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Claims  |
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I claim:
1. In a system for monitoring the location of movable objects relative to
multiple prescribed locations:
an emitter carried by each movable object for automatically transmitting
coded signals uniquely identifying that movable object, transmission from
said emitters being at some nominal power level;
a plurality of sensor stations, at least one at each of said prescribed
locations, each sensor station being arranged to receive signals from an
emitter located within a predetermined distance from that sensor station,
said prescribed locations being spaced sufficiently to prevent the signal
transmitted by an emitter at said nominal power level from being received
at more than one sensor station at a time;
a central processing station; and
means for automatically transferring received coded signals and sensor
station location signals from said sensor stations to said central
processing station;
wherein each emitter is characterized by the transmission of code tones
during plural successive intervals, plural code tones being transmitted
simultaneously during each interval, each emitter including:
a plurality of oscillators for providing code tones;
actuable control means for switching the frequency of at least one of said
oscillators between plural discrete frequencies;
timing means for defining said plural intervals; with pre-established time
durations
transmitter means responsive to said timing means for transmitting said
code tones simultaneously during at least two successive intervals of said
plural intervals; and
means responsive to said timing means for actuating said control means to
change the frequency of said at least one oscillator from one to the other
of said two successive intervals.
2. The system according to claim 1 wherein said at least one oscillator
operates at the same frequency during the first of said successive
intervals in each emitter, said same frequency identifying said first of
said successive intervals.
3. The system according to claim 2 wherein all of said oscillators are
arranged to have their frequencies switched by said control means in said
two successive intervals.
4. The system according to claim 1 wherein the combined code tones
transmitted during at least one of said intervals represent the identity
of the movable object from which the tones are transmitted, and wherein
the combined code tones transmitted during at least another of said
intervals represent a condition associated with the movable object from
which the tones are transmitted.
5. The system according to claim 1 wherein said movable objects are
motorized vehicles each having an odometer, each odometer including a
movable member which moves at a rate proportional to vehicle speed, and
wherein said transmitter means includes logic means responsive to said
movable odometer member for repetitively transmitting said sequence of
code tones at a repetition rate determined by the rate of movement of the
movable member.
6. The system according to claim 5 wherein said movable member comprises an
odometer cable arranged to rotate in response to movement of said vehicle,
and wherein said logic means comprises:
means for generating a count pulse for each complete rotation of said
odometer cable;
divider means for counting each count pulse and providing an actuator pulse
each time a predetermined number of count pulses is counted; and
gating means for gating on said transmitter means in response to said
actuator pulse.
7. The system according to claim 6 further comprising means for
periodically gating on said transmitter means in response to a
predetermined time delay between successive actuator pulses.
8. The system according to claim 1 wherein said sensor stations are
connected to said central processing station via respective telephone
lines used solely for transmission of signals between said central
processing station and a respective sensor station.
9. The system according to claim 1 wherein said sensor stations are
connected to said central processing station via telephone party lines.
10. The system according to claim 1 wherein said sensor stations
communicate with said central processing station via long distance
telephone lines, wherein each sensor station includes: means for
temporarily storing received code tones; means responsive to reception of
code tones for automatically attempting to establish connection to said
central processing station on said long distance telephone lines; and
means responsive to establishment of said connection for transmitting the
temporarily stored code tones to said central processing station via said
established connection on said long distance telephone lines.
11. The system according to claim 1 wherein said sensor stations
communicate with said central processing station via a radio channel, said
sensor stations each including: a radio transmitter tuned to said channel;
and means responsive to reception of code tones for actuating said radio
transmitter to transmit said code tones to said central processing
station.
12. The system according to claim 1 further characterized in that said
transmitter means is inhibited from transmitting code tones when its
emitter is proximate a further emitter having a further transmitter means
which is in the process of transmitting code tones, each emitter
including:
a blanking transmitter for transmitting a blanking pulse when said
transmitter means is transmitting code tones;
a blanking pulse receiver for receiving blanking pulses transmitted from
other emitters which are located within a predetermined distance from said
each emitter; and
delay means for inhibiting said transmitter means for a period of time
greater than the duration of said plural successive intervals.
13. In a vehicle locator system of the type in which vehicles carry
emitters which transmit code signals for reception at individual sensor
stations dispersed throughout a prescribed area or route, vehicle
speedresponsive apparatus associated with each emitter for assuring that a
vehicle passing a sensor station transmits said code signals at least once
while in the receiving range of the passed sensor station, said apparatus
comprising:
an odometer cable;
pulsing means for sensing rotation of said odometer cable and providing a
gating pulse for every n rotations of said odometer cable, where n is
greater than one; and
gating means responsive to each gating pulse for actuating said emitter to
transmit said code signals.
14. The system according to claim 13 further comprising means responsive to
elapse of at least a predetermined period of time between successive
gating pulses for actuating said emitter to transmit said code signals.
15. The system according to claim 13 wherein each emitter includes:
a plurality of code tone oscillators, each switchable to operate at least
at two frequencies;
timing means for dividing the transmission of code tones into at least two
intervals; and
means for switching the frequencies of said oscillators so that each
operates at two different frequencies in said two intervals, respectively.
16. A vehicle identification system of the type in which vehicles carry
emitters for transmitting code signals to sensor stations spaced along a
prescribed route or within a prescribed geographic area, said system being
characterized in that said sensor stations communicate with a central
processing station via long distance public telephone lines, said system
including:
at said sensor stations:
means for detecting when code signals have been received from a vehicle
emitter;
means for temporarily storing the received code signals until they are
transmitted to said central processing station;
means responsive to detection of received code signals for automatically
dialing said central processing station on said long distance public
telephone lines to attempt to establish a long distance telephone
connection between said sensor station and central processing station; and
means responsive to establishment of a long distance telephone connection
between said sensor station and said central processing station for
transmitting the temporarily stored code signals to said central
processing station via said long distance public telephone lines.
17. The system according to claim 16 further comprising, at each sensor
station, delay means for delaying automatic dialing for a predetermined
period after received code signals are detected to permit code signals
from other nearby vehicle emitters to be temporarily stored and
transmitted to said central processing station during a common long
distance connection.
18. The system according to claim 17 further comprising, at each sensor
station, auxiliary means for temporarily storing code signals received
from vehicles while a long distance transmission of previously-received
code signals is in progress; wherein said means for automatically dialing
is responsive to storage of code signals in said auxiliary means at the
termination of a long distance call for automatically dialing said central
processing station to establish a long distance connection therewith.
19. The system according to claim 16 wherein each emitter includes:
a plurality of oscillators for providing code tones;
actuable control means for switching the frequency of said oscillators
between plural discrete frequencies;
timing means for defining plural successive time intervals;
transmitter means responsive to said timing means for combining and
transmitting said code tones during at least a portion of each of said
intervals; and
means responsive to said timing means for actuating said control means to
change the frequency of said oscillators in different intervals.
20. In a system for monitoring the location of movable objects relative to
multiple prescribed locations:
an emitter carried by each movable object for automatically transmitting
coded signals uniquely identifying that movable object, transmission from
said emitters being at some nominal power level;
a plurality of satellite sensor stations, at least one at each of said
prescribed locations, each satellite sensor station being arranged to
receive signals from an emitter located within a predetermined distance
from that satellite sensor station, said prescribed locations being spaced
sufficiently to prevent the signal transmitted by an emitter at said
nominal power level from being received at more than one satellite sensor
station at a time;
a plurality of main sensor stations, each associated with a respective
group of said satellite sensor stations;
radio transmitter means in each satellite sensor station for transmitting
coded signals received by said satellite sensor station to the main sensor
station associated therewith;
radio receiver means at each main sensor station for receiving coded
signals transmitted from satellite sensor stations associated with that
main sensor station;
a central processing station; and
means at each main sensor station for automatically transferring coded
signals received from said satellite sensor stations to said central
processing station.
21. The system according to claim 20 wherein the last-mentioned means
includes telephone lines.
22. The system according to claim 20 wherein the last-mentioned means
comprises:
means responsive to reception of coded signals from said satellite sensor
stations for automatically dialing said central processing station via
public telephone lines to attempt to establish telephone contact between
said main sensor station and said central processing station; and
means responsive to establishment of said telephone contact for
transmitting the coded signals received at said main sensor station to
said central processing station. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to systems for locating vehicles travelling
within a prescribed area or over a prescribed route, and particularly to
such systems wherein road side sensors receive emitted signals from
vehicles and transmit the signals to a central decoding station. The
invention as described herein is an improvement over the system described
in my prior U.S. Pat. No. 3,568,161 which is incorporated herein by
reference.
The system described in my prior patent employs a coded emitter located in
each vehicle and provides street-side sensors installed at pre-selected
locations within an area or region being monitored. The emitter is a very
low power RF transmitter which continuously radiates a signal modulated by
audio coding tones which identify the vehicle and/or its status. The
signal is demodulated at the sensor and automatically transmitted to a
terminal center by telephone lines or the like. Processing at the center
permits display or other type readout of the location of each vehicle
since the particular vehicle code has been received at a particular sensor
location. Vehicle location is updated each time the vehicle passes a
sensor. The number of vehicles which can be unambiguously identified in
such a system depends on the number of coding tones utilized in each
identification code. It is of course possible to increase the vehicle
capacity of the system by using a sequence of coding intervals wherein
different combinations of coding oscillators are gated on or not during
each coding interval. The problem with this approach, however, is that
failure of a coding oscillator can provide an erroneous identification
signal, resulting in the anomaly of the same vehicle showing up at the two
locations within the monitored region. The anomaly may be avoided by using
a parity oscillator which is gated on or not during each coding interval
to assure that an even (in the case of even parity) or odd (in the case of
odd parity) number of coding tones are gated on at any time. However, it
is desirable to avoid the expense of an additional parity oscillator. In
fact, it is desirable to minimize the number of oscillators required
overall so that the cost of the system can be minimized.
It is therefore an object of the present invention to provide a coding
arrangement in the system of the type described wherein the number of
coding oscillators, and therefore the system expense, is kept to a
minimum.
It is another object of the present invention to provide a coding sequence
in a vehicle locator system of the type described wherein the number of
oscillators is kept to a minimum and wherein the sequence is repeated
sufficiently often to assure that a complete coding sequence is received
by each sensor station passed by the vehicle.
It is another object of the present invention to provide a party line
arrangement in the connections between the sensor stations and the central
office to thereby minimize the cost of the system.
It is another object of the present invention to adapt the system of the
type described to large geographic regions by utilizing long distance
telephone interconnections between the sensor stations and the central
processing office.
It is another object of the present invention to employ radio links between
call boxes and decoding stations in a vehicle locator system.
It is still another object of the present invention to provide an
arrangement in a vehicle locator system of the type described wherein
transmission of a vehicle coding signal is delayed when two vehicles are
in close proximity so that confusion is minimized at the decoder.
SUMMARY OF THE INVENTION
In accordance with the present invention, the vehicle capacity of a vehicle
locator system is increased by employing a plural interval coding sequence
and by arranging each coding tone oscillator to generate more than one
tone. In this manner any oscillator can provide different tones during
different intervals in the sequence, thereby minimizing the number of
oscillators required to provide the various tone combinations. The
repetition rate of the pulse modulated transmitted tones is synchronized
to the vehicle odometer to assure that a complete coding sequence is
transmitted while the vehicle is within the receiving range of a sensor
station.
In order to permit vehicle location in regions encompassing more than one
local telephone office, long distance telephone lines and automatic dial
up connections are employed.
To maximize efficient utilization of telephone lines, a party line system
is employed wherein the locator system shares telephone lines with other
telephone system users.
Other features are disclosed, such as: the use of radio links between
sensor stations and a central office (in place of telephone lines);
delayed from one or more vehicle emitters which are proximate the same
sensor station to avoid garbling of two or more simultaneously received
codes; and adaptation of the system for use in predestrian (rather than
vehicle) location.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and still further objects, features and advantages of the present
invention will become apparent upon consideration of the following
detailed description of specific embodiments thereof, especially when
taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a functional block diagram of the overall system in which the
improvements of the present invention are employed;
FIG. 2 is a schematic diagram of a vehicle emitter of the present
invention;
FIG. 3 is a schematic diagram of a circuit for controlling a vehicle
emitter pulse repetition rate in response to the vehicle odometer;
FIG. 4 is a schematic diagram of sensor and decoder circuits which are
interconnected by dedicated telephone lines;
FIG. 5 is a schematic diagram of sensor, holding and dialing circuits which
are selectively and automatically interconnected to decoder circuitry via
long distance telephone lines;
FIG. 6 is a schematic diagram of a decoder circuit for use with the sensor
circuitry of FIG. 5;
FIG. 7 is a schematic diagram of a sensor station suitable for use with a
party line telephone connection to a decoder station;
FIG. 8 is a schematic diagram of a decoder station for use with a party
line telephone connection to a sensor station of the type illustrated in
FIG. 7;
FIG. 9 is a schematic diagram of a sensor station modified to transmit
information to a decoder station via a radio link;
FIG. 10 is a block diagram of a portion of a decoder circuit, illustrating
the modification required to permit the decoder to accept
radio-transmitted signals;
FIG. 11 is a schematic diagram of a circuit employed in conjunction with
vehicle or pedestrian emitters to prevent two such emitters from
transmitting codes simultaneously;
FIG. 12 is a diagrammatic illustration of a modification of the present
invention wherein satellite sensor stations are employed in conjunction
with main sensor stations; and
FIG. 13 is a schematic diagram of a satellite sensor station for use in the
system of FIG. 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now specifically to FIG. 1 of the accompanying drawings, two
vehicles 11 and 13 are illustrated as representing a fleet of vehicles
(i.e., police cars, buses, taxi cabs, trucks, trains, etc.) whose
locations are to be monitored. Each vehicle carries a coded emitter which
transmits an RF carrier signal which is modulated by audio frequency
coding tones. The emitter, which may be of the type described in my
aforementioned U.S. Pat. No. 3,568,161, is a very low power transmitter
designed to provide a signal with a limited range, on the order of 100
feet or less. Such transmitters can be operated without an FCC license on
many frequencies, and because of their limited range do not contribute to
the problem of spectrum congestion.
Multiple sensor stations, exemplified by sensor stations 15, 17, 19, 21, 23
and 25, are disposed at preselected locations within a prescribed area
through which vehicles 11 and 13 are to travel. For example, in the case
of police cars, the sensor stations may comprise police call boxes located
within the various beats or sectors to be patroled by the fleet of patrol
cars. Similarly, fire call boxes, traffic control boxes or other public
installations may be utilized as sensor stations, or in the alternative,
special sensor station installations may be provided. In the case of
buses, the street-side sensor stations would be spaced along the
prescribed bus routes, and in the case of taxi cabs the sensor stations
would be disposed at preselected locations in the area within which the
taxi company's franchised to operate. For trains, the sensor stations
would be disposed at various intervals along the track, and for trucks the
sensor stations would be located at specific points along the prescribed
truck route. As vehicle 11 passes the location of sensor 17, its coded
signal is received by a suitable receiver unit located in sensor station
17. The low power signals received by the sensor station are demodulated
to recover the audio frequency of coding tones which are then
automatically transmitted to sensor line termination centers 27 and 29.
Termination center 27 is illustrated as receiving the coding frequency
signals from sensor stations 15, 17 and 19, whereas sensor line
termination center 29 is illustrated as receiving the coding frequency
signals from sensor stations 21, 23 and 25. The number of sensor line
termination centers provided, as well as the number of sensor stations
feeding an individual termination center, depends upon the deployment of
sensor stations in any given system. It is conceivable, for example, that
all of the sensor line termination centers and the equipment located
therein may be merged into a signal unit located at a central control
installation which may or may not be at the same location as the
dispatcher. In the case of a police patrol car locator system, the police
call boxes are often connected by telephone lines to the various police
precinct houses located throughout the city. Accordingly, the present
system, when used to locate police cars, contemplates utilization of these
telephone lines in transmitting the coded frequencies from the call box
sensor stations to sensor line termination centers located within the
precinct houses. In some cases, it may be possible to use the telephone
lines in conjunction with carrier-derived circuits which can be
superimposed on existing physical circuits such as fire and police cables
without impairing the existing service. The carrier or radio frequency
technique is widely used in the field of telephony, radio and power line
telemetry and control. In the field of telephony, a significant portion of
all trunk and subscriber circuits are carrier derived without impairment
of the physical services and at a cost well below that which would result
from the utilization of additional physical circuitry.
At sensor line termination centers 27 and 29, the coding frequency signals
are sequentially scanned and decoded to provide information indicating
that a particular vehicle has passed the sensor station being scanned. The
decoded information is then fed to a computer 31. Each time a vehicle
passes a sensor the decoded signal updates a vehicle location memory file
in the computer. The computer in turn drives a display programmer 33 which
correlates computer information applied thereto so that appropriate lamps
on display map 35 are illuminated. Each of the lamps on map 35 corresponds
to a sensor station location and is illuminated in response to the sensing
of a vehicle at the respective sensor station. Other vehicle data, for
example passager loading on a bus, may also be displayed. By means of a
control panel 37 the dispatching officer can interrogate the computer to
determine the identity of a vehicle known to be at a given location, or
conversely to determine the last reported sensor station location of a
particular vehicle at any time.
Referring now to FIG. 2 of the accompanying drawings, there is illustrated
a schematic diagram of a typical vehicle emitter device which is carried
by all vehicles (11, 13) in a fleet of vehicles whose locations are being
monitored by the system of the present invention. The ultimate emitted
signal is transmitted by a low power radio frequency transmitting device
136 which, for example, may be essentially the same type of device as that
which is found in conventional radio-controlled garage door opening
systems. An example of a suitable transmitter is illustrated and described
in my aforementioned U.S. Pat. No. 3,568,161. An amplitude modulator 137
is utilized to amplitude modulate the RF signal generated within
transmitter 136. Each vehicle emitter circuit includes a plurality of
audio oscillators which serve as sources for coding tones. In the
particular embodiment illustrated in FIG. 2, four oscillators 128a, 128b,
128c and 128d are illustrated; however, it is to be understood that fewer
or more oscillators may be utilized, depending upon the coding
requirements in a particular system. The oscillators are preferably
plug-in type oscillators, each having a different nominal frequency. In
addition, each oscillator is rendered operative (i.e. oscillatory) only
when a return path is completed to ground through a variable resistor (R10
through R20) and transistor switch (120, 121, 124, 125). The oscillation
frequency is determined by the setting on the variable resistor through
which the oscillator is returned to ground. More specifically, oscillator
128a is returned to ground through either the series combination of
variable resistor R10 and transistor switch 120a or the series combination
of variable resistor R14 and transistor switch 121a. If R10 and R14 are
set to provide different series resistances, oscillator 128a oscillates at
a different frequency when switch 120a is closed than when switch 121a is
closed. Likewise audio oscillator 128b is returned to ground through
resistor R11 and switch 121b or through resistor R15 and transistor switch
121b. Audio oscillator 128c is returned to ground through resistor R12 and
switch 120c or resistor R16 and switch 121c. Oscillator 128d is returned
to ground through resistor R13 and switch 120d or through one or more of
resistors R17 through R21 and their appropriate status switches 124a,
124b, 124c, 124d, 125, the combination in series with switch 121d.
A primary clock source 123 provides a train of master timing pulses at a
repetition rate which is determined by the setting of a timing adjustment
potentiometer 119. The clock source pulses are applied to a bistable
multivibrator 122 which simply alternates between its Q and Q states upon
receiving successive pulses from the clock source. The Q output signal
from multivibrator 122 is applied to the base electrodes in each of
switches 120a through 120d. The Q signal from multivibrator 122 is applied
to the base electrodes of each of switches 121a through 121d. In this
manner, each of switches 120a through 120d are activated simultaneously
and in alternation with each of switches 121a through 121d which are also
actuated simultaneously. The Q output signal from multivibrator 122 is
also applied to a differentiating circuit 142 for purposes to be described
subsequently.
Whereas the coding arrangement described in my aforementioned U.S. Pat. No.
3,568,161 employs continuous modulation of the RF signal with
identification tones, the present invention utilizes a sequence of two or
more coding intervals wherein the modulating tones may differ in each
interval. More specifically, clock source 123 determines the rate at which
flip-flop 122 alternates between its Q and Q states. When the flip-flop is
in its Q state, transistors 120a through 120d are actuated and tone
control resistors R10 through R13 determine the frequencies of oscillators
128a through 128d. When flip-flop 122 is switched to its Q state,
transistors 121a through 121d are actuated and resistors R14 through R16
determine the operating frequencies of oscillators 128a through 128c. In
addition, resistors R17 through R21 determine the frequency of audio
oscillator 128d during this second coding interval. In the exemplary
embodiment illustrated in FIG. 2, switches 124a through 124d are
operator-actuated switched which represent a particular status condition
in the vehicle, such as passager loading in the case of a bus, on-call
condition in the case of a taxi cab, etc. and permit respective resistors
R17 through R20 to determine the frequency of audio oscillator 128d in
accordance with the vehicle status. An emergency switch 125 permits
resistor R21 to determine the frequency of oscillator 128d when actuated
during an emergency condition for the vehicle. Resistors R17 through R21
have a cumulative effect in determining the frequency of oscillator 128d.
More particularly, if more than one of switches 124a through 124d and 125
are actuated at one time when switch 121d is closed, it is the combined
parallel resistance of the corresponding resistors in series with the
active switches which determines the frequency of oscillator 128d. For
example, if switches 124a and 124b are closed when switch 121d is closed,
it is the combined parallel effect of resistors R17 and R18 which
determines the frequency of oscillator 128d. In this regard, it is
important that resistors R17 through R21 be selected so that all possible
combinations of these resistors produce a unique parallel resistance and
thereby a unique frequency for oscillator 128d.
Resistors R11 through R16, in the example of FIG. 2, determine the tones
which identify the vehicle. In other words, the tones produced by audio
oscillators 128b, 128c and 128d during the first coding interval and the
tones produced by oscillators 128a, 128b, 128c during the second coding
interval combine to uniquely identify the particular vehicle from which
these tones originate. On the other hand, the tone controlled by resistor
R10 is the same for all vehicles and is utilized to identify the first
coding interval in a coding sequence. In this manner the decoding
circuitry can properly synchronize its operation to the start of a coding
sequence.
In a preferred although not necessarily required feature of the present
invention, the oscillators in each vehicle are the same. This feature
permits a significant cost reduction in a multi-vehicle system by virtue
of the fact that advantage can be taken of the low cost characteristics of
large volume production and/or purchases.
The output signals from oscillators 128a through 128d are passed through
respective amplitude adjustment potentiometers 129a through 129d and are
summed at summing potentiometer 110 from which point the tones are applied
to amplitude modulator 137.
As mentioned above, one tone during the first interval of a coding sequence
is common to all vehicle emitters so that the first interval in the
sequence can be identified by the decoding circuitry. It is also possible
to utilize a second common tone to identify the second coding interval, or
a third common tone to identify a third coding interval, or two common
tones may be utilized to identify the first and last coding intervals
respectively. In general, the tones may be selected to suit the
requirements of the system. For example, the status coding tones may be
split so that one selected tone is actuated by a transistor and transistor
bank 120 so that it is transmitted with the first group of tones, and a
second selected status tone is actuated by a transistor in transistor bank
121 so that it would be transmitted with the second group of tones. If
three groups of tones are used, the first two groups might be utilized for
vehicle identification and the third for vehicle status, or if four groups
of tones are used the first two might be used for vehicle identification
and the last two for vehicle status, etc. In any case, an important
feature of the present invention, and one which is applicable no matter
how many coding intervals are employed, is the fact that the same
oscillators are capable of providing different frequencies during
different coding intervals depending upon the particular resistor
connected in its ground return path during that interval.
As noted from FIG. 2, the differentiating circuit 142 converts the leading
edge of the Q output signal from flip-flop 122 to a voltage spike which is
applied to AND gate 141. The other input signal to this AND gate is
derived from the vehicle odometer-controlled emitter pulser described
below in reference to FIG. 3. The output signal from the AND gate is
applied to a monostable multivibrator 143 which provides a pulse of fixed
duration each time AND gate 141 is actuated. This fixed duration pulse is
applied to the base of transmitter enabler transistor 44 to actuate the RF
transmitter 136 during the period of monostable multivibrator 143. This
portion of the circuit pulses the RF transmitter at a rate determined by
the distance travelled by the vehicle, which is equivalent to having the
pulsing rate of the transmitter controlled directly by the rate of speed
of the vehicle. This is important because the vehicle must send its signal
out frequently enough so that the vehicle cannot pass by a road side
sensor without having transmitted its signal within the receiving range of
the sensor. If the emitter and sensor antennas have an omni-directional
antenna pattern so that the maximum emitter-to-sensor range is a constant
of X feet in all directions, then the vehicle emitter must theoretically
pulse at least once every time the vehicle traverses 2X-ST feet, where S
is the speed of the vehicle in feet per second and T is the duration of
the emitter coding sequence. In the example described in reference to FIG.
2, T is equal to the time required to transmit the two coding intervals,
or, more precisely, twice the repetition frequency of clock 123. In
practice, however, since antenna patterns would most likely not be exactly
omni-directional, and in order to account for other system variables, the
vehicle emitter should be pulsed more frequently than once every 2X-ST
feet. If the "dual look" feature of my U.S. Pat. No. 3,568,161 is
employed, then the emitter would be pulsed twice as frequently.
As the speed of the vehicle increases, the time required for the vehicle to
traverse 2X-ST feet decreases so that the rate of the emitter pulsing must
increase accordingly. At very high speeds the pulsing rate may increase to
a point where the pulses run together, in which case the emitter
continuously pulses. The main object of having the emitter pulsing
controlled as a function of the distance traversed by the vehicle is to
limit the number of transmissions by the vehicle emitter to only the
number which is required for the system to operate reliably. In this way,
the probability of vehicle emitters interferring with each other is kept
to a minimum.
There are a variety of techniques which may be employed to control pulsing
of the emitter as a function of the distance traversed by the vehicle. By
way of example, one such technique is illustrated in FIG. 3 of the
accompanying drawings. Specifically, a small magnet 138 is attached to the
vehicle odometer cable 135. Each time the odometer cable completes one
rotation, magnet 138 rotates past a pick-up coil 139, thereby causing the
field of the magnet to be cut by the pick-up coil and causing a voltage
pulse to be | | |
