Radar-operated vehicle safety apparatus
FIELD OF THE INVENTION 
     The present invention relates generally to radar-operated vehicle safety aparatus and in particular to such apparatus in which the detected objects are discriminated in terms of their speeds relative to the roadway so as to allow reduction of the minimum spacing between successively moving vehicles. 
     BACKGROUND OF THE INVENTION 
     Detection of objects by radar produces valid as well as invalid return signals. Prior art radar-operated braking systems provide discrimination between such valid and invalid signals based on the relative distance of the object and the signal intensity, and give an alarm only when the relative distance becomes smaller than a preset minimum distance, regardless of the rate of change of the distance between it and the object. From the safety standpoint, the preset minimum distance must be decided on the assumption that the detected object is stationary. However, if the detected object is one that moves in the same direction and at substantially the same speed as the vehicle, the brake will be operated unnecessarily before the vehicle is actually approaching that object at a dangerous speed. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved radar-operated vehicle safety apparatus which discriminates a detected object between a stationary object and an object moving in the same direction as the vehicle and provides first and second minimum, or safety limit distances for comparison with the detected range for the respective objects. 
     Another object of the invention is to provide radar-operated vehicle safety apparatus which avoids unnecessary operation of braking system so as to allow reduction of the relative distance between successively moving vehicles to a minimum interval. 
     A further object of the invention is to provide a radar-operated vehicle safety apparatus which avoids the congestion of vehicles on roadways by allowing them to keep minimum spacing therebetween. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be understood from the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a circuit block diagram of an embodiment of the present invention; and 
     FIG. 2 illustrates details of the circuit blocks of the embodiment of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A radar-operated vehicle safety assurance system of the invention is schematically illustrated in FIG. 1. A vehicle speed sensor 10 of conventional design is provided to measure the speed of the roadway vehicle relative to the roadway surface and generate a corresponding signal Va which is applied to a computing circuits 12 and 14 and also to a target discrimination circuit 16. A radar device 18 of conventional design is provided to furnish information as to the vehicle speed relative to a moving target such as preceding vehicle or stationary target such as signpost or guardrail, and as to the range of the detected target. The radar device 18 provides a signal Vr indicating the vehicle&#39;s relative speed to the target and a signal R indicating the the measured range. 
     The target discrimination circuit 16 receives the range signal R from the radar devide 18 and determines whether the target of interest is stationary or moving in the same direction as the vehicle, and enables an AND gate 20 when the target is stationary or AND gate 22 when the target is one that is moving. The range signal R is then compared in comparators 24 and 26 with the output from the computing circuits 12 and 14, respectively, and applied to AND gates 20 and 22. 
     The first computing circuit 12 processes the signal Va from the vehicle speed sensor 10 to derive a first minimum, or safety limit distance on the assumption that the detected object is stationary such that the vehicle under any possible conditions can safely decelerate to a stop with a marginal distance left between it and the object. The second computing circuit 14 processes the signals Va and Vr and derives a second minimum, or safety limit distance on the assumption that the detected object is moving such that the vehicle under any possible conditions can safely decelerate to a stop with a marginal distance left between it and the moving object which is also assumed to have been decelerated to a stop. 
     When the measured range of the object is equal to or smaller than the first safety limit distance, it is considered appropriate to alert the driver that the vehicle is approaching at a dangerous speed to a stationary object in front. Under such circumstances, the comparator 24 will deliver an output, and the AND gate 20 is enabled by the signal from the discrimination circuit 16 indicating that the object is stationary, to pass the output from the comparator to an alarming device 28 and also to a brake actuator 32 through an OR gate 34. 
     Likewise, when the measured range is equal to or smaller than the second safety limit distance, it is considered appropriate to give an alarm signal indicating that the vehicle is approaching a moving object at a dangerous speed. Under such circumstances, the comparator 26 will deliver an output, and the AND gate 22 is enabled by the discrimination circuit 16 to pass the output from the comparator 26 to the alarming device 28 and to the brake actuator 32. 
     The operation of the invention will be fully comprehended by the following description. When the signals Va and Vr will substantially be equal, the detected target can be recognized as a stationary object, and conversely when the signal Va is greater than signal Vr, the detected target can be considered as a moving object. As illustrated in FIG. 2, the discrimination circuit 16 comprises a comparator 40 which discriminates the types of objects by comparing the input signals Va and Vr and provides an output to one of the AND gates 20 and 22 depending upon the result of the comparison. 
     Assuming that the detected target is a stationary object, and the vehicle running at a speed Va is assumed to have been decelerated at a rate of α to a speed Vat after the elapse of an interval &#34;t&#34;. Since Vat = Va - αt, the time T 1  taken to stop the vehicle is Va/α and the distance Rat travelled by the vehicle during deceleration is ##EQU1## 
     Assume that the vehicle come to a stop with a safety margin Rs left between it and the stationary object, and let Ro to denote the distance between the two at the instant when brake pedal is depressed, the following relation should hold: ##EQU2## from Equation (1) it will be appreciated that a warning signal should be given at the instant when the vehicle speed has decreased and the detected range R has reached a minimum distance represented by Ro = (1/2α) Va 2  + Rs. The computing circuit 12 is designed to find the minimum distance value Ro by computing the input data applied thereto from vehicle speed sensor 10. 
     The computing circuit 12 includes a squaring circuit 50 to provide an output representing Va 2  which is divided by 2α in the division circuit 52 to provide an output Va 2  /2α. An adder 54 is connected to the output of divider 52 to add a voltage representing the value of Rs to the output from the divider 52. The comparator 24 receives the output from the adder 54 for comparison with the detected range signal R and provides an output only when the output from the computing circuit 12 is reached. 
     Consider next a situation in which the detected target is a vehicle ahead which is assumed to have been moving at a speed of Vb relative to the roadway surface in the same direction as the vehicle behind and decelerated until it comes to a stop. Let Rb 1  to represent the distance travelled by the preceding vehicle during the deceleration operation, then the distance Rb 1  is given by (1/2α)Vb 2 . 
     Upon recognition of the deceleration operation, the driver in the following vehicle would immediately depress the brake pedal. There is however an inherent delay time T 1  before the vehicle actually begins to decelerate. Therefore, the distance Ra 1  travelled by the following vehicle during the time interval T 1  is given by: ##EQU3## Since the following vehicle starts deceleration at a speed Va + (dVa/dt)T 1 , the distance Ra 2  travelled by the following vehicle from the instant it starts deceleration to the instant it comes to a halt, will be given by ##EQU4## The safety margin Ra is thus given by 
     
         Rs ≦ Ro + Rb.sub.1 - (Ra.sub.1 + Ra.sub.2)          (4) 
    
     Since the speed of the following vehicle relative to the preceding one is Vr = Va - Vb, Equation (4) can be rewritten as follows: ##EQU5## Therefore, it will be understood that an alarm should be given when the following condition is met: ##EQU6## Therefore, it is understood that Ro is the minimum distance for the vehicle to be spaced from a moving object. 
     The computing circuit 14 is designed to compute Equation (6). In FIG. 2, the computing circuit 14 comprises a differentiator 60 connected to the vehicle speed sensor 10 to differentiate its output to produce dva/dt which is multiplied by T 1  /2 by a circuit 62 whose output is connected to an adder 64. The adder 64 provides summation of the two input signals applied thereto, one received from the multiplier 62 and the other from the vehicle speed sensor 10, the combined output being supplied to one input of a multiplier 66. 
     The differentiator 60 also supplies its output to multiplier 68 which multiplies the differentiated signal by ##EQU7## T 1  is then added to the output from the multiplier 68 in an adder 70 which supplies its output to the other input of the multiplier 66. In an adder 72 a voltage representing the safety margin Rs is added to the output from multiplier 66 and the added output is fed to the noninverting input of a differential amplifier or subtractor 74. 
     On the other hand, a voltage Rr representing the vehicle speed relative to the preceding one is applied to a multiplier or attenuator 76 where the input signal is multiplied by the factor of 1/α and fed into an input of a multiplier 78. The voltage Vr is also applied to a multiplier 80 to be multiplied by the factor of 1/2 and supplied to the noninverting input of a comparator or subtractor 82 for comparison with the output Va from vehicle speed sensor 10 and thence to the other input of the multiplier 78 to be multiplied by the output Vr/α from the multiplier 76. The output from the multiplier 78 is fed to the inverting input of the subtractor 74 which subtracts the output of multiplier 78 from the output of adder 72. The output from the subtractor 74 is thus a representation of the value Ro of Equation (6) and fed to the comparator 26 for comparison with the range representing signal R from the radar device 18, and if the former has reduced to a value equal to the latter the comparator 26 will provide an output through AND gate 22 to the alarm 30 as well as to the brake actuator 32 through OR gate 34. 
     Therefore, an alarm is given by the result of comparison between the actual and computed target ranges separately in respect of stationary and moving targets taking into account the safety marginal distance. The radar-operated braking system of the invention is thus operated by the valid signal which allows reduction of spacin between successively moving vehicles to a minimum without endangering safety.