Application Note

What Is TPMS?

A TPMS is mounted inside the tire valve, measures tire pressures and temperatures with a pressure sensor and a temperature sensor and sends their status to a receiver in vehicle via wireless communication. If any abnormality is detected in tire pressure or temperature, information is displayed on the instrument panel to warn the driver.

Use of TPMS is rapidly expanding globally and was mandated from September 2007 in the United States, from November 2012 in Europe, and January 2013 in Korea. It is rumored that they will be mandated in Japan and China as well in the near future. TPMS will continue to be a part of our daily lives.

TPMS was developed to improve automotive safety while vehicles are in motion, after an investigation of a series of accidents caused by tire bursts due to low tire pressure in the United States took place during 2000. It was found out later that TPMS is an ecological and environment-friendly technology contributing to energy saving and CO2 reduction from improved fuel efficiency by helping maintain proper tire pressure.

It further contributes to environment by extending the life of tires through proper tire pressure maintenance, consequently reducing use of resources as well as waste. The market for TPMS will continue to grow due to improved safety and environment friendliness from their use.

What Is a Surface Mountable Accelerometer Device?

Surface mountable accelerometer devices (shock sensor, Fig. 1) are a type of accelerometers using piezoelectric ceramics.

Piezoelectric ceramics (barium titanate, lead zirconate titanate, etc.) are ceramics that generate electric energy when pressure is applied (piezoelectric effect) and ceramics that generate mechanical strain when electric energy is applied (inverse piezoelectric effect) . Their excellent features are high electromechanical conversion efficiency, relatively free configuration, flexible characteristics and ease of mass production. Piezoelectric ceramics are widely used for ultrasonic sensors and other sensors in addition to accelerometers.

We call accelerometers with especially high resonant frequencies (over 20kHz approximately) , enabling them to detect rapid acceleration, surface mountable accelerometer devices. On the other hand, they are not suitable for detecting smooth accelerations such as a free fall by the force of gravity. They have a sandwich construction with substrates sandwiching a device (piezoelectric ceramic) from top to bottom (Fig. 2) . When this device receives a shock (acceleration) , the ceramic inside deforms, generating electric energy from the piezoelectric effect, to communicate information regarding the detected shock to the equipment. Their detection capabilities range from a small shock a human body receives while walking, to a big shock from an automotive collision, allowing for various applications.

Fig. 1

Fig. 2

Features of Surface Mountable Accelerometer Device

There are three major features for Murata's surface mountable accelerometer device.

Flexible detection angles

Optimal angles for shock detection by surface mountable accelerometer devices differ depending on the direction of the device. Murata makes surface mountable accelerometer devices with various detection directions available. Customers may select ideal sensors according to their specifications (Fig. 3) .

Part number	Appearance	Detection axis [degree]	Sensitivity		Capacitance [pF]	Resonance frequency [kHz]
			Voltage [µV/(m/s2)] {mV/G}	Charge [fC (m/s2)] {pC/G}
PKGS-00TAV-R		0	81.6｛0.800｝	-	245	39
PKGS-45TAV-R		45	78.5｛0.77｝	-	195	37
PKGS-25TA-R		25	-	20.9｛0.205｝	240	39

Fig. 3:Lineup of surface mountable accelerometer device for TPMS

Roles Surface Mountable Accelerometer Devices Play

Initially used for airbags, surface mountable accelerometer device applications for automobiles are expanding especially for TPMS. We will probably see TPMS installed in Japanese cars in the near future.

What kind of roles, then, do surface mountable accelerometer devices play in TPMS?
As mentioned before, TPMS was originally developed to improve drivers' safety. In that sense, TPMS does not play a vital role while the car is stopped, or actually wastes energy if it stays active.

By mounting a surface mountable accelerometer device, TPMS can operate only when there is more than a certain amount of shock (Fig. 4) . Energy consumption for data-transmitting wireless communication circuits can especially be reduced by limiting tire pressure detection and pressure data transmission only while in motion. This feature, to power the equipment only during operation and keep them in the sleep mode automatically while not in operation, is called "wake-up"feature ^*1, contributing to significant power saving in the equipment.

Fig. 4 Tire rotation and sensor output

In addition to the surface mountable accelerometer device-based wake-up feature for TPMS market, there is also a semiconductor MEMS-based^*2 (micro electro mechanical systems) wake-up feature, and two of them together comprise the majority share of the market. Since current TPMS are powered by batteries that are required to last between a few years to 10 years, low power consumption is a very important requirement by TPMS manufacturers.

Also the mounting position of TPMS being inside tire valves requires very high reliability. Murata's surface mountable accelerometer device satisfies both low power consumption and high reliability requirements and commands over 50% share of the market currently. Although the wake-up feature with the surface mountable accelerometer is currently only implemented in TPMS, as the low power consumption trend continues, there will be other situations where surface mountable accelerometer devices can play a role in the future. They may include sensing applications such as a building and energy management system (BEMS^*3) , a home energy management system (HEMS) , servers to archive data from BEMS and HEMS, home electrical appliances and mobile terminals.

Future of TPMS

TPMS is becoming smaller as with many other electronic equipment in the market. The largest advantage to downsizing TPMS is tire balance. As described earlier, TPMS are mounted inside tire valves, and the larger they are the more they would affect the tire balance. Therefore, we can predict that TPMS will continue to downsize, and surface mountable accelerometer devices will be required to downsize while maintaining the current level of reliability.

What will happen next, if surface mountable accelerometer devices become smaller? It is conceivable that TPMS will be mounted directly to the tire itself (in tire treads) to transmit warnings to drivers more directly and precisely for improved safety (Fig. 5) . This way, TPMS will able to immediately warn the driver of dangers based on information such as abnormalities caused by bumpy unpaved roads, road surface temperature approaching 100°Cfrom the summer sun and frozen ground of the winter.

Fig. 5

Smaller size and higher reliability will be required for TPMS mounted in tire tread, compared with valve-mounted TPMS. Since semiconductor MEMS are not very strong against temperature changes, we will need to keep proposing surface mountable accelerometer devices to our customers.

We will continue to help make TPMS smaller and lower profile, by coming up with total solutions through utilization of Murata's module technology combined with other sensor technologies such as pressure sensors.

Final Remarks

In the light of environmental issues such as advancing global warming, initiatives to save energy are conducted throughout the world. As Kyoto Protocol was extended until the end of 2020, requirements and interests for energy saving will become stronger for sure.

Murata's surface mountable accelerometer devices have a potential of contributing to energy saving for many types of equipment in addition to TPMS. It is Murata's wish to continue being a supplier that can propose attractive features to our customers and end users at all times, in terms of performance and environment friendliness.

Glossary