VWGoA - Ex. 1005
`Volkswagen Group of America, Inc., Petitioner
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`

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`U.S. Patent
`
`June 28, 1994
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`Sheet 3 of 5
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`5,325,096
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`FIG. 3A
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`

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`U.S. Patent
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`June 28, 1994
`
`Sheet 4 of 5
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`5,325,096
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`FIG. 3B
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`5
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`U.S. Patent
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`June 23, 1994
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`Sheet 5 of 5
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`5,325,096
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`FIG.1.
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`POWERSUPPLY
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`WARNNGCONTROL
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`1
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`SMART BLIND spor SENSOR
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`5,325,096
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`This is a continuation of application Ser. No.
`07/930,079 filed on Aug. 14, 1992, now abandoned.
`BACKGROUND OF THE INVENTION
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`1. Field of the Invention
`This invention relates to automotive radar systems,
`and more particularly to a radar system for sensing the
`presence of obstacles in a vehicle’s “blind spots”.
`.
`2. Description of Related Art
`A continuing problem that presents itself to operators
`of automotive vehicles is the difficulty in seeing obsta-
`cles near the vehicle but in a location that is difficult to
`observe from the driver’s seat. Such regions are com-
`monly referred to as “blind spots”. For example, the
`angles between 90° and 170° from the forward direction
`of a vehicle (i.e., to the right of the vehicle and slightly
`behind the operator thereof) is a common blind spot,
`particularly for large vehicles such as buses and trucks.
`This right-side blind spot is a source of numerous acci-
`dents when a drive makes a right-hand turn or a right
`lane change and does not see another vehicle in the
`blind spot. Another common blind spot is the rear of a
`vehicle when backing up.
`The most common solution to the problem of blind
`spots has been to use mirrors to aid the operator of the
`vehicle in determining whether obstacles are present in
`a blind spot. Such mirrors have been made in a variety
`of shapes and mounted in various locations to provide
`the operator with the greatest ability to detect obstacles
`in particular blind spots. For example,
`it
`is common
`place today to see a concave mirror mounted to the
`right side of a vehicle aimed at the right-side blind spot.
`While mirrors provide the operator with some informa-
`tion regarding the presence of obstacles in certain of a
`vehicle’s blind spots, but they are less useful at night and
`under adverse weather conditions. Hence, a more com-
`plete and satisfactory solution is still sought by many.
`A known alternative to the use of mirrors to detect
`obstacles in a vehicle’s blind spot is to mount a camera
`on the vehicle to provide the operator with a visual
`image of obstacles in the vehicle’s blind spot. However,
`this solution is complex and expensive, requiring a video
`camera and video monitor. Further, a video monitor
`can present a complex image that must be interpreted by
`a driver, and such monitors can be distracting. More-
`over, like mirrors, such camera systems are less useful at
`night and under adverse weather conditions.
`Therefore, there is presently a need for a simple, and
`inexpensive solution to the problem of detecting haz-
`ardous obstacles in the blind spots of a vehicle. Such a
`solution should also be useful at night and under adverse
`weather conditions. The present
`invention provides
`such a solution.
`
`SUMMARY OF THE INVENTION
`
`The present invention is a simple, compact, and inex-
`pensive radar detection system configured to detect the
`presence of an obstacle in a vehicle’s blind spots and
`generate a signal to the vehicle operator indicative of
`the presence of such an obstacle.
`The system uses a common radar transceiver that
`transmits a radio frequency (RF) signal directed at a
`blind spot of the vehicle. The signal is reflected off any
`obstacles that are present in that blind spot region. The
`frequency of the transmitted signal is compared with
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`the frequency of a reflection of the transmitted signal to
`determine whether the reflected signal has been Dop-
`pler shifted. A Doppler shift in the frequency generally
`indicates that an obstacle has moved into the blind spot.
`Analog filters and digital circuits are used to filter out
`Doppler frequencies attributable to objects which are of
`no interest, such as stationary objects (for example,
`parked cars, road signs, and road side trees). Only obsta-
`cles that are traveling at approximately the same speed
`and direction as the vehicle are considered to be of
`interest. Therefore,‘ it is only these obstacles that will
`cause the blind spot sensor to generate an indication that
`an obstacle is present in the blind spot.
`The indication that is communicated to the vehicle
`operator is preferably an unobtrusive illuminated indi-
`cator which, in the preferred embodiment of the present
`invention, is affixed to or mounted near one of the vehi-
`cle’s side mirrors. Having the indicator affixed in this
`manner allows it to be seen by a normal, practiced mo-
`tion of the driver’s head. However, the operator is not
`distracted or disturbed by the frequent indications of
`obstacles which may occur under normal traffic condi-
`tions, and which are of little or no interest to the opera-
`tor unless a maneuver is planned which would cause the
`vehicle to come into contact with the obstacle. In addi-
`tion to the illuminated indicator affixed to or mounted
`near a side mirror, an obtrusive audible indicator is
`provided in the preferred embodiment of the present
`invention which creates an audible tone, whistle, or
`buzz when an obstacle is present and the vehicle’s turn
`signal is active.
`A malfunction detector is also included in the inven-
`tive blind spot sensor. The malfunction detector moni-
`tors an output of a sample and hold circuit to ensure that
`an output voltage from the sample and hold circuit is
`within expected limits, thereby determining whether
`the system is functioning properly.
`The details of the preferred embodiments of the pres-
`ent invention are set forth in the accompanying draw-
`ings and the description below. Once the details of the
`invention are known, numerous additional innovations
`and changes will become obvious to one skilled in the
`art.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a simplified block diagram of the present
`invention.
`
`FIG. 2 is a detailed block diagram of the signal pro-
`cessing section of the present invention.
`FIGS. 3a and 3b are flow charts of the procedure
`followed by the preferred embodiment of the present
`invention upon detection of an obstacle.
`FIG. 4 is a simplified schematic of the indicator cir-
`cuit of the preferred embodiment of the present inven-
`tion.
`
`Like reference numbers and designations in the vari-
`ous drawings refer to like elements.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Throughout this description, the preferred embodi-
`ment and examples shown should be considered as ex-
`emplars, rather than as limitations on the present inven-
`tion.
`FIG. 1 is a block diagram of the preferred embodi-
`ment of the present invention. The preferred embodi-
`ment shown in FIG. 1 includes a radar transceiver
`which determines the presence or absence of a target.
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`However, in an alternative embodiment of the present
`invention, the transceiver may emit and receive electro-
`magnetic signals to other frequencies, or signals other
`than electromagnetic radiation, such as ultrasonic radia-
`tion. Such ultrasonic transceivers are well known in the
`art and are used for detection of objects in the context of
`alarm systems, for example.
`'
`In FIG. 1, a Gunn diode 1 generates an radio fre-
`quency (RF) transmit signal based upon an input pro-
`vided to the Gunn diode 1 from a timing control circuit
`3. The timing control circuit 3 pulses for a duration of
`10 us at a rate of 10 kHz (i.e., the timing control signal,
`and consequently the RF transmit signal output by the
`Gunn diode 1, has a 10% duty cycle). A 10% duty cycle
`was chosen to optimize the energy efficiency of the
`system. The RF transmit signal is coupled to an RF
`coupler circuit 5 which permits RF energy to be cou-
`pled from the Gunn diode 1 to an antenna 7 and an RF
`mixer diode 9.
`The antenna 7 directs the RF transmit signal along a
`side of a vehicle upon which the radar system is
`mounted. In the illustrated embodiment of the present
`invention, a single antenna is used to transmit a single
`RF signal, and is mounted to provide the most effective
`coverage of a blind spot of a particular vehicle. How-
`ever, in an alternative embodiment of the present inven-
`tion intended for use with large vehicles, such as busses,
`a plurality of antennas may be used to ensure that obsta-
`cles which are present anywhere within the vehicle’s
`blind spots are detected. The RF transmit signal is re-
`flected off obstacles in the path of the signal. The an-
`tenna 7 receives a portion of the reflected signal. If an
`obstacle which reflects the transmit signal is in motion
`relative to the antenna 7, a Doppler frequency shift
`occurs between the transmitted signal and the received
`signal. Doppler shifting is a well-known phenomenon
`by which a signal which is reflected off an object which
`is approaching the source of the signal is compressed,
`thereby causing the frequency of the signal to be shifted
`upward. Likewise, the frequency of a signal that is re-
`flected off an object that is moving away from the
`source is shifted downward.
`
`The reflections of the RF transmit signal which are
`received by the antenna 7 are coupled to the RF coupler
`circuit 5, which in turn couples the received reflections
`to the RF mixer diode 9. The mixer diode 9 generates an
`output which has a frequency equal to the difference
`between the frequency of the RF transmit signal and the
`received reflections of the RF transmit signal. In the
`preferred embodiment of the present invention a Dop-
`pler detection module, such as part no. DRO2980 mar-
`keted by Alpha Industries, includes the RF antenna 7,
`the RF coupler circuit 5, the Gunn diode 1, and the
`mixer diode 9 in a single housing.
`The output of the mixer diode 9 is coupled to a signal
`processing section 11. The signal processing section 11
`amplifies, time demultiplexes, and filters the output of
`the mixer diode 9. The signal processing section 11 is
`coupled to a central processing unit (CPU) 31 that de-
`termines whether the output of the signal processing
`section 11 represents an obstacle of interest in the blind
`spot. The CPU 31 is coupled to an indicator circuit 41
`which presents warnings to the vehicle operator.
`FIG. 2 shows a detailed diagram block of the signal
`processing section 11 of the preferred embodiment of
`the present invention. An adjustable preamplifier (pre-
`amp) 21 receives the output from the mixer diode 9. The
`preamp 21 has a very low frequency response of ap-
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`proximately 5 Hz, thereby permitting very low frequen-
`cies to be amplified. By adjusting the gain of the preamp
`21, the sensitivity of the system is set to permit only
`those obstacles which are'in the immediate presence of
`the vehicle to be detected. Since the signal strength of
`the reflection drops by the square of the distance (i.e.,
`P= l/dz), proper adjustment of the preamp 21 is very
`effective in limiting the range of the blind spot sensor.
`For example, experimentation has shown that a motor-
`cycle will be detected in the lane adjacent to a vehicle
`equipped with the’ present invention at a distance of
`approximately 3 feet, while an automobile of average
`size will not be detected as being present if there is an
`empty lane between the automobile and the radar-
`equipped vehicle.
`In an alternative embodiment of the present inven-
`tion, the distance to an obstacle can be detected and
`obstacles that are beyond a specified range can be disre-
`garded. Thus, obstacles that are outside the blind spot
`(i.e., two lanes from the vehicle) but which are highly
`reflective will not cause the blind spot sensor to falsely
`indicate the presence of an obstacle in the blind spot. In
`one such alternative embodiment, a continuous wave
`(CW) frequency-modulated (FM) ramped modulation
`signal is applied to the input of the Gunn diode 9, caus-
`ing the Gunn diode 9 to change frequency in a linearly
`proportional relationship to time for a first period. After
`the first period, the CW FM ramped modulation signal
`causes the Gunn diode 9 to change frequency in an
`inverse linearly proportional relationship to time for a
`second period, which may be equal to the first period.
`Because there is a time delay between the transmis-
`sion of a signal and the receipt of the reflection of that
`signal off an obstacle, the delay being proportional to
`the distance from the transceiver to the obstacle, the
`frequency of the received reflection differs from the
`transmit frequency by an amount that is proportional to
`the time required for the signal to travel to the obstacle
`and return. Therefore, the frequency is also propor-
`tional to the distance between the antenna 7 and the
`obstacle. Because the CW FM ramped modulation sig-
`nal causes the frequency of the transmit signal to rise for
`a period of time and then to fall for a period of time, the
`frequency shift caused by the Doppler phenomenon can
`be distinguished from the frequency shift caused by the
`range of the obstacle. CW FM ramping modulation
`range detection schemes, such as described here, are
`well known in the art.
`In another such alternative embodiment of the pres-
`ent invention, the receiver circuitry is gated off a speci-
`fied amount of time after the beginning of a transmission
`pulse. If the specified amount of time is equal to the
`amount of time required for the transmit signal to reach
`the outer limits of the range of interest, only those ob-
`stacles that are within the range of interest are detected.
`In the preferred embodiment of the present invention,
`the output of the preamp 21 is coupled to a sample and
`hold circuit 23. The sample and hold circuit 23 samples
`the output of the preamp 21 at a rate and for a duration
`equal to the rate and duration at which the transmit
`signal is pulsed by the Gunn diode 1 (i.e., for 10 its at a
`rate of 10 kHz in the preferred embodiment). The sam-
`pling is synchronized to the transmission of the transmit
`signal by applying the same synchronization signal from
`a pulse generator circuit 25 to both the Gunn diode 1
`and the sample and hold circuit 23. The synchronization
`signal causes the Gunn diode 1 to generate the transmit
`signal when the synchronization signal is at a relatively
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`high voltage level, and also gates the sample and hold
`circuit 23 to sample the output of the preamp 21 during
`the same period. Each time the sample and hold circuit
`23 samples the output of the preamp 21, a voltage level
`is recorded. Thus, the output of the sample and hold
`circuit 23 is a series of voltage levels which increment
`or decrement every 100 us. The voltage levels represent
`the phase difference (i.e., Doppler shift) between the
`transmit signal and the received signal applied to the
`mixer diode 9 during each sample period.
`The output of the sample and hold circuit 23 is cou-
`pled to a low pass filter 27. The low pass filter 27 of the
`preferred embodiment of the present invention has a 3
`dB cutoff frequency of about 100 Hz. The low pass
`filter 27 serves three purposes: 1) to smooth the signal
`output by the sample and hold circuit 23 by removing
`high-frequency components of the output waveform; 2)
`to reduce noise,
`thus improving sensitivity without
`increasing RF power; and 3) to eliminate signals which
`represent objects moving rapidly relative to the vehicle,
`including stationary objects. Since the purpose of the
`present invention is to determine whether an obstacle
`which would otherwise go undetected by the operator
`is present in a blind spot of the vehicle, those obstacles
`which move rapidly through the blind spot are not of
`interest. It is assumed that obstacles that are moving
`rapidly through one of the vehicle’s blind spots will be
`seen before entering the blind spot, or will pass through
`the blind spot before the operator causes the vehicle to
`perform a maneuver which would present a danger due
`to the presence of that obstacle.
`The low pass filter 27 is coupled to a square wave
`generator 29 which generates a square wave signal that
`alternates between 0 volts and 5 volts. The frequency of
`the signal output by the square wave generator 29 is
`determined by the frequency of the input to the square
`wave generator 29 from the low pass filter 27. A square
`wave transition is output by the square wave generator
`29 whenever an obstacle has been detected.
`In the preferred embodiment of the present invention,
`the square wave generator 29 is a comparator circuit
`with hysteresis. The hysteresis provides noise immu-
`nity, prevents the comparator from oscillating, and
`limits range detection to a defined distance. Thus, when
`the input to the square wave generator 29 rises to cross
`a first relatively high threshold, the output of the square
`wave generator 29 transitions to a 5 volt level. When
`the input to the square wave generator 29 falls below a
`second relatively low threshold,
`the output of the
`square wave generator 29 transitions to a 0 volt level.
`The creation of a square wave output provides noise
`immunity and allows the output to be further processed
`by the CPU 31.
`Because some of the circuitry used in the present
`invention operates more efficiently when power is sup-
`plied from a bipolar power supply (i.e., both positive
`and negative voltages), a virtual ground circuit 33 is
`included in the illustrated embodiment of the present
`invention. The virtual ground circuit 33 works in con-
`junction with a voltage regulator 35 to supply the
`power requirements of the illustrated embodiment of
`the present invention. Most automotive vehicles today
`include a 12 volt battery which powers the starter
`motor and the electrical system when the engine of the
`vehicle is not operating, and a voltage generator or
`alternator which recharges the battery and supplies
`current to the vehicle electrical system when the engine
`is operating. The voltage regulator 35 of the present
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`invention receives power from the 12 volt vehicle
`power source and generates a stable 5 volt output. The
`5 volt output of the voltage regulator 35 is applied to
`those components of the present invention which oper-
`ate from a positive 5 volt source, and to the virtual
`ground circuit 33. The virtual ground circuit 33 creates
`a 2.5 volt output which acts as a virtual ground refer-
`ence for those components within the present invention
`that require both positive and negative supply voltages.
`Thus, the 5 volt output of the voltage regulator 35 is 2.5
`volts positive with respect to the virtual ground refer-
`ence, and earth ground (0 volts) is 2.5 volts negative
`with respect to the virtual ground reference. Such vir-
`tual ground circuits are well known in the art.
`A malfunction detector circuit 39 is coupled to both
`the sample and hold circuit 23 and the square wave
`generator 29. The malfunction detector circuit 39 gen-
`erates an output that indicates whether the present in-
`vention is operating properly. When the present inven-
`tion is operating properly, a direct current (DC) offset is
`present at the analog output of the sample and hold
`circuit 23. The DC offset is stripped from the analog
`output by capacitively coupling the analog output from
`the sample and hold circuit 23 to the low pass filter 27.
`However, the DC portion of the output of the sample
`and hold circuit 23 is present in the output that is cou-
`pled to the malfunction detector 39. In the preferred
`embodiment of the present invention, if the DC offset is
`not above a specified voltage, the malfunction detector
`39 generates and sends a gate control signal to the
`square wave generator 29 which decouples the square
`wave generator 29 from output circuitry of the signal
`processing section 11. A voltage divider circuit coupled
`to the signal processing section 11 output causes the
`output of the signal processing section 11 to be 2.5 volts.
`Since, under normal conditions, the square wave gener-
`ator 29 outputs only 0 volts or 5 volts, the presence of
`a 2.5 volt output from the square wave generator 29
`indicates a problem.
`The output of the square wave generator 29 is cou-
`pled to a dual edge-triggered memory register (flip-
`flop) 37, which is used to establish a “persistence per-
`iod”, as described below. A “persistence period” is
`defined in the preferred embodiment as the amount of
`time that it takes the vehicle upon which the radar
`system in mounted to travel 15 feet. When an obstacle is
`first detected, as determined by a transition at the out-
`put of the square wave generator 29, the CPU 31 waits
`the persistence period before responding to additional
`transitions. During the persistence period, no warnings
`are sent to the driver indicators. After the end of the
`persistence period, a warning is sent after each such
`transition if the transition occurs either within one sec-
`ond after the end of the last persistence period or two
`seconds after a prior warning was sent. Otherwise, a
`new persistence period cycle begins.
`If it is determined that an obstacle persists in the blind
`spot, a indication is presented to the operator of the
`vehicle. In the preferred embodiment of the present
`invention, three types of indications are used. If the
`vehicle’s turn signal becomes active (as detected by a
`position sensor coupled to an input of the CPU 31), and
`an obstacle is detected in the blind spot, an audible
`alarm sounds (e.g., emits an audible tone, whistle, or
`buzz) and a red visual indicator illuminates. If the turn
`signal is not active and an obstacle is detected in the
`blind spot, the audible alarm is not activated, but the red
`visual indicator illuminates. If no obstacle is detected, a
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`yellow visual indicator illuminates and the red indicator
`is inactive (illumination of the yellow indicator signifies
`that the blind spot sensor and circuit are active.)
`In an alternative embodiment of the present inven-
`tion, sensors to detect the steering wheel position and-
`/or the position of the turn signal are used to provide an
`indication that the operator is attempting to turn or
`change lanes. Other sensors may also be used to aid in
`the determination as to when the operator is attempting
`to cause the vehicle to enter a blind spot region. The
`system can be configured, if desired, to detect turning
`indicated by the position of the turn signal and/or by
`sensing the position and movement of the steering
`wheel, and to activate the audible alarm only if a turn is
`indicated in the direction of a blind spot in which an
`obstacle is present.
`FIG. 3 is a flow chart of the procedure followed by
`the preferred embodiment of the present invention for
`determining whether to warn the vehicle’s operator of
`the presence of an obstacle in a monitored blind spot.
`When a transition from 0 volts to 5 volts or from 5 volts
`to 0 volts occurs, a flag within the register 37 is set. In
`the illustrated embodiment of the present invention, the
`CPU 31 polls the register 37 at regular intervals to
`determine whether the register 37 has been set (STEP
`301). (In an alternative embodiment of the present in-
`vention, the CPU 31 is interrupted when the flag within
`the register 37 is set.) Once the CPU 31 detects that the
`flag within the register 37 has been set, the CPU 31
`resets the flag (STEP 302) and ceases polling the regis-
`ter 37. The CPU 31 is coupled to a speedometer which
`measures the ground speed of the vehicle. The CPU 31
`uses the vehicle speed to calculate how long it will take
`the vehicle to travel 15 feet (i.e., the persistence period)
`(STEP 303), and sets a timer to “time-out” at the end of 35
`the calculated amount of time (STEP 304). Once the
`timer times out (STEP 305), the CPU 31 sets a one
`second and a two second flag timer (STEP 306), and
`resets the flag in register 37 to ensure that any new
`transitions that may have occurred during the persis-
`tence period are cleared (STEP 307).
`In an alternative embodiment of the present inven-
`tion, one timer is used to indicate the amount of time
`elapsed after the flag in the register 37 is reset. Thus, the
`same timer which was used to determine when the
`persistence period has elapsed is reset and can be read at
`any time to determine the amount of time elapsed since
`the flag in the register 37 was reset.
`The timers of the preferred embodiment of the pres-
`ent invention are integrated into the CPU 31. However,
`one or more of the timers may be implemented in exter-
`nal circuitry.
`In the illustrated embodiment of the invention, the
`CPU 31 once again begins polling the flag within the
`register _37 after the persistence timer has timed out
`(STEP 308). By suspending the polling of the register
`37 for the persistence period, and resetting the register
`37 at the end thereof, the system effectively ignores
`transitions at the output of the square wave generator 29
`caused by reflections of the RF transmit signal off sta-
`tionary obstacles, such as parked cars and road signs,
`which are present in the blind spot for less than the
`persistence period.
`The CPU 31. checks whether a warning is presently
`being displayed (i.e., in the preferred embodiment of the
`present invention, whether the red indicator is illumi-
`nated) (STEP 317) while waiting for the flag in the
`register 37 to be set. If a warning is presently being
`
`v
`
`8
`displayed, the CPU 31 determines how long it has been
`since the warning was last activated. If the warning has
`been on display for more than one second without being
`reactivated (STEP 318), the CPU 31 causes the warning
`to cease being displayed (STEP 319). The CPU 31 also
`determines whether an audible alarm has been sounding
`for more .than one second without being reactivated
`(STEP 320), and causes the audible alarm to cease if
`reactivation of the alarm has not occurred in the last
`one second (STEP 321).
`If the CPU 31 determines that the flag in the register
`37 is set (STEP 308), the CPU 31 resets the flag (STEP
`309) and checks how long it has been since the persis-
`tence timer timed-out (STEP 310). If more than two
`seconds have passed since the persistence timer timed-
`out, the process returns to STEP 303 and suspends the
`polling of the register 37 once again. Thus, if an obstacle
`reflects the RF transmit signal back to the antenna 7,
`causing the output of the square wave generator 29 to
`transition, but no further reflections are detected for
`over two seconds, the system behaves as if the next
`transition of the square wave generator 29 is unrelated
`to the last transition, i.e., polling is suspended to ensure
`that the obstacle that caused the transition persists for
`more than the time required to travel 15 feet.
`However, if the transition has occurred within two
`seconds of the tirne-out of the persistence timer (i.e., the
`two-second persistence timer has not timed-out), then
`the CPU 31 checks whether one second has elapsed
`between the end of the persistence period and the latest
`transition (STEP 311). If more than one second has
`elapsed, then the CPU 31 checks whether more than 2
`seconds have elapsed since the last warning has been
`reactivated (STEP 312). If more than two seconds have
`elapsed, then the system returns to STEP 303 and sus-
`pends polling'of the flag in the register 37 for the dura-
`tion of a newly calculated persistence period. Other-
`wise, a one second warning timer and a two second
`warning timer are set (STEP 313), and the warning is
`reactivated (i.e., in the preferred embodiment of the
`present invention, the yellow indicator is turned off and
`the red indicator is turned on) (STEP 314). It should be
`understood that, as with the flag timers of step 306, the
`warning timers may be implemented as a single timer
`and may be either discrete timers or integrated into the
`CPU 31.
`
`In the preferred embodiment of the present invention,
`the CPU 31 determines whether the vehicle turn signal
`is active (STEP 315). If so, an audible alarm is activated
`to indicate that an obstacle is present in the blind spot
`and that turning the vehicle may be hazardous (STEP
`316). After reactivating the warning and resetting the
`warning. timer, the system returns to STEP 308 to await
`the next setting of the flag in the register 37.
`By determining whether a warning has been acti-
`vated within the last two seconds, and if so, then ex-
`tending the period before which the system resets the
`persistence timer, an obstacle in the blind spot which is
`moving at a very slow speed relative to the vehicle is
`not filtered out of the system due to the long duration
`between transitions of the square wave generator 29
`output. For example, an obstacle in the blind spot mov-
`ing at a relative speed that produces a Doppler fre-
`quency of less than 5 Hz generates transitions at the
`output of the square wave generator 29 at twice the
`Doppler frequency, i.e., less than 1 Hz. Therefore, the
`time between transitions is greater than 1 second. In-
`creasing the amount of time allowed between the time-
`
`10
`
`5,325,096
`
`l0
`
`15
`
`20
`
`25
`
`30
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`10
`
`

`
`9
`out of the persistence timer in STEP 305 and the next
`occurrence of a transition (as determined by detecting
`that the flag in the register 37 has been set) increases the
`low frequency response of the system. If it is already
`determined that an obstacle was very recently present
`(i.e., the warning timer has not yet tirned-out), then the
`possibility that an obstacle of interest caused the transi-
`tion is much greater.
`FIG. 4 is a simplified schematic of a preferred indica-
`tor circuit 41 for controlling the illumination of two
`warning indicators 407, 408, one of which would be
`yellow and the other red. A power supply is coupled to
`two resistors 401, 402 and a photo switch 403. A warn-
`ing control input 404 coupled to a control output of the
`CPU 31 controls the conductivity of a bipolar transistor
`405, which in turn controls the conductivity of a field
`effect transistor (FET) 406. By controlling the bipolar
`transistor 405 and the FET 406, the warning control
`input 404 controls the current flow through the two
`warning indicators 407, 408. The photo switch 403 is
`capable of bypassing the current limiting resistors 401
`and 402, and thus increasing the luminance of each of
`the warning indicators 407, 408. The photo switch 403
`is turned on (i.e., conducts current) when the ambient
`light is greater than a predetermined threshold amount.
`Therefore, the luminance is automatically controlled as
`a function of the ambient light, such that the warning
`indicators 407, 408 are visible in full sunlight, and are
`dimmed for nighttime conditions. Diodes 409, 410 di-
`vide the current that passes through the photo switch
`403 when the photo switch is conducting, while isolat-
`ing the current that flows through the resistors 401, 402
`and indicators 407, 408 when the photo switch is not
`conducting.
`In the preferred embodiment of the present invention,
`the visual warning indicators 407, 408 are very high
`luminance light emitting diodes (LEDs) placed on or in
`close proximity to a mirror on the same side of the
`vehicle as the blind spot sensor, such that when the
`operator looks in the mirror the warning indicators 407,
`408 are prominent. Having the warning indicators 407,
`408 affixed to an existing mirror allows it to be seen by
`a normal, practiced motion of the driver’s head. How-
`ever, the operator is not distracted or disturbed by the
`frequent
`indications of obstacles which may occur
`under normal traffic conditions, and which are of little
`or no interest to the operator unless a maneuver is
`planned w