FEATURE FOCUS: MEMS
pumped
up
Micro-device manufacturers
shift into high gear, as new federal rules give them millions of tires
to ride.
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.
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| Inside a MEMS tire-pressure monitor, a piezo-resistive pressure sensor and accelerometer are hard-wired to an application-specific integrated circuit. |
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.
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| A tire-pressure sensing display gives numerical pressure readings based on a pressure sensor installed on each wheel of a passenger car. |
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.
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.
 |
| 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. |
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.
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.
 |
| 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. |
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.
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.
 |
| MEMS sensors undergo liquid immersion, pressure cook, and other testing in an environmental qualification lab before they are shipped. |
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.
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 of VTI Technologies, a supplier of
micro-accelerometers 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."
 |
| 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. |
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 Dave Juzswik of EnTire Solutions. "Sealing
of the module is really critical," he said.
Juzswik said there are basically two ways to seal a tire-sensing 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.
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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Two Tracks 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.
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. — John DeGaspari
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© 2005 by The American Society of Mechanical Engineers