U.S. patent application number 09/737538 was filed with the patent office on 2002-06-20 for obstacle detection sensor using synchronous detection.
This patent application is currently assigned to PROSPECTS, CORP.. Invention is credited to Hawley, Stephen A., O'Connor, Christopher J..
Application Number | 20020074528 09/737538 |
Document ID | / |
Family ID | 24964310 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020074528 |
Kind Code |
A1 |
O'Connor, Christopher J. ;
et al. |
June 20, 2002 |
Obstacle detection sensor using synchronous detection
Abstract
An object detection system employs a photo-emitter and
photo-detector for synchronously detecting and processing an
optical signal reflected from an object in a pinch zone of a window
or door opening. A photo-emitter light signal is modulated by a
modulation signal having an active phase and an inactive phase. The
optical detector provides an optical detector signal that is a
function of the intensity of the received light. The detected light
signal is synchronously detected using a switching amplifier that
multiplies the reflected modulated light signal by a first gain
during the active phase and by a second gain during the inactive
phase. The duration of the active and inactive phases and the first
and second gains are selected such that the system gain will
average to zero for ambient light when integrated over a
predetermined measurement period. The synchronously detected signal
is subtracted from a predetermined offset voltages and this
difference is then integrated over the measurement period. The
output of the integrator is then compared to a predetermined
threshold value.
Inventors: |
O'Connor, Christopher J.;
(Northville, MI) ; Hawley, Stephen A.; (Bedford,
MA) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Assignee: |
PROSPECTS, CORP.
|
Family ID: |
24964310 |
Appl. No.: |
09/737538 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
250/559.4 |
Current CPC
Class: |
G01V 8/12 20130101 |
Class at
Publication: |
250/559.4 |
International
Class: |
G01N 021/86 |
Claims
1. An apparatus for detecting an obstacle in a pinch zone, the
apparatus comprising: a modulator providing a first modulation
signal having an active phase having a first duration and an
inactive phase having a second duration; a photo-emitter coupled to
said modulator for receiving said first modulation signal and
operative so as to provide an optical signal during an active phase
in response to said modulation signal and to be turned off during
an inactive phase; an optical detector optically coupled to said
photo-emitter and configured to receive a portion of said optical
signal that is reflected from said pinch zone and responsive
thereto, said optical detector providing a detector output signal
indicative of one characteristic of said received optical signal; a
switched amplifier having a first input coupled to said detector
output signal and a second input connected to a reference voltage,
said switched amplifier also being coupled to said first modulation
signal and having a first gain corresponding to said active phase
and a second gain corresponding to said inactive phase, said
switched amplifier providing an output signal that includes said
detector output signal multiplied by said first gain during said
active phase and said detector output signal multiplied by said
second gain during said inactive phase; an integrator coupled to
said switched amplifier and receiving said switched amplifier
output signal, said integrator being configured and arranged to
integrate said switched amplifier output signal over a
predetermined measurement period corresponding to at least one pair
of active and inactive phases, and to provide an integrator output
signal; and a detector for receiving said integrator output signal
and configured and arranged to provide indicia of the presence or
absence of an object within said pinch zone based on said
integrator output signal.
2. The apparatus of claim 1 wherein said photo-emitter is a
LED.
3. The apparatus of claim 2 wherein said LED emits light in the
infrared region.
4. The apparatus of claim 1 wherein the photo-emitter is a laser
diode.
5. The apparatus of claim 1 wherein the photo-emitter is a
flash-tube or flash-lamp.
6. The apparatus of claim 1 wherein said photo-emitter provides
said optical signal in a form selected from the group consisting of
a plane of light, a fan of light having a plurality of fingers, a
narrow beam of light, and a multifaceted beam of light.
7. The apparatus of claim 1 wherein said optical detector is a
photodiode.
8. The apparatus of claim 7 wherein said photodiode is responsive
to infrared light.
9. The apparatus of claim 8 wherein said photodiode includes a
visible light filter.
10. The apparatus of claim 1 further including a preamplifier
having an input coupled to said optical detector and an output
providing a preamplified detector output signal to said switched
amplifier.
11. The apparatus of claim 10 wherein said preamplifier includes a
current-to-voltage converter coupled to said photodiode.
12. The apparatus of claim 11 wherein said preamplifier includes a
gain stage amplifier coupled to said output of said a
current-to-voltage converter and provides an amplified detector
output signal to said switched amplifier.
13. The apparatus of claim 1 further comprising a subtraction
module having a first input coupled to said output of said switched
amplifier and a second input coupled to an adjustable offset
voltage, said subtraction module having an output coupled to said
integrator, wherein said adjustable offset voltage is selected such
that the integrator output signal can be maintained at a
substantially constant value during the measurement period.
14. The apparatus of claim 13 further comprising a control element
coupled to a memory, said control element configured and arranged
to adjust said offset voltage to maintain the output of the
integrator substantially constant during the measurement
period.
15. The apparatus of claim 1 wherein the modulation signal is
substantially a square wave with substantially a 50% duty cycle,
and wherein and the first and second gains of the switching
amplifier have the same magnitude and opposite polarity.
16. The apparatus of claim 1 wherein said modulator further
provides a second modulation signal having an active phase having a
third duration and an inactive phase having a fourth duration, said
modulator providing said first modulation signal to said
photo-emitter and said second modulation signal to said switched
amplifier, wherein said third duration is larger than said first
duration, and said fourth duration is larger than said second
duration.
17. The apparatus of claim 1 wherein the duration of the active
phase of the modulation signal is substantially less than the
duration of the inactive phase, and wherein the first gain of the
switched amplifier has a nonzero value and the second gain of the
switched amplifier has a substantially zero gain.
18. The apparatus of claim 1 further comprising a voltage reference
signal coupled to a second input of said switched amplifier, said
switched amplifier providing as an output during the active phase,
the difference between said detector output signal multiplied by
said first gain and said reference signal voltage multiplied by
said second gain, and providing as an output during the inactive
phase the difference between the voltage reference signal
multiplied by the first gain and said detector output signal
multiplied by the second gain.
19. The apparatus of claim 1 further comprising a threshold voltage
signal, and wherein said detector is a comparator having a first
input coupled to said integrator output signal and a second input
coupled to said threshold voltage signal, wherein said comparator
provides a signal indicative as to the presence or absence of an
object within the pinch zone.
20. The apparatus of claim 1 wherein said detector includes an A/D
converter coupled to a microcontroller.
21. The apparatus of claim 20 wherein the integrator is a
microcontroller performing the numerical integration of the
signal.
22. The apparatus of claim 1 wherein the modulator is a
microcontroller.
23. The apparatus of claim 1 further comprising a reset switch
coupled to the integrator to reset the integrator to a
predetermined value.
24. The apparatus of claim 23 wherein the reset switch is a
microcontroller.
25. The apparatus of claim 23 wherein the reset switch is an analog
switch.
26. The apparatus of claim 23 wherein the reset switch is a
resistor.
27. A method of detecting an optical signal in the presence of
ambient light, the method comprising the steps of: receiving an
optical signal having a first frequency and a first phase in the
presence of an ambient light signal; amplifying said received
optical signal and said ambient light signal; synchronously
detecting said received optical signal and ambient light signal to
provide a synchronously detected signal; subtracting said
synchronously detected signal from a predetermined offset voltage
to provide a subtracted signal; integrating said subtracted signal
over a predetermined period of time to provide an integrated
signal; and detecting a change in said integrated signal indicative
of a change in said optical signal.
28. The method of claim 27 wherein the step of amplifying includes
filtering said received signal through a band-pass filter.
29. The method of claim 27 wherein the step of synchronously
detecting includes the steps of: switching said amplified received
optical signal and said ambient light signal at said first
frequency and substantially at said first phase between first and
second outputs; amplifying, with a first gain, said optical signal
received at a first input; and amplifying, with a second gain, said
optical signal received at a second input.
30. The method of claim 27 further including the step of
initializing the integrator.
31. The method of claim 30 wherein the step of initializing further
includes the step of generating an adjustable offset voltage.
32. The method of claim 31 wherein the step of generating an
adjustable offset voltage includes the steps of generating an
optical signal during an initialization period when no obstacle is
present and adjusting the offset voltage such that the integrated
signal is substantially zero during the initialization period.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] N/A
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] N/A
BACKGROUND OF THE INVENTION
[0003] The present invention relates to a method and apparatus for
providing an indication of the presence of an object within a pinch
zone located in the path of an automated closure device such as a
powered window, powered sunroof, powered door or hatch using an
optical sensor, and more particularly to the use of an optical
sensor that incorporates a synchronous detection amplifier to
selectively amplify the desired light signal in the presence of
ambient light and electronic noise.
[0004] Closures for apertures such as vehicle windows, sunroofs and
sliding doors are now commonly motor driven. As a convenience to an
operator or passenger of a vehicle, power windows are frequently
provided with control features for the automatic closing and
opening of an aperture following a simple, short command from the
operator or passenger. Alternatively, automatic closing and opening
of an aperture may be in response to an input from a separate
device, such as a rain or temperature sensor. For instance, a
driver's side window may be commanded to rise from any lowered
position to a completely closed position simply by momentarily
elevating a portion of a window control switch, then releasing the
switch. This is sometimes referred to as an "express close"
feature. This feature is also commonly provided in conjunction with
vehicle sunroofs. Auto manufacturers may also provide these
features in conjunction with power doors, hatches or the like. Such
automated aperture closing features may also be utilized in various
other home or industrial settings.
[0005] In addition to providing added convenience, however, such
features introduce a previously un-encountered safety hazard. Body
parts or inanimate objects may be present within an opening when a
command is given to automatically close the window or door. For
example, an automatic window closing feature may be activated due
to rain impinging on an interconnected rain sensor while a pet in
the vehicle has its head outside the window. A further example
includes a child who has placed its head through a window or
sunroof that is activated to close by the driver, another passenger
or accidentally by the child.
[0006] In order to avoid potentially tragic accidents or property
damage involving intervening objects entrapped by power windows or
sunroofs, systems have been developed which detect the circumstance
in which a window has been commanded to express close but closure
has not occurred within a given period of time. As an example, a
system may monitor the time it takes for a window to reach a closed
state. If a temporal threshold is exceeded, the window is
automatically lowered. Another system monitors the electrical
current drain attributed to the motor driving the window. If it
exceeds a predetermined threshold at an inappropriate time during
the closing operation, the window is again lowered.
[0007] The problem with such safety systems is that an intervening
object must first be entrapped and subject to the closing force of
the window or other closure device for a discrete period of time
before the safety mechanism lowers the window or reverses the
sunroof or other closure device. Personal injury or damage to
property may still occur in such systems. In addition, if a
mechanical failure in the window driving system occurs or if a fuse
is blown, the person or object may remain entrapped.
[0008] Non-contacting object detection systems are known which
detect the presence of an intervening object within an open area.
Such systems include, for example, security systems and garage door
safety interlocks, to detect interruption of a light beam across an
opening. Other systems are used with automotive apertures having
motorized closure members such as windows, sunroofs, and sliding
doors, to detect an intervening object proximate or extending
through the respective aperture. Undesired operation of an aperture
closure member is therefore prevented when an intervening object
such as a finger or arm is extended through the opening during
closure; the closure member is not required to come into contact
with the intervening object for the object to be detected.
[0009] Such object detection systems typically measure the
magnitude of a reflected signal to determine the presence or
non-presence of an intervening object. A photo-emitter emits a
light beam which an optical system directs across the opening that
is being monitored. An uninterrupted opening may result in the
reflection of at least a some portion of the emitted beam from the
opposing side of the aperture. A photo-receiver disposed in an
appropriate location receives the reflected light beam and
generates an output signal indicative of the intensity of the
reflected beam. Reflection from the opposing side ordinarily
results in a reflected signal of a well-defined intensity being
returned to the receiver. Alternatively the emitted beam may be
directed so that it may graze or not strike an opposing member in
which case little or no light energy may be returned in the absence
of an object in the opening. An intervening object located in the
path of the light beam changes the intensity of the reflected light
beam, a condition reflected in the detector output signal. The
detector output signal with an object in the opening being
monitored will thus differ from the detector output signal in the
absence of an object. Depending upon the reflectivity of the
intervening object and the reflectance characteristics of the
aperture environment, the detector output signal will be greater or
less than the nominal output signal from the detector.
[0010] These optical systems, however, are vulnerable to
interference by ambient light such as sunlight as well as
fluorescent and incandescent overhead illumination. Prior art
solutions have included the use of synchronous detectors and
"judgment circuits" consisting of a number of logic circuits
coupled together. These judgment circuits however, are still
susceptible to interfering sunlight. In addition, these judgment
circuits typically include several steps each of which contains
several digital logic circuits. The large number of parts
associated with the judgment circuits can increase both the power
that is consumed and dissipated as heat, and also can increase the
cost associated with the object detection circuitry.
[0011] It would therefore be desirable to provide an apparatus and
method for detecting the presence of an object by measuring a
change in a light signal that is received in the presence of
ambient light, and which can be calibrated or initialized in such a
way so as to cancel the portion of the signal that is not
associated with an obstacle. Preferably, such an apparatus provides
enhanced accuracy by reducing the effect of the interfering ambient
light while using fewer parts and consuming less power than then
prior art.
BRIEF SUMMARY OF THE INVENTION
[0012] A method and apparatus are disclosed for sensing an object
by an optical sensor that utilizes synchronous detection and an
integrator for separating a desired optical signal from ambient
light and electronic noise as well as a means for canceling
modulated energy from features of the environment not associated
with an object in the opening.
[0013] In one embodiment, the system includes a modulator driving a
photo-emitter and a switched amplifier with first and second
modulation signals respectively. A photodetector receives a portion
of light reflected from the pinch zone and/or an object therein and
provides an optical detector signal to the switched amplifier.
[0014] The switched amplifier has a first input coupled to the
optical detector signal and a second input connected to a reference
voltage. This amplifier alternately switches between two phases
thus providing a first gain corresponding to the active state of
the photo-emitter and a second gain of opposite polarity
corresponding to the inactive state of the photo-emitter. The
switched amplifier provides an output signal that includes a first
voltage that results from the difference between the amplified
optical detector signal at the first input and the reference signal
at the second input multiplied by the first gain during the active
phase followed by second voltage that results from the difference
between detector output signal and the reference signal multiplied
by said second gain during the inactive phase. The first and second
gains and the duration of the active and inactive phases of the
measurement period are selected in such a way that the gain of said
switched amplifier has an average value of zero when no optical
signal is present when averaged over a predetermined measurement
period which will include at least one pair of active and inactive
phases and may include many such pairs.
[0015] The obstacle detection system further includes a means to
electronically integrate the difference between output of the
switched amplifier and an adjustable reference voltage for a
predetermined measurement time. The integrator is configured and
arranged to integrate the output signal of the switched amplifier
over at least one active phase and inactive phase, and to provide
an integrator output signal. A detector receives the integrator
output signal and is configured and arranged to provide indicia of
the presence or absence of an object within said pinch zone. In a
preferred embodiment the reference voltage input to the integrator
is selected so that the output voltage changes significantly only
when an object is present in the monitored opening. A detector
element monitors the integrator output signal and is configured and
arranged to provide indicia of the presence or absence of an object
within said pinch zone.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0016] The invention will be more fully understood with reference
to the following detailed description in conjunction with the
drawings of which:
[0017] FIG. 1 is a block diagram of the optical sensing system as
employed in the presently disclosed invention;
[0018] FIG. 2 is a schematic diagram of the photo-detector and
input amplifier of FIG. 1;
[0019] FIG. 3 is a schematic diagram of the switched amplifier of
FIG. 1;
[0020] FIG. 4 is a schematic diagram of the subtraction module and
integrator of FIG. 1;
[0021] FIG. 5 is block diagram of the modulator and photo-emitter
driver/amplifier of FIG. 1;
[0022] FIG. 6 is a schematic diagram of the driver/amplifier
circuit 508 of FIG. 5 along with a typical temperature compensation
circuit 510;
[0023] FIG. 7 is a timing diagram of a modulation signal provided
to the switched amplifier and photo-emitter of FIG. 1; and
[0024] FIG. 8 is a timing diagram of a modulation signal provided
to the switched amplifier and photo-emitter of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0025] A method and apparatus for detecting the presence of an
object within a pinch zone of an automated closure device such as a
power sunroof, power window, or a powered door or hatch is
disclosed. The definition of the pinch zone varies depending upon
the nature of the automated closure device. For example, if the
automated closure device comprises a power-assisted, sliding
closure member such as a power sunroof, a power window, or a power
door, the pinch zone is defined by a leading edge of a closure
member and a portion of the aperture defining a terminal portion of
the aperture opening with the closure member leading edge. If the
closure device comprises a powered, hinged door or hatch or a
powered revolving door, the pinch zone is generally a plane defined
by an edge of the aperture approached by the leading edge of the
door or hatch and a line adjacent the aperture edge in the path of
travel of the leading edge of the door or hatch.
[0026] In each of the embodiments described herein, a measurement
period is a predetermined period of time containing at least one
active phase and at least one inactive phase. As used herein, an
active phase is a period of time in which a photo-emitter provides
illumination. Similarly, as used herein, an inactive phase is a
period of time in which a photo-emitter does not provide
illumination.
[0027] The obstacle detection system includes an optical sensor
that receives a modulated light signal provided by a photo-emitter
temporally controlled by modulation signal. The photo-emitter
provides illumination of the window or other opening during the two
or more active and inactive phases of a measurement period. An
intervening object within the pinch zone reflects a portion of the
illumination provided by the photo-emitter and provides a reflected
light signal. In addition, depending on the direction and the shape
of the emitted light beam, the opposing side of the opening may, or
may not, also reflect light back to the photo detector. One or more
photo-detectors receive the reflected light signal and provide a
detector output signal indicative of one or more characteristics of
the received reflected light signal. The detector output signal may
be an analog signal such as a voltage or an electrical current, or
a digital output signal. The obstacle detection system processes
the detector output signal to provide an indication of the presence
or absence of an intervening object within the pinch zone.
[0028] The detector output signal is first pre-amplified then
synchronously detected by a switching amplifier. The resulting
signal is averaged by integrating over a predetermined measurement
period prior to comparison to a predetermined threshold value. The
predetermined threshold value is determined to represent the effect
of an object in the opening. As used herein, synchronous detection
utilizes a switching amplifier having first and second inputs. Each
input is switched between first and second gains in a manner that
is synchronous to the active phase(s) and inactive phase(s) of a
measurement period. The switching amplifier switches the detector
output signal from the first gain to the second gain synchronously
with the illumination provided by the modulated light source.
[0029] The first and second gains, the duration of the active and
inactive phases, and the number of active and inactive phases
within a measurement period are selected to reduce the level of
background interference from ambient light sources. The above
values are selected such that, in the absence of a modulated light
signal, the average or integrated value of the switching amplifier
output will not change from its initial value over a measurement
period. Thus, any signals that are not synchronous with the
modulated light signal, i.e., signals that are present during both
the active and inactive phases of a measurement period, will be
averaged or integrated to zero over the measurement period.
Averaging or integrating the switched amplifier output will remove
the non-synchronous background signals from further processing so
that they will not interfere with the detection of obstacles within
the pinch zone. The output from the switching amplifier is
integrated over a measurement period that may include one or more
active and inactive phases. By allowing the measurement period to
include multiple phases, spurious signals that are coincidentally
correlated with the modulated light signal over only one or two
activation periods can be removed and the number of false alarms
reduced. The averaged or integrated value is compared to a
predetermined threshold value to determine the presence or absence
of an obstacle within the pinch zone, and an appropriate indication
can be generated.
[0030] Referring to FIG. 1, a modulator 110, or alternatively a
micro-controller 122 which may have an internal timing system,
provides a modulation signal 109 to the photo-emitter driver 111.
The photo-emitter driver 111 provides the power to the
photo-emitter 102 that provides the modulated light signal 103. The
photo-emitter 102 may be configured and arranged with an optical
system to provide various patterns of illumination. For example,
the various patterns of illumination may be in the form of a plane
of light, a multifaceted fan of light having a plurality of
fingers, or a narrow beam of light. An object 101 reflects at least
a portion of the incident light 103 and some portion of the
reflected light signal 105 impinges upon a photo-detector 104. The
photo-detector 104 generates a detector output signal 107 that is
indicative of at least one characteristic of the reflected light
signal 105. The photo-detector 104 provides the detector output
signal 107 to the input of preamplifier 106. The preamplifier 106
amplifies the detector output signal 107 and provides the amplified
detector output signal to the switched amplifier 108.
[0031] The switched amplifier 108 is coupled to a modulation signal
123. The modulation signal 123 controls the switched amplifier 108
such that the switched amplifier 108 switches the preamplified
detector output signal and a reference voltage signal 125 between
first and second gains. The switched amplifier 108 provides an
output that is the difference between the two inputs multiplied by
the corresponding gain. As will be explained below, in some
embodiments the duration of the inactive phase of modulation signal
123 is slightly longer in duration than the corresponding active
phase of the modulation signal 109 provided to the photo-emitter
driver 111. This longer duration prevents small phase shifts that
might occur in the photo-signal in the preamplifier 106 from
shifting in time into the inactive phase of the amplifier and
causing self-interference. A non-zero reference voltage signal may
be used to establish a "no-signal" voltage level when a single rail
voltage supply is used in the illustrated embodiment. As will be
explained below, the preamplified detector output signal is
multiplied by the first gain during the active phase of the
measurement period, and by the second gain during the inactive
phase of the measurement period. The switching amplifier 108 driven
by the modulation signal 123 performs synchronous detection of the
preamplified detector output signal.
[0032] An adjustable predetermined offset voltage 112 can be
arithmetically combined with the synchronously detected signal to
reduce the difference between the output of the switched amplifier
and the reference voltage 113 to provide a substantially zero
average value when no obstacle is within the pinch zone. Therefore,
even though some reflected light may be returned from the opposing
members of the monitored opening, little or no signal will be
present. As will be explained below, the adjustable offset voltage
112 may be determined during a calibration or initialization
process prior to use or it may be determined on a periodic or
as-needed basis.
[0033] An integrator 114 receives the difference between the
synchronously detected signal and the adjustable offset voltage
112. The integrator 114 integrates this difference signal over a
measurement period and provides an integrated signal to the
detector 116. A measurement period is initiated by resetting the
integrator to a known value.
[0034] The output of the integrator is monitored by detector 116
that compares the integrated signal during a measurement period to
a predetermined threshold and generates indicia of the absence or
presence of an object within the pinch zone. In another embodiment,
an object is detected if the detector measures the time required
for the output of the integrator stage to change by a predetermined
amount and the measured time is less than a predetermined value. In
another embodiment a microcontroller 122 may monitor either the
voltage or the time as discussed above. In yet another embodiment
the detector 116 may be configured to generate the reset pulse
independent of microcontroller intervention. In this embodiment,
the frequency at which reset pulses are generated can be used as
indicia of an obstacle in the opening.
[0035] FIG. 5 illustrates one embodiment of a modulator 110 and a
photo-emitter driver 111 that are suitable for use with the
obstacle detection system 100. Modulator 110 includes an oscillator
502 that is operative to provide a train of suitable pulses at a
predetermined frequency and a predetermined duty cycle to a
frequency divider 504. In the illustrated embodiment, the frequency
divider 504 is a D flip-flop configured as a frequency divider. The
D flip-flop provides an output of square wave pulses at a frequency
that is one-half the predetermined frequency of the oscillator and
sets the duty cycle at substantially 50%. This signal is used to
modulate the switching amplifier. By making the duty cycle
substantially 50%, the active phases and the inactive phases of the
switching amplifier will have the same duration. Consequently, the
condition of zero average gain of the switching amplifier will be
achieved when the amplifier gain during the active phase is equal
in magnitude but opposite in sign to the amplifier gain during the
inactive phases of the modulation. If some errors associated with
signal phase shifts can be tolerated, the signal 123 can also be
used to modulate the light source directly. Otherwise, as
illustrated, the 50% duty cycle square wave modulation can be used
to trigger a pulse of shorter duration than the active phase of the
signal used to modulate the switching amplifier. As mentioned above
the use of this secondary pulse generator permits the active phase
of the light output to be constrained entirely to the active phase
of the switching amplifier in the presence of amplifier induced
phase shifts and prevents self-interference.
[0036] In other embodiments the amplifier can be periodic but have
a duty cycle other than 50% as shown in FIG. 7. The requirement for
zero average gain over one pair of cycles in this case is satisfied
when the product of the amplifier gain during active phase times
the duration 708 of the active phase is equal in magnitude but
opposite in sign to the product of the gain of the inactive phase
times its duration 710. As is the case for a 50% duty cycle
modulation, the duration 706 of the active phase of the
photo-emitter can be set to be shorter than the corresponding phase
708 of the switching amplifier to avoid errors due to phase shifts.
The switching amplifier 108 receives this output 123 of square
waves 702 from the frequency divider 504. The switching amplifier
108 uses this square wave pulse train 702 for switching the
switched amplifier inputs between the first and second gains. In
addition, a monostable multivibrator 506 receives the square wave
pulse train 702. In the illustrated embodiment, the monostable 506
triggers on a positive going transition of each pulse. The
monostable 506 produces a pulse having a width that is slightly
less in duration than the positive pulses 708 provided by the
frequency divider 504 to the switched amplifier 108. FIG. 7
illustrates this where the pulse train 702 is provided to the
switched amplifier 108 and the monostable 506. It can be seen that
the output 109 of the monostable 506, the pulse train 704, has a
shorter duration 706 when compared to the duration 708 of the pulse
train 702. This difference in the duration of the activation phases
ensures that the illumination time of the photo-emitter 102 is
smaller than the active phase of the switched amplifier. This
prevents any "spill over" into the inactive phase of the
measurement period by the active phase. In a preferred embodiment,
the oscillator 502 can be a 555 timer configured for astable
operation. The monostable can be a 555 timer configured for
monostable operation, and in one embodiment a 556 dual timer can be
used for both the astable portion and the monostable portion of the
circuit.
[0037] In some cases, more complex modulation waveforms may be
useful. For example, in applications where security is required it
will be useful that the modulation scheme cannot be defeated by a
simple external device. In this case, a microcontroller 122 may be
used to generate more complex modulation waveforms having different
characteristics in terms of the duty cycle and the periodicity and
symmetry of the waveforms. For example, the microcontroller 122 may
be programmed to provide an aperiodic or pseudo random pulse train
in which the duty cycle is much less or much more than 50%. In
these cases, there may be more than one pair of active and inactive
phases in each measurement period. As long as the condition for
zero average switching amplifier gain over the measurement period
is achieved, the detection scheme will provide the desired
selectivity.
[0038] In this embodiment, a microcontroller may be employed to
select the phase duration(s) from a predetermined table of
pseudo-random values that have been calculated so that the total
duration of the inactive and active phases is the same when summed
over the measurement period. Alternatively, RDAC's could be used as
R312 and R 308 and programmed by the microcontroller to balance the
signal on a phase by phase basis.
[0039] In some applications it may be desirable to employ a light
emitter such as a flash lamp and certain types of LED's and laser
devices for which the active phase is characterized as events of
very short duration. In these instances the duty cycle associated
with the active phase may approach 0%. When this condition applies,
a very simple implementation of the switched amplifier becomes
possible. Instead of switching between positive and non-positive
gains during the respective phases, the amplifier need only switch
between zero gain during the inactive phase and a non-zero gain
when the light sources is active. In other words the switching
amplifier is simply an amplifier that is turned on when the light
is on and off when the light is off. The average gain will now be
approximately zero over the measurement period. Waveforms 802 and
804 show modulation signal 123 and 109 respectively, wherein
waveform 804 includes the shorter duration activation phase pulses
806. As illustrated this waveform has a duty cycle that is much
less than 50% and in which there may be more than one active phase
and inactive phase per measurement period 808.
[0040] The monostable 506 provides the shorter duration activation
phase pulses to the photo-emitter amplifier/driver 508, which in
turn drives the photo-emitter 102 into illumination. The
photo-emitter 102 may be one, two, or more photo-emitters and the
configuration of the photo-emitters is such that the pinch zone is
fully illuminated. In a preferred embodiment, the photo-emitters
are light emitting diodes (LED's) that operate in the infrared
region of the optical spectrum.
[0041] In the illustrated embodiment, the amplifier driver includes
an optional temperature compensation module 510. As is known, a
change in the ambient temperature changes the intensity of the
illumination of an LED. To avoid a false alarm or a missed
intervening object due to fluctuations in the intensity of the
illumination caused by temperature, a stable constant level of
illumination is desirable. If there is a significant amount of
background energy the temperature induced change in the
illumination signal can be confused with that arising from an
obstacle. In one embodiment, a temperature compensation module 510
can maintain a constant drive level to the LED's in response to
signals 512 from a temperature sensor (not shown).
[0042] FIG. 6 illustrates one embodiment of a temperature
compensating circuit 510 and LED driving/amplifying circuit 508
suitable for use with the obstacle detection system 100. The LED
driver/amplifier circuit 508 comprises a pair of inverters, 610 and
612 coupled to the modulator 110 in order to convert the active
output to a signal that is suitable for switching the base of the
drive transistor 618. The voltage dividing circuit of resistors 614
and 616 sets a nominal bias voltage at the base of transistor 618.
In a preferred embodiment, the transistor 618 is a Darlington
transistor having a current gain of at least 300. The emitter
voltage of the Darlington transistor 618 will be two diode drops,
approximately 1.3 volts, below the bias voltage present at the base
of transistor 618. This emitter voltage is applied to resistor 634,
and determines the collector current flowing through both the
transistor 618 and the three LED's 626, 628, and 620. The collector
current in turn determines the intensity of the illumination of the
LED'S. The resistors 620, 622, and 624 act as transient snubbing
elements which eliminate high frequency radiation by providing a
current path when the LED's are turned off abruptly. These
resistors are typically of high resistance and do not conduct a
substantial portion of the LED ON current.
[0043] A temperature sensor (not shown) provides a temperature
signal to the non-inverting amplifier comprised of op-amp 604 and
resistors 606 and 608. The output of the non-inverting amplifier is
provided to the base of transistor 618 via resistor 635. This
output causes the DC bias voltage at the base of transistor 618 to
change with temperature, and therefore adjusts the emitter voltage
and collector current through resistor 634 as discussed above. In
this manner, the intensity of the illumination of the LED's may be
maintained at a constant value across a predetermined temperature
range.
[0044] However, as known to one skilled in the art, various other
means may be used to drive the LED's and provide for temperature
compensation thereof.
[0045] The photo-emitter 102 is selectively positioned such that a
substantially planar light beam emitted from photo-emitter and the
associated optical system 102 traverses at least a portion of the
pinch zone and impinges upon the vehicle interior, potentially
including the associate trim elements that surround opening (not
shown).
[0046] In the event an object is present in the field of the
emitted light beam, the amplitude of the signal reflected off the
object (not shown) is likely to vary based upon the size,
orientation, and reflectivity of the object. The obstacle detection
system detects variations in the output signal from the
photo-detector 104 and compares the output to a known threshold
value to determine the absence or presence of an object within the
pinch zone. The obstacle detection system provides a signal
indicative of the presence or absence of an object within the pinch
zone.
[0047] FIG. 2 illustrates one embodiment of a suitable
photodetector 104 and input amplifier stage 106. Photo-detector 104
may be a photo-diode 201 produces a photo-current that is a
function of the intensity of the incident light. A suitable
photodiode will typically be a PIN photodiode that has a
sufficiently fast response time to allow satisfactory operation at
the desired modulation frequency. In addition, the PIN photodiode
may be operated in a reverse bias mode as shown in FIG. 2 in order
to increase the width of the depletion region, thus providing a
greater bandwidth of operation. In the illustrated embodiment, a
photodiode was chosen that is operable in the infrared wavelengths
and has visible light blocking filters for wavelengths associated
with sunlight.
[0048] Op-amp 202 and resistor 204 form a current-to-voltage
converter 228 which converts the photo-current produced by
photo-diode 201 into a voltage. In a preferred embodiment, resistor
204 is 10 k resistor. Op-amp 202 preferably has an input bias
current that is substantially less than the signal current received
from photo-diode 201, and in addition, should have slew rate of at
least 3-4 volts/microsec in order to operate at modulation
frequencies above 10 KHz. In the illustrated embodiment, the op-amp
202 is a single supply op-amp, and therefore a non-zero voltage,
reference rail must be set about which the signal will swing. This
reference rail is set by the combination of op-amp 206, resistors
208 and 210, and Vcc, the supply voltage 203. The voltage division
of the supply voltage 203 will set the reference rail by the pair
of resistors 208 and 210. In a preferred embodiment, resistors 208
and 210 are of equal value and the reference rail 226 is therefore
set at Vcc/2 volts. Op-amp 206 is configured as a unity gain
amplifier and provides a low impedance, reference rail 226 to AC
amplifier 230 and other parts of the system as will be discussed
below.
[0049] The output signal from op-amp 202 is AC coupled, via
capacitor 229, to the AC amplifier 230 to block low frequency and
DC signals arising from ambient illumination, particularly
sunlight. Capacitor 299 and resistor 222 form a single-pole high
pass filter with cutoff frequency of a few hundred Hertz. The next
stage in this embodiment is an AC amplifier that is comprised of:
op-amp 212; resistors 214, and 218; and capacitors 216 and 220. AC
amplifier 230 provides an amplified signal on line 224. The
preferred type of op-amps 202, 206, and 212 for single supply
operation are rail-to-rail input and output Op-amps, having a low
offset voltage and low noise. Because a single rail voltage supply
is used, the common mode input range of the op-amps should include
both ground and Vcc. In a preferred embodiment, resistor 214 is 100
k, capacitor 216 is 3 pf, resistor 218 is 5.1 k, capacitor 220 is
0.01 uf, and capacitor 229 is 0.22 uf. Op-amp 212 is a MAX4126,
op-amp 206 is a TLC082, and op-amp 202 is a TLC082. All values are
examples of a preferred embodiment.
[0050] FIG. 3 illustrates one embodiment of a switched amplifier
108 in combination with the modulator 110 output signal 123 that
provides synchronous detection of the pre-amplified detector output
signal that is suitable for use with the obstacle detection system
disclosed herein. Switching amplifier 108 employs a double-pole,
double-throw switch pair 302 that simultaneously switches both
input lines 224 and 226 between a normally open (NO) position and a
normally closed (NC) position in response to the modulation input
123. The input line 224, which is connected to the preamplified
detector output signal, and input line 226, which is connected to
the reference voltage, are switched between the op-amp 310 input
terminals 305 and 307 of difference amplifier 316. The switch 302
provides the preamplified detector output signal on input line 224
to the negative input 305 during the active phase of the modulation
signal 123 and to the positive input 307 during the inactive phase
of the modulation signal 123. Similarly, the reference signal
voltage will be provided to the positive input 307 during the
active phase of the modulation signal, and to the negative input
305 during the inactive phase of the modulation signal 123.
[0051] Op-amp 310, and resistors 304, 306, 308, and 312 comprise a
conventional differential amplifier. Resistors 308 and 312 may have
the same value as do resistors 304 and 306. When the switched
amplifier is in the active phase, the output of the difference
amplifier 316 is given by
Vo=V(reference)+G[V(signal)-V(reference)]
[0052] where the gain, G=R308/R304. When the switched amplifier is
in the inactive phase, the inputs to the differential amplifier are
reversed so that the output voltage will have the same magnitude
but opposite polarity and is given by
Vo=V(reference)-G[V(signal)-V(reference)].
[0053] The output voltage over at least one measurement period will
then average to V(reference) when duration of the active phase of
the switched amplifier equals the duration of the inactive
phase.
[0054] In the illustrated embodiment, resistors 308 and 312 are 100
k and resistors 304 and 306 are 10 k. Thus, the difference
amplifier in the illustrated embodiment provides a nominal output
of 10(V.sub.+-V.sub.-), where V.sub.+ is the voltage on the
positive input 307, and V.sub.- is the voltage on the negative
input 305.
[0055] FIG. 4 illustrates one embodiment of the difference module
118 and the integrator 114 that are suitable to use with the
obstacle detection system. In the illustrative embodiment,
difference module 118 and integrator 114 are combined into a
circuit 120 comprising op-amp 406, capacitor 402, resistor 404, and
offset voltage source 126. Thus, the signal present on the output
line 408 will result from the integration of the current that flows
through resistor 404 as a result of the difference between the
voltage of the input signal on line 314 and the adjustable offset
voltage 416 which is applied to the non-inverting terminal of op
amp 406.
[0056] Circuit 112 in FIG. 4 shows one embodiment of an adjustable
voltage source which provides the adjustable offset voltage 416 for
the integrator. In this embodiment a resistive digital to analog
converter (RDAC) 414 such as an AD8400 is used. This device is the
equivalent to a digital controlled potentiometer with 256 possible
resistance settings. It is configured as a voltage divider in
conjunction with resistors 413 and 415. By using an RDAC as opposed
to conventional digital to analog converter (DAC) it is possible to
confine the total voltage adjustment to a narrow range on either
side of the system reference voltage. In the illustrated
embodiment, for example, resistors 413 and 415 have the same value
and the values of 413, 414, and 415 are selected to provide an
increment of 200 microvolts for each of the 256 steps. In other
embodiments, circuit function 112 may be implemented with the use a
DAC, a manually set potentiometer or any combination of an RDAC,
DAC and manual potentiometer.
[0057] The single-pole-single-throw switch illustrated in FIG. 4
serves the function of discharging the integration capacitor 402 of
circuit 120. This may be a normally open or normally closed switch
such as the MAX4502 and MAX4501, respectively. Both devices are
responsive to operation by the control system such as the
microcontroller 122. A measurement cycle commences with the switch
set to the closed position for a sufficient time to discharge
capacitor 402. The discharge time is a small multiple of the time
constant associated with capacitor 402 and resistor 406. Resistor
406 is represents the combination of the intrinsic resistance of
the switch and any resistors which may be added to prevent the
current discharge of capacitor 402 from exceeding the specified
limits associated with the switch 410. In the illustrated
embodiment, the reset event requires switch 410 to be closed for a
minimum of one millisecond.
[0058] In some embodiments, particularly those in which cost is a
primary factor and high performance is not required, switch 410 may
be replaced by a resistor. The value of the resistor substitute is
selected such the resulting time constant represented by the
product of the selected resistor value times the value of capacitor
402 is commensurate with the measurement time.
[0059] In the absence of a signal on line 314 caused by the
presence of an object in the opening, it is desirable to have the
integrated output of 120 remain at a constant value so any change
in the state of the integrator output can be used to indicate that
there is an obstacle in the opening. The presence of an adjustable
reference voltage for the electronic integration 120 of the signal
314 makes it possible to initialize the sensor so as to cancel any
modulated signals that are returned to the sensor from opposing
members of the window or door opening or fixed features in the
vicinity that do not result from an obstacle. Initialization can
take place at any moment when it is known that there no obstacle in
the opening. A typical instance for this to occur is at the time of
assembly of the integrated system comprised of sensor and the
associated door or window. There may be other times, however, at
which initialization can take place over the lifetime of the system
and will depend on the nature of the application. The process of
initialization takes place with the sensor operating in the normal
mode by adjusting the reference voltage from 112 while
simultaneously monitoring the output of the integrator 408. In the
absence of an object in the opening the offset voltage is adjusted
to a value for which the integrator output 408 remains constant
over the measurement period. Thus any object which appears in the
opening which causes a change in the reflected signal from the
initialized value will provide indicia of its presence.
[0060] The sensitivity of the detection scheme described above will
increase with the duration of the measurement time. However, the
application also requires that the measurement process occur with
sufficient rapidity that movement of the window can be arrested
quickly without striking or entrapping any object between
measurement cycles. In the illustrated embodiment the maximum
measurement time required to detect the smallest obstacle of
interest was set to 100 ms. It should be recognized that larger
obstacles will the provide an indication in less time.
[0061] Initialization as described above takes place with the
photo-emitter switching between the active and inactive states thus
providing modulated illumination into the opening. In some
embodiments requiring greater sensitivity or operation over a wider
range of environmental conditions it is useful to also initialize
the sensing system with the photo-emitter inactive. In a manner
similar to the initialization described above, the offset voltage
is adjusted with the photo-emitter inactive while maintaining the
modulation input to the switched amplifier until the output voltage
is unchanged over the measurement period. In this way signals that
are intrinsic to the circuit itself such as leakage currents and
offset voltages may be canceled prior to making a subsequent
measurement with the photo-emitters active. This secondary
initialization process may take place just prior to an active
measurement or at any time when the sensor is dormant.
[0062] Detector 116 may be a voltage comparator having a first
input coupled to a reference threshold voltage and a second input
coupled to the integrator 114 output 408. The comparator changes
the output state when the integrator output exceeds the reference
threshold voltage. In this case the length of time required for the
integrator to reach the threshold voltage as measured from the time
of reset of switch 410 of FIG. 4 may used to infer the relative
size of the obstacle. In a preferred embodiment, a threshold of
1.25 V is used. As described above, the output of the comparator
may used to trigger a pulse generator that resets switch 410
directly and thereby initiating a new measurement cycle. In this
case a free-running oscillator will be formed whereby the frequency
of reset pulses can be used as an indication of the state of the
monitored opening.
[0063] Alternatively, the detector may be a micro-controller or
microprocessor having an internal analog-digital (A/D) converter,
or controlling an external A/D converter, in which the output from
integrator 114 is converted to a binary number and compared to a
threshold number previously stored in a memory.
[0064] Those of ordinary skill in the art should further appreciate
that variations to and modification of the above-described methods
and apparatus for providing object detection in an aperture in the
path of a closure member may be made without departing from the
inventive concepts disclosed herein. Accordingly, the invention
should be viewed as limited solely by the scope and spirit of the
appended claims.
* * * * *