the useful range of the sensor and to avoid the detection of unwanted objects by moving the sensor, for example by rotating the antenna of a radar sensor, as the vehicle travels through a non-linear path, so as to move the beam of the sensor in the direction of the vehicle's path. However, when the beam is moved so as to maximize the useful range of the sensor, there is the possibility that another vehicle or object within the vehicle's path becomes undetectable due to an effective lateral shift of the object or other vehicle out of the beam for some arc length of the non-linear path as the range to the object or other vehicle decreases from the maximum range of the sensor along this path, or not to be detected, such as when another vehicle enters into this arc length from another trajectory.

The present invention seeks to provide an improved method and apparatus for adjusting such sensors.

Accordingly, an aspect of the present invention provides a method of adjusting a sensor for use in detecting objects in the path of a vehicle as defined in claim 1.

According to another aspect of the present invention, there is provided apparatus for adjusting a sensor for use in detecting objects in the path of a vehicle as defined in claim 12.

The invention can increase the effective field of detection of the sensor as compared to the fixed beam case and can prevent the loss of detection of an obstructing object resulting from the effective lateral shift of the field of detection relative to the vehicle's path as the field of detection is steered into the curved path of the vehicle.

The invention can also provide for the simultaneous steering of transmitted and received beams of a forward looking sensor of a vehicle as the vehicle travels through a curvilinear path.

Preferably, the field of detection of the sensor is steered at an angle which limits the effective lateral shift of the field of detection relative to the projected path of the vehicle such that a predetermined minimum detection criterium is achieved. In one embodiment, the minimum detection criterium is represented by a predetermined portion of the field of detection intersecting an obstruction located at the point of maximum lateral shift of the field of detection relative to the vehicle's projected path. In another embodiment, the minimum detection criterium is defined by a predetermined maximum lateral shift of the field of detection relative to the projected path.

Advantageously, the predetermined maximum lateral shift is related to the width of a small sized vehicle such that the centreline of the field of detection is not shifted beyond the edge of an obstructing vehicle.

Preferably, the angle by which the field of detection is moved is controlled on the basis of the determined radius of curvature of the vehicle's projected path.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 illustrates a prior art detection system in which the sensor is fixed;</li><li>Figure 2 illustrates a prior art detection system in which the sensor is steered to maximize the detection range of the sensor;</li><li>Figures 3 and 4 illustrate an embodiment of detection system;</li><li>Figure 5 is a block diagram of an embodiment of sensor system; and</li><li>Figure 6 is a flow diagram illustrating the operation of the system of Figure 5 in moving the radar beam.</li></ul>

Referring to Figure 1, the situation resulting from a detection system having a fixed sensor (such as a radar sensor) transmitting a fixed beam, can be seen for the case where the vehicle to which it is attached travels through a curved path. The detection system is carried on a vehicle 10 and transmits a beam 14 having a beam width BW and a maximum detection range S. The path of the vehicle 10 is represented by the line 12, passing through the centreline of the vehicle, which has a radius R. As can be seen, due to the curvature of the path away from the beam 14, there is an effective lateral shift between the beam 14 of the sensor and the path of the vehicle. This shift increases with distance from the vehicle 10. In the case where an obstructing vehicle 16 is in the path of the vehicle 10, the effective lateral shift between the sensor beam 14 and the obstructing vehicle 16 results in the obstructing vehicle 16 falling out of the detection zone of the beam 14 even at relatively small distances from the vehicle 10. Further, the sensor may pick up objects such as vehicles following different paths or barriers adjacent the road which are not within the vehicle's path.

Figure 2, on the other hand, illustrates the situation resulting from a detection system in which the sensor beam 14 is steered along the path of the vehicle 10 so as to increase the detection range of the sensor. This is accomplished by steering the beam 14 so as to intersect the path of the vehicle 10 at the maximum range of the sensor. As can be seen, there is an effective lateral shift between the beam 14 and the path that would result in obstructing vehicles (such as the vehicles 16' and 16'') falling out of the detection zone of the beam 14 over the arc length L of the path 12. Similarly, objects coming onto the path 12 in this region from another trajectory would also not be detected.

Figures 3 and 4 illustrate an embodiment of the invention, applied to a sensor 22 which transmits and receives a beam 14, having a predetermined field of detection, and in which the beam 14 of the sensor is shown steered on the basis of the curved, substantially circular, path of the vehicle, at a sensor steering angle Y.

The parameters illustrated in Figures 3 and 4 are as follows : (1) Y is the steering angle of the beam 14 and is defined as the angle between the centreline 18 of the beam 14 and the velocity vector of the vehicle 10; (2) S_limit is substantially the limit of the detection range of the sensor and in use is the distance from the vehicle 10 to the point at which the beam centreline 18 intersects the substantially circular line 12 of the path of the vehicle 10; (3) A_max is the maximum lateral distance, or shift between the path line 12 and the beam centreline 18 for a given steer angle Y and path curvature and occurs at a distance S_limit/2 for the path 12 shown; (4) B is half the angle of the arc segment defined by the beam centreline 18; (5) R is the radius of the path 12 of the vehicle 10; (6) b is half the width of the beam 14 at the distance S_limit/2; (7) d is half the width of the obstructing vehicle 16; (8) e is the width of the beam 14 irradiating the obstructing vehicle 16 (the beam/vehicle overlap); and (9) BW is the angular beam width.

The path ahead of the vehicle 10 along the line 12 is a projected path determined in the preferred embodiment from the angle to which the vehicle 10 is steered and the velocity of the vehicle 10. In another embodiment, the projected path is determined from the steering angle alone.

In general, the beam 14 is steered to an angle of Y so that the value A_max falls within a predetermined minimum detection criterium. In one embodiment, A_max is made equal to half the width of the vehicle 10.

In another embodiment, the value of A_max is computed such that in use a desired minimum proportion of the beam width BW irradiates the obstructing vehicle 16 over the range S_limit. In other words, the value of A_max is chosen such that a predetermined minimum proportion of the field of detection of the sensor falls onto a predetermined area about the path of the vehicle 10. This could be in, to one side, or above the path of the vehicle 10, or some other area near or in the path of the vehicle 10.

The following relationships between the foregoing parameters can be shown: <maths id="math0001" num=""><math display="inline"><mrow><mtext>Y = B,   (1)</mtext></mrow></math><img file="EP0443643A2_D0001.tif" /></maths><maths id="math0002" num=""><math display="inline"><mrow><msub><mrow><mtext>S</mtext></mrow><mrow><mtext>limit</mtext></mrow></msub><mtext> = 2RsinY + △S   (2)</mtext></mrow></math><img file="EP0443643A2_D0002.tif" /></maths> where △S generally accounts for the path width, <maths id="math0003" num=""><math display="inline"><mrow><msub><mrow><mtext>Y</mtext></mrow><mrow><mtext>limit</mtext></mrow></msub><msub><mrow><mtext> = cos⁻¹[(R-A</mtext></mrow><mrow><mtext>max</mtext></mrow></msub><mtext>)/R]   (3)</mtext></mrow></math><img file="EP0443643A2_D0003.tif" /></maths> where Y_limit is the value of the beam angle Y resulting in the maximum lateral distance value A_max, <maths id="math0004" num=""><math display="inline"><mrow><msub><mrow><mtext>e = d-A</mtext></mrow><mrow><mtext>max</mtext></mrow></msub><mtext>+b,   (4)</mtext></mrow></math><img file="EP0443643A2_D0004.tif" /></maths><maths id="math0005" num=""><math display="inline"><mrow><msub><mrow><mtext>b = Rsin{cos⁻¹[(R-A</mtext></mrow><mrow><mtext>max</mtext></mrow></msub><mtext>)/R]}tan(BW/2).   (5)</mtext></mrow></math><img file="EP0443643A2_D0005.tif" /></maths>

In the case where the beam 14 is to be steered so that a minimum proportion PCT of the beam will irradiate the obstructing vehicle 16 when the obstructing vehicle is at the distance S_limit/2, then <maths id="math0006" num=""><math display="inline"><mrow><msub><mrow><mtext>PCT = e/2b = (b+d-A</mtext></mrow><mrow><mtext>max</mtext></mrow></msub><mtext>)/2b   (6)</mtext></mrow></math><img file="EP0443643A2_D0006.tif" /></maths> where <maths id="math0007" num=""><math display="inline"><mrow><msub><mrow><mtext>+b ≧ (A</mtext></mrow><mrow><mtext>max</mtext></mrow></msub><mtext>-d) ≧ -b</mtext></mrow></math><img file="EP0443643A2_D0007.tif" /></maths> and b is defined by expression (5).

From expressions (5) and (6), it can be seen that since the values of PCT, d and BW are known or are assigned desired values, only A_max and R are unknowns. However, the radius R of the projected path of the vehicle 10 can be determined from the driver's chosen steering angle D_d (the steering wheel position angle relative to a reference angle, such as a centreline position, divided by the steering gear ratio) of the vehicle 10; and more accurately by also taking into account vehicle dynamics by use of the general linear vehicle dynamics equation <maths id="math0008" num=""><math display="inline"><mrow><msub><mrow><mtext>D</mtext></mrow><mrow><mtext>d</mtext></mrow></msub><mtext> = 57.3(A+B)/R + KtV²/R,   (7)</mtext></mrow></math><img file="EP0443643A2_D0008.tif" /></maths> where A and B are the distances from the front and rear wheels of the vehicle 10 to its centre of gravity, Kt is a vehicle specific constant and V is the velocity of the vehicle 10. The vehicle specific constant Kt is a scaling factor related to the lateral acceleration term V²/R and the steering angle, and is also a function of the vehicle suspension characteristics. An approximation of the constant Kt can be determined by experiment, for example by driving the vehicle through a curve of known radius at a known velocity and by measuring the steering angle.

Since D_d and V can be measured, the expression (7) can then be solved to find the radius of curvature R of the projected path of the vehicle 10 taking into account the motion of the vehicle. Using the obtained value of R, expression (6) can then be solved for A_max as a function of the obtained curvature R of the projected path. The determined value of A_max provides the desired minimum proportion PCT of the beam 14 which will intersect the obstructing vehicle 16 (the minimum detection criterium). Once the value of A_max is determined, the steer angle Y_limit which will provide the minimum proportion can then be determined from expression (3).

It can be seen that for known or assigned values of beam width BW, obstructing vehicle width d and minimum irradiating proportion PCT, a corresponding value of Y_limit can be determined from a measurement of the radius R of the projected path of the vehicle 10 which, in this embodiment is obtained via expression (7) from measurements of vehicle speed V and driver input steering angle D_d.

In another embodiment, the minimum detection criterium is chosen to be a constant value for A_max, such as half the width of the narrowest obstructing vehicle that may be encountered, for example the width of a small sized vehicle. This value is then substituted in expression (3) along with the determined radius R to obtain the steer angle Y_limit.

By controlling the steer angle of the beam 14 to the angle Y_limit, the effective lateral shift of the beam 14 relative to the projected path is limited so that no obstructing vehicle will fall out of the beam 14 as the beam is steered while travelling along a curved path. At the same time, the detection range S_limit of the sensor is maximized. The value of this range can be computed by expression (2) and used to cut-off signals from the sensor beyond this range so as to prevent sensing of unwanted vehicles in different paths, or other objects such as railings, etc.

Referring to Figure 5, there is illustrated a system fitted to the vehicle 10 and used for sensing the presence of, for example, an obstructing vehicle and for steering the beam 14 on the basis of the equations described above. The system includes a radar system 20 of conventional design for determining distance and/or speed relative to an object such as the obstructing vehicle 16. The radar system 20 transmits a microwave signal of known beam width BW ahead of the vehicle 10 by means of an antenna 22 mounted on the front of the vehicle 10. This signal forms the beam 14 previously referred to.

A reflected microwave signal from an object in the vehicle's projected path is picked up by the antenna 22 and detected by the radar system 20 which in turn provides distance and/or relative speed information to a cruise computer 24.

The cruise computer 24 may provide vehicle control functions in response to the information provided from the radar system 20. For example, the cruise computer 24 may control the speed of the vehicle 10 to maintain a desired distance from the obstructing vehicle 16 in a similar manner to that described in our copending application No (RJ/3436) filed the same day as this application.

The antenna 22 is rotatable to steer the radar beam transmitted and received thereby by means of a stepper motor 26. The antenna may, for example, be mounted on a platform mounted on an input shaft that is rotatable about a vertical axis on rotation of an output shaft of the motor 26 to which it is coupled. The stepper motor 26 is controlled by the cruise computer 24 via a stepper motor driver 28 to rotate antenna 22 to steer the radar beam 14. In this embodiment, the stepper motor driver 28 causes the stepper motor 26 to rotate by a predetermined incremental angle for each step command from the computer 24, in the direction commanded by the computer 24.

The cruise computer 24 determines a desired radar beam limit steer angle Y_limit in the manner described above, in response to vehicle speed V, provided by a vehicle speed sensor 30, and a driver input steering angle D_d, provided by a steering wheel position sensor 32. Based on the angular position of the antenna (the beam angle Y), provided by an antenna position sensor 34, the cruise computer 24 determines direction and the number of angle increments by which the stepper motor 26 is to be rotated in order to establish the desired steer angle Y_limit. Control signals representative of this information are sent to the stepper motor driver 28 which causes the stepper motor 26 to be moved by the required number of angle increments in the determined direction.

The cruise computer 24 is in this embodiment a conventional general purpose digital computer, for example a Motorola MC-68HC11 microcomputer chip, programmed to control the angle of the antenna 22 and therefore the radar beam angle Y on the basis of the equations described above. The computer 24 comprises a read-only memory, a random access memory, an analog to digital converter, a central processing unit and an input-output section for interfacing with the peripheral devices. The read-only memory of the computer 24 contains the instructions necessary to implement its control algorithms in addition to storing constants, look-up tables, etc. utilized in carrying out its various tasks.

The flow diagram of Figure 6 illustrates the control algorithm of the cruise computer 24 used in this embodiment in controlling the steering of the radar beam 14. The specific program instructions are not described in detail as they will be apparent to those skilled in the art from the flow diagram in Figure 6. Of course, electronic circuitry may be used in place of a microprocessor.

Referring to Figure 6, the program for causing the cruise computer 24 to control the steer angle Y of the radar beam 14 by control of the angular position of the antenna 22 is executed at repeated interrupt intervals established by an internal timer. Upon the occurrence of a timer interrupt, the routine in entered at step 36 and proceeds to read the various input values including the vehicle speed V, the steering wheel position D_d and the antenna angular position Y. This step may also filter the input values if required.

At the step 40, the desired antenna position Y_limit is determined so as to establish the desired minimum detection criterium. In this embodiment, Y_limit is chosen so as to obtain a maximum lateral shift value A_max which establishes a predetermined minimum proportion PCT, in this case percent (such as 30%), of the beam width falling on, for example, an obstructing vehicle. In this embodiment, the antenna position Y_limit is obtained from a look-up table storing values of antenna steer angle Y_limit as a function of vehicle speed V and steering wheel position D_d. These values are obtained by means of the expressions described above, summarized as follows. For each combination of vehicle speed V and steering wheel position D_d, there is a corresponding projected path curvature radius R obtained from the expression (7). For each obtained radius R, there is a corresponding value of A_max for the known beam width BW of the radar beam, assumed obstructing vehicle width d and desired minimum proportion PCT of the radar beam to irradiate the obstructing vehicle, obtained from expression (5). Finally, for each value of A_max and radius R, there is a corresponding steering angle Y_limit determined from the expression (3). Using these relationships, a look-up table of values of antenna steering angle Y_limit is stored in the read-only memory of the cruise computer 24, as a function of vehicle speed V and steering wheel position D_d. As mentioned above, the predetermined minimum proportion of the beam width BW is determined as a predetermined minimum proportion of the beam to irradiate a predetermined area about the path of the vehicle. This may be in, above or to the side of the path, or any other area related to the path of the vehicle in which it is desired to detect an obstructing vehicle or other object.

It is not desirable to steer the antenna to the steer angle Y_limit for normal straight path steering corrections made by the driver. These normal steering corrections are represented by steering wheel angles D_d less than a predetermined value. To prevent the antenna being rotated at steering wheel angles less than the predetermined value, a dead-band function is stored in a look-up table in the the read-only memory as a function of the steering wheel angle relative to the reference angle.

In this embodiment, the dead-band function has a value of zero for values of steering angle D_d less than the predetermined value, and increases to unity as the steering angle increases above the predetermined value.

The dead-band function DB is obtained from the look-up table at step 42. At step 44, the steering angle Y_limit determined at step 40 is multiplied by the dead-band function determined at step 42 so as to prevent the antenna 22 from being rotated if the steering angle D_d represents a straight path steering correction. The resulting value is thus limited to a maximum allowable antenna steer angle Y_limit.

At the next step 46 the error e in the position of the antenna steering angle is determined on the basis of the expression e = Y_limit - Y. At step 48 the required stepper motor direction (based on the sign of the error e) and the number of angle increments for correcting the antenna steering angle error determined at step 46 is determined and output to the stepper motor driver 28. The number of angle increments is determined according to the expression    number of increments = G*e, where G is a gain function dependent on the error, G = f(e), with a value from 0 to 1. In another embodiment, G is a constant.

An error dead-band may be provided wherein no corrections in the steer angle of the antenna is provided for errors within the dead-band.

In the preferred embodiment, the step 48 outputs the angle increments to the stepper motor driver 28 sequentially at a predetermined rate to limit the slewing rate of the antenna. For example, step 48 may output the increment commands to the stepper motor driver 28 at 30 millisecond intervals by repeatedly executing a 30 millisecond delay loop until the required number of increments have been made. This method of rotating the stepper motor 26 limits the slewing rate of the antenna 22 to a predetermined vehicle steering wheel rotation rate and functions to filter rapid steering wheel movements. Following the step 48, the program exits the routine.

By the foregoing routine, the antenna steering angle and its radiated radar beam is controlled as the vehicle 10 travels through a curved path to rotate the radar beam angle such that the desired minimum detection criterium is established.

As will be apparent to the person skilled in the art, the sensor 22 may be replaced by other types of sensor.