FMVSS 138 is a regulation created by the National Highway Traffic Safety Administration (NHTSA). The objective of this regulation is to alert drivers if the pressure in a tire is down by 25%. All vehicles built since September 2007 are required to have a tire pressure monitoring system (TPMS). As Dr. Thomas Roscher, development engineer, Tire Pressure Monitoring Systems, Product Quality Assurance, Audi of America, points out, in Europe, where there isn't an FMVSS 138 on the books, drivers tend to check their tire pressure every two or three months. In the U.S...well, let's just say that the frequency is not that high.
Early on, when the regulation was being developed, it was thought that there would be two ways to accomplish the necessary TPMS functionality: direct and indirect. The standard is really technology neutral. The issue is accomplishing the task, not stipulating how it is to be accomplished.
According to Urban Forssell, president and CEO of NIRA Dynamics AB (Linkoeping, Sweden; www.niradynamics.se), the first generation of indirect systems, of which there were a handful in production, "had in common a comparison of wheel speed pairwise. You make an assumption that only one tire at a time loses air, and you make the comparison." He points out, "If all four tires lost air at the same time, you'd never know it."
Consequently, indirect TPMS generation one didn't cut it since FMVSS 138 also requires pressure loss detection on all four tires. So all vehicle manufacturers have opted for what is a more straightforward approach, which is the direct TPMS. What this means is that there are pressure sensors mounted inside each tire; there is transmission of that information to a processor that then activates the alert if necessary. While this approach certainly meets the requirements, Roscher acknowledges that drivers pretty much don't care whether their TPMS system is indirect or direct—until there are some changes. For example, the driver may decide that he wants to swap summer tires for winter tires. Or the driver wants to buy some new tricked-out wheels. Or the batteries in the sensors run out. All of this results in additional charges to the customer. Not only would there be the cost for new sensors for the new tires or wheels, but also the additional cost related to the service at the tire dealer, as the regulation stipulates that the tire dealer and the customer are certain that the TPMS system works as required after any changes (e.g., this may necessitate a calibration phase during which time the vehicle would need to be driven, and undoubtedly hourly rates would apply).
Audi is opting for the indirect route. That's right: indirect TPMS. Although the generation 1 approach didn't work, NIRA Dynamics, which didn't participate in that technology, has developed a software-based system that, explains Forssell, has sufficiently robust algorithms such that it can handle the NHTSA requirement of detecting 25% of tire pressure loss within 20 minutes while driving at a speed of 31 to 62 mph; the detection time is generally much faster than NHTSA's 20-minute requirement. There are no wheel-mounted battery-powered sensors. No radio frequency transmitters or receivers. Rather, the NIRA Dynamics system, designated TPI (for "Tire Pressure Indicator"), makes use of the wheel speed sensors that are part of the antilock braking system (ABS) and electronic stability control system (ESC). A processor is needed as well.
"Wheel speed sensors have been around since the introduction of ABS, and have proven to be mature, stable and reliable," Forssell says. They are used as input to the NIRA Dynamics software. At first, the processor that will make the calculations is a stand-alone system. For example for the '09 Audi A6, there will be what Roscher describes as a "little gray box." That's the processor. "Eventually," he says, "it will be integrated into the ESC processor." It is simply a matter, going forward, of making accommodations for the indirect TPMS system in the controller.
Essentially, the NIRA Dynamics TPI system "learns" the radii and frequency characteristics of properly inflated tires. Then, it is able to detect whether there are deviations from normal, which would be a result of the loss of tire pressure (e.g., when a tire loses pressure, there is a change in the radius as the tire rotates, which has a consequent effect on the measured parameters). The system can detect a change in any or all of the tires (remember that the previous indirect systems measured pairs of tires unlike this one, which has inputs from each of the tires).
Because there aren't additional sensors, there is less cost and complexity with this system, Roscher explains. He goes on to point out that there is a customer benefit because as this is a software-based system, when wheels or tires are changed, all the driver and/or tire dealer needs to do is recalibrate the system (this essentially consists of selecting a reset function, then driving the car with properly inflated tires—period). Forssell adds, "You can change and even supersize your tires and the system will learn about them."
According to Forssell, Audi is the first vehicle manufacturer that is standardizing on this indirect TPMS system. It is anticipated that the full lineup of Audi vehicles will be equipped with the system by the 2011 model year.