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Freescale Single-Package Tire Pressure Monitoring System



Remember kicking tires to see if they were getting low? Now, with each of your tires sporting a Tire Pressure Monitoring System (TPMS) containing an 8-bit MCU, an RF transmitter, and a two-axis (XZ) accelerometer, you're out of excuses for not knowing better.

Many people seem to think that maintaining proper tire pressure is a nuisance and that keeping our cars clean deserves more attention. In fact, a Rubber Manufacturers Association survey discovered that nearly 70 percent of American drivers wash their vehicles at least once a month while only 15 percent check their tire inflation that frequently. Unfortunately, driving on improperly inflated tires can be deadly. The National Highway Traffic Safety Administration (NHTSA) estimates that 23,000 accidents and 535 fatalities per year involve flat tires or blowouts, both of which can be the result of improperly inflated tires.

Improperly inflated tires are a global safety issue. In France, the Sécurité Routière estimates that nine percent of all fatal road accidents are the result of under-inflated tires. The German DEKRA found that tire problems are linked to an estimated 41 percent of all injury accidents. DEKRA's recent study in Germany found that 75 percent of vehicles on the road had at least one tire under-inflated by three pounds per square inch (psi) or more.

Under-inflated tires can also impose an economic hardship on the owner, particularly in businesses that rely heavily on road transportation. The economic impact can be especially serious in the commercial trucking industry. Just a few pounds of under-inflation can cost trucking firms thousands of dollars per year in heightened fuel costs, increased retread, and tire replacement expenses.

Safety considerations

Properly inflated tires reduce blowouts and braking distances, mitigate hydroplaning on wet surfaces, and improve overall vehicle handling. Proper inflation also increases the tire life and improves fuel efficiency, lowering the cost of vehicle operation. These benefits are frequently forfeited by motorists who fail to monitor their tire pressure to maintain proper inflation levels.

Under-inflation reduces tire stiffness, which degrades cornering forces and can result in skidding or loss of control in tight cornering or quick lane-changing maneuvers. Under-inflated tires also increase stopping distances because each tire's footprint and the pressure it exerts on the road has been altered.

Combining under-inflation with high-speed and/or overloaded conditions can also result in tire blowout, a catastrophic event that can cause the vehicle to veer into oncoming traffic or spin out of control. Even if control of the vehicle is maintained in such cases, changing a tire on the roadway often exposes the motorist to the dangers of oncoming traffic, particularly at night or in adverse weather conditions.

Though over-inflated tires are generally not considered to be as dangerous as under-inflated tires, they can still jeopardize the safety of vehicle occupants and increase the chances of tire failure. An over-inflated tire is stiffer with a smaller footprint in contact with the road. The tire is less responsive to irregularities in the road surface and can experience uneven tread wear, further diminishing the tire's safety characteristics.

According to the NHTSA, data from the tire industry indicates that 85 percent of all tire pressure losses are through slow leaks. Without suffering any road damage at all, tires can lose as much as one psi per month through natural leakage. Every drop of ten degrees Fahrenheit in ambient temperature can cause an additional loss of one psi per month. As a result, tire performance can degrade over an extended period of time while exhibiting little, if any, external indications of under-inflation.


Economic considerations

Properly inflated tires are not only safer to operate, but offer the owner a number of economic advantages. It's important to remember that the vehicle manufacturer, not the tire maker, determines proper inflation levels, which are usually listed on a placard on the inside of the driver compartment door. Keeping to the recommended tire pressure levels can mean:

  • More precise vehicular control for safer cornering and emergency maneuvering
  • Optimized road contact and rolling characteristics that improve fuel efficiency and extend the tire's lifetime
  • Improved traction in rain or snow conditions
To maintain proper inflation, tires should be checked at least once a month using a tire gauge. Drivers cannot "eyeball" proper inflation, since being off by just a few psi can mean that safe and economical vehicle operation has been compromised. Since most drivers are not diligent at regularly checking their tire pressures, automatic, remote Tire Pressure Monitoring Systems (TPMS) have been introduced.

Nowhere is there a stronger economic incentive for TPMS implementation than in the commercial trucking industry. Tests conducted by Goodyear Tire and Rubber Company have shown that just a 15 percent under-inflation of the steer, drive, and trailer tires can result in an eight percent drop in expected tire life and an over two percent decrease in miles-per-gallon fuel efficiency.

Keeping truck tires properly inflated can save thousands of dollars in fuel and tire servicing costs per vehicle over the course of its several hundred-thousand-mile lifetime, particularly when you consider that heavy-duty tractor/trailer trucks can have 18 or more tires. TPMS, deployed across an entire fleet of trucks, is a reliable tool to help maintain proper tire inflation under all driving conditions.

Direct and indirect tire pressure monitoring systems

In response to the dangers posed by widespread tire safety abuse, in particular following a number of highly publicized episodes around the turn of the millennium, the US government enacted the Transportation Recall, Enhancement, Accountability, and Documentation (TREAD) Act. As part of the TREAD Act, a federal motor vehicle safety standard, FMVSS 138, requires all vehicles, trucks, and buses with a gross vehicle weight rating of 10,000 pounds or less to be manufactured with TPMS by model year 2008.

There are two kinds of TPMS employed in motor vehicles, indirect and direct. Indirect TPMS uses the speed sensors in the ABS (anti-lock braking system) to compare wheel rotation speeds to determine under-inflated conditions. An under-inflated tire has a slightly smaller diameter than correctly inflated tires, thus it rolls at a different rate from properly inflated tires when the vehicle is in motion. The system alerts the driver when it detects an under-inflation condition. However, it cannot generally detect tire deflation until the tire is at least 25 percent under-inflated.

Indirect TPMS is inexpensive and easy to install, but it is not as accurate as direct TPMS. What's more, the user has to reset the system every time the tires are changed, rotated, or re-inflated.

Direct TPMS employs pressure sensors installed on the wheel rims to provide independent, real-time air pressure measurements for each tire that can be transmitted to the vehicle instrument cluster to instantly inform the driver. Direct TPMS can also employ additional components, such as MCUs and RF devices to expand the system's capabilities. The optimal TPMS solution includes:

  • Sensors for measuring pressure and temperature inside the tire
  • Controller with time base for periodic measurements
  • Means to identify which tire is providing the data
  • Data output to the vehicle chassis
  • Command input for diagnostics and wake up
  • Power source
The Freescale direct tire pressure monitoring system

Freescale's MPXY8300 TPMS (see Figure 1) is the first of its kind to integrate a pressure sensor, an 8-bit MCU, an RF transmitter, and a two-axis (XZ) accelerometer all in one package.

Figure 1: Block diagram of the single-package MPXY8300.

By combining the necessary features into one package, developers designing wheel-mounted TPMS modules do not have to deal with multiple packages, larger PCBs, or the costs of additional external components. This gives them the opportunity to use one solution in a wide variety of models that employ different rim sizes.

The MPXY8300 TPMS benefits the motorist by providing timely driver warning for improperly inflated tires. It benefits the manufacturer because it can be more easily integrated into an automaker's overall vehicle designs, regardless of make or model, since it doesn't require any additional components. The MPXY8300 TPMS offers:

  • Precise pressure measurements: The low-power, surface micromachined, capacitive pressure sensor's p-cell (pressure cell) measures pressure range from 100 to 800 kilopascale (kPA). 101.3 kPa is equal to one atmosphere pressure. The TPMS also offers high-pressure range p-cells for truck tires (100 to 1,400 kPa) and optional reduced accuracy calibration for lower cost applications. Freescale's capacitive surface Microelectromechanical Systems (MEMS) pressure sensing technology also offers significant power usage advantages over piezoresistive bulk MEMS. These include 0.14 µA supply current (3 V, 30 KHz) versus 600 to 10,000 µA and 0.9 nano-amp-second (nAs) minimum charge per reading versus 60 to 1,000 nAs for piezoresistive bulk MEMS.
  • Full integration: The fully integrated MPXY8300 TPMS modules (see Figure 2) provide independent pressure measurements for each individual tire, including the spare. Each module integrates a 315/434 MHz phase-locked-loop (PLL)-based RF transmitter that ensures continuous communication even after the tires have been rotated or replaced. The integrated motion sensors can also be programmed to transmit measurements at certain speeds (tire rotations), including for measurements for tires that are not rotating at all.
  • Extended battery life: Some regulations require a 10-year battery life for TPMS solutions. The MPXY8300 incorporates a number of low-power techniques to ensure robust operation over a long lifetime using minimal battery resources. Low-frequency oscillations driving a low-power wake-up timer and periodic reset driver plus TPMS specific power management techniques ensure a long battery life for more convenient and economical operation.
Figure 2: Typical TPMS architecture.

The MPXY8300 is the first TPMS to employ acceleration sensors. This is essential to enable the system to be customized for specific driving conditions and speeds. The sensor offers X-axis sensitivity to plus or minus 10 g and Z-axis sensitivity from 0 to 60 g with adjustable offset. Developers have the option to utilize an MPXY8300 TPMS with X- and Z-axis g-cells, Z-axis g-cells only, or a system without g-cells.

Conclusion

The MPXY8300 TPMS sets a new standard for improved vehicle safety. Freescale utilizes its advanced integration techniques to provide a compact System-in-a-Package (SiP) that provides real-time tire pressure monitoring and alerts the driver to improperly inflated tires. Compliant with FMVSS 138 guidelines, the MPXY8300 TPMS can be integrated with existing remote keyless entry systems, helping automakers and their suppliers more easily deploy TPMS in new model cars and trucks around the world,
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