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# Pumped UpPUBLIC ACCESS

Micro-Device Manufacturers Shift into High Gear, as New Federal Rules give them Millions of Tires to Ride.

[+] Author Notes

Associate Editor.

Mechanical Engineering 127(04), 34-39 (Apr 01, 2005) (6 pages) doi:10.1115/1.2005-APR-2

## Abstract

This article focuses on microelectromechanical systems that have been one of the hot topics of the engineering world for years now. Tire-pressure monitoring systems have been available on select passenger vehicles since the late 1990s. Tire-pressure sensor modules contain several components. A MEMS pressure sensor is the key element, but the package may also include a temperature sensor, voltage sensor, accelerometer, microcontroller, radio-frequency circuit, antenna, and battery. Some suppliers of MEMS tire-pressure sensors are seeking to eliminate batteries and power the tire-pressure sensing module by an alternative power source. In the sensor module, packaging must expose the sensor to air pressure and protect the rest of the components. Freescale, for example, protects its sensor with a Teflon filter, which makes sure that only dry air can enter and pressurize the diaphragm.

## Article

Inside a MEMS tire-pressure monitor, a piezo-resistive pressure sensor and accelerometer are hard-wired to an application-specific integrated circuit.

Microelectromechanical systems have been one of the hot topics of the engineering world for years now. After all, it’s fascinating to see the ingenuity by which inventors can create transducers from the stuff that semiconductors are made of.

Proponents say the technology has plenty of advantages. They talk, for instance, about its ability to yield masses of small, efficient, robust devices dirt-cheap.

MEMS has achieved high mass market demand in a number of industries, from automotive to biomedical to industrial to telecommunications. The automotive industry has seen several high-volume applications, each numbering in the tens of millions. The biggest automotive application is airbag accelerometers, of which some 90 million units were installed on vehicles in 2004, according to Roger Grace, a MEMS marketing consultant based in Naples, Fla.

The buzz in the business now is that the automobile is about to deliver another killer application. And safety legislation again is the cause. Just as airbags were put into new cars by law, so tire-pressure monitoring systems will start to become standard equipment on most passenger vehicles in the not-too-distant future.

A tire-pressure sensing display gives numerical pressure readings based on a pressure sensor installed on each wheel of a passenger car.

Although there are different ways to keep tabs on tire inflation, the method of choice—placing a tiny sensor in each wheel—promises to create an overnight demand for millions of microelectromechanical transducers. So for MEMS manufacturers, it’s like airbags all over again.

Tire-pressure monitoring systems have been available on select passenger vehicles since the late 1990s.

In the fall of 2000, however, Congress stepped in and passed the Transportation Recall Enhancement, Accountability, and Documentation Act.

The significance of the cryptic title is that it can be reduced to “TREAD Act.” Although the name may be frivolous, the purpose of the law is not. Congress was responding to the public climate after several rollover accidents involving Ford Explorers were linked to the underinflation of their tires.

The law calls for a federal rule that new vehicles weighing under 10,000 pounds be equipped with tire-pressure warning systems. The rule has not been issued, but the National Highway Traffic Safety Administration is working on it. When the rule kicks in, it will affect about 17 million vehicles sold in the U.S. every year. Each one will have a pressure-monitoring system at each wheel. The way the market is shaping up now in anticipation of the new rule, it is possible this one law could create an instantaneous market for perhaps 70 million MEMS devices a year.

That represents a huge and sudden jump in the number of MEMS tire-pressure sensors on the road today. Mark Fitzgerald, a senior industry analyst with Strategy Analytics in Boston, said there were only about four million MEMS tire-pressure sensors on passenger cars and light trucks in this country in 2003. He said that the number could balloon to as many as 76 million in 2008.

The rule is expected to have some influence on foreign markets, too. One reason is that all automakers will have to equip their vehicles with tire-pressure sensors destined for export to the United States. This year, around 13.7 percent of the 67.3 million passenger cars and light trucks will have MEMS tire- pressure sensors. That translates to 9.2 million vehicles, or 36.8 million sensors, according to Simon Schofield, senior analyst in the Strategy Analytics’ U.K. office. By 2011, MEMS tire-pressure sensors will account for more than three-quarters of tire-pressure sensing systems found on passengers and light trucks produced worldwide, he said.

One approach to tire monitoring is software-based and is designed to work with the vehicle’s anti-lock brake system. Known as the inferred, or indirect, method, it determines differences in pressure by comparing the rotational speeds of the wheels.

The alternative is the direct tire-pressure monitoring method, which places a sensor module at each wheel. The sensors measure the pressure in each tire and transmit the data wirelessly to a central receiver in the vehicle, which analyzes the information and displays it to the driver. The information varies from simple warning lights when pressure gets too low to readouts of pressure measurements. Some systems may also include pressure information about the spare tire.

Typically, a tire-pressure sensing module is located inside the rim of the wheel. The MEMS package must stand up to vibration, heat, and corrosive fluids.

The main advantage of the inferred method is that it is relatively inexpensive, since it requires no extra hardware. It has no battery life concerns or remote sensors that can be damaged by tire mounting or road hazards. On the other hand, it won’t detect significant underinflation when all four tires are equally soft or when two tires on the same side of the vehicle are underinflated, according to a NHTSA test report.

A typical tire-pressure monitoring system integrates many functions. Sensors in each wheel measure temperature and pressure at regular intervals. That information is sent by radio-frequency signal to an electronic control unit inside the vehicle. The unit analyzes the data it receives. Initiators interrogate sensors as needed to rapidly confirm possible warnings and to ensure that accurate information is sent to the driver. A display warns the driver in real time of any critical deviations from normal conditions.

NHTSA found direct tire-pressure systems can measure the pressure of each tire, have more consistent warning thresholds, and are quicker to provide warnings than inferred systems are.

Producers of sensors and other components, eager to get a foothold in the potential market, have been jockeying for position with suppliers of automotive system modules. With the expanding supplier base have come new developments in engineering to improve component integration, packaging, and power consumption—three key requirements that, along with cost, will determine which devices make it into vehicles.

## Slice and Dice

The sensor companies that succeed in getting their MEMS pressure sensors on wheels will be those coming up with the cheapest products, according to Roger Grace. The ramp-up in manufacturing of direct tire-pressure monitoring systems has helped drive down their cost. Grace pegs the current cost per wheel of a tire-pressure-sensing module at $12 to$15. He expects the cost to drop to $5 per wheel by 2007 as volumes increase. When tire-pressure monitors first came on the scene, they cost$25 to \$30 per wheel.

Tire-pressure sensor modules contain several components. A MEMS pressure sensor is the key element, but the package may also include a temperature sensor, voltage sensor, accelerometer, microcontroller, radio-frequency circuit, antenna, and battery. Each of these micro-devices performs a task that allows a tiny integrated module weighing 30 to 40 grams to measure the pressure, condition the signal, and transmit the data.

There are many ways to group the various components together on chips. In the interest of reducing cost, MEMS designers are working to integrate more components on a piece of silicon to reduce the number of chips in the package. And that is a very tough challenge on a micro-device that combines sensing, signal processing, and data transmission into one unit.

According to Grace, at least one MEMS company is working on a monolithic, or one-chip, design. “The cheapest solutions can only come about with total integration,” he said.

Freescale Semiconductor offers a surface micromachined capacitive pressure sensor for tire-pressure monitoring. Some sensor suppliers claim that capacitive sensors help to conserve battery power.

Bishnu Gogoi, a distinguished member of the technical staff at the sensor and analog products division of Freescale Semiconductor Inc. in Tempe, Ariz., said that the company’s tire-pressure sensing module consists of four basic parts: sensor, microcontroller, a wakeup switch, and a communications chip. The module also uses a battery. System designers must determine the best way to combine the various components.

“There are difficult technological challenges in trying to put it all together on what we could call a system on a chip,” Gogoi said. “Today one would call it a system in a package.” Integrating components in a single chip encounters limitations in materials, thermal budget, complex process integration, and other design issues, he said. It may also make testing the components more difficult. One advantage of a multichip device is that it is more manageable and provides greater flexibility, he said.

Steve Hendry, marketing manager for sensor products at Freescale Semiconductor, said the company currently supplies tire-pressure sensor modules to the large-vehicle market and is developing a second-generation sensor that combines a pressure sensor, microcontroller, RF transmitter, and motion detector in a single package. Consolidating the components in a single package will help the company to reduce the unit cost, he said.

Other sensor module suppliers are following suit. SmarTire Systems Inc. is a tire-pressure sensor supplier based in Richmond, British Columbia. It assembles sensor modules of components from several suppliers. Shawn Lammers, the company’s vice president of engineering, said that suppliers are combining the microprocessor, pressure sensor, and analog-to-digital circuitry in one integrated device, simplifying the assembly tasks for the company. “It’s getting a lot more condensed now,” he said.

Lammers added that its pressure sensor supplier, GE NovaSensor in Fremont, Calif., calibrates the pressure sensor and microprocessor before shipping the package. That service removes a major bottleneck in SmarTire’s production cycle, he said.

Powering the micro-devices is another challenge. Tire-pressure monitoring modules used on vehicles today are self-powered, energized by a coin-type battery in the module. Automakers are demanding a 10-year service life—a high expectation, given the battery’s proximity to high heat, moisture, and road salt. “The limitation of battery life is in the batteries themselves,” Lammers said. SmarTire has opted for a more robust battery, which can operate at 125°C continuously and survive up to 170°C for 72 hours. Many lithium batteries are designed to operate at 85°C, and the more robust batteries are roughly twice the cost, he said.

To conserve as much battery power as possible, tire-pressure sensor modules operate in a wake-sleep mode, in which the sensor is activated only when a pressure reading is called for. Either an accelerometer or a mechanical switch such as a ball-and-spring is used to alert the system that the vehicle is moving. Some suppliers say that a ball-and-spring motion switch has the advantage of being completely passive, and therefore is not a drain on battery power.

According to Lammers, accelerometers consume from one to four milliamps to make a measurement, and so use quite a bit of energy to determine if a vehicle is moving or not.

MEMS sensors undergo liquid immersion, pressure cook, and other testing in an environmental qualification lab before they are shipped.

## Pressure Sense

The type of pressure sensor also figures into the power mix. Some suppliers of sensor modules use traditional piezo-resistive MEMS sensors. Others use capacitive MEMS sensors. Proponents of capacitive sensors, including Freescale Semiconductor, say they consume less power. According to Gogoi, capacitive pressure sensors consume very little power themselves; the signal is just a change in capacitance, and the circuitry converts the capacitance to a voltage.

One of the companies whose products use piezo-resistive sensors is EnTire Solutions of Farmington Hills, Mich., a joint venture formed in 2003 between TRW Automotive and Michelin Tire in France. The company supplies tire-pressure sensor modules to a number of automakers. According to Dave Juzswik, chief engineer at TRW Automotive, the company finds piezo-resistive sensors more linear than capacitive sensors and a bit more accurate over wide pressure and temperature ranges.

Hatto Schick is product manager for tire-pressure monitoring systems at SensoNor, a subsidiary of Infineon Technologies AG in Horten, Norway, a major supplier of MEMS tire-pressure sensors to the automotive market. He said that, while a piezo-resistive sensor may draw more power than a capacitive one, the sensor still accounts for a small percentage of the overall power consumed in the system. Radio-frequency transmission, and operating the microcontroller and electronic circuits drain more power, he said.

Two established producers of MEMS accelerometers are preparing to enter the tire-pressure sensing market with product offerings of MEMS capacitive sensors. One company is Analog Devices, which has designed a capacitive pressure sensor that it intends to serve as the core part of a tire-pressure module it is developing. The company does not yet have a commercial pressure sensor, and plans to supply a working prototype to a customer in the next month, according to Kieran Harney, business development manager of the company’s MEMS division in Cambridge, Mass.

Harney believes that the low power consumption properties of MEMS capacitive sensors are important to the tire-pressure sensing application. He said the MEMS group is working with Analog’s high-resolution converter group in Limerick, Ireland, on a capacitive-to-digital converter technology that would be used with the new MEMS pressure sensor. John Wynne, a marketing manager based in Limerick, said the MEMS group is working with other divisions in the company to develop RF, microcontroller, and packaging technology for the integrated module.

## Two Track to Tire-Pressure Monitoring Systems

The mandate coming from the U.S. National Highway Traffic Safety Administration that will require tire-pressure monitoring systems has been in the works for more than four years, and still hasn’t taken its final form.

MEMS-based tire-pressure sensors should meet an upcoming NHTSA rulemaking, say automakers and sensor suppliers.

Congress called for NHTSA to issue a rule when the TREAD Act became law in the fall of 2000. The Transportation Recall Enhancement, Accountability, and Documentation Act was passed by Congress after rollover accidents involving Ford Explorers were linked to underinflation of their Firestone tires. The law calls for a federal rule that new vehicles weighing less than 10,000 pounds be equipped with a tire-pressure warning system.

NHTSA issued its first draft of the rule in June 2002. The document did not specify a technology, but did establish two performance standards. One would allow a direct-measurement approach, in which a sensor is placed in each wheel. The other standard applied to a software-based method designed to work with a vehicle’s anti-lock braking system.

Following a court challenge by public safety groups, a U.S. Court of Appeals ruled that the software approach did not meet legal safety requirements. The initial rule was sent back for a rewrite in August 2003, and the revision is still under way.

Because a new draft has yet to be issued, NHTSA has revealed few details about it. Unlike its predecessor, however, it will specify one performance standard: that car manufacturers be required to install a four-tire tire-pressure monitoring system that can warn a driver when a tire is more than 25 percent underinflated.

According to Mike Wolterman, manager of the tire and wheel group at Toyota Technical Center in Ann Arbor, Mich,, telling when a tire has lost 25 percent of its air is difficult to do with an indirect system, which infers relative tire inflation by comparing the rotation of the wheels. A direct system, which has a pressure sensor on each wheel, is able to perform a system check, indicating that all tires are inflated and that the system is operational.

Steve Hendry, marketing manager for sensor products at Freescale Semiconductor in Tempe, Ariz., said that, although the new rule will not specifically disallow the indirect method, he believes that indirect systems will no longer be able to meet the specifications to be set by NHTSA.

—John DeGaspari

Some suppliers of MEMS tire-pressure sensors are seeking to eliminate batteries and power the tire-pressure sensing module by an alternative power source. This is the approach ofVTI Technologies, a supplier of microaccelerometers in Dearborn, Mich. Rick Russell, the company’s director of marketing and sales, said the company has developed a line of MEMS capacitive sensors with an eye on the tire-pressure monitoring market, among other applications. The idea is to place the tire-sensing module inside the rubber of the tire. A battery-less system would operate on very low power, for which a capacitive sensor is suited because it operates on minimal power, he said.

Russell said the company plans to offer both battery-powered and battery-less modules. He believes that automakers are interested in battery-less MEMS tire-pressure sensors, and that demand will increase for future generations of the product. He said VTI has signed on to several programs with tier-one automotive suppliers working with tire OEMs on battery-less systems. The company’s strategy is to supply the sensing element and rely on tier-one suppliers to manufacture the entire module, Russell said. The company has not yet supplied any of its MEMS pressure sensors to the tire-pressure sensor market.

Other supplier companies, including SmarTire, are looking at the battery-less concept. Lammers explained that an external radio-frequency field would energize the system. An antenna in the wheel well would take RF energy and convert it to voltage. This would power the internal circuitry, convert it to pressure measurement, and send it back to the receiver in the wheel well.

Dirk Leman is the automotive product manager for Melexis Microelectronic Integrated Systems in Tessenderlo, Belgium. The company has developed a MEMS pressure sensor, and is developing integrated tire-pressure sensor modules, which it plans to introduce into the automotive market later in this decade. He believes that battery-based MEMS tire-pressure modules will dominate the industry in the near future. But he added that “there are some clever ideas popping up from people to generate power by other means.”

However they are designed, pressure sensors and delicate electronics in tire-pressure sensor modules must stand up to extreme temperatures, vibration, and corrosive fluids on the road, not to mention rough handling in the tire shop. Temperatures on the wheel could reach 100°C, enough to turn water into steam, according to Davejuzswik ofEnTire Solutions. “Sealing of the module is really critical,” he said.

Juzswik said there are basically two ways to seal a tiresensing module. One is to pot it with a fluid, such as silicone, which will set up and become hard, but not brittle, allowing for thermal expansion. The alternative, which is used by EnTire, is to weld a cover over the sensor. The main advantage of the latter approach is weight saving, he said.

Then there is mounting the sensor on the valve stem. EnTire Solutions uses two versions: fixed-angle and adjustable-angle mounts. The latter gives more flexibility and allows the sensor to be rotated down, making contact with the rim. The angle at which the sensor sits in the tire rim is important to minimize potential damage to the valve assembly, Juzswik said.

A piezo-resistive pressure sensor and RF transmitter are packaged together on the same PC board as part of a tire-pressure sensing module under development by Melexis.

In the sensor module, packaging must expose the sensor to air pressure and protect the rest of the components. Freescale, for example, protects its sensor with a Teflon filter, which makes sure that only dry air can enter and pressurize the diaphragm, according to Bishnu Gogoi. The rest of the components are under plastic.

Suppliers of direct tire-pressure monitoring systems are focused on making sure that there is enough manufacturing capacity and quality to meet the mandates of legislation in the United States, according to John McGowan, director of sensing product controls at Infineon’s Livonia, Mich., office. And, for now, those conditions appear to be met.

John Maxgay, the lead engineer for tire-pressure sensing modules at General Motors Technical Center in Warren, Mich., said the company expects to be in compliance with the mandate for the monitoring systems available today. GM currently buys most of its tire sensor modules from Schrader Electronics in Rochester Hills, Mich.

Maxgay said he is open, yet cautious, about new technologies to lower power consumption, reduce weight, or eliminate batteries from the mix. One thing is for certain: There are more than enough ideas out there to keep it interesting.

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