Currently, many of the advancements being made in automotive electronics depend directly on the development of advanced sensors and sensor systems. Sensor technology is vital to the development of many emerging automotive systems, which require not only accurate and reliable sensor elements, but also numerous electronic functions. These include signal conditioning and interface circuits, as well as the hardware and software required to process sensor data and prompt the necessary system responses.
Packaging, physical interconnect, and test needs also figure prominently in sensor system development. All elements of the system must be designed to withstand the harsh environmental conditions encountered in the automotive world while maintaining high reliability. In fact, industry veteran Joe Giachino of Visteon Corp. observes that the requirements imposed by automotive safety applications are actually tougher than those found in the military environment. At the same time, automotive components and systems must lend themselves to high-volume, low-cost production.
Despite these challenges, though, automotive applications are spawning the development of a host of sensor types for use in systems related to rider safety; engine and drive train; comfort, convenience, or security; and vehicle diagnostics or monitoring.1 In many cases, these are MEMS devices because of their potential for high performance, integration, and low cost. Nevertheless, other types of sensors are in development too. Quite a few of these new sensors—both MEMS and other styles—are being designed to implement advanced automotive safety systems beginning to come to market now.
In planning these new systems, automakers are striving to build collision detection and avoidance systems that will protect the vehicle from harm. That's the long-term goal. But in the short term, automakers are looking to lower risks. To this end, they're starting to implement occupancy seat sensors that improve airbag deployment and angular-rate sensors for enhancing vehicle dynamic controls.
The need to deploy airbags more intelligently is spawning development of several sensing technologies. Until recently, the decision of when to fire airbags depended only on collision detection, which was accomplished primarily with accelerometers that rapidly detect the occurrence of front- and side-impact collisions. But the realization that airbags pose a risk to infants, children, small adults, and even average-size adults who aren't properly seated has created the demand for a more sophisticated airbag deployment scheme.
In developing new airbag systems, it's no longer sufficient to trigger airbag inflation solely on the basis of collision detection. What's required are "smart" airbag systems that take into account the size of the passenger and—in more advanced designs—whether the passenger is in position for a safe deployment of the airbag.
In their efforts to create smart airbag systems, the Tier One automotive suppliers designing and building them take their cues from organizations like the National Highway Traffic Safety Administration (NHTSA). The NHTSA, which determines standards for testing crash protection equipment, has established a new set of rules for how airbag systems will be tested. Whereas in the past it was sufficient to test airbags using a crash test dummy that corresponded in size to an average (fiftieth percentile) male, automakers in the near future will have to ensure that their safety systems protect a range of occupant types.
Systems will be tested with a family of crash test dummies representing a one-year-old infant, a three-year-old child, a six-year-old child, a small (fifth percentile) female, and the fiftieth-percentile male. In addition to protecting children and small adults, new airbag systems must reduce the risk of injury to passengers who are near the airbag when it deploys. The new regulations give designers different technical options, such as weight sensors and dual-stage inflators, to protect occupants. The NHTSA will begin to phase in the new rules for airbag systems in 2003.2
With these requirements in mind, automotive suppliers are actively developing a number of occupancy seat sensors based on different approaches. Techniques under development include weight, pressure pattern, infrared (IR), ultrasonic, electric field (capacitive), and video image sensing. For now, the industry is still evaluating the merits of each of these methods and hasn't come to any consensus on which will provide the optimum solution for occupancy sensing. As Roger Grace, a marketing consultant close to the automotive sensor technology scene, comments, "It's still early in the game for one strategy to be considered a standard."
Plus, given the complexity of the task and its tough reliability requirements, it's unlikely that any single methodology will emerge as the sensing solution. Craig Bezek, manager of Advanced Technologies at Motorola's Automotive and Industrial Electronics Group (AIEG), Northbrook, Ill., notes that there are drawbacks with each approach to occupancy seat sensing.
For instance, weight and pressure pattern sensing, which are already being deployed, can categorize the occupant by weight class. But these systems are limited in their ability to determine the position of the occupant. So to measure position, designers must turn to IR, ultrasonic, and electric field sensing.
Yet none of these methods is perfect. With each, tradeoffs will be made in terms of sensor mounting location, size, speed of measurement, and life cycle. Environmental factors, such as ambient light, sound, temperature, and humidity, also are concerns.
According to Bezek, electric field and IR methods tend to be the fastest. When it comes to coverage area, though, electric field and ultrasonic sensing cover a wider area than IR. It's possible to increase coverage with multiple beams, but that adds cost.
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