Ultrasonic ground speedometer utilizing doppler effect of ultrasonic waves

An ultrasonic ground speedometer utilizing Doppler effect of ultrasonic waves comprises an ultrasonic transmitter for outputting an ultrasonic wave at an emitting angle against a road surface, an ultrasonic receiver for receiving a reflected ultrasonic wave caused by reflection of the output ultrasonic wave from the road surface, a signal processor for deriving a Doppler shift frequency between the frequencies of the output ultrasonic wave from the ultrasonic transmitter and the reflected ultrasonic wave received by the ultrasonic receiver, a first arithmetic circuit for deriving the angle difference between the emitting angle and the reception angle defined between the road surface and the propagation direction of the reflected ultrasonic wave received by the receiver, on the basis of the Doppler shift frequency, and a calculating circuit for calculating a ground speed of the vehicle, on the basis of both the Doppler shift frequency and the angle difference.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ultrasonic ground speedometer utilizing 
the Doppler effect of ultrasonic waves, which is, for example, adapted for 
detection of vehicle speed over the ground, and specifically to a 
speedometer being capable of providing a high accuracy of ground speed 
measurement over the en&ire ground velocity range to be measured. 
2. Description of the Prior Disclosure 
Recently, there have been proposed and developed various ultrasonic Doppler 
speed measurement devices. One such conventional ultrasonic Doppler ground 
speedometer has been disclosed in U.S. Pat. No. 5,054,003 issued on Oct. 
1, 1991 (corresponding to Japanese Patent Application No. 1-107319) and 
U.S. Pat. No. 5,097,453 issued on Mar. 17, 1992 (corresponding to Japanese 
Patent Application No. 1-107320). As shown in FIG. 1, the above-mentioned 
conventional ultrasonic Doppler ground speedometer generally includes an 
oscillator 1 for generating an output signal having a particular 
wavelength within a wavelength range of 2.6 mm to 3.4 mm essentially 
corresponding to an oscillator output frequency range of 131 kHz to 100 
kHz, an echo sounder transmitter 3 for transmitting an ultrasonic wave 
having the above noted particular wavelength, and a drive circuit 2 for 
amplifying the oscillator output signal and for driving the echo sounder 
transmitter 3 by the amplified signal output therefrom. The transmitter 3 
emits an ultrasonic wave 4 with the previously noted particular wavelength 
in a vehicle forward direction at a predetermined emitting angle against a 
road surface 5. The conventional ground speedometer also includes an 
ultrasonic receiver 6 for receiving a reflected ultrasonic wave caused by 
reflection of the output ultrasonic wave on the road surface 5, and for 
generating a reflected ultrasonic wave signal. In the prior art ground 
speedometers, the ultrasonic transmitter 3 and the ultrasonic receiver 6 
are both arranged on the same plane. In FIG. 1, reference numeral 7 
denotes an amplifier for amplifying the reflected ultrasonic wave signal. 
The conventional ultrasonic ground speedometer also comprises a signal 
processor including a multiplier 8 for deriving the frequency difference 
between the oscillator output signal frequency and the reflected 
ultrasonic wave signal frequency by multiplying both of the frequencies, a 
low-pass filter 9 for filtering undesirable noise from the frequency 
difference signal generated from the multiplier 8, a zero-crossing 
comparator 10 for waveform-shaping the filtered frequency difference 
signal representative of the Doppler shift, a pulse counter 11 for 
counting pulses in the Doppler shift signal from the comparator 10 and for 
deriving a Doppler frequency, and an arithmetic circuit 12 serving as a 
Doppler frequency/ground speed convertor for deriving the ground speed on 
the basis of the output from the pulse counter 11, representative of the 
Doppler frequency. A frequency controller 13 is also provided for 
controlling the oscillator output frequency on the basis of the output 
signal generated from the arithmetic circuit 12 in such a manner as to 
keep the reflected ultrasonic wave frequency represented by the sum of the 
output ultrasonic wave frequency and the Doppler shift to a constant value 
in response to change in the ground speed derived by the arithmetic 
circuit. 
FIG. 2 shows a principle of the ultrasonic Doppler ground speed measurement 
of the conventional ultrasonic ground speedometer, in which f.sub.o, 
f.sub.d and F respectively designate an output ultrasonic wave frequency 
emitted from the transmitter 3, a Doppler shift, and a received ultrasonic 
wave frequency received by the ultrasonic receiver 6. The received 
ultrasonic frequency F is equivalent to the sum of the output ultrasonic 
wave frequency f.sub.o and the Doppler shift f.sub.d. In the previously 
described conventional ultrasonic ground speedometers, the ground velocity 
is derived on the assumption that the transmitter 3 and the receiver 6 are 
both arranged on the same plane and in addition an emitting angle defined 
by the emitting direction of the output ultrasonic wave and the road 
surface is equal to a reception angle defined by the reflected direction 
of the reflected ultrasonic wave and the road surface. As is generally 
known, supposing that the acoustic velocity represented by c, the 
arithmetic circuit 12 employed in the conventional ground speedometer 
derives the ground velocity v from the Doppler frequency f.sub.d according 
to the following equation. 
EQU v=cf.sub.d /(2F-f.sub.d)cos.theta. 
As appreciated from the above equation, since the emitting angle .theta. is 
preset to a predetermined angle, the ground velocity v can be calculated 
by deriving the two values, namely the Doppler shift frequency f.sub.d and 
the received ultrasonic frequency F. 
However, there is a problem that linearity of the calculated ground speed 
to the actual ground speed is deteriorated at a high speed range, since 
the calculated ground speed is derived on the assumption that the emitting 
angle .theta. of the output ultrasonic wave is identical to the reception 
angle of the reflected ultrasonic wave. That is, since the above 
assumption is not satisfied at the high vehicle speed range, the 
difference between the actual vehicle speed and the calculated ground 
speed tends to become greater according to the increase in velocity at the 
high speed range in which the vehicle speed substantially approaches to 
the acoustic velocity. 
Referring now to FIG. 3, there is shown a linearity of the calculated 
ground speed derived by the conventional ultrasonic Doppler ground 
speedometer with respect to the reference ground speed measured with a 
spatial filter type ground speedometer. In general, such a spatial filter 
type ground speedometer can measure the vehicle ground speed with a 
considerably high measurement accuracy of .+-.0.5 km/h. In the case of 
test results illustrating the linearity of the calculated ground speed of 
FIG. 3, the emitting angle .theta. is preset to 45.degree.. As will be 
appreciated from the graph of FIG. 3, the linearity of the calculated 
ground speed to the measured reference ground speed is deteriorated at a 
high speed range of 90 km/h or more. The calculated value of ground speed 
is 160 km/h at the reference ground speed of 180 km/h. In such an 
excessively high speed range of 180 km/h, there is an error of 
approximately 20 km/h, resulting from the previously noted assumption. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an 
ultrasonic ground speedometer utilizing Doppler effect, which can provide 
a high accuracy of ground speed measurement over the entire ground 
velocity range to be measured. 
It is another object of the invention to provide an ultrasonic ground 
speedometer utilizing Doppler effect, which can precisely compensate the 
angle relationship between an emitting angle of an output ultrasonic wave 
generated by an ultrasonic transmitter and a reception angle of a 
reflected ultrasonic wave received by an ultrasonic receiver. 
It is a further object of the invention to provide an ultrasonic ground 
speedometer utilizing Doppler effect, which can provide a S/N ratio of 
Doppler frequency signal representative of Doppler shift between an output 
ultrasonic wave generated by an ultrasonic transmitter and a reflected 
ultrasonic wave received by an ultrasonic receiver. 
It is a still further object of the invention to provide an optimal 
arrangement of an ultrasonic transmitter and an ultrasonic receiver both 
employed in an ultrasonic ground speedometer utilizing Doppler effect, for 
insuring a high sensitivity of the ultrasonic receiver. 
In order to accomplish the aforementioned and other objects, an ultrasonic 
ground speedometer utilizing Doppler effect of ultrasonic waves, comprises 
ultrasonic transmitting means for outputting an ultrasonic wave at an 
emitting angle against a road surface, ultrasonic receiving means for 
receiving a reflected ultrasonic wave caused by reflection of the output 
ultrasonic wave from the road surface, means for deriving a Doppler shift 
frequency between the frequencies of the output ultrasonic wave from the 
ultrasonic transmitting means and the reflected ultrasonic wave received 
by the ultrasonic receiving means, first arithmetic means for deriving a 
first angle difference between the emitting angle and a reception angle 
defined between the road surface and a propagation direction of the 
reflected ultrasonic wave received by the ultrasonic receiving means, on 
the basis of the Doppler shift frequency, and calculation means for 
calculating a ground speed of a vehicle mounting the ultrasonic ground 
speedometer, on the basis of both the Doppler shift frequency and the 
first angle difference. The ultrasonic ground speedometer may further 
comprise second arithmetic means for compensating the emitting angle 
depending on the ground speed. The second arithmetic means compensates the 
emitting angle by a second angle difference between an actual emitting 
angle obtained in a vehicle running state and a preset emitting angle 
obtained in a vehicle stopped state. The second arithmetic means derives 
the second angle difference, on the basis of the Doppler shift frequency. 
The first arithmetic means derives the first angle difference based on the 
Doppler shift frequency from a predetermined 
first-angle-difference/Doppler-shift-frequency characteristic data stored 
in a first memory, while the second arithmetic means derives the second 
angle difference based on the Doppler shift frequency from a predetermined 
second-angle-difference/Doppler-shift-frequency characteristic data stored 
in a second memory. 
According to another aspect of the invention, an ultrasonic ground 
speedometer utilizing Doppler effect of ultrasonic waves, for an 
automotive vehicle comprises ultrasonic transmitting means for outputting 
an ultrasonic wave at a preset emitting angle against a road surface, in 
the vehicle forward direction, the preset emitting angle corresponding to 
an actual emitting angle obtained in a vehicle stopped state, ultrasonic 
receiving means for receiving a reflected ultrasonic wave caused by 
reflection of the output ultrasonic wave from the road surface, means for 
deriving a Doppler shift frequency between the frequencies of the output 
ultrasonic wave from the ultrasonic transmitting means and the reflected 
ultrasonic wave received by the ultrasonic receiving means, first 
arithmetic means for deriving a first angle difference between the preset 
emitting angle and a reception angle defined between the road surface and 
a propagation direction of the reflected ultrasonic wave received by the 
ultrasonic receiving means, on the basis of the Doppler shift frequency, 
and calculation means for calculating a ground speed of the vehicle 
mounting the ultrasonic ground speedometer, on the basis of both the 
Doppler shift frequency and the first angle difference. An emitting point 
of the ultrasonic transmitting means and a reception point of the 
ultrasonic receiving means are spaced apart from each other by a 
predetermined distance. In addition, the reception point of the ultrasonic 
receiving means is arranged backwardly of the emitting point of the 
ultrasonic transmitting means in the vehicle forward direction. It is 
preferable that an ultrasonic emitting point of the ultrasonic 
transmitting means and an ultrasonic reception surface including the 
reception point of the ultrasonic receiving means are both inclined in the 
vehicle forward direction with a predetermined inclination, and the 
ultrasonic reception surface is arranged backwardly of the ultrasonic 
emitting surface in the vehicle forward direction, so as to provide a high 
S/N ratio for the Doppler shift indicative signal. 
According to a further aspect of the invention, an ultrasonic ground 
speedometer utilizing Doppler effect of ultrasonic waves, for an 
automotive vehicle comprises ultrasonic transmitting means for outputting 
an ultrasonic wave at a preset emitting angle against a road surface, in 
the vehicle forward direction, the preset emitting angle corresponding to 
an actual emitting angle obtained in a vehicle stopped state, ultrasonic 
receiving means for receiving a reflected ultrasonic wave caused by 
reflection of the output ultrasonic wave from the road surface, means for 
deriving a Doppler shift frequency between the frequencies of the output 
ultrasonic wave from the ultrasonic transmitting means and the reflected 
ultrasonic wave received by the ultrasonic receiving means, first 
arithmetic means for deriving a first angle difference between an actual 
emitting angle and a reception angle defined between the road surface and 
a propagation direction of the reflected ultrasonic wave received by the 
ultrasonic receiving means, on the basis of the Doppler shift frequency, 
second arithmetic means for compensating the actual emitting angle 
depending on the ground speed, the second arithmetic means compensating 
the actual emitting angle by a second angle difference between the actual 
emitting angle obtained in a vehicle running state and a preset emitting 
angle obtained in a vehicle stopped state, the second arithmetic means 
deriving the second angle difference, on the basis of the Doppler shift 
frequency, and calculation means for calculating a ground speed of the 
vehicle mounting the ultrasonic ground speedometer, on the basis of the 
Doppler shift frequency, the first angle difference and the second angle 
difference. An emitting point of the ultrasonic transmitting means and a 
reception point of the ultrasonic receiving means are spaced apart from 
each other by a predetermined distance and the reception point of the 
ultrasonic receiving means is arranged backwardly of the emitting point of 
the ultrasonic transmitting means in the vehicle forward direction. An 
ultrasonic emitting surface including the emitting point of the ultrasonic 
transmitting means and an ultrasonic reception surface including the 
reception point of the ultrasonic receiving means are both inclined in the 
vehicle forward direction with a predetermined inclination, and in 
addition the ultrasonic reception surface and the ultrasonic emitting 
surface are both arranged on the same plane. 
The ultrasonic transmitting means may include an oscillator for generating 
the output signal having a predetermined wavelength range, an ultrasonic 
transducer for converting electric signals to acoustical signals and for 
emitting the output ultrasonic wave having the particular wavelength, and 
a drive circuit for amplifying the oscillator output signal and for 
driving the ultrasonic transducer via the amplified signal therefrom, and 
the ultrasonic receiving means may include an ultrasonic transducer for 
receiving the reflected ultrasonic wave and for converting acoustical 
signals to electric signals. The ultrasonic transducer is preferably 
comprised of a piezoelectric crystal unit. The ultrasonic transducer for 
the ultrasonic receiving means is preferably comprised of a high resonance 
characteristic ultrasonic transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
Referring now to FIGS. 4 through 9, particularly to FIG. 8, the ultrasonic 
Doppler ground speedometer of the first embodiment is designed on the 
assumption that an emitting angle of an output ultrasonic wave emitted 
from an ultrasonic transmitter is different from a reception angle of a 
reflected ultrasonic wave reflected from the road surface and received by 
an ultrasonic receiver, and in addition the emitting angle is not 
influenced by the vehicle speed. In FIG. 8, a received ultrasonic wave 
frequency F is defined as the sum of an output ultrasonic wave frequency 
f.sub.o and the Doppler shift frequency f.sub.d, i.e., f.sub.o +f.sub.d 
and v designates a vehicle velocity. As in FIG. 8, during measurement of 
ground speed of the vehicle, the ultrasonic transmitter/receiver unit is 
actually displaced from the right position (viewing FIG. 8) wherein the 
output ultrasonic wave is emitted from an emitting point P of the 
ultrasonic transmitter to the left position wherein the reflected 
ultrasonic wave is received by the ultrasonic receiver at a reception 
point Q'. The right position will be hereinafter referred to as a 
"transmission position", while the left position will be hereinafter 
referred to as a "reception position". As seen from the transmission 
position or the reception position, the emitting point P or P' and the 
reception point Q or Q' are spaced apart from each other by a distance d. 
Both the emitting point and the reception point are set at the same level 
h with respect to the road surface. Assuming that the propagation of the 
ultrasonic wave is similar to that of light beam, and the emitting angle 
.theta. is not influenced by the vehicle speed, the output ultrasonic wave 
is reflected from a reflection point O on the road surface with the 
emitting angle .theta.. An angle .alpha. represents the angle difference 
between the emitting angle .theta. and the reception angle defined between 
the propagation direction of received ultrasonic wave and the road 
surface. In other words. the reception angle is obtained by the sum of the 
angle .alpha. and the emitting angle .theta., i.e., (.theta.+.alpha.). 
.theta.' represents an angle between two line segments OP and OQ and 
corresponds to the value of the angle .alpha. when the vehicle speed v is 
equal to zero. A time t.sub.1 corresponds to a propagation time from the 
emitting point P to the reflection point O, while a time t.sub.2 
corresponds to a propagation time from the reflection point O to the 
reception point Q'. 
In the ultrasonic Doppler ground speedometer of the first embodiment shown 
in FIG. 8, the Doppler shift frequency f.sub.d is obtained by the 
following equation. 
EQU f.sub.d =v{cos.theta.+cos(.theta.+.alpha.)}F/{c+vcos(.theta.+.alpha.)}(1) 
As appreciated from the above equation (1), the Doppler shift frequency 
f.sub.d is represented as a function of the angle difference .alpha. as 
well as the vehicle speed v. Furthermore, as will be appreciated from FIG. 
8, the angle difference .alpha. varies depending on the vehicle speed v. 
Since the Doppler shift frequency f.sub.d can be measured and calculated 
by the ground speedometer of the first embodiment, the ground velocity v 
of the vehicle is easily derived by determining the relationship between 
the angle difference c and the Doppler shift frequency f.sub.d. 
As shown in FIG. 8, the ultrasonic transmitter/receiver unit is displaced 
from the transmission position to the reception position by a distance 
v(t.sub.1 +t.sub.2), in one cycle of ground speed measurement. With the 
above noted arrangement of the ultrasonic transmitter/receiver unit, since 
the reception point is backwardly offset from the emitting point by the 
distance d, the distance between the actual emitting point in the 
transmission position and the actual reception point shifted from the 
transmission position to the reception position is represented by the 
following equation. 
EQU v(t.sub.1 
+t.sub.2)-d=(cos.theta./sin.theta.)h-{cos(.theta.+.alpha.)/sin(.theta.+.al 
pha.)}h 
The propagation times t.sub.1 and t.sub.2 can be geometrically obtained 
from FIG. 8, as follows. 
EQU t.sub.1 =h/csin.theta., t.sub.2 h/csin(.theta.+.alpha.) 
wherein, c represents an acoustic velocity. On the basis of the above three 
equations, the following equation is derived. 
EQU v{h/csin.theta.+h/csin(.theta.+.alpha.)}-d=(cos.theta./sin.theta.)h-{cos(.t 
heta.+.alpha.)/sin(.theta.+.alpha.)}h 
Subsequently, the distance d between the transmitter and the receiver can 
be geometrically obtained from FIG. 8, as follows. 
EQU d=h/tan(.theta.-.theta.')-h/tan.theta. 
The following equation is derived by substituting the above distance d into 
the right side of the previously derived equation. 
EQU v{sin(.theta.+.alpha.)+sin.theta.}-csin.theta.sin(.theta.+.alpha.)cot(.thet 
a.-.theta.') +csin.theta.cos(.theta.+.alpha.)=0(2) 
In the above equation (2), the ground velocity v is included only in a 
first term v{sin(.theta.+.alpha.) + sin.theta.)56 but not in a second 
term csin.theta.sin(.theta.+.alpha.)cot(.theta.-.theta.') and a third term 
csin.theta.cos(.theta.+.alpha.). Therefore, the ground velocity v can be 
calculated on the basis of the above equation, assuming that the angle 
difference .alpha. is univocally determined from a predetermined 
relationship between the angle difference .alpha. and the Doppler shift 
frequency f.sub.d represented by the previously noted equation (1), i.e., 
f.sub.d =v{cos.theta.+cos(.theta.+.alpha.)}F / {c+vcos(.theta.+.alpha.)}. 
Since the equations (1) and (2) respectively include a term of the 
acoustic velocity c, such as the second and third terms of the equation 
(2), the equations (1) and (2) are dependent on air temperature, because 
of the acoustic velocity c itself being a function of air temperature. For 
example, the relationship between the angle difference .alpha. and the 
vehicle speed v varies depending on change in the acoustic velocity c. The 
following tables 1, 2, 3 and 4 represent the relationship between the 
vehicle speed v, the angle difference .alpha. and the Doppler shift 
frequency f.sub.d, at four air temperatures of -25.degree. C., 0.degree. 
C., 25.degree. C., and 50.degree. C., respectively. These test results 
shown in the tables 1 through 4 are experimentally assured by the 
inventors of the invention. These tests are made under a particular test 
condition in which the emitting angle .theta. of an output ultrasonic wave 
is set at a particular angle of 45.degree. and the received ultrasonic 
wave frequency F is controlled to be kept at a particular frequency of 130 
kHz by means of an output ultrasonic wave frequency controller disclosed 
in the previously described U.S. Pat. Nos. 5,054,003 or 5,097,453 entitled 
"ULTRASONIC GROUND SPEEDOMETER UTILIZING DOPPLER EFFECT". The disclosure 
of the above-identified U.S. Patents respectively assigned to the same 
assignee as the invention is herein incorporated by reference for the sake 
of disclosure. The setting value 130 kHz of the received ultrasonic wave 
frequency F is determined according to the teaching of the above U.S. 
Patents, as as to assure the highest possible S/N ratio of a Doppler shift 
signal. As described in the above U.S. Pat. Nos. 5,054,003 or 5,097,453, a 
frequency controller which is hereinafter described in detail is employed 
in the ground speedometer, for varying the output ultrasonic wave 
frequency output from the transmitter within a particular frequency range 
of 100 kHz to 131 kHz, so as to keep the received ultrasonic wave 
frequency at a constant value (130 kHz). 
TABLE 1 
______________________________________ 
(-25.degree. C.) 
v(km/h) .alpha.(deg) 
fd(kHz) 
______________________________________ 
0 -7.18486 0 
10 -6.605 1.69143 
20 -6.01814 3.34583 
30 -5.42408 4.96395 
40 -4.82286 6.54652 
50 -4.21422 8.09426 
60 -3.59813 9.60783 
70 -2.97444 11.0879 
80 -2.34307 12.535 
90 -1.70394 13.9498 
100 -1.05687 15.3328 
110 -0.401718 16.6846 
120 0.261482 18.0056 
130 0.933033 19.2964 
140 1.61288 20.5574 
150 2.3013 21.789 
160 2.99832 22.9917 
170 3.70409 24.1658 
180 4.41867 25.3117 
190 5.14232 26.4297 
200 5.87499 27.5202 
210 6.61697 28.5834 
220 7.36831 29.6196 
230 8.1291 30.6291 
240 8.89959 31.6121 
250 9.67981 32.5687 
______________________________________ 
TABLE 2 
______________________________________ 
(0.degree. C.) 
v(km/h) .alpha.(deg) 
fd(kHz) 
______________________________________ 
0 -7.18481 0 
10 -6.63187 1.61427 
20 -6.07256 3.19481 
30 -5.5067 4.74228 
40 -4.93424 6.25732 
50 -4.35518 7.74055 
60 -3.76926 9.19253 
70 -3.17656 10.6139 
80 -2.57686 12.0051 
90 -1.97011 13.3666 
100 -1.35616 14.6991 
110 -0.734947 16.0029 
120 -0.106419 17.2785 
130 0.529541 18.5264 
140 1.17312 19.7469 
150 1.82433 20.9404 
160 2.48333 22.1073 
170 3.15019 23.248 
180 3.825 24.3628 
190 4.50792 25.4519 
200 5.19906 26.5157 
210 5.89855 27.5544 
220 6.60635 28.5684 
230 7.3227 29.5578 
240 8.04771 30.5229 
250 8.78139 31.4638 
______________________________________ 
TABLE 3 
______________________________________ 
(25.degree. C.) 
v(km/h) .alpha.(deg) 
fd(kHz) 
______________________________________ 
0 -7.18484 0 
10 -6.65638 1.54385 
20 -6.12209 3.05684 
30 -5.58187 4.53956 
40 -5.03566 5.99258 
50 -4.48337 7.41642 
60 -3.92488 8.8116 
70 -3.36015 10.1786 
80 -2.78905 11.518 
90 -2.21162 12.8301 
100 -1.6276 14.1154 
110 -0.03699 15.3744 
120 -0.439754 16.6074 
130 0.164253 17.8149 
140 0.775131 18.9971 
150 1.39296 20.1544 
160 2.01785 21.2873 
170 2.64981 22.3959 
180 3.28905 23.4806 
190 3.93554 24.5417 
200 4.58951 25.5794 
210 5.25088 26.594 
220 5.91993 27.5857 
230 6.5966 28.5548 
240 7.28112 29.5014 
250 7.97355 30.4258 
______________________________________ 
TABLE 4 
______________________________________ 
(50.degree. C.) 
v(km/h) .alpha.(deg) 
fd(kHz) 
______________________________________ 
0 -7.18481 0 
10 -6.6788 1.47931 
20 -6.16746 2.93029 
30 -5.65063 4.35346 
40 -5.12831 5.74931 
50 -4.60044 7.11831 
60 -4.06693 8.46094 
70 -3.52768 9.77761 
80 -2.98261 11.0688 
90 -2.43172 12.3348 
100 -1.87493 13.5761 
110 -1.31205 14.7931 
120 -0.743134 15.9861 
130 -0.168049 17.1555 
140 0.413311 18.3015 
150 1.00098 19.4245 
160 1.59507 20.5249 
170 2.19564 21.6028 
180 2.80272 22.6586 
190 3.41651 23.6925 
200 4.03699 24.7048 
210 4.66427 25.6958 
220 5.2984 26.6655 
230 5.93953 27.6143 
240 6.58777 28.5423 
250 7.24311 29.4497 
______________________________________ 
As will be appreciated from the tables 1 through 4, the relationship 
between the vehicle speed v and the angle difference .alpha. varies 
depending on change in the acoustic velocity c, while the relationship 
between the angle difference .alpha. and the Doppler shift frequency 
f.sub.d is essentially constant, irrespective of change in the acoustic 
velocity c. On the basis of the above experimentally obtained test data of 
the tables 1 through 4, the relationship of the angle difference .alpha. 
versus the Doppler shift frequency f.sub.d is univocally determined by a 
sole characteristic curve shown in FIG. 9. As set forth above, the ground 
speed of the vehicle is derived from the measured Doppler shift frequency 
f.sub.d and the angle difference .alpha. read out from the .alpha. - 
f.sub.d characteristic curve of FIG. 9, on the basis of the following 
equation derived from the previously noted equation (1). 
EQU v=cf.sub.d /{Fcos.theta.+(F-f.sub.d)cos(.theta.+.alpha.)} (3) 
Referring now to FIG. 4, there is shown a block diagram illustrating the 
ultrasonic Doppler ground speedometer of the first embodiment. In the 
block diagram of FIG. 4, the same reference numerals used in the prior art 
of FIG. 1 will be applied to the corresponding elements used in the first 
embodiment, for the purpose of comparison between the prior art and the 
first embodiment. As shown in FIG. 4, the ultrasonic ground speedometer of 
the first embodiment comprises an oscillator 1 for generating an output 
signal having a particular wavelength, an ultrasonic transmitter 3 for an 
output ultrasonic wave, a drive circuit 2 for amplifying the oscillator 
output signal and for driving the transmitter 3, an ultrasonic receiver 6 
for receiving a reflected ultrasonic wave caused by reflection of the 
output ultrasonic wave on a road surface 5 and for generating a received 
ultrasonic wave signal, an amplifier 7 for amplifying the received 
ultrasonic wave signal from the receiver 6, a multiplier 8 for deriving 
the frequency difference between the oscillator output signal frequency 
and the received ultrasonic wave signal frequency, a low-pass filter 9 for 
filtering undesirable noise from the frequency difference signal from the 
multiplier 8, a zero-crossing comparator 10 for waveform-shaping the 
filtered frequency difference signal representative of the Doppler shift 
frequency, a pulse counter 11 for deriving a Doppler shift frequency by 
counting pulses in the Doppler shift signal from the comparator 10, and a 
frequency controlling circuit 13 for controlling the oscillator output 
frequency in such a manner as to keep the received ultrasonic wave 
frequency to a constant value, in response to change in the measured 
ground speed of the vehicle. The ground speedometer of the first 
embodiment is different from the prior art shown in FIG. 1 in that the 
transmitter 3 and the receiver 6 are spaced apart from each other and the 
angle difference .alpha. is introduced into the calculation of the ground 
speed. 
As shown in FIG. 5, the transmitter/receiver unit of the first embodiment 
is designed such that the receiver 6 is arranged backwardly of the 
transmitter 3 in the travelling direction of the vehicle. An ultrasonic 
wave emitting surface including the emitting point P of the transmitter 3 
is inclined in the vehicle forward direction with respect to the road 
surface, while an ultrasonic wave reception surface including the 
reception point Q of the receiver 6 is also inclined in the vehicle 
forward direction in the same manner as the emitting surface. A high 
resonance characteristic ultrasonic transducer having a high Q value in 
the vicinity of a designated constant frequency, such as 130 kHz, is often 
utilized as an ultrasonic receiver, so as to enhance the. S/N ratio of the 
received ultrasonic wave signal generated by the receiver 6 and 
consequently to enhance the S/N ratio of the Doppler shift indicative 
signal. For example, such a resonance type ultrasonic transducer exhibits 
a relatively narrow directivity as shown in FIG. 6. In general, a 
reception sensitivity is dependent on the receiver's size and shape. In 
the ultrasonic receiver having a reception sensitivity characteristic 
shown in FIG. 6, when the reception sensitivity obtained in a direction of 
the central axis of the receiver is compared with the sensitivity obtained 
in a direction inclined from the central axis by .+-.10.degree., the 
latter is deteriorated by approximately 20 dB. Therefore, when the 
ultrasonic receiver having a relatively narrow directivity is used, it is 
desirable that the receiver receives the reflected ultrasonic wave 
substantially in a direction of the central axis of the receiver, so as to 
assure a high S/N ratio of the Doppler shift indicative signal. In the 
case of the ultrasonic transmitter/receiver arrangement of the 
conventional ultrasonic ground speedometer shown in FIG. 2, the receiver 
must receive the reflected ultrasonic wave in a direction angularly 
displaced from the central axis of the receiver, since the angle 
difference .alpha. is gradually increased from 0.degree. to 20.degree. in 
accordance with an increase in the vehicle speed from 0 km/h to 250 km/h. 
This results in a low S/N ratio of the Doppler shift indicative signal. 
In view of the above, the receiver 6 is arranged backwardly of the 
transmitter 3 in the vehicle forward direction in consideration of the 
angle difference .alpha. varied depending on the vehicle speed, so as to 
insure a high reception sensitivity of the receiver. 
In the first embodiment, the measuring vehicle speed range is set within a 
particular range of 0 km/h through 250 km/h. The distance d and the level 
h are determined such that the angle difference .alpha. becomes 0.degree. 
at 125 km/h being a mean value of 0 km/h and 250 km/h. Therefore, when the 
vehicle speed is varied within a range of 0 km/h through 250 km/h, the 
angle difference .alpha. fluctuates within a range of -7.degree. through 
8.degree.. With the arrangement of the ultrasonic transmitter/receiver 
unit of the first embodiment, it will be appreciated that the receiver 
receives the reflected ultrasonic wave in a direction closer to the 
central axis of the receiver. This insures more accurate measurement of 
the ground speed. 
Returning to FIG. 4, the ultrasonic ground speedometer of the first 
embodiment also includes a memory 14 for deriving a value of 
cos(.theta.+.alpha.) on the basis of the Doppler shift frequency f.sub.d 
derived from the pulse counter 11 and an arithmetic circuit 15 for 
calculating the ground velocity v of the vehicle from the equation (3) on 
the basis of both the Doppler shift frequency f.sub.d derived from the 
pulse counter 11 and the value of cos(.theta.+.alpha.) derived from the 
memory 14. As described in detail, the memory 14 stores values of 
cos(.theta.+.alpha.) versus various Doppler shift frequencies, in the form 
of a data map previously derived on the basis of the .alpha. - f.sub.d 
characteristic curve of FIG. 9. 
Referring now to FIG. 7, there is shown the relationship between the 
reference ground speed measured by the conventional spatial filter type 
ground speedometer and the calculated ground speed obtained through the 
ultrasonic ground speedometer of the first embodiment. As appreciated from 
the graph of FIG. 7, the linearity of the calculated ground speed obtained 
according to the first embodiment to the measured ground speed obtained by 
the spatial filter type ground speedometer is remarkably enhanced. 
As will be appreciated from the above, since in the ultrasonic ground 
speedometer of the first embodiment, the receiver 6 is arranged backwardly 
of the transmitter 3 to permit the receiver 6 to receive the reflected 
ultrasonic wave in a direction closer to the central axis of the receiver, 
and in addition the vehicle ground speed is calculated through the 
arithmetic circuit 15 in consideration of the angle difference .alpha. 
between the emitting angle of the output ultrasonic wave and the reception 
angle of the reflected ultrasonic wave, the vehicle velocity is enhanced 
and consequently more accurate measurement of the ground speed of the 
vehicle is assured. 
In the first embodiment, the value of cos(.theta.+.alpha.) is read out from 
the memory 14 based on the value of the Doppler shift frequency f.sub.d 
derived from the pulse counter 11. Alternatively, the value of 
cos(.theta.+.alpha.) may be derived on the basis of the value of .alpha. 
calculated based on an approximate expression representative of the 
.alpha. - f.sub.d characteristic curve, since the angle difference .alpha. 
can be represented as a function of the Doppler shift frequency f.sub.d, 
as appreciated from FIG. 9. 
The .alpha. - f.sub.d curve of FIG. 9 can be approximately represented by 
the following quadratic function of the Doppler shift frequency f.sub.d. 
EQU .alpha.deg=7.16.times.10.sup.-3 f.sub.d.sup.2 +2.84.times.10.sup.-1 f.sub.d 
-7.18 
wherein, the unit of the Doppler shift frequency f.sub.d is kHz. 
In the Doppler ground speedometer of the first embodiment, the frequency 
controller 13 controls the received ultrasonic wave frequency F to be kept 
to a constant value, such as 130 kHz, so as to more enhance the S/N ratio 
of the Doppler shift signal. In this case, the Doppler shift frequency 
f.sub.d and the ground speed v of the vehicle are obtained by substituting 
(f.sub.o + f.sub.d) into F of the equation (1), as follows. 
EQU f.sub.d =v{cos.theta.+cos(.theta.+.alpha.)}f.sub.o /(c-vcos.theta.) 
EQU v=cf.sub.d /{(f.sub.o +f.sub.d)cos.theta.+f.sub.o cos(.theta.+.alpha.)} 
Second Embodiment 
Referring now to FIGS. 10 through 15, particularly to FIG. 12, the 
ultrasonic Doppler ground speedometer of the second embodiment is designed 
on the assumption that the emitting angle of the output ultrasonic wave 
generated from the transmitter is different from the reception angle of 
the reflected ultrasonic wave received by the receiver, and in addition 
the emitting angle is actually influenced by the vehicle speed. Since the 
second embodiment is similar to the first embodiment, the same reference 
numerals used in the first embodiment of FIGS. 4, 5 and 8 will be applied 
to the corresponding elements used in the second embodiment of FIGS. 10, 
11 and 12, for the purpose of comparison between the first and second 
embodiments. In FIG. 12, the emitting angle .theta. represents an emitting 
angle only when the vehicle is stopped. As seen in FIG. 12, the level of 
the transmitter 3 is different from that of the receiver 6, and only the 
transmitter is set at a level h with respect to the road surface. An arrow 
A represents an acoustic velocity indicative vector in a vehicle stopped 
state, while an arrow B represents a vehicle speed indicative vector. A 
point O represents an actual reflection point. Assuming that the vehicle 
is stopped, the output ultrasonic wave emitted from the emitting point P 
is propagated along the broken line (or the arrow A) and reflected on the 
road surface with the emitting angle .theta.. However, when the vehicle is 
travelling at a certain vehicle speed, the output ultrasonic wave advances 
in a direction determined by the sum of the two vectors A and B and 
reflected from an actual reflection point O on the road surface with the 
emitting angle slightly decreased from the emitting angle .theta.. In the 
second embodiment, the angle difference .alpha. represents an angle 
between the actual emitting angle slightly reduced from the emitting angle 
.theta. obtained in the vehicle stopped state and the reception angle. On 
the other hand, .beta. represents the angle difference between an actual 
emitting angle obtained in the vehicle running state and a preset emitting 
angle .theta. obtained in the vehicle stopped state. As set forth above, 
the ultrasonic Doppler ground speedometer of the second embodiment is 
designed to derive the calculated ground speed in consideration of the 
angle difference .beta. as well as the angle difference .alpha. as 
considered in the first embodiment. When compared with the first 
embodiment, the ground speedometer of the second embodiment may provide 
more accurate measurement of ground speed of the vehicle. 
In the ultrasonic Doppler ground speedometer of the second embodiment shown 
in FIG. 12, the Doppler shift frequency f.sub.d is obtained by the 
following equation. 
EQU f.sub.d 
=v{cos(.theta.-.beta.)+cos(.theta.-.beta.+.alpha.)}F/}c+vcos(.theta.-.beta 
.+.alpha.)} (4) 
As appreciated the above equation (4), it is necessary to determine two 
values of the angle differences .alpha. and .beta., in order to obtain the 
ground speed v from the equation (4). The two angles .alpha. and .beta. 
can be geometrically obtained from FIG. 12, as follows. 
##EQU1## 
As appreciated from the above equation, the angle difference .alpha. 
varies depending on both the level h of the transmitter 3 and the distance 
d between the emitting point P and the reception point Q, in addition to 
the vehicle speed v. Since the Doppler shift frequency f.sub.d can be 
measured and calculated by the ground speedometer of the second 
embodiment, the ground velocity v is easily derived by determining the 
relationship between the angle difference .alpha. and the Doppler shift 
frequency f.sub.d and the relationship between the angle difference .beta. 
and the Doppler shift frequency f.sub.d. 
As previously described in the first embodiment, the relationship between 
the angle difference .alpha. and the Doppler shift frequency f.sub.d is 
univocally determined, irrespective of change in acoustic velocity c, if 
the emitting angle .theta., the level h, the distance d and the received 
ultrasonic wave frequency F are preset to given values. Likewise, the 
relationship between the angle difference .beta. and the Doppler shift 
frequency f.sub.d is also determined, irrespective of change in acoustic 
velocity. These test results are assured by the inventors of the present 
invention. In order to experimentally obtain the angle difference data 
.alpha. and .beta. used in the ground speedometer of the second 
embodiment, tests was made under a particular condition in which the 
emitting angle .theta. obtained in the vehicle stopped state is set to 
45.degree., the level h is set to 200 mm, the distance d between the 
emitting axis of the transmitter and the reception axis of the receiver is 
set to 35 mm, and the received ultrasonic wave frequency F is preset to 
130 kHz. On the basis of the above test results, the relationship of the 
angle difference .beta. versus the Doppler shift frequency f.sub.d was 
obtained as shown in the graph of FIG. 13, while the relationship of the 
angle difference .alpha. versus the Doppler shift frequency f.sub.d was 
obtained as shown in the graph of FIG. 14. Therefore, the ground speed of 
the vehicle is derived from the measured Doppler shift frequency f.sub.d 
and the angle difference .beta. read out from the .beta. - f.sub.d 
characteristic curve of FIG. 13, and the angle difference .alpha. read out 
from the .alpha. - f.sub.d characteristic curve of FIG. 14, on the basis 
of the following equation derived from the previously noted equation (4). 
EQU v=cf.sub.d 
/{Fcos(.theta.-.beta.)+(F-f.sub.d)cos(.theta.-.beta.+.alpha.)}(5) 
Returning now to FIG. 10, there is shown a block diagram illustrating the 
ultrasonic Doppler ground speedometer of the second embodiment. As 
appreciated from comparison of the two block diagrams, namely the block 
diagram of the first embodiment shown in FIG. 4, and the block diagram of 
the second embodiment shown in FIG. 10, the construction of the first 
embodiment including the elements 1 through 11 and 13 is similar to that 
of the second embodiment. However, the second embodiment is different from 
the first embodiment in that the memory 14 for the value of 
cos(.theta.+.alpha.) is replaced with two memories, namely a memory 14a 
for the value of cos(.theta.-.beta.) and a memory 14b for the value of 
cos(.theta.-.beta.+.alpha.), i.e., both angle differences .alpha. and 
.beta. are introduced into the calculation of the ground speed. 
In addition to the above, as shown in FIG. 11, the transmitter/receiver 
unit 16 of the second embodiment is designed such that the receiver 6 is 
arranged backwardly of the transmitter 3 in the vehicle travelling 
direction, and the ultrasonic wave emitting surface including the emitting 
point P of the transmitter 3 and the ultrasonic wave reception surface 
including the reception point Q of the receiver 6 are both inclined in the 
vehicle forward direction with a predetermined inclination, and the 
previously noted emitting surface and the reception surface are both 
arranged on the same plane. With the above arrangement of the 
transmitter/receiver unit of the second embodiment, the receiver having a 
relatively narrow directivity shown in FIG. 6 can exhibit a high reception 
sensitivity, since the receiver 6 can receive the reflected ultrasonic 
wave in a direction closer to the central axis of the receiver. In these 
constructions, the S/N ratio of the Doppler shift indicative signal is 
enhanced and consequently more accurate measurement of ground speed is 
insured. 
Returning to FIG. 10, the ultrasonic ground speedometer of the second 
embodiment includes the memories 14a and 14b, respectively deriving values 
of cos(.theta.-.beta.) and cos(.theta.- .beta.+.alpha.) on the basis of 
the Doppler shift frequency f.sub.d derived from the pulse counter 11 and 
an arithmetic circuit 15 for calculating the ground velocity v from the 
equation (5) on the basis of the Doppler shift frequency f.sub.d, the 
value of cos(.theta.-.beta.) derived from the memory 14a, and the value of 
cos(.theta.-.beta.+.alpha.) derived from the memory 14b. As described in 
more detail, the memory 14a stores values of cos(.theta.-.beta.) versus 
various Doppler shift frequencies, in the form of a data map previously 
derived on the basis of the .beta.-f.sub.d characteristic curve of FIG. 
13, while the memory 14b stores values of cos(.theta.-.beta.+.alpha.) 
versus various Doppler shift frequencies, in the form of a data map 
previously derived on the basis of the .alpha.-f.sub.d characteristic 
curve of FIG. 14. 
Referring nov to FIG. 15, there is shown the relationship between the 
reference ground speed measured by the conventional spatial filter type 
ground speedometer and the calculated ground speed obtained through the 
ultrasonic ground speedometer of the second embodiment. As seen in the 
graph of FIG. 15, the linearity of the calculated ground speed obtained 
according to the second embodiment to the measured ground speed obtained 
by the spatial filter type ground speedometer is remarkably enhanced. 
As will be appreciated from the above, the second embodiment is superior to 
the first embodiment, since the angle difference .beta. as well as the 
angle difference .alpha. is introduced into calculation of ground speed. 
When compared with the first embodiment, the ultrasonic Doppler ground 
speedometer of the second embodiment can provide more accurate measurement 
of ground speed of the vehicle. 
Similarly to the first embodiment, the frequency controller 13 employed in 
the ground speedometer of the second embodiment controls the received 
ultrasonic wave frequency F to be kept constant, so as to more enhance the 
S/N ratio of the Doppler shift signal. The ground speed v are obtained by 
substituting (f.sub.o + f.sub.d) into F of the equation (5), as follows. 
EQU v=cf.sub.d /f.sub.o 
{cos(.theta.-.beta.)+cos(.theta.-.beta.+.alpha.)+f.sub.d 
cos(.theta.-.beta.)} 
Although it is preferable to arrange the receiver backwardly of the 
transmitter in the vehicle travelling direction as previously described in 
the first and second embodiments, the transmitter and the receiver may be 
arranged at the same location. 
In the previously described first and second embodiments, an ultrasonic 
transducer, such as a piezoelectric transducer, is traditionally used as 
the ultrasonic transmitter or the ultrasonic receiver. As is generally 
known, such a piezoelectric transducer may be a piezoelectric crystal unit 
for converting acoustical or mechanical signals to electric signals, or 
vice versa. In order to provide possible highest S/N ratio for the Doppler 
shift indicative signal, it is preferable to utilize a high resonance 
characteristic ultrasonic transducer. 
While the foregoing is a description of the preferred embodiments for 
carrying out the invention, it will be understood that the invention is 
not limited to the particular embodiments shown and described herein, but 
that various changes and modifications may be made without departing from 
the scope or spirit of this invention as defined by the following claims.