Title: System and method for determining a distance of an object using emitted light pulses
Abstract: A system and method for determining a distance of an object is provided. The method includes transmitting a light pulse to a polymeric light reflector at a first time. The method further includes reflecting the light pulse from the reflector. The method further includes receiving a portion of the light pulse reflected from an object at a second time. Finally, the method includes determining a distance of the object from the reflector based on a time difference between substantially the first and second times.
Patent Number: 6,897,465 Issued on 05/24/2005 to Remillard, et al.
| Inventors:
|
Remillard; Jeffrey Thomas (Ypsilanti, MI);
Weber; Willes H. (Ann Arbor, MI);
Lippa; Allan J. (Northville, MI)
|
| Assignee:
|
Ford Global Technologies, LLC (Dearborn, MI)
|
| Appl. No.:
|
065579 |
| Filed:
|
October 31, 2002 |
| Current U.S. Class: |
250/559.38; 356/5.01 |
| Intern'l Class: |
G01N 021/86; G01V008/00 |
| Field of Search: |
250/55938,559.19,230
356/501,3
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Luu; Thanh X.
Assistant Examiner: Sohn; Seung C.
Attorney, Agent or Firm: Lippa; Allan J., Bir Law, PLC
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-In-Part of U.S. patent application Ser. No.
09/598,284 entitled NIGHT VISION SYSTEM UTILIZING A DIODE LASER ILLUMINATION MODULE
AND A METHOD RELATED THERETO filed Jun. 22, 2000, now U.S. Pat. No. 6,429,429 issued
Aug. 6, 2002.
Claims
1. A system for determining a distance to an object, comprising:
a light source generating a light pulse at a first time;
a polymeric light reflector receiving said light pulse and reflecting said light
pulse;
a light detector configured to receive at least a portion of said light pulse
reflected off the object, said portion being received at a second time; and,
a controller operably connected to said light source and said light detector,
said controller configured to determine a distance of the object based on a time
difference between said first and second times.
2. The system of claim 1 wherein said light source comprises a near-infrared
light source.
3. The system of claim 1 wherein said polymeric light reflector includes a first
and second plurality of reflective facets, said first plurality of reflective facets
receiving said light pulse from said light source and reflecting said light pulse
to a second plurality of reflective facets that further reflect said light pulse
toward the object.
4. The system of claim 1 wherein said polymeric light reflector includes a transparent
portion and a reflective surface, said light pulse moving through said transparent
portion to said reflective surface, said surface reflecting said light pulse toward
the object.
5. The system of claim 1 wherein said light detector comprises a near-infrared
light detector.
6. The system of claim 1 wherein said controller is further configured to generate
a received waveform based on said received light pulse, said controller being further
configured to indicate the object is detected when any portion of said waveform
has an amplitude greater than a predetermined threshold at said second time.
7. The system of claim 6 wherein said predetermined threshold has a first value
at a first elapsed time after said transmission and a second value at a second
elapsed time, said second elapsed time being after said first elapsed time, said
second value being less than said first value.
8. The system of claim 1 wherein said controller is further configured to generate
a received waveform based on said received light pulse, said controller being further
configured to multiply an amplitude of said received waveform by a gain value to
obtain a gain adjusted value, said controller being further configured to indicate
the object is detected when said gain adjusted value is greater than a predetermined
threshold at said second time.
9. An article of manufacture, comprising:
a computer storage medium having a computer program encoded therein for determining
a distance of an object, said computer storage medium comprising:
code for inducing a light source to emit a light pulse at a first time that is
reflected by a polymeric light reflector toward an object;
code for storing values indicative of a received portion of said light pulse
reflected from the object at a second time; and,
code for calculating a distance of the object from said reflector based on a
time difference between said first and second times.
10. A method for determining a distance to an object disposed in an environment, comprising:
transmitting a light pulse to a polymeric light reflector at a first time;
reflecting said light pulse from said reflector;
receiving a portion of said light pulse reflected from said object, said portion
being received at a second time; and,
determining a distance of said object based on a time difference between said
first and second times.
11. The method of claim 10 wherein said reflecting step includes:
reflecting said light pulse from a first reflective surface in said reflector
to a second reflective surface in said reflector; and,
reflecting said light pulse outwardly from said second reflective surface.
12. The method of claim 11 wherein the step of reflecting said light pulse from
a first reflective surface includes reflecting said light pulse to illuminate a
width of a roadway.
13. The method of claim 10 wherein said determining step includes:
generating a received waveform based on said received light pulse;
indicating the object is detected when any portion of said waveform has an amplitude
greater than a predetermined threshold at said second time; and,
calculating said distance based on said time difference between said first and
second times.
14. The method of claim 13 wherein said predetermined threshold has a first value
at a first elapsed time after said transmission and a second value at a second
elapsed time, said second elapsed time being after said first elapsed time, said
second value being less than said first value.
15. The method of claim 10 wherein said determining step includes:
generating a received waveform based on said received light pulse;
multiplying an amplitude of said received waveform by a gain value to obtain
a gain adjusted value; and,
indicating said object is detected when said gain adjusted value is greater than
a predetermined threshold at said second time; and,
calculating said distance based on said time difference between said first and
second times.
16. The method of claim 10 wherein said light pulse comprises a near-infrared
light pulse.
17. The method of claim 10 wherein the time difference is an average time difference.
18. A method for determining distance from an object, comprising:
transmitting a plurality of light pulses to a polymeric light reflector;
reflecting said light pulses from said reflector;
receiving said light pulses reflected off said object using a light detector;
determining an average travel time of said plurality of pulses; and,
determining a distance of said object based on said average travel time.
19. The method of claim 18 wherein said step of determining an average travel
time includes:
generating a plurality of received waveforms responsive to said light pulses
received by said light detector;
aligning said plurality of received waveforms in a common time interval;
determining an averaged received waveform by averaging said plurality of received
waveforms over said common time interval; and,
calculating said average travel time of said light pulses based on said averaged
received waveform.
20. The method of claim 18 wherein said plurality of light pulses comprises a
plurality of near-infrared light pulses.
21. A method for determining distance to an object disposed in an environment,
the method comprising:
transmitting a plurality of light pulses to a polymeric reflector that directs
at least a portion of the light pulses to illuminate the environment;
receiving reflected light pulses from the environment;
detecting the object based on elapsed time from transmitting the light pulses
and intensity of the reflected light pulses; and
determining distance to the object based on a time difference between transmitting
the light pulses and detecting the object.
22. The method of claim 21 wherein the step of detecting comprises comparing
a waveform based on the received reflected light pulses to a threshold that decreases
as elapsed time increases.
23. The method of claim 22 wherein the threshold decreases in a stepwise manner.
24. The method of claim 21 wherein the step of detecting comprises:
generating a waveform based on the received reflected light pulses and a gain
that increases as elapsed time increases; and
comparing the waveform to a constant threshold.
25. The method of claim 21 wherein the step of determining distance comprises
determining distance based o&an average time difference between transmitting the
light pulses and detecting the object.
26. The method of claim 21 wherein the polymeric reflector comprises a transparent
thin sheet optical element.
Description
BACKGROUND OF INVENTION
1. Field of the Invention
This invention relates to a system and methods for determining a distance of
an object using emitted light pulses.
2. Background of the Invention
A known laser range finding apparatus is disclosed in U.S. Pat. No. 5,669,174.
The apparatus utilizes an infrared laser to emit pulses of infrared light along
a narrow beam path toward an object. The infrared pulses are reflected from the
object and are received by a photoelectric diode which generates electrical pulses
responsive thereto. The electrical pulses are used to determine a distance of the object.
The known system, however, has a substantial drawback. In particular, because
the infrared laser emits a beam of light along a narrow beam path, automatically
detecting objects over a relatively wide area is not possible. For example, if
the known system were mounted in an automotive vehicle, objects in front of an
automotive vehicle on a roadway that are outside of the narrow beam path would
not be detected and thus their distance could not be calculated.
Another known laser range finding apparatus is disclosed in U.S. Pat. No.
5,949,530. The apparatus utilizes a laser to transmit a light pulse to relatively
large reflectors that reflect the light pulse toward an object. The apparatus,
however, requires relatively large reflectors which cannot be packaged in relatively
small package spaces. Accordingly, vehicle designers would have extreme difficulty
in utilizing such an apparatus in an automotive vehicle where only small packaging
spaces would be available.
The inventors herein have recognized that there is a need for a system and method
that minimizes or reduces one or more of the above-mentioned deficiencies.
SUMMARY OF INVENTION
The system for determining a distance of an object in accordance with a first
aspect of the present invention is provided. The system includes a light source
generating a light pulse at a first time. The system further includes a polymeric
light reflector receiving the light pulse and reflecting the light pulse. The system
further includes a light detector configured to receive at least a portion of the
light pulse reflected off the object at a second time. Finally, the system includes
a controller operably connected to the light source and the light detector. The
controller is configured to determine a distance of the object based on a time
difference between substantially the first and second times.
A method for determining a distance of an object in accordance with a second
aspect
of the present invention is provided. The method includes transmitting a light
pulse to a polymeric light reflector at a first time. The method further includes
reflecting the light pulse from the reflector. The method further includes receiving
a portion of the light pulse reflected from the object at a second time. Finally,
the method includes determining a distance of the object based on a time difference
between substantially the first and second times.
A method for determining a distance of an object in accordance with a third aspect
of the present invention is provided. The method includes transmitting a plurality
of light pulses to a polymeric light reflector. The method further includes reflecting
the light pulses from the reflector. The method further includes receiving the
light pulses reflected off the object using a light detector. The method further
includes determining an average travel time of the plurality of pulses propagating
from the light reflector to the object and then to the light detector. Finally,
the method includes determining a distance of the object based on the average travel time.
The system and methods for determining a distance of an object represent a significant
improvement over conventional systems and methods. In particular, the system may
be packaged in a relatively small package space since the polymeric light reflector
is extremely thin as compared with conventional reflectors and lenses. Accordingly,
the inventive system may be located in a relatively large number of locations in
an automotive vehicle. Further, the polymeric light reflector provides a wider
beam path—as compared to a narrow beam path transmitted directly from a
laser—that can illuminate a roadway for automatically determining a distance
of objects on the roadway.
These and other features and advantages of this invention will become apparent
to one skilled in the art from the following detailed description and the accompanying
drawings illustrating features of this invention by way of example.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic of a system for determining a distance of an object in
accordance with a first aspect of the present invention.
FIG. 2 is a perspective view of a polymeric light reflector utilized in the
system of FIG. 1.
FIG. 3 is a front view of the polymeric light reflector of FIG. 2.
FIG. 4 is an enlarged fragmentary sectional view of the polymeric light reflector
of FIG. 2 taken along lines 4—4.
FIG. 5 is an enlarged fragmentary sectional view of the polymeric light reflector
of FIG. 2 taken along lines 5—5.
FIG. 6 is a signal schematic of control signals for inducing a diode laser to
generate near infrared (NIR) light pulses.
FIG. 7 is a schematic of a waveform generated from a NIR light pulse reflection
and a threshold used to detect an object.
FIG. 8 is a schematic of a waveform generated from a NIR light reflection and
a signal gain.
FIG. 9 is a schematic of a resultant signal generated from the waveform and
gain of FIG. 8 and a threshold used to detect an object.
FIG. 10 is a schematic of an averaged waveform generated from two NIR light
pulse reflections and a threshold used to detect an object.
FIG. 11 is a schematic of an averaged waveform generated from two NIR light
pulse reflections and a signal gain.
FIG. 12 is a schematic of a resultant signal generated from the waveform and
gain of FIG. 11 and a threshold used to detect an object.
FIG. 13 is a flowchart of a method for determining a distance of an object in
accordance with a second aspect of the present invention.
FIG. 14 is a flowchart of a method for determining a distance of an object in
accordance with a third aspect of the present invention.
DETAILED DESCRIPTION
Referring now to the drawings wherein like reference numerals are used
to identify identical components in the various views, FIG. 1 illustrates a system
10 for determining a distance of an object
26. System
10 may
be utilized in a plurality of applications where the distance of an object is desired.
For example, system
10 may be used in an automotive vehicle (not shown)
to detect the distance of objects on a roadway from a vehicle.
As illustrated, system
10 includes a housing
12 which can hold
the
remaining components of system
10. It should be understood, however, that
the components of system
10 contained in housing
12 could be disposed
at different locations wherein housing
12 would not be needed. For example,
the components of system
10 could be disposed at different operative locations
in the automotive vehicle so that a single housing
12 would be unnecessary.
System
10 further includes a light source
14, a fiber optic
cable
15, a light reflector
16, a narrow-band optical filter
17,
a focusing lens
18, a light detector
19, and a controller
20.
Light source
14 is provided to generate one or more light pulses to
illuminate the environment and objects in the environment. The light pulses can
either be in the visible light spectrum or the non-visible light spectrum. For
purposes of discussion below, the light pulse may comprise a near infrared (NIR)
light pulse, whose wavelength is in the 700-1500 nm range. Further, light source
14 may comprise a NIR diode laser. In alternate embodiments, however, light
source could comprise other devices capable of emitting relatively short duration
light pulses having a pulse duration of 10-100 ns for example.
As illustrated, light source
14 receives one or more voltage pulses (V
T)
from controller
20 and generates an infrared light pulse responsive thereto.
In particular, light source
14 may comprise a Single Stripe Diode Laser,
Model No. S-81-3000-C-200-H manufactured by Coherent, Inc. of Santa Clara, Calif.
As illustrated, light source
14 may be disposed in housing
12. Further,
light source
14 may be connected to a first end of fiber optic cable
15
using a conventional light coupler (not shown) as known by those skilled in the
art. The second end of fiber optic cable
15 is operatively disposed adjacent
to polymeric light reflector
16.
Fiber optic cable
15 is utilized to transmit light from light source
14 to polymeric light reflector
16. Because of the high brightness
(candela per unit area) of light source
14, cable
15 preferably is
a relatively small diameter (0.1-1.0 mm) glass fiber. The use of a small diameter
glass fiber provides several benefits over monofilament plastic pipes and glass
fiber bundles used in non-laser based remote lighting systems. A small diameter
glass fiber is less bulky than plastic pipes or glass fiber bundles that typically
are 10-12 mm in diameter. Further, a small diameter glass fiber is significantly
less expensive than monofilament plastic pipes or glass fiber bundles. Still further,
a small diameter glass fiber is easier to package, handle, and to install than
monofilament plastic pipes or glass fiber bundles.
Light reflector
16 is provided to reflect and expand light (represented
by arrows A) generated by light source
14 generally in a first direction
from reflector
16. In a preferred embodiment, shown in FIGS. 2 and 3, reflector
16 comprises a unitary sheet of optical material extending generally along
a first axis
27. Reflector
16 preferably has a thickness range from
3-9 mm. It should be understood, however, that reflector
16 may have a thickness
less than 3 mm or greater than 9 mm. Reflector
16 is preferably constructed
from a transparent, solid piece of plastic such as polycarbonate and utilizes the
principle of total internal reflection (TIR) to reflect light. TIR is explained
in more detail hereinbelow. Reflector
16 may also be constructed from other
transparent materials such as acrylics. Referring to FIGS. 1,
2 and
3,
reflector
16 includes a front surface
28, a back surface
30,
a bottom surface
32, a top surface
34, side surfaces
36,
38,
and an aspheric lens
40.
Referring to FIGS. 3 and 4, bottom surface
32 of reflector
16
defines a first plurality of reflective steps
42 extending generally along
the axial length of reflector
16. Each of reflective steps
42 includes
a reflective facet
44 and a tread portion
46. As illustrated, each
tread portion
46 is generally parallel to axis
27. Each reflective
facet
44 is oriented at approximately a 45° angle relative to the adjacent
tread portion
46. It should be understood, however, that the angle of each
reflective facet
44 may vary depending upon the critical angle (discussed
further hereinbelow) of respective facet
44. Further, reflective facet
44
may have a curved shape (not shown) to further direct the light in a desired direction.
Still further, the number of reflective steps
42 may vary, and correspondingly,
the number of reflective facets
44 may vary.
Reflective facets
44 utilize the principle of TIR to reflect light
received from aspheric lens
40 towards reflective facets
50. Total
internal reflection of the light occurs when the incident angle θ exceeds
the critical angle θ
C given by the equation θ
C=sin-1(n
1/n
2)
wherein n
1 is the index of a refraction of air and n
2 is
the index of a-refraction of the polymeric material used to construct reflector
16. In an alternate embodiment (not shown), reflective facets
44
can be metalized if the light strikes facets
44 at an angle less than the
critical angle.
Referring to FIGS. 1,
2 and
5, back surface
30 defines
a second plurality of reflective steps
48 extending generally perpendicular
to axis
27. Each of reflective steps
48 includes a reflective facet
50 and a tread portion
52. As illustrated, each tread portion
52
is generally perpendicular to axis
27 and parallel to front surface
28.
Each reflective facet
50 is oriented at approximately a 45° angle relative
to the adjacent tread portion
52. It should be understood, however, that
the angle of each reflective facet
50 may vary depending upon the critical
angle of respective facet
50. Further, each reflective facet
50 may
have a curved shape (not shown) to further direct the light in a desired direction.
Still further, the number of reflective steps
48 may vary, and correspondingly,
the number of reflective facets
50 may vary. Referring to FIGS. 4 and 5,
facets
50 are aligned to receive light reflected from one or more reflective
facets
44, and, like facets
44, utilize the principle of TIR. Facets
50 reflect the received light through the front surface
28 of reflector
16 as will be described in further detail hereinafter. In an alternate embodiment
(not shown), reflective facets
50 can be metalized if the light from reflective
facets
44 strikes facets
50 at an angle less than the critical angle.
Referring to FIG. 2, aspheric lens
40 is provided to collimate the
light exiting fiber optic cable
15. The axial distance between cable
15
and lens
40 is chosen such that the light diverging from cable
15
fills the aperture of lens
40. Lens
40 is preferably constructed
to have a surface of revolution about axis
27 with a circular or hyperbolic
cross section. As illustrated, element
40 is disposed on side surface
36
of reflector
16 and may be integral with reflector
16. In an alternate
embodiment of reflector
16, lens
40 may comprise a separate lens
disposed in operative proximity to reflector
16.
Referring to FIGS. 1 and 2, the light pulses generated by light source
14 are received by reflector
16 from the second end of fiber optic
cable
15. The light being emitted from the second end of cable
15
preferably has a spread angle between 20-50°. It should be understood, however,
that the spread angle may be less than 20° or greater than 50° depending
upon the light characteristics of cable
15. The emitted light enters reflector
16 through aspheric lens
40 disposed on the side surface
36
of reflector
16. As discussed previously, element
40 collimates the
light, which then propagates toward reflective facets
44. Reflective facets
50 receive the light reflected from facets
44 and further reflect
the light through the front surface
28 of reflector
16 generally
in a first direction toward an object
26.
In an alternate embodiment of system
10, light reflector
16 could
be replaced by a fan-shaped reflector described in commonly owned U.S. Pat. No.
6,422,713, which is incorporated by reference herein in its entirety. In this alternate
embodiment, light source
14 could be directly coupled to the fan-shaped
reflector. Thus, fiber optic cable
15 would not be needed.
Narrow-band optical filter
17 is provided to allow light at a
wavelength substantially equal to the wavelength of light generated by light source
14 to pass therethrough. For example, when NIR light pulses are generated
by light source
14, filter
17 allows only light within the NIR emission
spectrum of the light source to pass therethrough and be received by light detector
19. In this case, filter
17 would prevent saturation of detector
19 by visible light emitted from the head lamps (not shown) of other automotive
vehicles. Filter
17 is conventional in the art and is preferably disposed
proximate focusing lens
18.
Focusing lens
18 is provided to focus NIR light pulses passing through
filter
17 onto light detector
19. Lens
18 may comprise an
aspherical lens, a doublet lens, or a triplet lens and can be constructed from
optical glass or plastics such as that used in standard camera lenses.
Light detector
19 is provided to generate a signal (V
R) responsive
to each reflected light pulse received by detector
19. Detector
19
may comprise a photodiode having a 1.0 nanosecond (ns) response time. Signal (V
R)
has an amplitude that is indicative of a power level or intensity of a received
light pulse and is received by controller
20.
Controller
20 is provided to implement the methods for determining
a distance of an object. Controller
20 includes a central processing unit
(CPU)
21, input/output ports
22, read-only memory (ROM)
23
or any suitable electronic storage medium containing processor-executable instructions
and calibration values, random-access memory (RAM)
24, and a data bus
25
of any suitable configuration. Controller
20 generates voltage pulses (V
T)
for generating light pulses and receives the voltage signals (V
R) corresponding
to received portions of the light pulses, as explained in greater detail below.
Referring to FIG. 13, a method for determining a distance of an object
in accordance with a second aspect of the present invention will now be described.
At step
54, a light pulse from NIR diode laser
14 is transmitted
to light reflector
16. As discussed above, controller
20 can generate
a voltage pulse
71 to induce light source
14 to generate a corresponding
light pulse. The voltage pulse
71 can have a duration (ΔT
D)
of 10-40 ns, for example. Controller
20 can store the time (T
1)
when the pulse was transmitted in RAM
24.
Next at step
56, light reflector
16 reflects the NIR pulse generally
in a first direction from reflector
16. Reflector
16 may be configured
to provide a horizontal light spread of 4-5 degrees and a vertical light spread
of 1-2 degrees. As shown in FIG. 1, the light pulse can propagate to an object
26 and be reflected from object
26.
Next at step
58, the light pulse reflected off object
26 can be
received by infrared light detector
19 which generates a voltage signal
(V
R) based on the power level or intensity of the light pulse. In particular,
the amplitude of signal (V
R) is proportional to the power level or intensity
of the light pulse. Referring to FIG. 7, waveform
74 generated by voltage
signals (V
R) over a monitoring period of 2000 ns (after transmission
of the light pulse) is illustrated.
Next at step
60, controller
20 can determine a distance of object
26 based on a time interval from a time (T
1) when the infrared
light pulse is transmitted to a time (T
2) when the light pulse reflected
off object
26 is received by light detector
19.
Referring again to FIG. 7, the substeps of step
60 will now be explained
in greater detail. Controller
20 can sample the voltage (V
R)
generated by light detector
19 over time using I/O ports
22. For
example, controller
20 can sample the voltage (V
R) every 40 ns
over a monitoring time period of 2000 ns. Each of the sampled values of voltage
(V
R) can be stored in RAM
24 of controller
20. The plurality
of stored values of voltage (V
R) over the monitoring time period defines
received waveform
74—comprising a set of points illustrated in FIG.
7.
Once waveform
74 is obtained, one of two methods can be utilized to detect
object
26. Referring to FIG. 7, a first method compares an amplitude of
each point of waveform
74 to a threshold (V
THRESH1) to determine
when an object
26 is detected. As shown, the threshold (V
THRESH1)
is decreased over an elapsed time of 2000 ns. The threshold (V
THRESH1)
is decreased because the transmitted light and reflected light pulses decrease
in signal strength by the square of the distance traveled, which leads to a return
signal for relatively distant objects being proportional to (1/Td′), where
T
d (i.e., T
d=T
2-T
1) is the total travel
time of the light pulse. The predetermined threshold (V
THRESH1) may
be defined using the following equation:
##EQU1##
where
A=predetermined constant having units of
##EQU2##
B=predetermined constant having units of
##EQU3##
C=predetermined constant having units of
##EQU4##
The constants A, B, C may be empirically determined and depend on the sensitivity
of detector
19, the field of view of detector
19, the transmission
power of light source
14, and the shape, size, and reflective characteristics
of the objects to be detected.
By decreasing the threshold (V
THRESH1) over the monitoring period,
the sensitivity of system
10 is increased for detecting relatively distant
objects that would have a reflection with a relatively small amplitude. Further,
the sensitivity of system
10 to fog is decreased by having a relatively
high threshold value for relatively small elapsed travel times (e.g., 0-500 ns)
of the light pulse. It should be understood, that the threshold (V
THRESH1)
could be implemented using equations different from the Equation (1). For example,
(V
THRESH1) could implemented using an equation that: (i) decreases (V
THRESH1)
in a stepwise manner (with two or more steps) over time, or (ii) decreases (V
THRESH1)
substantially linearly.
Referring to FIGS. 8,
9, a second method for detecting an object
26 is graphically illustrated. As shown, waveform
74 is obtained
from portions of a received light pulse as described above. Further, a signal gain
(G) is illustrated that increases over an elapsed time (or travel time) of a light
pulse. Controller
20 can multiply the amplitude of each point of waveform
74 at a predetermined elapsed time by a corresponding signal gain value
to obtain a gain-adjusted value. For example, the amplitude of waveform
74
at time T=1000 ns (value
78) can be multiplied by gain value
80 to
obtain the gain-adjusted value
82. When the gain adjusted value is greater
than a predetermined threshold (V
THRESH2), controller
20 indicates
object
26 is detected. Thus, by increasing the gain (G) over the elapsed
monitoring time, the sensitivity of system
10 is increased for detecting
relatively distant objects that would have a return light pulse with a relatively
small amplitude.
The gain (G) may be defined by the following equation:
where:
D=predetermined dimensionless constant;
E=predetermined constant having units of (seconds)
-2
F=predetermined constant having units of (seconds)
-4
The constants D, E, F in Equation (2) may be empirically determined and depend
on the sensitivity of detector
19, the field of view of detector
19,
the transmission power of light source
14, and the shape, size and reflective
characteristics of the objects to be detected. It should be understood, however,
that gain (G) could be defined by equations different from the foregoing equation.
For example, (G) could defined by an equation that: (i) increases gain(G) in a
stepwise manner (with two or more steps) over time, or (ii) increases gain (G)
substantially linearly.
After the object
26 has been detected by step
60, step
60
determines a distance of object
26 based on a time difference between time
(T
1) when the light pulse is transmitted and time (T
2) when
the light pulse reflected from object
26 is received by light detector
19.
In particular, a distance value (DIST) can be calculated using the following equation:
where C=speed of light (3.0E 8 meters/second).
Referring to FIG. 14, a method for determining a distance of an object
in accordance with a third aspect of the present invention will now be described.
At step
62, a plurality of light pulses from NIR diode laser
14
are transmitted to polymeric light reflector
16. As discussed above, controller
20 can generate a plurality of voltage pulses (V
T), such as pulses
71,
72 to induce light source
14 to generate the NIR light
pulses that are transmitted to light reflector
16. Although only two pulses
71,
72 are shown, controller
20 can generate as many pulses
as needed to obtain a desired signal-to-noise ratio. The voltage pulses
71,
72 can each have a duration of 10-40 nanoseconds with a repetition frequency
of 100-500 kHz. The repetition frequency is defined as 1/ΔT
p,
(where ΔT
p is the time duration between light pulses) and determines
the maximum detection range of the system. For example, if the repetition frequency
is 100 kHz, the system
10 would have a maximum detection range of 1500 meters
(e.g., detection range=3.0E8/100,000/2) where 3.0E8 meters/second is the speed
of light. Alternately, at a repetition frequency of 500 kHz, system
10 would
have a maximum detection range of 300 meters.
Next at step
64, light reflector
16 can reflect the light pulses
outwardly. As discussed above, reflector
16 may be configured to provide
a horizontal light spread of 4-5 degrees and a vertical light spread of 1-2 degrees
to illuminate the width of a roadway. The light pulses can then propagate to an
object
26 and be reflected from object
26.
Next at step
66, the light pulses reflected off object
26 can
be received by light detector
19 which generates a plurality of voltage
signals (V
R) based on the power level or intensity of the received light
pulses. The data values for each of the received waveforms generated by each of
the pulses, respectively, can be stored in RAM
24 of controller
20.
For example, referring to FIG. 10, the data values for waveform
84 produced
from reflected light from pulse
71 can be stored in RAM
24. Similarly,
the data points for waveform
86, produced from reflected light from pulse
72 can be stored in RAM
24. Thereafter, controller
20 can
generate a plurality of data values defining a waveform
88 in which each
data value of waveform
88 corresponds to an average value of data values
in waveforms
84,
86. For example, data value
90 of waveform
88 may correspond to the average value of data values
94,
92
of waveforms
84,
86 respectively, which are aligned in a common time interval.
Referring to FIG. 10, although only two waveforms
84,
86
generated from two light pulse reflections are shown, it should be understood that
controller
20 could produce an averaged waveform from more than two waveforms.
For example, controller
20 could average 2000 waveforms—generated
by 2000 light pulses having a repetition rate of 200 kHz—to obtain an averaged
waveform
88 every 10 milliseconds.
Once averaged waveform
88 is obtained, one of two methods can be utilized
to detect object
26. Referring to FIG. 10, a first method compares waveform
88 to threshold (V
THRESH1). The characteristics of threshold
(V
THRESH1) were discussed above. When the averaged waveform
88
has a voltage value greater than the threshold (V
THRESH1), controller
20 indicates object
26 is detected.
Referring to FIGS. 11 and 12, a second method for detecting an object
26
is graphically illustrated. Controller
20 can multiply an amplitude of each
point of waveform
88 at a predetermined elapsed time by a corresponding
signal gain value to obtain a gain-adjusted value. For example, the amplitude of
waveform
88 at time T=1000 ns (value
90) is multiplied by gain value
92 to obtain the gain-adjusted value
94. When the gain adjusted value
94 is greater than a predetermined threshold (V
THRESH2), controller
20 indicates object
26 is detected at that time. The detection time
is designated as time (T
3). As shown in FIG. 11, the average travel
time for the NIR pulses
71,
72 is approximately 1000 ns.
When controller
20 determines detection time (T
3) when object
26 is detected, controller
20 can calculate the average travel time
(T
AVG) of the pulses
71,
72 using the following equation:
Referring to FIG. 14, at step
70, controller
20 can calculate
a distance value (DIST) indicative of the distance of object
26 based on
an average travel time of the light pulses using waveform
88. In particular,
the distance value (DIST) can be calculated using the following equation:
where C=speed of light.
The system
10 and the methods for determining a distance of an object
represent a significant improvement over conventional systems and methods. In particular,
system
10 may be packaged in a relatively small volume since the polymeric
light reflector is extremely thin as compared with conventional reflectors and
lenses. Accordingly, the inventive system may be readily located in relatively
large number of locations in an automotive vehicle. Further, the polymeric light
reflector can spread the light pulses along a wider beam path to illuminate a width
of a roadway as opposed to spot illumination by conventional lasers. Thus, system
10 is able to automatically determine a distance of object that would be
undetectable by known systems.
*
