Magnetometer vehicle detector

A magnetometer vehicle detector for detecting various parameters of traffic on a roadway. A number of sensors, having a compact package, along with connecting cables, may be placed in road way with a small number of standard width sawcuts. Alternatively, sensors may be placed in the roadway within tubes under the external surface of the roadway. The package design of the sensor is such that the sensor can be placed in the sawcut or tube only in a certain way or ways resulting in the most sensitive axis of the sensor being most likely affected by just the traffic or vehicles desired to be detected and measured. The sensor may be a magnetoresistive device having a permalloy magnetic sensing bridge. Multiple sensors may be placed in single or multiple lanes of the roadway for noting the presence of vehicles and measuring traffic parameters such as average speeds, vehicle spacings, and types and numbers of vehicles. Such information is processed from the shapes, times and magnitudes of the signature signals from the sensors.

BACKGROUND OF THE INVENTION 
This invention pertains to roadway vehicle detectors, and particularly, the 
invention pertains to magnetometer detectors for detecting vehicles on 
roadways. 
Present traffic vehicle detectors consist of wire loops that act as an 
electrical inductor, along with a capacitor, in an oscillator circuit that 
detects the presence or absence of a vehicle such as an automobile, truck 
or bus. This kind of detection system requires the wire loop to be 
installed below the pavement by the insertion of the loop into typically 
eight saw cuts into the surface of the pavement. The four-sided loop must 
be about four feet on a side to provide enough sensitivity to detect 
smaller vehicles. 
The failure rate of wire loops themselves is unacceptably high. The 
failures are the result of pavement upheaval and the differential in 
coefficients of thermal expansion between the pavement material and the 
wire. The wire breaks when the temperatures go too high or too low. A 
failure of the wire loop requires the installation of a replacement loop 
which is offset in location with respect to the first loop which has 
failed. This offset location is used because it is quite difficult to 
repair an in-place loop. However, having to offset the replacement loop 
causes some loss of optimum placement which results in some loss of 
vehicle detection accuracy and certainty. 
Traffic engineers who use wire loops for obtaining information, not only 
want presence information, but want to obtain other information, including 
vehicle count, speed, headway or direction, occupancy, and identity. 
Vehicle count is obtainable with a wire loop, but obtaining speed from a 
single loop is not feasible since speed is determined by the time it takes 
a vehicle to pass between two points. Two loops do not provide sufficient 
time resolution of passing vehicles for obtaining accurate speed 
indications. Headway is a spacing between vehicles in the same lane and 
the present loops do not have the spatial resolution to determine vehicle 
spacing, particular vehicles at close distances from one another, with 
useful accuracy. Occupancy is the measure of the presence of a vehicle in 
a lane, whether moving or stationery. Present wire loop detectors are poor 
for accurately detecting vehicles below a certain speed thereby not being 
always able to detect traffic that has come to a standstill. Further, wire 
loops also are incapable of providing information about the type of 
vehicle passing over the loop since the measurement coil cannot resolve 
the vehicle features, especially if detection signals have relatively low 
signal-to-noise ratio characteristics. 
SUMMARY OF THE INVENTION 
The invention involves placing one or more magnetometers, particularly 
magnetoresistive detectors, in each lane of a roadway or highway. These 
detectors are laid in a standard saw cut groove in the highway or may be 
inserted under the highway through a tube installed across the road bed 
under the pavement. The magnetoresistive transducer is advantageous in 
view of other magnetometer approaches. The magnetoresistive sensor is a 
permalloy magnetometer which is small and can be made to fit within a 
standard-width pavement saw-cut. Multiple permalloy magnetometers can be 
fabricated on one cable and spaced at pre-measured separations for 
measuring particular kinds of parameters of vehicles. The permalloy 
magnetoresistive sensor is a solid-state sensor. It can be produced at 
very low cost. Unlike some related-art fluxgate magnetometers, the 
transducer support electronics of the present magnetoresistive sensor is 
packaged within the magnetometer unit; and wire loops have added loss of 
sensitivity as multiple loops are added on the same cable in an 
installation. 
The advantages and features of a magnetometer in contrast to a wire loop 
detector are numerous. A magnetometer can be functional on bridge decks 
having steel present and where cutting of the deck pavement for a loop is 
not permitted. The magnetometer survives better in crumbly pavements for a 
longer period of time than an ordinary wire. A magnetometer requires fewer 
pavement cuts and significantly fewer linear feet of cut for roadway 
installation. The magnetometers have much higher sensitivity (i.e, they 
can detect bicycles) than a wire loop sensor. Such higher sensitivity 
provides for a high signal-to-noise ratio thereby resulting in the 
collection of more accurate data. A magnetometer can separately detect two 
vehicles spaced only about a foot apart. Also, motion of the vehicle is 
not required for an magnetometer to accurately sense the vehicle. With 
shallow placement of a magnetometer, identification of vehicles according 
to types or models can be attained from the different magnetic signatures 
that occur as major components of a vehicle pass over the magnetometer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates the typical roadway 12 installation for loop detectors 
14. Loop 14 requires four pavement sawcuts of at least four feet long and 
four corner pavement sawcuts of about one foot long each in order to 
accommodate the laying down of the wire coil for sensor 14. Also, there is 
required a long pavement cut 16 from the edge of roadway 12 to loop 14 of 
which for interlane groups the pavement cut may cross one or more other 
lanes of roadway 12. 
FIG. 2 shows an installation of one configuration of the present invention 
10 on roadway 12. One magnetoresistive (MR) sensor 10 is installed per 
lane. MR sensor 10 is connected to the edge of the roadway via a sawcut 
slot 16 with a connection wire to the hand hole 18 for each of the lanes 
21, 22 and 23. The lines from the sensors go from hand holes 18 to 
controller 20 which may be a '386 Dell computer acquisition system which 
is a standard model 170 controller/emulator. From system 20 the line goes 
onto a traffic management center. 
FIG. 3 is a closer view of the installation of an MR sensor 10 embedded in 
roadway 12. A standard diamond saw cut slot 16 in roadway 12 is about 3/4 
to 1 inch deep and 3/8 inch wide. This is sufficient for inserting MR 
detector 10 which is about 1/4 inch wide, 5/16 inch deep and 2 inches 
long. Once detector 10 and its corresponding connection leads 24 are 
inserted in slot 16, then slot 16 is filled in with an epoxy filler or 
other suitable material. Sensor 10 is physically quite small, especially 
if an open-loop magnetometer approach 80 of FIG. 14 is used. A single-axis 
sensor 10, oriented in the vertical direction to intercept the maximum 
component of earth's field, provides good vehicle signatures. The length 
of cable 24 is not critical. Sensors 10 can withstand the full range of 
weather conditions, including temperature extremes, water, and various 
chemicals. 
FIGS. 4a-c illustrates package types of the MR sensor 10. The package of 
the sensor 10 is designed so that the sensor fits only in a vertical 
position, the most sensitive axis is situated in the direction of the 
vehicles to be detected, in a standard sawcut 16 of roadway 16 that sensor 
10 is to be embedded in. FIG. 4a shows the packages for an in-line MR 
sensor 10 and FIG. 4b shows an end-unit MR sensor 10. FIG. 4c indicates 
the arrangement of the contents in MR sensor 10. Shown in sensor 10 of 
FIG. 4c are permalloy magnetic sensor 26 and integrated circuits 28. 
FIG. 4c reveals a single permalloy transducer 26, with signal-conditioning 
and data-communication electronics 26 on a small, narrow printed wiring 
board 29. Board 29 is attached and sealed to cable 24 with epoxy, 
neoprene, polyurethane or other suitable potting material. Multi-wire 
cable 24 provides both power and signal paths for sensors 10. Sensor 10, 
mounted on the cable, is small enough to fit within the standard 3/8 inch 
wide slot as shown in FIG. 5. For each of lanes 21, 22 and 23, three of 
these sensors 10 are strung along the same cable 24 and share common power 
lines. Sensors 10 are be spaced a few feet apart to generate the 
time-delayed signatures needed to determine the vehicle length and speed. 
FIG. 5 illustrates another installation approach which employs a standard 
schedule 40 or custom extruded PVC tube 30 installed across roadway 12. 
Tube 30 has internal diametrical guide slots 36 to carry and maintain the 
position of detector boats 31, 32 and 33 in a vertical position relative 
to the horizontal surface of roadway 12. Extruded PVC (plastic) pipe 30 
may be pre-installed during a pavement pour of the highway. Sensors 10 may 
be installed later. In an existing roadway 12, wide slots may be cut and 
the pipe or tube may be dropped into slot 16 and covered with an epoxy, 
concrete or other filler. The advantage of this kind of installation is 
that MR sensors 10 may be removed from tube 30 at the edge of roadway 12 
to perform maintenance or add more MR sensors 10. Sensors 10 are situated 
on lane boats 31, 32 and 33 which are to be positioned under lanes 21, 22, 
and 23, respectively. The lane boats are connected with 1/16th inch 
stainless steel cable for detector 10 boat 31, 32 or 33 entry or 
withdrawal from tube 30. Boats 31, 32 and 33 slide into tube 30 along 
guiding slots 36. Connected to respective sensors 10 are detector leads 24 
for conveyance of signals and power. When tube 30 is laid on a concrete 
roadway 12 bed it may be tied down with nylon tie 38 to a reinforcement 
bar 40 to prevent float of tube 30 during the fill of roadway 12 with 
concrete or other substance. 
FIG. 6 reveals the sensor layout for roadway 12 wherein multiple sensors 10 
exist for each of lanes 21, 22 and 23 of roadway 12. At most, each lane 
requires two slots 16 and 42. Slot 16 provides a way for sensor lead 24 
from hand hole 18 to slot 42 which incorporates three sensors 10 in a line 
parallel to its respective lane 21, 22 or 23. Each of all the lanes have 
three sensors. However, multiple sensors 10 for each lane may instead 
incorporate two or four or more MR sensors 10. Multiple sensors for each 
lane can provide extensive traffic information such as vehicle length, 
speed and headway. The sensitive axes of sensors 10 are aligned in the 
vertical direction or a direction perpendicular to the surface of roadway 
12. Sensors in slot 42 are spaced at specific distances (e.g., 1 to 5 
yards) apart so as to generate the time-delayed signatures sufficient to 
determine vehicle length and speed. As a vehicle passes over each MR 
sensor, it generates a signal "shadow". With all of sensors 10 in slot 42 
for a given lane, 21, 22 or 23, connected to a data station 20 via sensor 
leads 24 along slots 16 and through hand holes 18 onto data station 20, a 
signal processor uses a threshold level to differentiate between vehicles 
in the lane of the monitored sensors 10 and the vehicles in the other 
lanes and to minimize the likelihood of "false alarms". 
FIG. 7 shows the three magnetic components Bx, By, and Bz which are 
labelled 86, 87 and 88, respectively, of a truck passing a three-axis 
magnetometer 10 from a distance of greater than 50 feet from roadway 12. 
Signatures 86, 87 and 88 are similar in shape, but are much larger in 
amplitude and detail when a magnetometer is placed within roadbed 12. For 
this application, where the size and cost of sensor 10 are a high 
priority, using only the z-axis signal 88 (Bz) provides high-integrity 
information to identify vehicle count, speed, headway, occupancy, and 
types of vehicles. 
FIG. 8 shows an example of vehicle signatures from a linear array of single 
axis MR sensors in slot 42 for a given lane. Time period T.sub.1 may be 
used to determine the speed of a vehicle passing over sensors 10, since 
the sensors 10 spacing is known. Vehicle speed may be confirmed and made 
more accurate by repeating the measurement of time T.sub.4 between 
"shadows" 46 and 48. The time differential between shadows 46 and 48 
should be approximately the same as the time differential between shadows 
44 and 46. 
Time period T.sub.2 in FIG. 8 may be used to determine the headway between 
vehicles, and since there are multiple signatures, the headway measurement 
may be corroborated. Vehicle count and roadway occupancy by vehicles can 
be tabulated versus time by using a real-time clock 91 in system 
controller 20 of FIG. 11. Computer 20 may accumulate data for a fixed 
period of time and then the data, at the computer operator's convenience, 
may be transferred, already tabulated in a "demographic" data format to a 
remote station 92 via a telephone-modem link. 
The width of each of the signature shadows, 44, 46, 48, 54, 56 and 58, 
correlate directly with vehicle size or length. It is evident that shadows 
44, 46 and 48 reveal a vehicle length or size substantially shorter than 
that of shadows 54, 56 and 58. The shadows themselves can reveal an 
identification of particular vehicles since major components such as an 
engine, transmission and axles of a passing vehicle may reveal distinct 
signatures, depending on the amount of sensitivity, the amount ferrous 
metal present in the vehicle and the proximity of sensor 10. For instance, 
shadows 54, 56 and 58 have an indentation 52 which may represent space 
between two axles of a large vehicle passing over each of sensors 10. With 
a particular kind of magnetometers, it is possible to differentiate even 
between different types of trucks or other vehicles. T.sub.3 is the signal 
period that represents the length of a vehicle. To get more detailed 
information, MR sensor 10 functions as a "point" sensor in that it 
generates a signal based on the magnetic field properties in a very 
localized region above sensor 10. 
The algorithms of micro-computer 90 are adaptive to account for variations 
in the detected signatures due to various detector positions and kinds of 
installations. For example, the signature of a vehicle going north-south 
varies from its signature when the vehicle is going east-west. The 
software accounts for these differences without having to retrain the 
system for each sensor 10 installation. Typically, a vehicle's signal is 
well above the sensor's electrical noise. The coupling of the signature of 
a vehicle into the next lane sensor is every small, as shown in FIGS. 9a-e 
and 10a-l, so inter-lane cross-coupling is not a problem. 
FIGS. 9a-e show representative sensor 10 signals caused by a five ton cargo 
truck traveling thirty miles per hours it passes over or near sensor 10. 
The front of the truck is to the left and the end of the truck is to the 
right. Curve 93 of FIG. 9a reveals the center of a truck passing over 
sensor 10. Curve 93 is a clear signature of the front axle and engine and 
then the undercarriage support. Curve 94 of FIG. 9b involves sensor 10 
halfway between the truck center and the tire track. Curve 94 reveals 
almost no signal before or after the truck. Curve 95 of FIG. 9c is when 
the truck tires are passing over sensor 10. Curve 95 shows a clear 
signature of the front axle, the engine and the tandem axle. Curve 96 of 
FIG. 9d involves the truck tire track passing 1.5 feet away from sensor 
10. Curve 96 can provide an estimate of the side position of the truck 
within the lane. Curve 97 of FIG. 9e shows the truck passing sensor 10 
with the outside tire track three feet from sensor 10. Curve 97 indicates 
almost no signature detected in the traffic lane next to the lane of the 
truck. 
FIGS. 10a-l show representative signatures for various vehicles travelling 
30 miles per hour. The front of the respective vehicles is to the left and 
the end of the vehicles is to the right. A vertical scale of one gamma 
equal 10.sup.-5 gauss for each signature is shown. Curve 98 of FIG. 10a is 
a signature of a VOLKSWAGEN having a rear-mounted engine, passing directly 
over sensor 10. Curve 99 of FIG. 10b is the signature from sensor 10 in a 
lane adjacent to the lane of the VOLKSWAGEN. Curve 100 of FIG. 10c is a 
signature of a VEGA station wagon having a front-mounted engine, passing 
directly over sensor 10. Curve 101 of FIG. 10d is the signature from 
sensor 10 in a lane adjacent to the lane of the VEGA. Curve 102 of FIG. 
10e is a signature of a four-door FORD sedan passing directly over sensor 
10. Curve 102 shows the engine in front followed by an undercarriage 
structure. FIG. 10f reveals signature 103 from sensor 10 in a lane 
adjacent to the lane of the FORD. Signature 104 of FIG. 10g is of a 
motorcycle. FIG. 10h shows signature 105 from sensor 10 in a lane adjacent 
to the lane of the motorcycle. FIG. 10i shows signature 106 of an 
eighteen-wheel semi-truck. Signature shows an engine in front followed by 
two main axle assemblies of the trailer. Signature 107 of FIG. 10j is from 
sensor 10 in a lane adjacent to the lane of the semi-truck. Signature 108 
in FIG. 10k is of a city passenger bus having an engine in the rear and 
two axles. FIG. 101 shows signature 109 from a sensor in a lane adjacent 
to the bus. 
Once the class of a vehicle is determined, the velocity, headway, and even 
the acceleration profile is determined by matching signatures from sensors 
10 placed along the lane. The acceleration profile coupled with the 
terrain (i.e., going uphill, downhill, etc.) gives an indication of the 
load on the detected vehicle. Signature detection and analyses can provide 
various kinds of information about the detected traffic. 
FIG. 11 is a block diagram of controller 20 and remote control/data station 
92. Controller 20 has inputs from sensor 10 to multiplexer 110. The sensor 
signals are multiplexed into one signal line to an analog-to-digital 
converter 111 for digitizing the signals for inputting into micro-computer 
90 to be time-tagged and processed. Real-time clock 91 provides the timing 
basis for computer 90. The processed outputs of computer 90 include 
vehicles counts 112, vehicle type classifications 113, speed distributions 
114, and vehicle spacings 115. Other parameter determinations may be 
processed. The outputs of computer 90 may go through a modem 116 in a 
parallel or serial format to be sent on to remote control/data station 92. 
Power supply 117 provides voltages to the sensor power bus. 
FIG. 12 shows the operations performed on the sensor 10 signals by 
micro-computer 90. Incoming signals 118 are digitized and time tagged. 
Signals 118 go to processing block 119 that determines the times (T1) 
between signal peaks 44 and 46 of the signals as illustrated in FIG. 8. 
Block 120 averages the T1's for a number of sensors 10. Then the vehicle 
speeds are determined by block 121 in accordance with sensor spacing/T1. 
Then the vehicle speeds may be averaged by processing block 122. Incoming 
signals 118 are also processed by block 123 which measures the times (T2) 
between signature groups 44, 46, 48 and 54, 56, 58, respectively, as 
illustrated in FIG. 8. Block 124 determines vehicle spacings by 
multiplying the vehicle speed or sensor spacing/T1 from block 121 by T2 
from block 123 to obtain a vehicle spacing determination. The vehicle 
spacings from block 124 may be averaged by processing block 125. Block 126 
provides predetermined signal threshold values which are compared with 
incoming signals from block 118 by block 127 to determine T3 values as 
illustrated in FIG. 8. The T3 values are averaged by block 128. The 
averaged T3 values are sent on to processing block 129 for sorting into 
vehicle types and determining the numbers of each type. Block 130 
categorizes the vehicle types in various fashions in accordance of the 
kind of information that is desired. For instance, the T3 information may 
be categorized with small T3's representing motorcycles, medium T3's 
representing automobiles, and large T3's representing trucks. The digital 
information of average vehicle speeds from block 122, average vehicle 
spacings from block 125 and vehicle categorizations from block 130 may 
processed into parallel or serial format by block 131 for sending to modem 
116 for transmission to control center or control/data station 92. 
FIG. 13 is a schematic of an example of a magnetoresistive sensor 10. 
Permalloy magnetoresistive sensing bridge 50 detects magnetic signals or 
field variations of a vehicle in the vicinity of sensor 50. Reset field 
coil 60, though not necessary, resets the magnetization of sensing bridge 
50 to its easy axis direction. The switching of the magnetization of 
sensing bridge 50 is back and forth from 0 to 180 degrees with respect to 
the easy axis, so that sensor 50 output will be insensitive to thermal 
drifts and to offsets of bridge 50 in large magnetic fields. The output 
signals from bridge 50 due to vehicle magnetic signals 62, are enhanced by 
amplifier 64. The signals from amplifier 64 are integrated by integrator 
66. Although sensor 10 can be an open loop system, Integrator 66 has an 
output that may be fed back through feedback coil 68 and through 
integrating capacitor 70 to the input of electronic integrator 66. A 
magnetic feedback from feedback coil is fed back to bridge 50. This 
magnetic feedback allows the output of sensing bridge 50 in a closed loop 
fashion. The closed loop configuration reduces cross-axis sensitivity and 
non-linearity, relative to magnetic signal 62, of the output of sensing 
bridge 50. Resistor 72 provides a load to integrator 66 output. Resistor 
72 provides a particular scale factor in the coil-current-to-voltage 
conversion. The analog output of integrator 66 goes onto analog-to-digital 
(A/D) converter 74. The digital signal output of converter 74 goes to a 
data transceiver 76 which manages digital data that is sent onto the 
digital data bus of system 20. Power and timing circuit 78 conditions 
power from a system bus for all the circuits of sensor 10 and provides 
reset signals to coil 60 and timing signals to integrator 66, A-D 
converter 74 and data transceiver 76. 
FIG. 14 shows a basic magnetoresistive sensor 80 having magnetoresistive 
bridge 50 and differential amplifier 84. Sensor 50 may be a permalloy 
bridge is "barber pole" biased so that no external magnetic bias is 
required. Power regulator 82 provides the necessary DC voltages for sensor 
80, from an AC power bus from a roadside station. Sensor 80 is more 
economical, though with the tradeoff of being less accurate, than sensor 
10 of FIG. 13. Trimmed-down versions of sensor 10 may be used, such with 
the absence of feedback coil 68 for open loop operation and/or the absence 
of the reset coil.