Tire pressure monitoring requirement kicks in
Austin, TX —Your next new car is going to have a direct-type tire pressure monitoring system (TPMS) consisting of a miniature data acquisition system and wireless data transmitter. So says the U.S. Congress. The Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act , which is Federal Motor Vehicle Safety Standard (FMVSS) No. 138, requires a tire pressure monitoring system to be installed in all 2008 model year passenger cars, multipurpose passenger vehicles, trucks, and buses. That means all new cars manufactured after Aug. 31, 2007, must have equipment installed capable of alerting drivers when tire pressure is 25% or more below the placard pressure for individual tires.
Direct TPMS installations present embedded system designers with a serious control challenge. The in-wheel component has to do its job without outside assistance throughout the vehicle’s expected service life of 10 years. It must be light weight and immune to extremes of temperature, shock, and vibration.
In November 1, 2000, Congress required the National Highway Traffic Safety Administration (NHTSA) to develop a rule requiring all new motor vehicles to be equipped with a warning system to indicate to the operator when a tire is significantly under-inflated. In response to this requirement, NHTSA undertook an evaluation of then-existing OEM and aftermarket tire pressure monitoring systems for light vehicles. At that time, there were two classes of TPMS technology: indirect and direct.
Indirect TPMS works with a vehicle’s ABS. The ABS uses wheel speed sensors to measure the rotational speed of each of the four wheels. As a tire’s pressure decreases, the rolling radius decreases, and the rotational speed of that wheel increases correspondingly. Most current indirect TPMSs compare the sums of the wheel speeds on each diagonal (that is, the sum of the speeds of the right front and left rear wheels as compared to the sum of the speeds of the left front and right rear wheels). Dividing the difference of the sums by the average of the four wheels speeds allows the indirect TPMS to have a ratio that is independent of vehicle speed.
Direct TPMSs use pressure sensors located in each wheel to directly measure the pressure in each tire. These sensors broadcast pressure data via a wireless radio frequency transmitter to a central receiver. The data are then analyzed and the results sent to a display mounted inside the vehicle. The type of display varies from a simple indicator light, which is how most vehicles are currently equipped, to a display showing the pressure in each tire, sometimes including the spare tire. Thus, direct TPMSs can be linked to a display that tells the driver which tire is under-inflated.
In its evaluation, NHTSA determined the capabilities of existing technologies and the methods of warning the driver that were under consideration by system manufacturers at that time. Based on this evaluation, NHTSA determined the minimum system performance criteria that were technically feasible and provided the most useful information to the driver for preventing unsafe conditions.
The VRTC tested four ABS-based indirect TPMSs. None met all the requirements of either alternative proposed in the NPRM. The VRTC determined that since reductions in tire diameter with reductions in pressure are very slight indirect TPMSs require a 20% to 30% drop in pressure before they can detect under-inflation. The VRTC also concluded that those thresholds were highly dependent on tire and loading factors. The VRTC also found that none of the tested indirect TPMSs could detect significant under-inflation when all four of the vehicle’s tires were equally under-inflated, or when two tires on the same axle or two tires on the same side of the vehicle were equally under-inflated.
Since direct TPMSs actually measure the pressure in each tire, they can detect when any tire or when each tire in any combination of tires is under-inflated, including when all four of the vehicle”s tires are equally under-inflated. Direct TPMSs also can detect small pressure losses. Some systems can detect a drop in pressure as small as 1 psi.
Through its testing, NHTSA found that systems that use sensors to directly measure tire pressure (pressure-sensor based systems) were better able to detect under-inflation, had more consistent warning thresholds, and were quicker to provide under-inflation warnings than the systems that infer tire pressure from monitoring wheel speeds (wheel-speed based systems).
As a result of this research, NHTSA will accept only direct TPMS to meet TREAD Act requirements. That means all vehicles manufactured after Aug. 31 that the act covers must be equipped with direct TPMS.
The RF transmitter operates at 315/434 MHz and uses ASK/FSK modulation. Units can have any one of three accelerometer options: no accelerometer, a single axis sensor, or a dual axis sensor.
The MCU characteristics necessarily include low power consumption and 8-bit architecture. Low power consumption is needed to provide acceptable battery life on the order of 10 years. An 8-bit architecture provides the relatively small computing requirements in an inexpensive device. In addition, the company’s architecture specifies 512b RAM, 16KB Flash memory, a low-frequency input to allow upgrades during the product’s service life without demounting the tire, and a temperature sensor.
P-Chip pressure sensors are available to cover three pressure ranges: A low range (100-450 KPa) for smaller vehicles, a standard range (100-800 KPa) for mid- and full-size vehicles, and a truck-tire range (100-1500 KPa) for commercial-size trucks and motorhomes that use high-pressure tires.
The in-wheel TPMS component must operate continually on a single, never-recharged battery. To do so, it must spend most of its time in sleep mode, waking periodically to report the tire’s pressure. It must also be ready to wake up quickly to report a sudden pressure loss that might occur between scheduled reports. And, of course, its software must be robust because it cannot crash.