WIRELESS SENSOR SYSTEM FOR TRAFFIC SIGNAL CONTROL

Technical Field

[0001] This disclosure relates to monitoring systems. In particular, this disclosure relates to a wireless system for monitoring vehicle traffic.

Background Information

[0002] There exists a need to monitor vehicular traffic so as to operate traffic signals efficiently, thereby reducing traffic congestion. Typical traffic monitoring systems include, for example, inductive loop technology, video detection technology, or magnetometer technology. Inductive loop technology has been the most widely used method of monitoring traffic. Inductive loops are simply a multi-turned loop of wire installed in a saw-cut in the roadway. An inductive loop creates its own inductive field when power is applied to the wire. A mass of metal that enters the field changes the inductance, which is read by a loop amplifier in a signal cabinet. Inductive loops have been found to be a reliable form of detection that can be configured to meet many detection situations.

[0003] However, inductive loop technology has inherent drawbacks. For example, cutting a road surface to install an inductive loop degrades the road surface, which causes failure to the pavement and results in expensive repaving. Further, inductive loops require hardwire connections to the signal cabinet, which results in the need to install conduit in lengths of approximately 200 feet to approximately 300 feet down each leg of an intersection. Thus, it is very difficult in many instances to retrofit an intersection with inductive loops due to the expense of installing conduit at an intersection that has already been constructed. [0004] Inductive loops often fail as a result of poor road surfaces and the dynamic nature of asphalt. When an inductive loop fails, a new loop must be cut in the asphalt, which further degrades the roadway. Inductive loops have an unreliable life expectancy and generally cannot be installed during poor weather conditions. Thus, when an inductive loop fails during the winter months, traffic monitoring at the intersection with the failed inductive loop may be ineffective for days, weeks or months.

[0005] Video detection is a relatively new technology for monitoring traffic. Video detection systems generally include a camera mounted on a mast arm over the roadway. The camera detects changes in its field of vision. Such changes in the

field of vision are interpreted as vehicles in particular lanes of traffic. However, video detection has not been widely accepted in the traffic industry. [0006] Video detectors have proven to be unreliable due to environmental situations that cannot be controlled. For example, cameras may be blinded by sunrise or sunset conditions. As another example, cameras may lose visibility at night, in rain, or in dense fog. Additionally, video systems do not have the ability for fail-safes, meaning that when a camera does not see a vehicle there is no alternate method of cycling the traffic signal. Thus, vehicles may potentially sit at an intersection without being detected and, therefore, without being given a green light. [0007] Further, cameras that are generally used in video detection systems do not have enough range for advance detection of vehicles approaching an intersection. Advance detection allows traffic signals to operate more efficiently. To provide advance detection in a video detection system, additional cameras are installed at locations in advance of the intersection. However, because the cameras are expensive, most traffic control agencies use inductive loops for advance detection, or forgo advance detection altogether.

[0008] Magnetometers are a form of detection that has been around for many years. Magnetometers measure the earth's magnetic field. A magnetometer detects the presence of a vehicle by detecting a change in the magnetic field caused by the magnetic pull of metal in the vehicle when it is near the magnetometer. In recent years, magnetometers have been configured using radio frequency (RF) interfacing to eliminate the need for saw-cuts and hardwiring to a signal cabinet. [0009] However, magnetometer detectors have generally proven to be unreliable and have not been widely accepted. Magnetometers are generally intolerant to environmental changes and nearby metal such as water mains, rebar and manhole covers. Newer technology using magnetometer detection and RF interfacing has not sufficiently addressed this problem. Additionally, the RF interfacing generally used in magnetometer systems does not have the range of transmission necessary to operate advance detection at most intersections. Further, magnetometer systems typically require manual setup of individual magnetometer detectors. Thus, changing settings on the individual magnetometer detectors requires extracting them from the road. The general design of magnetometer systems also requires one antenna for every four magnetometer detectors in the road. Because typical intersections need approximately thirty or more individual magnetometer detectors, many antennae are

required to operate such a system. Thus, magnetometer systems may be quite expensive to purchase.

Summary of the Disclosure

[0010] The embodiments disclosed herein provide a wireless vehicle traffic detection system. In one embodiment, a system is implemented with wireless signal communication among remotely located system components to monitor ground vehicle traffic at a vehicle traffic flow-controlled intersection. The system includes a road sensor subsystem operable to sense an ambient magnetic field level in a vicinity of the road sensor subsystem. The road sensor subsystem samples the ambient magnetic field to sense a change from the ambient magnetic field level to a magnetic field level corresponding to the presence of a vehicle in the vicinity of the road sensor subsystem. The road sensor subsystem also provides a vehicle traffic state signal representing a vehicle traffic state of the vicinity of the road sensor subsystem.

[0011] The system also includes a master transceiver operable to send a poll signal to, and receive the vehicle traffic state signal from, the road sensor subsystem. The master transceiver and the road sensor subsystem communicate by way of a wireless signal communication link to continually update the master transceiver as to whether the vehicle traffic state signal indicates the magnetic field level corresponding to the presence of a vehicle. The system also includes a spread spectrum communication subsystem providing the wireless signal communication link and operating on a changing frequency to which the road sensor subsystem is synchronized during its communication with the master transceiver. [0012] In some embodiments, the system also includes a spread spectrum compatible repeater transceiver operating in the wireless signal communication link to receive the poll signal and the vehicle traffic signal and in response transmit signals corresponding to the poll signal and the vehicle traffic signal for receipt by the road sensor subsystem and master transceiver, respectively. Further, in some embodiments, the road sensor subsystem includes a passive magneto-resistive sensor.

[0013] In another embodiment, a method for monitoring ground vehicle traffic includes sensing ambient magnetic field level components in three dimensions, and sensing a change in the ambient magnetic field level components. The change corresponds to the presence of a vehicle. The method also includes generating a

vehicle traffic state signal representing a vehicle traffic state based on the change in the ambient magnetic field level components, receiving a poll signal and in response to the poll signal transmitting the vehicle traffic state signal using a spread spectrum wireless signal communication link operating on a changing frequency. [0014] In another embodiment, a system for monitoring ground vehicle traffic includes sensing means for sensing a change from an ambient magnetic field level to a magnetic field level corresponding to the presence of a vehicle. The sensing means provides a vehicle traffic state signal. The system also includes wireless communication means for sending a poll signal to, and receiving the vehicle traffic state signal from, the sensing means. The wireless communication means provides spread spectrum communication operating on a changing frequency to which the sensing means is synchronized during its communication with the wireless communication means.

[0015] Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.

Brief Description of the Drawings

[0016] FIG. 1 is a block diagram of a wireless traffic detection system according to one embodiment.

[0017] FIG. 2 is a schematic diagram illustrating the wireless traffic detection system of FIG. 1 configured to detect traffic approaching, entering, and/or stopped at an intersection according to one embodiment.

[0018] FIG. 3 is a flowchart illustrating an example RF communication protocol used for an RF link according to one embodiment.

[0019] FIGS. 4A and 4B are tables illustrating example RF communication packet structures for various communication packets shown in FIG. 3 according to one embodiment.

[0020] FIG. 5 is a block diagram of an example road sensor unit according to one embodiment.

[0021] FIG. 6 is a block diagram of an example master transceiver unit or repeater transceiver unit according to one embodiment.

[0022] FIG. 7 is a block diagram of an example master interface control card or slave interface control card according to one embodiment.

[0023] FIG. 8 is a schematic diagram illustrating user controls and indicators of a master interface control card according to one embodiment.

Detailed Description of Preferred Embodiments

[0024] Technological advances in RF technology, magnetic detection, battery technology, and low power microprocessors have created an environment that allows for the production of a small detection device that is not hardwired to a traffic signal controller, is easy to install, and does not negatively impact the roadway. In one embodiment, a vehicle detection system includes one or more detectors that are installed using a small core drill or road surface installation with no additional saw- cuts or conduit installation.

[0025] Thus, the detectors are easy to install at a substantial cost savings as compared, for example, to inductive loops. Further, the vehicle detection system is easily integrated into existing intersections by not needing infrastructure construction. Individual detectors and/or batteries can also be easily replaced without further cutting of the road. Temporary detectors may be installed on a road surface, for example, in poor weather to replace failed inductive loops or for use during reconstruction of traffic signals to allow for detection throughout reconstruction. Thus, substantially uninterrupted vehicle detection may be achieved to allow for more efficient traffic flow.

[0026] In one embodiment, long range transmission coupled with repeater technology allows for a flexible configuration of individual detectors and advance detection used for efficient traffic flow as vehicles approach an intersection. The individual detectors are self-monitoring and provide feedback to one or more receiver units in a cabinet. Thus, the vehicle detection system can monitor battery levels and send a constant call in the case of a failed detector. In one embodiment, a constant call is a preferred fault environment over no call if an individual detector fails to detect traffic.

[0027] In one embodiment, the vehicle detection system uses passive magneto- resistive technology for detecting vehicles. Magneto-resistive technology allows for reliability and accuracy in detecting vehicles and is not affected by environmental shift or environmental constants such as metallic water mains or rebar. Robust transmitting and receiving coupled with a repeater system allow for antenna configurations that eliminate or reduce problems that may be specific to an

intersection such as curves in the road, buildings, vehicle shadowing, and/or other physical characteristics of the intersection.

[0028] Reference is now made to the figures in which like reference numerals refer to like elements. For clarity, the first digit of a reference numeral indicates the figure number in which the corresponding element is first used. In the following description, numerous specific details are provided for a thorough understanding of the embodiments disclosed herein. However, those skilled in the art will recognize that the embodiments can be practiced without one or more of the specific details, or with other methods, components, or materials. Further, in some cases, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring aspects of the embodiments. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. I. SYSTEM OVERVIEW

[0029] FIG. 1 is a block diagram of a wireless traffic detection system 100 according to one embodiment. The wireless traffic detection system 100 includes one or more road sensor units 110 (one shown), a master transceiver unit 112, one or more repeater transceiver units 114 (one shown), a master interface control card 116, one or more slave interface control cards 118 (one shown), and a traffic control unit 120.

[0030] For illustrative purposes, FIG. 2 is a schematic diagram illustrating the wireless traffic detection system 100 of FIG. 1 configured to detect traffic approaching, entering, and/or stopped at an intersection 200 of two streets 210, 212 according to one embodiment. The wireless traffic detection system 100 is used to efficiently control traffic signals (e.g., attached to poles 214). However, an artisan will recognize from the disclosure herein that the wireless traffic detection system 100 is not limited to detecting traffic at an intersection and may be used to detect ground vehicles or moving traffic in a wide variety of situations. For example, the wireless traffic detection system 100 may be used to detect ground traffic at one or more locations of a street or freeway. The wireless traffic detection system 100 may also be used, for example, to detect a ground vehicle approaching or stopped at a gate located at the entrance of a parking lot or a gated community. [0031] Referring to FIGS. 1 and 2, the wireless traffic detection system 100 is relatively easy to setup and modify as compared to conventional detection systems.

The road sensor units 110 are installed above the road surface or flush with the road surface. Thus, one or more road sensor units 110 may be quickly installed or removed. Further, a user may use software control to quickly add or delete one or more road sensor units 110 from the wireless traffic detection system 100. The wireless traffic detection system 100 is less likely to have bad connections or broken sensor loop problems. Thus, the wireless traffic detection system 100 also requires less maintenance than that required for conventional hard-wired loop systems. [0032] The road sensor units 110 are remotely located, battery powered devices that detect automobile traffic by sensing local disturbances in the earth's ambient magnetic field attributable to the presence of automobiles. To save power, a user can temporarily or permanently shut down one or more road sensor unit 110 when not needed. Each of the road sensor units 110 includes an antenna (see antenna 544 in FIG. 5) for bi-directional, wireless communication with the master transceiver unit 112 and/or the repeater transceiver unit 114. In one embodiment, each of the road sensor units 110 dynamically tunes its respective antenna 544 to match an RF frequency currently in use.

[0033] The road sensor units 110 are mounted in or on the roadway and sense traffic states within their respective sensing ranges. For example, FIG. 2 shows two road sensor units 110 located so as to detect vehicles traveling along the street 210 toward the intersection 200 (from the bottom to the top of the drawing), three road sensor units 110 located so as to detect vehicles traveling along the street 210 toward the intersection 200 (from the top to the bottom of the drawing), and four road sensor units 110 located so as to detect vehicles traveling along both directions of the street 212 (two for each direction). An artisan will recognize from the disclosure herein that more or fewer road sensor units 110 may be used. For example, additional road sensor units 110 may be placed at greater distances from the intersection 200 so as to provide advance detection of vehicles approaching the intersection 200.

[0034] As discussed in detail below, each road sensor unit 110 periodically measures its ambient magnetic field in three dimensions and compares the ambient three-axis magnetic field against a reference measurement to determine the presence of a stationary automobile or moving traffic. In one embodiment, the road sensor units 110 measure their respective ambient three-axis magnetic field approximately every 83.25 ms. However, an artisan will recognize from the

disclosure herein that other measurement periods are possible and may depend, for example, on the expected traffic speed.

[0035] The road sensor units 110 report their respective traffic states to the master transceiver unit 112 and/or the repeater transceiver unit 114. The master transceiver unit 112 handles the interface between the road sensor units 110, the repeater transceiver unit 114, and the master interface control card 116. In one embodiment, the master transceiver unit 112 is mounted as close as possible to its antenna 216 and the repeater transceiver unit 114 is mounted as close as possible to its antenna 218. The repeater transceiver unit 114 is optional and is used in some embodiments to enhance the range of the master transceiver unit 112 by repeating the master transceiver unit's commands and sending system responses back to the master transceiver unit 112. The repeater transceiver unit 114 acts as a slave device to the master transceiver unit 112. In one embodiment, the repeater transceiver unit 114 is solar powered for ease of installation.

[0036] In one embodiment, the master transceiver unit 112 and/or the repeater transceiver unit 114 uses a bi-directional RF link (e.g., through the antennae 216, 218) to poll the road sensor units' traffic states approximately every 333 ms, or another appropriate time interval, so as to check system integrity. Each of the road sensor units 110 responds to the poll via the RF link by sending its respective traffic status information in a time-multiplexed, frequency hopping packet structure. In one embodiment, the frequency hopping is in a frequency range between approximately 902 MHz and approximately 928 MHz. However, other frequency ranges may also be used. The master transceiver unit 112 provides a robust data link by dynamically changing a frequency hopping table so as to establish a best or adequate communication link for current environmental conditions. Each individual installation of the wireless traffic detection system 100 has its own unique system address so as to avoid RF interference with other similar installations in nearby locations. [0037] The master transceiver unit 112 receives the data from the road sensor units 110 (either directly or through the repeater transceiver unit 114) and processes the data before sending it to the master interface control card 116. The master transceiver unit 112 communicates with the master interface control card 116 through a wired communication link. For example, the master transceiver unit 112 may communicate with the master interface control card 116 via an RS-485 link or other physical connection.

[0038] As shown in FIG. 2, the master interface control card 116, the slave interface control card 118 and the traffic control unit 120 are located in a controller cabinet 220. The master interface control card 116 distributes the detected traffic states received from the master transceiver unit 112 to the traffic control unit 120 through an existing loop card slot in a control card backplane (not shown) in the traffic control unit 120. In one embodiment, the master interface control card 116 is capable of processing inputs from eight road sensor units 110 and controlling two "loop" outputs (discussed below) at the control card backplane. An artisan will recognize from the disclosure herein that the master interface control card 116 may also be configured to process inputs from a different number of road sensor units 110 and/or control a different number of loop outputs.

[0039] As discussed in detail below, the master interface control card 116 includes all of the functions of the slave interface control card 118 with the addition of seven-segment displays and additional user buttons. The master interface control card 116 handles the interface to the master transceiver unit 112 and the interface to all of the slave interface control cards 118. In one embodiment, the master interface control card 116 and slave interface control cards 118 communicate with one another via an RS-485 link. In one embodiment, there is one master interface control card 116 per traffic control intersection.

[0040] The one or more slave interface control cards 118 receive the respective status of the road sensor units 110 from the master interface control card 116 and convert this data to signals that activate loop inputs to the traffic control unit 120. In one embodiment, the slave interface control cards 118 accept user inputs in the form of eight buttons and provide status information via eight light emitting diodes (LEDs). However, an artisan will recognize from the disclosure herein that a different number of buttons and/or LEDs can also be used.

[0041] In one embodiment, each master interface control card 116 or slave interface control card 118 includes a loop signal output that can activate a loop "trigger" within the traffic detection system 100. Four road sensor unit signals are assigned to each loop signal on the master interface control card 116 or the slave interface control card 118. There are two loop signal outputs on each master interface control card 116 or slave interface control card 118. [0042] The traffic control unit 120 includes a microprocessor (not shown) for controlling the traffic signals based on the respective traffic status received from the

road sensor units 110. For example, after detecting that a vehicle is waiting at a red light at the intersection 200, the traffic control unit 120 may provide the vehicle with a green light. In one embodiment, the traffic control unit 120 can handle 64 road sensor units 110. However, an artisan will recognize from the disclosure herein that the traffic control unit 120 may be configured to handle any number of road sensor units 110.

[0043] The traffic detection system 100 includes several advantages over conventional detection systems. For example, system firmware for the road sensor units 110, the master transceiver unit 112, the repeater transceiver unit 114, the master interface control card 116, the slave interface control card 118, and/or the traffic control unit 120 are field upgradeable. The system firmware is field upgradeable via a hardwired data link and/or an RF data link. Individual measurement parameters, such as detection threshold (discussed below), are also field upgradeable. Further, the road sensor units 110 may be individually added and/or removed from a site installation when necessary or desired. [0044] The traffic detection system 100 also includes supporting functions not directly related to traffic flow control. For example, each road sensor unit 110 is capable of measuring and reporting an ambient road temperature, a battery voltage, a device serial number, a device software version, a traffic detection threshold level and any sensor errors that occur. A user may access this information from the master interface control card 116. II. EXAMPLE RF COMMUNICATION PROTOCOL

[0045] As discussed above, the master transceiver unit 112, the one or more repeater transceiver units 114, and the one or more road sensor units 110 communicate with one another through a bi-directional RF link. FIG. 3 is a flowchart illustrating an example RF communication protocol 300 used for the RF link according to one embodiment. For illustrative purposes, FIGS. 4A and 4B are tables illustrating example RF communication packet structures for various communication packets shown in FIG. 3 according to one embodiment. An artisan will understand from the disclosure herein that the number of bytes, times and other results shown in FIGS. 4A and 4B are provided by way of example only and are not intended to limit the disclosure. [0046] Referring to FIGS. 3, 4A and 4B, the example protocol 300 starts at a step

310 and proceeds to a step 312 where the master transceiver unit 112 sends a first

poll and a second poll to the road sensor units 110. This first poll is referred to herein as a master poll primary and the second poll is referred to herein as a master poll secondary. Example data structures and packet times are shown in FIG. 4A for both the master poll primary and master poll secondary data packets. The repeater transceiver unit 114 receives the master poll primary and the master poll secondary. If in range, one or more of the road sensor units may also receive the master poll primary and the master poll secondary.

[0047] At a step 314, the repeater transceiver unit 114 repeats the master poll primary and the master poll secondary as a repeater poll primary and a repeater poll secondary, respectively. Example data structures and packet times are shown in FIG. 4A for both the repeater poll primary and repeater poll secondary data packets. [0048] At a step 316, the road sensor units 110 respond (with sensor response data packets) to one or more of the polls by transmitting their respective data to the master transceiver unit 112 or the repeater transceiver unit 114 in a time-multiplexed response. The time multiplexing is based on a system index for each of the road sensor units 110. In an example embodiment, 64 road sensor units 110 are used. Thus, for such an embodiment, 64 indexes are available. An example data structure and packet times are shown in FIG. 4B for the sensor response data packet. [0049] At a step 318, the repeater transceiver unit 114 sends acknowledgements (with repeater ACK data packets) to the road sensor units 110 from which it received sensor response data packets. At a step 320, the repeater transceiver unit 114 also sends received data from any road sensor units 110 to the master transceiver unit 112 (with repeater data transfer data packets). Example data structures and packet times are shown in FIG. 4B for both the repeater ACK and repeater data transfer data packets.

[0050] At a step 322, the master transceiver unit 112 sends acknowledgements (with master ACK data packets) to the road sensor units 110 from which it received sensor response data packets. An example data structure and packet time are shown in FIG. 4B for the master ACK data packet. At a step 324, the master transceiver unit 112 processes the data received either directly or indirectly from the road sensor units 110, as discussed herein. At a step 326, the master transceiver unit 112 transmits the processed data to the master interface control card 116. In one embodiment the communication between the master transceiver unit 112 and

the master interface control card 116 is via a hard-wired RS-485 connection at approximately 115.2 KB/s.

[0051] At a step 326, the master transceiver unit 112 transmits a master sync data packet to any road sensor units 110 in communication therewith that may need to be resynchronized to the master polls. Similarly, at a step 330, the repeater transceiver unit 114 transmits a repeater sync data packet to any road sensor units 110 in communication therewith that may need to be resynchronized to the repeater polls. Example data structures and packet times are shown in FIG. 4B for both the master sync and repeater sync data packets.

[0052] At a step 332, the protocol 300 queries whether there are additional frequency index values available in a frequency table. If yes, the protocol 300 proceeds to a step 334 where the master transceiver unit 112, the repeater transceiver unit 114, and the road sensor units 110 each increment the communication frequency to the next index value in the frequency table. The protocol 300 then returns to the step 312. If there are no additional frequency index values available in the frequency index table, at a step 333, the protocol 300 resets the frequency selection index so as to restart with the first frequency index at the beginning of the frequency table.

[0053] Thus, the master transceiver unit 112 and the repeater transceiver unit 114 send their respective poll signals at the beginning of each new frequency increment. In one embodiment, there are approximately 50 frequency values used in one complete sequence out of a total of 128 possible frequency values (e.g., a 50 byte frequency table index). In such an embodiment, the road sensor units 110 resynchronize their respective internal response timers on the reception of each poll signal from the master transceiver unit 112 or the repeater transceiver unit 114, which occurs at least once approximately every 25 frequency steps or approximately every 4.7 seconds.

[0054] Once a road sensor unit 110 receives one of the poll signals from the master transceiver unit 112 or the repeater transceiver unit 114, and receives the ACK data packet from the master transceiver unit 112 or the repeater transceiver unit 114, the road sensor unit 110 enters a low power mode and ignores the rest of the frequency increments within the current frequency hop sequence for as long as the sensed traffic state does not change. Upon sensing a traffic state change or at the beginning of the next frequency sequence, the road sensor unit 110 "re-

awakens" and responds to each poll it receives until it receives the ACK data packet from the master transceiver unit 112 or the repeater transceiver unit 114. If a road sensor unit 110 loses synchronization with the primary poll signal, it waits for a sync signal (328, 330) to regain synchronization.

[0055] Referring to FIGS. 4A and 4B, the "packet type" byte is part of the hardware address section of the communication packet. In one embodiment, the master transceiver unit 112, the repeater transceiver unit 114, and the road sensor units 110 each include an nRF905 RF Transceiver available from Nordic Semiconductor ASA of Trondheim, Norway that defines the packet types according to Table 1 :

Table 1

Each of the nRF905 RF Transceivers is programmed to listen for one of the packet types at the appropriate time.

[0056] The "intersection ID" byte is associated with the current packet and is part of the hardware address section of the nRF905 RF Transceiver communication packet. The intersection ID holds up to 256 intersection values. The "sensor command" byte defines a DATA REQUEST to all of the road sensor units 110 or a request specific to a single road sensor unit 110 (e.g., DELETE, REPROGRAM, RESET, WRITE). The sensor commands are shown in Table 2:

Table 2

[0057] For a data request, the "command data" byte defines the type of data that is being requested from the road sensor unit 110. The command data for a data request are shown in Table 3:

Table 3

For everything but a data request, the command data byte represents the sensor ID that is used to target a specific road sensor unit 110.

[0058] The "frequency table index" byte defines the next frequency index and the frequency table index counter. A two-bit table index counter (e.g., bits 7 - 6) increment (and overflow) when a new frequency table is in use. A road sensor unit 110 checks this table index count for a change before loading new frequency table information from the secondary poll. The other bits (e.g., bits 5 - 0) hold the next index for the current frequency table being used.

[0059] The "frequency table" is a 25-byte table of frequency index values to be written to the road sensor units 110. This is one-half of the 50 total frequency index values. The packet index parameter discussed below indicates whether this is the upper or lower half of the frequency index table. A road sensor unit 110 loads each half of this table if it sees that the table index counter has changed from its last

received state. These parameters are ignored by the road sensor unit 110 if the table index counter has not changed since the last received master poll.

[0060] The "CRC" is a 16-bit cyclic redundancy check (CRC) of the previous data in the corresponding data packet. The CRC bytes are part of the hardware CRC section of the nRF905 communication packet.

[0061] The "packet index/sensor ID" byte includes a flag (bit 7) that indicates if the secondary packet contains the lower half (e.g., bit 7 = 0) or the upper half (e.g., bit 7 = 1) of the frequency index table. Another flag (bit 6) indicates if the frequency table information should be loaded (e.g., 0 = no load, 1 = load new table information). The sensor ID index has a range of 1 - 63 (bits 5 - 0) to cover the maximum number of sensors in the system. When a road sensor unit 110 is added to the traffic detection system 100, the master transceiver unit 112 sets a new sensor

ID index in the road sensor unit 110.

[0062] The "data write address" byte is a pointer to an area within a road sensor unit 110 where custom parameters are stored. The data write address parameter is used if the "write data to sensor" command was received in the primary poll packet.

The data write address parameter pointer is set to 0 if there is no data to write. The

"data write" byte includes the data to write to the road sensor unit 110. The data write parameter is ignored by the road sensor unit 110 if the data write address pointer is set to 0.

[0063] The "response type" byte indicates the type of data that is being returned from a road sensor unit 110. The response types are shown in Table 3 above. The

"response data" includes two bytes of data returned from the road sensor unit 110.

The value of the response data depends on the type of information that the road sensor unit 110 was instructed to send.

[0064] The "dead time between responses" is a time-period between sensor response packets. The value for this parameter shown in FIG. 4B was determined from a crystal parts-per-million (PPM) resolution that could be sustained between each cycle through twenty different frequencies. It has been found that the sensor transmissions can stay properly time-synchronized for this period.

[0065] The ACK flags ("repeater ACK" and "master ACK") are bit flag acknowledgements for each road sensor unit's index in the current intersection.

Eight bytes cover all 64 road sensor unit index possibilities. The "valid data flag" byte includes a validity bit flag for every sensor data response in each of the

received repeater transceiver unit 114 packets. There are eight repeater transceiver unit 114 packets received in each cycle. The least significant bit in the valid data flag byte corresponds to the least significant word of the sensor responses.

III. EXAMPLE COMMUNICATION SCENARIOS

[0066] The following tables indicate example master poll packet setups for communication to and from the road sensor units 110 according to one embodiment.

Table 4 shows normal master polling of traffic status:

Table 4

[0067] Table 5 shows normal master polling with a data request:

Table 5

[0068] Table 6 shows a special command to a specified road sensor unit 110:

Table 6

[0069] Table 7 shows a write data to a specific road sensor unit 110:

Table 7

[0070] Table 8 shows a response from a road sensor unit 110:

Table 8

IV. EXAMPLE HARDWARE EMBODIMENTS

[0071] The following descriptions of road sensor unit 110, master transceiver unit 112, repeater transceiver unit 114, master interface control card 116 and slave interface control card 118 embodiments are provided by way of example. An artisan will understand from the disclosure herein that the specific values and elements of each embodiment may be changed or combined for a specific traffic detection system.

A. Example Road Sensor Unit

[0072] FIG. 5 is a block diagram of an example road sensor unit 110 according to one embodiment. As discussed above, the road sensor unit 110 detects automobile traffic by sensing a temporary disturbance of the measured magnetic field in the vicinity of the road sensor unit 110. The road sensor unit 110 measures X, Y, and Z magnetic field strength components. The sum of the absolute values of the X, Y and Z magnitude signals is compared to a sensing threshold to determine whether traffic is within range of the road sensor unit 110. The sensing threshold is initialized when the road sensor unit 110 is first installed in the wireless traffic detection system 100. The threshold may be periodically recalibrated. When loaded with application software, the road sensor unit 110 is initialized with a unique two-byte serial number. Further, the road sensor unit 110 includes hardware to facilitate the download of new or updated application software via the master transceiver unit 112 or repeater transceiver unit 114 RF link.

[0073] An automobile "traffic status" is sent to the master transceiver unit 112 or the repeater transceiver unit 114 immediately after the road sensor unit 110 receives a data-poll signal from one of these devices. Six different information packets can be transmitted by the road sensor unit 110 including "No car," "Car," "Traffic moving," "Requested data," "Error data," and "ID mode." The ID mode may be entered by

sensing a magnetic field larger than the ambient earth's field strength or by receiving a command via the RF link. When in the ID mode, the road sensor unit 110 repeatedly sends its serial number (at maximum intervals of approximately 60 seconds).

[0074] In this example embodiment, the road sensor unit's data communications are performed using a spread-spectrum, frequency-hopping method centered at approximately 915 MHz. Fifty communication frequencies are selected out of 128 possible frequencies. The road sensor unit 110 uses an SPI-type protocol at approximately 50 Kb/s for RF communication with the master transceiver unit 112 and the repeater transceiver unit 114. The road sensor unit 110 also synchronizes its internal timing with reference to polling signals from the temperature-stabilized master transceiver unit 112 and/or repeater transceiver unit 114. [0075] The road sensor unit 110 is battery powered and operates with the lowest power consumption possible. In addition to measuring the ambient magnetic field, the road sensor unit 110 also measures and transmits road temperature and battery level. The road sensor unit 110 also reports internal error conditions back to master transceiver unit 112 upon request.

[0076] As shown in FIG. 5, the example road sensor circuit includes sensor circuitry 510, a master microcontroller unit 512 (master MCU 512), RF circuitry 514, and a slave microcontroller unit 516 (slave MCU 516). The sensor circuitry 510 includes an X-axis magnetic sensor 518, a Y-axis magnetic sensor 520, and a Z-axis magnetic sensor 522. An artisan will recognize from the disclosure herein that two or more of the magnetic sensors 518, 520, 522 may be combined in a single device. For example, in one embodiment, the X-axis magnetic sensor 518 and the Y-axis magnetic sensor 520 comprise a 2-axis magnetic sensor (Honeywell part no. HMC1002) and the Z-axis magnetic sensor 522 comprises a 1-axis magnetic sensor (Honeywell part no. HMC1001), both available from Honeywell International Inc. of Morristown, New Jersey. The master MCU 512 may separately enable each of the magnetic sensors 518, 520, 522.

[0077] Calibration of the X-axis magnetic sensor 518, the Y-axis magnetic sensor 520, and the Z-axis magnetic sensor 522 is needed before the road sensor unit 110 device can be put in to operation. The master transceiver unit 112 or the repeater transceiver unit 114 sends calibration commands to the road sensor unit 110 to initiate the calibration process.

[0078] The road sensor unit 110 also includes degaussing circuitry 524 to reset or degauss each of the magnetic sensors 518, 520, 522. For illustrative purposes, the degaussing circuitry 524 is schematically illustrated in FIG. 5. The degaussing circuitry includes two field effect transistors 526, 528 (FETs 526, 528) controlled by the master MCU 512 through "set" and "reset" control lines. During a set, the FET 526 switches the charge stored in the capacitor 530 through respective coupling capacitors 532, 534, 536 to reset coils (not shown) in the magnetic sensors 518, 520, 522. During a reset, the FET 528 switches to allow a reverse current to flow via the coupling capacitors 532, 534, 536 from the reset coils in the magnetic sensors 518, 520, 522.

[0079] Each of the magnetic sensors 518, 520, 522 produces a sensor output voltage that is individually measured by the master MCU 512. The master MCU 512 controls an analog switch 538 so as to select between one of three sensor outputs or a reference voltage Vref to be measured by the master MCU 512. The selected output of the analog switch 538 is then provided to one or more amplifiers 540 before being provided to an analog-to-digital converter (ADC) input of the Master MCU 512 for measurement. The master MCU 512 is configured to individually enable the analog switch 538 and the one or more amplifiers 540.

[0080] The master MCU 512 controls the 3-axis magnetic field measurements and handles the RF communication. The master MCU 512 provides a signal to a digital-to-analog (D/A) converter 542 to generate an offset voltage Voffset that is used to null a bridge signal offset into the one or more amplifiers 540. As discussed below, the offset voltage Voffset is also used to tune an antenna 544 in the RF circuitry. Although not shown, the master MCU 512 is clocked at approximately 4 MHz using an external crystal operating at 32,768 Hz as a reference. The master MCU 512 connects directly an RF transceiver 546 via port lines. [0081] The RF transceiver 546 according to this example embodiment is a single- chip RF transceiver that is controlled directly by the master MCU 512. The RF transceiver 546 is clocked at approximately 16 MHz by an external crystal (not shown). An RF signal is received (RF In) by the antenna 544 and provided through a first filter 548, a first RF switch 550, an amplifier 552, a second RF switch 554, and a second filter 556 into an antenna input of the RF transceiver 546. In this example embodiment, the first filter 548 and the second filter 556 each comprise a surface acoustic wave (SAW) filter at approximately 915 MHz. Further, the receiver

sensitivity is approximately -105dBm. The master MCU 512 enables and/or controls the RF switches 550, 554 and the amplifier 552.

[0082] The RF transceiver 546 transmits an RF signal (RF Out) through the second filter 556 and the second RF switch 554 to a two-stage discrete RF amplifier 558. The two-stage RF amplifier 558 includes a pre-amplifier section that drives a main amplifying stage to provide approximately +13dBm of total signal amplification. The master MCU 512 enables the two-stage RF amplifier 558, which provides the RF Out signal to the antenna 544 through the first RF switch 550 and the first filter 548. The antenna 544 is electrically coupled to antenna tuning circuitry 560. For illustrative purposes, the antenna tuning circuitry 560 is schematically illustrated in FIG. 5 and includes an inductor 562 coupled in series with a capacitor 564. The master MCU 512 tunes the antenna 544 to a selected operating frequency by adjusting the offset voltage Voffset that is coupled between the inductor 562 and the capacitor 564, changing the impedance of the antenna 544.

[0083] The slave MCU 516 is used to reprogram the master MCU 512 with new or updated application firmware. The slave MCU 516 initiates a "bootloader" mode in the master MCU 512 and has full control of the RF transceiver 546 during the firmware download process. In this example embodiment, the slave MCU 516 is clocked at approximately 4 MHz using a 32,768 Hz crystal (not shown) as a reference.

[0084] Although not shown, the road sensor unit 110 includes a memory for storing parameters including, for example, a sensor serial number (e.g., two bytes stored in a flash ROM), an intersection number (e.g., 1 byte), a sensor ID index (e.g., 1 byte), a sensor detection threshold value (e.g., 1 byte), and other information discussed herein. In one embodiment, a portion of the memory is set aside for user or custom setup parameters.

B. Example Master or Repeater Transceiver Units

[0085] FIG. 6 is a block diagram of an example master transceiver unit 112 or repeater transceiver unit 114 according to one embodiment. As discussed above, the master transceiver unit 112 serves as an interface between the road sensor unit(s) 110, the repeater transceiver unit(s) 114, and the master interface control card 116. The master transceiver unit 112 also handles the adding and/or deleting of a road sensor unit 110 in an intersection and includes a temperature stabilized oscillator (not shown) for system timing. In this example embodiment, the oscillator

provides a signal at approximately 32 KHz. Further, the master transceiver unit 112 includes hardware to facilitate the download of new or updated application software to the master transceiver unit 112 via an RS-485 link.

[0086] The master transceiver unit 112 communicates with the road sensor unit(s) 110 and the repeater transceiver unit(s) 114 to obtain road sensor information during each frequency hop. The master transceiver unit 112 communicates with the road sensor unit(s) 110 and the repeater transceiver unit(s) 114 using an SPI-type protocol at approximately 50 Kb/s. In addition to communicating road sensor information with the road sensor unit(s), the master transceiver unit 112 also sends degauss and offset neutralize commands to the road sensor unit(s) 110. [0087] The master transceiver unit 112 also determines whether the strength of a received signal from a road sensor unit 110 is adequate for communication. The master transceiver unit 112 determines this communication threshold by decreasing the receive amplification to see whether it still receives the signal from the road sensor unit 110. The master transceiver unit 112 also sends a "poll" signal to the road sensor unit(s) 110 and the repeater transceiver unit(s) 114. The road sensor unit(s) 110 respond to the poll with traffic or status information. The master transceiver unit 112 sends this information to the master interface control card 116 to be distributed to the proper sensor loop input.

[0088] The master transceiver unit 112 communicates with the master interface control card 116 to provide road sensor state information and receive user commands. The road sensor state information may include, for example, "car," "no- car," and "traffic" indicators. The master interface control card 116 displays the "car" and/or "traffic" states on corresponding green light emitting diode (LED) displays (see FIG. 8). The master transceiver unit 112 combines the road sensor signals received from the repeater transceiver unit 114 with the road sensor signals received from the road sensor units 110 before sending this information to the master interface control card 116. In this example embodiment, the master transceiver unit 112 communicates with the master interface control card 116 using RS-485 protocol at approximately 115.2 KB/s.

[0089] In addition to road sensor state information, the master transceiver unit 112 sends system errors, requested road sensor addresses, road sensor battery voltage levels, road temperatures, road sensor magnetic detection threshold values, road sensor software versions, repeater transceiver software versions, and master

transceiver unit software version to the master interface control card 116. As discussed below, the master interface control card 116 displays system errors on a seven-segment digit display (see FIG. 8) and displays road sensor errors on corresponding red LED displays (see FIG. 8). The master transceiver unit 112 also holds an intersection ID value set by the master interface control card 116 (uniquely identifying an intersection as one of 256 possible intersections). [0090] As discussed below, the master transceiver unit 112 may also be configured as a repeater transceiver unit 114 by selecting values for certain resistors (not shown). The repeater transceiver unit 114 repeats commands from the master transceiver unit 112 and relays road sensor information back to the master transceiver unit 112. The repeater transceiver unit 114 also reports internal error conditions back to the master transceiver unit 112 for proper error handling. The repeater transceiver unit 114 includes hardware to facilitate the download of new or updated application software to the device via the RF link.

[0091] As shown in FIG. 6, the master transceiver unit 112 (or repeater transceiver unit 114, if configured as such) includes master/repeater selection circuitry 610, a master transceiver unit interface 611 , a master microcontroller unit 612 (master MCU 612), RF circuitry 614, and a slave microcontroller unit 616 (slave MCU 616). The master MCU 612 handles the RS-485 communications to the master interface control card 116 via the master transceiver unit interface 611. In this example embodiment, the master transceiver unit interface 611 comprises an RS-485 transceiver, part no. SN65HVD12D, available from Texas Instruments Inc. of Dallas, Texas.

[0092] The master MCU 612 also handles the RF communications to the road sensor units 110 and the repeater transceiver units 114 via the RF circuitry 614. Although not shown, the master MCU 612 is clocked using two external crystals operating at 8 MHz and 32,768 Hz. The master MCU 612 connects directly to the RF transceiver 618 via port lines.

[0093] The master/repeater selection circuitry 610 configures the master MCU 612 and the slave MCU 616 to operate either as a master transceiver unit 112 or a repeater transceiver unit 114. In one embodiment, the firmware in the master MCU 612 and/or the slave MCU 616 determines functionality as either a master transceiver unit 112 or repeater transceiver unit 114. In such an embodiment, the master/repeater selection circuitry 610 includes one or more resistors that are used

during the firmware programming operation to configure the firmware as a master or slave.

[0094] The RF transceiver 618 according to this example embodiment is a single- chip RF transceiver that is controlled directly by the master MCU 612. The RF transceiver 618 is clocked at approximately 16 MHz by an external crystal (not shown). An RF signal is received (RF In) by an antenna 620 and provided through a first filter 622, a first RF switch 624, an amplifier 626, a second RF switch 628, and a second filter 630 into an antenna input of the RF transceiver 618. In this example embodiment, the first filter 622 and the second filter 630 each comprise a SAW filter at approximately 915 MHz. Further, the receiver sensitivity is approximately -105 dBm. The master MCU 612 enables and/or controls the RF switches 624, 628 and the amplifier 626.

[0095] The RF transceiver 618 transmits an RF signal (RF Out) through the second filter 630 and the second RF switch 628 to a two-stage discrete RF amplifier 632. The two-stage RF amplifier 632 includes a pre-amplifier section that drives a main amplifying stage to provide approximately +16 dBm of total signal amplification. The master MCU 612 enables the two-stage RF amplifier 632, which provides the RF Out signal to the antenna 620 through the first RF switch 624 and the first filter 622.

[0096] The slave MCU 616 is used to reprogram the master MCU 612 with new or updated application firmware. The slave MCU 616 initiates a "bootloader" mode in the master MCU 612 and has full control of the RF transceiver 618 during the firmware download process. In this example embodiment, the slave MCU 616 is clocked at approximately 4 MHz using a 32,768 Hz crystal (not shown) as a reference.

C. Example Master or Slave Interface Control Cards

[0097] FIG. 7 is a block diagram of an example master interface control card 116 or slave interface control card 118 according to one embodiment. The master interface control card 116 or slave interface control card 118 plugs in to a backplane connector (not shown) in the traffic controller cabinet 220 shown in FIG. 2. Firmware in the master interface control card 116 or the slave interface control card 118 may be field upgraded via an RS-485 connection to an interface control card bus. [0098] The master interface control card 116 includes electrical circuitry and functionality for both a master interface control card 116 and a slave interface control

card 118. However, the master interface control card 116 includes a display card 710 that a slave interface control card 118 does not have. Thus, attaching the display card 710 to a slave interface control card 118 converts the slave interface control card 118 to a master interface control card 116. As discussed in detail below, the display card 710 allows entry of user selections and provides user information and system feedback to a user.

[0099] The master interface control card 116 handles communication between the master transceiver unit 112, the slave interface control cards 118, and the traffic control unit 120 via an RS-485 serial link. The master interface control card 116 receives road sensor information via the master transceiver unit 112 (which is external to the traffic controller cabinet 220), processes the received road sensor information, and distributes the processed results to the slave interface control cards 118. The master interface control card 116 communicates with the slave interface control cards 118 and activates corresponding loop sensor outputs at the backplane connector. The master interface control card 116 also includes hardware to facilitate the download of new or updated application software via the RS-485 link. [00100] As discussed below, the master interface control card 116 also handles the integration of new or additional road sensor units 110 into the system 100 during an integration mode. After an assignment of the new road sensor unit 110 to the system 100, the master interface control card 116 cancels the integration mode. The master interface control card 116 also senses the number of slave interface control cards 118 installed at a current intersection.

[00101] The slave interface control cards 118 each include a unique serial number and communicate with the master interface control card 116 via the RS485 link. Each of the slave interface control cards 118 is capable of handling two sets of road sensor units 110 with up to four road sensor units 110 in each set. The slave interface control cards 118 provide a loop output signal to the traffic control unit 120 for each set of road sensor units 110 so as to provide the combined sensed traffic state.

[00102] The slave interface control cards 118 are also configured to provide information to a user. For example, the slave interface control cards 118 may provide indicia of traffic states for their respective road sensor units 110. The slave interface control cards 118 also allow the user to initiate a test mode of their respective road sensor units 110. The slave interface control cards 118 also provide

indicia of the test results to the user. The slave interface control cards 118 include hardware to facilitate the download of new application software via the RS485 link. [00103] As shown in FIG. 7, the master interface control card 116 (or slave interface control card 118 if the display card 710 is not included) includes a master microcontroller unit 712 (master MCU 712), a slave microcontroller unit 714 (slave MCU 714), a control card interface 716, a master transceiver unit interface 718, sensor position controls 720, and sensor indicators 722.

[00104] The master MCU 712 handles the RS-485 communications between the master interface control card 116 and the slave interface control cards 118 via the control card interface 716. The master MCU 712 also handles the RS-485 communications between the master interface control card 116 and the master transceiver unit 112 via the master transceiver unit interface 718. In this example embodiment, the control card interface 716 and the master transceiver unit interface 718 each comprises an RS-485 transceiver, part no. SN65HVD12D, available from Texas Instruments Inc. of Dallas, Texas. The master MCU 712 is clocked using two external crystals (not shown) at 6.144 MHz and 32,768 Hz.

[00105] The master MCU 712 controls the sensor indicators 722 and measures the state of the sensor position controls 720. The master MCU 712 also controls the display card 710 via a display card interface 724. The display card interface 724 controls multiplexing and current drive levels of a numerical display 726 on the display card 710. The numerical display 726 displays user information and system feedback. The display card 710 also includes sensor controls 728 that relay user selections to the master MCU 712. The sensor position controls 720, sensor indicators 722, numerical display 726, and sensor controls 728 are discussed in detail below.

[00106] FIG. 8 is a schematic diagram illustrating the user controls and indicators of the master interface control card 116 according to one embodiment. For illustrative purposes, FIG. 8 shows an example layout of the sensor position controls 720, sensor indicators 722, numerical display 726, and sensor controls 728. In this example, the numerical display 726 comprises four 7-segment LED displays 810 and the sensor controls 728 include an up button 812, a down/degauss button 814, an add button 816, and a del button 818. [00107] As discussed above, in this example the master interface control card 116 or each slave interface control card 118 controls two sets of road sensor units 110

with up to four road sensor units 110 in each set, for a total of eight possible road sensor units 110. The master interface control card 116 and slave interface control cards 118 assign each of their corresponding road sensor units 110 a position or sensor ID slot (e.g., slots 1-8). The sensor position controls 720 include a position button 820 (eight shown) for each sensor ID slot. The sensor indicators 722 include a pair of LEDs 822, 824 for each sensor ID slot. For each slot, the LEDs 822, 824 comprise two different colors. For this example, the LED 822 is referred to as a red LED 822 (eight shown) and the LED 824 is referred to as a green LED 824 (eight shown).

[00108] When a road sensor unit 110 detects traffic, a corresponding green LED 824 is turned on. When a fault is detected in a road sensor unit 110, a corresponding red LED 822 is turned on. When the red LED 822 is on, a user may press the corresponding position button 820 to initiate a test mode that tests communication with the corresponding road sensor unit 110. If the display card 710 is electrically coupled to the master MCU 712, pressing the corresponding position button 820 will also display an error code on the 7-segment LED displays 810. The red LED 822 flashes while the 7-segment LED displays 810 display the error code. In this example, the error code is displayed for approximately 10 seconds after the user presses the position button 820.

[00109] Successful communication during the test mode causes the green LED 824 to illuminate briefly. Failure to communicate with the road sensor unit 110 during the test mode causes the red LED 822 to illuminate. After testing, the master interface control card 116 or slave interface control card returns to normal operation. The user may clear an error condition by pressing the del button 818. In response, the corresponding road sensor unit 110 is sent a "reset sensor" command on the next master poll.

[00110] During normal operation, the user may press a position button 820 on the master interface control card 116 to display (on the 7-segment LED displays 810) the serial number of the corresponding road sensor unit 110 assigned to that position. In one embodiment, signals from four road sensor units 110 in a set assigned to a loop position on the master interface card 116 are logically ORed together. When the position button is pressed, the corresponding green LED 824 will also flash to indicate that the slot has been selected. The other green LEDs 824 illuminate (solid) if they have been assigned to a respective road sensor unit 110. If no road sensor

unit 110 has been assigned to the sensor ID slot corresponding to the pressed position button 820, the 7-segment LED displays 810 display dashes instead of a serial number.

[00111] In addition to viewing serial numbers, the user may display additional sensor information on the 7-segment LED displays 810 by pressing the up button

812 or the down button 814 to scroll through sensor information including, for example, software version, errors, traffic detection threshold levels, battery voltage levels, and road temperatures. The system senses vehicles and otherwise operates normally while the 7-segment LED displays 810 are active. Pressing the corresponding position button 820 a second time cancels the information display and returns the system to normal operation. The 7-segment LED displays 810 timeout after approximately one minute and return to normal operation if no further button presses are sensed.

[00112] Returning to FIG. 7, the master MCU 712 provides two loop signal outputs

(CH 1 Loop Output and CH2 Loop Output) to the traffic control unit 120. Each loop signal output corresponds to up to four road sensor units 110 and provides sensed traffic state information. In one embodiment, each loop signal output is optically isolated.

[00113] The slave MCU 714 monitors the control card interface 716 for a reprogramming command sent on the RS-485 card bus. Once the slave MCU 714 receives the correct command, the slave MCU 714 initiates a bootloader mode in the master MCU 712. In this example embodiment, the slave MCU 714 is clocked using a 32,768 Hz crystal (not shown).

V. ADDING/DELETING ROAD SENSOR UNITS

[00114] A road sensor unit 110 is added to the system 100 before it can be polled for data. A user may place a road sensor unit 110 in an "add sensor" mode by exposing the road sensor unit 110 to an external magnetic field that is greater than the earth's magnetic field (e.g., placing a magnet near the road sensor unit 110). In the add sensor mode, the road sensor unit 110 looks for an "add sensor" poll packet from the master transceiver unit 112 or repeater transceiver unit 114 during a normal secondary poll time slot. Table 9 shows an example add poll packet format:

Table 9

[00115] The road sensor unit 110 responds to the add sensor poll packet with its serial number in the position indicated by the sensor ID byte. No frequency table updates or road sensor unit 110 memory modifications can be performed during the add sensor procedure because the secondary poll signal time slot is being used for the add sensor poll packet.

[00116] Referring to FIG. 8, after the road sensor unit 110 enters the add sensor mode, the user presses the add button 816 on the master interface control card 116. The 7-segment LED displays 810 then blink and display dashes until the serial number is received from the road sensor unit 110 being added to the system 100. Positions (e.g., sensor ID slots) with sensors already assigned will have the corresponding green LEDs 824 illuminated. Only one road sensor unit 110 may be added to a road sensor position. In one embodiment, a road sensor unit 110 must first be deleted from a position before another road sensor unit 110 may be added to the position. In addition, or in another embodiment, the add sensor mode will timeout after approximately 20 seconds if no road sensor unit 110 is added to the system within that time.

[00117] After the serial number is received from the road sensor unit 110 being added to the system 100, the 7-segment LED displays 810 display the serial number. The user may then press a position button 820 on the master interface control card 116 where no road sensor unit 110 is yet assigned. The serial number displayed on the 7-segment LED displays 810 stops blinking when the road sensor unit 110 is assigned to the new position. The green LED 824 for that position momentarily illuminates to indicate that the road sensor unit 110 has been assigned to the selected position. The master transceiver unit 112 then sends a command to

the road sensor unit 110 to degauss and initialize. The system 100 then returns to normal operation mode.

[00118] When in the ADD sensor mode, the road sensor unit 110 defaults to a known frequency-hop index table. The fifty-index frequency-hop sequence is changed to five constant index values (e.g., 1 , 32, 64, 96, 128) that are repeated ten times to complete the fifty index sequence. The road sensor unit 110 monitors the first frequency in the list for the add sensor poll packet, which includes the road sensor unit's ID number and the intersection number. In response to the add sensor poll packet, the road sensor unit 110 returns its own serial number in the designated sensor ID slot. If the add sensor poll packet is not received after approximately two seconds, the road sensor unit 110 hops to the next frequency in the list and listens for approximately two more seconds. An artisan will understand from the disclosure herein that the road sensor unit 110 may hop frequencies after waiting for a different time period.

[00119] The road sensor unit 110 remains in the add sensor mode until the user removes the external magnetic field therefrom or upon acknowledgment by the master transceiver unit 112 or the repeater transceiver unit 114. The road sensor unit's serial number data are returned at the time index indicated by the sensor ID byte in the add poll master/repeater secondary poll signal. When the master interface control card 116 assigns the road sensor unit 110 to a loop position, the road sensor unit's index then changes so as to respond in the next available highest index location (e.g., the highest index number is 64). Responding in the higher index locations reduces the latency in detecting the road sensor state. [00120] To delete a road sensor unit 110 from the system 100, the user presses the position button 820 corresponding to the position of the road sensor unit 110 to be deleted. When the user presses the position button 820, the 7-segment LED displays 810 display the serial number of the road sensor unit 110 assigned to that sensor position or dashes if no road sensor unit 110 has been assigned. The corresponding green LED 824 also flashes to indicate the position that has been selected. The other green LEDs 824 for the other positions illuminate (solid) if a road sensor unit 110 has been assigned to that corresponding position. [00121] The user then simultaneously presses the selected position button 820 and the del button 818. In response, the master transceiver unit 112 sets the command action bits (in the sensor command byte of the primary polling signal) to

"delete sensor" and sets the selected road sensor unit's index ID in the "command data" byte. The road sensor unit 110 with the index ID is deleted from the system. [00122] The deleted road sensor unit 110 is then put into a low-power operating mode and no longer responds to poll signals from the master transceiver unit 112. The user may reactivate the deleted road sensor unit 110 by following the add sensor procedure discussed above. After deleting the road sensor unit 110, the 7- segment LED displays 810 display dashes to indicate that the road sensor unit 110 has been deleted from the system. The green LED 824 for the corresponding position is extinguished to indicate that the road sensor unit 110 was deleted from the position. The system 100 then returns to normal operation mode. The road sensor unit 110 that was deleted returns to an initialized mode (e.g., as if it were new out-of-the-box).

VI. MANUAL ROAD SENSOR UNIT RESET/DEGAUSS

[00123] To apply a manual reset that will degauss and offset neutralize a road sensor unit 110, the user presses and holds the down/degauss button 814 on the master interface control card 116 while the 7-segment LED displays 810 are blank. The down/degauss button 814 is a dual function button. As discussed above, the down/degauss button 814 may be used to scroll through sensor information displayed on the 7-segment LED displays 810. However, while the 7-segment LED displays 810 are blank, the down/degauss button 814 sends a reset/degauss command to the master MCU 712. The user selects the road sensor unit 110 to degauss/reset by simultaneously pressing the down/degauss button 814 and the corresponding position button 820. The master transceiver unit 112 sends the command to degauss the selected road sensor unit 110. The selected road sensor unit 110 responds by degaussing and offset neutralizing its sensor circuitry, as discussed above.

VII. SETTING THE DETECTION THRESHOLD VALUE

[00124] In one embodiment, all of the road sensor units 110 in the system 100 are set to the same detection threshold value. The master poll secondary data packet addresses a specific road sensor unit 110 using the specific road sensor unit's ID value. New threshold data are included in the "data write" byte in the master poll secondary data packet. In another embodiment, the detection threshold values may be individually set for each road sensor unit 110.

VIII. TRAFFIC DETECTION SCHEME

[00125] In one embodiment, the road sensor unit 110 reports a "car" traffic state and a "no car" traffic state. When the detection threshold value is exceeded, the road sensor unit 110 sends a "car" signal. When the detection state changes, the road sensor unit 110 sends a "no-car" signal. In one embodiment, time hysteresis is built into the switching between the two states.

[00126] In one embodiment, the road sensor unit 110 reports the "car" state when the detection threshold value is continuously exceeded for more than approximately two seconds or another predetermined time interval. Similarly, the road sensor unit 110 reports the "no car" state when the detection threshold value has not been exceeded for the predetermined time interval.

[00127] In one embodiment, the road sensor unit 110 also reports a "traffic moving" traffic state. In certain such embodiments, the road sensor unit 110 reports the "traffic moving" state when the detection state has changed within a two second time frame or another predetermined time interval. The road sensor unit 110 reports a change of status at the next available reporting interval within a frequency increment polling from the master transceiver unit 112. The master interface control card 116 and/or slave interface control cards 118 toggle the corresponding loop slot signal when the road sensor unit 110 reports the "traffic moving" status. The road sensor unit 110 continues to send the new status information until it receives an ACK from the master transceiver unit 112.

[00128] It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.