Note: Descriptions are shown in the official language in which they were submitted.
20~686
The present invention relates to a radar safe stopping
distance detector and associated antenna for use in an
automobile. More specifically, the present invention relates to
a device for detectin~ the distance between an automobile and an
object in proximity thereto in order to determine a safe
stopping distance therebetween and to issue a visible and/or
audible alarm when the actual distance therebetween is less than
the safe distance between the vehicle and an object ahead.
Automobile traffic increases each year in most countries.
~0 With this increased traffic, there is a corresponding rise in
automobile accidents. E~urthermore, increases in traffic continue
to tax overcrowded road systems far beyond their designed
capabilities. Increased loads, crumbling roadways and faster
moving traffic all increase the risk of an accident.
Nevertheless, in most situations/ drivers are either unaware of
or ambivalent to the fact that they are travelling too close to
a vehicle in front of them to stop in case of a sudden stop. It
is estimated that one half of the total cost of automobile
accidents is a result of rear end collisions.
An efficient collision warning system can alert drivers
to react and adjust their driving habits in order to avoid most
collisions and save at least $25 billion annually in traffic
accident costs, injuries and fatalities.
Many researchers have investigated the automotive
collision warning ~ield. However, the de~ices produced in past
for this purpose suffer from several problems, such as high false
alarms, real targets being ignored, target ambiguity in a dense
environment and slow operation.
~62~86
Radar is best known in a pulse form. A pulse of radio
energy is emitted and the radar receiver listens ~or a reflected
return pulse from a target. When this pulse is received, the-time
interval is noted so that the distance can be calculated. The
ability of the radar to resolve distance is limited to the length
of the pulse. Automotive radar requires a range resolution of
approximately 3 meters. This would require the pulse to be less
than lO nanoseconds long, given the average space between
automobiles and potential obstacles. A system of this type would
be very costly to implement in a millimeter wave transceiver. A
number of devices have been proposed which utilize pulsed radar.
These include Kubota et al., United States Patent No. 3,787,845,
issued January 22, 1974 and san et al., United States Patent No.
3,898,653, issued August 5, 1975. Each of these devices shares
~5 this shortcomings of pulsed radar.
The high rate of false alarms, which is generally
associated with these pulse radar devices, is primarily caused
by the multitude of road features that do not present a danger
to the vehicle but which may momentarily appear to be a dangerous
object to a detection system. These include guard rails, signs,
buildings, overpasses and vehicles in other lanes. These objects
reflect and scatter radio waves. The amount of reflection and
scattering that is produced by these objects cannot generally be
predicted or modeled. For example, it is very difficult for small
2~ pulsed radar units to distinguish between a car and a sign or
guard rail.
The size of previous radar units has also been a problem.
The radar's ability to distinguish angular direction is primarily
20~686
dependent on the size of the antenna as compared to the
wavelength of the radio frequency used. A larger antenna has a
narrower beam and can therefore distinguish objects with greater
precision. The antenna can be made smaller, but this requires
much higher frequencies to be used and high frequency devices are
usually very expensive and difficult to produce.
Lastly, the time required by the radar system to acquire
a target and determine its velocity and distance is critical to
the effectiveness of the unit. The mul~itude of targets and the
high relative speeds found in highway driving introduce a time
constraint on the speed of the signal processor. In the past,
these units have been slow to react and easily confused.
There re~ains, therefore, a need in the art for a compact
and simplified radar system which can be utilized to accurately
predict an optimum space cushion between a vehicle and potential
obstacles.
The present invention has a small antenna, and a digital
signal processor with a special method of target detection and
characterization to significantly reduce false alarms and improve
~0 target selectivity.
According to one aspect of the invention, there is
provided a safe stopping distance detector for a host vehicle.
The detector comprises an oscillator for generating continuous
wave electromagnetic radar output signals. A modulator is
operably coupled to the oscillator to modulate the output
signals. A signal processor is operably coupled to the modulator
to cause the modulator to produce multiple slope output signals
to emanate from the oscillator. A transmit antenna is adapted to
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be located for the transmission of radar signals in a
predetermined direction. A power splitter is electrically
connected between the oscillator and the transmit antenna. A
receive antenna is adapted to be located to receive radar signals
S emanating from the transmit antenna and reflected off an
obstacle A mixer is electrically coupled between the power
splitter and the receive antenna for processing bea-t frequencies
for each modulation slope. The signal processor is electrically
coupled to the mixer, the signal processor including means for
~0 analyzing the beat frequencies to identify the obstacle, means
for calculating the distance to the obstacle, means for
determining if the host vehicle can stop within such distance.
Also, means is coupled to the signal processor for producing an
alarm if the host vehicle cannot stop within such distance.
~5 According to another aspect of the invention, there is
provided a method of determining a safe stopping distance between
a host vehicle and a target object. The method comprises the
steps of providing a continuous wave, frequency modulated radar
device on the host vehicle to transmit and receive radar signals
~0 reflected off the object. The radar signal is modulated to
produce a plurality of modulation slopes. The transmitted signal
is matched with the received signal to produce beat frequencies
corresponding to the delay between such signals for each
modulation slope. The beat frequencies are converted to digital
data. A Fast Fourier Transform is performed on the digital data
to produce frequency domain data. The frequency domain data
relative to each slope is matched to identify the target object.
The distance to the target object and the distance to stop the
2~6268~
host vehicle is calculated, and an alarm is emitted if the
distance to the target object is less than the host vehicle
stopping distance.
According to yet another aspect of the invention, there
is provided a planar phased array radar antenna comprising a
linear microstrip feed line having a plurality of spaced-apart,
hybrid taps located therealong. Each tap has opposed branches.
A corporate feed array is coupled to each branch of the hybrid
tap, each corporate feed array having a pair of branches. Also,
ln a microstrip patch is coupled to each corporate feed array
branch.
Preferred embodiments of the invention will now be
described by way of example with reference to the accompanying
drawings, in which:
~5 Figure 1 is a diagrammatic representation of a preferred
embodiment of an automotive safe stopping distanc~ detector
according to the present invention;
Figure 2 is a graphical representation of the filter
attenuation utilized by the device;
Figure 3 is a diagrammatic representation of an antenna
utilized in the detector;
Figure 4 is an enlarged view of a portion of the antenna
of Figure 3 showing a hybrid tap feed arrangement;
Figure 5 is an enlarged view of an alternative embodiment
of feed arrangement;
Figure 6 is a graphical representation of the detector
output and input;
Figure 7 is a graphical representation of the detector
2a~686
output and Doppler shifted input;
Figure 8 is a graphical representation of the voltage and
timing of the device output during a single modulation slope;
Figure 9 is a graphical representation of the output
frequency change during a single cycle;
Figure 10 is a graphical representation of the processor
activity during multiple c~cles;
Figure ll is a graphical representation of the
unprocessed ou~put of the radar receiver having a single target
1~ and an extraneous object;
Figure 12 is a graphical representation similar to Figure
11, but wherein the single target is farther away;
Figure 13 is a graphical representation of the
unprocessed output of the extraneous object of Figures 11 and 12;
Figure 14 is a graphical representation of the FFT output
of the signal processor operating on the signal shown in Figure
11;
Figure 15 is a diagrammatic representation of the FF~
output of the signal processor operating on the signal shown in
Figure 12;
Figure 16 is a diagrammatic representation of the FFT
output of the signal processor operating on the signal shown in
Figure 13;
Figure 17 is a graphical representation of the
~5 unprocessed output of the radar receiver which is viewing three
targets; and
Figure 18 is a graphical representation of the FFT output
of the signal processor operating on the signal shown in Figure
2 ~ 8 ~
17.
The safe stopping distance detector of the present
invention is generally indicated by reference numeral 8 in Figure
l and operates using frequency modulated, continuous wave (FM-CW)
radar, in which the frequency of the radar is modulated linearly
over time. Referring firstly to Figure l, a signal processor lO,
which is preferably a di~ital signal procassor, chooses a desired
modulation slope for the radar signal and commands a modulator
12, which in turn alters the radar transmission of an oscillator
:lO 14 in a sloped fashion as shown in Figure 6. A slope up or down
in frequency is produced by sending a rising or falling control
voltage from modulator 12 to oscillator 14. The oscillator 14
char.ges the frequency of oscillations proportionally to the
control voltage. If oscillator 14 does not produce the desired
~5 frequency, an optional frequency multiplier 16 multiplies the
frequency of the oscillator l~ as will be described further
below.
The oscillator output signal is passed through a bandpass
filter 17 and into a power splitter 18, which sends a portion of
~0 the output of bandpass filter 17 to a mixer 20 and the remainder
to a transmit antenna 2l. The received signal is received by a
reception antenna 22 and mixed by mixer 20 with the current
output signal from bandpass filter 17. The difference between the
transmitt2d and received signals, or beat frequency, is passed
2~ to an amplifier 24, then to filters 28 ~the purpose of which will
be described further below). The output from filters 28 passes
through another amplifier 26 and goes to an analog to digital
converter 30. The digitized values produced by A/D converter 30
2~268S
are read by the processor 10 and stored in memory until 128 are
collected. The processor then calculates a Fast Fourier Transform
or FFT on the sampled data. The FFT calculation c~nverts the
signals from ~ime domain data to frequency domain data. The data
is then analyzed to determine what frequencies are present and
their ~requency and power are recorded.
The processor lO then commands another slope to begin and
the above process repeats until a predetermined number of slopes
have been completed. The frequencies are matched to yield the
distance and Doppler information about each target. The processor
lO then reads the speed of the host vehicle from a tachometer 32
and calculates the minimum or safe stopping distance. If the
distance to the target is less than the safe stopping distance,
an alarm 33 is triggered. Five levels of alarm are provided, each
1~ indicating a higher level of danger. For example, each successive
alarm state could be triggered by each addition of two meters of
penetration into the safe stopping distance.
A control 31 i5 optionally provided for sensitivity
adjustment to adjust the parameters of the safe stopping distance
~0 calculation. The values used can be reduced or increased as
desired, to compensate for road conditions, visibility, vehicle
physical characteristics and driver reaction time.
The frequency of operation of safe stopping distance
detector 8 is preferably about 24.125 GHz. This is high enough
to significantly reduce the size of the antenna to about 4 cm by
12 cm. The transmit signal is generated by oscillator 14 which
preferably operates at about 12 GHz. ~his frequency is doubled
by multiplier 16 up to about 24 GHz. If desired, a three times
2 ~ f
multiplier 16 could be used so that detector 8 operates at about
35 GHz. Alternatively, a suitable oscillator 14 could be chosen
to operate at any desired frequency, thus 01iminating the need
for a multiplier 16.
Signal processor 10, preferably is a Texas Instruments
digital signal processor, part number TMS 32015. This is a first
generation 16 bit digital signal processor that uses a modified
Harvard structure with a hardware multiplier. The processor can
execute 5 million instructions per second, has 4K of program and
data ROM and 256 words of RAM. The data being sent will provide
a 64 point complex FFT using internal memory.
Referring next to Figure 2, filters 28 include three sub-
filters. A high pass filter 40 at 100 HZ is used to provide a 40
dB/Decade roll up, and block DC voltages and very low frequency
system generated clutter or SGC. A second high pass filter 42 at
10 KHz is used to provide a 20 dB/Decade roll up over the
frequency band. This is done to partially compensate for the
target having a 40 dB/Decade roll off with distance. Finally, a
low pass filter 44 at 10 KHz is used for anti-aliasing since the
A/D sampling frequency is 20 KHz. This filter provides an 80
dB/Decade roll off. The effec~ of these filters on the signal is
graphically represented on the lower portion of Figure 2. This
is a form of automatic gain control, although other forms of
automatic gain control could be used if desired.
2~ Referring next to Figures 3, 4 and 5, an antenna
utilizing a two dimensional array of microstrip patches is
illustrated in Figure 3 by reference numeral 10Q. Antenna 100
uses a "hybrid tap" feed structure as shown in Figure 4, and
2~2~8g
consists of a linear microstrip ~eed line 122 with periodic
microstrip hybrid taps or power dividers 124 each tap having
opposed branches 125. The power in the ith divider or tap 124 is
split and is then fed to a patch or an array above and below
linear feed line 122 as indicated by arrows 126. A certain
predetermined amount of power is then transmitted down the feed
line by the microstrip transmission line 128 to the (i+l)~h power
divider 124. This transmission line 128 can be a fixed phase
shift for beam forming or can be replaced by an active phase
shifter to add beam steering capabilities to the array. This
power splitting and phasing continues until the power reaches the
final nth power tap, and any remaining power is either terminated
in a matched load or is reflected. This type of feeding structure
minimizes microstrip feed losses, thus increasing the efficiency
1~ of the array antenna.
An amplitude taper and phase taper can be easily achieved
across the array. An ampli~ude taper varies the amount of current
flowing in each patch of the array, and thus varies the amount
of power radiated by the elements. This variation in radiated
power is used to minimize the side lobe levels of the antenna
pattern. The i~h power divider can tap the required power to
implement the desired amplitude taper across the array. In beam
steering, the phase shift between power dividers determines the
direction that the antenna beam is pointing.
Several different antenna s~ructures can be made using
the hybrid tap feeding configuration of Figure 4. Figure 5 shows
the hybrid tap feed used with four rows 136 of corporate fed
arrays 134. The feed point 130 is connected to power dividers or
20~26~6
taps 132, which are similar to dividers 124 in Figure 4, and
which are tapped to feed corporate structures 134 above and below
the feed line. These corporate structures or arrays have branches
135 that feed the individual microstrip patches 137. In Figure
5, the E field is horizontal, but in Figure 3, the E field is
vertical. The latter is preferred in an automotive radar to
eliminate unwanted signals from the road and overpasses.
Alternatively, a structure like Figure 5 could be used by turning
it 90 degrees.
If desired, the upper portion of antenna 100 could be
divided in the center into two halves and fed separately to
create a beam steered antenna. If the signal from one half of the
array is delayed slightly the antenna can be made to "look" off
to one side.
Referring next to Figure 6, is a plot of the frequency
transmitted and received by antenna 100 versus time as shown. The
frequency is modulated linearly with time. The solid line in
Figure 3 illustrates the transmitted frequency. The dotted line
shows the received frequency. The vertical distance A is the
~0 deference or beat frequency. The horizontal distance C is the
time it took the radio wave to propagate to the target and back.
The range to the target is a function of this beat frequency and
for steady state conditions, is expressed as:
fb = 2S R/C,
~5 where:
fb = Beat Frequency
S = Modulation Slope in Hz/s (Hz per second) as determi~ed
by the hardware
2 0 ~ 6
R = Range
C = speed of light
If the target has relative velocity, the received signal
frequency is shifted by the ~oppler effect. Figure 7 illustrates
a graph of frequency verses time of a transmitted frequency
(solid line) and a simulated received signal (dotted line) with
Doppler. The steady state expression for the beat frequency under
these conditions is:
~o fd + fb = 2SR - 2r/~,
where:
f~ = Doppler Frequency
r = Relative, Radial Velocity
A = Wavelength of Radar Transmission
f~" 5, R, C are as above
Since there are two unknowns, to reach a solution for
range and relative radial velocity, in the above equation, two
equations are needed, hence tha need for the second sloping
~0 frequency shown in Figures 6 and 7. The two equations can now be
solved to determine the Doppler portion and the range portion.
From the Doppler portion or relative radial velocity, th~ speed
of the target vehicle can be determined by subtracting the speed
of the host vehicle.
The safe stopping distance (SDD) is calculated using the
following formula:
SSr) = (VO) (tR) + DD + (VO1 -- (VT) ( IVTI )
2~o 2aT
2 ~
where:
VO = own speed (m/s)
VT = target speed (m/s)
tR = reaction time (s)
aO = own or host vahicle deceleration rate (m/s2)
a~ = target deceleration rate (m/s2)
Do = safety margin (distance left over when both
vehicles have come to a stop).
The deceleration capability of the target vehicle must be
determined and a typical, average value may be utilized if
measured data is not available to for each vehicle.
Alternatively, an assumption can be made that the deceleration
rate of the target vehicle is the same as the host vehicle.
As mentioned above, control 31 can be used to adjust
sensitivity of detector 8. Some of the adjustments could be to
the assumed deceleration rate of the host or target vehicle, or
the reaction time of the driver, or the safety margin. An
~0 automatic adjustment can also be built in. For example, in the
process of the vehicles's own speed measurement, a history can
be remembered over a preset time period. If the microprocessor
discovers the vehicle is decelerating quickly, it is assumed that
the driver has depressed the brake. Because the driver already
~5 has the brake depressed, the reac~ion time needed to depress the
brake further is not as great as the reaction time n~eded to
remove the foot from the accelerator and then apply the brake.
Alternatively, if the driver is accelerating, it will take more
time to apply the brakes. So, if the microprocessor detects
braking or acceleration, the coefficient of reaction time in the
safe stopping distance equation is changed accordingly.
Detector 8 preferably is designed to operate in the range
of 2 to 100 meters. The preferred minimum and maximum ranges of
2~2686
operation, ranges of resolution and relative velocity resolution
are shown in the following Table:
Minimum Range 2 m
Maximum Range 100 m
Range Resolution ~/- 1.5 m
Relative Velocity Resolution +/- 2.25 mph
Antenna seamwidth, Horizontal 5 degrees
Antenna Beamwidth, Vertical 15 degrees
Maximum Reaction Time 0.100 seconds
Signal processor 10 typically has other input/output
ports that are not used by the main microprocessor function and
~hese may be interfaced with temperature sensors, braking
systems, cruise controls, motion sensors and speech synthesizers
as will be appreciated by those skilled in the art.
As will be described in detail below, in the operation of
detector 8, the signals received by signal processor 10 are
sampled and a list of data points is recorded which describes the
changes in the signal versus time. The processor then calculates
~0 the Fast Fourier Transform of this data. The new data gives the
strength of the signal at each frequency and the enkire frequency
spectrum is, therefore, known. The target fre~uencies can thus
be identified. This techniqua gives the device the ability to
detect multiple targets and reject clutter and interference
unlike comparable analog systems which cannot achieve this goal.
The radar thus has the ability to discriminate and select
specific targets for further processing.
Figures 8 to 10 show a typical timing diagram used in the
operatlon of detector 8. Referring to Figure 8, at time T1, a
start pulse 49 is sent to modulator 12 as indicated on line 50.
Modulation occurs in the shaded time period indicated by line 51.
When the data is sent to the modulator, the frequency modulation
14
2~6~
ramp or slope begins at point 53. After a present delay, i.e.,
at time point T2, processor 10 starts to r~ad A/D converter 30
as shown by line 54. After 128 samples are read into the
processor, the A/D sampling stops, the modulation stops, and an
FFT is calculated.
Figure 9 shows the frequency versus time graph of
multiple modulation cycles similar to the slope in Fi~ure 8. Each
modulation slope 60, 61, 62 etc. is analogous to the modulation
slope 52 in Figure 8, except that each one has a different slope
1~ in ~Iz/second. For example, slope 60 could have a slope of 45
GHz/s, slope 61 could have a slope of - 45 GHz/sec, slope 62
could be 32 GHz/sec, etc. Notice that the time for each slope is
constant, T2 to T3 in Figure 8. Any number of slopes can be used
in one cycle if desired, but six slopes are usually sufficient
.l5 for detector 8.
Figure 10 shows the timing diagram for A/D converter 30
and the calculation of the FFT by processor 10. During a
frequency slope, 60 for example, the beat frequency received by
processor 10 is sampled 128 times as indicated by bar 56. During
~0 this time, the processor accepts the samples from A/D converter
30 and stores them in memory. After all 128 samples have been
read by processor 10, the processor calculates an FFT as
indicated by bar 55 converting the signals from the time domain
to the frequency domain. The processor ~hen finds the peaks in
~5 the frequency spectrum and records the corresponding frequency
and power of the peak for further prosessing as will be
illustrated below. ~he processor then triggers another slope and
A/D convertor 30 begins another sampling cycle indicated by bar
20~686
63. The process of trigger, sampling, FFT calculation, frequency
peak storage and trig~er again continues un~il all the desired
modulation slopes have been performed and the necessary data
stored.
Figure 11 is a typical IF ~oltage waveform from a single
slope with a test target at 100 feet and a s~op sign at 328 feet
plotted as amplitude or voltage versus time. Figure 12 has the
same stop sign at 328 feet, but with the test target at 200 feet.
Figure 13 shows the IF signal from the stop sign alone. The
signal from the stop sign in Figure 13 is lost in the much
stronger signal from the test targets in Figures 11 and 12. The
AGC of filters 28 makes the signal appear to be about the same
voltage in the figures, but it is really 20 dB down. This
alternate target recognition is the major problem for analog
~5 signal processors.
Figure 14 to 16 are plots of the FFT amplitude or power
versus frequency of the very same data as shown in Figures 11 to
13, respectively. The stop sign is clearly visible in the
fxequency spectrum at point 57, the test target can be seen in
~0 Figures 14 and 15 at point 58. System generated clutter or noise
can be seen in Figures 16 along line 59.
From the frequancy spectrum calculated by the FFT, the
peaks are found as indicated in Figures 14 to 16. Normally, each
peak will be a target. The processor compares each frequency
point in the FFT with the neighboring points. If the upper and
lower frequency neighbors are decreasing steadily in power, this
frequency is identified as a peak or target and stored for
further use.
16
2~2~8~
Figures 11 to 16 illustrate target recognition using a
single slope. However, if only one slope is taken, the frequency
detected normally contains information about range and Doppler
speed. There is no way to determine how much is contributed by
the range part and how much is contributed by the Doppler part.
The unambiguous determination of range and Doppler speed of one
target requires two slopes, and two resultant FFT spectrums, so
that the equation on page 15 can be solved. If the two slopes are
equal in magnitude, but differ in sign as shown in Figure 6, the
average frequency in the detected spectrum of both slopes would
be the range part. The Doppler part would be one half the
difference of the two frequencies in the two spectrums.
If multiple targets are present, each target would
contribute a signal in the FFT spectrum. ~esolving the range and
Doppler ambiguities is therefore more complicated. For n targets,
there are 2n variables, and n frequencies in each frequency
modulation slope. In the preferred embodiment, a matrix of
simultaneous equations would be constructed and solved to give
the range and Doppler information about each target. However, if
~0 the signal processing capacity is not sufficient, a trial and
error approach can be taken. In this latter approach the
frequencies detected in the slope spectrums are first compared
two by two, pairing the frequencies detected in adjacent slopes
that differ by only their sign, for example an up slope of 45
GHz/s and a down slope of 45 GHz/s. If there are three
frequencies contained in each slope, each frequency in one slope
is compared against each of the three frequencies ~rom the other
slope and nine possible targets result, each with a speed and
17
2~2~86
Doppler. Each of the nine possible targets from each pair is
compared to the nine targeks from the other slope pairs resulting
in twenty-seven possible target pairings, each compared with each
other to yield 729 different combinations.
Target pairs with a speed greater than 200 mph may be
discarded and target pairs with a negative range may also be
discarded. Additionally, target pairs with a range of greater
than 400 meters may be discarded. This significantly reduces the
729 possible combinations to a few dozen.
1~ Once each of the nine pairings has been investigated from
each slope pair, and the undesirable pairings have been
eliminated, the remaining pairings are compared to the other
slope pairs to find two pairs, i.e., data that matches over four
slopes, that have the same range and Doppler and return power.
This target is stored as a suspect. When all the comparisons are
complete, each suspect is checked against the third slope pair.
The suspect is then checked against the third remaining pair. If
no match is found, it may mean that two target frequencies have
collided in one Fourier frequency bin. This has a one in 32
2a chance of happening. Although small, the chance of this happening
is still significant. If the suspect target, which is identified
by the target speed, Doppler, and power return is not found in
the third pairing, it is still retained but given a low priority.
This means that if too many suspects match, and the
microprocessor runs out of memory to store all these suspects,
this one can be discarded. If it was a real target, it will
reappear in the next cycle.
When the table of target suspects is complete, the safe
18
2~626~
stopping distance is calculated ~or each one. The actual
distances to the targets are then compared to the safe stopping
distances and for each one where the actual distance is less than
the safe stopping distance an alarm signal is produced. The
5 high st alarm is stoxed, or displayed,and the measurement begins
again with a brand new set of slopes and FFTs.
In the preferred embodiment, the alarm from each
measurement cycle which comprises six slopes, six FFTs, the
suspect target table creation and the alarm calculation is
la averaged over four cycles to eliminate random targets due to
noise and momentary targets, which can be created when the
vehicle is turning or is momentarily looking directly at a
peripheral object such as a tree or sign. It has been found that
all vehicle targets will return a power in a certain range.
Bridges and buildings return too much power, and trees and guard
rails return too little power. Target suspects with a power that
is not within an accepted range can therefore be ignored.
Figures 17 and 18 illustrate another typical example of
the operation of detector 8. In this e~ample a vehicle with
detector 8 is travelling about 60 kilometers per hour with three
targets present. A stationary first target is about 75 meters
away. A second target is 100 meters away with a speed of about
110 kilometers per hour moving in the same direction as -the host
vehicle. A third target is approaching at a ground speed of about
90 kilometers per hour and is 31 meters away. Figure 17 is a plot
of the voltage waveform of the three target frequencies added
together. Figure 18 shows the results of the FFT output. The weak
signal at frequency 5.5 corresponds to target 1. This weak signal
19
2~626~6
might go unnoticed in an analog detection system. The other two
targets are clearly identified by the processor. This process is
repeated with other slopes so that target ranges and Dopplers can
be determined, safe stopping distances calculated and alarms
sounded or displayed as desired. The whole procedure from start
to finish using six slopes takes about 32 ms.
Having described preferred embodiments, it will be
appreciated that various modifications could be made to the
apparatus and methods described herein. For example, instead of
.l~ using a tachometer for determining the host vehicle speed, a
radar or Doppler signal could be picked up from the road or other
stationary objects to determine the vehicle speed. Instead of
determining safe stopping distances, detector ~ could simply be
used to sense or locate obstacles in proximity to the host
vehicle. Other types of antenna could be used, as well as other
types of signal processors, filters, automatic gain controls and
oscillators. Also, it will be appreciated that the detection of
this invention could be used in any type of vehicle or other
moving object, such as a boat, or train or even an airplane. The
term vehicle for the purposes of this specification is intended
to include any moving object that is desired to be stopped safely
before hitting another object.
While preferred embodiments of the invention have been
described, it is to be distinctly understood that the invention
is not limited thereto, but may be otherwise embodied and
practised within the scope of the following claims.
2Q
