Distance measuring device

A distance detecting device having a transmitter for intermittently transmitting measuring waves toward an object, a pair of receivers provided so as to be spaced from each other by half the wave length of the waves in a direction of the object, each receiver receiving reflected waves from the object and transmitting a reception signal responsive to the amplitude of the reflected waves, an addition circuit for adding the reception signal from each of the pair of receivers and transmitting an addition signal, a peak detection circuit for detecting a peak of the addition signal and transmitting a reflected waves reach signal at the time of peak detection, and an operation device for calculating the distance to the object based on the return time from the time when the waves are transmitted to the time when the reflected waves reach signal is outputted.

BACKGROUND OF THE INVENTION 
The present invention relates to a distance measuring device, especially a 
device for intermittently transmitting ultrasonic waves or electric waves 
toward an object, measuring the time required until the transmitted 
ultrasonic waves or electric waves return to the device after being 
reflected on the object, and detecting the distance to the object from the 
measured time. 
The present inventors previously have proposed a device for simply and 
certainly detecting a peak value of even the reflected waves formed like a 
series of smooth mountains due to damping or the like which occurs while 
they are being transmitted, and for measuring the distance based on the 
time required until a peak value of the reflected waves is detected 
(Japanese Unexamined Patent Publication No. Sho 60-46478). This device 
enables the measurement of the distance to the object, which is not 
affected by the hardness of the object, or the temperature and humidity of 
the transmission medium. 
The present inventors have employed the above-described device for 
detecting the air pressure of tires as shown in FIGS. 5 and 6. In FIGS. 5 
and 6, the drop of the air pressure of a tire W is detected by measuring 
the distance h between an axle A and a road surface E. An ultrasonic 
distance detector 1 is provided on the axle A and comprises an ultrasonic 
transmitter and an ultrasonic receiver (not shown), each facing the road 
surface E. The ultrasonic distance detector 1 further comprises a circuit 
for detecting a peak value of a reception signal of the ultrasonic 
receiver and for transmitting a reflected waves reaching signal. 
The ultrasonic distance detector 1 also is connected to an operation device 
2. The operation device 2 measures the time t needed from the time when 
the ultrasonic waves are transmitted until the reflected waves reaching 
signal is received by the ultrasonic distance detector 1, calculates the 
distance h to the road surface E based on the measured time t and gives an 
air pressure drop alarm when the distance h becomes smaller than a 
predetermined value. 
However, a large number of experimental results show that the air pressure 
drop alarm is sometimes erroneously given although the air pressure of the 
tire is sufficiently high. This is caused by the fact that the time t 
varies not linearly in proportion to the distance h but periodically at 
distances of .lambda. as shown by the line y in FIG. 4. This periodical 
variation of the time t seems to result from the interference between 
waves reflected on the object. In this case the distance .lambda. is equal 
to the wave length of waves such as ultrasonic waves used in this device. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a distance measuring 
device which transmits waves toward an object and calculates the distance 
to the object from a return time required until the waves return after 
being reflected on the object. 
Another object of the present invention is to provide a distance measuring 
device which calculates the distance to the object from a return time 
needed from the time when waves are transmitted until peaks of the 
reflected waves return to the device. 
Still another object of the present invention is to provide a distance 
measuring device which accurately calculates the distance to the object, 
cancelling the variation of the return time due to the interference 
between the reflected waves. 
The distance measuring device according to the present invention comprises 
transmission means for transmitting measuring waves, each having a 
constant frequency, toward an object intermittently, a pair of reception 
means provided so as to be spaced from each other in a direction of the 
object by a predetermined distance which is an odd multiple of half the 
wave length of the measuring waves, each of the reception means receiving 
reflected waves of the measuring waves on the object and outputting a 
reception signal responsive to an amplitude of the received reflected 
waves, addition means for adding reception signals from the reception 
means to each other and transmitting an addition signal having vibration 
periods, peak detection means for detecting a peak of the addition signal 
and transmitting a reflected wave reach signal at the time of peak 
detection, and operation means for calculating the distance to the object 
based on a return time from the time when the measuring waves are 
transmitted to the time when the reflected waves reach signal is 
outputted. 
According to the distance measuring device having the above-described 
construction, the return time varies not periodically but linearly in 
proportion to the distance to the object. Therefore, the distance to the 
object can be accurately detected from the return time.

DETAILED DESCRIPTION OF THE EMBODIMENT 
FIG. 1 is a block diagram of an ultrasonic distance detector 1 shown in 
FIG. 5. The ultrasonic distance detector 1 is provided with one ultrasonic 
transmitter 11 and two ultrasonic receivers 12A and 12B, each facing a 
road surface E. Both of the ultrasonic transmitter 11 and the ultrasonic 
receiver 12A are opposed to the road surface E at the same distance h. The 
ultrasonic receiver 12B is located above the receiver 12A so as to be 
spaced therefrom by a predetermined distance d. In this case, the distance 
d is set to .lambda./2, wherein .lambda. is the wave length of the used 
ultrasonic waves. 
The transmitter 11 is connected to a transmission circuit 101. The 
transmission circuit 101 receives a transmission command signal 2a from an 
operation device 2 and transmits an excitation signal 1c to the 
transmitter 11. The ultrasonic waves from the transmitter 11 are reflected 
on the road surface E and return to the receiver 12A. A little later, the 
reflected waves also reach the receiver 12B. Then, the receivers 12A and 
12B output signals 12a and 12b, respectively, each being respective to the 
amplitude of the received reflected ultrasonic waves. 
The signals 12a and 12b are added to each other in an addition circuit 102 
to obtain an addition signal 1d. This addition signal 1d is fed to a peak 
detection circuit 13 which detects the peak of the detection signal 1d and 
outputs a reflected wave reach signal 1k at the time of peak detection. 
The operation device 2 calculates the distance h to the road surface E 
based on the time t needed from the time when the ultrasonic waves are 
transmitted until the reflected wave reach signal 1k is outputted, and 
operation device 2 emits an air pressure drop alarm when the distance h 
becomes smaller than a predetermined value. 
FIG. 2 is a circuit diagram of the ultrasonic distance detector of FIG. 1. 
As shown in FIG. 2, the peak detection circuit 13 is composed of a first 
sample-and-hold circuit 103, a second sample-and-hold circuit 104, a 
comparison circuit 105 and the like. As also shown, receivers 12A and 12B 
are connected to a differential amplifier 102A composing the addition 
circuit 102 and acting also as an active filter. The differential 
amplifier 102A adds the signals 12a and 12b from the receivers 12A and 12B 
to each other. 
Hereinafter, the operation of the above described circuits will be 
explained with reference to the time chart of FIG. 3. In FIG. 3, fine 
lines indicate the expansion or the contraction of the axis of time. 
In FIG. 2, the transmission command signal 2a (FIG. 3(1)) transmitted from 
the operation device 2 (FIG. 1) through a terminal T is turned into a 
transmission timing signal 1a (FIG. 3(3)) by way of fip-flops 101A and 
101B, each being composed of gates. The timing signal 1a rises 
synchronously with the standard pulse 1b (FIG. 3(2)) outputted from an 
oscillation circuit 101C. The standard pulse 1b is divided by counters 
101D and 101E to obtain a transmitter excitation signal 1c (FIG. 3(4)) 
having a predetermined frequency. This transmitter excitation signal 1c is 
fed to the transmitter 11 synchronously with the rising of the signal 1a. 
This transmitter excitation signal 1c is intermittently outputted for a 
constant time t.sub.1 until a terminal Q of a D type flip-flop 101F is 
inverted. When the terminal Q is inverted, a transmission confirmation 
signal 11 (FIG. 3(16)) is outputted from a monostable multivibrator 105C, 
composing the comparison circuit 105, to the operation device 2 through 
the terminal T. 
When ultrasonic waves from the transmitter 11 are reflected on the road 
surface E (FIG. 1) and enter into the receivers 12A and 12B, the reception 
signals 12a and 12b are added to each other by the amplifier 102A, and 
inputted to an amplifier 102C through the amplifier 102B to be 
impedance-converted (signal 1d (FIG. 3(5),(6)). The standard level of the 
addition signal 1d is a constant voltage Va outputted from an amplifier 
106. 
The addition signal 1d is inputted to an analog switch 103A composing the 
first sample-and-hold circuit 103. The addition signal 1d is also inputted 
to an amplifier 107 composing a differentiator and is shaped in an 
amplifier 108. As a result, a rectangular pulse signal 1e (FIG. 3(7) is 
produced having a predetermined pulse width .DELTA.t, which is turned to 
1 level in every vibration period of the signal 1d. 
A gate 103E responsive to pulse signal 1e outputs a sample signal 1f (FIG. 
3(8)) having a pulse width determined by a resistor 103F and a condenser 
103G, synchronously with the rising of the signal 1e. A gate 104B 
composing the second sample-and-hold circuit 104 outputs a sample signal 
1g (FIG. 3(9)) having a pulse width determined by a resistor 104C and a 
condenser 104D synchronously with the falling of the signal 1e. 
The analog switch 103A conducts at every time when the sample signal 1f is 
inputted to a terminal C thereof. This results in the peak values of the 
addition signal 1d in vibration periods thereof being successively held 
and outputted from an amplifier 103B as a first hold signal 1h (FIG. 
3(10)). The first hold signal 1h is sampled again by an analog swich 104A 
operated by a sample signal 1g outputted after lapse of a constant time 
.DELTA.t from the outputting time of the sample signal 1f and is held as a 
second hold signal 1i (FIG. 3(11). The first hold signal 1h and the second 
hold signal 1i then are compared with each other by a comparator 105A 
composing the comparison circuit 105, which outputs a peak detection 
signal 1j (FIG. 3(12), (13)) which turns to 1 level when the second hold 
signal 1i becomes larger than the first hold signal 1h. 
Upon receiving the peak detection signal 1j, a D type flip-flop 105B is 
set, and the reflected wave reach signal 1k (FIG. 3(15)) having a 
predetermined pulse width is outputted from the monostable multivibrator 
105C to the operation device 2 through the terminal T. In fact, the peak 
detection signal 1j is outputted in the vibration period next to that 
wherein the reception signal 1d actually has a peak value. However, since 
the vibration period of the ultrasonic waves is very short and constant, 
the above-described fact does not incur any practical problems. 
A comparator 109 also compares the first hold signal 1h with a constant 
voltage Vb obtained by rectifying and smoothing an output of a terminal Q9 
of counter 101E. When the first hold signal 1h is not larger than the 
constant voltage Vb, a signal 1m (FIG. 3(14)) inhibits the flip-flop 105B 
from being set and accordingly, the reflected wave reach signal 1k is not 
outputted. 
The operation device 2 calculates the distance h to the road surface E 
based on the return time t needed from the time when the ultrasonic waves 
confirmation signal 1l is received to the time when the reflected waves 
reach signal 1k is received. 
In this embodiment, in order to cancel error due to the ups and downs of 
the road surface E, a large number such as ten thousand calculations are 
performed, and the obtained calculation results are averaged to obtain the 
distance h between the detector 1 and the road surface E. 
The measured return time to so detected linearly varies relative to the 
obtained distance h to the road surface E as shown by the line x of FIG. 
4. In FIG. 4, the line y indicates the variation of the return time of the 
receiver 12A, and the line z indicates the variation of the return time of 
the receiver 12B located above the receiver 12A to the road surface E by 
.lambda./2. 
By adding the return times of the receivers 12A and 12B to each other, the 
fluctuation thereof is cancelled by each other to obtain the return time t 
varying in linear proportion to the distance h as shown by the line x. An 
accurate distance h can be detected from this return time t. 
As described above, in the present embodiment, the return time t is 
detected based on the addition signal 1d obtained by adding the reception 
signals 12a and 12b to each other. Instead, by measuring the return times 
of the receivers 12A and 12B, respectively, and adding the measured return 
times to each other, the same result can be obtained. But, the former 
method of the present embodiment is more practical than the latter method. 
The device of the present invention can be employed for measuring various 
distances other than detecting the air pressure drop of tires. In such 
cases, electric waves can be employed in place of ultrasonic waves. 
Furthermore, a structure such as transmitter 11 also acting as one of the 
receivers 12A and 12B will do. In this case, after ultrasonic waves are 
transmitted, the connection of the transmitter 11 must be changed from the 
transmission circuit 101 to the addition circuit 102. 
The distance between the receivers 12A and 12B is not limited to half of 
the wave length of the measuring waves (.lambda./2). Any odd multiples of 
half of the wave length of the measuring waves will do.