From multi-axis micro electromechanical systems (MEMS) to vision and radar sensors, and from exotic to well-established or mature technologies, sensors initiate the control system process. Advanced packaging, sensor interfaces, combined sensors and advanced system concepts are among the latest improvements that sensor and sensor-based system suppliers are developing to achieve increased control as well as reduce costs in safety systems.
According to the recent iSuppli report “Multi-sensor Packages Come into Vogue,” the use of multi-sensor MEMS packages in automotive Electronic Stability Control (ESC) systems will begin in 2010. Initially offered in systems from companies like Bosch and Continental, these multi-sensor products will increase to 25.9 million units in 2014 and address more than 50% of the market.
The reasons for the increasing popularity of multi-sensor packages or combo sensors that include an accelerometer and gyroscope are rather straightforward. “Combo sensors save costs through fewer packages and probably in later versions through a shared ASIC,” says Richard Dixon, senior analyst for MEMS at iSuppli. The package-level integration also allows carmakers to reduce the number of modules inside vehicles.
Based on the U.S. National Highway Traffic Safety Administration’s (NHTSA) requirement for electronic stability control systems on all passenger vehicles under 10,000 pounds by the 2012 model year, ESC will be a widely deployed system. ESC has been phased into U.S vehicles to meet the Federal requirement since 2009 models. However, ESC was already implemented in 80% of the registered vehicles in Germany and will be required for all new vehicles by the end of 2014. By comparing the driver’s request (input) with actual vehicle motion from accelerometers, gyroscopes and wheel speed sensors, an ESC system helps the driver detect and minimize skidding.
One of the MEMS sensor manufacturers offering a gyro-accelerometer combo sensor is VTI Technologies. Packaged in an 18.65 mm x 8.5 mm x 4.53 mm leadframe-based package, the SCC1300-D02 is a ±100º/s x-axis and ±2 g 3-axis design, and the SCC1300-D04 is ±300º/s x-axis and ±6 g 3-axis unit (see Fig. 1). The three-axis accelerometer and single-axis gyro combo are based on VTI's 3D MEMS manufacturing technology, which enables the company to produce extremely robust, stable and low-noise sensors.
The System Dictates the Sensor(s)
System requirements shape the specifications for the sensor or sensors used in the system and ultimately determine which sensor technology meets those requirements. Electric power steering (EPS) provides a good case in point. EPS is one of the energy saving measures being implemented by carmakers.
With the capability to reduce fuel consumption by as much as 4%, EPS is gaining in popularity. According to an article in The Detroit News, Ford Motor Co. has said that 90% of its vehicles will have EPS by 2012. Nexteer Automotive, a Saginaw, Mich., company, expects full-size pickup trucks to begin using electric power steering within the next three years.
EPS is also an essential feature for other advanced features such as automatic parking assist, adaptive cruise control (ACC), hybrid vehicles and vehicles with stop-start systems. EPS could also be part of future collision avoidance systems.
In an EPS system, a sensor indicates the driver's steering requirements, so a microcontroller can actuate an electric motor on the steering column or the steering rack. Hall-Effect devices, one of the elder statesmen of semiconductor sensors, continue to improve for EPS and other motion control applications. Two suppliers have taken rather different approaches to using Hall-Effect technology for EPS systems.
Melexis designed its second generation of automotive Hall-Effect latch sensors as a successor to its USx88x series. The initial product, the MLX92211, operates from 2.7 to 24V and provides latching magnetic behavior. South and North magnetic fields applied perpendicular to the package top cause the open drain output to switch ON and OFF. Rotary applications such as electric power steering commonly use shaft-mounted multi-pole ring magnets for this purpose.
In addition to protection features such as very high electro-static discharge (ESD) rating, reverse supply voltage protection and output short circuit protection, the platform’s magnetic core was designed to minimize sensitivity to temperature. To compensate for the decrease in a magnet’s strength with an increase in temperature, Melexis implemented a negative temperature coefficient of -1000 ppm/°C. This results in very stable and accurate magnetic switching points, typically ±3 mT (milliteslas) at 25°C. The output can be updated every 6 µsec with a jitter of only ±3 µsec for increased repeatability. In addition to EPS applications, the Hall Effect device can be used for window lifters, sunroof, seat adjusters, wipers or motor commutation.
In contrast, Bosch engineers chose the Hall-Effect approach for ESC systems. This replaced—for cost reduction reasons—the absolute measurement obtained from GMR (giant magneto resistance) technology that the company employed in its LWS5 steering-angle sensor. Intended for basic functions in compact-class and smaller vehicles, the newer LWS6 can also be used in electro-hydraulic power steering or ACC systems. The lower cost approach still provides typical steering-angle signal resolution of 1.5°. Using the square wave output from the sensor, the control unit calculates the position, rotation direction and rotation speed of the steering wheel.
Bosch engineers also used Hall Effect technology for torque sensor steering (TSS) specifically targeting EPS applications. The sensor consists of two Hall sensing elements that measure the steering force exerted by the driver through the action of two concentric rings that enclose the torsion bar. An increase in steering force caused by the frictional resistance between the tires and the road causes a directly proportional output from the TSS that can resolve twisting angles as small as 0.002° from changes in the magnetic flux (see Fig. 2).
EPS and ESC are just two of the vehicle applications for Hall Effect sensors. Hall-Effect sensing technology has been employed by Preh as a contactless sensing method integrated into Pierburg’s throttle body for the Ford Coyote V-8 engine. Used in the Ford F-150 truck and Mustang passenger car, the Hall-Effect design provides an alternative to potentiometric sensors. For the underhood application, the sensor has an operating temperature range from -58°C to +160°C.
Sensor Interface Requirements
Sensors normally connect to the vehicle’s bus through a Controller Area Network (CAN) or Local Interconnect Network (LIN) protocol. Two emerging interfaces specifically developed for sensors include the SAE J2716 SENT (for Single Edge Nibble Transmission) protocol and the PSI5 organization’s Peripheral Sensor Interface 5 (PSI5).
SAE’s SENT encoding scheme targets applications where high-resolution sensor data needs to be communicated to an electronic control unit (ECU) such as the engine control module. Instead of multiple analog-to-digital conversions required with traditional approaches, SENT requires only one conversion providing a cost savings for the sensor and the ECU. In addition to easier implementation of diagnostics functions with SENT, the protocol also provides a cyclic redundancy check (CRC) for reliable data transmission.
Targeting throttle and valve position sensing and other powertrain sensing applications, austriamicrosystems used Hall Effect sensing technology for its AS5165 angular position sensor that adds the ability to transmit the data using the SENT protocol. The magnetic encoder combines integrated Hall elements, an analog front-end and digital signal processing into a single package. In addition to the ease of transmitting high-resolution sensor information, the sensor also integrates several protection functions including: protection for the power supply and output pins against overvoltage up to 27 V; power supply pins protection for reverse polarity to -18 V; and protection for continuous short circuit detection and broken wire detection.
The PSI5 is a two-wire interface that specifically targets airbag systems. The Manchester coded digital data transmission allows a data transmission speed of 125 kbps with high EMC robustness and low emission. In addition to handling a wide range of sensor supply current, the protocol allows alternative connectivity (see Fig. 3). The PSI5 organization consists of system suppliers Autoliv, Bosch Continental, Hella and TRW as well as component suppliers such as ELMOS, Freescale and STMicroelectronics.
Freescale Semiconductor’s recently introduced MMA5xxxW accelerometer is an X- or Z-axis satellite inertial sensor in a Quad Flat No-Lead (QFN) package that is compatible with the PSI5 rev 1.3 standard protocol. In addition to the special interface, the MEMS accelerometer’s over-damped design provides improved protection against parasitic vibrations for more robust front and side airbag solutions. The QFN package provides a smaller footprint than other available SOIC-based designs.
Cameras and Radar Sensors Help Drivers See More
Perhaps the most sophisticated sensors in today’s vehicles are the ones that enhance the driver’s vision. Cameras and radar sensors usually are found on high-end vehicles, but several suppliers are working to extend these sensors to a broader range of vehicles. Delphi, TRW and Continental have different approaches.
In its parking guidance system (PGS), Delphi uses a rear-view camera instead of ultrasonic sensors. The company claims it is a lower cost solution to the parking problem. Advanced algorithms take the information provided by the camera, determine if there is sufficient room to park the vehicle and calculate the backing path. The driver receives spoken step-by-step instructions required to park the car and a visual indication on the vehicle's display. Scalability allows the system to provide a variety of parking assistance possibilities depending on the requirements of the vehicle manufacturer.
TRW Automotive Holdings Corp. expects its AC100 24 GHz radar technology, priced at about half that of a 77 GHz radar sensor, to make its Collision Mitigation Braking (CMB) system attractive to a wide vehicle market. Designed to solve problems associated with city traffic and traffic jams, TRW’s CMB technology is applied at close distance (6-7 meters or 20-23 feet). Within this distance, a 20 kph (12.4 mph) speed reduction can be achieved by braking, with limited deceleration, before a crash occurs.
With a range of up to 150 m (492 feet), the 24 GHz radar can also enable driver assistance functions such as Adaptive Cruise Control. This allows the technology to address high-speed highway driving up to 160 kph (99.4 mph). The combination of city and high-speed driving satisfies many of the needs of today’s mass market vehicles.
The benefits of combining radar and camera-based vision sensors have been exploited in several vehicles. However, Continental engineers are looking to take the combination even further to prevent accidents. In those instances with insufficient braking room, the ability of the driver to steer around an obstacle to prevent a collision can provide another collision avoidance alternative. Continental’s Emergency Steer Assist (ESA) concept employs long range information from radar and expects to add video images from camera systems similar to those already used for Intelligent Headlamp Control. The combination gives the driver a far ranging threat assessment on a high-speed highway.
Using the EPS in conjunction with ESC, the driver can steer to the right or left and avoid a collision, especially in an unexpected stopped traffic situation. The ESC’s roll is to keep the vehicle on the road during the rapid steering maneuver and to stabilize it by the selective and early application of initial braking pressure to individual wheels. Since the necessary components for ESA are already present in many vehicles, Continental engineers feel confident that vehicle manufacturers can implement this feature relatively inexpensively (see Fig. 4).
No Camera. No Radar. No Problem.
For systems without radar and cameras, Nexteer Automotive, formerly Delphi Steering, has another possible solution. Nexteer engineers expect EPS to improve vehicle safety and stability by simply adding another control feature to ESC. By taking inputs from the stability control system, EPS can provide wheel torque and assist the driver to regain control quicker than with the brakes or engine alone. These systems will continue to rely on Hall Effect sensors in EPS systems to increase safety.