Vehicle Tyre Pressure Monitoring

Tyre pressure monitoring systems (TPM or TPMS) were implemented a number of years ago as a factory installed feature found only on high-end vehicles.

TPMS, as an ambedded electronic system is expected to be standard equipment in the next few years.

System overview
To do real time sensing of the exact pressure inside the tyre, the sensing device must be located in the tyre. This pressure measurement information must then be carried to the driver and displayed in the cabin of the car. The remote sensing module is comprised of a pressure sensor, a signal processor, and a RF transmitter. The system must compensate pressure variations due to emperature. Hence a temperature sensor is also required.

The power supply is provided by a long life battery that the embedded intelligence helps to manage as effectively as possible. The receiver could be either dedicated to TPM use, or shared with the other functions in the car.

Remote Sensing Module (RSM)
Once mounted in the tyre, the RSM is a stand-alone device. Its embedded intelligence has to independently manage the sensing functions, the measurement processing, the RF transmission, and the power management.

To address each of these functions, Motorola offers two new components as a solution. The TPMS Sensor is an integrated monolithic chip device. It is comprised of both a temperature and pressure sensor with on board circuitry.

The second component is a microcontroller and a RF transmitter, with both chips housed in the same package.

The TPM sensor
The Motorola TPM pressure sensor uses less than 0.5µA in standby mode. The pressure-sensing cell is capacitive and requires a C to V (capacitance to voltage) conversion stage. The sensor's built-in non-volatile memory can store calibration data while the ADC allows a direct digital serial connection to the controller.

In standby mode, all analogue and digital blocks are switched off, except an internal low frequency oscillator that sends a wake up pulse over an output pin to the controller periodically.

A pressure measurement mode allows the pressure cell, and the C to V converter to be activated. The temperature measurement mode activates the temperature cell (a PTC resistor) and its conditioning block.

Finally, the read mode enables the measurements to be stored in a sampling capacitor. The read mode activates the A to D converter and enables the controller to read serially the measurement. These four modes are coded through two input pins controlled by the microcontroller. The coding is chosen so as to make the standby mode coded with logic zero on both pins.

The microcontroller
The 68HC08RF2 device was chosen for its combination of an HC08 micro together with a RF transmitter in a single 32-pin LQFP package. The dual chip HCO8RF2 has no internal connections between the controller die and the RF die, but the pinout is optimised to shorten the necessary external connections. The 2Kbytes of user Flash memory with embedded charge pump allow designers to implement the necessary software routines to address the TPMS application's functional requirements.

The RF transmitter is PLL based, addressing both ASK (amplitude modulation) and FSK (frequency modulation) and its transmission rate is configurable up to 9600 baud. With a reference quartz oscillator of 13.56MHz, the PLL is able to generate 315, 433, 868MHz carriers.

The system architecture
The HCO8RF2 controls the sensor-state by setting the different operating modes. When the sensor is set in standby mode, its internal low frequency oscillator periodically wakes up the controller. After each wake up, the controller may run different and configurable tasks according to the software program. Between two wake up pulses, the microcontroller is in the Stop mode, all functions are disabled to minimise the power consumption, and only an external stimulus can wake it up again.

To improve the battery management, an inertial switch can be employed to detect the parking mode. In parking conditions, the RF transmissions can be stopped or reduced, improving power management and reducing the data collision risk between RKE and TPM transmissions. The RSM must be as small and lightweight as possible since it is mounted inside the tyre. An oversized RSM could result in wheel imbalance.


The receiver
A single receiver can be shared between both the RKE and TPM systems since the same transmitting format is used in both. The TPM function must use as little CPU time as possible and to achieve this, a highly integrated RF receiver such as the MC33591, also called Romeo 2, is required.

This RF receiver was developed in order to provide a comprehensive RF link that is integrable in RKE and TPM systems with Romeo 2 at one end, and the HCO8RF2 at the other end. Thanks to its embedded RF decoding and data registers, the chip minimises the communication with the receiver microcontroller. The MCU is not called until a valid data frame is received, validated, and stored by the Romeo 2 device.

Tyre identification
The simplest to perform tyre identification is the manual initialisation performed in the factory, or in the garage each time a tyre is replaced or moved (rotated). The second method is by automatic identification. Using this method, the system locates each tyre automatically by a learning procedure that is activated regularly, or upon request.

Combining different information sources could be the path taken to meet these needs. TPM is in fact, destined to become more integrated into the vehicle architecture.

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