Abstract:
An optical radar system for vehicles has a stationary receiver and a transmitter which scans the road surface ahead of the vehicle, the scan area being shifted in accordance with the direction of steering of the vehicle. The scan area is covered by periodically sweeping a narrow-beam pulse laser through the designated scan area and monitoring the return lag of pulses reflected by objects in the scan area. The scan area is centered over the longitudinal axis of the vehicle while the latter is moving straight ahead. The axis of the scan area is shifted in proportion to the orientation of the steering wheel of the vehicle.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an optical radar system for sensing the distance to obstacles using a search beam, and more particularly, to an optical radar system having a variable sensing area and hence improved performance using a search beam which can swivel in accordance with the steering angle of a steering wheel. 
     One conventional radar system for vehicles is disclosed in Tokkaisho (Japanese unexamined published patent application) No. 55-86000 published on June 28, 1980. The radar system disclosed in this application is an electromagnetic wave radar system. However, recently, it has been suggested to use optical radar employing a laser search beam in vehicles. 
     This optical radar system includes a control unit which produces and processes electrical signals, an optical transmitter which radiates a search beam having a predetermined wavelength, and an optical receiver which collects light reflected by reflective objects and transduces the collected light into a corresponding electrical signal. 
     Furthermore, the control unit includes a pulse modulator which produces a trigger signal input to a signal processing unit, the signal being generated simultaneously with a drive pulse signal. Drive pulse signal causes a light-emitting element fixed to the optical transmitter to produce a corresponding light pulse. The light pulse is then focused by a lens into a beam which is transmitted in front of the vehicle. Weak light reflected by an object is focused by a large-diameter lens. The reflected light focused by the lens passes through an optical filter which filters out background noise (external lights such as solar light, artificial illumination, etc.), enters a light-receiving surface of a light-sensitive element positioned at the focal point of the lens, and is transduced into a corresponding electrical reflection signal consisting of narrow, low-amplitude pulses. The reflection signal is input to, amplified and shaped by a wide-band amplifier of the control unit into a high-amplitude pulse signal which is then output to the signal processing unit. The processing unit derives the propagation delay of reflection light pulse relative to the light pulse radiated by the optical transmitter from the time relationship between the trigger signal and the pulse signal, thereby allowing calculation of the distance to the object in a well-known manner. 
     In the above radar, the beam transmitted by the optical transmitter is designed so as to have a slight angle of divergence in order to ensure detection of objects in front of and relatively close to the vehicle. 
     When the angle of divergence and direction of the beam are fixed, the detection distance limit (beyond which objects are not regarded as obstacles even if sensed) is restricted so as not to detect objects in the adjacent lanes while the vehicle travels along a straight road. An object present outside of the detection distant limit in front of the vehicle cannot be detected. 
     On the other hand, when the vehicle is travelling along a curved road, the beam is greatly offset out of the vehicle lane, so that a preceding vehicle travelling along the curved lane cannot be tracked. The deviation of the beam from the curved lane increases as the radius of curvature of the road decreases. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an optical radar system which is capable of increasing the detection distance limit without sensing objects in the adjoining lanes and which can reliably detect the preceding vehicles even through curves. 
     This invention provides an optical radar system for a vehicle, comprising: an optical transmitter means for transmitting a search beam toward possible objects in a road along which the vehivle is travelling; means for repeatedly scanning the search beam parallel to and across the road surface; an optical receiver means for receiving the light of the search beam reflected by the objects; and means for determining the distances from the vehicle to the objects on the basis of the timing relationship between the transmitted search light and the received light. 
     This invention also provides a method of detecting and ranging objects in a road on which a vehicle travels, comprising the steps of (a) transmitting a search beam toward the objects; (b) scanning the transmitted search beam periodically across the road surface; (c) receiving light of the search beam reflected by the objects; and (d) determining the distances from the vehicle to the objects on the basis of the timing relationship between the transmitted search beam and the received light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art optical radar system for vehicles; 
     FIG. 2 is a timing chart of the main output waveforms of the radar system of FIG. 1; 
     FIG. 3 is a diagram of the detection area of the prior art optical radar along a straight road; 
     FIG. 4 is an illustration of a detection area along a curved road; 
     FIG. 5 is a block diagram of the radar system of the present invention; 
     FIG. 6 is a more detailed block diagram of the optical radar system of FIG. 5; 
     FIG. 7 is a timing chart of the main output waveforms of the radar system of FIG. 6; 
     FIG. 8 is a cross-sectional view of an optical transmitter and receiver portion of the radar system of FIG. 6; 
     FIG. 9 is a cross-sectional view of the main part of the transmitter; 
     FIG. 10 is a circuit diagram of an embodiment of a pulse drive circuit of FIG. 9; 
     FIG. 11 is a diagrammatic perspective view of a deflection mirror and a mirror rotating device of the radar system of FIG. 6; 
     FIG. 12 is a diagrammatic perspective view of the combined internal components of the mirror drive device; 
     FIG. 13 is a perspective view of the radar system of FIG. 6 as mounted in the vehicle; 
     FIG. 14 is a graph of the relationship between the electric current supplied to the mirror rotating device and the rotating angle of the deflection mirror; 
     FIG. 15 is a graph illustrating changes in the direction of the search beam mediated by the deflection mirror; 
     FIG. 16 is a block diagram of an embodiment of a distance determinator unit of FIG. 6; 
     FIG. 17 is a flowchart of the program executed by a microcomputer of the radar system of FIG. 6; 
     FIG. 18 is a circuit diagram of a deflector drive unit; 
     FIG. 19 is a graph of the waveform of a signal fed to the deflector drive unit; 
     FIG. 20 is a diagram of the operation of the radar system when the vehicle is travelling along a curved road; and 
     FIG. 21 is a diagram of the operation of the radar system when the vehicle is travelling along a straight road. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to facilitate understanding of this invention, first, a prior art optical radar system will be described. An exemplary prior art optical radar is illustrated in FIG. 1 of the drawings. 
     This optical radar includes a control circuit X which produces and processes electrical signals, an optical transmitter Y which radiates a search beam having a predetermined wavelength, and an optical receiver Z which collects light reflected by objects and transduces the collected light into a corresponding electrical signal. 
     In addition, a pulse modulator 1 sends a trigger signal b to a signal processing unit 3, the pulses of signal b being generated simultaneously with those of a drive pulse signal a, which has a period Tp (about 100 μs), a pulse width Tw (about 50 ns) and a peak value Vo, as shown in FIG. 2. 
     Drive pulse signal a causes a light-emitting element 2 fixed to optical transmitter Y to produce a corresponding light pulse Lt having a wavelength λ and a pulse width Tw. Light pulse Lt is then focused by a lens 4 into a beam having a angle of divergence θt. The weak light Lr reflected by objects within the path of the beam is focused by a large-diameter lens 5. Reflected light Lr collected by lens 5 passes through an optical filter 6 which filters out background noise (external lights such as solar light, artificial illumination, etc.), enters a light-receiving surface of a light-sensitive element 7 positioned at the focal point of lens 5, and is transduced into a corresponding electrical reflection signal c consisting of narrow, low-amplitude pulses. Reflection signal c is amplified and shaped by a wide-band amplifier 8 into a high-amplitude signal d which is then output to signal processing unit 3. Unit 3 derives the propagation delay τ of reflection light pulse Lr relative to light pulse Lt radiated by optical transmitter Y from the time relationship between trigger signal b and pulse signal d, thereby allowing calculation of the distance R to the reflection object from the following formula: 
     
         R=c·τ/2 
    
     where the units of R and τ are meters and seconds, respectively, and c is about 3×10 8  meters/second. 
     In the above radar system, the beam Lt transmitted by optical transmitter Y is designed so as to have a angle of divergence of about 70 mrad and an optical axis g normal to the front of a vehicle J as shown in FIG. 3 in order to ensure detection of an objects in front of and relatively close to vehicle J. 
     When the angle of divergence θt and direction of beam Lt are fixed, as above, a detection distance limit r 0  (beyond which objects are not regarded as obstacles even if sensed) is restricted when the vehicle is travelling along a straight road as shown in FIG. 3. That is, since one vehicle lane has a width W of about 5 m on an average road, and assuming the distance covered by the beam Lt on either side of the lane is the detection distance limit r 0 , r 0  ≈W/θ t  =70 m, so that an object present 100 m or more in front of the vehicle cannot be sensed. 
     As shown in FIG. 4, when vehicle J is travelling along a curved road, the beam Lt falls well outside of the vehicle lane, so that a preceding vehicle Jf travelling along the curved lane cannot be sensed. The deviation of the beam Lt from the curved lane increases as the radius of curvature Ra decreases. 
     In FIG. 5, an optical radar system for vehicles according to this invention includes an optical transmitter 11 which transmits a search beam substantially parallel to a road surface, an optical receiver 12 which receives light reflected by objects and a control unit 10 which senses the distance to reflective objects in accordance with the time relationship between the transmitted search beam and the received light, and a scan unit SU which repeatedly swivels the axis of the search beam to cause the light signal to sweep repeatedly through a scanning field. 
     In more detail, in FIG. 6, control unit 10 includes a clock generator 13, a wide-band amplifier 14, a distance determiner 15, a microcomputer 16, a D/A converter 17 and a deflector drive unit 18. Microcomputer 16 receives a signal S 5  indicative of the actual steering angle of the steering wheel from a steering angle sensor 19. As shown in FIG. 7, clock generator 13 produces a trigger signal S 1  having a period Tp (≈0.1 ms) and a pulse width Tt (≈100 ns) supplied to optical transmitter 11. Transmitter 11 is responsive to trigger signal S 1  to produce a search beam pulse Lt having a wavelength λ, a pulse width Tw (≲50 ns) and a repetition period Tp. 
     The part Lr of search beam Lt reflected by an object (not shown) is sensed by optical receiver 12 which produces an electrical reception signal S 2  which is in turn amplified and shaped by wide-band amplifier 14 and then sent to distance determiner 15. Determiner 15 produces a distance data signal S 4  corresponding to the distance to the object in accordance with a delay time τ of reception signal S 2  relative to trigger signal S 1  and supplies the data signal S 4  to microcomputer 16. 
     FIG. 8 is a cross-sectional view of optical transmitter 11 and receiver 12, taken parallel to the road surface. As shown in the figure, the housings of transmitter 11 and receiver 12 are juxtaposed into an integral unit. Optical receiver 12 includes a large-diameter convex lens 121 which collects light Lr reflected by the object, an optical filter 122 which filters out background noise such as solar light, artificial illumination, etc., (filter 122 may be of an interference type having a pass band, the central wavelength of which is λ), a photoelectric transducer 123 (such as a PIN photodiode) having a relatively large light receiving surface (the diameter of which is 7-10 mm) which receives the reflection light Lr passing through filter 122, and a preamplifier 124 which has a band width of tens of MHz and a gain of 20-30 dB and which amplifies the signal from transducer 123 and outputs a signal S 2  via a coaxial connector 125. 
     Transmitter 11 includes an optical transmitting unit 111, a deflection mirror 116 which pivots or swivels optical signal Lt transmitted by transmitting unit 111 parallel to the road surface to cause the signal Lt to scan across the road, and a mirror rotating device 117. As shown in FIG. 9, transmitting unit 111 includes a pulse drive unit 114 and laser diode 113 in a hollow cylindrical housing and a convex lens 112 mounted in an opening in the housing. Pulse drive circuit 114 is so designed, as shown in FIG. 10, that when trigger signal S 1  from clock generator 13 is output to pulse drive unit 114 via coaxial connector 115, it triggers a transistor Q1 to render same conductive, thereby supplying a charge (of about 150 V) stored in a capacitor C 1  connected to the collector of transistor Q1 to a laser diode 113 connected between the emitter of transistor Q1 and ground. 
     Search beam Lt transmitted by transmitting unit 111 is reflected along a predetermined path by a reflection mirror 118 mounted directly in front of convex lens 112 and then reflected by deflection mirror 116 and radiated through a window 119 in the front end of the vehicle. 
     Deflection mirror 116 is mounted on a shaft 32 of mirror rotating device 117, as shown in FIG. 11. Shaft 32 is rotated through a predetermined angle corresponding to the electric current Id supplied by deflector drive unit 18 to mirror drive device 117, thereby rotating deflection mirror 116. The deflection angle θ of search beam Lt reflected by mirror 116 is twice the rotation angle θ 2  of mirror 116, that is, 
     
         θ=2θ.sub.2                                     (1) 
    
     FIG. 12 briefly illustrates the principle of mirror rotating device 117 which includes two opposing permanent magnets 30 (S-polarity) and 31 (N-polarity) and a movable electromagnet 33 with a coil Ci wound thereon disposed between the two permanent magnets and a vertical shaft 32 extending through the midpoint of magnet 33. Deflection mirror 116 is fixed to the top of shaft 32 and a spring 35 is mounted between the lower end of shaft 32 and the casing. 
     Coil Ci is supplied with electric current Id by deflector drive unit 18, D/A converter 17 and microcomputer 16 via slip rings 36 and 37 provided on the upper and lower ends of shaft 32. Current Id magnetizes movable electromagnet 33, resulting in a force F having a direction shown by the arrows in the figure, so that movable magnet 33 stops at the position where force F and the return force of spring 37 balance after shaft 32 has rotated through a corresponding angle. 
     FIG. 13 shows the state in which optical transmitter 11, optical receiver 12 and control unit 10 are actually mounted and search beam Lt is transmitted. 
     As described above, mirror rotating device 117 rotates mirror 116 by an angle θ 2  corresponding to current Id. The relationship between current Id and angle θ 2  is shown in FIG. 14. 
     Id supplied by drive unit 18 changes in the form of a triangular waveform with a period of 2T 0  and an amplitude 2Ip so that mirror 116 repatedly swivels with a period of 2T 0 , as shown in FIG. 14. 
     If the scan range (sweep width) of search beam Lt is θ t  and the central angle γ of the scan field (the angle subtended by the center line of the scan field and the optical axis g of the transmitter 11) is γ, the relation between current value Id and rotating angle θ 2  is represented as follows: 
     
         Id=Iγ+2Ip(t/T.sub.0)-2n)                             (2) 
    
     where 2n≦(t/T 0 )≦2n+1, and Iγ is a constant current. 
     
         Id=Iγ+2Ip{2(n+1)-(t/T.sub.0)}                        (3) 
    
     where 2n+1≦(t/T 0 )≦2(n+1) ##EQU1## where 2n≦(t/T 0 )≦2n+1 ##EQU2## where 2n+1≦(t/T 0 )≦2(n+1) where n=0, 1, 2, 3, . . . , Iγ is a constant current, and t is time. 
     Thus, the orientation of search beam Lt changes as shown in FIG. 15. At this time, the relation between the angle θ subtended by search beam Lt and optical axis g and the swing angle θ t  is represented as follows: ##EQU3## where 2n≦(t/T 0 )≦2n+1 ##EQU4## where 2n+1≦(t/T 0 )≦2(n+1) 
     The central angle γ is adjusted according to the steering angle of the vehicle which changes as the vehicle moves through a curve and is monitored by microcomputer 16. Microcomputer 16 controls the scanning of search beam Lt and performs various controls on the basis of distance data S 4  from distance determiner 15, as will be described later. 
     As shown in FIG. 16, distance determiner 15 includes an RS flip-flop 151 set by trigger signal S 1  generated by clock generator 13 and reset by reception signal S 3  produced by wide-band amplifier 14, a high-frequency generator 152 which produces a high-frequency pulse train, an AND gate 153 and a high-speed counter 154 which counts the pulses of the pulse train. AND gate 153 is opened by the output of RS flip-flop 151 during the time lag between trigger signal S 1  and reception signal S 3 , thereby supplying the high-speed pulse train to counter 151. 
     Thus, counter 154 outputs count data corresponding to this time lag which is supplied as distance data signal S 4  to microcomputer 16. 
     Steering sensor 19 senses the angle of the steering wheel and includes, for example, a potentiometer mounted on the steering wheel, the potentiometer output being supplied as a binary code signal S 5  to microcomputer 16. 
     FIG. 17 is a flowchart of the operations executed by microcomputer 16 which can be broken down into three main procedures, namely, a scan control which controls the deflection of search beam Lt in accordance with the radius of curvature of the road along which the vehicle travels i.e., on the basis of steering angle data from steering angle sensor 19 to sense objects present ahead within the curved road portion; a detection distance limit value control which controls the detection distance limit value along both sides of the vehicle lane so as not to detect objects present in adjacent lanes; and a distance discrimination which discriminates whether the distances to the objects present within the detection range of the radar system are beyond a safe inter-vehicle distance and thereby whether there is any dange of rear-end collision. 
     The above scan control process includes reading data S 5  corresponding to the steering angle θ s  of the steering wheel from steering angle sensor 19 at step (1) and deriving the central angle γ of the scan angle θ t  of search light Lt on the basis of angle θ s  at a step (2). At this time, microcomputer 16 send a control signal S 6 , which sets the central angle of search beam Lt to γ, via D/A convertor 17 to deflector drive circuit 18, thereby rotating mirror 116 through a corresponding angle. 
     Calculation of the central angle γ for use in calculating voltage signal Ea will be described below. As shown in FIG. 20, the central angle γ represents the angle subtended by central axis i of scan angle θ t  of search beam Lt and optical axis g of optical transmitter 11. Changing the central angle γ in accordance with the sensed steering angle of the vehivle enables objects in front of the vehicle to be detected, even around curves. The central angle γ is set such that the radially inward extreme of search beam Lt (achieved when the deflection angle θ reaches its inward extreme) will be tangent to the radially inward edge li of the road lane. If the angle subtended by an axis h, which is perpendicular to a line segment OQ connecting the center Q of the front of the vehicle J and the center of curvature O of the curved lane, and the tangent to the lane boundary li is labelled &#34;α&#34;, then the central angle γ can be expressed as follows: 
     
         γ=β+α-θ.sub.t /2                    (8) 
    
     where β is referred to as actual steering angle, that is, the angle between the axis h of the front wheels and the longitudinal axis of the vehicle, i.e., optical axis g. This actual steering angle β is represented according to Ackerman&#39;s revolution theory as follows: 
     
         |β|≈Lh/Ra                   (9) 
    
     where Ra is a radius of curvature of the curve and Lh is the length of wheel base of the vehicle. Actual steering angle β can also be expressed as follows: 
     
         β≈k·θ.sub.s                    (10 ) 
    
     where k is a constant and θ s  is the actual steering angle as measured by sensor 19, and β is proportional to the steering angle θs of steering wheel. 
     The values of α and β of formula (8) cannot be measured directly, so that the central angle γ must be calculated from other known data, which will be described briefly below. 
     If vehicle J is assumed to travel along the centerline of a lane having a width of W, then, since angle α is equal to the angle at the center of curvature O of the lane between the center of the front Q of the vehicle J and the tangential point R b  of the search beam . . . 
     
         cos α=ORb/OQ=Ra/(Ra+W/2)                             (11) 
    
     In formula (11), since Ra&gt;&gt;W, 
     
         Ra/(Ra+W/2)≈1-(W/2Ra) 
    
     In addition, α&lt;1, thus, cos α≈1-(α 2  /2) therefore, 
     
         α=√W/Ra                                       (12) 
    
     thus, substituting formulae (9) and (12) into formula (8), ##EQU5## Since radius of curvature Ra of formula (13) is an unknown value, and from formulas (9) and (10), 
     
         Ra≈Lh/k|θs|                (14) 
    
     substituting Ra into (13), 
     
         γ≈k·θ.sub.s +√k·θ.sub.s ·W/Lh-(θ.sub.t /2)                         (15) 
    
     Therefore, once the steering angle θs of steering wheel is known, the central angle γ can be calculated. 
     Deflector drive circuit 18, which includes an operational amplifier OP, changes the current Id flowing through coil Ci wound on movable magnet 33 of mirror rotating device 117 in accordance with changes in the voltage signal Ea supplied thereto, as shown in FIG. 18. The voltage signal Ea is a triangular waveform signal which has a period of 2T 0 , an amplitude of Ep and a lower voltage lever Er, as shown in FIG. 19. In this embodiment, the value of period 2T 0  is selected such that T 0  =512Tp, so that during time T 0 , 512 pulses of search beam Lt are transmitted. Amplitude Ep of voltage signal Ea and lower voltage level Er have the following relationship and are calculated at steps (3) and (4) of FIG. 17: 
     
         Ep=Ip·Ri=K·θ.sub.t /2              (16) 
    
     
         Er=Ir·Ri=K·r-Ep                          (17) 
    
     where θ t  is the scan angle of ≈70 mrad of search beam Lt, Ri is the input resistance of operational amplifier OP of deflector drive circuit 18 and K is a constant determined by the characteristics of mirror drive circuit 117. 
     Calculation of voltage Ea is performed at steps (5), (6) and (7) in correspondance with elapsed time in order to conform voltage signal Ea to a triangular waveform such as shown in FIG. 19, as follows: 
     
         Ea=Er+2Ep(t/T.sub.0)-2n)                                   (18) 
    
     where 2n≦(t/T 0 )≦2n+1 
     
         Ea=Er+2Ep{2(n+1)-(t/T.sub.0)}                              (19) 
    
     where 2n+1&lt;(t/T 0 )&lt;2(n+1) 
     In this way, voltage signal Ea such as shown in FIG. 19 is fed to deflector drive unit 18, thereby rotating deflection mirror 116 repeatedly (step (8)). 
     Next, the detection distance limit value rc is calculated in order to prevent detection of objects outside out of the lane. 
     This is calculated on the basis of the deflection angle θ of search beam Lt. At a step (9) of the flowchart of FIG. 17, the deflection angle θ is calculated from voltage Ea derived at step (6) or (7) on the basis of the following formula (20) derived from formulae (16) and (17): 
     
         θ=2θ.sub.2 =Ea/k                               (20) 
    
     After signal S 4  indicative of the distance R to the detected object is received from distance determinator 15 in step (10), then the magnitude of the steering angle θs is checked at step (11). If |θs|≦0.05, then the vehicle is determined to be moving along a straight road whereas if the result is NO, the vehicle is determined to be moving along a curved road. 
     When the vehicle is determined to be moving straight ahead at step (11), the program goes to a step (12) to derive a detection distance limit rc by the following formula (21): 
     
         rc=W/2|θ|                          (21) 
    
     That is, if vehicle J is travelling along the centerline of a straight road, as shown in FIG. 21, the detection distance limit rc is the distance to the point P where search beam Lt transmitted at a deflection angle of θ intersects the vehicle lane border li. If the width of the vehicle lane is W, rc is given by formula (21). 
     On the other hand, when the vehicle is determined to be travelling along a curved road, the program goes to step (13) to derive the detection distance limit rc in accordance with the calculations described below. 
     When the vehicle is travelling along a curved road, the deflection angle θ of search beam Lt, as shown in FIG. 20, is in the following range: 
     
         γ-θ.sub.t /2≦θ≦γ+θ.sub.t /2 (22) 
    
     whereby search beam Lt scans through a range of θ t  centered on angle γ. 
     If a point where search beam Lt transmitted at a deflection θ intersects the outer curved borderline l 0  of the vehicle lane is P, and the point where a line extending from P normal to a line segment OQ intersects segment OQ is H, the following expressions hold: 
     
         Line segment QH=rc·sin (θ-β)           (23) 
    
     
         Line segment PH=rc·cos (θ-β)           (24) 
    
     Since the length of OQ=Ra+W/2, from formula (23): ##EQU6## 
     Applying the Pythagorean theorem to triangle ΔOHP, 
     
         (Segment PH).sup.2 +(Segment OH).sup.2 =(Segment PO).sup.2 (26) 
    
     where 
     
         Segment PO=Ra+W                                            (27) 
    
     Substituting formulae (24), (25) and (27) into formula (26), 
     
         rc.sup.2 ·cos.sup.2 (θ-β)+{Ra+W/2-rc·sin (θ-β)}.sup.2 =(Ra+W).sup.2                     (28) 
    
     From formulae (22), (8) and (16), 
     
         θ-β≦γ-β+θ.sub.t /2=α≈√W/Ra                           (29) 
    
     If, for example, the radius of curvature of the curve Ra≧1,000 m and the width of the vehicle lane W≈4 m, formula (29) gives: 
     
         θ-β≦0.06&lt;&lt;1                              (30) 
    
     Thus, 
     
         cos.sup.2 (θ-β)≈1-(θ-β).sup.2 (31) 
    
     
         sin (θ-β)≈θ-β                (32) 
    
     Substituting these formulae (31) and (32) into formula (28), the detection distance limit rc can be derived from the following formula: ##EQU7## 
     Performing an approximation on formula (33) and rearranging the result, since rc≧0 
     
         rc≈{(θ-β)+√(θ-β).sup.2 +W/Ra}Ra (34) 
    
     Since the radius of curvature Ra is a unknown numerical value, if Ra is eliminated using formula (18), the detection distance limit rc could then be derived as a function of steering angle θs as shown in the following formula: ##EQU8## 
     Calculation of rc in accordance with formula (35) results in reliable detection of objects in the curved lane concerned without detecting other objects in the adjoining lanes. 
     The avoidance of objects outside of the lane in which the vehicle is travelling is actually effected at a step (14) wherein the distance R to the object in front of the vehicle is checked to see whether it is less than rc derived from formula (21) or (35), and when R is greater than rc, the object is determined to be outside of the lane concerned, and accordingly, the distance data is ignored thereafter, i.e. the program ends immediately. 
     When an object is detected within the lane of the vehicle, the probability of collision is determined according to the distance to the object. 
     That is, at a step (15), the distance R to the object is compared to the minimum braking distance Rs derived in correspondance with to the current vehicle speed. When distance R to the object is determined to be less than braking distance Rs, there is a high probability of collision and at a step (16), an output is produced to generate an alarm. 
     The braking distance Rs is a distance that the vehicle will cover before coming to rest after the driver notices the object and begins to brake the vehicle as hard as possible, and is calculated on the basis of the vehicle speed sensed by a speed sensor (not shown), using another processing routine (not shown), employing the following expression: 
     
         Rs=Va·Td+Va.sup.2 /2α.sub.0                 (36) 
    
     where Va is the vehicle speed, α 0  is the deceleration setting, and Td is a response delay time. 
     Thus, whether the vehicle is travelling along a straight road or a curved road, the above processes enable objects present in the lane in which the vehicle is moving to be detected reliably and enable the distances to the objects to be measured, thereby resulting in a high-performance optical radar. 
     Connection of the radar illustrated in the above embodiment to an automatic vehicle travel controller will facilitate more accurate follow-up travel control which could allow the preceding vehicle to be followed at an appropriate inter-vehicle distance. 
     The distance R to the object and the direction toward the object derived on the basis of the deflection angle θ of search beam Lt when distance R is sensed are known, which further improves the radar performance. 
     As described above in detail, the optical radar system for vehicles according to the present invention ensures detention of objects in the vehicle lane in front of a vehicle moving through a curve and increases the maximum detection distance while preventing detection of objects outside of the lane in question. That is, the present invention provides a high-performance optical radar. 
     While this invention has been described and shown in terms of a preferred embodiment thereof, it will be apparent to those skilled in the art that various modifications and changes of this invention could be made without departing from the scope of this invention as set forth in the attached claims.