Modular TPMS systems provide economical, functional versatility

Technology News | January 24, 2012

By eeNews Europe

The modern automobile is feature rich in electronic systems. These range from safety and control functions such as airbags, antilock brakes, and tire pressure monitoring systems (TPMSes) to infotainment features integrating consumer devices such as phones and MP3 players into the car and navigation applications that not only provide directions but locations of points of interest, restaurants, and service stations. All such systems are governed by burgeoning numbers of microcontrollers (MCUs). High end (i.e. luxury) vehicles can have well over half a hundred MCUs.

This trend of growing MCU numbers (which drives up costs) is being countered in some quarters by a push toward greater integration of processing functions and sensors—resulting in fewer MCUs, but with multicore functionality, and sensors that serve several functions (i.e. a camera used for lane departure warning as well as obstacle detection).

However, like many of us have learned early in life playing with building blocks, a modular approach to certain problems can provide versatility in features tailored to how much functionality is desired for a given cost.

Modular TPMS: Affordable options

Let’s take for an example a tire pressure monitoring system. These originally came about after a series of control accidents involving SUVs with under-inflated tires. Besides a safety benefit, TPMSes can also help drivers maintain correct pressures which increase fuel efficiency, thus cutting emissions as well.

Automotive OEMs can install two basic types of TPMS systems. The first might be termed a simple or low-level system, consisting of a basic pressure sensor in each wheel and a transmitter IC to send that pressure data to a central receiver. A low pressure reading from one of the wheels results in illuminating a warning light in the driver’s instrument cluster—but does not indicate which tire is under-inflated. (From experience, such a system is all well and good in summer weather, but in harsh, winter conditions, one appreciates not having to manually take the pressure reading of each tire to find the under-inflated one, or ones.) The second, high-end TPMS would indicate the individual, problematic wheel—but requires more components (sensors) in the wheel.

Now the necessary transmitter and pressure sensor in each type of TPMS can be the same. However, if the sensors and transmitter are integrated into a single unit, then a high-end unit would contain components not needed for a low-level system and a simple system couldn’t fulfill more complex functionality. Thus, having a modular system where the TPMS IC can be connected to any number of required sensors used in a high end system, or just a pressure sensor, offers flexibility without unnecessary added expense. In looking to the future, in a modular system, even the RF transmitter IC can be changed out in later production versions to accommodate any desired or required system frequency changes.

The tough get going

It has to be noted that the environment a TPMS system is exposed to in a tire is far from benign. For instance, Melexis TPMS sensor and transmitter IC components (all qualified to AEQ-100 standard) are mounted in a dedicated package design of robust material; and are tested to verify functionality under high shock and vibration loads, as well as exposure to heat, solvents, and other media inside the tire. This is all the more impressive when it is realized that the ruggedized surface-mount pressure sensor is ported and exposed directly to the "air" inside the tire.

Equally important, such performance under adverse conditions has to be accomplished with minimal energy consumption drain on the TPMS module’s battery to allow a seven to ten year lifetime specification. Of the system energy drain, about 60% of this comes from standby, or sleep, current draw; 15% is used in powering up for the periodic pressure measurements and MCU execution; about 15% is allocated to transmitting data and 10 % for motion and LF commands detection .

Real world TPMS

The latest TPMS component offerings from Melexis allow such flexibility in robust packages for low- or high-end systems.

For instance, Melexis second generation modular-approach solution for TPMS, combines its MLX91801 Pressure Sensor (System in a Package (SIP) analog pressure sensor, and sensor interface micro-controller) and MLX72013 RF Transmitter. This TPMS module can be used in pair with the MLX71122 Multi-Channel RF Receiver to build a complete TPMS system, compliant with existing remote keyless entry (RKE) systems. Adding an external roll sensor and peripheral passive support components yields a complete, cost effective, TPMS system.

The MLX91801 contains blocks for control of the sensor interface, communication interface, and power management blocks supporting very low power consumption in standby mode. The device measures pressure, temperature, and supply voltage. The pressure sensing element is a MEMS-based absolute resistive-bridge pressure sensor. The communication interface provides commands for receiving and sending data to the host system. A standard software library with low-level routines gives access to all the IC’s functions. Melexis also provides Application Examples that offer an introduction to the Melexis microcontroller programming and facilitate eased customized developments.

The MLX91801 pressure sensor block offers 1% full-scale accuracy. The default pressure range is 100 – 800 kPa, with other ranges available by request.

The MLX72013 ASK/FSK (amplitude shift keying/frequency shift keying) transmitter IC is designed for applications in the license-free 433 MHz ISM (Industrial-Scientific-Medical) band air interface for wireless communications (433 MHz is available for use worldwide). The transmitter has been designed to meet the latest evolution of the EN 300 220 telecommunications standard; but it can also be used in other countries with similar standards, e.g. FCC part 15.231.

The MLX71122 multi-channel receiver IC is based on a double-conversion super-heterodyne architecture. This receiver is designed to receive FSK and ASK modulated RF signals either in eight predefined frequency channels or is frequency programmable via a 3-wire serial programming interface (SPI). Thanks to its highly linear RSSI (Received Signal Strength Indication, as noted in the sidebar previously), the MLX71122 allows for automatic wheel identification hence providing users with further integration capabilities.

Thus, despite the current trend for automotive TPMS systems to move in the direction of fully integrated solutions, the capabilities and flexibility noted with the Melexis components above show there is significant advantage in flexibility and cost of a modular solution.

Design examples

Figure 1 below shows an application example of the modular solution available with MLX91801 and a Melexis RF Transmitter IC, MLX72013.

Fig. 1 Schematic example of the tire pressure sensor based on the MLX91801

Figure 2 below illustrates a block diagram of the Melexis TPMS solution demo kit that is now available. This demo board uses the MLX91801 TPMS sensor, the MLX72013 RF Transmitter IC and the MLX71122 RF Receiver IC. The kit comes with an LF/RF transceiver and antenna, with USB connection to a computer and read out software.

Fig. 2 DK91801 block diagram. For better resolution click here.

Additional component details and specifications

MLX91801 Pressure Sensor

The MLX91801 is a System in a Package (SIP) combining an analog pressure sensor and a low-power sensor interface with an MLX16 micro-controller in a custom, pressure ported, plastic SO16 package. The device measures pressure, temperature and supply voltage. The pressure sensing element is a MEMS-based absolute pressure sensor.

Features and benefits:

Low-power microcontroller-based pressure-temperature-battery monitor designed for TPMS
Different available pressure ranges up to 1,400 kPa
Measurement of car and truck tire pressure with 1% precision
Sleep current 0.25 µA Run-mode current 340 µA
Temperature compensation of pressure sensor and oscillator
Diagnostics for system error detection
Motion detection with external shock sensor
Robust package
Shock survival up to 4,000G
7 GPIO pins (digital IN/OUT, analog IN)
Compatible with existing remote keyless entry (RKE) systems
Very sensitive LF interface for short-range wireless communication using 125 kHz
Automotive qualified

MLX72013 Transmitter

The MLX72013 transmitter’s carrier frequency fc is determined by the frequency of the reference crystal fref. The integrated PLL (phase-locked loop) synthesizer ensures that carrier frequencies, ranging from 425 to 445 MHz, can be achieved. This is done by using a crystal with a reference frequency according to: fref = fc/N, where N = 16 is the PLL feedback divider ratio.

Features and benefits:

Fully integrated PLL-stabilized VCO (voltage controlled oscillator)
Frequency range from 425 MHz to 445 MHz
Single-ended RF output
FSK through crystal pulling allows modulation from DC to 40 kbit/s
High FSK deviation possible for wideband data transmission
Wide power supply range from 1.95 V to 5.5 V
Operating temperature range -40°C to +125°C
Very low standby current
On-chip low voltage detector
High over-all frequency accuracy
FSK deviation and center frequency independently adjustable
Adjustable output power range from -16 dBm to +12 dBm
Adjustable current consumption from 3.5 mA to 15.9 mA
Conforms to EN 300 220 and similar standards 8-pin
Small Outline Integrated Circuit (SOIC)

MLX71122 Receiver

The MLX71122 receiver IC is designed for a variety of applications, for example in the European bands at 433 MHz and 868 MHz or for the use in North America or Asia, e.g. at 315 MHz, 447 MHz. or 915 MHz.

Features and benefits:

Programmable PLL synthesizer
8-channel pre-configured or fully programmable
SPI mode Double super-heterodyne receiver architecture with 2nd mixer as image rejection mixer
Reception of FSK, FM, and ASK modulated signals
Low shut-down and operating currents
AFC: Automatic frequency correction
AGC: Automatic gain control
On-chip IF filter
Fully integrated FSK/FM demodulator
RSSI for level indication and ASK detection
Second order low-pass data filter
Positive and negative peak detectors
Data slicer (with averaging or peak-detector adaptive threshold) 32 pin Quad Flat No-Lead 32L QFN 5×5 Quad
Evaluation board available