While there is much excitement about the role sensors are playing in wearables and smart phones, consumers are already surrounded by sensors in their vehicles -- sensors that are checking their driving efficiency and safety, reporting the condition of their vehicle to them, and adjusting vehicle systems for maximum performance. In fact, the car is one of the most sensor-rich systems in common use today.
And it’s only the beginning. In the not-too-distant future, cars will be able to act as chauffeurs, automatically driving you to a destination without you having to ever touch the gas pedal or steering wheel, as they carefully avoid obstacles and select the most efficient route while keeping you entertained. To get to that state, cars will add a variety of new sensing capabilities that will work in concert to allow intelligent choices and actions.
Today’s Automotive Sensors
Today, cars rely on sensors for a myriad of functions. Among the most commonly used sensors are airbag accelerometers, manifold absolute pressure sensors (MAPS), yaw-rate sensors, and tire-pressure monitors.
Airbags have been required by U.S. law for passenger safety since 1984. Today, the majority of airbag systems use MEMS-based accelerometers to detect rapid deceleration (crash), and some vehicles use as many as 12 for front, side, and rear crashes. Major producers of crash accelerometers include Analog Devices, Texas Instruments, Freescale Semiconductor, and Infineon.
The U.S. Environmental Protection Agency was responsible for the introduction of MAP sensors to reduce air pollution and maximize fuel economy. A MAP sensor measures pressure in the intake manifold. Those measurements are then used by an on-board computer to determine the optimum air and fuel mixture. These sensors are manufactured by Honeywell, Freescale, EPCOS, and others.
Yaw-rate sensors, which measure the rate of rotation about a central axis, are used in conjunction with a Global Positioning System (GPS) in determining vehicle location; they are also used to determine vehicle orientation as it begins to roll over or go into an uncontrolled skid. Analog Devices, Omron and Murata have all introduced MEMS-based solutions for rollover, navigation, and vehicle dynamic control applications.
Since tire under-inflation generates safety issues — creating excessive tire heat and wear, increasing the likelihood of a blowout — and decreases fuel efficiency, the U.S. National Traffic Highway Safety Administration (NHTSA) dictated the introduction schedule for tire-pressure monitoring systems starting in model year 1995. These monitoring systems have been mandatory for new cars sold in the United States since 2007.
Today’s typical TPMS sensor uses MEMS to directly measures tire pressure. It is installed on the rim of each tire, and sends readings wirelessly to a control module for analysis and interpretation. This does not require a new sensor each time a tire is changed, though it may be wise to recalibrate the sensors every so often, the way vehicles may get their tires rebalanced.
Recently, Freescale introduced the FXTH8715 TPMS; designed for trucks and other large vehicles (Figure 1). The series is said to be the industry’s smallest (7 mm x 7 mm x 2.2 mm) and highest accuracy (±17 kPa) fully integrated wireless TPMS. In a single QFN package, the sensor system integrates pressure and temperature sensors and a single- (Z axis) or dual-axis (X and Z axes) accelerometer with an RF transmitter, a 125 kHz low-frequency receiver, and an 8-bit microcontroller (S08 core) with SIM, interrupt, and debug/monitor. This system can intelligently collect and transmit pressure, temperature, and acceleration data to enable sophisticated analytics to not only improve fleet maintenance, but also transform the TPMS into value-added end-node on the Internet of Things.
Sensors For Alternate Fuels
Major vehicle manufacturers and their first-tier suppliers are working on new sensor-based applications for next-generation vehicles. Of major interest are sensors for alternate fuel vehicles (such as diesel, hybrid, electric, natural gas, and hydrogen), improved navigation and performance monitoring, and, ultimately, autonomous operation.
More stringent emission requirements have led manufacturers of diesel-powered vehicles to adopt several new systems requiring specialized sensors—for instance, sensors that measure in-cylinder pressure. Sensata, in conjunction with Beru, has developed an in-cylinder pressure sensor that employs a piezoresistive strain gage technology; it is currently used in VW production vehicles. Optrand is developing robust sensors that use fiber-optic sensing; the goal is to have sensors that survive temperatures up to 350 °C while operating in at pressures to 30,000 psi bar with an accuracy of 1 percent.
Additionally, selective-catalytic-reduction (SCR) systems are used in the exhaust after-treatment application. A typical SCR system has 14 sensors including 10 temperature sensors, three pressure sensors, and one urea quality sensor. The urea quality sensor checks for the necessary urea concentration as well for the presence of unwanted liquids.
Measurement Specialties (a TE Connectivity company) has recently introduced the FPS2800B12C4, a fluid property sensor ideal for automotive applications. It can be directly installed in the engine oil gallery or fuel system for quality and lubricant monitoring. It is a fully integrated module with all necessary sensors and an on-board microprocessor that can accept typical automotive voltages (12V or 24V and is enclosed in a highly rugged package for use in corrosive or high pressure/flow environments. The module comes factory calibrated with NIST traceable fluids and uses a universal CAN J1939 protocol to connect to a host controller.
Figure 2: TE Connectivity's sensor uses patented tuning fork technology to monitor multiple physical properties of a fluid and simultaneously process dynamic relationships between the properties.
A key component of hydrogen-fuel-cell-powered vehicles is tank-pressure monitoring. American Sensor Technology provides an extreme-environment pressure sensor that uses piezoresistive strain-gauge technology, and is available in a 300 psi format for the low side and 3,000 psi for the high side of the hydrogen fuel tank systems. Theses sensors are able to withstand the corrosive nature of the hydrogen media at temperatures to 145 °C and have ± 1.0 percent accuracy. Currently installed on large transport vehicles, these sensors are in beta test in a European Mercedes Benz rental fleet.
For electric and conventional vehicles, Freescale Semiconductor introduced a smart battery-monitoring sensor, the MM912x637, compatible with 12-V lead and 14-V Lithium-ion batteries. It determines the battery’s state of charge, state of health, and state of function by measuring the battery current, voltage, and temperature and feeding the data to a microcontroller with embedded battery performance algorithms. It also provides early warning of unusual battery discharge and wear-out (Figure 3).
Figure 3: Shown inside a battery-cable connector, Freescale’s MM912x637 battery-monitoring sensor measures and battery current, voltage, and temperature to provide early warning of battery discharge and wear-out. (Source: www.electronicproducts.com)
Sensors and Autonomous Vehicles
At the Consumer Electronics Show last January, Daimler showed its Mercedes-Benz F015 concept car and Audi demonstrated a Q7 loaded with what it called ‘piloted driving’ gear. To realize autonomous vehicles, a broad spectrum of new sensors will be required. For instance, various types of ranging and image sensors will be needed for adaptive cruise and emergency-breaking control, for lane changing/blind-spot sensors, and for parking assist. Many technologies are currently being considered for what are called automotive driver assistance, to increase vehicle safety today.
One such ranging system is Leddar. Originally discovered by the National Optics Institute (INO) in Quebec City and developed and commercialized by LeddarTech, Leddar uses the time of flight of LED-generated light signals and unique algorithms to detect, locate, and measure objects in its field of view. To do so, it sends very short light pulses about 100,000 times per second to actively illuminate an area of interest. The light backscattered from objects (either fixed or moving) over Leddar’s detection area is captured with P-I-N or avalanche photodiode detectors (or others), and analyzed with patented signal-processing IC technology, LeddarCore, which provides highly efficient algorithms to precisely map objects’ location and other attributes (Figure 4). A particular advantage of Leddar is that it offers both short and long-range performance, and so can be used for a number of driver-assistance applications.
Figure 4: LeddarTech’s Leddar system uses time-of-flight measurement of light pulses to determine object range and other attributes using its LeddarCore signal processing hardware/software technology. (Source: www.azosensors.com)
Texas Instruments currently offers the TIDA-00151 reference design for developing such applications as ultrasonic park assist, self-parking, and blind-spot detection. The module uses TI’s PGA450-Q1, which is a System-on-Chip (SoC) sensor interface IC for automotive ultrasonic sensors. The SoC provides all signal conditioning and processing for the transducer echo signals and for calculating the distance between the transducer and objects. The SoC’s MCU and program memory allow for full configurability for the specific end application.
Navigational systems also need to be more accurate and reliable for autonomous vehicles. Bulk acoustic wave (BAW) MEMS gyroscopes from Qualtré provide several compelling performance advantages over other MEMS gyros for this application: accuracy, resistance to shock and vibration, high-frequency operation, high Q without vacuum packaging, and high reliability through immunity to stiction. The manufacturing process, HARPSS, makes BAW devices reliably manufacturable in volume.