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Tire Pressure Monitoring System
Tire Safety System - TSS
Current and Future Developments in
Tire Pressure Monitoring Systems
Special reprint from
Automobiltechnischen Zeitschrift (ATZ)
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ATZ

DEVELOPMENT Tires and Wheels
Current and Future
Developments
in Tire Pressure
Monitoring Systems
By Harald Bochmann,
Ralf Kessler and
Gunter Schulze
Tire pressure monitoring systems offer more safety, greater
comfort and increased efficiency. They are also increasingly in
demand for high-volume cars. In principle, two different technologies
are competing in this field: direct measurement and
indirect measurement systems. With its Tire Safety System
(TSS), Beru AG is throwing its weight behind direct measurement
technology and is now presenting its third generation.
2 ATZ 2/2005 Jahrgang 107

1 Introduction
Tire pressure monitoring systems (TPMS)
are increasingly being seen as standard features
in cars. Complex insurance cases with
defect tires lend added impetus to this development.
For example, this issue prompted
the government in the United States to
introduce legislation for all cars and light
commercial vehicles to be fitted with these
systems in the future. The minimum requirements
according to the National Highway
Traffic Safety Administration (NHTSA)
include the specification that an alarm be
sounded if there is a 25 % loss in pressure
compared to the target pressure. The reduction
in pressure must be identified and displayed
within no more than ten minutes
[1].
The speed at which the NHTSA aims to
introduce these requirements is remarkable:
from 1. 9. 2005 onwards, 50 % of all
new vehicles are to be fitted with TPMS;
this figure will rise to 90 % from 1. 9. 2006
and to 100 % from 1. 9. 2007 onwards. It is
not clear whether other countries will also
follow, but it is fair to assume that it will be
the case given the practical experience
with other safety-related products.
Both the government and customers
alike expect systems of this kind to primarily
contribute to vehicle safety. The first indications
of pressure loss are indicated in
advance and the driver is prompted by an
alarm to replenish air in the tire or to stop
the vehicle, which in most cases will prevent
subsequent damage or accidents. This
kind of control facility is indispensable for
vehicles with run flat tires, so as to prevent
the driver unknowingly proceeding with
defective or insufficiently inflated tires.
Over and above the safety element, drivers
appreciate the benefits of tire pressure
monitoring. The pressure display in the vehicle
is updated at the press of a button.
In view of the fact that tires are designed
to last for their service life if they are properly
inflated, TPMS users benefit from the
increased efficiency in addition to the safety
aspect.
System costs have plummeted over the
last few years. Accordingly, TPMS is rapidly
becoming a standard feature even in midsize
vehicles.
2 System Variations
In principle, there are two standard methods
of determining tire pressure:
■ Indirect measurement systems draw on
measurement values provided by the ABS
and/or dynamic driving systems in order to
calculate changes in the tire pressure on
the basis of algorithms.
ATZ 2/2005 Jahrgang 107
■ Direct measurement systems measure
the actual pressure and temperature values
of the air in the tires, which is then sent to a
module for evaluation.
2.1 Indirect Measurement
Systems
Indirect measurement TPMS utilize the
ABS speed sensors in the vehicles to assess
differential speeds of the wheels. If the relative
differential speed of one wheel suddenly
exceeds a calculated threshold compared
to the other wheels, the system assumes
that the air pressure in the wheel in
question has fallen. An alarm is then triggered.
In this context, the threshold value
must be aligned dynamically in order to
compensate for the changes in wheel circumference
due to tire wear and stress. For
example, the stress differs when driving
round bends or in a winter environment,
in which the wheels turn at different
speeds due to the differing friction. In
these cases, the warning capacity of the indirect
measurement TPMS is significantly
restricted.
Although this type of operation can be
integrated cost-efficiently, such as in a
software module within the ESP or ABS
control units, its use is restricted to warnings
in the event of a sudden drop in pressure
in individual tires due to damage.
However, the system will not recognize a
gradual loss in pressure in all tires – for
example due to diffusion – nor deviations
of less than 25 % from the target value.
Furthermore, it is not possible to measure
the absolute pressure. Therefore, the indirect
measurement systems currently on
offer do not satisfy the NHTSA demands.
In all probability, this applies equally
to another indirect measurement TPMS
version, which is currently under development.
It draws on changes in the resonant
frequency of the wheel/tire system
depending on the tire pressure. In this
system, a measurement of the rotary oscillation,
which overlies the wheel rotation
as interference, is used to calculate
the pressure values. However, the computing
of the Kalman filter used to identify
the parameters is considerable and will
make itself noticeable in the hardware
costs [2].
Accordingly, the application possibilities
for indirect measurement systems are fundamentally
limited.
2.2 Direct Measurement
Systems
Drawing on sensors located on the inside of
the wheels, direct measurement systems
determine the actual individual wheel values
for pressure and temperature. The mea-
The Authors
Dr. Harald Bochmann,
is technical director at
Beru Electronics
GmbH, Bretten.
Ralf Kessler, is head
of the product group
development of tire
pressure monitoring
systems at Beru
Electronics GmbH,
Bretten.
Gunter Schulze, he is
head of the component
development for Tire
pressure monitoring
systems at Beru
Electronics GmbH,
Bretten.
sured values are sent by remote transmission
together with an identification code to
a control unit fitted in the vehicle [3].
The pressure sensor elements generally
operate on a micro-mechanical basis as
part of an integrated circuit. The temperature
is measured on the chip using a bandgap
reference. Depending on the remote
transmission conditions in the country of
use, the data transmission takes place via
frequency shift keying modulation (FSK) as
a burst with 19 kbit/s in each case in the 433
MHz or 315 MHz band. The telegram length
is 96 bits, Table 1. Error protection is provided
by the cyclic redundancy check algorithm
(CRC).
Special lithium batteries are used to
power the tire electronics. Combined with
the energy-optimised sensor electronics,
they allow a service life of up to ten years.
The required service life of the batteries
can be achieved by careful circuit design
and a needs-based control of the measurement
cycles and transmitter activity.
3

DEVELOPMENT Tires and Wheels
2.2 Direct Measurement Systems
Byte 1 Synchronisation 1111 1111
Byte 2 Synchronization 1111 1110
Byte 3 Identifier MSB
Byte 4 Identifier
Byte 5 Identifier
Byte 6 Identifier LSB
Byte 7 Druck / Pressure
Byte 8 Temperatur / temperature
Byte 9 Batterie-Restlebensdauer /
Battery power indicator
Byte 10 Status
Byte 11 CRC MSB
Byte 12 CRC LSB
Termination-Bit „0˝
Within this context, the measurement cycles
are defined depending on of the vehicle
movement. Longer measurement cycles
are possible if the vehicle is station-
Table 1: Data telegram of
the wheel electronics
Table 2: Comparison of indirect measuring TPMS with the three Beru versions
Functional features ABS supported TSS generation 2 TSS generation 2 TSS generation 3
series systems without trigger with trigger without trigger
Pressure measurement
in four wheels
– + + +
Temperature measurement
in four wheels
– + + +
Alarm for tire defect
Soft alarm (low sub-
+ + + +
standard pressure)
Hard warning (sub-
– + + +
standard pressure 25 %)
Alarm for even loss of
– + + +
pressure in all wheels – + + +
Alarm issued with position
Automatic learning
of new wheels
– – + +
(e. g. winter wheels)
Automatic learning
– +* + +
of new wheel positions
Rapid diagnosis of
– +* + +
the components
Target pressure transfer
for changes in pressure
+ + + +
due to driver actions
Pressure display menu
with measurement
+ + + +
values and position
Immediate availability
of the pressure values
– – + +
before start of driving
Satidfies NHTSA
minimum alarm
thresholds and
– +** + +**
response time – + + +
*With user invention; **when parked, the control unit must be powered via terminal 30
ary; the cycles are then cut appropriately
if the vehicle is in motion and may even
be measured in seconds if significant
changes in the pressure are identified
over time – i.e. if there is a tire defect or
the tire is being inflated. The tire electronics
autonomously recognizes vehicle
movement using a roll switch.
An alternative is a needs-based triggering
of the measurement via the reverse
channel to the wheel electronics. The trigger
transmitters required for this are installed
in the wheel arches.
In addition to the clocking of measurement
procedures, the trigger transmitters
enable the wheel positions to be learned in
a simple manner. The frequency band of
around 125 kHz used for the trigger channel
ensures that the range of the transmitted
trigger signal remains low due to the small
antenna structure in a ratio to the wavelength,
which permits reliable control of
the individual wheel electronics. Neighbouring
wheels remain unaffected, thus
enabling clear identification.
Therefore, in a comparison of the direct
and indirect measurement systems, Table
2, Beru recognizes the clear advantages of
the direct measurement system. This was
also the basis for the decision in 1995 to develop
a TPMS on this technological basis.
3 Functional Components
The functional components in the solution
that Beru developed as an example of a
modern tire pressure monitoring system
include the wheel electronics, (digital) antenna,
trigger transmitter and control unit.
Depending on the application, user interfaces
and displays are also necessary. The
summary of the individual elements is presented
in the schematic diagram shown in
Figure 1.
3.1 The Wheel Electronics
The wheel electronics unit represents the
core component. This unit, which is fitted
to the valve on the inside of the wheel, Figure
2, must be able to cope with extreme
ambient conditions. It must be capable of
withstanding temperatures of between
–40°C and + 120 °C – in bursts even as high
as + 150 °C – with acceleration values of up
to 2,000 g.
The form of the housing in contact with
the valve connection is by means of a
moulded spherical surface and, as such, it
can be fitted to nearly all known rim types
with only one design, Figure 3.
The unit is encapsulated to protect it
against the penetration of water and to
provide resistance to chemical residue in
the tires. A Teflon filter, patented by Beru,
beneath a “chimney”, Figure 4, guarantees
the required permeability for the interior
pressure to the sensor element.
The wheel electronics consist of three in-
4 ATZ 2/2005 Jahrgang 107

3 Functional Components
TSS generation 2 without trigger TSS generation 2 with trigger TSS generation 3 without trigger,
with compact control unit
Figure 1: System description
The components: wheel electronics (1), (digital) antenna (2), central control unit (3), trigger transmitter (4), compact control unit - control
unit with integrated digital antenna (5)
tegrated circuits. The first circuit draws on
micro-mechanical structures in order to
measure and digitise the values for pressure,
temperature and acceleration. The value for
acceleration is used as an ancillary factor. It
recognizes whether the wheel is moving, so
that the system can respond by switching to
an aligned transmission mode.
The second circuit contains a 433 MHz
(or 315 MHz) transmission power stage to
control the process. The transmission power
is selected in such a way that the system
is on the one hand suitable for certification
ATZ 2/2005 Jahrgang 107
under radio regulations, while on the other
hand guaranteeing the greatest possible
system reserves for operations.
Another module wakes up the system
when it receives a trigger signal of 125 kHz,
thus setting it to an active mode.
3.2 The Digital Antenna
The digital antenna contains a 433 MHz (or
315 MHz) RF receiver with corresponding demodulator
and data decoder. The received
measurement values are transmitted to the
control unit via a LIN bus interface. The ad-
vantage of this concept compared to previous
systems is that no RF wiring is required
in the vehicle to control the tire pressure,
which considerably improves the impact of
electro-mechanical interference. The receiver
initially transforms the 433 MHz (or 315 MHz)
signal to an intermediate frequency of 10.7
MHz after preliminary filtering. This is then
followed by demodulation, decoding and reformatting
to the LIN protocol. The concept is
designed in such a way that an adjustment
of the RF circuits in series production is not
necessary.
5

DEVELOPMENT Tires and Wheels
3.1 The Wheel Electronics
Figure 3: The cross-section of the wheel electronics shows
how the system can be adapted to suit various rims
Figure 2: The wheel
electronic system
is combined with
the Tire valve and
installed in the rim
Figure 4: Micro-mechanical pressure
sensor as an integrated module – the
chimney is visible on the right
3.3 The Trigger Transmitter
As the third component, the trigger transmitter
contains the evaluation of the LIN
telegram in order to control and generate
the trigger data and the 125 kHz generator
with a ferrite antenna as a transmission element.
Both the digital antenna and the
trigger transmitter are waterproof and impact-protected
in a robust housing and can
therefore be fitted to the outside of the vehicle
without difficulty.
3.4 The Control Unit
The control unit processes the information
received from the digital antenna and displays
it on the monitor. Alarm algorithms
and display strategies are implemented as
a customer-specific element of the overall
system. The software is based on the OSEK
operating system and can be updated by
Flash.
3.5 The Operating and Display
Unit
The respective operating and display unit is
coordinated with the vehicle manufacturers
and is aligned to suit the interior design
of the vehicle.
In this, there are various versions of information.
There can be a simple light control
or even an alphanumerical display of
the pressure and temperature values with
colour-coded status of the individual wheels
in the driver information system.
4 Current System Concepts
For reasons of installation and costs, it is
more practical to fit TPMS with a central receiver
unit, at least in the countries in
which the radio regulations permit this.
The spectrum of versions currently developed
by Beru covers three variations.
4.1 Basic System
The basic system does not identify the
wheel position, which means that it can be
implemented at low cost, yet still offer immediate
accumulated pressure information.
An alarm is triggered as soon as insufficient
tire pressure is identified, but without
providing the information as to which
wheel it refers to.
Individual wheel position pressure display
is not possible either. Individual wheel
detection takes place exclusively via the
identification code of the wheel electronics.
However, the system alarm thresholds must
either be uniform for all wheels or must on
one occasion be derived from the measured
pressure values in the wheel electronics using
a calibration switch and then allocated
to the matching identification codes of the
wheel electronics. The alarm display takes
6 ATZ 2/2005 Jahrgang 107

place using the driver information system
without positional allocation.
4.2 Trigger System
If positional allocation is required in addition
to the wheel electronics, the trigger system
is the best technical solution compared
to the other systems currently on offer.
The added number of components provides
fast and reliable detection of the
wheel positions. The transmitter is controlled
by the central control unit. A trigger
signal causes the allocated wheel electronics
to record a new pressure and temperature
measurement value and to transmit it
as a radio telegram encoded as requested
information.
Allocation to the wheel position is possible
very quickly due to the correlation between
a radio data set registered (marked)
in the central receiver and the trigger request.
The trigger-controlled system provides
the pressure information even before
the driver sets off.
In this system, the target values for the
tire pressures can be specified for each axle
by control unit programming and can also
be derived directly from the set pressures
using a calibration modulator.
A visual indication is given in the display,
which highlights the positional information,
Figure 5.
4.2 Trigger System
4.3 Compact System
Figure 6 shows the components of the actual
TSS.
The latest development continues to reduce
the number of system components.
The Beru compact system (TSS 3rd generation)
will consist only of a central receiver
unit with an integrated control unit function
and four wheel electronic systems.
Positional allocation takes place on
the basis of the rotary direction signal,
which the wheel electronic systems provide
in the radio data, relating the information
on the direction in which the vehicle
is moving, in combination with the
axle-specific separation from the fitted
wheel electronic systems to a comparison
of the various RF reception levels.
This is achieved by the installation of the
central receiver unit at the front or rear of
the vehicle in order to provoke these differences
in the RF level. Definition of the
target value for tire pressure takes place
as described above. A monitor displays
the warning and the positional information.
5 Integration in the Vehicle
Figure 5: Display in the Beru-TSS as used in the Audi A6 showing the
pressure and temperature values in the individual tires
ATZ 2/2005 Jahrgang 107
System integration in the vehicle is divided
into two areas. On the one hand, the best
positions for the hardware in terms of tech-
nical and design aspects must be defined,
and on the other hand the system must be
coupled to the vehicle wiring.
5.1 Installation Space
The potential installation spaces are surveyed
in terms of their radio frequency reception
characteristics in order to identify
the best possible position for installing the
receiver antenna. The most favourable position
is usually in the underbody, in the
bumpers or in the wheel arch. However, if
the vehicle manufacturer so wishes, the antenna
can also be installed in the vehicle interior.
The system is fundamentally designed
so that the reception of the RF signal can also
be guaranteed if the tires provide considerable
attenuation. Additional damping by
environmental influences (snow, rain) is also
considered in the system design.
5.2 Networking
Another factor of the system integration is
the connection of the system to the vehiclespecific
wiring. In most applications, the
control unit is operated in the existing comfort
CAN.
The components of the tire pressure
monitoring system all have a self-diagnosis
capability. The specific manufacturer’s diagnosis
services provide the required diagnosis
information.
The vehicle bus also deals with the operation
and display of the pressure information.
5.3 Technical Specifications
Statutory requirements, for example the
NHTSA initiative and the demands voiced
by vehicle manufacturers, define the
framework for alarm thresholds and response
times.
5.4 Special Features of the
Beru TSS
In addition to the tire pressure, the Beru
Tire Safety System also evaluates the tire
temperature. This is based on the general
gas law. This temperature compensation
means that the alarm threshold can be
aligned with the tire pressure.
A message is issued if the alarm threshold
was exceeded on two successive samples.
Given a typical transmission interval
of one minute, this means that detection
takes no longer than two minutes. If there
are high changes in pressure, the wheel
electronics transmit signals in cycles of
merely a second. In these cases, the detection
period is only two seconds.
In addition to tire defects with rapid loss
of pressure, the system also detects a gradual
loss of pressure – for example due to dif-
7

DEVELOPMENT Tires and Wheels
4.3 Compact System
Figure 6: Components of the Beru-TSS generation 2 without housing, with visible
construction - from the left: trigger transmitter, control unit, wheel electronics,
digital antenna
fusion or a leaky valve. The standard
threshold value for this is 0.3 bar. However,
it can be varied to suit manufacturers’ requirements.
This threshold is used to remind
the driver as quickly as possible that a
correction of the tire pressure is necessary.
5.5 Examples of Applications
Beru marketed its first tire pressure monitoring
system in 1997 as an option for the
BMW 7-Series. At the moment, the Beru
TSS is currently available as a series or ad-
BERU Aktiengesellschaft
Mörikestrasse 155 · D-71636 Ludwigsburg
P.O. Box 229 · D-71602 Ludwigsburg
Phone: ++49/7141/132-693
Telefax: ++49/7141/132-220
www.beru.com
ditional option to over 20 different models
by various manufacturers. It is factory
fitted as standard in top vehicles such as
the Bentley Continental GT, all Ferraris,
the Maybach, the Mercedes SLR and the
Porsche Carrera GT.
A special Beru TPMS can also be provided
as an option for the heavy-duty truck
from Mercedes-Benz, the Actros, with super-single
tires. The Beru subsidiary company
F1-Systems Ltd. has introduced TSS
to motor racing and even to Formula 1.
6 Outlook and Future
Developments
Promoted by the current legislative initiative
by the NHTSA, it is fair to expect that
all new vehicles in the United States will be
fitted with tire pressure monitoring systems
over the coming years. The automotive
industry and the suppliers are working
round the clock to prepare for this. As the
systems have been introduced as options in
the premium segment for some years now,
the current development activities are targeted
primarily at simplifying the installation
effort in the vehicles and on achieving
a high level of component integration.
These measures should lead to a further cut
in the system price. Various semiconductor
manufacturers have already announced
highly integrated modules for wheel electronic
systems.
A combination of the control unit functions
for signal evaluation with other components
in the vehicle, for example the remote
locking system or the ABS system, are
also possible.
Technical solutions that do not require a
battery for the wheel electronics are also at
a preliminary development stage. Development
over the next few years will show
whether the familiar transponder solutions
are suitable, or whether concepts that use
the moving energy of the wheel electronics
during the rotary phase for power
generation will assert themselves.
References
[1] NHTSA-Richtlinie FMVSS 138. Tire Pressure
Monitoring Systems (Stand 09/2004)
[2] International Patent Application WO
01/87647, World International Property
Organization, 2001
[3] Normann, N.: Reifendruck-Kontrollsystem
für alle Fahrzeugklassen. In: ATZ 102
(2000), Nr. 11
Printed in Germany 02.03.05 Bestell- Nr. 5 000 001 069