Patent Grant
8374573
This invention relates to automatic vehicle identification (AVI) systems used to detect a specific vehicle over an inductive loop embedded in a roadbed. More particularly, this invention relates to a technique for improving the performance and reliability of such systems.
AVI systems have been used for a substantial period of time to generate information specifying the presence or absence of a specific vehicle at a particular location sometimes termed a detection zone and to control access to restricted areas, such as an area providing restricted access via an automatically operated gate. Such systems have been used, for example, to limit ingress and egress at police impound lots to only authorized vehicles, to enable emergency vehicles (such as fire trucks and ambulances) to gain access to a gated residential or industrial area, and to monitor the progress of omnibuses along a city route.
A typical AVI system has a transmitter mounted on a vehicle which broadcasts over a limited range an encoded signal, usually a modulated carrier signal at a predetermined specific frequency (e.g., 375 kHz.), serving to identify the vehicle. A receiver connected to a loop antenna detects signals sensed at the specific frequency when a vehicle having such a transmitter is within the detection range of the loop-receiver combination. The receiver processes the detected signal to recover the encoded information, determines whether the encoded information matches a permissible code sequence stored in the receiver (which specifies a vehicle authorized in the system), and generates appropriate supervisory and control signals depending on the result of this determination. For example, in a system application in which access to a gate-guarded area is controlled by a receiver, the receiver may generate a gate operating signal in response to a match between the encoded information detected by the receiver and the permissible code sequence stored in the receiver. In a vehicle progress monitoring application, the receiver may time stamp the passage of the specific vehicle through the loop and store this time information and the identity of the vehicle in a local memory or transmit this information to a central processing unit, either instantaneously or on a periodical batch processing basis.
In a typical U.S. AVI system, information is encoded on a single frequency carrier signal by carrier burst modulation using a serial bit trinary encoding technique. According to this trinary encoding technique, a bit clock having a period of 0.1706 msec. Is used to define a bit period of 0.68250 msec. (four bit clock cycles); a trinary bit period of 1.3650 msec consisting of two consecutive bit periods. (eight bit clock cycles), and a nine bit trinary sequence of 12.285 msec (seventy-two bit clock cycles). The bit clock is also used to define two different types of bits—a short bit and a long bit. A short bit is defined as a binary signal asserted for the duration of one-half of a bit clock cycle (0.08530 msec.). A long bit is defined as a binary signal asserted for the duration of three and one-half cycles of the bit clock (0.59720 msec.). A ZERO value is defined as a two short bits during a trinary bit period consisting of a short bit at the beginning of a bit period followed by another short bit at the beginning of the next consecutive bit period. A ONE value is defined as two long bits during a trinary bit period consisting of a long bit at the beginning of a bit period followed by another long bit at the beginning of the next consecutive bit period. A TWO value is defined as one long bit followed by one short bit during a trinary bit period consisting of a long bit at the beginning of a bit period followed by a short bit at the beginning of the next consecutive bit period. In a nine bit trinary sequence, the least significant bit is transmitted first, followed by the next most significant bit, etc., until the most significant bit has been transmitted. The order of the bits is weighted according to the trinary encoding system, so that a transmitted value of ONE for the first trinary bit in the trinary sequence (bit 0) is interpreted as ONE, and a transmitted value of TWO for trinary bit 0 is interpreted as TWO; a transmitted value of ONE for the second trinary bit in the trinary sequence (bit 1) is interpreted as THREE, a transmitted value of TWO for trinary bit 1 is interpreted as SIX; etc. (For the last trinary bit in the trinary sequence [bit 8], a transmitted value of ONE is interpreted as 6561, and a transmitted value of TWO is interpreted as 13,122). As an example, to transmit a permissible code sequence of 13, 762, the sequence of transmitted values, beginning with the least significant bit, is ONE, ZERO, TWO, TWO, ONE, TWO, ZERO, ZERO, TWO.
The binary code values are encoded on a single frequency carrier in the following manner. A zero is signified by a short carrier burst followed by a short carrier burst; a one is signified by a long carrier burst followed by a long carrier burst; and a two is signified by a long carrier burst followed by a short carrier burst. The timing and positioning of the carrier bursts follow precisely the timing and sequencing constraints noted above. Each trinary sequence is separated from the next by a guard band consisting of a time period during which the carrier is inactive.
An AVI receiver determines the numerical value of a valid received code by adding the values for the trinary bit sent at each bit position in the trinary sequence using the weighting factors noted above. In order to validate the reception of a permissible code sequence, known AVI receivers are designed to require that the identical code sequence be decoded from two or more successive received trinary sequences.
In many loop locations, ambient electro-magnetic radiation can be present, either continuously or sporadically. This radiation is usually referred to as electrical noise signals, or simply noise signals. Some of these noise signals can contain a frequency component having the same frequency as the frequency of the AVI carrier signal generated by AVI transmitters. Given the precise timing constraints resident in the standard AVI trinary encoding process, the presence of such ambient noise signals at the AVI carrier frequency can adversely affect the operation of the AVI detection system, since the AVI receiver must be configured to detect all signals at the predetermined specific frequency. If present at a given loop, these carrier frequency noise signals will pass through the AVI receiver processing circuitry (since it must accept signals at the carrier frequency). The AVI receiver will attempt to process these noise signals, usually with a negative result—e.g., no comparison match with a permissible code sequence. These carrier frequency noise signals can possess sufficient amplitude to mask a permissible code sequence encoded in the carrier frequency. When the carrier frequency noise signals appear at the receiver during the same time as the carrier frequency signals, the AVI system cannot detect and take appropriate action in response to the arrival of an authorized vehicle at the loop. In the case of a fire truck responding to an emergency call in a gate-guarded community, for example, the AVI receiver can fail to generate the necessary gate operating control signal, thus denying the fire truck immediate access to the secured area. In the case of a bus route monitoring application, the AVI receiver can fail to detect the passage of a particular bus, resulting in the loss of important bus location information.
Efforts to devise an AVI system devoid of the above noted disadvantages have not met with success to date.
The invention comprises an AVI system signal processing technique providing improved performance and reliability in the presence of ambient noise signals. This improved performance and operation affords reliable receiver operation in the presence of electrical noise.
In a broadest apparatus aspect, the invention comprises an automatic vehicle identification (AVI) receiver for processing signals received thereby to recover information encoded in carrier frequency signals generated by a transmitter at a specific frequency for the purpose of identifying an authorized vehicle. The receiver comprises an input terminal adapted to be coupled to an inductive loop, which defines a detection zone, for receiving signals from the loop a filter unit coupled to the input terminal for permitting signals at the specific frequency present on the input terminal to pass therethrough and for substantially attenuating all other frequency components of signals present on the input terminal, the filter unit having an output; a variable gain amplifier having an input coupled to the output of the filter unit for amplifying signals input thereto and for limiting the amplitude of signals amplified thereby to a maximum value, the variable gain amplifier having a gain control signal input and an output, the variable gain amplifier having an operating range with a substantially linear portion; an amplitude detection circuit having an input coupled to the output of the variable gain amplifier and a gain control signal output coupled to the gain control input of the variable gain amplifier for sensing the amplitude of signals received from the variable gain amplifier and for generating a gain control signal for controlling the gain of the variable gain amplifier so that so that the signals input to the variable gain amplifier are operated on within the substantially linear portion and the amplitude of signals amplified by the variable gain amplifier are limited to the maximum value; a carrier-to-pulse conditioning circuit having an input coupled to the output of the variable gain amplifier for converting carrier frequency signals present at the output of the variable gain amplifier to a binary pulse train signifying the temporal length of each active carrier period and the temporal length of each quiescent carrier period, the carrier-to-pulse conditioning circuit having an output; and a decoder unit having an input coupled to the output of the carrier-to-pulse conditioning circuit for generating an authorized vehicle signal when the binary signal train matches a permissible code sequence contained in the decoder unit, the decoder unit having an output for manifesting the authorized vehicle signal.
The filter unit preferably comprises a multi-stage tuned filter circuit having a narrow pass band centered on said specific carrier frequency.
The maximum value to which the amplified signals are limited is selected to be less than the supply voltage for the receiver, and is preferably selected to be a value which lies within the linear operating range of the variable gain amplifier.
The gain control signal generated by the gain control circuit preferably enables the variable gain amplifier to operate at maximum gain in the absence of any carrier frequency signals input thereto.
The carrier-to-pulse conditioning circuit preferably includes biasing means for establishing a trigger threshold for input carrier frequency signals, and binary level circuitry for establishing the signal on the output of the carrier-to-pulse conditioning circuit at a first binary level when the carrier frequency input signal rises above the trigger threshold at the beginning of an active carrier period and for establishing the signal on the output of the carrier-to-pulse conditioning circuit at a second binary level when the carrier frequency input signal falls below the trigger threshold at the end of an active carrier period.
The binary level circuitry preferably includes a first comparator having a first input coupled to the biasing means, a second input for receiving the input carrier frequency signals, and an output; a second comparator having a first input coupled to the output of the first comparator, a second input coupled to the biasing means, and an output; a switching transistor having a control input coupled to the output of the second comparator and an output element serving as the output of the carrier-to-pulse conditioning circuit; and an R-C circuit having a capacitor coupled between ground and the first input of the second comparator and a resistor coupled between the first input of the second comparator and supply voltage.
The binary level circuitry further preferably includes a second R-C circuit coupled between the output of the second comparator and the first input of the first comparator for preventing small carrier frequency noise signals from affecting the operation of the first comparator.
The receiver also may further include an amplifier coupled to the output of the variable gain amplifier for establishing a quiescent value for signals output from the variable gain amplifier.
In a broadest process aspect, the invention comprises a method of processing signals received by an automatic vehicle indicator (AVI) receiver to recover information encoded in carrier frequency signals generated at a specific frequency by a transmitter and identifying an authorized vehicle, the method comprising the steps of:
The method further preferably includes the steps of:
The step (a) of filtering preferably includes the step of passing the signals received by the receiver through a multi-stage tuned filter circuit having a narrow pass band centered on said specific carrier frequency.
The step (c) of processing preferably includes the step of enabling maximum gain amplification in the absence of any carrier frequency signals
The step (d) of converting preferably includes the steps of:
The step (ii) of generating preferably includes the steps of initially charging a capacitor through a resistor coupled to the supply reference voltage, discharging the capacitor when the amplified signal rises above the first trigger threshold at the beginning of an active carrier period, permitting the capacitor to charge at a rate determined by the time constant of the resistor and capacitor, establishing a second trigger threshold, discharging the capacitor if the amplified signal again rises above the first trigger threshold before the capacitor is charged to the second trigger threshold, and generating the binary signal at the second level when the capacitor is charged to the second threshold level before the amplified signal rises above the first trigger threshold. The time constant of the resistor and capacitor is at least greater than the time length of one cycle of the amplified signal.
The step (ii) of generating preferably includes the step of preventing any small noise components present in the amplified signal from influencing the generation of the binary signal.
The AVI receiver incorporating the invention processes the carrier frequency signals input thereto in such a manner that the binary pulse train generated by the carrier-to-pulse conditioning circuit faithfully replicates any information encoded on a carrier signal by the associated AVI transmitter, even in the presence of noise signals at the carrier frequency. The combined operation of the variable gain amplifier and the amplitude detection circuit ensures that the maximum amplitude of the carrier frequency signals processed by the variable gain amplifier will be maintained at a constant value less than supply voltage and that the variable gain amplifier will operate on the incoming carrier signals essentially over the linear range of the variable gain amplifier. This in turn assures that the duration of the active carrier intervals and the passive carrier intervals (i.e., the ON time and the OFF time of the incoming carrier signals) will be faithfully replicated in the binary signal generated by the carrier-to-pulse conditioning circuit.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.
Turning now to the drawings,
The circuit parameters of amplitude detection circuit 20 are selected to ensure that the gain feedback signal generated thereby will force the variable gain amplifier 16 to maintain the maximum amplitude of the carrier frequency signals at a predetermined value less than the supply voltage and lying within the linear range of the variable gain amplifier 16. For the specific embodiment illustrated in
The carrier frequency signals present at the output of single stage amplifier 18 are coupled along a second signal path to the input of a carrier-to-pulse conditioning circuit 22 comprising the circuit components located within the region enclosed by broken lines designated by reference numeral 22 in
In the quiescent state, when no carrier frequency signals are presented to the inverting input of first comparator 31 the output of first comparator 31 is open (HIGH). The non-inverting input of second comparator 33 is held HIGH by the voltage on capacitor 36 and the output of second comparator 33 is also open (HIGH). Transistor 38 is switched ON and the signal on the collector output is LOW. When a carrier frequency signal above the trigger threshold is first presented to the inverting input of first comparator 31, the output of comparator 31 transitions LOW which causes the output of second comparator 33 to transition LOW. Transistor 38 is switched off and the signal on the collector output transitions HIGH. When the output of first comparator transitions LOW, capacitor 36 is discharged and begins to charge through charging resistor 35. When the signal on the collector output of transistor 38 transitions HIGH, R-C feedback network 39 forces the level of the bias voltage applied to the non-inverting input to first comparator 31 to a lower value (4.0 volts in the specific embodiment of
The binary signal train present at the output of carrier-to-pulse conditioning circuit 22 is coupled to the input of a conventional pulse train decoder 24 comprising the circuit components located within the region enclosed by broken lines designated by reference numeral 24 in
The AVI receiver 10 incorporating the invention processes the carrier frequency signals input thereto in such a manner that the binary pulse train generated by the carrier-to-pulse conditioning circuit 22 faithfully replicates any information encoded on a carrier signal by the associated AVI transmitter, even in the presence of noise signals at the carrier frequency. The combined operation of the variable gain amplifier 16 and the amplitude detection circuit 20 ensures that the maximum amplitude of the carrier frequency signals processed by the variable gain amplifier 16 will be maintained at a constant value less than supply voltage and that the variable gain amplifier will operate on the incoming carrier signals essentially over the linear range of the variable gain amplifier 16. This in turn assures that the duration of the active carrier intervals and the passive carrier intervals (i.e., the ON time and the OFF time of the incoming carrier signals) will be faithfully replicated in the binary signal generated by the carrier-to-pulse conditioning circuit 22.
In operation, as the associated AVI transmitter approaches the loop, the amplitude of the valid encoded carrier frequency signals increases. When the amplitude of the encoded carrier frequency signals reaches the maximum permitted threshold value, amplitude detection circuit 20 generates a gain feedback signal which results in a reduction of the gain of variable gain amplifier 16 so as to maintain the maximum amplitude of the signals output from variable gain amplifier 16 to the maximum permitted value. This reduces the amplitude of both the valid encoded carrier frequency signals and any concurrently present carrier frequency noise signals. As the amplitude of the valid encoded carrier frequency signals continues to rise (as the vehicle-mounted transmitter approaches closer to the loop), the gain reduction signal generated by amplitude detection circuit 20 causes further reduction of the gain of variable gain amplifier 16, which further reduces the amplitude of any carrier frequency noise signals. Eventually, the magnitude of the valid encoded carrier frequency signals will be so much greater than that of the carrier frequency noise signals that the gain reduction applied to variable gain amplifier 16 will result in the reduction of the magnitude of any carrier frequency noise signals below the trigger threshold of carrier-to-pulse conditioning circuit 22. Further, this condition will always persist for a sufficiently long period of time that pulse train decoder 24 has sufficient time to recognize a valid permissible code sequence from two or more successively received sequences. In addition, the carrier-to-pulse conditioning circuit 22 ensures that each cycle of a burst of valid carrier frequency signals is reliably detected, and that the termination of a burst of valid carrier frequency signals is faithfully reflected in the binary pulse train generated from the sequence of valid encoded carrier frequency bursts.
While the above provides a full and complete disclosure of the preferred embodiments of the invention, various modifications, alternate constructions and equivalents will occur to those skilled in the art. For example, while the invention has been described with reference to a specific carrier frequency, other carrier frequencies can be employed depending on the requirements of a particular application. In such cases, the time constants of the charging resistor 35-capacitor 36 combination, as well as the R-C network 39, may be changed to match the timing parameters of the different frequencies. In addition, different maximum permitted amplitude values for variable gain amplifier 16 and different trigger threshold values for carrier-to-pulse conditioning circuit 22 may be selected, depending on the requirements of particular applications, especially when other supply voltage values are required. Further, different circuit components may be employed, such as those specifically described with reference to variable gain amplifier 16 and pulse train decoder 24. Therefore, the above should not be construed as limiting the invention, which is defined by the appended claims.
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