US3614729A

US3614729A - Vehicle-detecting apparatus using electromagnetic wave

Info

Publication number: US3614729A
Application number: US809843A
Authority: US
Inventors: Kyohei Fujimoto; Nobuyuki Suyama
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1968-03-29
Filing date: 1969-03-24
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19
Also published as: DE1915813B2; DE1915813A1

Abstract

An all solid-state microwave vehicle-detecting apparatus specially equipped with time-setting circuits to attain a high accuracy in registering the number of moving vehicles on a roadway, said time-setting circuits processing signals to obtain an output well correlated with an actual number of vehicles regardless of the state of traffic, said vehicle-detecting apparatus having a high reliability and compactness attained by completely transistorized circuits and radiofrequency circuits of special design comprising a circuit which functions as a frequency multiplier as well as a signal detector.

Description

ilnited States Patent Inventors KyoheiFujimoto Yokohama; Nobnyuki Suyama, Chigasaki-shi, both of Japan Appl. No. 809,843

Filed Mar. 24, 1969 Patented Oct. 19, 1971 Assignee Matsushita Electric Industrial Co., Ltd.

Osaka, Japan Priority Mar. 29, 1968 Japan 21 134 VEHICLE-DETECTING APPARATUS USING [50] Field of Search 343/5, 5 PD, 8; 340/38 [56] References Cited UNITED STATES PATENTS 3,182,312 5/1965 Daniels 343/8 X Primary Examiner-T. I-I. Tubbesing Atlorney-Stevens, Davis, Miller & Mosher ABSTRACT: An all solid-state microwave vehicle-detecting apparatus specially equipped with time-setting circuits to attain a high accuracy in registering the number of moving vehicles on a roadway, said time-setting circuits processing signals gz g g s g WAVE to obtain an output well correlated with an actual number of alms rawmg vehicles regardless of the state of traffic, said vehicle-detect- U.S. Cl 340/38 R, ing apparatus having a high reliability and compactness at- 343/5 R, 343/5 PD, 343/8 tained by completely transistorized circuits and radiofrequen- Int. Cl 608g l/015, cy circuits of special design comprising a circuit which func- G08g l/06 tions as a frequency multiplier as well as a signal detector.

fir 5 4x 5% f" ;Z;ZEZCFT ro BANDH ISS DETECTOR l MULT/PL/Ef? I AMPL/HER *SHAP/NG I I 6 V i I I] OSC/LLATOR I /9 Chm/,7 OUTPUT I L H R 21A) 1 PATENTEU 0121291971 3,514,729

sum 3 or 3 FIG. 5a

L= 4.5m 50 E s .5 2.0 E E R m /.0 2 E 05 a a b a a2 1 1 l l l 1 l l l "/0203940506070 /0203040506070 AVERAGE spa-0 v (km/h) AVERAGE SPEED v /h) FIG. 6

' 29 32 AUX I57 T/ME 26 2N0 TIME cowsm/vr 30\ co/vsm/vr CIRCUIT CIRCUIT 7 ATTORNEY s VEHICLE-DETECTING APPARATUS USING ELECTROMAGNETIC WAVE The present invention relates to vehicle-detecting apparatuses, and more particularly to those of type using an electromagnetic wave.

Recently, an undesirable excess concentration of trafiic has quickly advanced on streets and especially in large cities almost all over the world this problem is becoming more and move serious.

So-called traffic control systems have been rapidly developed these years as a means to solve such traffic problems on streets. As a terminal equipment of the system various vehicle-detecting apparatuses have also been developed, and the ones now in practical use utilize inductance loops, pressure tubes and ultrasonic waves respectively.

A vehicle-detecting apparatus of the type using an electromagnetic wave has a number of advantages as compared with those of the other type, but it is at the same time defective in that when applied to a plurality of traffic lanes the vehicle detecting apparatus often cannot count the actual number of vehicles. This is true in case vehicles of many different shapes are running.

In principle, a vehicle-detecting apparatus of the type using an electromagnetic wave continuously radiates an electromagnetic wave from the antenna toward a roadway on which vehicles pass receives a reflected wave from vehicles passing within a detection zone of the apparatus, and detects the reflected wave having a frequency modulated by a Dopplershifted frequency so that the presence of the passing vehicles is detected to deliver the corresponding count output.

Advantages provided by such a vehicle-detecting apparatus of the type using an electromagnetic wave as compared with the other types of vehicle-detecting apparatus are as follows:

1. No construction work is needed for the roadway on which vehicles pass.

in an inductance loop-type apparatus, for example, inductance loops must be buried under roads or streets, sometimes new roads have to be constructed, and therefore not only a large amount of expense would be incurred for such work but also the traffic flow would be disordered.

2. Available detection range is flexible.

In inductance loop-type and pressure tube-type apparatuses, the available detection range is limited by the location and/or the dimensions or size of the inductance loops, so that the location and/or the dimensions of the inductance loops to be buried have to be selected depending upon each separate trafiic lane and the number of lanes. On the other hand, in an electromagnetic wave-type apparatus, the available detection range is adjustable by changing the mounting position and/or the angle of an antenna so that only one apparatus suffices for either a single traffic lane or a multiplicity of traffic lanes.

3. The apparatus is not susceptible to the environment.

Deformation of roads and weather conditions such as rain and wind which the inductance loop-type and ultrasonic wavetype apparatuses have suffered from respectively do not influence the operation of the apparatus of this type at all.

4. No mechanical abrasion is encountered.

Mechanical troubles such as breakage of pressure tubes, change of characteristics of inductance loops due to deformation of roads, etc. need not be considered.

5. The size is small, power consumption is small and production cost is low.

The power consumption is smaller than that of the ul trasonic wave-type apparatus, the size is so small that production and installation cost is low. This is especially appreciable when cost per traffic lane is considered.

6. The reliability as a whole is high.

The reliability, when the (counting) operation, maintenance, etc. of the apparatus are taken into consideration, is high.

Notwithstanding the above-mentioned various advantages, the electromagnetic wave-type vehicle detecting apparatus has in general the following drawbacks:

l. Relating to resolution.

The above-mentioned advantage (2) is accompanied by the following inconveniences especially when the detection zone of the apparatus is rather broadly extended.

l-A. Multiple counting for one vehicle: If a passing vehicle has large dimensions or if a vehicle passes the detection zone at a rather low speed, electromagnetic waves reflected back from the passing vehicles are intermitted, resulting in splitting one signal.

l-B Decreased counting for a group of vehicles: In contrast to the above case, when a group of vehicles pass simultaneously within the detection zone, the counting number derived from the apparatus is less than the actual number of vehicles. This problem takes place in the case where a single antenna is used in responding to the passing vehicles on more than one trafiic lanes or where the traflic is rather heavy. 2. Undesirable output for traffic control due to failure of the output of apparatus.

In addition to the above-recited drawbacks, inconveniencies are encountered with the vehicle-detecting apparatus in carrying out satisfactory trafiic control. In other words, if the high-frequency oscillator section which is one of the most essential parts of the apparatus stops the oscillating operation for some reason, the counting operation is ceased and the number of vehicles counted by the apparatus or the traffic is in appearance none. Thus, this kind of trouble will cause undesirable traffic control on the roadway.

The present invention is intended to provide a novel vehicle-detecting apparatus which includes specific circuitries and is capable of performing improved functions.

The present invention will next be described referring to a preferred embodiment in conjunction with the accompanying drawings, in which:

FIG. 1 is an electrical systematic diagram of a vehicle-detecting apparatus in accordance with an embodiment of the present invention;

FIG. 2 is an electrical connection diagram of the major part of the apparatus shown in FIG. 1;

FIG. 3 is a graph showing a relationship between the vehicle speed and the signal duration time;

FIG. 4 is a diagram showing a relation between the travel of vehicles and the output signals;

FIGS. 5a and 5b show counting rates calculated; and

FIG. 6 is a somewhat detailed circuit diagram useful for explaining the time-setting operation of the present apparatus.

Referring to FIG. 1, and FIG. 2 the body of a vehicle-detecting apparatus of the present invention is designated by reference numeral 1 and includes a high-frequency section 2 and a low-frequency amplifier and signal-processing section (hereinafter referred to as low-frequency section) 3. Numerals 4 and 5 denote an antenna and a moving vehicle respectively. In the high-frequency section 2: numeral 6 is an oscillator including an oscillation-active element for generating a signal at a frequency of f,/n; 7, a frequency multiplier and detector which feeds a power at a frequency of f,, and which includes a resonant circuit 9 to which is fed the output of the oscillator 6, another resonant circuit 10 one terminal of which is connected to the antenna 4, a frequency-multiplyingactive element 11, a detection output delivering coil 12 through which a detected output from the active element is derived, and a detector circuit 13; and 14, a fail-safe circuit including a gate circuit consisting essentially of a diode 21 connected to a commercial power source 22 through a variable resistor 23 and adapted for sensing the presence of the oscillation output of the frequency multiplier and detector 7 by monitoring the self-bias voltage to the active element 11 so that when the output of the oscillator 7 is decreased or becomes zero and therefore the bias voltage is lowered, an artificial signal at a frequency of the commercial power source 22 is fed to the low-frequency section 3, without any additional oscillator. In the low-frequency section 3: numeral 15 is a lowlfrequency band-pass amplifier 16, a detector circuit; 17, a wave-shaping circuit; iii, an output relay circuit; 19, a timesetting circuit; and 263, an output terminal.

The operation of the apparatus will next be explained. The output at a frequency of f,/n of the oscillator 6 of the highfrequency section is supplied to the frequency-rnultiplying active element 111 through the first resonant circuit 9 of the frequency multiplier and detector 7. The radiofrequency output from the resonant circuit is n-times multiplied by means of the active element 111 so that a predetermined output at a frequency of f, may be fed to the antenna 4 through the second resonant circuit 10. An electromagnetic wave radiated from the antenna 4 is reflected by a vehicle with a Dopplershifted frequency when it passes the detection zone of the apparatus. Presence of a moving vehicle within the detection zone can be detected as follows. Since the frequency-multiplying active element 11 possesses nonlinear characteristics, it functions as a detector as well as a frequency multiplier, so that if the reflected wave is mixed with a multiplied wave at the element ill, the output of the element 11 gives Dopplershifted components to effect the homodyne detection. Thus, the detected output is derived from the detector circuit 13 through the detection output delivering coil 12. The function of the detector circuit 13 is to detect the Doppler deviation in tenns of amplitude and the frequency component of the deviation is not detected, but it is possible to further provide an additional circuit for deriving such frequency component. The output signal from the detector circuit 13 is then fed to and amplified with a high gain at the low-frequency band-pass amplifier 115, and the amplified signal is subjected to envelope detection. The detected signal is transformed into a rectangular wave by the wave-shaping circuit 17 to thereby actuate the output relay circuit 13 to deliver an on-ofi' output at the output terminal 20 representative of the number of detected vehicles.

Referring to FIG. 6 wherein a circuit arrangement for performing the time-setting operation is illustrated, the output from the wave-shaping circuit 17, when including more than one pulse for one vehicle, is integrated at a first time-constant circuit 25 for converting the output from the shaping circuit 17 into a signal having a single pulse for one vehicle, and the integrated signal is applied to a transistor 26. The transistor is then activated, so that a relay 27 is energized with its contacts 28 and 29 closed. Thus, at the terminal 20 there appears only one signal for one vehicle even though more than one pulse is supplied for one vehicle to the low-frequency section 3.

Inasmuch as the contact 29 is closed so long as a signal is fed to the transistor 26, if the signal includes a smaller number of pulses than the actual number of vehicles, a circuit portion consisting of a second time-constant circuit 30 for dividing the duration time of the smaller number of pulses and a unijunction transistor 31 functions to generate pulses at a predetermined interval so that the generated pulses trigger a silicon controlled rectifier 33 through a coupling circuit 32 to render the rectifier 33 conductive. As a result, a relay 34 in series with the silicon controlled rectifier 33 is energized and opens its contact 35. Therefore, the terminals 20 are opened without any output. Upon closure of a contact 38 to be efiected after lapse of a time determined by an auxiliary circuit for resetting the relay 34, a relay 39 is energized with its contact 24 opened, which closes the contact 35. Accordingly, the output terminals 20 are closed, and when the relay 27 is maintained operative, the circuit consisting of the members 29, 30 and 31 functions to continue generation of pulses at a predetermined frequency. if, however, the pulse duration time of a signal fed to the circuit 30 is shorter than the duration time of the output of the circuit consisting of the members 29, 30 and 31, the contact 29 is opened, so that the circuit consisting of the members 29, 30 and 31 no longer generates its output. Then, no input is applied to the silicon controlled rectifier 33 and the operation for dividing the signal duration is stopped.

If there is no vehicle within an effective range covered by a radiated wave, no reflected wave which contains a Doppler shifted component is returned back to the antenna 4. Therefore, no input is applied to the low-frequency section 3, thus delivering no output from the output terminal 20.

The present invention is featured in the frequency multiplier and detector 7 and fail-safe circuit 14 both in the highfrequency section 2 and the time-constant circuit 19 in the low-frequency section 3. The combination of these in accordance with the present invention makes it possible to provide a small-sized vehicle-detecting apparatus with a high sensitivity, obviating the drawbacks experienced with the conventional vehicle-detecting apparatus.

The time-setting circuit performs the dual function of removing the above-mentioned drawbacks (l-A) and (l-B) thus making the counting accuracy of the apparatus almost equal to percent. The probability and the frequency of intermission of a signal at the time when a vehicle passes along a road is determined by the size and kind of the vehicle, the width of the road, the number of trafiic lanes, the location where the antenna is placed and the sensitivity of the vehicledetecting apparatus. In order to prevent such intermission, the duration time of a signal has to be lengthened. For example, suppose that the length of a vehicle is l, the moving speed of the vehicle is v, and the detection zone of the vehicle-detecting apparatus is L. Then, the effective signal duration time I, is

1# while considering that vehicles are advancing at a constant average speed v (group velocity) the interval 1, between two adjacent signals is where X represents a headway. Since the headway with respect to the speed of a vehicle can be known from the measurement heretofore made, I, is easily determined. For example, in case l=3m or 4m and L=3m, t, is as indicated in FIG. 3. It will be seen that the efi'ective signal duration time 1; becomes shorter as a vehicle speed goes higher and the time interval 1, between two adjacent signals becomes longer as a vehicle speed goes higher. Thus, it is necessary to select the signal duration time so as to be shorter than t, and almost as long as a normal signal duration corresponding to one vehicle such as indicated by t, in FIG. 4. As seen from FIG. 3, supposing that a minimum vehicle speed v during the measurement is 10 km./hr., and since a pause time interval t, between signals is approximately 200 msec., the signal duration may be set between 50 and 200 msec. taking into consideration possible speed range of vehicles. Thus, a circuit means is provided to vary the signal duration time stepwise, a maximum duration time 200 msec., msec., 100 msec. and 50 msec. By setting the signal duration at one of those values in accordance with trafiic condition, corrected counting can be attained.

Next, consideration will be given to the case where vehicles are successively passing through the detection zone of the apparatus. In this case there appears a very complex phenomenon, so that here the phenomenon is first symbolized. FIG. 4 indicates a symbolized relationship between the travel of vehicles and the lapse of time as well as corresponding output signal duration time. The headway is sometimes l which is less than L and is sometimes 1,, which is larger than L, and at this time the former situation is considered (i.e., l L).

The signal with respect to a vehicle A is represented by S having a duration time a, while the signal with respect to a vehicle B is represented by S having a duration time 1,. Since the vehicle B enters the detection zone L before the vehicle A leaves the zone, the signal 8,, will overlap the overlap the signal 8,, during a time n. Thus, the resulting signal is a single signal S with respect to the two vehicles A and B. B. Suppose now that such overlap takes places n-times for Q vehicles. Then, the counted number m of vehicles is This frequencyfis equal to m of equation (3). The counting rate a is defined as co unted number i,

actual number of vehicles Q Meanwhile, if a signal (duration time: t,,) is intermitted at a period of I and is divided intoB,

Z {N (when quotient is integer) or i N+1 (when quotient is not integer) (6) so that the corrected counting rate a is after the overlap of signals has been compensated for, the mean duration time lSm ofone signal becomes Thus, from equations (6) and 7) 1 can be derived to increase the counting rate. In equation (8),

FIG. 5a shows a case where L=4.5 m. and l =3m. FIG. 5b shows another case where L=7 m. and 1=3 m. A curve or represents the counting rate which is the ratio of the number of counts obtained without forcibly dividing produced signals at every period t (namely, without increasing the number of pulses) to the actual number of passing vehicles. As a vehicle speed is lowered, the headway becomes short to thereby increase the probability of successive entrance of vehicles into the detection zone L of the vehicle-detecting apparatus, consequently resulting in a reduction in the counting rate a. If an output signal having duration time exceeding the time T is forcedly divided at every period i the number of counts, of course, goes higher than the case with no such division to thereby increase the counting rate a. FIG. 5 shows a result of calculation of the counting rate in the case of T =O.5, 1.0, 1.5 and 2.0 sec., L=4.5 m. and !--3 m. For example in FIG. 5a, when t =l.5 sec. and the vehicle speed is 10 km./hr., the counting rate becomes approximately 1.0, and when t,,-l.5 sec. and the vehicle speed is 32 km./hr. the counting rate becomes 0.6. When the vehicle speed exceeds 32 km./hr., passing vehicles tend to overlap one another to exclude the probability of producing signals having duration time exceeding 1.5 sec., thereby causing the counting rate curve to coin- 'cide with the curve (1. Because there are cases where B in the expression (6) does not take a value of an integer depending on the relation between the duration T of a signal and the period a in the expression (7) does not take the form of a continuous curve. Reading values out from these figures, t is selected so as to be 1.25 to 1.5 sec. for 22330 Km./h.

1.0 see. for W230 to 40 Km./h., and 0.75 to 0.5 sec. for 11240 Km./h.

as the most suitable values. Thus, it can be said that the intermitting period can be known theoretically in combination with data based upon various experiments with regard to the traffic condition on a roadway under consideration. The apparatus furnishes a means for adjustment to vary 1,, stepwise and set it to one of the values: 0.5, 1.0, 1.5 and 2.0 sec. to correct the counting rate a, so that one of the drawbacks l-B) can be overcome.

As mentioned above, the time-setting circuit 19 is provided with two circuit means capable of setting the signal duration time and the signal intermission time at suitable values which are supported by theoretical calculation and experiments. A proper combination of the dual setting operation of the timeconstant circuit, that is, expansion and intermission of a signal, makes it possible to achieve satisfactory counting which is correlated with the actual number of vehicles to such an extent that the counted number is within I5 percent of the actual number of moving vehicles in the detection zone. In the conventional vehicle-detecting apparatuses, the counting rate is what is susceptible to the above-mentioned drawbacks (l-A) and l-B) and is at most multiplied by a fixed correction factor for any traffic condition. In the present apparatus, the counting rate a can be modified by means of time-setting circuits so as to be correlated with the actual number of vehicles; further by multiplication of the counting rate by a correction factor, the corrected counting rate a'will be almost representative of the actual number of vehicles.

The high-frequency section 2 functions to multiply the frequency of the oscillation output from the oscillator 7 and at the same time to detect the Doppler-shifted component contained in the reflected wave. conventionally, these two functions have been carried out separately, or sometimes simultaneously using the grid circuit of a vacuum tube, but in the latter case, workability has been questionable due to low sensitivity and short lifetime of vacuum tube. In the present apparatus, since the high frequency section includes an active element which may be a semiconductor such as a variablecapacitance diode for frequency multiplication, the Dopplershifted component can be simultaneously detected by the active element because the element is of a nonlinear characteristic. That is, by mixing a reflected wave with a portion of an electromagnetic wave to be transmitted, the resultant output is representative of a Doppler-shifted component. Further by making impedance match properly at the output circuit from which the Doppler-shifted component is derived, the sensitivity of the apparatus is much improved. (If a vacuum tube is used, such impedance matching is impossible.) The circuit construction using an active element is very effective for performing the two functions, the frequency multiplication and the mixing, so that the size and cost can be reduced and the construction is simplified, thus making the vehicle-detecting apparatus economical and highly reliable.

In addition, the present vehicle-detecting apparatus includes a fail-safe circuit 14 for the purpose of preventing the erroneous traffic control when the oscillator fails to transmit the power, as it gives no output, indicating zero traffic. The fail-safe circuit 14 operates to monitor the power output of the frequency multiplier and detector 7 by means of the bias voltage to the semiconductor active element 11 and includes a gate circuit such that upon decrease or cease of the oscillation output the corresponding variation in the bias voltage to the element is detected and the detected variation is used to actuate the gate circuit adapted to supply a signal at a commercial power source frequency to the output relay circuit 18 through a low-frequency amplifier. Therefore, the fail-safe circuit is capable of effectively producing an artificial signal at a commercial power source frequency without an additional oscillator. As a result of supplying an artificial signal at a commercial power source frequency to the low-frequency section 3 which signal has a fixed duration time, the artificial signal is intermitted by the time-setting circuit 19 at a period to which the circuit 19 is set. Therefore, even if the oscillator ceases its output, the present vehicle-detecting apparatus provides an artificial output corresponding to an output representative of a certain number of vehicles, so that trafiic control can be effected by the artificial output without bringing about any serious problem even when the oscillator fails its output.

As has been described above, in the vehicle detecting device of the present invention: the high-frequency section serves to feed a radiation power to an antenna so that an electromagnetic wave is radiated from the antenna and at the same time serves to detect a Doppler-shifted component contained in a reflected wave from vehicles; the low-frequency section serves to selectively amplify only the Doppler-shifted component at a bandpass amplifier without introducing noise, to subject the amplified signal wave-shaping process at a wave-shaping circuit and to process the shaped signal at a time-setting circuit; and the fail-safe circuit is operative to monitor the bias voltage to the Doppler-shifted component detecting active element so that upon detection of a decrease or cease of the oscillation output from the high frequency section in terms of the corresponding variation in the bias voltage, an artificial signal may be produced from a commercial power source and fed to the low-frequency section to prevent nil output of the apparatus in spite of absence of the oscillation output from the oscillator.

As a consequence, irrespective of whether only one vehicle passes or in the event of very slow-moving or closely spaced vehicles passing through an effective range of a radiated beam of electromagnetic waves the counting rate is always almost representative of the actual number of vehicles, and further even though trouble takes place by accident in the highfrequency section the apparatus can provide an artificial output signal rather similar to an output signal representative of the actual traffic.

Further, it is possible that the fail-safe circuit is so constructed as to be responsive to a variation in the bias voltage to the oscillator which produces an oscillation output signal.

What is claimed is:

1. A vehicle-detecting apparatus, comprising, in combination:

a high-frequency section including antenna means for radiating an electromagnetic wave and for receiving a reflected wave from vehicles and means for detecting a Doppler-shifted component in terms of amplitude contained in said reflected wave;

a low-frequency section, including: circuit means for selectively amplifying only a part of said Doppler-shifted component which is within a particular range; means means for wave-shaping said amplified signal component; and a time-setting circuit, including first circuit means for converting converting a plurality of shaped signals from a wave-shaping circuit into a smaller number of count signals, means extending the duration time of said plurality of shaped signals, and second circuit means operatively connected to said first circuit means for converting at least one shaped signal from said wave-shaping circuit into a larger number of count signals and means for dividing said at least one shaped signal, the number of said count signals being nearly representative of the actual number of vehicles passing through an effective range of a radiated beam of electromagnetic waves from said antenna; and

fail-safe circuit means monitoring said low-frequency section and supplying an output signal to said low frequency section upon detection of an abnormal operation of said high-frequency section.

2. A vehicle-detecting apparatus according to claim 1,

wherein said Doppler-shift detecting means includes means applying said reflected wave to an active element and means for mixing a portion of said reflected wave with a portion of said radiated signal.

3. A vehicle-detecting apparatus as set forth in claim 1, characterized in that said fail-safe circuit includes a gate circuit for sending an artificial si al at a frequency of a commercral power source to said lowequency section in the event of a power failure in said high frequency section detected in terms of a variation in the bias voltage of either one of a frequency-multiplying active element and an oscillation active element in said high-frequency section.

4. A vehicle-detecting apparatus comprising a high frequency section including a high-frequency oscillator circuit, an antenna for radiating an output of said high frequency oscillator circuit toward a ground surface and for receiving a radio wave reflected by a moving vehicle on the ground surface and a detector circuit for detecting a Doppler-shifted frequency component in the received wave;

a low-frequency section including a bandpass amplifier circuit for selectively amplifying a component lying within a predetermined frequency band of said Doppler-shifted frequency component, a detector circuit for detecting only an amplitude of an output of said bandpass amplifier circuit, a wave-shaping circuit for producing a constantamplitude output for a waveform having a varying amplitude greater than a predetermined value in an output of said detector circuit and producing zero output for a wave form having an amplitude smaller than said predetermined value, time-setting circuit means including a time-setting circuit A for holding an output of said wave-shaping circuit for a predetermined time to prevent a plurality of count outputs from being produced by a single vehicle and a time-setting circuit B for dividing, at every predetermined time interval, a single output of long duration of said wave-shaping circuit produced when a plurality of vehicles successively pass to simulate the number of passing vehicles, and an output circuit for con trolling the output of said wave-shaping circuit by said time-setting circuit means to deliver a signal as an output thereof; and

a fail-safe circuit for monitoring the operation of said highfrequency section to produce an artificial signal having a substitutive frequency corresponding to said Dopplershifted frequency when said high-frequency section becomes inoperative, thereby simulating the passing of vehicles in succession, and to send the artificial signal to said low-frequency section, thereby maintaining the operation of said vehicle-detecting apparatus in a state of detecting a predetermined number of passing vehicles by utilizing said time-setting circuit B.

5. A vehicle-detecting apparatus as defined in claim 4, wherein said detector circuit in said high frequency section for detecting the Doppler-shifted frequency component includes means to supply a part of a transmission output produced by a high-frequency oscillator circuit comprising an active element and a Doppler-shifted frequency wave received by said active element through said antenna and to detect by utilizing a nonlinear characteristic of said active element.

6. A vehicle-detecting apparatus as defined in claim 4, wherein said fail-safe circuit comprises a gate disposed in an output circuit of an oscillator of a predetermined frequency within the Doppler frequency band, said gate being opened by a reduction in a bias voltage of an active element.

7. A vehicle-detecting apparatus as defined in claim 6, wherein a voltage source of a commercial line frequency 50 Hz. or 60 Hz. is utilized in place of said oscillator of a predetermined frequency within the Doppler frequency band.

Claims

1. A vehicle-detecting apparatus, comprising, in combination: a high-frequency section including antenna means for radiating an electromagnetic wave and for receiving a reflected wave from vehicles and means for detecting a Doppler-shifted component in terms of amplitude contained in said reflected wave; a low-frequency section, including: circuit means for selectively amplifying only a part of said Doppler-shifted component which is within a particular range; means means for wave-shaping said amplified signal component; and a timesetting circuit, including first circuit means for converting converting a plurality of shaped signals from a wave-shaping circuit into a smaller number of count signals, means extending the duration time of said plurality of shaped signals, and second circuit means operatively connected to said first circuit means for converting at least one shaped signal from said wave-shaping circuit into a larger number of count signals and means for dividing said at least one shaped signal, the number of said count signals being nearly representative of the actual number of vehicles passing through an effective range of a radiated beam of electromagnetic waves from said antenna; and fail-safe circuit means monitoring said low-frequency section and supplying an output signal to said low frequency section upon detection of an abnormal operation of said high-frequency section.

2. A vehicle-detecting apparatus according to claim 1, wherein said Doppler-shift detecting means includes means applying said reflected wave to an active element and means for mixing a portion of said reflected wave with a portion of said radiated signal.

3. A vehicle-detecting apparatus as set forth in claim 1, characterized in that said fail-safe circuit includes a gate circuit for sending an artificial signal at a frequency of a commercial power source to said low-frequency section in the event of a power failure in said high frequency section detected in terms of a variation in the bias voltage of either one of a frequency-multiplying active element and an oscillation active element in said high-frequency section.

4. A vehicle-detecting apparatus comprising a high frequency section including a high-frequency oscillator circuit, an antenna for radiating an output of said high frequency oscillator circuit toward a ground surface and for receiving a radio wave reflected by a moving vehicle on the ground surface and a detector circuit for detecting a Doppler-shifted frequency component in the received wave; a low-frequency section including a bandpass amplifier circuit for selectively amplifying a component lying within a predetermined frequency band of said Doppler-shifted frequency component, a detector circuit for detecting only an amplitude of an output of said bandpass amplifier circuit, a wave-shaping circuit for producing a constant-amplitude output for a waveform having a varying amplitude greater than a predetermined value in an output of said detector circuit and producing zero output for a wave form having an amplitude smaller than said predetermined value, time-setting circuit means including a time-setting circuit A for holding an output of said wave-shaping circuit for a predetermined time to prevent a plurality of count outputs from being produced by a single vehicle and a time-setting circuit B for dividing, at every predetermined time interval, a single output of long duration of said wave-shaping circuit produced when a plurality of vehicles successively pass to simulate the number of passing vehicles, and an output circuit for controlling the output of said wave-shaping circuit by said time-setting circuit means to deliver a signal as an output Thereof; and a fail-safe circuit for monitoring the operation of said high-frequency section to produce an artificial signal having a substitutive frequency corresponding to said Doppler-shifted frequency when said high-frequency section becomes inoperative, thereby simulating the passing of vehicles in succession, and to send the artificial signal to said low-frequency section, thereby maintaining the operation of said vehicle-detecting apparatus in a state of detecting a predetermined number of passing vehicles by utilizing said time-setting circuit B.