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FIELD OF THE INVENTION
This invention relates to a transponder for an automatic vehicle
identification system and more particularly to a receiver-transmitter
circuit arrangement including a receiving coil tuned to the frequency of
an interrogating signal, a rectifier for rectifying the interrogating
signal and providing d.c. operating potential, a multistage digital
divider for dividing the frequency of the interrogating signal and
providing a data rate signal to a shift register, a logic network for
receiving the divided frequency signal and supplying an amplifier, a
latching network for enabling the shift register so that the stored binary
encoded data phase modulates the divided frequency signal which is coupled
by the amplifier to a tuned transmitting coil.
BACKGROUND OF THE INVENTION
In order to maximize the usage of the vehicles or rolling stock of various
transportation systems, it has been found to be highly advantageous to
automatically derive the identity and other necessary information from a
moving train, mass transit car or bus as it passes a wayside point along
its route of travel. It will be appreciated that an automatic vehicle
identification system finds particular utility in railroad and mass and/or
rapid transit operations. For example such a system has been employed in
identifying the cars of a moving train entering a classification yard, a
station or a switching conjunction or any other location along its route
of travel. Similarly, such an automatic identification system may be
utilized to obtain certain information from mass transit vehicles to
establish and verify the times of arrival at stations or stopping points
as well as to monitor the positions of the vehicles along this route of
travel.
While the prior art discloses a number of arrangements for automatically
identifying moving vehicles, these previous systems have suffered from one
or more shortcomings, such as, being unreliable in operation due to
climatic condition being expensive to maintain due to environmental
adversities or being unacceptable due to inability to conform with certain
requirements. In one previous system, it is proposed that each moving
vehicle be equipped with one or more color coded label members which would
be scanned by a source of white light so that reflected radiant energy
would be directed back to a wayside scanning apparatus. The received
reflected radiant energy is divided into two separate color paths for
processing, decoding and providing signals which identify the particular
vehicle carrying the coded label. The vehicle-carried labels include a
plurality of reflective or non-reflective markings which are carried by a
suitable backing member. The backing member is suitably attached to the
side or sides of the given vehicle. In practice it has been found that the
spectral response characteristics of the coded markings are greatly
impaired by climatic and environmental conditions to the point where
little if any intelligible information is received by the wayside scanner.
That is, the build-up of dirt, grease, tar, oil, dust and other foreign
matter covers and obliterates the coded markings so that the coded
identity is unreadable unless the labels are frequently cleaned and
reconditioned. Similarly the emitted rays of white light cannot
effectively penetrate fog and mist and are blocked and dispersed by snow
flakes and rain drops so that unacceptable and unsatisfactory readings
occur during adverse climatic conditions. Another problem in reliable
reading of the coded markings arises when the car-carried labels are
skewed or tilted by uneven loading, swaying and vibrational movement which
occurs as the moving vehicle passes a wayside scanner. Thus, it will be
appreciated that the above noted vehicle identification system is
expensive to maintain as well as unreliable in operation due to the
outdoor milieu in which it is required to function:
In another prior art arrangement, it is suggested that a depending portion
of a railway vehicle, such as, the truck or the like, be magnetized with a
preselected polarity pattern to form the coded identity which is unique to
the particular vehicle. Such a magnetic identification system is
impractical for several reasons. First, it will be appreciated that
railway vehicles are exposed to severe shock and vibration and experience
continuous pounding which causes the alignment of dipoles in the cast iron
trucks thereby creating magnetic regions having a much stronger intensity
than that of the coded magnetic area. Hence, the significance of the
magnetic code was destroyed or obliterated which has little, if any,
relationship with the identity of the vehicle. Second, it is necessary to
mount a magnetic reading head extremely, if not, illegally close to the
track rail in order to detect the magnetic coded regions. Thus, the
wayside reading device would not have the required clearance with the
vehicle so that the system could not be approved and accepted by the
railroad industry.
Yet another prior art system employs an electromagnetic scheme having a
vehicle-carried transponder and a wayside stationed interrogator. The
transponder includes passive elements which are inductively activated as
they pass an interrogating station. Hence, the transponder provides a
uniquely coded response signal when interrogated by an interrogating
station which is thereafter decoded by the interrogation station to
establish the desired input data relating to the characteristics of the
particular moving vehicle upon which the transponder is mounted. It will
be appreciated that passive responsive elements require precise physical
alignment and are adversely influenced by a variety of environmental
factors which affect the reliability and accuracy of the system. In
addition, extraneous noise signals have an adverse effect on prior types
of transponder-interrogator systems and result in the development of
inaccuracies in the coded information transmitted by the transponder and
received at the interrogating station.
Accordingly, it is an object of this invention to provide a new and
improved transponder for an automatic vehicle identification system.
A further object of this invention is to provide an automatic vehicle
identification system having a unique batteryless carborne transponder
energized by an interrogating signal to produce coded signals peculiar to
the particular vehicle.
Another object of this invention is to provide an improved transponder for
generating a phase modulated coded signal for identifying a given object
as it passes an interrogating area.
Still a further object of this invention is to provide a novel inert
receiver-transmitter circuit arrangement responsive to an interrogating
signal for providing d.c. operating potential for energizing the
receiver-transmitter whereby coded information is transmitted so long as
the receiver-transmitter is under the influence of the interrogating
signal.
Still another object of this invention is to provide a unique inductively
powered type of transponder for transmitting a phase modulated message
upon the reception of an interrogating signal.
Yet a further object of this invention is to provide a novel transponder
having a pick-up coil supplying a rectifier network to produce d.c.
operating power, a frequency divider, a shift register, a latching
network, a logic network, and an amplifier fed transmitting coil for
propagating a coded message upon the reception of an interrogating signal.
Yet another object of this invention is to provide a new and improved phase
modulation transponder which is economical in cost, simple in design,
reliable in operation, durable in service and efficient in operation.
SUMMARY OF THE INVENTION
In accordance with the present invention, the inductive type of transponder
is employed in an automatic vehicle identification system for producing a
coded message as the vehicle passes an interrogation location along its
route of travel. The transponder includes a tuned receiver coil for
picking up interrogating signals having a given frequency. A full-wave
rectifier network is coupled to the receiver coil for rectifying the
interrogating signals into d.c. operating potential. The interrogating
signals are fed to a multistage frequency divider which is powered by the
d.c. operating potential from the full-wave rectifier network. A shift
register is coded with binary information and is powered by the rectified
d.c. operating potential. The frequency divider produces a clock pulse
which establishes the data rate for the shift register. The frequency
divider also produces a frequency divided signal which is coupled to a
"NOR" gate latching circuit and is fed to a NOR gate logic network. The
shifter register is also coupled to the logic network. The logic network
is connected to the input of a balanced two stage switching transistor
amplifier which has its output coupled to a transmitting coil which is
tuned to the frequency divided signal. When the moving vehicle enters the
interrogation location, the interrogating signals are picked up and
rectified to produce d.c. operating potential. The presence of d.c.
operating voltage causes the frequency divider to divide the incoming
interrogating signals into the frequency divided signal and also to
produce data rate clock pulses which are fed to the shift register. The
shift register is initially inhibited by the latching circuit so that the
clock pulses have no effect, and a logical zero appears on the output of
the shift register. The frequency divided signals are applied to the input
of the amplifier via the logical network. Thus, one stage of the balanced
switching amplifier is driven in phase with the frequency divided signals
while the other stage is 180.degree. out of phase due to the inverting
action of the logic network. Hence, an a.c. output signal which is in
phase with the frequency divided signal is developed in the transmitting
coil and is propagated into the interrogating area. At a present threshold
voltage the latching circuit enables the shift register so that the coded
bits of the message are shifted out in serial fashion. In practice, the
initial bits of the coded message are binary "0's" so that the a.c. output
signal remains in phase with the frequency divided signal. Upon the
appearance of a binary "1" in the serial output message, the logic network
reverses the conduction of the two stages of the amplifier so that a phase
shift occurs in the output and in the voltage developed in the
transmitting coil. Thus, the logic network inverts the frequency divided
signal when a logical 1 appears on the output of the shift register while
a logical 0 results in no change in the phase relationship. Hence, the
frequency divided signal is phase modulated in accordance with the binary
coded data that is stored in the shift register. The phase modulated
signals will continue to be transmitted so long as the vehicle is in the
interrogating area and the receiver coil picks up the interrogating
signals. In practice the total time that the transponder is in the
interrogating area is sufficient to receive the entire coded message which
may include synchronization data and error checking bits. When the vehicle
passes beyond the interrogating area, the interrogating signals will no
longer be picked up by the receiver coil. The lack of interrogating
signals results in the loss of the d.c. operating potential so that the
transponder becomes inactive and ceases to transmit the phase modulated
signals.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing objects and other attendant features and advantages of this
invention will become more fully evident from the detailed description
when analyzed and considered in conjunction with the accompanying drawings
wherein:
FIG. 1 is a schematic circuit diagram partially in block form of an
inductive type of transponder for an automatic vehicle identification
system in accordance with the present invention.
FIG. 2 is a timing diagram and graphical representation of the wave forms
obtained from the receiving coil, the rectifier, the frequency divider,
the shift register and the transmitting coil of the transponder
illustrated in FIG. 1.
DESCRIPTION OF THE INVENTION
Referring now to the drawings and in particular to FIG. 1, there is shown
an inductive type of transponder or receiver-transmitter circuit
arrangement for use in an appropriate automatic vehicle identification
system, such as, railroad, mass and/or rapid transit operations and the
like. The transponder includes a receiver coil L1, a full-wave rectifier
network RN, a frequency divider FD, a shift register SR, a latching
circuit LC, a logic network LN, a balanced amplifier BA and a transmitter
coil L2. The receiver and transmitter coil L1 and L2 of the transponder
are incapsulated and hermetically sealed in a weather-proof fiberglass
unit which is bolted to a rigid metal protective frame. The coil unit and
frame are mounted to the underside of the vehicle so that interrogating
signals are picked up by the receiver coil L1 when the vehicle passes over
an interrogator transmitter coil which is embedded in or located on the
roadway along the route of travel. The vehicle-carried coils are connected
to a programmable module or selector control box via pretuned coaxial
cables. The module or box houses the various components or elements of the
transponder and is mounted on the control panel or deck in the motorman's
cab or operator's location. The programmable module includes a plurality
of thumb wheels or selection dials to set the number or identity of the
particular vehicle carrying the transponder. For example, a units, tens,
hundreds, thousands etc. dial may be manually manipulated to provide data
entry, namely, the numerical designation of the vehicle which forms part
of the encoded message stored in the shift register as will be described
hereinafter.
In viewing FIG. 1, it will be noted that the receiving or pick up coil L1
is tuned to the frequency, for example 100KHZ of an interrogating signal
by a tuning capacitor C1. The inductor and capacitor form a parallel tuned
circuit for picking up interrogating signals from the roadway as the
vehicle passes over an interrogation loop suitably located at a station or
at some other check point along which the vehicle travels. The picked up
interrogating signals have the multiple role of providing d.c. operating
power for the component parts or circuits of the transponder as well as
establishing the carrier frequency signal and the clock pulses. As shown,
a portion of the interrupting signals is rectified by full-wave rectifier
network including diodes D1 and D2. The anode electrode of diode D1 is
connected to the upper junction point J1 of inductor-capacitor L1-C1 while
the anode electrode of diode D2 is connected to lower junction J2 of the
inductor-capacitor L1-C1. The cathode electrodes of diodes D1 and D2 are
connected in common to positive junction point J3. A common or ground lead
is connected from a center tap on inductive loop L1 to ground terminal G.
A smoothing capacitor C3 is connected from positive d.c. junction point J3
and ground point G to removed ripple voltage and spurrious noise. As shown
the positive supply terminal provides d.c. operating potential for the
various networks and circuits of the transponder. It will be seen that the
multi-stage frequency divider FD is supplied d.c. operating voltage via
conductor or leads W1 and W2 while the shift register SR is supplied
positive voltage via conductive wires W1, W3 and W4. Likewise, the center
tap of coil L2 of transistor balanced amplifier BA is connected to the
positive d.c. voltage junction point via leads W1, W3, W5 and current
limiting resistor R2. It will be appreciated that d.c. operating power is
also supplied to the latching circuit and the logic network. However, in
order to avoid confusion and for the purpose of convenience the supply
connections and the grounds have not been illustrated in FIG. 1 of the
drawings.
It will be noted that a current-liminting resistor R1 and a voltage
breakdown device, such as, a zener diode Z1 are serially connected across
d.c. terminals J3 and G. The junction point J4 between resistor R1 and
zener diode Z1 is connected by lead W6 to the latching circuit LC the
purpose of which will be described hereinafter.
In practice, the frequency divider is a conventional multi-stage unit
employing a plurality of cascaded bistable circuits, such as,
semiconductive flip-flop circuit transistor multivibrators. Each stage or
multivibrator of the frequency divider will halve the frequency of the
applied input signal. Thus, the initial multivibrator stage will produce
an f/Z carrier signal where f is the frequency of the receiver
interrogating signals, namely, the 100KHZ is divided by Z. It will be
observed that the interrogating signals appearing at junction J3 are
applied to the input of the frequency divider FR via lead W7. It will be
seen that a clock or timing signal is derived from the final flip-flop
stage of the frequency divider and has a frequency of f/Zn where n is an
interger. As shown, the clock frequency signal which establishes the data
rate is derived from the final stage of the divider FD and is coupled to
the shift register via conductor W7.
The shift register SR is a suitable parallel-input serial-output network
including a plurality of bistable devices, such as, transistorized
multivibrators. In practice, the clock pulses are applied over lead W7
jointly to the appropriate inputs of the respective solid-state flip-flop
circuits. The vehicle identification number is binarily encoded by hard
wiring at the factory or is established by manual manipulation of
thumbwheels by the operator which encodes the information into the shift
register SR via leads W8, W9 and W10. While a three digit identification
number is sufficient in a transit operation having less than a thousand
(1,000) vehicles, it is understood that the shift register may be expanded
to accommodate a transportation system having a much greater number of
vehicles. In addition, the encoded message includes other stored
characters, such as start of test, carriage return, line feed, end on
feed, parity check, stop or any other data for instructions or control
functions to a computer and/or teletype printer employed at the
interrogation location or at central control. In practice, the shift
register remains initially disenabled so that a logical zero (0) appears
at its output terminal when the transponder reaches and begins to pass the
forward end to the interrogation loop. In viewing FIG. 1, it will be noted
that the latching circuit LC is employed to enable the shift register SR
to allow the clock pulses to shift the binary data in serial form. As
shown, the latching network includes a pair of NOR logic gates G-7 and
G-8. As mentioned above, lead W6 provides a positive or high signal to one
of the inputs of NOR gate G-7 which has its output fed back to one of the
inputs of NOR gate G-8. The output of NOR gate G-8 is fed back to the
other input of the NOR gate G-7. The other input of the NOR gate G-7 is
connected to the output of the first stage of the frequency divider via
conductive lead W11.
It will be seen that the lead W11 also is connected to the input of the
logic network LN which includes the NOR gates G-1, G-2, G-3, G-4, G-5 and
G-6. Specifically, the conductor W11 is connected to one input of the NOR
logic gate G-4 as well as to both of the inputs of the NOR gate G-1. As
shown, the output from the shift register SR is connected to the other
input of NOR gate G-4 as well as to both of the inputs of the NOR gate G-2
via lead W12. The output of NOR logic gate G-2 is connected to one of the
inputs of NOR gate G-3 while the output of NOR gate G-1 is connected to
the other input of the NOR gate G-3. The output of NOR gate G-3 is
connected to one input of the NOR gate G-5 while the output of NOR gate
G-4 is connected to the other input of NOR gate G-5. The output of NOR
gate G-5 is connected to both of the inputs of the NOR gate G-6. The
output of the NOR gate G-5 is also connected to the input of balanced
switching amplifier while the output of NOR gate G-6 is also connected to
the input of the balance switching amplifier BA.
The two stage balance switching amplifier BA includes a first PNP
transistor Q1 having a base electrode b1, collector electrode c1 and an
emitter electrode e1 and a second PNP transistor Q2 having a base
electrode bZ, a collector electrode c2 and an emitter electrode eZ. As
shown, the output of NOR logic gate G-5 is connected to the base electrode
b1 of transistor Q1 via resistor R3 while the inverted output of NOR gate
G-6 is connected to the base electrode bZ of transistor Q2 via resistor
R4. The collector electrodes c1 and c2 of transistors Q1 and Q2,
respectively, are connected in common to ground. The emitter electrode e1
of transistor Q1 is connected to one end of the parallel tuned circuit
formed by transmitting coil L2 and capacitor C2 while the emitter
electrode eZ of transistor Q2 is connected to the other end of the
parallel tuned circuit. In practice, the parallel tuned circuit L2-C2 is
tuned to the transmission frequency which is the f/Z frequency derived
from the first stage of the frequency divider FD.
Turning now to the operation of the subject invention, it will be assumed
that a vehicle carrying the transponder of FIG. 1 is approaching an
interrogation location along the route of travel. It will be appreciated
that the vehicle-carried transponder remains inert so that no energy is
consumed until power is received from the wayside. When the transponder
passes over the leading edge of the interrogation loop signals, such as,
those illustrated by the waveforms (a) of FIG. 2 having a frequency of a
100KHZ are picked up by the receiving coil L1. The parallel resonant
circuit including inductor-capacitor L1-C1 maximizes the amplitude of the
received interrogating signals so that the full-wave rectifier including
diodes D1 and D2 rectifies the a.c. signals. The d.c. voltage at junction
joint J3 begins to rise in a positive direction as shown by curve (b) of
FIG. 2. As previously mentioned, the capacitor C3 removes a.c. ripple
voltage and noise, signals and the d.c. operating voltage developed at
point J3 is applied to the respective frequency divider FD, shifter
register SR and amplifier circuit via leads W1, W2, W3, W4 and W5 as well
as to the latch circuit and logic network via suitable conductors (not
illustrated). It will be appreciated that the a.c. interrogating signals
are also applied to the input of the frequency divider FD via lead W7, and
as the d.c. supply voltage reaches the operating level, the frequency
divider FD is activated thereby causing a clock frequency signal to be
applied to the shift register SR via lead W7. Likewise, the first stage of
the frequency divider FD supplies an f/Z frequency signal over lead W11 to
the logic network LN and in turn to the balanced switching amplifier BA.
Initially, the shift register is not enabled for serial operation so that
a logical zero appears on lead W12 at this time. This causes the NOR gate
G-2 to produce a logical 1 which inhibits the NOR gate G-3. The f/Z signal
applied to NOR gate G-4 is initially inverted and then is again inverted
by NOR gate G-5. Thus, the output for NOR gate G-5 which is in phase with
the f/Z signal is applied to the input of transistor Q1 via resistor R3.
Accordingly, the transistor Q1 is driven in phase with the output of the
frequency divider. It will be seen that the output of NOR gate G-5 is
inverted by the NOR gate G-6. Thus, the output from NOR G-6 which is
180.degree. out of phase with the f/Z signal is applied to the input of
transistor Q2 via resistor R4. Accordingly, the balanced switching
amplifier BA develops in phase amplified f/Z output signals in the
resonant circuit L2-C2 which are propagated into the interrogation area by
the transmitter coil L2.
As the vehicle continues to move into the interrogation area and as the
transponder picks up the interrogating signals, the level of the rectified
d.c. voltage reaches a threshold point at which the latching circuit LC
including NOR gates G-7 and G-8 is enabled. That is, when the positive
d.c. voltage at junction exceeds the breakdown voltage of the zener diode
Z1 a constant potential is applied to one input of NOR logic gate G-7 via
lead W6. Thus, the latching circuit enables the shift register SR so that
serial data of the storage message begins to be applied to the NOR logic
gates G-2 and G-4 via leads W12. As previously mentioned, the data rate is
dictated by the clock pulses which are applied to the shift register SR
over lead W7. As shown in the waveform (c) of FIG. 2 several of the
initial bits serially shifted out of the shift register SR are logical 0's
so that the voltage developed across inductor LZ remains in phase as is
shown by the waveform (d) of FIG. 2. When a logic 1 is shifted from the
shift register SR, as shown by the third bit of curve (c), the NOR gate
G-4 is inhibited while the NOR gate G-1 is enabled. The enabling of gate
G-1 results in an inversion of the signals at the outputs of NOR gates G-5
and G-6 so that the conduction of the transistors Q1 and Q2 are reversed
in phase. Thus, a logic 1 produces a 180.degree. phase shift in the
voltage developed in inductive coil L2 as shown by curve (d) of FIG. 2.
Accordingly, a logic 0 of the stored message produces an output voltage
which is in phase while a logical 1 of the stored message produces an
output voltage which is 180.degree. out of phase. Hence, the f/Z signal is
phase modulated in accordance with the logic significance of the data
message in shift register SR. The transmitted signals will continue to be
phase modulated in accordance with the encoded data in shift register SR
as shown by waveforms (c) and (d) of FIG. 2 so long as the vehicle is in
the interrogation area and the transponder is over the interrogating loop.
The period of interrogation is of a sufficient time to permit the entire
message including the synchronization, data and error checking bits to be
transmitted at least once and preferably several times for redundancy
purposes. It will be appreciated that the number of total message readouts
is a function of the maximum speed of the moving vehicle, the length of
the interrogating loop, the frequency of the interrogating signal and the
rate of the clock pulses which may obviously be varied as desired. When
the vehicle moves out of the interrogating area and the transponder passes
beyond the trailing edge of the interrogating loop, the f/Z signals
induced into pickup coil L1 are markedly reduced and eventually disappear
as shown in (a) of FIG. 2. This causes a rapid decrease in the level of
the d.c. operating potential so that the f/Z signal and the clock pulses
cease to be produced. Thus, the transponder reverts to an inert condition
since no d.c. power is available for operating the various circuits. The
transponder will remain inactive until the vehicle again enters an
interrogation area where interrogating signals are picked up to cause the
propagation of the encoded message.
It is obvious that it is possible to transmit the signal at other
frequencies, such as, f/n frequencies where n is an integer. Since f/n can
be derived from various stages of the frequency divider, it would be
possible to have four phase states, for example, (0.degree., 180.degree.,
.+-. 90.degree.) or (.+-. 45.degree., .+-. 90.degree.) where the frequency
of the signal is f/4.
It will be appreciated that while the present invention finds particular
utility in an identification system for railroad and mass and/or rapid
transit operations, it is understood that the invention may be employed in
various other environments and fields, such as, trucking, taxi and other
moving object facilities.
In addition, it will be understood that various changes, modifications and
alterations may be made without departing from the spirit and scope of the
subject invention. For example, the logic may take the form of OR, AND or
NAND gates, and the disclosed rectifier and amplifier may be replaced by
other configuration in practicing the invention. Other changes and
ramifications will undoubtedly occur to those skilled in the art that are
deemed to fall within the purview of the present invention which is
intended to be limited only as set forth in the appended claims. Thus, it
is understood that the showing and description of the present invention
should be taken in an illustrative or diagrammatic sense only.
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