Tire pressure monitoring system utilizing a pressure activated transducer and sensor

A wireless tire pressure monitoring system is disclosed to warn a driver of a vehicle of low pressure in a vehicle tire. The monitoring system includes a transducer to which wires or other physical connections cannot be easily made. The transducer is attached to a wheel rim and a magnetic field can be selectively produced by elements of the transducer in response to changes in tire pressure. A sensor is mounted for sensing the magnetic field with the sensor producing an output that can be coupled with a monitor in the vehicle cab.

FIELD OF INVENTION 
The invention pertains to systems for continually monitoring tire pressure 
for a vehicle that is at rest or travelling at any speed and sounding an 
alarm at the driver's instrument panel when the air pressure of a tire 
falls below a preset minimum pressure level. 
BACKGROUND OF THE INVENTION 
Many vehicles have one or more tires that may deflate or go completely flat 
during travel without the driver being immediately aware of the condition. 
These include tractor-trailer combinations, motor homes, buses and various 
types of vehicles and trailers being towed that have axles mounted with 
two or more tires, or tandem rear axles that may have a total of four, six 
or eight fires. Unlike those of motorcars, drivers of these vehicles 
cannot sense that a tire has low pressure or suffered a blowout. The large 
size and heavy weight of these vehicles isolate a driver from noise and 
vibration created by a blowout. Likewise, a driver towing another vehicle 
is insulated from any sensation of a deflating or flat fire on the towed 
vehicle. 
Low tire pressure can often lead to a blowout of the tire if it goes 
undetected. While traveling on an interstate highway at speeds of between 
55 and 65 m.p.h., a blowout can cause extensive damage, especially when it 
goes undetected. The tire disintegrates and its debris damages the vehicle 
or trailer, often extensively. The wheel hits against the pavement, and 
usually suffers damage. In the case of dual rear wheels, where there is 
four wheels per axle, continued motion with one of the tires deflated may 
even cause the deflated tire to catch fire. There have been cases reported 
in which such a fire has caused total loss of the vehicle or trailer. Most 
disconcerting about a loss of pressure or low pressure in a vehicle's tire 
is, however, that it affects the load carrying capability, steering, 
breaking and overall control of the vehicle. Loss of a load bearing tire 
may result in a driver losing control and causing damage to the vehicle, 
passengers and other property and passengers of other vehicles. 
Needless to say, the continual prospect of suffering a blown tire makes 
drivers of buses, motorhomes, tractors and other large or towing vehicles 
very uneasy. 
Because a tire with abnormally low pressure is a candidate for a blowout, a 
system which monitors all the tires and provides early warning of an 
abnormally low pressure would go a long way toward providing peace of mind 
to the drivers, as well as provide improved safety, prolong the life of 
tires, prevent tire destruction and fires, and save time. However, though 
instrument panels in vehicles are over-populated by gauges and lights for 
providing all sorts of warning, most vehicles do not have a tire pressure 
monitor that drivers who spend a lot of time on the road desperately want 
and need. In addition to the failure to recognize the importance of 
detecting at an early stage low tire pressure to prevent blowouts, there 
is at least one other reason for the absence of tire pressure monitoring 
systems: prior art tire pressure monitors have adopted expensive and 
impractical approaches to this problem. 
In the art, there are several examples of tire pressure monitors and alarm 
systems. These are typically fastened to the rim of the wheel and require 
that a hole be drilled through the wheel. See U.S. Pat. No. 4,954,677 of 
Alberter et al.; U.S. Pat. No. 4,894,639 of Schmierer; U.S. Pat. No. 
4,866,982 of Gault; U.S. Pat. No. 4,768,375 of Echardt et al.; and U.S. 
Pat. No. 4,784,993 of Lothar et al. These systems include a transducer of 
some sort that converts the pressure to a signal for communicating the 
pressure to a remote display. 
The disadvantage of these tire pressure monitors is that the transducers 
are mounted through the wheel rim. Thus, the wheel must be either 
specially manufactured or adapted (if possible) with holes that are 
drilled in the wheel to receive the transducers. As holes cause undue 
stress on the wheel retrofitting preexisting wheels, it gives rise to 
safety and liability problems. Thus, they must be manufactured for these 
systems as original equipment. However, they must meet strict Department 
of Transportation guidelines and undergo stress tests before approval. 
These systems also require that the wheel be removed from the vehicle and 
disassembled to gain access to the transducers for service. Furthermore, 
they require some sort of electrical connection between the transducer and 
any remote monitoring device. With a rotating wheel, this electrical 
connection requires special contacts, complicating the system, introducing 
added cost and reducing reliability. 
The problem of connecting the transducer to a monitor has been solved in 
part by radio frequency communications. As shown in U.S. Pat. No. 
4,890,090 of Ballyns, a pressure transducer is coupled to a radio 
frequency transmitter that is mounted within the tire and secured to the 
wheel rim. Although it has the advantage of wireless communication of the 
pressure to a remotely placed monitor, it suffers from the same 
disadvantages of the rim mounted transducers: it is difficult to install 
and service, and requires special adaption of the wheel. 
To avoid this communication problem, it is possible to indirectly monitor 
the condition of the tire using tire rotation sensors like those installed 
as original equipment on vehicles with anti-lock braking and some 
all-wheel drive systems. To detect a deflating tire, these sensors are 
monitored for abnormal changes in rotation speeds of the tire indicating 
deflation. Doing so requires sophisticated sensors, data processing 
equipment and algorithms, and a vehicle originally equipped with this 
advanced and expensive technology. It is a sophisticated approach, but one 
that is not feasible for most vehicles such as buses, trucks and motor 
homes currently being manufactured and on the road that are not using this 
technology. 
Another approach avoids altogether mounting transducers on a tire. Yet it 
is just as complicated and expensive. An elaborate, and extremely 
expensive air pressure line is built into the car that runs from a wheel, 
through a hub and down an axle to a sensor located within the vehicle. 
This approach is generally available only to the most sophisticated and 
expensive vehicles and must be installed as original equipment. 
Despite previous substantial efforts to improve the safety of tires, 
current tire pressure monitoring systems continue to run in the vein of 
being expensive and elaborate; they require substantial modification to 
wheels and to the car for their use; and they offer methods having little 
to no feasibility for retrofitting the millions of ordinary wheels that 
are in use and will continue to be manufactured and used. 
SUMMARY OF THE INVENTION 
The invention overcomes the aforementioned and other disadvantages of prior 
art systems and provides for an elegantly simple, low cost, reliable and 
easily repairable system for warning a driver of an abnormally low 
pressure in a tire and preventing unexpected tire blowouts. It is designed 
to be added or retrofitted to tires of existing vehicles such as busses, 
motor homes, trucks and trailers. 
The major components of the system are a pressure sensing device, a hall 
effect sensor, and a control box. In a preferred embodiment, the pressure 
sensing device consists of a fixed magnet, a pneumatic cylinder and a 
steel flag in a common housing. The pneumatic cylinder is pressurized by 
the pressure of the tire being monitored through a small capillary tube 
attached to the pneumatic cylinder on one end and to an extension of the 
tire's valve stem on the other. The steel flag is attached to the piston 
rod of the pneumatic cylinder and moves with the stroke of the piston. The 
flag is arranged relative to the magnet so that the flag covers the magnet 
at a preset low pressure condition and exposes the magnet at a preset high 
pressure condition. The unit is then simply screwed onto a standard tire 
stem. Installation and service is thus very simple and the device fits 
almost every type of tire and thus can be used with almost any type of 
vehicle. No alterations to the wheel or difficult installation procedures 
are required. The hall effect sensor in the preferred embodiment is 
mounted to a fixed member near the wheel so that it is within sensing 
distance of the magnet inside the pressure sensing device housing. When 
the steel flag is not coveting the magnet, indicating a high pressure 
condition, the hall effect sensor will detect the magnetic field and send 
the appropriate electrical signal to the control box. When the magnet is 
covered by the steel flag, the hall effect sensor will not detect the 
magnetic field. The hall effect sensor is connected to the control box by 
wires which are routed from each wheel to the control box. 
The control box in the preferred embodiment receives the electrical signals 
from each wheel and determines if any of the wheels is showing a low 
pressure condition. If a low pressure condition exists, the control box 
will alert the driver of the condition. 
In one form of the invention, a wireless monitoring system for indicating 
the occurrence of a predefined condition with a transducer to which wires 
and other physical connections cannot be easily made is disclosed, the 
monitoring system comprising transducer means for sensing an occurrence of 
a predefined condition, means for selectively producing a magnetic field 
coupled to the transducer and operable to emit a magnetic field when the 
transducer is in a first state and further operable to not emit a magnetic 
field when the transducer is in a second state and means for sensing a 
magnetic field positioned so as to be in at least occasional proximity to 
the means for selectively producing a magnetic field, the means for 
sensing a magnetic field producing an output only when the magnetic field 
is present. 
In another form of the invention, a tire pressure monitoring system for 
warning a driver of a vehicle when pressure in a tire falls below a 
predetermined pressure level is disclosed, comprising a pressure chamber 
having a pneumatic inlet, means for mechanically securing the pressure 
chamber to an inside rim of a wheel carrying the tire and mounted to an 
axle of the vehicle, the means for mechanically securing including air 
passage means for communicating pneumatic pressure from a stem of the tire 
to the pneumatic inlet, a piston slidingly engaged in the pressure and 
operable to move within the pressure chamber in response to an increase of 
the pneumatic pressure, a rod having a first end coupled to one side of 
the piston, the rod sealingly engaged with the pressure chamber and 
operable to extend into and out of the pressure chamber, biasing means 
coupled to a second end of the rod and operable to extend the rod into the 
pressure chamber when a biasing force of the biasing means is greater than 
a force on the piston created by the pneumatic pressure, a permanent 
magnet disposed in proximity to the pressure chamber; the magnet producing 
a magnetic field, a Hall effect sensor mounted on the axle and operable to 
sense the magnetic field and produce an output when the permanent magnet 
is in substantially close proximity thereto, magnetic shield means coupled 
to the rod and operable to selectively deflect the magnetic field away 
from the Hall effect sensor when the pneumatic pressure is below the 
predetermined pressure and to leave the magnetic field substantially 
unaffected when the pneumatic pressure is above the predetermined pressure 
and indicator means operable to alert the driver based upon the output. In 
another form of the invention, a method for detecting a fault in one of a 
pair of devices is disclosed, comprising the steps of (a) producing a 
first output pulse periodically while there is no fault in a first device 
of the pair; (b) producing a second output pulse periodically while there 
is no fault in a second device of the pair, wherein a first period of the 
first output is substantially equal to a second period of the second 
output; (c) producing a first fault detect pulse when two second output 
pulses are produced without the first output pulse being produced between 
the two second output pulses; (d) producing a second fault detect pulse 
when two first output pulses are produced without the second output pulse 
being produced between the two first output pulses; (e) accumulating the 
first fault detect pulses in a field fault detect accumulator; (f) 
accumulating the second fault detect pulses in a second fault detect 
accumulator; and (g) indicating a fault when the first or second fault 
detect accumulator has accumulated a predetermined number of respective 
fault detect pulses. 
These and other advantages, objects and aspects of the invention are 
exemplified by the preferred embodiments of the invention shown in the 
accompanying drawings. Following is a description of the preferred 
embodiments, made with reference to the drawings, for enabling one of 
ordinary skill in the art to practice the best mode of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, integrated sensor and transmitter assembly 101 is a 
major component of the preferred embodiment of the tire pressure warning 
system. The integrated sensor and transmitter assembly 101 has housing 
comprised of a body 103 fabricated from glass filled ABS plastic and a 
cover 105 made from ABS plastic. The cover 105 is securely fitted to the 
body 103 by use of retainer ring 107. Two "O" ring seals 109 and 111 
provide a water-resistant seal between the body 103 and the cover 105. 
A brass insert coupler 113 is molded into the body 103 for coupling the 
integrated sensor/transmitter to a Shrader valve of a tire stem (not 
shown). It has a threaded bore 115 and tongue 117 for depressing a valve 
core of a Shrader valve. Brass insert 113 screws onto the outside threaded 
end of a tire stem (not shown). The brass insert 113 is secured within the 
body 103 by use of a molded-end flange 119 that fits within a groove 
circumscribing the brass insert 113. The threaded bore 115 of the brass 
insert 113 secures the integrated sensor and transmitter assembly 101 to a 
tire stem. When the integrated sensor and transmitter assembly 101 is 
screwed onto a Shrader valve of a tire stem, valve core pressure release 
117 depresses the valve core of the Shrader valve, thereby releasing 
pressurized air into tire stem extension cavity 121 and air line 123. 
A second Shrader valve 125 is screwed into a tire stem extension portion 
127 of body 103 and seals the tire stem extension cavity. The second 
Shrader valve and the tire stem extension provide the ability to check 
tire pressure with a manual tire pressure gauge or to fill the tire with 
air without removing the integrated sensor and transmitter assembly. So 
that a cap can be screwed onto the fire stem extension, its exterior 
surface is threaded. 
The air line 123 terminates at a pocket defined within body 103 for 
receiving pressure sensor assembly 129. The pressure sensor 129 fits 
snugly within the pocket and seals it. When air line 123 is pressurized 
from air in a tire, the sensor accurately measures the pressure of air in 
the tire. 
The pressure sensor 129 is mounted in a plane oriented perpendicularly to 
the tire stem. This reduces the overall height of the integrated sensor 
and transmitter assembly, as compared to one having a pressure sensor 
mounted parallel to the stem. In this configuration, the integrated 
pressure sensor and transmitter assembly must be attached to a Shrader 
valve of a tire stem, when the wheel rotates, that travels within a plane 
perpendicular to the ground. Otherwise, centrifugal force generated by a 
rotating tire will act against the diaphragm, causing inaccurate pressure 
readings. 
The body 103 also includes an electronics cavity 131 integrally formed 
within the body and enclosed by cover 105. For purposes of illustration, 
it is shown without any electronics. The electronics that are placed 
within the cavity 131 are coupled to the pressure sensor assembly 129 for 
transmitting a signal indicative of an abnormally low pressure sensed by 
the pressure sensor 129. The area between the cover 105 and the body 103, 
including the electronics cavity, is open to the pressure sensor, and 
therefore the pressure of air within the area serves as a reference 
pressure for the pressure sensor 129. To accurately measure tire pressure, 
the pressure within the area should be maintained at ambient pressure. No 
means are provided for ensuring an ambient pressure is maintained within 
this area. However, because only abnormally low tire pressures need to be 
sensed, and because it is unlikely that the pressure will deviate 
substantially from the ambient pressure, a non-ambient pressure within the 
area will generally not detrimentally affect the pressure sensor's 
performance for this purpose in most cases. 
The dimensions of the integrated sensor and transmitter assembly 101 are 
very small, approximately 1.064 inches in diameter by 0.76 inches high. 
The ABS plastic body 103 and cover 105 are very light weight and are of 
minimal height. The integrated sensor and transmitter assembly 101 
therefore may be sufficiently secured to a tire stem sufficiently to 
prevent it from being thrown off during movement and rotation of the tire 
by simply screwing it onto the stem. 
Referring now to FIG. 2, a top view of body 103, with cover 105 removed, 
shows electronics cavity 131 and pressure sensor assembly 129. The 
pressure sensor assembly 129 is mounted within an integrally formed 
circular opening to air passage 123 in body 103 shape. The pressure sensor 
assembly is electrically coupled to electronics circuitry (not shown) 
mounted within the electronics cavity 131 by leads 201. 
Cavity 131 is large enough to accommodate the electronics and any antennas 
(not shown) required to transmit a signal from the integrated sensor and 
transmitter assembly 101 to an antennae mounted within a vehicle for 
receiving a signal. In order to provide cavity 131 with a volume large 
enough to accommodate electronics and antennas while keeping the diameter 
and height of body 103 as small as possible, the electronics cavity 131 
wraps around, in a circular fashion, tire stem extension portion 127 of 
body 103. Keeping the size of the base small also reduces the weight of 
the integrated sensor and transmitter assembly 101 (FIG. 1). 
Referring now to FIG. 3, illustrating in detail the cross-section of the 
pressure sensor assembly 129 in FIG. 1, diaphragm 301 sets snugly within 
the circular opening 303 of body 103 and rests on annular-shaped ledge 
305. The diaphragm is constructed from ABS plastic. Air under pressure 
within air passage 123 acts against an underside surface of the diaphragm. 
The pressure tends to deflect upwardly in the diaphragm, causing a nub 307 
integrally molded into an upper surface of the diaphragm to be displaced 
linearly and upwardly when the pressure sensor is assembled to hold the 
diaphragm in place around its circumferential edges. 
Partially overlaying diaphragm 301 is a first annular-shaped spacer 309 
that separates diaphragm 301 from lower metal contact 311. The lower metal 
contact is annularly shaped, having a hole defined within its center 
through which nub 307 passes. A second annularly-shaped spacer 313 
partially overlays lower metal contact 311 to space the contact apart from 
an upper metal contact 315. The upper metal contact has a "U" shaped 
depression that extends downwardly within the opening defined by second 
annularly-shaped spacer 313 to make contact with the lower metal contact 
311. 
A wave washer 317 cooperates with retainer ring 319 to apply a downward 
force to a top side of lower metal contact 311. The retainer ring 319, the 
waver washer 317 and the ledge 305 cooperate to hold together the pressure 
sensor assembly 129. Wave washer 317 further acts as a spring, exerting a 
downward force when compressed between an upwardly displaced upper metal 
contact and the retainer ring 319. 
When assembled, the diaphragm 301, nub 307, first and second spacers 309 
and 313 and waver washer 317 function as a pressure switch that is 
normally closed, but open under normal tire pressures. When air pressure 
within air line 123 is greater than a first predetermined value, the 
pressure displaces the diaphragm 301. The nub 307 on the diaphragm thereby 
moves upwardly and linearly, making contact with upper metal contact 315 
and displacing it upwardly so that it is no longer in contact with lower 
metal contact 311. Movement of the upper metal contact compresses the wave 
washer 317, creating an oppositely acting force to balance the force of 
the air pressure and stop movement of the diaphragm. When the air pressure 
falls below the predetermined pressure, the force applied by the 
compressed wave washer 317 moves the upper contact downwardly to meet with 
the lower metal contact 311. Upper and lower contacts 315 and 311 are 
essentially a switch that is opened and closed by the movement of 
diaphragm 301 in response to the pressure of the air within passage 123. 
A lead is attached to each metal contact for connecting the pressure sensor 
to transmitter circuitry. Lead 321 is attached to the upper surface of 
upper metal contact 315. Though it is not shown, lead 323 is attached to 
the upper surface of lower metal contact 311 and passed through an opening 
defined within upper metal contact 315. 
To test the integrated sensor and transmitter assembly when it is mounted 
to a tire stem and the air pressure in the tire is normal, a 
"push-to-test" button 325 is provided. The button is mounted through an 
opening defined in the cover 105. An "O"-ring seal 327 seals the opening. 
Referring now to FIG. 3A, pressure sensor 329 replaces the pressure sensor 
129 shown in FIGS. 1 and 3. Pressure sensor 329 does not function or 
operate like pressure sensor 129. Instead, metallic contacts 331 and 333, 
separated by insulating spacer ring 335, remain normally opened. Pressure 
greater than a preselected level of the air within air line 123 acts 
against diaphragm 337, causing it to flex upward and push against 
disc-shaped portion 339 of "push-to-test" button 341. The button is 
thereby displaced. When displaced, button 341 acts against lower contact 
331, flexing it and displacing its annular-shaped raised ridge portion 343 
so that it makes contact with the upper contact disc. Lower contact 331 is 
a spring disc and generates a force tending to open the contacts so that 
the contacts are open when pressure acting against the diaphragm falls 
below a preselected level. 
Referring now to FIG. 4, an active transmitter circuit 400 for transmitting 
air pressure information from pressure sensor 129 is shown that will be 
inserted into the electronics cavity 131 (FIGS. 1 and 2) of the integrated 
sensor and transmitter assembly 101. The transmitter circuit included a 
printed circuit board (PCB) 401 that is shaped to snugly fit within the 
electronics cavity 131. A metallic loop antenna 403 is "printed" onto the 
PCB for transmitting radio frequency (RF) signals. Mounted to the board is 
a microtransmitter 405, such as a RFM-MX Series microtransmitter 
manufactured by RF Monolithits, Inc. of Dallas, Tex. The microtransmitter 
has five inputs/outputs: an RF output 1, a modulation input 2, a power 
control input 3, a direct current power supply input 4, and a ground 5. 
Power is supplied by a suitable, long-life battery 407. The battery power 
supply input is connected to the positive terminal of the battery 407. A 
switchable power supply circuit is created by connecting the negative 
terminal of the battery to one of the two leads 201 from the pressure 
sensor 129, and connecting the other lead to the ground pin of the 
microtransmitter 405. When the pressure sensor 129 senses a normal 
pressure within a tire, the contacts within the pressure sensor are 
opened, thereby opening the power supply circuit and disabling the 
microtransmitter. When the pressure falls below a preselected, abnormal 
pressure level, the contacts within the pressure sensor to close (see FIG. 
3) power is supplied to the microtransmitter. The microtransmitter then 
begins to transmit a radio frequency signal. 
The RF output of the microtransmitter is connected to the loop antenna in a 
manner described by the manufacturer of the microtransmitter. If desired, 
the RF output can be modulated with an identifying code by a signal 
provided by additional circuitry (not shown) to the modulating signal 
input. Appropriate circuitry (not shown), as specified by the manufacturer 
of the microtransmitter, is provided on the PCB for setting the power of 
the RF output with the power control input. 
Referring now to FIG. 5, a passive radio frequency receiver and transmitter 
assembly may be used in place of the active microtransmitter circuit 400 
shown in FIG. 4 in the electronics cavity 131 (FIGS, 1 & 2). It is used in 
connection with a normally open pressure sensor switch, such as pressure 
sensor 329 of FIG. 3A. The main component is a surface acoustic wave 
(SAW), tapped delay line (TDL) transmitter/receiver 501. The other 
component is an antenna 503. The antenna in this case is a sheet of foil 
505 mounted to a piece of cardboard 507. The surface area of the foil 
antenna is determinative of its gain, and therefore should be maximized. 
The SAW device acts like a transponder. It is excited by a pulsed radio 
frequency (RF) signal at a particular frequency received on its antenna 
503. The received signal is delayed and processed by passive tapped delay 
line circuitry and is then re-radiated on the antenna 503. In effect, the 
SAW device echoes back or responds with an RF signal having known 
characteristics when interrogated by an RF signal having predetermined 
characteristics. No power is required or consumed by the device; it is 
entirely passive. 
In the present application, the antenna 503 is connected to the RF 
input/output pin of the SAW device. The antenna is also connected by one 
of the two leads 201 to one of the contacts of pressure sensor 329. The 
other of the two leads 201 couples the metal package of the SAW device, 
which is ground, to the other contact in the pressure sensor 329. When the 
contacts are closed in the pressure sensor 329 by normal tire pressure in 
a tire to which the integrated transmitter/sensor is attached, the antenna 
is grounded, thus disabling the SAW device. When the tire pressure falls 
to abnormal levels and opens the contacts in the pressure sensor 329, the 
SAW device is able to transmit or echo back. Alternatively, it is also 
possible to mount a SAW device directly on the diaphragm of a pressure 
sensor so that the diaphragm stresses this SAW device and thereby alters 
its response characteristics. The response characteristics of the SAW 
device thereby represent the tire pressure. Once calibrated to the actual 
tire pressure, the response characteristics of the SAW device become 
representative of the actual pressure sensed by the pressure sensor, 
thereby providing the ability, if desired, to monitor a continuous range 
of tire pressures. 
SAW devices are manufactured by a number of different firms, such as RF 
Monolithits, Inc. of Dallas, Tex. and SAWTEK, Inc. of Orlando, Fla., and 
are available in a wide range operating frequencies. Generally, higher 
frequency devices are preferred, as they are smaller and require less 
antenna area. However, they are more expensive. Further, frequency of 
operation of these devices is governed by Federal Communications 
Commission regulations. For application in remote monitoring of tire 
pressure, the permitted frequency range of operation is 900 to 945 MHz. In 
this operating range, the size of the antenna 503 is preferred to be at 
least 1.25 square inches. 
Referring now to FIG. 6, a schematic diagram illustrates the electronic 
components of the tire pressure monitoring system using a passive 
transmitter, such as the SAW device 501, in the integrated 
sensor/transmitter 101. An RF transmitter 601 generates a pulsed RF signal 
at an operational frequency and having characteristics in accordance with 
those required for the SAW device 501 to respond. The pulsed RF signal is 
transmitted over an antenna 603. Antenna 603 is mounted within a wheel 
housing of a vehicle in close proximity to the wheel to which the 
integrated sensor/transmitter housing 101 (FIG. 1) holding the SAW device 
501 is mounted. 
Each transmission of the pulsed RF signal is part of a cycle that includes 
a "transmit" and a "listen" interval. The pulsed RF signal is transmitted 
during the "transmit" interval of the cycle. It is followed by a "listen" 
interval. Each cycle is periodically repeated sixty times a second in the 
preferred embodiment. Typically, the radiation pattern of pulsed RF signal 
will be narrowly shaped or focused and aimed at a hub of a wheel to 
provide an area of high gain through which the integrated 
sensor/transmitter assembly 101 (FIG. 1) passes as a tire to which it is 
mounted rotates. When radiated from a flat, planar antenna mounted in a 
wheel well it assumes a football shaped pattern. Sixty cycles per second 
ensures that an integrated sensor/transmitter assembly mounted on a wheel 
of a vehicle travelling at highway speeds will be within this high gain 
area for at least five cycles. 
During the listen interval, the transmitter stands by and receiver 605 
listens for a responding signal from SAW 501 on antenna 603. Signals 
received on antenna 603 and provided to receiver 605 are first filtered 
with band pass filter 607 and amplified with preamplifier 609 before being 
passed to decoder 611. The function of the decoder is to identify a signal 
originating from a SAW device. In its simplest form, identifying the 
signal requires detection of a signal having a frequency equal to that of 
a signal originating from the device and having a predetermined phase 
relationship with the pulsed RF signals from the transmitter. The receiver 
then provides to an alarm unit 613 mounted near a driver of a vehicle a 
signal indicating detection of a responding or echoing SAW, and the alarm 
unit either sounds an audible alarm or flashes a visual alarm, or both. 
Before providing the signal to alarm unit 613, the receiver confirms 
detection of a responding SAW by listening for and detecting a responding 
signal in each of a predetermined number of consecutive listen intervals. 
Typically, more than one tire will be monitored in most systems. Receiver 
605 listens for responses from any SAW device in integrated pressure 
sensor/transmitter assemblies mounted to the tires. 
In more sophisticated systems having a multiple number of integrated 
pressure sensor/transmitter assemblies 101 (FIG. 1), each coupled to a 
tire stem of a separate tire, the signal echoed by the SAW device is, if 
desired, modulated with a predetermined binary cede. Which SAW is 
responding is then determined by the decoder demodulating the signal and 
looking up the binary code. The tire with abnormally low pressure is 
thereby identified. The receiver 605 then indicates with a signal to the 
alarm unit 613 the tire with the abnormal tire pressure. Though the 
decoder 611 looks up with the binary code which tire corresponds to the 
demodulated code, this could be handled by an alarm unit that looks up the 
tire from a table that has been programmed with the codes and the 
corresponding tires. In this case, where it is possible to identify which 
tire in a multiple-tire vehicle is transmitting, the pressure sensor 
switch 329 may be changed to a normally closed switch 129. The SAW thus 
responds when the tires are under normal pressure and fall to respond when 
the pressure drops below a predetermined value. A failure to relieve a 
response from a SAW indicates either low pressure or a failed pressure 
sensor and transmitter assembly. 
An alternate method (not shown) for identifying uniquely which tire of a 
plurality of tires is suffering from abnormally low tire pressure is to 
poll and listen to each integrated sensor/transmitter separately. This can 
be done by selectively transmitting from each of a plurality of 
interrogating antennas 603, each focused on one or two integrated pressure 
sensor/transmitter assemblies in a time division multiplexed fashion. A 
switching circuit for selectively coupling the transmitter 601 and 
receiver 605 to a line going to each antenna 603 would be required. It is 
also conceivable that this may be done by utilizing SAW's responding to 
unique frequencies and stepping the frequency of the pulsed RF signal from 
transmitter 601 through the range of frequencies and listening after each 
step for a response. 
Referring now to FIG. 7, a schematic representation of a motorhome or bus 
701, the alarm unit 613, transmitter 601 and receiver 605 of FIG. 6, 
collectively represented by monitoring unit 703, are mounted near the 
driver, as represented by steering wheel 705. A coaxial cable 706 couples 
monitoring unit 703 to a plurality of antennas 603 and 603A mounted within 
each wheel well. Antenna 603 is a flat panel type, but it can be built 
with a slight radius to fit the curve of the wheel well. It is mounted at 
the top of single axle wheel wells so that it emits or broadcasts 
downwardly toward the hub of the wheel. 
Each integrated sensor/transmitter assembly 101 is screwed onto the Shrader 
valve (not shown) of a stem of a tire 707. Please note that it is standard 
that tire stems for inside tires of a multiple tire axle are coupled to a 
reinforced air hose that is brought through the outside wheel and 
terminates at a Shrader valve connected to the outside wheel. Thus, the 
outside tires of the rear axles 708 have a pair of integrated 
sensor/transmitters, each connected to one of two Shrader valves (not 
shown). 
Antenna 603 has a conical or football-shaped radiation pattern. This 
pattern delivers high gain in exchange for a narrower coverage area. 
Although it is desirable to have coverage over the entire path of rotation 
of an integrated sensor/transmitter 101, it is not necessary and it is 
more desirable to provide for high gain considering the relatively weak 
signals given off by SAW devices. It is sufficient that an integrated 
sensor/transmitter passes through the RF radiation at least twice during 
the rotation of the wheel, remaining, as previously indicated, within the 
radiation pattern for at least five consecutive transmit/listen cycles. 
Maximum gain occurs when an integrated sensor/transmitter assembly is 
closest to the antenna 603 during revolution of the tire. 
Antenna 603A is comprised of two flat panel antennas. It is designed to be 
used in wheel housings for vehicles having two rear axles 708, each 
typically with two tires, such as motorhomes, buses and trailers. The two 
panels are secured to the top of the wheel well in a manner so as to form 
a shallow triangle, with the football-shaped radiation pattern of each of 
the antennas aimed separately toward (though not necessarily exactly at) 
the hub of the outer wheel of one of the two rear axles 708 in the wheel 
housing. 
Referring now to FIG. 8, a schematic representation of a tractor 801 and 
trailer 803 combination, the set up of the on-board monitoring unit 703, 
antennas 603 and 603A and the integrated sensor/transmitters 101 are 
installed on a tractor and trailer combination in the same manner they are 
installed on the bus or motorhome 701 (FIG. 7). However, because the 
trailer 803 will be disconnected from the tractor 801, the branch of 
coaxial cable 706 running to the antennas 603A in the rear 805 of the 
trailer is coupled through a disconnect coupling 804 to the monitoring 
unit 703 in the cab. For other towing arrangements, such as motorhomes 
towing boats or other vehicles, the low tire pressure monitoring system is 
installed in a like manner. 
Referring now to FIG. 9, shown is a top view of brass insert 113 of FIG. 1, 
illustrating the position of the tongue 117. 
Referring now to FIG. 10, a cross-section taken along section line 10--10 
of FIG. 9, the brass insert is sealed against the top of a Shrader valve 
(not shown) with "O"-ring seal 1001 when screwed onto the Shrader valve. 
Tongue 117 extends downward to depress the valve core in the Shrader 
valve. 
Referring now to FIG. 11, an alternate embodiment for a body 1101 and cover 
1103 of an integrated transmitter/sensor assembly is used in place of the 
body 103 and cover 105 of integrated transmitter/sensor assembly 101 in 
FIG. 1. An opening 1105 is defined in the cover through which extends a 
push-to-test button (not shown) such as the push-to-test button 325 shown 
in FIG. 3 and push-to-test button 341 in FIG. 3A. Like the tire extension 
section 127 of body 103 (FIG. 1), body 1101 includes a tire stem extension 
1107 section protruding, when assembled, through an opening in cover 1103. 
Note that, for purposes of illustration, a valve core assembly is not 
shown within the opening of the tire stem extension section. 
Unlike that of the body 103 (FIG. 1), body 1101 has an integrally formed 
planar section 1109 for an electronics cavity in which are mounted an 
antenna 1111 (shown in phantom) and SAW device or microtransmitter 1113 
(shown in phantom), as the case may be. The antenna 1111 is a rectangular 
piece of conductive foil mounted to cardboard, to which the RF input and 
output of the SAW or microtransmitter 1113 is coupled. When the integrated 
sensor and transmitter assembly 101 is mounted to a Shrader valve of a 
tire stem that is substantially parallel to the ground, this planar 
section 1109 of body 1101 properly orients the plane of the antenna 1111 
so that it is parallel to the Shrader valve. With this orientation, the 
antenna is orthogonal twice during a revolution of a tire with respect to 
a line between the antenna 1111 and an antenna mounted near the wheel in a 
wheel housing, shown in FIGS. 7 and 8. Because the gain of the antenna 
1111 is dependent on its area orthogonal to the direction of incidence of 
a radio frequency radiation, this body geometry maximizes the gain of the 
antenna 1111. Maximizing gain increases reliability. This is especially 
desirable if a SAW device is used as a transponder, as signals transmitted 
by it are relatively weak. 
Referring now to FIG. 12, a section taken along line 12--12 of FIG. 11 of 
the assembled body 1101 and cover 1103 reveal a cavity 1201 for receiving 
the pressure sensor assembly 129, shown in FIG. 3, or pressure sensor 
assembly 329 shown in FIG. 3A. The pressure sensor assembly is not shown 
for purposes of clarity. Like the body 103 (FIG. 1), body 1101 includes an 
air passage 123 for communicating air pressure to the pressure sensor 
assembly from the brass insert 113 that is screwed onto the tire stem. 
Tire stem extension section 1107 also includes a screw in valve core 
assembly such as valve core assembly 125 shown in FIG. 1. However, it too 
is not shown for clarity. 
The planar antenna section 1109 of body 1101 is substantially square to 
allow antenna 1111 to have a large surface area. The antenna is made of a 
foil of conductive material 1111A that is mounted to the back side of a 
plastic antenna cover panel 1111B for protection against weather and other 
hazards. The antenna cover panel fits snugly between flanges 1109A of 
antenna section 1109 of body 1101 and is, if desired, fastened with an 
adhesive to the antenna section 1109 of body 1101. 
A cavity 1203 is integrally formed in the antenna section 1109 to receive 
SAW device or microtransmitter 1113. Antenna cover 111 lB effectively 
seals this cavity against hazards. Though not shown, the RF input and 
output leads of the SAW device are connected to antenna 1111. Furthermore, 
the two leads 201 (FIGS. 2, 4 and 5) from either pressure sensor assembly 
129 (FIG. 3) or 329 (FIG. 3A) when mounted in cavity 1201, are routed 
around tire stem 1107, first within cover 1103 and then down into a well 
1205 integrally formed within base 1101. The leads then pass from well 
1205 through a narrow passage 1207 to the SAW device or microtransmitter 
1113. 
An alternative embodiment of the low tire pressure monitoring system of the 
present invention which utilizes a hall effect sensor is illustrated in 
FIGS. 13A-B and indicated generally as 1300. A valve stem extension 1302 
is illustrated schematically, but is analogous in design to the valve stem 
extension 101 illustrated in FIG. 1 in that it is adapted to screw onto an 
existing Shrader type valve stem and provide both a capillary air path for 
sensing the tire pressure and a second Shrader valve for normal tire 
operations (such as manually checking tire pressure and adding air to the 
tire). A small brass air line 1304 is attached to the valve stem extension 
1302 and is open to the inside of the valve stem extension 1302. When the 
valve stem extension 1302 is attached to the tire valve stem, the pressure 
inside air line 1304, the valve stem extension 1302, the valve stem and 
the tire are equal. 
The other end of the air line 1304 is attached to the inlet port 1305 of a 
pneumatic cylinder 1306 (see FIGS. 14A-B). The cylinder contains a piston 
1308 that is adapted to sealingly engage the walls of the pneumatic 
cylinder 1306 and slide longitudinally therein. The piston 1308 is carried 
by a rod 1310 that is adapted to sealingly engage an opening in an end 
wall of the pneumatic cylinder. The piston 1308 is coupled to an internal 
spring 1314 which biases the piston 1308 and piston rod 1310 to its full 
extension into the pneumatic cylinder 1306, as shown in FIG. 13B. Air 
pressure from the monitored tire appearing at the inlet port 1305 acts 
upon the piston 1308 and tends to extend the piston rod 1310 from the 
pneumatic cylinder 1306, thereby acting against the spring force and 
compressing the spring 1314. The piston 1308 will therefore move within 
the pneumatic cylinder with changing air pressures of the monitored tire. 
A steel flag 1316 is attached to the piston rod 1310 by means of nuts 1318 
and 1320. Steel flag 1316 is shaped so that it completely covers a south 
pole magnet 1322 when the piston 1308 is extended to a first predetermined 
point within the pneumatic cylinder 1306 (see FIG. 14B). The steel flag 
1316 is shaped so that it covers the magnet 1322 half way when the piston 
1308 is extended to a second predetermined point within the pneumatic 
cylinder 1306. Additionally, the steel flag 1316 is shaped so that it 
leaves the magnet 1322 completely uncovered when the piston 1308 is 
extended to a third predetermined point within the pneumatic cylinder 1306 
(see FIG. 14A). The spring constant of spring 1314 is selected so that the 
piston 1308 will move to the first, second and third predetermined 
positions at known air pressures appearing at the inlet port 1305. 
Specifically, the spring 1314 is sized so that the piston 1308 will be 
located at the first predetermined point when a pressure that is below the 
minimum allowed pressure of the monitored tire is reached. The spring 1314 
is also sized so that the piston 1308 will be located at the second 
predetermined point when a pressure that is equal to the minimum allowed 
pressure of the tire is reached. Finally, the spring 1314 is also sized so 
that the piston 1308 will be located at the third predetermined point when 
a pressure that is above the minimum allowed pressure of the tire is 
reached. In this way, the magnet 1322 is covered by the steel flag 1316 at 
all fire pressures below the preset minimum allowable pressure, halfway 
covered at the minimum allowable pressure, and uncovered above the minimum 
allowable pressure. 
The steel flag 1316 will block the magnet flux from the magnet 1322 when it 
is covering the magnet. The magnet 1322 is situated in the device 1300 
such that the south pole of the magnet 1322 is facing the cover 1324 of 
the device. When the steel flag 1316 is fully extended (as in FIG. 14A), 
magnetic flux from the south pole is present at the cover 1324 of the 
device. When the steel flag 1316 is fully retracted (as in FIG. 14B), most 
of the magnetic flux is blocked from reaching the front cover 1324 of the 
device 1300. When the magnet 1322 is half covered by the steel flag 1316, 
the level of magnetic flux present at the cover 1324 of the device 1300 is 
less than half of the level when the flag 1316 is fully extended. 
The pneumatic cylinder 1306, the steel flag 1316 and the magnet 1322 are 
housed in an ABS plastic box 1326. The box 1326 is sealed with an ABS 
plastic cover 1324 to protect the contents and to seal out moisture and 
dirt. 
From the above, it will be appreciated by those skilled in the art that the 
pressure sensing device 1300 is adapted to produce differing amounts of 
magnetic flux near the cover 1324 in proportional relationship to the 
pressure of air within the tire to be monitored. In order to measure the 
amount of magnetic flux generated by the device 1300, a hall effect sensor 
assembly 1500 is provided, as illustrated in FIG. 15. The hall effect 
sensor assembly 1500 contains a hall effect sensor 1502 oriented parallel 
to the cover of the pressure sensing device 1300. The hall effect sensor 
1502 is housed inside a metal tube 1504 which provides a rigid mounting 
and protection from dirt and moisture. Wires 1506 and 1508 provide power 
and ground for the sensor 1502 to operate electrically and a wire 1510 
transmits the electrical signal from the sensor 1502 to the control box. 
The sensor 1502 and wires 1506-1510 are sealed inside the metal tube 1504 
with epoxy resin in order to protect against dirt and moisture. 
The hail effect sensor 1502 is preferably a unipolar south actuating 
current sinking sensor, such as SS443A manufactured by Honeywell 
Microswitch, Inc. Hall effect sensor 1502 will sense the presence of 
magnet flux from the south pole of magnet 1322. Specifically, when the 
magnet flux is present, the signal on wire 1510 from the sensor 1502 will 
be less than 0.5 VDC. When the magnet flux is not present, the signal on 
wire 1510 from the sensor 1502 will be approximately 5.0 VDC. The signal 
on wire 1510 will switch from 0.5 VDC to 5.0 VDC when the steel flag 1316 
is covering approximately half of the magnet 1322. 
In operation, as the pressure sensing device 1300 moves in a plane parallel 
to the face of the hall effect sensor 1502, the signal on wire 1510 from 
the hall effect sensor 1502 will begin at 5.0 VDC when the hall effect 
sensor 1502 is too far away from the magnet 1322 to sense any lines of 
magnetic flux. As the magnet 1322 of the pressure sensing device 1300 
aligns with the hall effect sensor 1502, the signal on wire 1510 will 
change to 0.5 VDC if the pressure in the tire is sufficient to move the 
piston 1308 and hence the steel flag 1316 to a position which does not 
cover the magnet 1322. As the pressure sensing device 1300 then moves past 
the hall effect sensor 1502 such that the lines of magnetic flux from 
magnet 1322 can no longer be sensed by the sensor 1502, the output on wire 
1510 returns to 5.0 VDC. If the pressure in the tire is insufficient to 
move the steel flag 1316 to a position which does not cover the magnet 
1322, the signal from the hall effect sensor 1502 on wire 1510 would 
remain at 5.0 VDC as the sensor 1502 aligns with the magnet 1322 in the 
pressure sensing device 1300. In other words, an electrical pulse cycling 
between 5.0 VDC to 0.5 VDC to 5.0 VDC is produced when the air pressure in 
the tire is above the predetermined minimum allowable pressure. On the 
other hand, if the air pressure in the tire is below the predetermined 
minimum allowable pressure, the output on wire 1510 remains at a steady 
5.0 VDC. 
FIG. 16 illustrates how this principle is applied to sensing pressure in 
the tires of a vehicle. A pressure sensing device 1300 is attached to the 
inside rim 1600 of each wheel on the vehicle. The valve stem extension 
1302 of each pressure sensing device 1300 is attached to the valve stem of 
each tire and thereby couples the tire pressure to the sensing device 1300 
via air line 1304. The hall effect sensor 1500 is mounted to a fixed 
member 1602 of the chassis of the vehicle and is oriented so that it is 
aligned with the magnet 1322 of the pressure sensing device 1300 once with 
each revolution of the wheel. In other words, the hall effect sensor 1500 
is mounted in a fixed position and the pressure sensing device 1300 
rotates with the rim of the tire, thereby bringing the pressure sensing 
device 1300 and the hall effect sensor 1500 into alignment once each 
revolution of the tire. One hall effect sensor 1500 is mounted for each 
wheel of the vehicle. It will be appreciated by those skilled in the art 
that the valve stem extension 1302 is on the outside of wheel 1600 and the 
pressure sensing device 1300 is on the inside of wheel 1600, therefore 
there is sufficient clearance for the hall effect sensor 1500 over the 
pressure sensing device 1300 without interference from the valve stem 
extension 1302. 
The hall effect sensor 1500 of each wheel on the vehicle is coupled via 
wire 1510 to a control box 1700 located inside the vehicle. Two 
embodiments of the control box operator panel are shown in FIG. 17. FIG. 
17A illustrates a control box operator panel 1702 for use with a vehicle 
having 14 tires, while the operator panel 1704 of FIG. 17B is for a 
vehicle having 6 tires. It will be appreciated by those skilled in the art 
that a suitable operator panel may be designed for a vehicle having any 
number of axles and any number of wheels per axle. The operator panel 
includes a power on indicator LED 1706 which indicates that the 
electronics are coupled to an active power supply. A pressure OK indicator 
LED 1708 is included for each monitored tire which blinks one rime each 
rime the hall effect sensor 1500 passes the magnet 1322 in the pressure 
sensing device 1300 if the pressure is above the maximum low pressure 
level. A red pressure error indicator LED 1710 is provided for each 
monitored tire which is illuminated when the control box 1700 has 
determined that the tire pressure is below the minimum acceptable tire 
pressure. The LED 1710 remains on continuously until the low pressure 
condition is corrected and the reset button 1712 is pressed. The reset 
button 1712 is used by the operator to reset the electronics after an 
error condition has been corrected. An alarm on/off button 1714 is used to 
enable/disable the audible alarm in the control box 1700. An alarm off 
indicator LED 1718 indicates that the audible alarm has been disabled. A 
fuse holder 1720 and fuse (not shown) is provided to protect the 
electrical components of the control box 1700 in case of an electrical 
short or voltage surge. 
The electronic circuitry of control box 1700 is illustrated in schematic 
block diagram form in FIGS. 18 and 19. The block diagrams of FIGS. 18 and 
19 are illustrative of a system employing two tire pressure sensors. It 
will be appreciated by those skilled in the art that analogous systems 
supporting a fewer or greater number of sensors may be easily constructed 
by analogy with reference to FIGS. 18 and 19. Referring now to FIG. 18, a 
first hall effect sensor 1802 is mounted to the chassis of the vehicle 
(1602) in the proximity of the rim of the wheel of tire one and a second 
hall effect sensor 1804 is mounted similarly near the rim of the wheel of 
tire two. The hall effect sensors 1802 and 1804 are of the design of 
sensor 1500 of FIG. 15. The output of hall effect sensors 1802 and 1804 is 
in the form of short pulses which coincide with the sensing of the 
corresponding pressure sensing device 1300 (mounted on the rim) during 
each revolution of the associated tire. This output is buffered by input 
buffers 1806 and 1808, respectively. The buffered outputs are input to 
respective edge detect circuits 1810 and 1812. The design of such edge 
detect circuits are well known in the art. The edge detect circuits 1810 
and 1812 require a system clock input provided by clock oscillator 1814, 
preferably a model ICM 72421PA manufactured by Harris Semiconductor. 5 VDC 
power is supplied to all of the powered components from a voltage 
regulator 1816 coupled to the 12 VDC line from the vehicle's battery. 
Voltage regulator 1816 is preferably a model 78SR1-05-HC manufactured by 
Power Trends. 
The outputs of buffers 1806 and 1808 are also input to pulse expander 
circuits 1824 and 1826, respectively, which are preferably a model 
SN74HC123 retriggerable multivibrators manufactured by National 
Semiconductor. Pulse expanders 1824 and 1826 operate to produce an output 
pulse upon the occurrence of each input pulse, however the output pulse 
has a uniform, longer period. The output of the pulse expanders 1824 and 
1826 are coupled to green LED indicators 1708 on the control box operator 
panel 1702 or 1704 of FIG. 17. The pulse expanders 1824 and 1826 are 
required because the buffered outputs of the hall effect sensors 1802 and 
1804 have relatively short duration pulse widths which may be too short to 
produce a visible "blink" of the indicator LEDs 1708. 
The edge detect circuits 1810 and 1812 produce an output pulse having a 
duration of one clock period upon the detection of a transition in the 
buffered hall effect sensor output. The output pulses of edge detect 
circuits 1810 and 1812 appear on lines 1828 and 1830, respectively. The 
outputs of the edge detect circuits are also input to an enable control 
circuit 1818 which produces an output enable signal on line 1820 when a 
signal is received from tire one. Similarly, a second output enable signal 
1822 is produced when a signal is received from tire two. Upon the receipt 
of this signal from tire two, the output enable signal on line 1820 is 
extinguished. This process repeats continuously as long as signals are 
received from the two tires. 
The detection of a fault (i.e. a tire having pressure below the 
predetermined minimum allowed pressure) is accomplished by comparing the 
edge-detected signals from two different rims. If an edge-detected pulse 
is received from a first rim and is followed by an edge-detected pulse 
from the second tire, no error output is generated (normal operation). 
However, if an edge-detected pulse is received from the first tire and is 
not followed by an edge-detected pulse from the second fire, an error 
output is generated for each subsequent occurrence of the edge-detected 
output of the first fire. In other words, each tire should produce an 
output once during every revolution, therefore each sensor should produce 
an output before the other sensor produces a second output. In order to 
account for differences in the speed of rim rotation, such as during 
cornering or wheel spin, the error outputs are input to a counter and no 
alarm is given to the driver until a predetermined number of consecutive 
error outputs occur. The error counter is reset if the second sensor 
begins producing output pulses before reaching the predetermined number of 
missing pulses for an alarm. This function is implemented in the schematic 
block diagram circuit of FIG. 19. 
Missing edge-detected pulses are detected by pulse comparators 1902 and 
1904, which may be any of a wide variety of pulse comparators known in the 
art. Pulse comparator 1902 detects missing pulses from the sensor of tire 
one, while pulse comparator 1904 detects missing pulses from the sensor of 
tire two. The outputs of pulse comparators 1902 and 1904 are input to 
fault detect accumulator 1906 which may be any suitable counter or shift 
register, such as model 74HC164 manufactured by National Semiconductor. 
Fault detect accumulator 1906 functions to record the number of missing 
pulses detected for both tire one and tire two. This count is reset if the 
tire begins producing output pulses again before fault detect accumulator 
reaches its predetermined threshold count. Pulse comparators 1908 and 1910 
are used to detect the occurrence of output pulses from the tire one 
sensor and the tire two sensor, respectively. The outputs of pulse 
comparators 1908 and 1910 are coupled to the reset of fault detect 
accumulator 1906. Fault detect accumulator is also reset by manual reset 
switch 1912 or the occurrence of a power on startup condition. 
Error enable circuit 1914 prevents false error accumulation when the 
vehicle is stopped with one of the hall effect sensors 1802, 1804 directly 
over the magnet of it's respective fire. If the vehicle is rocked back and 
forth while stopped (as when maintaining position on an incline by 
repeatedly engaging the clutch of the vehicle for brief moments), one hall 
effect sensor (the one directly over its associated sensor) will 
continuously produce output pulses while the other does not, thereby 
accumulating false error conditions in the fault detect accumulator 1906. 
When fault detect accumulator 1906 reaches a predetermined count for one of 
the tires, it produces an output to the appropriate error latch 1916 or 
1918. Error latch 1916 and 1918 are operative to illuminate respective 
fault indicator LEDs 1710 on the control box operator panel 1702 or 1704. 
Error latches 1916 and 1918 also produce an output to the audible fault 
alarm 1920 which will produce an alarm that the driver of the vehicle can 
hear, so that he does not have to constantly monitor the LED indicators. 
Signal lines 1922 and 1924 are provided for connection to switch 1714 (see 
FIG. 17) to allow the driver to disable the audible alarm 1920. 
It will be appreciated by those skilled in the art that because the present 
invention utilizes the comparison of two tire sensors in order to 
determine a fault, the circuitry of FIGS. 18 and 19 may be reproduced in 
any number of corresponding pairs to provide error detection for any 
number of tires on a given vehicle. 
While the present invention has been described with a reference to a few 
specific embodiments, the description is made for the purposes of 
illustrating of the invention. It is not intended, nor should it be 
construed as limiting, the invention. Various modifications may occur to 
those skilled in the art without departing from the true scope and spirit 
of the invention as set forth by the appended claims.