U.S. patent application number 11/267775 was filed with the patent office on 2007-05-24 for tire pressure sensor system with improved sensitivity and power saving.
This patent application is currently assigned to Silicon Valley Micro C Corporation. Invention is credited to Su Shiong Huang, Shengbo Zhu.
Application Number | 20070113636 11/267775 |
Document ID | / |
Family ID | 38052171 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070113636 |
Kind Code |
A1 |
Huang; Su Shiong ; et
al. |
May 24, 2007 |
TIRE PRESSURE SENSOR SYSTEM WITH IMPROVED SENSITIVITY AND POWER
SAVING
Abstract
A battery powered tire pressure sensor system with a high
sensitivity stretch sensor assembly having a variable resistance
longitudinal displacement characteristic. The stretch sensor
assembly has at least two juxtaposed stretch sensors, each with a
first layer bearing the variable resistance element and a second
support layer. The sensor assembly is mounted on or in the side
wall of a pneumatic tire so that the assembly is displaced by the
tire side wall and the resistance is a function of internal tire
pressure. The assembly is coupled to a processor which samples the
resistance of the stretch sensor assembly periodically. When the
processor determines that the pressure is outside a safe range, an
r.f. generator is activated by the processor to generate an unsafe
tire pressure signal. This signal is converted by a receiver to a
warning for the driver. A power saving unit controls application of
electrical power to the system as a function of tire speed to
prolong battery life.
Inventors: |
Huang; Su Shiong; (Bellevue,
WA) ; Zhu; Shengbo; (San Jose, CA) |
Correspondence
Address: |
Warren P. Kujawa
461 Indigo Springs St.
Henderson
NV
89014
US
|
Assignee: |
Silicon Valley Micro C
Corporation
San Jose
CA
|
Family ID: |
38052171 |
Appl. No.: |
11/267775 |
Filed: |
November 4, 2005 |
Current U.S.
Class: |
73/146 |
Current CPC
Class: |
B60C 23/06 20130101 |
Class at
Publication: |
073/146 |
International
Class: |
G01M 17/02 20060101
G01M017/02 |
Claims
1. In a tire pressure sensor system having a variable resistance
displacement sensor for providing resistance values indicative of
internal tire pressure when coupled to a pneumatic tire; a
processor coupled to said displacement sensor element for
converting resistance values corresponding to inadequate internal
tire pressure to r.f. generator activation signals; and an r.f.
generator circuit for transmitting an unsafe tire pressure warning
signal when activated by said processor; the improvement wherein
said variable resistance displacement sensor comprises a variable
resistance sensor assembly having first and second individual
stretch sensors, each said stretch sensor having a first flexible
layer containing a variable resistance element and a second
flexible support layer, said individual stretch sensors being
arranged with said first flexible layer of said first stretch
sensor in facing relation with said first flexible layer of said
second stretch sensor.
2. The invention of claim 1 further including an electrical bridge
circuit having four branches, a first pair of said branches having
fixed resistance elements connected in series, a second pair of
said branches having said variable resistance elements of said
first and second stretch sensors connected in series.
3. The invention of claim 1 wherein said variable resistance sensor
assembly further includes third and fourth individual stretch
sensors, each of said third and fourth stretch sensors having a
first flexible layer containing a variable resistance element and a
second flexible support layer, said third and fourth individual
stretch sensors being mutually arranged with said second flexible
support layer of said third stretch sensor in facing relation with
said second flexible support layer of said fourth stretch sensor,
said first flexible layer of said third stretch sensor being
arranged in facing relation with said second flexible support layer
of said second stretch sensor.
4. The invention of claim 3 further including an electrical bridge
circuit having four branches, a first one of said branches having
said variable resistance element of said first stretch sensor, a
second one of said branches having said variable resistance element
of said fourth stretch sensor, a third one of said branches having
said variable resistance element of said second stretch sensor, and
a fourth one of said branches having said variable resistance
element of said third stretch sensor, said first and second
branches being connected in series, said third and fourth branches
being connected in series.
5. The invention of claim 1 wherein said processor, said r.f.
generator circuit, and said variable resistance sensor assembly are
mounted on a support substrate having a flexible portion underlying
said variable resistance sensor assembly.
6. The invention of claim 1 wherein said improvement further
includes a power saving unit for limiting the application of
electrical power to said variable resistance sensor assembly, said
power saving unit including an input terminal adapted to be coupled
to a source of electrical power, an output terminal for supplying
electrical power to said variable resistance sensor assembly, and a
vehicle speed sensitive switch for connecting said input terminal
to said output terminal when said tire attains a first
predetermined speed and for disconnecting said input terminal from
said output terminal when the speed of said tire drops below said
first predetermined speed.
7. The invention of claim 6 wherein said switch comprises an
electrically conductive contact member having a first portion
connected to said output terminal and a free end, and an
electrically conductive pivot member having a first portion
connected to said input terminal and a mass member mounted on a
free end, said mass member being mounted to make physical contact
with said free end of said contact member when said tire attains
said first predetermined speed.
8. The invention of claim 7 wherein said mass member has opposing
ends; and wherein said switch includes first and second contact
members connected to said output terminal, said first contact
member having a free end located in the path of one of said
opposing ends, said second contact member having a free end located
in the path of the other one of said opposing ends.
9. The invention of claim 6 wherein said power saving unit further
includes a control signal output terminal coupled to said
processor; and wherein said vehicle speed sensitive switch includes
control signal means for connecting said input terminal to said
control signal output terminal when said tire attains a second
predetermined speed different from said first predetermined speed
and for disconnecting said input terminal from said control signal
output terminal when the speed of said tire drops below said second
predetermined speed.
10. The invention of claim 9 wherein said switch comprises an
electrically conductive contact member having a first portion
connected to said output terminal and a free end, and an
electrically conductive pivot member having a first portion
connected to said input terminal and a mass member mounted on a
free end, said mass member being mounted to make physical contact
with said free end of said contact member when said tire attains
said first predetermined speed; and wherein said control signal
means comprises a second contact member having a first portion
connected to said control signal output terminal and a free end,
said mass member being mounted to make physical contact with said
free end of said contact member when said tire attains said second
predetermined speed.
11. The invention of claim 10 wherein said mass member has opposing
ends; and wherein said control signal means includes first and
second contact members connected to said control signal output
terminal, said first contact member having a free end located in
the path of one of said opposing ends, said second contact member
having a free end located in the path of the other one of said
opposing ends.
12. For use in a tire pressure sensor system having a variable
resistance sensor for providing resistance values indicative of
internal tire pressure when coupled to a pneumatic tire; a
processor coupled to said sensor for converting resistance values
corresponding to inadequate internal tire pressure to r.f.
generator activation signals; and an r.f. generator circuit for
transmitting an unsafe tire pressure warning signal when activated
by said processor; a power saving unit for limiting the application
of electrical power to said variable resistance sensor, said power
saving unit including an input terminal adapted to be coupled to a
source of electrical power, an output terminal for supplying
electrical power to said variable resistance sensor, and a vehicle
speed sensitive switch for connecting said input terminal to said
output terminal when said tire attains a first predetermined speed
and for disconnecting said input terminal from said output terminal
when the speed of said tire drops below said first predetermined
speed.
13. The invention of claim 12 wherein said switch comprises an
electrically conductive contact member having a first portion
connected to said output terminal and a free end, and an
electrically conductive pivot member having a first portion
connected to said input terminal and a mass member mounted on a
free end, said mass member being mounted to make physical contact
with said free end of said contact member when said tire attains
said first predetermined speed.
14. The invention of claim 13 wherein said mass member has opposing
ends; and wherein said switch includes first and second contact
members connected to said output terminal, said first contact
member having a free end located in the path of one of said
opposing ends, said second contact member having a free end located
in the path of the other one of said opposing ends.
15. The invention of claim 12 wherein said power saving unit
further includes a control signal output terminal coupled to said
processor; and wherein said vehicle speed sensitive switch includes
control signal means for connecting said input terminal to said
control signal output terminal when said tire attains a second
predetermined speed different from said first predetermined speed
and for disconnecting said input terminal from said control signal
output terminal when the speed of said tire drops below said second
predetermined speed.
16. The invention of claim 15 wherein said switch comprises an
electrically conductive contact member having a first portion
connected to said output terminal and a free end, and an
electrically conductive pivot member having a first portion
connected to said input terminal and a mass member mounted on a
free end, said mass member being mounted to make physical contact
with said free end of said contact member when said tire attains
said first predetermined speed; and wherein said control signal
means comprises a contact member having a first portion connected
to said control signal output terminal and a free end, said mass
member being mounted to make physical contact with said free end of
said contact member when said tire attains said second
predetermined speed.
17. The invention of claim 16 wherein said mass member has opposing
ends; and wherein said control signal means includes first and
second contact members connected to said control signal output
terminal, said first contact member having a free end located in
the path of one of said opposing ends, said second contact member
having a free end located in the path of the other one of said
opposing ends.
18. A method of reducing power consumption in an electrically
powered tire pressure sensor system having a variable resistance
sensor for providing resistance values indicative of internal tire
pressure when coupled to a pneumatic tire, a processor coupled to
the sensor for converting resistance values corresponding to
inadequate tire pressure to r.f. generator activation signals, and
an r.f. generator circuit for transmitting an unsafe tire pressure
warning signal when activated by the processor, said method
comprising the steps of: (c) providing a source of electrical
power; and (d) applying the electrical power to the variable
resistance sensor for a tire pressure measurement period whose
duration is related to tire speed.
19. The method of claim 18 wherein said step (b) of applying
includes the steps of (i) preventing the application of electrical
power to the variable resistance sensor until the tire speed
reaches a first tire speed threshold, (ii) furnishing electrical
power to the variable resistance sensor for a measurement period
related to the period of time required for a preselected number of
tire revolutions at the first tire speed threshold when the tire
speed reaches the first tire speed threshold, and (iii) terminating
the application of electrical power to the variable resistance
sensor when the tire speed falls below the first tire speed
threshold.
20. The method of claim 19 wherein said step (b) of applying
further includes the step of changing the length of the measurement
period to a different value when the tire speed reaches a second
tire speed threshold, the different value being related to the
period of time required for a preselected number of tire
revolutions at the second tire speed threshold.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to automotive tire pressure sensors.
More particularly, this invention relates to a battery powered
method and system for monitoring internal tire pressure of vehicle
tires using a sensor system with improved sensitivity and a power
saving device.
[0002] Tire pressure sensor systems are known and are commonly used
to monitor the internal air pressure in individual pneumatic tires
of a vehicle and to provide a warning signal to the driver whenever
the internal air pressure in one or more of the vehicle tires is
dangerously low or high. The warning signal is typically generated
by an r.f. signal generator controlled by a microprocessor
connected to the tire pressure sensor, the warning signal being
generated whenever the internal tire pressure measured by the
sensor lies outside a predetermined normal operating range,
signifying either a high or a low pressure condition. This r.f.
signal is transmitted to a vehicle-mounted receiver, which uses the
warning signal to alert the driver either visually (by activating a
warning lamp or display) or audibly (by activating an audible
alarm) or both. Electrical power to the sensor circuitry is
provided by a battery, which must be replaced when the available
power drops below a useful level.
[0003] Known tire pressure systems, such as that disclosed in
commonly assigned, co-pending patent application Ser. No.
10/346,490 filed Jan. 21, 2003 for "External Mount Tire Pressure
Sensor System", the disclosure of which is hereby incorporated by
reference, use a mechanical strain sensor having an essentially
linear variable resistance characteristic in one branch of an
electrical bridge circuit to measure the internal pressure of a
tire to which the sensor is attached. This type of sensor is
relatively insensitive to mechanical vibrations, which are
regularly encountered in an automotive environment. In addition,
the configuration of the electrical circuitry (i.e., the electrical
bridge circuit) is relatively simple, has well-known performance
characteristics, and has been found to be reasonably reliable in
operation.
[0004] In spite of the effectiveness of the known sensor circuitry
using the strain sensor and bridge circuit, there are inherent
limitations which limit the performance of such devices. Firstly,
due to the fact that only a single variable resistance element (the
strain gauge) is incorporated into one branch of the bridge
circuit, the sensitivity of the sensor circuit is limited to the
variable resistance range of the single strain gauge used. This
limits the potential measurement range of the sensor system. In
addition, the known sensor circuitry is susceptible to measurement
inaccuracies due to different coefficients of thermal resistivity
of the variable resistance strain sensor and the fixed resistances
forming the bridge circuit. Secondly, since the sensor circuitry is
continuously powered by the essential battery, the useful lifetime
of the battery is limited by the battery energy capacity. This
drawback is compounded by the need for components having relatively
small physical size due to installation constraints. As a
consequence, battery replacement is a major constraint to the
efficacy of such known sensor systems.
[0005] Efforts to provide a simple yet accurate and durable tire
pressure monitoring system devoid of the above-noted disadvantages
have not been successful to date.
SUMMARY OF THE INVENTION
[0006] The invention comprises a method and system for monitoring
internal vehicle tire pressure employing a variable resistance
sensor assembly having greater sensitivity than known devices and
more tolerant of temperature fluctuations; and a power saving unit
providing extended useful battery life.
[0007] From a first apparatus aspect, the invention comprises an
improvement for a tire pressure sensor system having a variable
resistance displacement sensor for providing resistance values
indicative of internal tire pressure when coupled to a pneumatic
tire; a processor coupled to the displacement sensor element for
converting resistance values corresponding to inadequate internal
tire pressure to r.f. generator activation signals; and an r.f.
generator circuit for transmitting an unsafe tire pressure warning
signal when activated by the processor. The improvement comprises a
variable resistance sensor assembly having first and second
individual stretch sensors, each stretch sensor having a first
flexible layer containing a variable resistance element and a
second flexible support layer, with the individual stretch sensors
being arranged with the first flexible layer of the first stretch
sensor in facing relation with the first flexible layer of the
second stretch sensor so that the variable resistance elements face
each other. The variable resistance elements are inserted in an
electrical bridge circuit having four branches: a first pair of the
four branches have fixed resistance elements connected in series,
while a second pair of the four branches have the variable
resistance elements of the first and second stretch sensors
connected in series.
[0008] In a preferred variation of this basic embodiment, the
variable resistance sensor assembly further includes third and
fourth individual stretch sensors, with each of the third and
fourth stretch sensors having a first flexible layer containing a
variable resistance element and a second flexible support layer.
The third and fourth individual stretch sensors are mutually
arranged with the second flexible support layer of the third
stretch sensor in facing relation with the second flexible support
layer of the fourth stretch sensor. Also, the first flexible layer
of the third stretch sensor is arranged in facing relation with the
second flexible support layer of the second stretch sensor. The
variable resistance elements are inserted in an electrical bridge
circuit having four branches: a first one of the branches has the
variable resistance element of the first stretch sensor, a second
one of the branches has the variable resistance element of the
fourth stretch sensor, a third one of the branches has the variable
resistance element of the second stretch sensor, and a fourth one
of the branches has the variable resistance element of the third
stretch sensor. The first and second branches are connected in
series, and the third and fourth branches are connected in
series.
[0009] In both of the above embodiments, the ohmic electrical
connections in the bridge circuit ensure that resistance changes
due to temperature changes are cancelled out by the configuration
of the resistance components.
[0010] The tire pressure sensor system components comprising the
processor, the r.f. generator circuit, the variable resistance
sensor assembly, and a battery are all mounted on a common support
substrate having a flexible portion underlying at least the
variable resistance sensor assembly. The support substrate can be
mounted on a tire side wall-either the outside wall or the inside
wall; or embedded in the tire side wall during the tire formation
process. In surface mount installations, a sensor guide secured to
a tire side wall slidably captures a free end of the sensor
assembly. The other end of the sensor assembly is secured to the
tire side wall. This arrangement prevents excessive longitudinal
stretching of the sensor assembly and premature failure.
[0011] From a second apparatus aspect the invention comprises a
power saving unit for use in a tire pressure sensor system having a
variable resistance sensor for providing resistance values
indicative of internal tire pressure when coupled to a pneumatic
tire; a processor coupled to the sensor for converting resistance
values corresponding to inadequate internal tire pressure to r.f.
generator activation signals; and an r.f. generator circuit for
transmitting an unsafe tire pressure warning signal when activated
by the processor. The power saving unit limits the application of
electrical power to the variable resistance sensor in a manner
related to tire speed so that power is only applied, and thus drawn
from the battery, for a measurement period related to tire speed
after the tire speed has reached a threshold speed value.
Preferably, this measurement period is related to the time required
for a tire of a given size to complete a preselected number of
revolutions.
[0012] The power saving unit has an input terminal adapted to be
coupled to a source of electrical power (the battery in a
particular embodiment), an output terminal for supplying electrical
power to the variable resistance sensor, and a vehicle speed
sensitive switch for connecting the input terminal to the output
terminal when the tire attains a first predetermined speed and for
disconnecting the input terminal from the output terminal when the
speed of the tire drops below the first predetermined speed. In one
embodiment, the switch comprises an electrically conductive contact
member, such as a spring, having a first portion connected to the
output terminal and a free end, and an electrically conductive
pivot member, such as a spring or a pivot arm, having a first
portion connected to the input terminal and a mass member mounted
on a free end. The mass member is mounted to make physical contact
with the free end of the contact member when the tire attains the
first predetermined speed, thus enabling the transfer of electrical
power from the input terminal to the output terminal. Preferably,
the mass member has opposing ends; and the switch is provided with
first and second contact members connected to the output terminal,
with the first contact member having a free end located in the path
of one of the opposing ends of the mass member, and the second
contact member having a free end located in the path of the other
one of the opposing ends of the mass member. With this
configuration, the positioning of the power saving unit on a
vehicle tire is facilitated.
[0013] In an alternate embodiment, a magnetically actuatable reed
switch is coupled between the input terminal and output terminal,
and a magnet is mounted on the free end of the pivot arm to
activate the reed switch when the tire attains the first
predetermined speed.
[0014] In an alternate embodiment, the power saving unit further
includes a control signal output terminal coupled to the processor;
and the vehicle speed sensitive switch includes control signal
means for connecting the power input terminal to the control signal
output terminal when the tire attains a second predetermined speed
different from (and preferably higher than) the first predetermined
speed and for disconnecting the input terminal from the control
signal output terminal when the speed of the tire drops below the
second predetermined speed. When received, the control signal
serves as an indication to the processor that a different smaller
measurement period can now be used. This different measurement
period is also related to the time required for the tire to
complete a preselected number of revolutions.
[0015] In this embodiment, the switch configuration is essentially
the same as the switch used in the first embodiment. The control
signal means comprises a contact member having a first portion
connected to the control signal output terminal and a free end, and
the mass member in the switch is mounted to make physical contact
with the free end of the contact member when the tire attains the
second predetermined speed. Similar to the first embodiment, the
mass member preferably has opposing ends; and the control signal
means includes first and second contact members connected to the
control signal output terminal, the first contact member having a
free end located in the path of one of the opposing ends of the
mass member, and the second contact member having a free end
located in the path of the other one of the opposing ends.
[0016] From a process aspect, the invention comprises a method of
reducing power consumption in an electrically powered tire pressure
sensor system having a variable resistance sensor for providing
resistance values indicative of internal tire pressure when coupled
to a pneumatic tire, a processor coupled to the sensor for
converting resistance values corresponding to inadequate tire
pressure to r.f. generator activation signals, and an r.f.
generator circuit for transmitting an unsafe tire pressure warning
signal when activated by the processor, the method comprising the
steps of:
[0017] (a) providing a source of electrical power; and
[0018] (b) applying the electrical power to the variable resistance
sensor for a tire pressure measurement period whose duration is a
related to tire speed. Step (b) of applying preferably includes the
steps of (i) preventing the application of electrical power to the
variable resistance sensor until the tire speed reaches a first
tire speed threshold, (ii) furnishing electrical power to the
variable resistance sensor for a measurement period related to the
period of time required for a preselected number of tire
revolutions at the first tire speed threshold when the tire speed
reaches the first tire speed threshold, and (iii) terminating the
application of electrical power to the variable resistance sensor
when the tire speed falls below the first tire speed threshold.
[0019] The method may further provide for a second measurement
period by modifying step (b) of applying to further include the
step of changing the length of the measurement period to a
different value when the tire speed reaches a second tire speed
threshold, the different value being related to the period of time
required for a preselected number of tire revolutions at the second
tire speed threshold.
[0020] The invention provides a convenient solution to the problem
of monitoring internal tire pressure in vehicles equipped with
pneumatic tires. The system can be installed either during
manufacture of a new tire, manufacture of a new vehicle or as an
aftermarket item. Further, existing vehicles without tire pressure
sensor systems can easily be retrofitted with a state-of-the-art
system at relatively low cost. This is particularly beneficial in
jurisdictions which mandate low tire pressure warning devices on
all road vehicles. The sensor assembly provides substantially
enhanced measurement sensitivity, and the power saving unit
substantially reduces power consumption, which is particularly
important in those installations which use a relatively
inaccessible battery as a source of electrical power.
[0021] For a fuller understanding of the nature and advantages of
the invention, reference should be made to the ensuing detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic view of a prior art single tire
pressure monitor circuit using a single stretch sensor in a bridge
circuit;
[0023] FIG. 2 is a perspective view of a single tire pressure
monitor circuit having four stretch sensors in a bridge circuit
according to the invention;
[0024] FIG. 3 is a perspective view similar to FIG. 2 showing an
alternate embodiment of a single tire pressure monitor circuit
having two stretch sensors arranged in series connected branches of
a bridge circuit;
[0025] FIGS. 4 shows the comparative sensitivity of the prior art
bridge circuit of FIG. 1 and the two embodiments of the invention
shown in FIGS. 2 and 3;
[0026] FIG. 5 is a schematic perspective view of a tire pressure
monitoring system according to the invention showing the physical
layout of the major components;
[0027] FIG. 6 is a perspective view showing a single tire pressure
monitoring system according to the invention mounted on the outside
wall of a tire;
[0028] FIG. 7 is a sectional view through a vehicle wheel and tire
showing two possible placements of the invention;
[0029] FIG. 8 is a sectional view similar to FIG. 7 showing an
internal placement of the invention;
[0030] FIG. 9 is a schematic view of a first embodiment of a motion
detector according to the invention;
[0031] FIG. 10 is a schematic view of an alternate embodiment of a
motion detector according to the invention;
[0032] FIGS. 10A and 10B are schematic views of an alternate
embodiment of a motion detector according to the invention having a
reed switch;
[0033] FIG. 11 is a schematic view of a multi-stage embodiment of a
motion detector according to the invention; and
[0034] FIGS. 12A and 12B are timing diagrams illustrating the
operation of a multi-stage motion detector of FIG. 11 at two
different wheel speeds.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Turning now to the drawings, FIG. 1 is a schematic view of a
prior art single tire pressure monitor circuit using a single
stretch sensor in a bridge circuit. As seen in this Fig., the
monitor circuit, generally designated with reference numeral 10,
includes a single stretch sensor 12 ohmically connected in one
branch of a bridge circuit having three additional branches each
with a fixed resistance R ohmically connected as shown. Stretch
sensor 12 is a known component having the property of an ohmic
resistance which varies in a predictable amount with linear
longitudinal displacement of the sensor body. Stretch sensor 12 has
a first layer 14 on which a thin variable resistance element 15 is
mounted; and a second, base layer which carries the first layer and
provides additional mechanical strength for sensor 12. The fixed
resistances R are all of equal value. A reference voltage Vin from
a source of D.C. electrical power (not shown) is applied to two
nodes of bridge circuit 10. Stretch sensor 12 is affixed to a
vehicle tire (not shown) in such a manner that the stretch sensor
12 will flex as a function of internal pressure. The resistance
value of stretch sensor 12 depends upon the amount of flexing due
to the internal tire pressure and the direction in which the
flexing occurs. As shown in FIG. 1, when sensor 12 flexes in a
first direction the value of the resistance increases (R+r), where
R is the at rest resistance value of sensor 12 and r is the
additional resistance value due to the flexing of sensor 12.
Similarly, when sensor 12 flexes in the opposite direction, the
value of the resistance decreases (R-r). As the resistance of
stretch sensor varies, the measuring voltage Vout will vary
accordingly, thus providing a measured value of internal tire
pressure.
[0036] FIG. 2 is a perspective view of a single tire pressure
monitor circuit having four stretch sensors 22a-22d in a bridge
circuit 20 according to the invention. As seen in this Fig., each
sensor assembly 22 comprises four two layer stretch sensors each
having a first layer 24 bearing the variable resistance element,
and a base layer 25. Two of the stretch sensors 22a, 22b are
arranged with the first layers in facing relation; while the two
remaining stretch sensors 22c, 22d are arranged in back-to-back
relation. The sensors 22a and 22c which face in the same direction
are designated in FIG. 2 with the annotation R+r; while the two
sensors 22b and 22d which face in the same direction but opposite
from the direction of sensors 22a and 22c are designated with the
annotation R-r. The sensors are ohmically connected as shown in
FIG. 2 with the R+r sensors arranged in two opposite branches of
the bridge circuit 20, and the R-r sensors arranged in the other
two opposite branches of the bridge circuit 20. With this
arrangement, any variations in resistance due to thermal effects
are totally cancelled out electrically, so that measured resistance
values are a pure function of internal tire pressure. The sensor
assembly 22 is physically mounted to the tire in the manner
described below.
[0037] FIG. 3 is a perspective view similar to FIG. 2 showing an
alternate embodiment of a single tire pressure monitor circuit
having two stretch sensors arranged in series connected branches of
a bridge circuit. As seen in this Fig., each sensor assembly 32
comprises two two layer stretch sensors each having a first layer
24 bearing the variable resistance element, and a base layer 25.
The two stretch sensors 32a, 32b are arranged with the first layers
in facing relation in an R+r, R-r configuration. The single layer
sensors are ohmically connected as shown in FIG. 3 with the R+r
sensor and the R-r sensor arranged in series connection in adjacent
branches of the bridge circuit 30. The other two branches of the
bridge circuit are provided with fixed resistance elements 26 of
equal value R. With this arrangement, any variations in the
variable resistance elements due to thermal effects are totally
cancelled out electrically, and any variations in the fixed
resistance elements R due to thermal effects are totally cancelled
out electrically so that measured resistance values are a pure
function of internal tire pressure. The sensor assembly 32 is
physically mounted to the tire in the manner described below.
[0038] FIGS. 4 shows the comparative sensitivity of the prior art
bridge circuit of FIG. 1 and the two embodiments of the invention
shown in FIGS. 2 and 3. As seen in this Fig., for the single sensor
prior art device shown in FIG. 1 the magnitude of the output
voltage Vout is a function of r/4R. For the two sensor embodiment
of FIG. 3 the magnitude of the output voltage Vout is a function of
r/2R. For the four sensor embodiment of FIG. 2, the magnitude of
the output voltage Vout is a function of r/R. As will be
appreciated by those skilled in the art, the FIG. 2 embodiment
provides an increase in sensitivity by a factor of four over the
prior art arrangement; while the FIG. 3 embodiment provides an
increase in sensitivity by a factor of two. This represents a
substantial improvement in measurement capability.
[0039] FIG. 5 is a schematic perspective view of a tire pressure
monitoring system 50 according to the invention showing the
physical layout of the major components. As seen in this Fig., the
major components of the tire pressure monitoring system 50 include
an integrated circuit 51, a battery 52, a stretch sensor assembly
22 or 32, an antenna 54, and a motion detector 55 (described
below). These components are secured in any desired fashion (such
as by using a suitable adhesive) to a substrate layer 56. The
integrated circuit 51 contains the active electronic components
usually found in an r.f. monitoring system and will not be further
described as this arrangement is well known to those skilled in the
art. The antenna 54 is coupled to the r.f. section of integrated
circuit 51 in the usual manner. The stretch sensor assembly 22, 32
is ohmically connected to a bridge circuit contained in integrated
circuit 51. Battery 52 is connected to the power input terminals of
integrated circuit 51. Substrate layer 56 is adhered to a mounting
layer 57 using a suitable adhesive. At least those portions of
substrate later 56 and mounting layer 57 underlying stretch sensor
assembly 22 or 32 should be sufficiently flexible to allow the
stretch sensor assembly to flex with the tire side wall in order to
provide an accurate resistance value. For surface mount
installations (described below), a generally U-shaped sensor guide
58 having anchor ends 59a, 59b slidably captures sensor assembly
22, 32 and the underlying portions of substrate layer 56 and
mounting layer 57. Sensor guide is dimensioned to maintain sensor
assembly 22, 32 closely adjacent the tire side wall, while at the
same time permitting sliding motion of sensor assembly 22, 32
within sensor guide 58.
[0040] Sensor assembly 22, 32 is fixed at the lower end thereof to
a first tire anchor point (the outer tire surface, the inner tire
surface or an internal anchor point-see below) by adhering the
generally rectangular lower portion of substrate layer 56 and
mounting layer 57 to the first tire anchor point. The anchor ends
59a, 59b of sensor guide 58 are fixed to a second tire anchor
point. When the contour of the tire side wall changes due to a
change in internal tire pressure, sensor assembly 22, 32 will flex
with the contour change due to the fact that sensor assembly 22, 32
is fixed to the tire anchor point at the lower end thereof and is
slidably retained in close proximity to the tire side wall by
sensor guide 58. However, since only the lower end of sensor
assembly 22, 32 is fixed to the first tire anchor point, sensor
assembly 22, 32 cannot be stretched to the breaking point, which
could occur if sensor assembly 22, 32 were firmly adhered along its
entire length. This mounting arrangement prevents premature failure
of sensor assembly 22, 32.
[0041] As shown in FIG. 6, a single tire pressure monitoring system
50 according to the invention can be mounted on the outside wall of
a tire 61 by attaching the mounting layer 57 (FIG. 5) to the tire
sidewall at an appropriate location. This can be done using a
suitable adhesive, such as an epoxy adhesive. Preferably, the
system 50 is adhered to the tire sidewall using a two component
hook-and-loop attachment system, such as that sold under the Velcro
trademark. This arrangement provides addition vibration damping to
an installed tire pressure sensing system.
[0042] FIG. 7 is a sectional view taken through a vehicle tire and
wheel assembly illustrating two alternate placements of the tire
pressure sensing system 50. As seen in this Fig., the system 50 can
be attached to the outside wall 61 of the vehicle tire using the
attachment mechanism described above. This placement allows for
easy replacement of an exhausted battery 52 since the battery 52 is
readily accessible. Alternatively, the system 50 can be attached to
the inside tire wall 62 prior to mounting the tire on the wheel 63.
This arrangement provides protection for the system 50 components
from mechanical abrasion and severe environmental conditions, but
has the disadvantage that the tire must be removed from the wheel
63 when battery 52 needs replacement.
[0043] FIG. 8 is a sectional view similar to FIG. 7 showing another
alternate placement of the tire pressure sensor system. As seen in
this Fig., sensor system 50 is molded into the interior of the tire
between outer side wall 61 and inner side wall 62. Since the
temperatures required for the tire molding process are relatively
low compared to the temperature tolerance of the components of
system 50, this internal placement is practical. The internal
arrangement shown provides the maximum protection for the
components of system 50 since they are entirely encased in the tire
material. However, when the battery 52 is exhausted, it cannot be
replaced with this arrangement.
[0044] The resistance measurement process used to determine
internal tire pressure is very similar to that disclosed in the
above-referenced pending U.S. patent application Ser. No.
10/346,490. The value of the measured resistance of stretch sensor
assembly 22, 32 varies between a maximum R max when the pressure
sensor system 50 is located a minimum distance from the pavement
and subject to maximum displacement (closest to the pavement), and
a minimum R min when the pressure sensor system 50 is at the
maximum distance from the pavement (farthest from the pavement) and
subject to minimum displacement. The parameter which is used to
compute tire pressure is the difference R=(R max)-(R min). This
parameter is calculated by programmed circuitry within integrated
circuit 51. When this value lies within a predetermined acceptable
range defined by two predetermined threshold values, no signal is
transmitted from antenna 54 since the internal tire pressure is
within the permitted range. When the value of R is greater than a
predetermined first threshold value-signifying a low pressure
condition, integrated circuit 51 activates an internal r.f.
transmitter, which causes a low pressure signal to be transmitted
from antenna 54. Similarly, when the value of R is less than a
predetermined second threshold value--signifying a high pressure
condition, integrated circuit 51 activates the internal r.f.
transmitter, which causes a high pressure signal to be transmitted
from antenna 54. The low pressure signal or high pressure signal is
received by conventional on-board receiver circuitry (not shown),
which converts the low or high pressure signal to a perceivable
warning signal, such as a visible indicator, an audible alarm, or
both. In general, the receiver circuitry includes a decoder for
decoding the low and high pressure signals to a form which can be
used to operate the warning indicator. Representative examples of
such receivers are illustrated and described in U.S. Pat. Nos.
5,900,808; 6,175,301; and 6,453,737. Since the receiver circuitry
is conventional and well-known to those skilled in the art, further
description is deemed unnecessary.
[0045] To conserve battery power, resistance measurements can be
made periodically, rather than continuously. For example, an
initial vale of R may first be calculated. If the value of R is
less than the first threshold value and higher than the second
threshold value (i.e. indicates that the tire pressure lies within
the acceptable range), integrated circuit 51 will wait for one
minute, and then proceed with another calculation of the parameter
R. If any calculation results in a value of R which lies outside
the range defined by the two threshold levels (i.e. higher than the
first threshold or lower than the second threshold), integrated
circuit 51 will wait for a shorter period (ten seconds) and then
perform another calculation of the parameter R. If the result is
another value of R which lies outside the range defined by the two
thresholds, integrated circuit 51 activates the r.f. transmitter to
generate a low or high tire pressure signal. If the result is a
successive value of R which does not lie outside the range defined
by the two thresholds, integrated circuit 51 will wait for one
minute, and then proceed with the next calculation.
[0046] To further conserve battery power, power from the battery 52
to integrated circuit 51 is selectively applied under control of a
motion detector 55, a first embodiment 55A of which is shown in
FIG. 9. As seen in this Fig., one terminal of battery 52 (the
positive terminal in this embodiment) is connected to a first
terminal 91 of motion detector 55A. Terminal 91 is ohmically
connected to a pair of contact springs 92, 93 disposed along a
pivot path 94 of a mass block 95. Mass block 95, which is
fabricated from an electrically conductive material, is mounted to
the upper end of a pivot spring 96, also fabricated from an
electrically conductive material. The lower end of pivot spring 96
is ohmically connected to a power output terminal 98. Power output
terminal 98 is connected to the power input terminal of integrated
circuit 51.
[0047] In operation, when the vehicle tire to which tire pressure
sensor system 50 is attached is at rest, mass block 95 is
positioned centrally of contact springs 92, 93 and maintained in
this position by the action of pivot spring 96. In this central
position, mass block is out of contact with contact springs 92, 93
and, as a result, power from battery 52 does not flow to output
terminal 98 and no power is consumed. As the vehicle tire starts to
rotate, mass block 95 is deflected along pivot path 94 under the
influence of centrifugal force in the direction of either contact
spring 92 or contact spring 93, depending on the orientation of
motion detector 55 on the tire side wall and the direction of
rotation of the tire. When the rotational speed of the tire reaches
a predetermined value (e.g. 10 m.p.h.), mass block 95 is deflected
a sufficient distance to make contact with one of the two contact
springs 92, 93. At this point, an ohmic electrical circuit is
established between power input terminal 91 and power output
terminal 98, and D.C. electrical current can flow from battery 52
to integrated circuit 51. It should be noted that the tire speed at
which power is applied to integrated circuit 51 is a matter of
design choice and can be set at a value deemed appropriate to one
of skill in the art. Once a power connection is established between
battery 52 and integrated circuit 51, the tire pressure measurement
process described above commences.
[0048] FIG. 10 illustrates an alternate embodiment of the motion
detector 55B. In this embodiment, pivot spring 96 is replaced by a
pivot arm 101, pivotally mounted at the bottom end thereof to a
fixed reference point and having a ferro-magneuc mass block 103
mounted on the upper end thereof. A permanent magnet 105 is secured
to a fixed reference point of motion detector 55B. Operation of the
embodiment of FIG. 10 is very similar to the embodiment of FIG. 9,
with the difference that the magnetic force between mass block 103
and permanent magnet 105 maintains mass block 103 out of contact
with spring 92 or spring 93 until the magnitude of the centrifugal
force due to the rotation of the tire exceeds the magnitude of the
magnetic holding force between mass block 103 and permanent magnet
105.
[0049] FIGS. 10A and 10B illustrate another alternate embodiment of
the motion detector 55C. FIG. 10A shows motion detector 55C in the
unactuated state, while FIG. 10B shows motion detector 55C in the
actuated state. In this embodiment, pivot arm 101 has a magnet 106
mounted on the upper end thereof. Permanent magnet 105 is secured
to a fixed reference point of motion detector 55C. Contact springs
92, 93 are replaced by a magnetically actuated normally open
contact reed switch 108 having a first terminal 109 ohmically
connected to input terminal 91 and a second terminal 110 ohmically
connected to terminal 98. Operation of the embodiment of FIGS. 10A
and 10B is as follows. When the magnitude of the centrifugal force
due to the rotation of the tire is less than the magnitude of the
magnetic holding force between magnet 106 and magnet 105, pivot arm
101 and magnet 106 are maintained in the attitude illustrated in
FIG. 10A, in which magnet 106 is sufficiently remote from reed
switch 108 that reed switch remains in the unactuated state and no
electrical power is transferred between terminal 91 and terminal
98. When the magnitude of the centrifugal force due to the rotation
of the tire exceeds the magnitude of the magnetic holding force
between magnet 106 and magnet 105, pivot arm 101 and magnet 106 are
rotated (counter-clock wise in FIG. 10B) so that magnet 106
approaches reed switch 108 and causes the contacts therein to
close, thereby ohmically connecting terminals 91 and 98 and
transferring electrical power from battery 52 to integrated circuit
51. While only one reed switch 108 is shown in FIGS. 10A and 10B,
it is understood that a pair of reed switches 108 may be used in
motion detector 55C positioned at locations similar to the
locations of contact springs 92, 93 in the embodiment of FIG.
9.
[0050] As will now be apparent, the inclusion of motion detector
55A-55C in the power circuit of tire pressure sensor system 50
prolongs the useful life of battery 52 by preventing the
application of D.C. electrical power to integrated circuit 51 when
the vehicle to which the tire is rotatably attached is at rest or
moving at a speed at which tire pressure is not a matter of
concern. Even further power savings can be achieved by the
multi-stage motion detector 55D shown in FIG. 11. As seen in this
Fig., multi-stage motion detector 55D has the same elements 91, 92,
93, 95, 96, and 98 incorporated therein as motion detector 55A. In
addition, motion detector 55D includes an additional pair of power
contact springs 112, 113 mounted along opposite ends of the pivot
path 94 of mass block 95 but arranged at points along the pivot
path 94 which are outboard of the inner contact faces of contact
springs 92, 93. Power contact springs 112, 113 are ohmically
connected in parallel to an additional output terminal 115, which
is connected to a dedicated input port of integrated circuit 51.
The purpose of contact springs 112, 113 and output terminal 115 is
to provide a control signal to integrated circuit 51 signifying
that the tire rotation speed has achieved a predetermined higher
value than that signified by contact between mass block 95 and
either contact spring 92 or contact spring 93. For example, the
mechanical parameters controlling the rotational speed at which
springs 92, 93 and mass block 95 make contact and permit the
application of electrical power to integrated circuit 51 to enable
the tire pressure measurement process may be set at 10 m.p.h.;
while the mechanical parameters controlling the rotational speed at
which springs 112, 113 and mass block 95 make contact and generate
the control signal may be set at 50 m.p.h. The control signal on
output terminal 115 is used for the following purpose.
[0051] During the tire pressure measurement process, a significant
amount of power is consumed from battery 52 when electrical current
is applied to sensor assembly 50. Multi-stage motion detector 55D
enables integrated circuit 51 to minimize the total amount of
current applied during the measurement process by limiting the
measurement period to the time period required to make an accurate
measurement of the tire pressure as a function of vehicle speed.
FIGS. 12A and 12B illustrate this power tailoring technique for a
205/65R15 tire having a radius of 0.32 m. For a vehicle speed of 10
m.p.h., the time required for one revolution of this specific tire
is 0.45 second. Thus, the minimum time period required to obtain a
measurement of Rmin and Rmax is 0.45 second. Since the angular
position of the tire at any given moment when electrical power is
applied to integrated circuit 51 is indeterminate with the present
system, it is prudent to enable the tire pressure measurement
process for two complete revolutions of the tire after power is
applied. With reference to FIG. 12A, at a vehicle speed of 10
m.p.h. the tire pressure measurement process is enabled for 0.90
second, which is the time required for two complete revolutions of
the tire after the measurement process is enabled. Thus, with
motion detector 55D, after electrical power is transferred from
battery 52 to integrated circuit 51 via the conductive path
terminal 91, spring 96, mass block 95, contact spring 92 or 93, and
terminal 98, the tire pressure measurement process is enabled for
0.90 second when the control signal on terminal 115 is inactive or
deasserted.
[0052] For a vehicle speed of 50 m.p.h., the time required for one
revolution of the same tire is 0.09 second; and two complete
revolutions require 0.18 second. Thus, the minimum time period
established to obtain a reliable measurement of Rmin and Rmax is
0.18 second. With reference to FIG. 12B, at a vehicle speed of 50
m.p.h. the tire pressure measurement process is enabled for 0.18
second, which is the time required for two complete revolutions of
the tire after the measurement process is enabled. Thus, with
multi-stage motion detector 55D, after electrical power is
transferred from battery 52 to integrated circuit 51 via the
conductive path terminal 91, spring 96, mass block 95, contact
spring 92 or 93, and terminal 98; and electrical power is
transferred from battery 52 to integrated circuit 51 via the
conductive path terminal 91, spring 86, mass block 95, contact
spring 112 or 113, and terminal 115 (thereby asserting the control
signal), the tire pressure measurement process is enabled for only
0.18 second.
[0053] As will now be apparent, multi-stage motion detector 55D
limits power consumption during the tire pressure measurement
process while still allowing an accurate measurement of tire
pressure to be obtained. It is understood that, although
multi-stage motion detector 55D has been described with reference
to the common elements of motion detector 55A, detector 55D may be
configured using the common elements of motion detectors 55B and
55C. Also, it is understood that additional stages may be added to
multi-stage motion detector 55D to incorporate more and different
speed thresholds than the two thresholds described above. For
example, an additional set of contact springs may be installed at
wider spacings than contact springs 112, 113 shown in FIG. 11 to
specify a third, higher speed threshold with a shorter power-on
time period. Further, it is understood that the measurement periods
can be based on a different number of revolutions of the tire than
the two revolution example in the preferred embodiment, if
desired.
[0054] While the preferred embodiments have been thus-far described
as a single unit for one tire, in practice each tire of a vehicle
will be equipped with a tire pressure sensor system 50. Various
encoding arrangements can be made to uniquely identify each
individual sensor, and the warning indicator can be configured to
identify the particular tire which is improperly inflated.
[0055] As will now be apparent, the invention provides a simple,
low cost tire pressure sensor system which is relatively simple in
construction and enjoys higher measurement sensitivity than known
systems using a single stretch sensor. In addition, the tire
pressure sensor system according to the invention can accommodate
various modes of installation, such as being incorporated into the
tire during manufacture, installed on the inside wall of the tire
before mounting on the wheel, and installed on the outer side wall
of the tire after mounting on the wheel. Further, the motion
detector portion of the invention limits power consumption and thus
prolongs battery life. Lastly, the invention provides an accurate
and reliable system for monitoring tire safety on all vehicles
using pneumatic tires.
[0056] While the invention has been described with reference to
particular preferred embodiments, various modifications, alternate
embodiments, and equivalents may be employed, as desired. For
example, paired springs 92, 93 may be replaced with a single spring
positioned along the pivot axis of the mass block support member,
if desired. If a single spring is used, care must be taken to
orient the sensor system in the proper direction on the tire to
ensure that application of electrical power to integrated circuit
51 will occur upon forward motion of the vehicle. Also, while the
invention has been described with reference to the use of adhesives
for attaching the sensor to the tire side wall, other known
techniques may be used, if deemed suitable, for the purpose of
attaching the sensor to the tire side wall. Therefore, the above
should not be construed as limiting the invention, which is defined
by the appended claims.
* * * * *