Automotive Sensors--The Future of System Integration

Solid-state sensors are increasingly being used in cars, driving the level of system integration. System designers are also sharing them between systems through so-called sensor fusion. However, the future of system integration remains unclear as developers weigh system cost and flexibility.

The rapid increase in the number of sensor applications is driving system integration to a deeper level. Safety systems such as electronic stability control and the growing number of airbags require a large number of sensors. In the cockpit, sensors help control the temperature and improve air quality.

System engineers are hesitant to decide when to integrate system components and when to use discrete packaging to gain more options. At the same time, they are trying to protect sensors while increasing the speed of data processing.

Packaging

When engineers need to expand functionality without using more sensors or processors, they mainly resort to electronic packaging. Packaging multiple chips together not only saves space, but also saves cost. It is difficult to find a sensor that does not use some smart packaging.

“Almost all of our products are two-chip or multi-chip packages, where the MEMS (micro-electromechanical system) chip and the ASIC (application-specific integrated circuit) chip do the calibration and filtering,” said Dave Monk, automotive sensor product manager at Freescale Semiconductor. “Sometimes there is a logic to whether it is necessary or not.”

Once the ASIC chip and sensor are packaged together, the next step is uncertain. Some design teams want to add more sensors to these multi-chip package modules.

At this point, they often follow the approach of many system designers and adopt a system architecture that puts several different sensors into a single inertial measurement unit. This centralized approach can help them eliminate one or two controllers and even eliminate power supplies or electrical enclosures. If costs allow, engineers can put several sensors on the same carrier board and package them into a multi-chip module.

Network connectivity is very important for these sensor modules—they will be activated in a collision and send out several bytes of data. A possible in-vehicle network connection is the LIN (local interconnect network) bus.

"Enabling all sensors in a multi-sensor module to share the same LIN interface chip is a major way to reduce costs," said Luc Buydens, product manager for infrared thermometers at Melexis Inc.

Some design teams are more cautious, however, pointing out that the price difference between sensors is quite large. Accelerometers cost much less than gyroscopes, which poses a potential problem - throwing the entire package away if an accelerometer fails will obviously increase the cost of the system significantly.

"We decide very carefully what products we package together," said Rob O'Reilly, a member of the MEMS and sensor technology team at Analog Devices. "It may be okay to package an accelerometer and a gyroscope together, but it's hard to say when something goes wrong."

▲More and more sensor modules share the small amount of data they provide to the controller via the LIN network.
▲As the number of sensors continues to increase, the dual-axis and tri-axis accelerometers and gyroscopes provided by Analog Devices are placed in the central inertial measurement unit.

Interior and Interference

For in-cabin applications, there are fewer such concerns, since many sensors for ride comfort are priced similarly. This allows engineers to add features, such as air quality monitoring systems in luxury cars or dual-zone climate control in consumer vehicles, at relatively low cost.

“For cabin comfort, you can combine air/infrared temperature sensors, humidity sensors, CO2 sensors, and windshield temperature sensors in one module—all connected via a LIN network,” Buydens said.

Regardless of where the sensor module is located or what it contains, packaging also extends the range of protection it provides. The primary purpose of these packages is to protect the sensors, which often contain trace amounts of silicon that sense motion. In addition, packaging also addresses electromagnetic interference (EMI). “EMI is the biggest problem with electronic packaging,” O’Reilly said.

Other factors make EMI protection more important. For one thing, the frequencies of electronic devices have increased significantly. In addition, vehicles are becoming more and more electronically rich, while more consumer electronics are being used in cars.

If EMI is not well controlled, intermittent interference problems will cause many problems.

Automakers are also changing test standards, from 200V/m at 1GHz to 400V/m at 4GHz. "Every 100V increase makes the test exponentially more difficult," O'Reilly said. "We used to test only the module, but now we have to test every component in the module."

Is integration possible?

Multi-chip components can reduce the pressure on chip users and suppliers to combine multiple sensors onto a single piece of silicon. However, integration can reduce costs, so automakers hope to benefit from the sensor integration approach used in the consumer market.

Gaming products such as Nintendo's Wii have opened up a huge market for MEMS sensors, which are not only low-cost but also resistant to vibration. Automakers hope to learn from this industry.

"Gaming products use a high level of integration of accelerometers and gyroscopes. In the future, we will see more and more technology from the consumer market being grafted into the car," said Hubert Conti-Geitner, technical manager for advanced powertrain and safety markets at STMicroelectronics.

He also pointed out that automakers are just starting to use two-axis gyroscopes, while the consumer market is already offering systems with three-axis gyroscopes. For consumer products, gyroscopes and accelerometers can be combined on a single chip.

However, it is not yet clear whether this integration approach can be applied to the automotive market. Integrating everything on a single silicon chip can reduce costs, but it also limits application flexibility. Different cars have different requirements, so automakers may not use the same chip for all brands or models.

"A high level of integration requires a stable market with few changes," Conti-Geitner said. "Integration requires a lot of money and time. If the integrated chip has a resolution of 30 degrees per second and you need 15 degrees per second, this will cause problems."

Before the final plan is determined, engineers will also need to carefully study other types of integration. Although it is possible to combine sensors and logic functions on the same chip, this practice is relatively rare. The line width of MEMS technology is much larger than that of logic devices, so it is not very efficient to integrate them.

Many MEMS suppliers have not yet integrated sensors used in different systems.

For example, accelerometers have a sensitivity that determines how the sensing device is built. Applications such as brakes and airbags require completely different sensitivities, so different accelerometers are used for each system.

"For brakes, many companies set the sensitivity to 2g," said Marc Osajda, global automotive strategy manager at Freescale Semiconductor. "For airbags, the sensitivity is about 40g."

System designers would like to use one sensor for multiple systems, but this is obviously not possible.

"Chassis stability usually requires a lot of optimization work to ensure that the driver cannot feel the bumps from the ground, which requires a lower sensitivity accelerometer," said Max Liberman, marketing manager for Analog Devices' automotive safety business. "Some developers try to use sensors in electronic stability control (ESC), but more people think that one or two accelerometers should be used separately."