Agilent Measurement Journal - Issue 3 - 2007 - (Page 40)

Looking inside a direct monitoring system Tire-pressure sensors are made from piezo-resistive materials that pick up pressure variances electrically through a diaphragm that ﬂexes with the pressure level. Sensor data is then electronically processed and encoded before being transmitted over the RF link. Most transmitters use radio frequencies within the industrial, scientiﬁc and medical (ISM) band, typically at 315 MHz, 434 MHz, 868 MHz or 915 MHz. In many cases, the transmitted signal is modulated using either asynchronous-shift keying (ASK) or frequency-shift keying (FSK). To illustrate the transmitted data, a unique serial number or ID for the pressure sensor is typically 32 bits in length, with the pressure data occupying eight bits or one byte. Depending on the design, the data stream may also include the battery condition and a status stream. In some designs, temperature is measured along with tire pressure and up to one byte of temperature data is included in the data 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 directmeasurement sensor units mounted inside tires. Lithium cells are a common choice because they provide long life and reduce the likelihood that the battery will require replacement during the life of the vehicle. The aforementioned short transmission times also contribute to longer battery life. Two more 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 each year. With typically four tires each, that represents more than 60 million pressure sensors and 15 million base receiver units to be manufactured and tested every year. Faced with such massive volumes, manufacturing organizations are looking for ways to accelerate their time-to-market performance. Leveraging existing RF platforms To reduce part count and vehicle cost, many auto manufacturers try to integrate the TPMS with another RF platform. One example is the remote keyless entry (RKE) system, which is a logical choice for two reasons. First, it is typically used when the vehicle is shut off and parked; the TPMS is in use when the vehicle is running and in motion. Second, the RKE and TPMS systems use the same frequency ranges within the ISM band. 40 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

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