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Make autos smarter with sensor systems

Posted: 01 Dec 2006 ?? ?Print Version ?Bookmark and Share

Keywords:digital bus system for centralized processing? automotive sensor technology? vehicle sensor technology? Rick DeMeis?

Sensors measuring pressure, temperature and acceleration have been a staple of automotive electronics for many years. But the myriad systems required for functions such as emissions control, fuel economy and safety go far beyond sticking a simple transducer where needed and wiring it back to a control box.

The advent of various digital bus systems allows for centralized processing and simplified wiring. It also makes it possible to take advantage of processing intelligence packaged with the sensor right where the data is acquired. But such an architecture brings reliability concerns and the classic automotive gremlin of ever-lower cost requirements.

A high-data-rate bus!such as the widely used CAN and the more powerful, deterministic FlexRay bus!are traditionally used for applications such as engine and chassis control where intensive, fast processing is required. The lower-cost, single-wire LIN was developed for body electronics applications, such as seat positioning and climate control. This is where speed is not of the essence, thus simplicity and low cost dominate. Single-wire LINs also mean less weight for greater fuel economy.

The ability to put a control IC along with a sensor on the mechanical part being monitored saves space and processing duty on a system's central processor, said Matthias Poppel, worldwide advanced embedded-control marketing manager for Texas Instruments (TI). But he added that the reliability of such a combined mechanical and electronic sensor is a concern.

In Poppel's view, "the move to 32bit MCUs and the construction of satellite processors/sensors attached to mechanical parts will have to be proven." He noted that a PCB with a mechatronic sensor attached to a component where the measurement takes place has to be tested and proven for any expected movement, loads, temperature and vibration besides the integrity of the attachment mechanism itself.

Also touching on sensor reliability concerns and the influence of packaging on them was Steve Henry, power market manager, programmable controllers, for Freescale Semiconductor's sensor operations. "The challenge with sensors!accelerometers, for instance!is that customers want smaller packages with a sensor and control IC."

Henry said that Freescale offers a MEMS device in a 6-by-6mm quad flat no-lead surface-mount package. The die has to be stacked, however, to meet the footprint requirements for a smaller overall package that can be mounted in tighter and tighter places.

Stacking the accelerometer!which raises concerns about mass and resonance!onto a processor chip demands attention to stress sensitivity from external influences (loads and vibrations), said Henry. "Coating the accelerometer with a silicon gel, room temperature vulcanization or other material can isolate it from the package," he noted. But then it becomes necessary to develop different die-attachment techniques because "it is like trying to wire-bond to a pillow," Henry said. And that affects reliability.

"Stacking dice to optimize the process is needed because you can't integrate all functions on one piece of silicon," said Mark Shaw, manager for marketing, applications and systems for Freescale sensor products. You can take advantage of high chip-logic density and high-standoff voltage (for the sensor processor), and not be limited by MEMS processes, he said.

CAN and FlexRay suit engine and chassis control processing.

Henry said the sensor should be looked at from a packaging point of view. The result is that instead of putting it on one piece of silicon, it is on two chip elements!a processor and a sensor.

Shaw pointed out that a big MCU chip is a large die in itself. "Because MEMS have a higher defect rate with lower yield," he noted, having both processor and MEMS sensor on the same chip would result in greater expense. Good processor sectors would end up being rejected due to MEMS defects in the sensor portion.

While also acknowledging that sensor yield has to come up, Frank Cooper, president of ZMD America, nevertheless sees an advantage in migrating to the simplified packaging offered by what he terms a "single-silicon solution." This methodology goes beyond having to wire-bond an ASIC chip to a sensor and connector, and then overmolding a package.

"The true single-silicon solution puts the G-sensor accelerometer, temperature or flow sensor on the same chip with the signal processing for the fewest wire bonds," said Cooper. Thus, there are fewer sites for cracking, shorting, fatigue and contamination of the fabricated sensor assembly.

Rubber meets road
Reliability is also an issue because sensors are being subjected to harsh environments they have never encountered. One such prominent application is tire pressure sensing (TPS).

John McGowan, head of Infineon Technologies' Sense and Control Group, said that sensors for TPS are in a "tight, hot place" and have to be rugged and long-lived, but still come in at a reasonable cost.

Freescale's Henry also cited the "media compatibility" issues to which TPS sensors can be exposed!"interesting chemicals" and fluids can splash on a tire in a garage, including battery acid, mounting lubricant, dust, chemical residues from the manufacturing process and moist air inside an inflated tire.

Infineon's McGowan said that placing processing at the sensor ensures accuracy from functions that include temperature compensation, self-calibration and detection of failure modes. Controlling cost comes from the integration of functions and features on the single chip (as opposed to the discrete passive components used in the past) as well as volume production. Finally, such intelligent satellite sensors allow for a smaller central processor that can be freed from number crunching to allow faster decision processing.

Current tire pressure monitoring sensors are either inflation stem-mounted or strapped to the interior of the wheel rim. Because these devices are powered by coin-cell batteries, McGowan said, tier-one suppliers are pushing for a 10-year battery life. "To achieve that goal, we use vehicle information within the processing algorithm to reduce the sample and transmission rates if the car is not moving," he added.

Future pressure monitoring may be done with sensors embedded directly in the tire structure. These would have to be powered by what McGowan termed "energy scavenging," using the tire flexing to drive a piezo for sensor energy. The concept could be extended, say, to using engine vibrations to power knock sensors. Alternatively, embedded tire pressure sensors could be powered inductively from outside the tire. Concerns here include the effects of any metal antenna loops in the tire walls on the physical characteristics of the tire.

Safety and engine efficiency will remain primary concerns in current and future automotive sensor apps.

Preliminary work on an "intelligent tire" done by a group headed by Magneti Marelli goes beyond merely measuring pressure. The group's results were reported at the SAE 2005 World Congress by project leader Andrea Neponte and strategic innovation manager Piero De La Pierre.

Down the road
Additional applications of sensors in the next five years will probably include more gyro-based components, according to Freescale's Shaw. These will supply angle-rate data for roll stability control and other axes' closed-loop control. These gyros will be based on MEMS, whose machining costs should come down with increased production volume.

Peter Knittl, marketing manager for pressure and Hall-effect sensors at Infineon, sees added performance for airbag-triggering impact sensors by going to pressure-based devices rather than the current G-sensors. "This changeover to 'active' sensors is being driven by new government mandates (FMVSS-201) for side impact," he said. "A G-sensor triggers after the structure has deformed. But a pressure sensor in a door will detect a sound wave sooner!in roughly 5-6ms, compared with up to 10ms for a G-sensor." A future airbag system could use both types of sensors to provide redundancy.

Automotive sensor systems trends revolve not just around where sensors might be used, but "how the various bus systems will have to work together and which domains each will respectively conquer," said TI's Poppel.

- Rick DeMeis
EE Times

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