A 1977 study at Indiana University estimated that 260,000 vehicle crashes occurred each year in the United States because of underinflated tires (out of a total 18 million crashes for all reasons). In November 2000, Congress enacted the Transportation Recall Enhancement, Accountability and Documentation Act (TREAD), in part due to the more than 250 fatalities linked to underinflation of Firestone tires on Ford Explorers. TREAD mandated tire pressure sensors (TPS) on all new passenger vehicles and light trucks; many new vehicles come equipped with these devices and all vehicles are required to have them by September 2007.TPS warn drivers of underinflation, leaks, and the loss of air pressure that occurs naturally; tires typically lose about 1 psi each month due to natural permeation, losing more in warm weather. In an under-inflated tire the sidewalls will flex excessively creating high temperatures that degrade the tire and make failure more likely.
It is ironic that, with all the sophisticated technology in today's vehicles, tires are one of the last systems to be instrumented. For decades sensors have existed to measure parameters such as oil pressure, coolant temperature, and electrical output. More recently, sensors have been added for seat belts, environmental systems, road temperature, back-up indicators, GPS locators, and other functions. While all of these systems are important, most of them do not have the direct impact on safety that tire sensors do. A recent survey by the Car Care Council found that about half of all vehicles inspected had improperly inflated tires. The National Highway Traffic Safety Administration (NHTSA) estimates that TPS systems on all vehicles would prevent about 120 fatalities per year.
The Challenges
The chief challenge to measuring tire pressure is the simple fact that the tire is rotating at high speeds and making a direct connection to a rotating tire is difficult. The tire is also exposed to unexpected hazards, water, and road chemicals and subjected to centrifugal forces that try to pull it off the wheel. All this in temperatures that can reach well over 100°F in summer and far below 0°F in winter. TPS systems must be designed to handle these harsh conditions and to meet four key requirements: They must secure the sensor in the tire or wheel, provide power to the sensor, extract data from the sensor, and display the information to the driver.
Securing the sensor in the tire or the wheel is the easiest condition to meet. Pressure sensors that can withstand the conditions in a tire are already available at reasonable cost. The real challenges are the next two—getting power to the sensor and extracting data from the sensor to the display. The most practical and cost-effective method of extracting the data from a rotating tire is to use a wireless signal and wireless communication has become the standard for TPS systems. In the complete package, the sensor measures the tire pressure and the circuit transmits it as a radio signal to the display for the driver. Power is typically supplied with a battery contained in the TPS package. A popular wheel-mounted TPS package is shown in Figure 1.
Figure 1: Wheel-mounted TPS system manufactured by Siemens VDO Corp.
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The small wireless transmitter in a TPS system is similar to that used in an auto key fob to lock your car. In the TPS system, it sends data from the rotating tire to the display reader. The antenna is located in the valve stem so that it is slightly outside of the wheel. This antenna design is an attempt to prevent the metal wheel from blocking too much of the signal.
TPS systems are designed to send a signal directly from the sensor on each wheel to a receiver and display the data for the driver. For cost reasons, a single receiver is preferred. This means that the signal from the TPS must be strong enough to overcome the attenuation and signal loss caused by the mass of the car. This requires a high-power circuit and larger battery. However, a large battery increases both size and cost and adds weight that must be balanced.
Battery life in TPS systems can be quite variable. Batteries can last for a reasonable time period when the transmitter is managed on a duty cycle and when everything goes right. However, batteries may not always last as long as expected. In many TPS systems, changing the battery requires replacing the entire sealed sensor package. Estimates of the cost to consumers for TPS maintenance run from a few hundred dollars to a thousand dollars over the life of the system.
Battery Alternatives
There is a great deal of interest in alternatives to batteries for TPS systems. One of the most attractive options is to capture energy from the moving tire with what are called "energy harvesters." A simple example of an energy harvester is the modern generator, or alternator, used in all vehicles. It is usually connected to the engine by a belt and as it turns it transforms some of the mechanical energy of the engine into electricity.
We believe that the best candidate for energy harvesting in TPS systems is the piezoelectric effect. Piezoelectric (PZ) materials can convert some of the vibration of the tire into electricity that is then stored in a capacitor and used to power the TPS system on the desired duty cycle. Potential advantages include:
- Longer life—an energy harvester could last as long as the vehicle
- Smaller size—a harvester could be smaller than a battery
- More rugged
- Lower cost—harvesters will be competitive with current batteries
- Less maintenance—harvesters will not normally need to be replaced
- No switch required for energy harvesters—batteries must be turned off when the vehicle is stationary and require a switch and motion sensor. PZ harvesters do not require this
- Lower environmental impact—harvesters are more environmentally benign when compared to the multiple batteries that would be required over the life of the vehicle
Unfortunately, energy harvesters also have one big disadvantage—they're difficult to manufacture. While PZ harvesters have been around in low-cost products, such as lighters, for a long time it is both difficult and costly to build a small, rugged system suitable for powering a TPS. Let's examine the anatomy of the device.
Anatomy of a Piezoelectric Energy Harvester
As shown in Figure 2, a TPS energy harvester is a beam of PZ material bonded to a metal conductor to form a bimorph. The bimorph is fixed at one end and free to vibrate along its length like a tuning fork. The electric current generated from the PZ material as the bimorph vibrates is captured by electrodes.
Figure 2: A basic piezoelectric bimorph
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A piezoelectric energy harvester usually consists of five basic design elements: A PZ ceramic material, typically a lead-zirconia-titania compound called PZT; conductors to carry the current, typically silver, gold, or aluminum; an insulating mount to hold the bimorph firmly in place; an open space for the free end of the bimorph to vibrate (typically air); and a package, typically ceramic or metal, to protect and hold the device.
All of the above elements need to be bonded into a strong package no larger than the batteries currently used in tire pressure sensors, which are somewhat smaller than a quarter coin but only about half as thick. This presents a challenge for conventional manufacturing techniques.
Manufacturing Methods
The design requirements of a PZ energy harvester could be met by using a layered manufacturing process, creating layers of both ceramic and metal simultaneously and leaving spaces where necessary. Approaches such as rapid prototyping can be used to create models of PZ harvesters, but not actual working parts. So far, successful assembly has relied on hand assembly and machining.
EoPlex has developed a relatively new process that is capable of manufacturing PZ harvesters. This high-volume technology builds parts in layers, but can produce thousands of parts simultaneously from many different materials. These parts include active elements (e.g., circuits, catalyst beds, mixing chambers, capacitors, and piezoelectric actuators) that are produced in one step.
The EoPlex method uses printing as a forming tool, with proprietary printing pastes or "inks" acting as the building blocks to create the 3D structure.
An EoPlex printing paste looks similar to a thick-film paste or a solder paste, and consists of engineered mixtures of inorganic powders, which create the final structure, and an organic portion, which acts as the liquid carrier, allowing the paste to be printed.
The inorganic portion of the paste is composed of a careful distribution of fine ceramics, glasses, metals, and modifiers that are sintered or fired together. Since these materials are all fired in the same part, the process is called cofiring. Not all material combinations can be cofired. For example, tungsten, a high-temperature metal that requires a firing temperature of 1500°C, cannot be used with tin, which melts at about 230°C. Other materials may not be candidates for cofiring due to mismatches in thermal expansion or other properties.
The organic compounds used are proprietary blends of binders, polymers, dispersants, viscosity and surface modifiers, and other additives. These organic materials allow EoPlex engineers to create printing pastes that cure quickly between layers, carry very high loadings of the inorganic powders, print to high accuracy, and sinter to very high density during the firing process.
For components with complex open areas we use special printing materials calledfugitive, negative, or sacrificial pastes that are designed to disappear at some point in the process. Fugitive pastes are printed like the other materials and are used to form complex structures within the part. During postprocessing, the fugitive structure is removed to leave a 3D structure of channels, chambers, and spaces. Good fugitive materials exhibit highly complex chemistry. Fugitive printing inks must do all of the following simultaneously:
- Print crisply and precisely
- Cure instantly to allow other layers to be printed without distortion or "smearing"
- Expand and contract with changes in temperature by an amount similar to the other materials in the structure
- Bond well with other materials, including conductors, ceramics, metals, and glasses
- Disappear cleanly, without leaving behind any residue, ash, or contaminants
- Be able to diffuse through the walls of the part without requiring a vent
- Not create any significant stresses during removal
The requirement to maintain low stress on the part during fugitive removal is critical. Any pressure generated by the fugitive will normally occur at a relatively low temperature before the ceramic and metal precursors have reacted. As a result, the structure will be weak and can be damaged by the pressure of the escaping fugitive, unless the process is carefully controlled.
Building an Energy Harvester
The process is a good match for the requirements of a PZ energy harvester. For this application it will use five different proprietary printing materials including a tough ceramic for the outer package, a PZT material to generate electricity, and a series of conductors and contacts to collect and carry the charge (Figure 3).
Figure 3. An EoPlex energy harvester design
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This energy harvester has two piezoelectric layers that are bonded to three metal layers to provide power as the beam vibrates up and down. A mass has been built into the beam to tune the arm to the available vibrations in the tire and generate more energy. The assembly is housed in a ceramic package with electrical contacts built into the top.
To build this part, EoPlex starts with the CAD model, which is sliced into a set of layers according to specific design rules. These CAD slices are used to create print screens or masks, similar to those used in circuit board printing. Screens are produced for all positive images—which are printed with ceramic or metal paste—as well as all negative images, which are printed with fugitive paste. The number of screens and the variety of images per screen depends on the number of layers that the part is sliced into and the number of materials in each layer.
Parts are printed using commercial screen printers modified to deposit the proprietary inks. At the end of the process, we will have a full slab of parts consisting of hundreds of layers made up of different images and materials. A test panel of parts 18 by 18 in. is shown in Figure 4. A red dye has been added to the fugitive material in this panel for clarity.
Figure 4. An 18 by 18 in. panel of EoPlex parts. The red material is the dyed fugitive compound
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When the fugitive material is removed and the parts sintered, the parts emerge separately and no cutting is required. In Figure 5, four different demonstration parts are shown with the dyed fugitive material still present (left) and with the fugitive material removed and the full parts revealed (right).
Figure 5. The demo parts on the left show the dyed fugitive material still present while those on the right show the parts after the fugitive material has been removed
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EoPlex currently develops energy harvesters for tire pressure sensors and other applications. The company also uses these same techniques for fuel cell components, microreactors, thermal management systems, and advanced circuits.
We are pleased to see that tire safety, fuel economy and the risks of underinflation are being aggressively addressed. This technology, unheard of 30 years ago when the 1977 Indiana study was done, has evolved to accommodate the development of TPS systems. We're delighted to help advance that technology even further.
