Automotive sensors are taking advantage of new materials and sensing principles as electronics penetrate further into automotive infotainment, safety, and comfort applications. Sensing technologies that go beyond the well-known piezoresistive, capacitive, and inductive sensing principles embodied in modern microelectromechanical systems (MEMS) sensors are under investigation.
Silicon carbide (SiC), indium antimonide, carbon nanotube (CNT), and fiber-optic materials are homing in on automotive applications, using infrared (IR), surface-acoustic-wave (SAW), variable reactance, and interferrometric sensing principles. One of the most innovative approaches to sensing involves the use of SiC for linear position sensors by Inprox Technology Corp. via its InGen Direct program, with major collaboration by NASA.
This all-digital variable-reactive approach provides the benefits of an ultra-high bandwidth, operation over a harsh temperature range of –100°C to 1400°C, high accuracy, and the elimination of the need for signal-processing circuitry. Inductive and capacitive methods both are being investigated within respective frequency bands of 10 to 500 kHz and 5 to 20 MHz, respectively.
The analog signal being sensed is converted into a frequency in the form of a square wave that’s fed directly into a microprocessor. The processor’s speed determines the measurement’s accuracy. One interesting application is quantifying the runout and wear of bearings in automotive gear teeth, in real time, by sensing the edges of the gear teeth.
The initial effort involves position sensing, though temperature, pressure, and speed sensing are also possible. Potential automotive applications include sensing the speed of engines and anti-lock braking systems (ABSs), detecting engine mis-firing, and monitoring and controlling emissions. These sensors can be used in dc/motor resolvers, turbocharger systems, steering wheels, powertrains, chassis electronics, safety systems, and diesel engines.
IR Sensing Gaining Traction
Recent advances in carbon-dioxide (CO2) sensing using IR technology make it more practical to use such sensors in many mass-market products for consumer and automotive electronics. Bosch’s Climate Control Sensor (CCS) monitors CO2 levels in the passenger compartment while increasing the efficiency of the car’s air-conditioning (A/C) system, leading to enhanced electronic control of the A/C system and a corresponding reduction in energy demand. The CCS uses IR spectroscopy for operation.
When fresh air is fed into the passenger compartment, a CCS-controlled A/C system switches to the recirculation mode. Energy is saved because the amount of power needed for cooling is reduced. That in turn means less required power for cooling, which means a lighter load on the engine to drive the A/C compressor. Bosch estimates fuel savings of up to 10% when the CCS is operated in the maximum cooling mode.
GE Sensing Telaire Products, which makes patented dual-beam absorption IR CO2 sensors for consumer household applications, believes this type of sensor is also cost-effective for automotive applications (Fig. 1). GE says the main features of its 7000 series of sensors—simplicity, reliability and long-term stability—make them very attractive for in-vehicle use.
The SAW torque sensing principle is an emerging technology for automotive sensing applications, as it offers wireless and batteryless operation from 30 MHz to 3 GHz. Certain engine, transmission, drive-line, and chassis processes can be controlled more precisely using SAW technology.
A joint effort by Honeywell and Transense Technologies LLC has yielded a single-quartz die with a pair of nominal 433-MHz single-port SAW resonators, which use the principal tensile and compressive strains that act at ±45° to the axis on the surface of a shaft in torsion for the measurement of torque.
Fiber optics are also gaining ground in automotive sensing. A fiber Fabry-Perot interferometer can be used to measure engine pressure, monitor sparkplug firing, and sense rotary bearing. BCS Advanced Technologies LLC has shown how such a sensor can be used in an auto engine’s cylinder head to measure engine pressure (Fig. 2).
Optical fibers can be used for other automotive sensor applications. Jonathan D. Weiss, senior member of the technical staff at Sandia National Laboratories, has invented a technique to use optical fibers for the state-of-charge monitoring of car lead-acid batteries. JSA Photonics Inc. has developed engineering prototypes of this concept (Fig. 3). Optical fibers can also be used for monitoring the concentration levels of automotive antifreeze solutions.
Researchers at the Massachusetts Institute of Technology (MIT) and the University of Chicago, with funding from the U.S. Department of Energy (DoE), demonstrated that magnetic sensors can operate at the high temperatures expected in future cars and aircraft. Conventional magnetic sensors lose their sensing ability when subjected to temperatures of a few hundred degrees Celsius. By either grinding up indium antimonide and fusing it with heat or introducing impurities of a few hundred parts per million into the material, the researchers introduced high-temperature sensing capability.
An ambitious automotive sensing effort using multi-walled carbon nanotubes (CNTs) is going on at YTC America Inc., a wholly owned subsidiary of Japan’s Yazaki Corp. This gas ionization sensor detects nitrous-oxide (NOx) gases in car passenger compartments and exhaust systems. The simple two-electrode structure consists of CNTs grown on a silicon substrate and covered with a top aluminum plate separated from the substrate by glass insulators (Fig. 4).
The design goal is to develop a robust sensor that features a fast response time, is highly sensitive, and can operate from relatively low voltages. Thus far, YTC researchers have achieved 1-ppm sensitivity levels and response times of less than 50 ms when detecting corrosive gases such as nitrogen dioxide (NO2). They also hope to lower the normally high voltages of more than 1 kV needed for ionization-type sensors down to 100 V or less, potentially allowing for sensor operation from a battery.
No matter what sensing materials or principles will be employed in automobiles, the ultimate goal will be the availability of low-cost sensors resulting in highly sensorized vehicles. They will allow cost-effective improvements in vehicle safety, productivity, and performance. One such low-cost MEMS device that many companies like Honeywell and Analog Devices are working on would be an inertial measurement unit (IMU), which has recently been incorporated in cars for vehicle stability control like correcting for under- and over-steering (Fig. 5).