Passenger safety is a major element of every automotive design. This focus is supported by ongoing efforts to enhance safety-related features, either as a competitive advantage in the marketplace or in response to government requirements. One such mandated feature is the tire-pressure monitoring system (TPMS), which alerts the driver by providing either real-time pressure readings or under-pressure warnings through readouts on the instrument panel.
Studies have shown that vehicles commonly have under-inflated tires. This issue has unwanted side effects such as increased stress on the steering system, accelerated tire wear and decreased fuel economy. Unfortunately, it also has some very sobering consequences. Statistics show that more than 400 fatal and up to 10,000 non-fatal accidents per year are caused by flat tires or blowouts. Twenty percent of flat tires and blowouts are the result of under-inflation.
For these reasons and more, the United States, the European Union, South Korea, and other nations have, in recent years, enacted legislation requiring that all new passenger cars be equipped with a TPMS. The main purpose is to ensure better handling and greater safety by giving drivers real-time warnings about lost tire pressure.
Comparing two approaches to monitoring
The owner’s manual of any recently produced vehicle includes the recommended tire pressure for the factory-installed tires. As an example, the US legislation sets a threshold of no less than 25 percent deflation from the recommended pressure. Any reading lower than the threshold triggers the TPMS, which will then activate a readout that warns the driver.
Two types of monitoring systems are currently in use: indirect and direct. Indirect systems utilize signals measured by typical antilock braking systems (ABS) that use wheel-speed sensors to regulate ABS operations. Data from those same sensors can be used to compare the rotational speeds of the tires: an under-inflated tire has a smaller circumference and therefore a faster rotational speed than the others. Unfortunately, this method requires that a car be in motion for a bit of distance before a problem becomes apparent. Also, it might go undetected in the rare event that all four tires have deflated by the same amount and are rotating at the same speed.
In comparison, direct monitoring has proven to be a more accurate and reliable way to measure tire pressure. This method uses one base receiver unit that monitors four transmitting pressure sensors, one mounted inside each tire. The receiver unit is commonly placed inside the vehicle and drives an indicator on either the dashboard or a separate display. Each sensor measures pressure and transmits its reading over a radio frequency (RF) link to the receiver. The transmitters may be programmed with a unique code or serial number, allowing the base receiver to identify each tire separately.
Looking at the embedded technology
Many tire-pressure sensors are made from piezo-resistive materials that pick up variations electrically through a diaphragm that flexes with changes in pressure level. Sensor data is electronically processed and encoded before being transmitted over the RF link (Figure 1).
The transmitters typically operate at 315, 434, 868 or 915 MHz. In many cases, the transmitted signal is modulated using either asynchronous-shift keying (ASK) or frequency-shift keying (FSK).
The transmitted data is usually 32-bits long. Each string includes the unique serial number or ID and the tire-pressure data, which usually occupies one byte. Depending on the design, the data stream may also include status information such as the power level of the transmitter battery. Some designs also measure the temperature inside the tire and include this data in the transmitted stream.
Many of today’s systems send data at 9600 bps using Manchester Code in which digital ones and zeros transition between high and low halfway through each bit period. A 9600-bps baud rate shortens the transmission time, which indirectly reduces interference.
Managing battery life
Batteries are presently the most common way to power direct-measurement sensor units mounted inside the tires. Lithium cells are a common choice because they provide long life and reduce the likelihood of battery replacement during the life of the vehicle.
Two techniques help extend battery life: keeping transmitter power low and avoiding full-time transmission. Given the short communication distances, low-power transmission is a viable approach. It also is practical and efficient to let the transmitter remain in standby mode, sending data at fixed intervals programmed by the base receiver unit (which also can transmit to the sensor units). Current drain is typically 100 nA in standby mode and 1 to 5 mA when transmitting.
Testing TPMS and other systems
In the United States, more than 7 million passenger cars and 8 million light trucks are sold in a typical year. This translates into at least 15 million base receiver units and, assuming four tires per vehicle, 60 million pressure sensors that must be manufactured and tested every year. Faced with such massive volumes, manufacturers are looking for ways to accelerate time-to-market.
Testing these devices requires a broad range of technologies: battery simulators, switching systems, RF signal analyzers, RF signal generators, and more. Test systems such as the Agilent TS-5020 provide a flexible platform that can be configured to test a variety of automotive subsystems—TPMS, remote-keyless entry (RKE), supplemental restraints, and others (Figure 3). In addition to the instruments, the TS-5020 can also be configured with a variety of interfaces and test fixtures, and the Agilent TestExec SL software. The result is a solution that provides flexibility, saves time and reduces the cost of testing many types of automotive systems.
Specific to TPMS, testing covers the transmitter and receiver. Transmitter testing includes measurements of signal power level, frequency deviation (for FSK), burst characteristics (for ASK) and demodulation of FSK and ASK signals. The process begins by sending the transmitter the required 125-kHz wakeup signal, which activates the microcontroller and initiates continuous RF transmission. An RF spectrum or signal analyzer is needed to make the required measurements.
Receiver testing requires an RF signal generator capable of producing a modulated carrier signal that simulates actual transmissions from the sensors. After the receiver detects a data frame, it compares the tire ID to the four IDs stored in memory. If an ID match is found, the pressure data will be processed and the relevant tire indicator will be illuminated if low pressure is detected.
In automated testing, factors such as cost, flexibility and reuse become increasingly important when production volumes reach into the tens of millions. The best solution is one that helps manufacturers meet the technical and business challenges in the automated testing of such high-volume devices and subsystems.