Vehicle detector with power failure information saving

A method and apparatus for storing reference information in a non-volatile memory unit when a power loss to a vehicle detector is imminent. A power monitor circuit senses an impending power loss. In response, the vehicle detector transfers reference information, including a reference count and the number of loop cycles over which the reference count was accumulated, as well as optional detector status information, from the vehicle detector volatile memory (typically RAM) to the non-volatile memory unit. When power is resumed, the information stored in the non-volatile memory unit is restored to the vehicle detector. The restored information prevents improper and potentially dangerous vehicle detector operation caused by the loss of reference information during unpredictable power losses. The transfer operation is not performed if the power loss is due to a mechanical disconnection of power. By limiting the information transfer operation to only an impending power loss condition, the use of non-volatile memory to store the information is made practically feasible.

BACKGROUND OF THE INVENTION
 This invention relates to vehicle detectors used to detect the presence or
 absence of a motor vehicle in an inductive loop embedded in a roadbed.
 More particularly, this invention relates to a vehicle detector with a
 reference information saving feature upon power failure.
 Vehicle detectors have been used for a substantial period of time to
 generate information specifying the presence or absence of a vehicle at a
 particular location. Such detectors have been used at vehicular traffic
 intersections, for example, to supply information used to control the
 operation of the traffic signal heads; have been used to supply control
 information used in conjunction with automatic entrance and exit gates in
 parking lots, garages and buildings; have been used in railway
 installations for railway car detection and control; and have been used in
 security barrier installations to prevent the sudden erection of a
 security barrier from underneath an overlying vehicle. A widely used type
 of vehicle detector employs the principle of period shift measurement in
 order to determine the presence or absence of a vehicle in or adjacent the
 inductive loop mounted on or in a roadbed. In such systems, a first
 oscillator, which typically operates in the range from about 10 to about
 120 kHz is used to produce a periodic signal in a vehicle detector loop. A
 second oscillator operating at a much higher frequency is commonly used to
 generate a sample count signal over a selectable number of loop cycles.
 The relatively high frequency count signal is typically used to increment
 a counter, which stores a number corresponding to the sample count at the
 end of the selected number of loop cycles. This sample count is compared
 with a reference count stored in another counter and representative of a
 previous count over the same number of loop cycles in order to determine
 whether a vehicle has entered or departed the region of the loop in the
 time period between the previous sample count and the present sample
 count. The number of loop cycles selected is related to the sensitivity of
 the vehicle detector, and this number is typically set manually by a field
 service technician when installing or re-initializing the detector. In
 some detectors, this selection process is aided by an automatic default
 setting built into the detector system.
 The initial reference value is obtained from one or more initial sample
 counts and stored in a reference counter. Thereafter, successive sample
 counts are obtained on a periodic basis, and compared with the reference
 count. If the two values are essentially equal, the condition of the loop
 remains unchanged, i.e., a vehicle has not entered or departed the loop.
 However, if the two numbers differ by at least a threshold amount in a
 first direction (termed the Call direction), the condition of the loop has
 changed and may signify that a vehicle has entered the loop. More
 specifically, in a system in which the sample count has decreased and the
 sample count has a numerical value less than the reference count by at
 least a threshold magnitude, this change signifies that the period of the
 loop signal has decreased (since fewer counts were accumulated during the
 fixed number of loop cycles), which in turn indicates that the frequency
 of the loop signal has increased, usually due to the presence of a vehicle
 in or near the loop. When these conditions exist, the vehicle detector
 generates a signal termed a Call Signal indicating the presence of a
 vehicle in the loop.
 Correspondingly, if the difference between a sample count and the reference
 count is greater than a second threshold amount, this condition indicates
 that a vehicle which was formerly located in or near the loop has left the
 vicinity. When this condition occurs, a previously generated Call Signal
 is dropped.
 In order to function properly, the initial reference value must be obtained
 while the loop is not under the influence of a vehicle. Past detectors
 obtain the initial reference count value by seeking the largest count
 obtained during the sample count process and using that number for the
 reference count value. Since the largest count value occurs when no
 vehicle is present over the loop, the detector cannot operate properly
 until the detector experiences the first vacant loop condition. The Call
 signals generated by a vehicle detector are used in a number of ways.
 Firstly, the Call signals are presented to an output terminal of the
 vehicle detector and forwarded to various types of traffic signal
 supervisory equipment for use in a variety of ways, depending on the
 system application. In addition, the Call signals are used locally to
 drive a visual indicator, typically a discrete light emitting diode (LED)
 or a multiple LED display or a liquid crystal display (LCD) to indicate
 the Call status of the vehicle detector, i.e. whether or not the vehicle
 detector is currently generating a Call signal.
 Vehicle detectors with the Call signal generating capability described
 above are used in a wide variety of applications, including vehicle
 counting along a roadway or through a parking entrance or exit, vehicle
 speed between preselected points along a roadway, vehicle presence at an
 intersection controlled by a traffic control light system, in a parking
 installation entrance gate, in a parking stall, in railroad yards and
 numerous other applications.
 Most present day vehicle detectors are designed and manufactured using a
 microprocessor-based architecture. This type of system architecture uses
 volatile random access memory (RAM) to store reference samples, status
 information and other information (such as system sensitivity) needed for
 the proper identification of vehicle arrival or departure from the
 location monitored by the vehicle detector. Such vehicle detectors are
 sensitive and vulnerable to power outages, particularly due to the severe
 environment in which they are typically installed. When power to a
 detector is interrupted, the reference information stored in the volatile
 system RAM is lost. When power is subsequently restored, the vehicle
 detector must resume operation without the lost reference information.
 This can lead to improper and dangerous operating conditions, as the
 following examples will demonstrate.
 In parking lot entrance installations, a vehicle detector is commonly used
 to provide advisory signals used in the operation of an automatic gate.
 When a vehicle enters a loop in front of the gate, the vehicle detector
 normally generates a Call signal, which is used to open the gate so that
 the vehicle can pass through the gated entrance and proceed to a parking
 stall. Once the vehicle has left the loop, the vehicle detector drops the
 Call signal and the gate is operated to the closed position. If a power
 outage occurs when the vehicle is over the loop and the reference
 information is lost from system RAM, the current reference value is lost.
 If the vehicle is still over the loop when power is resumed, a new
 reference value is obtained which prevents the detection of the continued
 presence of the vehicle over the loop. As a result, the automatic gate is
 closed while the vehicle is in the gate area, usually damaging the
 vehicle.
 In rail yard applications, vehicle detectors are commonly used to help
 control the operation of track switches. More particularly, in such
 installations the need frequently arises to move rail cars from one track
 to another for traffic management purposes. In order to move a rail car
 from one track to another, the car is propelled along the present track
 toward a track switch. Before the railcar reaches the track switch, the
 switch is operated to divert the approaching car from the present track to
 a desired different track. The vehicle detector is usually connected to a
 loop positioned to monitor the track switch region in order to detect the
 presence of a rail car over the switch region. If a rail car is present
 over the switch region, the Call signal generated by the vehicle detector
 is used to prevent operation of the track switch in order to preclude
 derailing of the rail car. If a power outage occurs while a rail car is
 present over the switch region and the reference information is lost from
 system RAM, the current reference value is lost. If the rail car is still
 over the loop when power is resumed, a new reference value is obtained
 which prevents the detection of the continued presence of the rail car
 over the loop in the track switch region. As a result, if the track switch
 is operated the rail car can be derailed.
 In a left turn lane at a controlled vehicle intersection, a vehicle
 detector is typically used to monitor the presence of a vehicle waiting
 for the control green signal (commonly a left-pointing arrow) so that the
 vehicle can proceed into the intersection and make a left turn. If no
 vehicle is detected at the time the left turn signal is normally turned
 green by the intersection controller, this phase of the traffic control
 cycle is typically skipped, so that the left turn signal remains red. If a
 vehicle is detected, the left turn phase is entered and the vehicle is
 given a green signal thereby permitting the vehicle to proceed into the
 intersection a make a left turn. If a power outage occurs while a vehicle
 is present over the left-turn loop and the reference information is lost
 from system RAM, the current reference value is lost. If the vehicle is
 still over the left-turn loop when power is resumed, a new reference value
 is obtained which prevents the detection of the continued presence of the
 vehicle waiting for the left turn signal to turn green. Since the vehicle
 waiting for the green left turn arrow is no longer detected following an
 interruption of power to the vehicle detector and traffic signal
 controller, the traffic signal controller provides a safe condition for
 the intersection upon the return of power. The traffic signal controller
 accomplishes a safe start up condition, following the return of power, by
 providing a minimum amount of green time for each traffic lane. This
 minimum amount of green time ensures that traffic in each lane is allowed
 to move so that each detector experiences, as a minimum, a momentary
 vacant condition; and therefore is able to obtain a valid reference count
 value. Without the traffic signal controller providing green time for each
 traffic lane following the application of power, the detectors would not
 be able to obtain a valid reference value; thus creating a very dangerous
 condition, which could result in an accident. In security barrier
 applications, vehicle detectors are used to condition the operation of
 retractable barrier posts or solid structures designed to prevent entry of
 vehicles into a secure area. Such barrier devices are typically designed
 to be in a normally erect position above the surface of the ground or
 pavement. Normally, an approaching vehicle encounters the erect barrier
 and cannot enter the area unless authorized to do so by a human operator
 (e.g. a security guard posted at the entrance to the area) or an automatic
 authorization system (such as a card-actuated barrier operating system). A
 vehicle detector system is typically installed in a position to generate a
 call signal whenever a vehicle is positioned over the barrier when in the
 retracted state in order to prevent the erection of the barrier from
 beneath the vehicle and consequent damage. If a power outage occurs when a
 vehicle is over a retracted barrier and the reference information is lost
 from system RAM, the current reference is lost. If the vehicle is still
 over the loop (and thus the barrier) when power is resumed, a new
 reference value is obtained which prevents the detection of the continued
 presence of the vehicle over the loop. As a result, the barrier is
 suddenly erected and the vehicle is usually severely damaged.
 As will now be apparent, the need exists for some mechanism to prevent the
 loss of reference information in a vehicle detector when a power outage
 occurs. In a microprocessor-based vehicle detector, a workable solution
 might appear to be to add non-volatile memory and store the reference
 information in this memory each time the values are updated. However, this
 approach is not practically feasible. Known non-volatile memory devices
 suffer from the limitation of possessing only a finite number of
 read/write cycles. After this limit has been reached, a typical
 non-volatile memory device cannot be relied upon to reliably store and
 retrieve information. The limit is typically about one million erase/write
 cycles, after which the device manufacturer will no longer guarantee
 reliability. In a typical microprocessor-based vehicle detector, the
 number of samples taken per second can be as high as one thousand,
 depending on the sensitivity setting of the detector (the sensitivity
 setting establishes the length of the sample period, usually defined by
 the number of loop cycles during which the high speed counter is permitted
 to accumulate counts). Consequently, the reliability limit, and thus the
 useful lifetime, of a non-volatile memory device in such a vehicle
 detector can be reached after only one thousand seconds of operation, or
 slightly less than seventeen minutes, if the detector is being operated at
 the lowest sensitivity. Even at higher sensitivities, the useful lifetime
 of a non-volatile memory device in a vehicle detector can be exceeded in
 less than forty hours of operation. Since vehicle detectors are expected
 to operate reliably in situ for years, the simple addition of non-volatile
 storage to permanently store reference information is not a practical
 solution to the problem.
 SUMMARY OF THE INVENTION
 The invention comprises a vehicle detector system which solves the problem
 of lost reference information upon power outage by providing non-volatile
 storage to save the reference information only when a power outage is
 imminent.
 From an apparatus standpoint the invention comprises an improvement in a
 vehicle detector having circuitry powered by a source of electrical power
 for sensing changes in an associated inductive loop related to the
 presence of a vehicle in the vicinity of the loop and for generating a
 Call signal in response to such changes; the improvement comprising a
 non-volatile memory device for receiving and storing reference information
 from the vehicle detector, a power monitor circuit for detecting an
 impending loss of power to the vehicle detector, and means responsive to
 the power monitor circuit for storing the reference information in the
 non-volatile memory device upon detection of an impending loss of power.
 The reference information includes a loop inductance reference count
 accumulated over a sample period and a sample period value, the sample
 period value preferably comprising the number of loop cycles over which
 the reference count was accumulated. The reference information optionally
 includes status information relating to the detector prior to the loss of
 power, such as the Call status (i.e. an indication whether the vehicle
 detector is generating a Call signal when the impending loss of power is
 detected by the power monitor circuit), any loop failures, and the status
 of any reference tracking routines. The reference information stored in
 the non-volatile memory device preferably includes status information
 generated by the vehicle detector during the impending power down routine
 specifying whether valid data is being stored in the non-volatile memory
 device.
 For multi-channel vehicle detectors designed for use with more than one
 loop, the reference information includes a plurality of channel
 identifiers each associated to a different one of the loop channels for
 correlating the reference information transferred to the non-volatile
 memory device with the channel associated thereto.
 The invention further includes means for enabling transfer of the reference
 information back to the vehicle detector when power is resumed after a
 loss, so that the vehicle detector can resume operation with reference to
 the operating history prior to power loss.
 Because the reference information is not operationally significant if the
 power loss resulted from the vehicle detector being unplugged from the
 power source, the invention preferably includes means for sensing an
 impending loss of power due to a mechanical interruption of power to the
 vehicle detector, and means for preventing operation of the means for
 enabling transfer in response to the operation of the sensing means. The
 sensing means preferably includes means for determining the absence of a
 connection between the loop and the vehicle detector.
 From a process standpoint, the invention comprises a method of saving
 reference information in a vehicle detector volatile memory prior to an
 impending power loss, the method comprising the steps of sensing an
 impending loss of power to the vehicle detector; and storing the reference
 information in a non-volatile memory device before the vehicle detector
 becomes inoperative due to a loss of power. The reference information
 preferably includes a loop inductance reference count accumulated over a
 sample period and a sample period value, while the sample period value
 preferably comprises the number of loop cycles over which the reference
 was accumulated. The reference information further preferably includes
 status information relating to the detector prior to loss of power, such
 as an indication whether the vehicle detector is generating a Call signal
 when the impending loss of power is detected by the power monitor circuit.
 The reference information further preferably includes status information
 specifying whether valid data is presently being stored in the
 non-volatile memory device when the power down routine is being performed.
 When the process is implemented with a multi-channel vehicle detector
 capable of operating more than one loop, the reference information
 includes a plurality of channel identifiers each associated to a different
 one of the loop channels for correlating the reference information stored
 in the non-volatile memory device with the channel associated thereto.
 The method further includes the step of enabling transfer of the reference
 information back to the vehicle detector when power is resumed after a
 loss.
 Because the reference information is not operationally significant if the
 power loss resulted from the vehicle detector being unplugged from the
 power source, the method further includes the steps of sensing an
 impending loss of power due to a mechanical interruption of power to the
 vehicle detector, and preventing performance of the step of enabling
 transfer when an impending loss of power due to a mechanical interruption
 is sensed. The step of sensing preferably includes the step of determining
 the absence of a connection between the loop and the vehicle detector.
 The invention enables the storage of valuable and necessary vehicle
 detector reference information in a non-volatile memory unit prior to the
 loss of that information due to a power outage, without requiring an
 excessive number of erase/write operations which have heretofore precluded
 the use of cost effective non-volatile storage of such information.
 For a fuller understanding of the nature and advantages of the invention,
 reference should be had to the ensuing detailed description taken in
 conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Turning now to the drawings, FIG. 1 is a block diagram of a vehicle
 detector incorporating the invention. As seen in this figure, an
 oscillator 12 operable over a frequency range of about 10 to about 120 kHz
 is coupled via a transformer 13 to a pair of output terminals 14. Output
 terminals 14 are adapted for connection to an inductive loop usually
 mounted within the roadbed or rail bed in a position such that vehicles to
 be sensed will pass over the loop. Such loops are well-known and are
 normally found installed at controlled locations in the highway system,
 such as at intersections having signal heads controlled by a local
 intersection unit, parking lots with controlled access, rail yard track
 switch locations, security barrier installations and the like.
 The oscillator circuit 12 is coupled via a squaring circuit 16 to a loop
 cycle counter 18. Loop cycle counter 18 typically comprises a multi-stage
 binary counter having a control input for receiving appropriate control
 signals from a control unit 20 and a status output terminal for providing
 appropriate status signals to the control unit 20, in the manner described
 below.
 A second oscillator circuit 22, which typically generates a precise,
 crystal controlled, relatively high frequency clock signal (e.g., a 6 mHz
 clock signal) is coupled via a second squaring circuit 23 to a second
 binary counter 25. Counter 25 is typically a multi-stage counter having a
 control input for receiving control signals from control unit 20 and a
 count state output for generating signals representative of the count
 state of counter 25 at any given time. The count state of counter 25 is
 coupled as one input to an arithmetic logic unit 26. The other input to
 arithmetic logic unit 26 is one or more reference values stored in a
 reference memory 28. Reference memory 28 is controlled by appropriate
 signals from control unit 20 in the manner described below.
 An input/output unit 30 is coupled between the control unit 20 and
 externally associated circuitry. I/O unit 30 accepts appropriate control
 signals via an upper input path 31 to specify the control parameters for
 the vehicle detector unit of FIG. 1, such as mode, sensitivity, and any
 special features desired. I/O unit 30 furnishes data output signals via
 lower path 32, the data output signals typically comprising Call signals
 indicating the arrival or departure of a vehicle from the vicinity of the
 associated loop. In the preferred embodiment, the implementation of the
 system of FIG. 1 has been done using a type 17C44 processor available from
 Microchip Corp. of Chandler, Ariz.
 Initially, control unit 20 supplies control signals to loop cycle counter
 18 which define the length of a sample period for the high frequency
 counting circuit comprising elements 22, 23 and 25. For example, if
 control unit 20 specifies a sample period of six loop cycles, loop cycle
 counter 18 is set to a value of six and, when the sample period is to
 commence, control unit 20 permits loop cycle counter 18 to begin counting
 down from the value of six in response to the leading edge of each loop
 cycle signal furnished via squaring circuit 16 from loop oscillator
 circuit 12. Contemporaneously with the beginning of the countdown of the
 loop cycle counter 18, control unit 20 enables high frequency counter 25
 to accumulate counts in response to the high frequency signals received
 from high frequency oscillator circuit 22 via second squaring circuit 23.
 At the end of the sample period (i.e., when the loop cycle counter has
 been counted down to zero), control unit 20 generates a disable signal for
 the high frequency counter 25 to freeze the value accumulated therein
 during the sample period. Thereafter, this sample count value is
 transferred to the ALU 26 and compared with the value stored in reference
 memory 28, all under control of control unit 20. After the comparison has
 been made, the sample process is repeated.
 The reference value in reference memory 28 is a value representative of the
 inductance of the loop oscillator circuit comprising elements 12-14 (and
 the associated loop) at some point in time. The reference is updated at
 the end of certain periods in response to certain comparisons involving
 the reference stored in memory 28 and successively obtained samples from
 counter 25. Whenever the difference between a given sample from counter 25
 and the reference from memory 28 exceeds a first threshold value in the
 Call direction, the control unit 20 senses this condition and causes the
 generation of an output signal--termed a Call signal--on conductor 32
 indicating the arrival of a vehicle within the loop vicinity. Similarly,
 when the difference between a given sample and the previous reference
 exceeds a second threshold in the No Call direction the control unit 20
 senses this condition and causes the Call output signal on conductor 32 to
 be dropped. In the preferred embodiment, the Call direction is negative
 and the Call direction threshold value is -8 counts; while the No Call
 threshold value is -5 counts.
 Power is supplied to the system elements depicted in FIG. 1 from a
 dedicated power supply 35 via appropriate power conductors suggested by
 arrows 36. Power supply 35 typically provides DC voltage to the electronic
 circuit components comprising the vehicle detector, and is usually powered
 by either AC or DC electrical power available at the installation site of
 the vehicle detector.
 Call signal conductor 32 is coupled to an indicator unit 40 having a
 visible indicator and, optionally, an audible indicator. Whenever the Call
 signal is asserted, both the visible indicator and the optional audible
 indicator of indicator unit 40 are activated. Whenever the Call signal is
 de-asserted, both the visible indicator and the optional audible indicator
 are de-activated.
 Reference memory 28 is coupled to a non-volatile memory device 50, such as
 a type 24C02C serial EEPROM device available from Microchip Corp. of
 Chandler, Ariz. Memory device 50 is used in the manner described below to
 store current reference information whenever a power outage is imminent,
 and to furnish such stored information to the vehicle detector system when
 power is resumed after an outage. Memory device 50 is coupled to and
 controlled by control unit 20.
 A power monitor circuit 60 monitors the value of the voltage supplied by
 power supply 35. Whenever the value of the supply voltage drops below a
 fixed threshold, power monitor circuit 60 generates a control signal,
 which is coupled to control unit 20, signifying an imminent power outage.
 When control unit 20 receives this control signal, a power down impending
 routine is initiated by control unit 20 as described more fully below,
 which results in the transfer of current reference information from
 reference memory 28 to non-volatile memory device 50. The value of the
 fixed threshold is chosen to be sufficiently high to afford sufficient
 time for control unit 20 to complete the power down impending routine
 prior to a drop in value of the supply voltage below the level required to
 sustain control unit 20, reference memory 28, and non-volatile memory
 device 50. When power is restored after a power outage, the reference
 information stored in non-volatile memory device 50 is initially
 transferred to reference memory 28 by control unit 20 prior to commencing
 normal operation.
 With reference to FIG. 2, power monitor circuit 60 comprises a comparator
 61 having two input voltage terminals 62, 63. Input voltage terminal 62 is
 coupled to one plate of a storage capacitor 64 and monitors the value of
 the supply voltage from power supply 35 by means of a first voltage
 divider circuit consisting of resistors 65, 66. Input voltage terminal 63
 is coupled to the output of a voltage regulator 68, from which the
 operating voltage for control unit 20 and the remaining vehicle detector
 system electronic elements is obtained. Input voltage terminal 63 monitors
 the value of the voltage output from regulator 68 by means of a second
 voltage divider circuit consisting of resistors 70, 71. Whenever the
 supply voltage from power supply 35 begins to drop, the voltage on input
 terminal 62 follows this trend immediately, while the voltage on input
 terminal 63 only follows this trend with a time delay. By comparing the
 two voltage values, an impending power outage is sensed by comparator 61.
 Whenever the difference between the two voltages indicates that the supply
 voltage on capacitor 64 has dropped below the fixed threshold, the level
 of the signal on control output terminal 67 of comparator 61 changes to
 signal control unit 20 to begin the power down impending routine. In a
 specific embodiment of the invention using the microprocessor noted above
 with a nominal operating voltage of 5.0 D.C. volts and a minimum operating
 voltage of 4.75 D.C. volts, a 12.0 D.C. volts power supply, and a
 regulated supply voltage of 5.0 D.C. volts, the fixed threshold is 7.1
 D.C. volts. Other threshold values may be selected, as appropriate.
 When control unit 20 receives the control signal from comparator 61
 signifying an impending power outage, control unit 20 transfers certain
 information stored in reference memory 28 to non-volatile memory device
 50. The reference information transferred to non-volatile memory device 50
 comprises two basic types: a reference count and the number of loop cycles
 over which the reference count was accumulated. The value of the reference
 count is the most recent value stored in reference memory 28. The value of
 the number of loop cycles over which the reference count was accumulated
 is a number calculated during the initialization of the vehicle detector
 and stored in reference memory 28 and non-volatile memory device 50.
 In addition to the two essential types of information noted above, other
 useful reference information may be transferred to non-volatile memory
 device 50 during the power down impending routine. For example, a status
 bit can be set to in a dedicated register location in non-volatile memory
 device 50 to signify that a power down failure occurred. Further, if the
 vehicle detector is in a Call state of operation--i.e., in the process of
 issuing a Call signal--this status information can be stored in
 non-volatile memory device 50. Additionally, if the vehicle detector had
 experienced a current or previous failure--such as a loop failure--this
 status information can be stored in non-volatile memory device 50. Also,
 the status of a given reference tracking routine being performed at the
 time can also be stored in non-volatile memory device 50, along with any
 reference value used in the performance of the tracking routine. In
 general, any useful status information can be saved for subsequent use
 when power is restored.
 After power is restored to the vehicle detector, control unit 20 begins
 operation by checking the value of the power down failure bit in the
 register of non-volatile memory device 50. If the value of this bit
 indicates that a power down failure occurred previously, any pertinent
 previously stored information is transferred to reference memory 28. In
 this way, the vehicle detector can restart operation with a valid
 reference count, the number of loop cycles over which the reference count
 was accumulated, and optionally, correct status information as of the time
 when power was previously lost. Thus, if a vehicle was over the loop when
 power was lost and is still present when power is restored, it will be
 detected. Similarly, if no vehicle was over the loop when power was lost
 and no vehicle is occupying the loop when power is restored, the vehicle
 detector will detect this condition. Further, if a vehicle was over the
 loop when power was lost and left during the power outage, the vehicle
 detector will detect this change. Also, if no vehicle was over the loop
 when power was lost and a vehicle is occupying the loop when power is
 restored, the vehicle detector will detect this condition. In addition,
 depending on the type of status information previously stored in
 non-volatile memory device 50, the vehicle detector will recapture the
 Call state, reference tracking state, loop failure history, and any other
 status information of interest.
 The vehicle detector illustrated in FIG. 1 is a single channel detector
 having one transformer 13 and associated loop. Vehicle detectors are known
 which incorporate two or more channels, i.e. two or more transformers 13
 and associated loops. In such multiple channel detectors, the status
 information stored in non-volatile memory device includes channel
 information which associates the reference count, sample period, and
 status information to the proper channel. FIG. 3 illustrates a preferred
 technique for implementing data-to-channel association in a two channel
 vehicle detector. As seen in this Fig., reference memory 28 and
 non-volatile memory device 50 each have registers allocated to the
 reference count, the sample period value (preferably comprising the number
 of loop cycles over which the reference count was accumulated), and status
 information for each channel. In addition, non-volatile memory device 50
 has a valid data register for storing status information regarding
 individual channel data and also a status register for the whole detector.
 In the two channel example illustrated, the valid data register has
 dedicated bit positions for channel one and channel two, as well as a
 power down bit position for indicating whether there is valid data
 regarding power down stored in non-volatile memory device 50. The purpose
 of the power down bit is to eliminate unnecessary reading, erasing and
 re-writing of the non-volatile memory device 50 when there is no data to
 be restored, thereby extending the useful life cycle of the non-volatile
 memory device. The purpose of the dedicated channel bit positions is
 similar: the value of channel bit indicates whether there is valid data
 stored in the registers associated to that bit channel. If not, then these
 associated registers are not examined. It is noted that this function of
 reading data only when valid data is present is also applicable to a
 single channel implementation of the invention to avoid unnecessary
 operation of the non-volatile memory device 50.
 In operation, when power is restored to the vehicle detector control unit
 20 accesses the valid data register of non-volatile memory device 50 and
 inspects the power down bit and the channel bits. If the value of the
 power down bit indicates that no valid data is stored in device 50, the
 data restore operation is by-passed. If the value of the power down bit
 indicates that valid data is stored in non-volatile memory device 50, then
 control unit 20 accesses and transfers to reference memory 28 the data
 from those registers with valid data as specified by the channel bits. If
 a channel bit indicates that no valid data is present for that channel,
 the associated registers are not accessed.
 It should be noted that transfer of the reference information from the
 non-volatile memory device 50 to the vehicle detector upon application of
 power to the vehicle detector is not always appropriate. For example, when
 a new vehicle detector is initially powered up, there is no historical
 reference information stored in non-volatile memory device 50 to be
 transferred. Either the power down status bit or the valid data bits (or
 both) ensure that no transfer will take place under this circumstance.
 Similarly, when an already installed vehicle detector is disconnected by
 physically removing it from the associated connector slot and then
 reconnecting it in the same or a different slot, the data restore
 operation would normally not be useful or appropriate. However, under this
 circumstance the value of the power down bit, and probably one or more of
 the valid data bits, would indicate the need to perform the transfer
 operation. To prevent this undesired operation, the power down impending
 routine includes an initial loop check to determine whether the impending
 power loss is due to a mechanical power disconnect between the vehicle
 detector and the loop. If not, the power down impending routine proceeds
 normally as described above. If so, the power down impending routine is
 aborted.
 The initial loop check may be performed in a variety of ways. In the
 preferred embodiment, the initial loop check is done by sensing whether
 the loop is present in the vehicle detector circuit. If not, the impending
 power down condition is assumed to be due to a physical disconnect between
 the vehicle detector and the loop, and the impending power down routine is
 aborted. Alternatively, the initial loop check may be performed by
 installing a continuity switch between the vehicle detector connectors and
 the detector connector socket, and sensing the state of this switch. The
 continuity switch may assume many different forms, such as a shortened
 connector pin for one of the two main power connectors, so that this pin
 will disconnect before the remaining conductor pins when the vehicle
 detector is physically removed from the detector socket.
 As will now be apparent to those skilled in the art, vehicle detectors
 provided with the invention reliably eliminate the uncertainties and
 potentially dangerous conditions encountered in the past when power has
 been lost and subsequently restored to a vehicle detector operating in the
 field. In particular, with the invention a vehicle cannot be "lost" in the
 interim between power loss and power resumption: if a vehicle was present
 when the power loss occurred and is still present in the loop when power
 resumes, it will be detected. If a vehicle was present and has departed
 from the loop during the power-down interim, this condition will also be
 detected by the normal Call routine. If a vehicle was not present in the
 loop, but one has entered the loop during the power-down interim, this
 condition will also be detected by the normal Call routine. If no vehicle
 was present in the loop, and none entered during the power-down interim,
 this condition will be detected by the normal Call routine. These
 advantages are accomplished at relatively low cost by using a non-volatile
 memory device to store the needed reference information only when there
 exists an impending power down condition--thereby substantially reducing
 the frequency of use of the erase-write cycle in the non-volatile memory
 device, which substantially prolongs the useful life time of such a
 device.
 Although the invention has been described with reference to the loss of
 power to the cabinet housing the vehicle detector and other traffic
 control equipment, the invention also provides protection in the event
 that only the vehicle detector loses power, while the reminder of the
 traffic control equipment remains powered up. Such a condition has
 occurred in the past in installations having one or more vehicle detectors
 provided with individual power supplies with only marginal capacity. In
 such installations, a drop in the externally-supplied line voltage can
 cause the vehicle detector power supply output voltage to drop below the
 threshold operating voltage required for the vehicle detector to function
 properly. When the line voltage recovers, the vehicle detector resumes
 operating with the preserved reference information as described above.
 Although 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.
 Therefore, the above should not be construed as limiting the invention,
 which is defined by the appended claims.