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BACKGROUND OF THE INVENTION
The present invention relates to mobile tracking units for a vehicle
tracking system and, more particularly, to mobile tracking units employing
motion sensors for reducing power consumption therein under predetermined
conditions.
A significant requirement for mobile tracking units used in vehicles which
generally lack a power source, such as freight railcars, shipping
containers and the like, is to have relatively low electrical power
consumption characteristics. Mobile tracking units used in a power-starved
environment must be substantially power efficient in order to provide
reliable and economical operation. Typically, the mobile tracking unit
includes a navigation set, such as a Global Positioning System (GPS)
receiver or other suitable navigation set, responsive to navigation
signals transmitted by a set of navigation stations which can be either
space- or earth-based. In each case, the navigation set is capable of
providing data indicative of the vehicle location based on the navigation
signals. In addition, the mobile tracking unit can include a suitable
electromagnetic emitter for transmitting to a remote location the vehicle
position data and other data acquired with sensing elements in the
vehicle. Since both the navigation set and the emitter are devices which,
when energized, generally consume a large portion of the overall
electrical power consumed by the mobile tracking unit, it is desirable to
control the respective rates at which such devices are respectively
activated so as to reduce the overall power consumption of the mobile
tracking unit. For presently available mobile tracking units, use of a
motion sensor, such as an accelerometer, has been suggested to detect
shock or impact events which the vehicle encounters during routine use.
However, no suggestion has been made of using the motion sensor for
controlling the respective rates at which the aforementioned devices are
activated so as to substantially reduce the overall power consumption of
the mobile tracking unit.
SUMMARY OF THE INVENTION
Generally speaking, the present invention fulfills the foregoing needs by
providing a mobile tracking unit capable of operating in a power-starved
environment and suitable for a vehicle tracking system. The tracking unit
includes a navigation set for generating data substantially corresponding
to a respective vehicle position. The navigation set is periodically
energized at a selected activation rate F.sub.G while the vehicle is
moving to generate the vehicle position data. An electromagnetic emitter
can be employed in the tracking unit for transmitting the vehicle position
data, in which case such emitter is periodically energized at a selected
activation rate F.sub.EM, while the vehicle is moving, to transmit at
least the vehicle position data. A motion sensor, such as an accelerometer
or vibration sensor, is employed for generating data indicative of vehicle
motion. A tracking unit controller is coupled to the motion sensor to
receive the vehicle motion data and is designed to control the navigation
set and the emitter based upon the vehicle motion data so that during
times when the vehicle is substantially stationary, each of activation
rates F.sub.G and F.sub.EM can be respectively decreased by a
predetermined factor, thereby substantially reducing the overall power
consumption of the tracking unit. The tracking unit controller is further
designed to revert to activation rates F.sub.G and F.sub.EM upon an
indication from the motion sensor of renewed vehicle motion, thereby
avoiding any substantial loss of vehicle position data during times of
renewed vehicle motion.
A method of operating a mobile tracking unit for a vehicle location system
in accordance with the present invention can include the following steps:
providing a navigation set capable of generating data substantially
indicative of a respective vehicle position; periodically energizing the
navigation set at a selected activation rate F.sub.G while the vehicle is
moving to generate vehicle position data; providing an electromagnetic
emitter (if optionally needed) capable of transmitting the vehicle
position data; periodically energizing the electromagnetic emitter at a
selected rate F.sub.EM, while the vehicle is moving, to transmit at least
the vehicle position data; generating data indicative of vehicle motion
using a motion sensor; controlling the navigation set and the emitter
based upon the vehicle motion data so that during times when the vehicle
is substantially stationary, each of the activation rates F.sub.G and
F.sub.EM is respectively decreased by a predetermined factor, thereby
substantially reducing overall power consumption of tracking unit; and
controlling the navigation set and the emitter to revert to activation
rates F.sub.G and F.sub.EM upon an indication from the motion sensor of
renewed vehicle motion, thereby avoiding any substantial loss of vehicle
position data during times of renewed vehicle motion.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with
particularity in the appended claims. The invention itself, however, both
as to organization and method of operation, together with further objects
and advantages thereof, may best be understood by reference to the
following description in conjunction with the accompanying drawings in
which like numbers represent like pans throughout the drawings, and in
which:
FIG. 1 is a block diagram of an exemplary vehicle tracking system which can
employ a mobile tracking unit in accordance with the present invention;
FIG. 2 is a block diagram illustrating further details of the mobile
tracking unit shown in FIG. 1; and
FIG. 3 is a graph showing respective exemplary activation rates for
components of the mobile tracking unit wherein the activation rates are
selected to reduce the overall power consumption of the mobile tracking
unit in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides mobile tracking units capable of operating
in a power-starved environment. The mobile tracking units can be
conveniently employed for a vehicle tracking or monitoring system which at
least provides vehicle location information using navigation data derived
from an existing navigation system, such as the Global Positioning System
(GPS) satellite constellation, thereby providing highly accurate,
real-time, vehicle tracking capability. It will be appreciated that the
present invention is not limited to GPS navigation, being that vehicle
tracking systems that use other navigation systems such as Loran, Omega,
Transit and the like, or even satellite range measurement techniques (as
respectively described in U.S. Pat. Nos. 4,161,730 and 4,161,734, both by
R. E. Anderson, issued Jul. 17, 1979, both assigned to the present
assignee, and herein incorporated by reference) can advantageously benefit
from the use of a mobile tracking unit that employs a motion sensor in
order to reduce the power consumption of the tracking unit. The tracking
system is particularly useful in fleet vehicle management, railcar
tracking, cargo location and the like. As used herein the term "vehicle"
includes shipping containers and other such means of carrying or
transporting goods on board a motorized vehicle or platform such as ships,
aircrafts, land vehicles, or other vehicles.
FIG. 1 shows, by way of example and not of limitation, mobile tracking
units which employ navigation signals from a GPS satellite constellation,
although, as suggested above, other navigation systems can be used in lieu
of GPS. FIG. 1 shows a set of mobile tracking units 10A-10D which are
installed in respective vehicles 12A-12D which are to be tracked or
monitored. A multiple communication link 14, such as a satellite
communication link using a communication satellite 16, can be provided
between each mobile tracking unit (hereinafter collectively designated as
10) and a remote control station 18 manned by one or more operators and
having suitable display devices and the like for displaying location and
status information for each vehicle equipped with a respective mobile
tracking unit. A constellation of GPS satellites, such as GPS satellites
20A and 20B, provides highly accurate navigation signals which can be used
to determine vehicle position and velocity when acquired by a suitable GPS
receiver. Briefly, the GPS was developed by the U.S. Department of Defense
and gradually placed into service throughout the 1980's. The GPS
satellites constantly transmit radio signals in L-Band frequency using
spread spectrum frequency techniques. The transmitted radio signals carry
pseudo-random sequences which allow users to determine location relative
to the surface of the earth (within approximately 100 ft), velocity
(within about 0.1 MPH), and precise time information. GPS is a
particularly attractive navigation system to employ, being that the
respective orbits of the GPS satellites are chosen so as to provide
substantially world-wide coverage and being that such highly-accurate
radio signals are provided free of charge to users by the U.S. federal
government. Communication link 14 can be conveniently used for
transmitting vehicle conditions or events measured with suitable sensing
elements, as will be explained shortly hereafter.
FIG. 2 shows that mobile tracking unit 10 includes a navigation set 50
capable of generating data substantially corresponding to the vehicle
position. The navigation set is chosen depending on the particular
navigation system used for supplying navigation signals to a given mobile
tracking unit. Preferably, the navigation set is a GPS receiver such as a
multichannel receiver. However, it should be apparent that other receivers
designed for acquiring signals from a corresponding navigation system can
also be employed. For example, the navigation set, depending on the
vehicle position accuracy requirements, can be chosen as a Loran-C
receiver or other such less highly-accurate navigation receiver than a GPS
receiver. Further, as described in the foregoing incorporated by reference
U.S. patents, the navigation set can conveniently comprise a transceiver
that inherently provides two-way communication with the control station
and avoids the need for separately operating an additional component to
implement such two-way communication. Briefly, such transceiver would
allow for implementation of the previously mentioned satellite range
measurement techniques being that the position of the vehicle is simply
determined at the control station by range measurements to the vehicle and
the control station from two satellites whose position in space is known.
In each case, it will be appreciated by those skilled in the art that the
power consumed by the navigation set imposes a severe constraint for
reliable and economical operation of the mobile tracking unit in vehicles
which do not have power supplies like shipping containers, railcars used
for carrying cargo and the like. For example, typical GPS receivers
currently available generally consume as much as 2 watts of electrical
power. In order for the GPS receiver to provide a position fix, the GPS
receiver must be energized for a certain minimum period of time in order
for the receiver to acquire sufficient signal information from a given set
of GPS satellites so as to generate a navigation solution. A key advantage
of the present invention is the ability to substantially reduce the
overall power consumption of the mobile tracking unit by selectively
reducing the activation or usage rate for the navigation set and other
components of the mobile tracking unit. In particular, if, during times
when the vehicle is stationary, the activation rate for the navigation set
is reduced, then the overall power consumption of the mobile tracking unit
can be substantially reduced, for example, by a factor of at least about
100. Mobile tracking unit 10 may include a suitable electromagnetic
emitter 52 functionally independent from the navigation set. Emitter 52 is
shown in dashed lines to indicate that such functionally independent
component is only optional depending on the particular design
implementation for the tracking unit. As suggested above, if the
navigation set comprises a transceiver, then emitter 52 would be a
redundant component. Emitter 52 is capable of transmitting the vehicle
position data by way of communication link 14 (FIG. 1) to the control
station. By way of example and not of limitation, if a GPS receiver is
used, the GPS receiver and the emitter can be conveniently integrated as a
single integrated unit for maximizing efficiency of installation and
operation. An example of one such integrated unit is the commercially
available Galaxy Inmarsat-C/GPS integrated unit available from Trimble
Navigation, Sunnyvale, Calif. which is conveniently designed for data
communication and position reporting between the control station and the
mobile tracking unit. A single, low profile antenna 54 can be conveniently
used for both GPS signal acquisition and satellite communication.
Regardless of the type of navigation set employed, and consistent with a
key advantage of the present invention, a motion sensor 56, such as a low
power accelerometer, vibration sensor, acoustical sensor or combination
thereof, is coupled to a tracking unit controller 58 so as to supply to
controller 58 data indicative of vehicle motion, preferably along three
mutually orthogonal axes, i.e., the motion sensor is preferably a triaxial
accelerometer. A set of three accelerometers individually integrated with
suitable signal conditioning circuitry in a respective single monolithic
integrated circuit, such as accelerometer model ADXL50 available from
Analog Devices, Norwood, Mass. or similar accelerometers, can be
conveniently mounted in the vehicle or in the tracking unit to provide
such triaxial sensing. The tracking unit controller may comprise a
multi-bit single chip digital microcontroller suitably programmed, as will
be explained shortly hereafter, to control operation of navigation set 50
and emitter 52. A real-time clock module 60 can be connected to tracking
unit controller 58 so as to periodically enable the controller to resume
operation after the controller is in a "sleep-mode" associated with a low
power mode of operation. Preferably, tracking unit controller 58 includes
sufficient memory and throughput capability to process data acquired from
a suite of respective sensing elements 68. A power source such as battery
62 is used to enable operation of mobile tracking unit 10. As shown in
FIG. 2, battery 62 can be a rechargeable battery, such as a nickel-cadmium
battery or a similar rechargeable battery, coupled to a suitable charging
circuit 64 which receives electrical power from an array of solar cells 66
or other such electrical power transducer. The charging circuitry
typically includes suitable charging regulators and voltage and current
sensors (not shown) monitored by the controller for determining the
condition of the battery. A backup battery (not shown) can be conveniently
provided to enhance reliable operation of the mobile tracking unit.
Alternatively, battery 62 can be a nonrechargeable battery replaced at
preestablished time intervals. In each case, it will be appreciated that
it is advantageous to reduce the overall power consumption of mobile
tracking unit 10 so that, for example, in the case of a rechargeable
battery, the size and cost requirements for charging circuit 64 and array
of solar cells 66 can be conveniently reduced. Similarly, in the case of a
nonrechargeable battery, reducing the power consumption of mobile tracking
unit 10 advantageously allows for extending the useful life of the battery
and to reduce maintenance costs associated with the mobile tracking unit.
As suggested above, the navigation set in order to provide the navigation
solution required for determining vehicle location and velocity must be
energized for a sufficient period of time to acquire the navigation
signals which in the case of GPS are transmitted from any available set of
GPS satellites. For example, depending on various initial conditions, such
as availability of satellite ephemeris and almanac data, the GPS receiver
may require to be energized for at least about 1 or 2 minutes in order to
generate data for establishing a position fix. Similarly, the emitter must
be energized for a respective period of time needed to transmit data
associated with the vehicle. In each case, it will be apparent that it is
advantageous to be able to selectively reduce the activation rate for the
navigation set and (if a functionally independent emitter is used) the
activation rate for the emitter depending on vehicle motion data generated
with the motion sensor. As used herein the expression "activation rate"
refers to the rate or frequency of use for a particular component in the
mobile tracking unit. FIG. 2 illustrates in block diagram from respective
exemplary sensing elements 68A-68D coupled to tracking unit controller 58
and used for measuring predetermined respective conditions associated with
a given vehicle. Temperature sensing element 68A provides an economical
way of measuring temperature in the vehicle. Pressure sensing element 68B
can be used in tanker vehicles to measure the weight of the cargo. Strain
gauge 68C can be used for measuring tensile or compressive forces in the
vehicle which can provide particularly useful data in an accident
postmortem investigation. Limit switch 68D can be used for actuation in
response to a suitable mechanical input such as can be obtained from
closing or opening doors. In each case, the data acquired with such
sensing elements (collectively designated as suite of sensing elements 68
in FIG. 2) can be stored in tracking unit controller 58 and transmitted
via communication link 14 together with the vehicle location data. Thus,
it should be appreciated that the mobile tracking unit in accordance with
the present invention conveniently provides enhanced useful data to users.
In FIG. 3, each rectangular block represents a period of time during which
navigation set 50 (FIG. 2) is energized to generate vehicle position data.
Similarly, each vertical arrow corresponds to respective times when
emitter 52 (FIG. 2) is energized to transmit vehicle position data and
other data associated with the vehicle if desired. It will be appreciated
that during times while the vehicle is moving, activation rates F.sub.G
and F.sub.EM can be respectively selected by tracking unit controller 58
(FIG. 2) in order to periodically energize the navigation set and the
emitter, respectively. By way of example and not of limitation, in the
case of a railcar vehicle using a GPS receiver, four GPS receiver
activations per hour sufficient to generate four respective position fixes
per hour may be adequate to substantially track the location of the
railcar vehicle. Again, in the case of a railcar vehicle, one emitter
activation per hour may be sufficient to transmit data acquired by the
mobile tracking unit within the last hour so as to adequately report the
vehicle position data (i.e., the last four position fixes) and other data
acquired with suite of sensing elements 68. FIG. 3 further illustrates, in
accordance with a key advantage of the present invention, that when the
vehicle motion data from the motion sensor indicates that the vehicle is
substantially stationary, the tracking unit controller can advantageously
decrease each of the respective activation rates (F.sub.G and F.sub.EM)
for the GPS receiver and the emitter by a predetermined respective factor
whose value can be conveniently chosen depending on the specific
application. Various schemes are possible, for example, the activation
rates may be gradually decreased so as to provide a gradual transition
from activation rates F.sub.G and F.sub.EM to the decreased activation
rates. In each case, such decreased activation rates individually
contribute to substantially reducing the overall power consumption of the
mobile tracking unit being that, in the case of railroad transportation
for example, the average railcar in North America is stationary for
approximately at least 90% of the time. Preferably, the navigation set and
emitter are not completely deactivated even when the vehicle is stationary
for a long period of time so as to provide increased reliability of
operation. For example, this would prevent an erroneous determination that
the vehicle is stationary when in fact the vehicle is moving, in case of a
failure associated with the motion sensor. FIG. 3 also illustrates that
upon the motion sensor indicating renewed vehicle motion, the tracking
controller can be programmed to control respective operation of the
navigation set and emitter so as to revert to activation rates F.sub.G and
F.sub.EM, thus avoiding any substantial loss of vehicle position data
during times of renewed vehicle motion.
A method of operating a mobile tracking unit in accordance with the present
invention can include the steps of providing a navigation set 50 (FIG. 2)
capable of generating data substantially corresponding to a respective
vehicle position; periodically energizing the navigation set at a selected
activation rate F.sub.G while the vehicle is moving to generate the
vehicle position data; providing, if needed, an electromagnetic emitter 52
(FIG. 2) capable of transmitting the vehicle position data; periodically
energizing the electromagnetic emitter at a selected rate F.sub.EM, while
the vehicle is moving, to transmit at least the vehicle position data;
generating data indicative of vehicle motion using a motion sensor such as
a triaxial accelerometer or vibration sensor; controlling the navigation
set and the emitter based upon the vehicle motion data so that during
times when the vehicle is substantially stationary each of the activation
rates F.sub.G and F.sub.EM can be respectively decreased by a
predetermined factor, thereby substantially reducing overall power
consumption of the tracking unit; and controlling the navigation set and
the emitter to revert to activation rates F.sub.G and F.sub.EM upon the
vehicle motion data from the motion sensor indicating renewed vehicle
motion, thereby avoiding any substantial loss of vehicle position data
during times of renewed vehicle motion.
While only certain features of the invention have been illustrated and
described herein, many modifications, substitutions, changes, and
equivalents will now occur to those skilled in the art. It is, therefore,
to be understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the invention.
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