Ever since Porsche introduced the first direct-type tire-pressure monitoring system (TPMS) in 1997, manufacturers have been struggling to solve two major technical challenges: developing a TPMS that requires as little power as possible to operate (either from a battery or an energy scavenging technique or both) and a form factor that better suits a tire’s shape.
On top of that, the need for a low-cost manufacturing approach overshadows both of these issues. The bill-of-materials cost has to be low enough to make TPMSs more attractive for automotive applications. The goal is to develop a more intelligent TPMS that will minimize if not eliminate battery operation.
The market for energy scavenging and intelligent TPMSs isn’t fully developed yet but has some potential, according to market research firm iSuppli Corp. The first intelligent tires will arrive in 2010, and they’ll likely use batteries (Fig. 1). The first priorities will be improving electronic circuitry within the TPMS and improving power management, using battery technology that has yet to reach its limits.
By 2012, iSuppli forecasts that the first energy scavengers for intelligent tires will appear in high-end cars as well some trucks, and larger volumes will occur by 2015. The iSuppli study forecasts a potential market by 2015 of 60 million TPMSs modules valued from $500 million to $600 million and scavengers ranging in price from $40 to $50.
In the past couple of years, some notable efforts have resulted in low-power, more intelligent MEMS pressure sensors that minimize the need for power scavenging. For example, the SmartSense sCAP3 tire-pressure sensor from Kavlico Corp., an operation of Schneider Electric, is a CMOS-based, single-chip, 1.2- by l.5-mm capacitive sensing element.
The sCAP3 operates on supplies as low as 1.8 V and draws a mere 20 µA in a 4- by 4-mm quad flat no-lead (QFN) package, so it can be powered by an inductive antenna as well as batteries. A temperature element on the chip allows temperature compensation of tire pressure measurements. A digital output eliminates the need for an analog-to-digital converter (ADC).
Siemens VDO (now part of Continental) introduced a slim form-factor TPMS that mounts next the tire’s wall, representing conventional TPMSs in tires. Working with the Goodyear Tire & Rubber Co., Siemens VDO has since introduced the intelligent Tire IQ pressure sensor. Unlike other TPMSs, it’s molded in the tire’s rim, not located within the wheel well (Fig. 2). This approach, while not supported by everyone in the automotive industry, signals a trend to move tire-pressure intelligence away from the wheel well and onto the tire itself.
Piezo Technology Gaining Importance
Many energy-harvesting techniques are under investigation for TPMSs. One of the most popular is the piezoelectric effect using PZT (lead zirconium titanate) compounds. Piezoelectricity results from the ability of crystals and certain ceramic materials to generate a voltage in response to mechanical stress. This type of energy can be generated within a wheel’s tire, stored in a capacitor, and used to power a TPMS.
A major part of the success of piezo materials for TPMSs lies in developing a mass-production process that enables cost-effective energy harvesters to be manufactured. EoPlex Technologies is using one such process with five different proprietary printing materials. Its goal is to develop coin-size, low-cost, PZT TPMSs that can be embedded in a tire, operate without a battery, and send information to a car’s dashboard control wirelessly.
The 3D printing process being used includes the manufacture of a tough ceramic outer package, the piezoelectric material to electrical power, a series of conductors and contacts to collect and carry the charge, a fugitive material to provide the space needed for the piezoelectric material to vibrate, and other specialized conductors or dielectrics (Fig. 3).
Advanced Cerametrics has developed a proprietary viscous suspension spinning process (VSSP) for making nearly any ceramic material into fiber form. It can be used to produce energy harvesting, sensing, and actuation devices for many applications, including TPMSs. The patented process handles a variety of cross-section fiber geometrics within a wide diameter range of 15 µm to 1.5 mm.
Also, the process features a clean burnout because it is cellulose-based. It’s a low-cost and robust manufacturing technology as well (Fig. 4). VSSP-made transponders have been successfully tested on Honda Civics, Lincoln Navigators, and Dodge Dakotas on the road, producing sufficient energy between 0.13 and 1.2 minutes for a single wireless transmission.
“The breakthrough we made was by making the piezoelectric ceramic material flexible. Prior to that, it was brittle and could not be bent. We’re now looking to be cost-competitive with automotive and rechargeable batteries, as well as using the product in a car’s wheel axle,” says Richard “Bud” Cass, chairman of the board of Advanced Cerametrics.
PiezoTAG Ltd. says that its piezoelectric TPMSs represent the “next generation in tire-pressure and temperature sensing.” They offer battery-less operation, harvesting power from a device within the tire using the tire’s rotation. The company also says the technology offers high RF transmission rates and frequencies for data reception (every 6 seconds). There’s no need for initiators in each wheel arch. And, there’s automatic wheel identification and location as well as easy OEM and retro-fit usage.
Some companies are even making development kits for evaluating and simplifying energy harvesting designs available for many applications. The EH301 kit and EH301 EPAD energy harvesting modules from Advanced Linear Devics accept energy from a variety of sources that operate from vibration, light, chemical reaction, fluid and air flow, environment heat, and other effects.
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