The increasing demand for robust and reliable vehicle safety systems has been and will continue to be a major factor in the growth of MEMS sensors in the automotive market.
The introduction and broad rollout of seat belts in the 1960s and 1970s and airbags in the 1980s and 1990s have resulted in significant reductions in injuries and fatalities from traffic accidents. The devices continue to evolve: cars now typically feature both front and side airbags, and many vehicles are fitted with seat belt pretensioners.
In an accident, the pretensioner spins the seat belt’s spool with an explosive charge to near-instantly draw up any slack in the belt. This action settles the car’s occupants solidly into their seats just before the full force of the impact hits, putting them into a position to gain maximum value from the ballooning airbag and to keep them from “submarining” under the bag and dashboard into the floor.
Accelerometers or inertial sensors and their associated electronics recognize any sudden deceleration from a frontal impact. In previous years, discrete accelerometers mounted at the front of the car communicated to electronics near the airbag. With MEMS technology, a tiny chip containing an accelerometer and microprocessor can be mounted near or within the airbag or seat belt pretensioner package. For side impacts, MEMS sensors evaluate a rapid increase in air pressure within the car door to decide whether the side airbags should be deployed. The fast response provided by MEMS is welcome considering that only milliseconds are available in which to trigger these safety devices and to protect the occupants.
Anti-lock braking systems keep the wheels rolling enough (even under heavy pressure from the driver on the brake pedal) to maintain control of steering while applying maximum stopping force without the tires slipping. Magnetic sensors determine which wheels are turning and at what speed, while gyros detect the rotation of the vehicle. The microprocessor then determines how much braking force each wheel should receive to help bring the vehicle under control.
Electronic stability control (ESC) combines input from gyros, accelerometers, and magnetic wheel speed sensors to recognize a skid or similar loss of control by comparing the car’s speed, motion (primarily the yaw, or spin around the car’s vertical axis), and steering angle. The ESC then adjusts the throttle and applies individual brakes as necessary to help the driver keep or regain control. With an effective ESC, the driver might not even be aware that the system was activated.
Sharp cornering and certain sudden steering actions can create the risk of rollover, especially in vehicles with a higher center of gravity such as SUVs, pickup trucks, and vans. Roll stability control combines the use of gyros and accelerometers to test whether the vehicle’s roll rate and angle have exceeded acceptable limits. If so, the controller may intercede to manage the brakes, throttle, engine torque, and/or active suspension systems (if available) to counteract the rollover forces. The system might also fire the side airbags if a rollover is imminent.
Tire pressure monitoring systems (TPMSs) help avoid unsafe driving conditions and poor fuel economy from tires that have lost significant pressure. Tire pressure can be monitored directly or indirectly. For direct measurement, a MEMS pressure sensor checks the inflation of each tire. In an indirect system, magnetic wheel speed sensors identify a wheel rotating faster than anticipated – an underinflated tire is a “smaller” tire and will have to rotate more often to keep pace with a properly inflated tire. In either case, the TPMS notifies the driver that there is a tire problem.
Adaptive cruise control and automatic emergency braking systems act on the driver’s behalf to adjust the vehicle’s throttle and/or apply the brakes, maintaining a safe distance from other vehicles and avoiding collisions. These systems rely on accurate distance measurements provided by technologies like LiDAR (light detection and ranging). LiDAR illuminates targets with pulsed laser light and determines the intervening distance based on the time required for the reflected laser light to return. By “painting” the environment in front of and around a vehicle in this manner, LiDAR can create a three-dimensional view of objects that the driver and automatic systems must be aware of. Optical MEMS, especially micromirrors, can play a necessary role in both the steering of the laser beam and the gathering of the reflected light.
Automatic emergency call systems combine input from inertial sensors and safety systems to determine whether an accident has occurred and, to some degree, the severity of the situation (e.g., have the airbags deployed?). The system can independently contact emergency services and, provide the accident’s location using navigational data. This can put first responders on the scene more quickly, especially when the vehicle’s occupants are unable to take action on their own.
Various sensors assist in maximizing engine performance and fuel efficiency while minimizing vehicle emissions.
Pressure and gas sensors monitor gases and fluids in and around the vehicle, including:
- Atmospheric pressure and air-fuel mixture, to ensure optimal fuel efficiency;
- Oil pressure, to maintain proper lubrication;
- Fuel tank pressure, to find potential vapor or fluid leaks; and
- Exhaust gas recirculation flow, to minimize NOx emissions.
Thermal sensors track engine temperature and prompt adjustments affecting performance (such as air-fuel mixture and ignition timing) and protection (cooling system).
Meanwhile, magnetometers gauge the speed and position of the camshaft and crankshaft to match the timing between valves and pistons and help manage effective fuel injection and consumption.
Comfort, convenience, and security
The driving public has an insatiable appetite for comfort, convenience, and security. MEMS sensors play a significant role in meeting the demand.
Drivers need not know how to reach their destination – they can hear step-by-step voiced instructions from the car’s GPS navigation system. However, GPS satellite signals are sometimes blocked by structures or dense foliage, causing a temporary disruption in data. Inertial navigation systems bridge that gap. MEMS inertial sensors track changes in speed and direction. This information is used by a processor to do “dead reckoning” calculations that predict the vehicle’s changing position until GPS is once again available.
Some automobiles now include the ability to hold a vehicle on a grade without the driver needing to hold the brake. This is a function of the electronic parking brake. This system senses an incline using data from MEMS accelerometers and holds the car temporarily from rolling. If the car is parked on a grade, the “hill hold” mechanism will keep the car from budging after the handbrake is released, freeing the driver from concern that the car will roll into any adjacent objects until the clutch is engaged and prepared to propel the vehicle forward.
The conditions within the vehicle can greatly affect occupant comfort. Climate control systems using MEMS thermal and gas sensors can monitor temperature, particulate matter, humidity, and carbon dioxide levels and adjust heating, cooling, and ventilation accordingly.
Anti-theft systems employ inertial sensors. These watch for a jostling of the car or for a new tilt that might come from an attempt to lift one end of the car. If such motion is found, the security system begins to wail and possibly transmits a radio signal calling for assistance.