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From air bags and electronic stability control to lane departure warning and blind spot-detection systems, sensing applications continue to proliferate in all types of automobiles. The role of sensors also continues to grow in emerging vehicles. That means new technologies and standards are constantly being developed to meet current and forthcoming challenges.
At this year's Automotive Sensors Symposium, which was held at the Rosemont Convention Center in Rosemont, Ill. on June 5 in conjunction with Sensors Expo & Conference, several papers highlighted advances on this front. In the full-day program, the morning session, “Sensors for Improving Established Vehicle Systems,” focused on advancements in sensors for established applications, as well as advancements on current technologies to improve future vehicle design systems. The afternoon session, “Sensing in Active Safety Systems,” addressed the sensor-intense systems for improving safety by preventing accidents. Early versions of these systems are appearing on luxury vehicles. Randy Frank, our contributing editor and president of Randy Frank & Associates was the organizer/chairman of the Automotive Sensors Symposium.
A paper by Eric Weir of Hella Electronics Corp. titled, “The SENT Protocol,” shed light on the emerging single-edge nibble transmission (SENT) encoding scheme (Figure 1) that is intended for use in applications where high-resolution sensor data needs to be communicated from a sensor to an engine control unit (ECU). It is in-tended as a replacement for the lower-resolution methods of 10-bit ADCs and PWM and as a simpler low-cost alternative to CAN or LIN data buses. The implementation assumes that the sensor is a smart sensor containing a microprocessor or dedicated logic device (ASIC) to create the signal. Designated by the SAE task force as J2716, SENT is a unidirectional communications scheme from sensor/transmitting device to controller/receiving device, which does not include a coordination signal from the controller/receiving device. The sensor signal is transmitted as a series of pulses with data measured as falling to falling edge times.
According to the Hella paper, some specs of the SENT protocol include:
- two 12-bit values (channels) in a single message, with possible loss of one channel to gain more resolution in a single channel design;
- message length to be < 1 ms for all possible messages and to include clock frequency maximums under worst case;
- must adhere to a three-wire interface (power, signal and return);
- to be as reliable, with an improved cost effectiveness to current technologies; and
- assumes a nominal clock frequency of 28 kHz with a ± 20% tolerance.
With SENT, the receiver calibrates the system on the fly by measuring the calibration pulse from falling edge to falling edge. A four-bit status and communications nibble follows terminated by the cyclical redundancy check (CRC) nibble. Each successive nibble contains four bits of data, which can be used to represent two 12-bit words. The status byte transmits serial data for sensor identification.
In the second morning paper, Melexis' Mark White and Vincent Hiligsmann investigated the growing market for electrical power steering (EPS) sensors. Its paper, “Triaxis Hall Solution for Steering Position Sensor Application,” indicated that more than 50% of new vehicles in Europe will incorporate EPS by 2007, while the United States would achieve such a figure by 2010. According to Melexis, primary drivers for EPS sensing include steering wheel position and speed, steering torque and EPS motor position and speed sensors (Figure 2), while vehicle and engine speeds were labeled as auxiliary needs. Although several sensing technologies like linear Hall, magneto-resistive (AMR or GMR), inductive and optical are available for these applications, Melexis' paper demonstrated the benefits of its Triaxis rotary position sensors. The paper presented the supplier's Hall effect rotary position sensor chip MLX90316 for absolute non-contacting 3608 angular position sensing. Unlike others, according to Melexis, it can sense all three magnetic flux density components at a single point, and provide non-contact position sensing in harsh automotive environments.
CMOS AND MEMS
The result of a combination of mixed-signal CMOS and the patented integrated magneto-concentrator (IMC) technology, the MLX90316 is suitable for a number of sensing applications, which include:
- accelerator/brake pedal position;
- throttle position;
- steering wheel position;
- float-level sensor;
- non-contacting potentiometer; and
- motor-shaft position.
Another paper in the same session by Melexis titled “Gyroscope for Navigation Applications” showed how the accuracy of GPS can be improved by combining GPS position sensing with MEMS-based angular rate sensor (gyroscope). This combination can enhance the accuracy of GPS instruments in situations where satellite reception is lost. The paper showed that its MLX90609-N2 gyroscope, based upon a thick silicon-on-insulator (SOI) high-performance microelectromechanical systems (MEMS) technology, enables the realization of compact, high accuracy and cost-effective GPS systems with dead-reckoning capabilities. As a direct result of low zero rate output drift, high resolution (better than 0.1 °/s) and high dynamic range, accurate positioning can be guaranteed even when satellite reception is lost over long distances. The MLX90609-N2 offers an analog (0-5 V) and digital (SPI) output. The digital (SPI) output simplifies the interface to the microcontroller eliminating the need for an additional ADC. The EEPROM allows programming of date code, serial number and calibration parameters. The factory set full-scale ranges available are: ±75 °/s, ±150 °/s or ±300 °/s. Programmable calibration parameters allow the temperature compensation of the bias and gain.
The mechanical structure of the MEMS gyroscope is sensitive to Coriolis forces created by movement. Designed with a differential mechanical structure, the MLX90609-N2 exhibits low vibration and angular rate cross sensitivity and a high immunity to external linear acceleration. Each section of the differential structure comprises a double-frame gyroscope. The use of two frames allows for better decoupling between the driven and sensing modes. Plus, it employs monocrystalline Si to ensure better long-term behavior and reliability than gyroscopes based on poly-Si micromachining.
In the final morning paper Roger Grace, president of Roger Grace Associates, stated that MEMS/MST have proven themselves as a viable technology with the risk-averse automotive electronic systems engineers in applications such as manifold absolute pressure (MAP) and airbag accelerometers. And, a large number of suppliers are constantly driving down the prices. Meanwhile, mechanical MEMS resonators are gaining ground to replace decades-old quartz crystal solutions in applications like keyless entry, tire pressure monitoring, entertainment systems, telematics and system networks. According to Grace's paper, MEMS frequency sources are slated to appear in 2009 model cars.
The afternoon session continued with MEMS and CMOS sensors. The first afternoon paper by VTI Technologies on “Advanced 3-D MEMS Acceleration Sensors for Vehicle Applications” pointed toward the strengths of 3-D MEMS sensors. In going from surface micromachined technology to 3-D structure using a single crystal, 3-D MEMS offer high mechanical shock endurance (>20,000 g), no sticking problems due to large proof mass, and are fully calibrated, according to VTI's paper. It also offers a large sensing element capacitance change, good stability and over-damped sensing element.
The VTI paper indicated the migration from analog signal conditioning to digital techniques for reduced system design complexity. It also indicated that the combination of bulk and micromachining with deep reactive ion etching (DRIE), wafer bonding and thicker structural layers are the wave of the future. Implementing such advances, VTI has developed multi-axis sensors for auto safety applications. As an example, the paper described the development of three-axis accelerometers with a measurement range of ±2 g and a zero point error of less than ±50 mg. Designed for electronic stability control (ESC), it breaks new ground in power consumption, which is rated at 3 mA at 3.3 V. The supplier recently launched its three-axis accelerometer family. There are plans to unveil the automotive version (SCA31xx) next year. Meanwhile, the company has released single-axis SCA-8xx accelerometers with SCA-810 for X-axis measurement and SCA830 and 820 for Y and Z axes measurements.
Micron's CMOS image sensors for advanced automotive safety systems followed the VTI presentation. Micron presented its wide VGA CMOS image sensor MT9V022 for high-quality scene-understanding and smart imaging applications. Designed to support the interior and exterior needs of automotive imaging, this wide-VGA CMOS image sensor features DigitalClarity, Micron's breakthrough, low-noise CMOS imaging technology that achieves CCD image quality (based on signal-to-noise ratio and low-light sensitivity) while maintaining the inherent size, cost and integration advantages of CMOS. In order to ensure that next generation of scene-understanding CMOS image sensors meet tomorrow's automotive needs, the automotive system designers must share their key requirements with sensor makers, the Micron paper concluded.
The focus of STMicroelectronics paper titled, “Advanced Silicon Sensors for Emerging Automotive Applications,” was advances in CMOS cameras and MEMS-based accelerometers and pressure sensors. In car applications, CMOS cameras are on the rise and within the next few years as many as six CMOS cameras will be found in a typical vehicle. With knowledge gained in volume production of low g accelerometers, ST is getting ready to enter the mid/high g accelerometer market. It is also developing one-, two- and three-axis gyroscopes.
Silicon valley startup Canesta described its 3-D camera-based sensing system called electronic perception technology (EPT) for active safety applications. According to Canesta's paper, “Electronic Perception Technology for Active Safety,” the design employs an infrared (IR) light source, a special optical sensor chip module and embedded imaging software. The software runs within the sensor chip module to provide the ranging and recognition from a single chip. By using the time-of-flight methodology and measuring the time it takes light to travel to and from the object, the sensor determines the distance of the object at that location as well as providing the imaging data. This single 3-D camera can replace a stereo-imaging system and lidar or radar sensors to reduce system cost and it consumes less power.
Potential uses of this technology include advanced active safety applications. Inside the vehicle, applications include occupant detection for smart airbag systems, anti-theft systems, driver drowsiness recognition, identification of a child or pet left in a parked vehicle, obstacle recognition when closing power doors, windows and sunroofs. Exterior applications include adaptive cruise control, blind spot monitoring, collision avoidance, lane departure warning, parking assist systems, and pedestrian detection. Some of these applications could share a sensor, reducing total cost.
The afternoon session concluded with Continental's “Active Safety & Sensor Technology Trends.” While the paper discussed several sensor trends, including radar, IR, CMOS cameras, optical and system integrated steering angle sensors, it indicated that the goal is to develop active passive integration approach (APIA) for accident and injury preventing vehicles of future.