System and method for diagnosing loss of pressure in tires of a vehicle

A system and method for diagnosing when a tire of a vehicle is beginning to lose pressure. The vehicle has four wheels, and four associated tires mounted thereon, each with an associated angular speed when the vehicle is in motion. Four wheel speed sensors are attached to one of the four wheels of the vehicle for sensing the angular speed of each wheel. A microprocessor is also included that is electrically coupled to each of the wheel speed sensors for receiving the angular speed of each wheel. The microprocessor integrates, over time, the angular speed of each wheel. An indicator is also provided that is coupled to the microprocessor for indicating that a tire of the vehicle is losing pressure if the microprocessor determines that the integral of the angular speed of one of the tires is greater than the integral of the angular speed of the other tires. A method is also provided for diagnosing when one of the vehicle tires is beginning to lose pressure, such as in the case of a slow leak.

BACKGROUND 
1. Field of the Invention 
In general, the present invention relates to diagnostic systems for 
vehicles, and in particular, the present invention relates to a system and 
method for diagnosing a low pressure tire of a vehicle. 
2. Description of the Related Art 
Modern vehicles have a large number of different systems, proper operation 
of which effects the vehicle's overall safety, operability, and 
drivability. It is important to inform the driver as quickly as possible 
if the vehicle is not operating at its peak performance. Currently, 
vehicles provide information to the driver by different diagnostic 
systems, which generally are restricted to critical engine parameters. Few 
existing systems, however, provide information about tire condition, which 
may be of great importance to the operation of the vehicle. 
Current tire monitoring systems are very costly and add weight to the 
vehicle via extra sensors and computer hardware. These systems typically 
monitor and measure tire pressure by the use of extra electromechanical 
devices that are added to a vehicle in the after-market, such as U.S. Pat. 
No. 5,289,160 to Fiorletta, U.S. Pat. No. 4,816,802 to Doerksen, and U.S. 
Pat. No. 4,894,639 to Schmierer. These patents and others in the area of 
tire monitoring systems, employ special hardware that is attached to the 
tire or wheel system of the vehicle for measuring tire pressure loss. In 
operation, the systems relay information via electromechanical means to 
warn a vehicle operator if pressure in one of the tires is getting low. 
The systems have, however, proved to be very expensive, difficult to 
install, and somewhat unreliable during practical applications. Further, 
devices have been provided for controlling differential driven wheel slip, 
such as U.S. Pat. No. 5,535,124 to Hosseini. These types of devices do not 
adequately detect and indicate to a vehicle operator when a tire of the 
vehicle has begun to go flat. 
It would, therefore, be desirable in the present art to have a system and 
method for diagnosing a low pressure tire of a vehicle that uses existing 
vehicle hardware such as an anti-lock braking system and that will 
indicate to an operator when one of the vehicle's tires has a relatively 
lower pressure and, perhaps, is beginning to lose pressure without 
directly measuring pressure in any of the tires. It would also be 
desirable to have a system and method that detects when a tire is losing 
pressure by simply integrating angular speed of each wheel that each tire 
of the vehicle is mounted on, over time, and determining from the 
integration whether one of the tires of the vehicle is beginning to loose 
pressure via a slow leak. 
SUMMARY OF THE INVENTION 
The present invention eliminates the disadvantages in the prior art by 
providing a system and method for diagnosing loss of pressure in tires of 
a vehicle. The vehicle preferably has four wheels, upon which the tires 
are mounted, each with an associated angular speed when the vehicle is in 
motion. A wheel speed sensor is attached to each of the four wheels of the 
vehicle, such as in an anti-lock braking system, for sensing the angular 
speed of each wheel. A microprocessor is also included that is 
electrically coupled to each of the wheel speed sensors for receiving the 
angular speed of each wheel. The microprocessor integrates, over time, the 
angular speed of each wheel. It is expressly understood that this 
integration of the angular speed over time will simply be referred to as 
the "angular speed" when stated below in the present invention. An 
indicator is also provided that is coupled to the microprocessor for 
indicating the presence of a tire with low air pressure, if the 
microprocessor determines that the angular speed of one of the tires is 
greater than the angular speed of the other tires. 
The method includes the steps of receiving the angular speed of each wheel 
and determining the angular speed of each wheel over time. Next, the 
method determines whether the angular speed of one wheel is greater than 
the angular speed of the other wheels and above a threshold value. If the 
answer is yes, the method indicates to an operator of the vehicle, via an 
indicator, that a tire of the vehicle is losing pressure. 
Other objects, features and advantages of the present invention will become 
apparent by reference to the following detailed description when 
considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Commencing with FIG. 1, a system 10 for monitoring and diagnosing loss of 
pressure in four tires 16a, 16b, 16c, and 16d of a vehicle 12 is shown. 
Each tire 16a, 16b, 16c, and 16d is mounted on an associated wheel 42a, 
42b, 42c, and 42d,respectively, of the vehicle 12. Each wheel 42d has an 
associated angular speed W.sub.f when the vehicle 12 is in motion. The 
angular speed of each wheel 42a, 42b, 42c, and 42d,is governed by the 
equations: 
EQU 2.pi.R.sub.f W.sub.f =2.pi.RW 
EQU R.sub.f W.sub.f =RW 
##EQU1## 
whereby R equals the radius of any particular wheel; R.sub.f equals the 
radial speed of the tire with low pressure; and W equals the angle of 
wheel rotation. Since radius R will always be greater than R.sub.f, thus 
W.sub.f is always greater than W though the difference can be infinitely 
small. Therefore, integration of the angular speed over time is required 
for the microprocessor to determine which tire is losing pressure. 
The vehicle 12 includes a frame 14 and a drive shaft 18 that runs 
longitudinally along the undercarriage of the vehicle 12. The wheels 42a, 
42b, 42c, and 42d are driven by an engine (not shown) through drive shaft 
18, respective first and second axles 20, and 30, and final drive devices 
24a, 24b, 24c, and 24d. The first axle 20 has an anti-lock braking systems 
26a and 26b, which includes braking mechanisms 22a and 22b and wheel speed 
sensors or transducers 28a and 28b. Similarly, the second axle 30 includes 
an anti-lock braking systems 36a and 36b having braking mechanisms 32a and 
32b and wheel speed sensors 38a and 38b, respectively. 
Each wheel speed sensor or transducer 28a, 28b, 38a, and 38b produces 
respective pulse signals having values responsive to, or representing, the 
angular speed W of the wheels 42a, 42b, 42c, and 42d of the vehicle 12. 
The wheel speed sensors of the present system 10 are each attached to one 
wheel of the vehicle 12 for sensing the angular speed. Each transducer 
28a, 28b, 38a, and 38b is preferably a Hall Effect Sensor. Other 
transducers, however, such as optical and electromagnetic devices may also 
be employed as alternatives in the present invention. Also, the phrase 
"wheel speed sensor" is to be construed in general terms and not as a 
strict scientific term because the ECU 50 typically has to compute or 
perform some calculations to determine the wheel speed and such 
information is usually not directly fed from the wheel speed sensors. 
The present system 10 further includes a microprocessor or Electronic 
Control Unit (ECU) 50 that is electrically coupled to each of the 
transducers 28a, 28b, 38a, and 38b for receiving the angular speed W of 
each wheel 42a, 42b, 42c, and 42d. The ECU 50 includes a CPU memory 
(volatile and non-volatile), bus lines (address, control, and data), and 
other hardware, software, and firmware needed to perform the task of data 
manipulation of an electrical signal. 
During movement of the vehicle 12, the ECU 50 is continuously receiving the 
angular frequency signals of the wheels 42a, 42b, 42c, and 42d and 
integrating, over time, the electrical signals representing the angular 
speed of each wheel 42a, 42b, 42c, and 42d, and comparing the results. If 
the pressure in one of the tires 16a, 16b, 16c, and 16d, is significantly 
less than in the others, then the corresponding integral for that tire 
will be the maximum of the integrals computed for each tire over time. 
This is a result of the "effective" diameter of the tire with less 
pressure being smaller than the others and so the corresponding angular 
speed for the wheel upon which it is mounted is higher than the others. 
The integral for each iterative pulse is governed by the equation: 
##EQU2## 
If the integral I.sub.k angular speed W.sub.f of the wheel is greater than 
the integral of the other three wheels by a predetermined threshold value 
that is stored in the memory of the ECU 50, the ECU 50 will send an 
electrical signal to an indicator 52 that is coupled to the ECU 50. The 
indicator 52 indicates when one of the fires 16a, 16b, 16c, and 16d,of the 
vehicle 12 is losing pressure if the ECU 50 determines that the angular 
speed of one of the wheels 42a, 42b, 42c, and 42d is greater than the 
angular speed of the other wheels. In the preferred embodiment, the 
indicator 52 is a Malfunction Indicator Light (MIL) for notifying a 
vehicle operator when pressure in one of the tires 16a, 16b, 16c, and 16d, 
becomes low. 
The present system 10 considers if the angular speed of one tire is greater 
than the other three tires taken from an average value integrated over 
time. It is understood that, external factors acting upon the wheels 16a, 
16b, 16c, and 16d,of the vehicle 12 may result in the angular speed of one 
of the wheels being greater than the others. For example, if one of the 
wheels was spinning on ice or snow. In addition, if a flat tire has been 
replaced by a smaller size spare tire such that the associated vehicle 
wheel will have an angular speed that is greater than the other wheels of 
the vehicle. But, in each of these instances, the vehicle operator will be 
aware of the external circumstances that may be causing the indicator to 
indicate to the driver that there may be a pressure problem with one of 
the tires. 
Another more common example, is shown in FIG. 4, which illustrates the 
vehicle 12 traversing a curved pathway 44. In this instance, the inner 
tires 16a, and 16d, which are following an arced path with radius R1, will 
have an angular speed that is less than the outer tires 16b and 16c, which 
are following an outer arced curve path with larger radius R2. The present 
system 10, however, will rectify this situation since two wheels of the 
vehicle will have an approximately equal angular speed as opposed to only 
one wheel when one of the tires has less pressure than the others. In 
operation, if it is determined by the ECU 50 that one of the tires 16a, 
16b, 16c, or 16d is losing pressure, the present invention may then check 
the tire opposite tire on the same side of the vehicle 12. If both tires 
on the same side of the vehicle are uniformly low in pressure, then it is 
determined that the vehicle 12 is traversing a curved pathway such as in 
FIG. 4, and an indication is not made to the vehicle operator. 
Referring to FIG. 2, a method for diagnosing the four tires 16a, 16b, 16c, 
and 16d, of vehicle 12 is shown. The method 58 begins or starts at bubble 
60 where the ECU 50 calculates four different integrals of the angular 
speed of each of the wheels 42a, 42b, 42c, and 42d, of the vehicle 12. In 
the preferred embodiment, the ECU 50 employs four separate 32 bit up-down 
counters (not shown) each for an associated wheel speed integral, but it 
is appreciated than any N-bit counter could be used depending on the 
application desired. The method 58 falls to block 62 whereby the four 
up-down counters are reset. Method 58 next falls to block 64 whereby the 
angular speed of each wheel 42a, 42b, 42c, and 42d,of the vehicle 12 is 
received by the ECU 50 via transducers 28a, 28b, 38a, and 38b. 
The method 58 next falls to decision block 66 where it is determined if a 
set time delay has expired. If the ECU 50 determines that is has not, the 
method 58 continues to loop back to decision block 66 until the time delay 
has expired. The method 58 then falls to block 68 where the time delay is 
reset in the memory of the ECU 50. The method 58 next falls to block 70 
where the max value, representing the maximum angular speed of one of the 
wheels 42a, 42b, 42c, and 42d, is determined from the four counters. The 
method 58 then falls to block 72 where the differences between the maximum 
value of one of the counters and the other counters is calculated by the 
ECU 50. The method 58 continues to decision block 74 where it is 
determined if the max value difference calculated in block 72 is greater 
than a set threshold value, such as a value stored in a data table that 
corresponds to a ratio of desired fire pressure values. If the difference 
is not, the method returns to decision block 66. If the answer, however, 
in decision block 74 is yes, the method falls to block 76 whereby the ECU 
50 has determined that pressure in one of the tires 16a, 16b, 16c, and 
16d, is getting low and the diagnostics indicator 52 is turned on. The 
method 58 next falls to bubble 78 where it is stopped or ended and the ECU 
50 performs other vehicle diagnostics. 
During operation of the method 58, each electrical signal pulse from one of 
the transducers 28a, 28b, 38a, and 38b, breaks the method 58 and sends it 
to interrupt sub-routine, or method 80 that is shown in FIG. 3. The method 
counts each revolution of each tire and stores these values in the 
respective up-down counters. The method 80 first determines in block 86 
that an interrupt request from an edge detector has been received. The 
method 80 continues to block 88 where a counter designated as CNT.sub.N is 
incremented. The method 80 of the interrupt sub-routine next falls to 
block 90 where an interrupt and request flag is cleared and returns to 
method 58. 
While the invention has been described in detail, it is to be expressly 
understood that it will be apparent to persons skilled in the relevant art 
that the invention may be modified without departing from the spirit of 
the invention. Various changes of form, design or arrangement may be made 
to the invention without departing from the spirit and scope of the 
invention. Therefore, the above mentioned description is to be considered 
exemplary, rather than limiting, and the true scope of the invention is 
that defined in the following claims.