3/29/2006

Tire Pressure Monitoring System (TPMS) An Introduction

Saurav Mukherjee Mukherjee.Saurav@gmail.com

Contents Target Readers.......................................................................................................................................... 1

Keywords .................................................................................................................................................. 1

Introduction .......................................................................................................................................... 2

Why TPMS ............................................................................................................................................ 2

TPMS Requirements ............................................................................................................................. 2

Types of TPMS - Indirect, Direct & Hybrid ............................................................................................ 2

Indirect TPMS ................................................................................................................................... 2

Direct TPMS ...................................................................................................................................... 3

Hybrid TPMS ..................................................................................................................................... 4

Abbreviations ....................................................................................................................................... 4

Target Readers Anybody who is interested in modern day automobiles.

Keywords TPMS, DDS, Wheel Module, Pressure sensor, Transmitter, Embedded System Application,

Automotive Braking System Application

Introduction The modern automobile comes with many safety & comfort features. Most of these features

are implemented via dedicated software and hardware. The software reside inside one or more ECU/s

(Electronic Control Units) mounted in the vehicles. Sleuths of such features are available today in the

market. Each of them aims at providing the rider with safer & enriched driving experience. ABS, TCS,

ESP, TPMS etc. are a few such examples.

TPMS is what its name says: it is a system to continuously monitor the air-pressure of the tires

of automobiles on the move. The tire pressure monitoring system provides the early warning that

allows the driver to correct simple problems before they become catastrophes.

Why TPMS Running a vehicle with under-inflated tires reduces the lifetime of the tire considerably. Under-inflated tires sometimes increase the braking distance and hence have a potential

safety concern. It increases the rolling resistance of tires and hence decreases the fuel-economy of the

vehicle. Research showed that the under-inflation of a vehicle’s radial tires by 10 pounds per sq. inch

(psi) reduced the fuel economy by almost 3%. Another study concluded that under-inflated tires were

probable cause of almost 1.4% of all motor vehicle crashes.

Following US legislation, TREAD (Transportation Recall Enhancement, Accountability, and

Documentation law), NHTSA (National Highway Traffic Safety Administration) has made it mandatory

for the vehicle manufacturers to equip all the new cars with TPMS. It offered two options for the vehicle

manufacturers to comply with the law. All the new cars are supposed to be equipped with Tire Pressure

Monitoring System. Manufacturers in Europe and Japan are taking notable steps in response to this

U.S. legislation.

TPMS Requirements Passenger cars, trucks, multipurpose passenger vehicles, and buses with a GVWR of 10,000

pounds or less, except those vehicles with dual wheels on an axle are supposed to be equipped with

TPMS. TPMS must warn the driver when the pressure in any single tire or in each tire in any combination

of tires, up to a total of four tires, has fallen to 25% or more below the vehicle manufacturer’s

recommended cold inflation pressure for the tires, or a minimum level of pressure specified in the

standard, whichever pressure is higher. The driver must be warned within 20 minutes of driving time

from ignition on. There should be no detection during vehicle standstill. There should be a malfunction

indicator for the system itself.

Types of TPMS - Indirect, Direct & Hybrid TPMS can be broadly divided into two categories, viz. Direct and Indirect. There is a third type

called Hybrid. Direct TPMS warns the drivers when air-pressure in any one of the rolling tires falls below

at least 25% of the recommended cold tire inflation pressure. On the contrary, an Indirect TPMS warns

the drivers when a single rolling tire has lost at least 30% of its inflation pressure as compared to other

three rolling tires. Hybrid TPMS is a combination of both the direct & indirect TPMS.

Indirect TPMS This is the earliest TPMS, minimizing the cost and development time. Indirect systems use the

vehicle's antilock braking system's (ABS) wheel speed sensors to compare the rotational speed of one

tire vs. the other three positions on the vehicle. If one tire is low on pressure, its circumference changes

enough to roll at a slightly different number of revolutions per mile than the other three tires. The

aggregate of speeds of diagonal tires are compared in the following way:

Speed Ratio, Vr = ((Vfr + Vrl) – (Vfl + Vrr)) / Vav

Where, Vfr = speed of front right wheel

Vrl = speed of rear left wheel Vfl = speed of front left wheel Vrr = speed of rear right wheel Vav = average speed of all four wheels

Dividing the difference in speeds in the diagonal wheels by the average wheel speed ensures

that the ratio becomes independent of the vehicle speed. Speeds of diagonally opposite wheels are

considered to take into the account the speed difference caused by curves & turns.

If the ratio, Vr deviates from a preset tolerance, one or more tires must be over or under-

inflated. In that case, the driver is warned via say, a warning lamp. This is system is also know as

Deflation Detection System or DDS.

But indirect tire pressure monitoring systems have several inherent disadvantages. Indirect

systems won't tell the drivers which tire is low on pressure, and won't warn the driver if all four tires

are losing pressure at the same rate (as occurs during the fall and winter months when temperatures

turn colder). Additionally, they sometimes generate false warnings. The false warnings occur when the

vehicle is driven around a long curve that causes the outside tires to rotate faster then the inside tires,

or when the tires spin on ice and snow-covered roads. In both of these cases, the false alarms would

train the driver to disregard the tire pressure monitoring system's warnings, negating its purpose

completely.

Direct TPMS In this case, a new system (comprising of new software and hardware) has to be built, as

against its indirect version where existing system (for example, Antilock Braking System, ABS) is used

to obtain the required system variables. This system displays the pressure values of each rolling tire on

the dashboard or at least indicates the status of each rolling wheel (under-inflated, over-inflated or

normal). In this system, one wheel module (sensor & transmitter) is attached to each wheel. The

module also rotates along with the wheel. These modules transmit accurate pressure, temperature

values etc. to the antennas/receivers present in the vehicle-network in form of telegram or data packets

via HF radio signals. There can be separate antennas for this purpose or antennas of other existing

system, like Remote Keyless Entry (RKE) system, may be utilized.

However, the direct TPMS needs some extra validations, like own-wheel-recognition, auto

location, spare wheel detection etc. Once these checks are successfully passed, the display may come

up. TPMS related information like tyre pressure values and validity information, pressure loss

information for each tire, system status (e.g., failure, warning) etc. are given as vehicle bus message.

Based on the vehicle topology and other parameters (like number of antennas present etc.) different

strategies have been developed to implement the direct TPMS. For example, there can be a single

antenna system or a four-antenna system or so.

The display may vary from car to car. It also alerts the driver instantly if pressure in any tires

falls below a predetermined level or in case of a rapid pressure loss (say, in case of tire-puncture).

Pressure differences in tunes of few psi can be taken care of.

TPMS, rather direct TPMS, can be realized in standalone mode, meaning a separate TPMS-ECU

may be designed, or it may be integrated in existing ABS/ESP ECU. In such cases, a separate processor

(as an add-on to the main ECU) may be required to handle the radio communication. However, in that

case the core TPMS algorithm stays in the main ECU itself and the add-on provides the relevant

information to it via say, a SPI link. The add-on receives the telegrams from different antennas and pre-

processes them. ECU controls the antennas via the add-on.

However, this system also suffers from some drawbacks. Climatic conditions & road-profiles

have a bearing on the quality of the received telegrams. Telegrams transmission may become little

intermittent in some cases. In those conditions, it takes longer to come up with correct display. Then

the wheel-modules are battery driven and the cells run dry in 5-10 years time frame. Therefore it

becomes mandatory to change the same after that time period. But attempts are being made to

produce battery-less wheel modules. The problem of false warning, rather false display still persists,

albeit it is much less as compared to indirect TPMS. Direct TPMS being based on radio-frequency

transmission, different problems related to that field are also present.

On a brighter side, there are manufacturing companies like Beru, Schrader etc, who

manufacture the wheel transmitter modules to transmit pressure & temperature value with high

resolution. Efforts are being made to produce economical battery-less modules. Big auto-OEMs are

investing in development of TPMS to make driving safer & more pleasurable.

Hybrid TPMS On an overall comparison, direct tire pressure monitoring systems are the better means to

warn the driver of low tire pressure before inconvenience becomes calamity. It is even possible to have

the car equipped with Direct TPMS and then have the DDS as the backup or fallback option. This is

known as a hybrid system. The hybrid system is a combination of both the above stated systems. The

basic idea is to enhance the limitations of indirect systems by using pressure sensors mounted on some

or all wheels. One possibility could be the use of two pressure sensors mounted on diagonal wheels,

which help in detecting the under-inflation in any combination (two or all four tires). For more accurate

localization, systems with one pressure sensor on each tire are also being planned. The pressure sensor

modules provide accurate tire pressure information while the localization of the under-inflated tire(s)

is done using indirect system. Together, direct and indirect monitoring systems offer a unique level of

safety. The indirect system acts as a permanent back up in case a defective TPMS wheel module fails to

trigger a warning when the actual pressure drops below the set value.

Abbreviations

Abbreviations Expansions

ABS Antilock Braking System

DDS Deflation Detection System

ECU Electronic Control Unit

ESP Electronic Stability Program

GVWR Gross Vehicle Weight Rating

HF High Frequency

NHTSA National Highway Traffic Safety Administration

OEM Original Equipment Manufacturers

psi pounds per square inch

RKE Remote Keyless Entry

SPI Serial Peripheral Interface

TPMS Tire Pressure Monitoring System

TREAD Transportation Recall Enhancement, Accountability, and Documentation