Seats of power

  • 10-Dec-2008 12:39 EST
Networked seat controls are becoming more common, with some seats having up to eight controllers.

­Seats are continuing to offer more functions, automatically moving into position and providing lumbar support while also offering heating, air conditioning, or even massages. That is prompting significant growth in the electronics that manage these functions.

These controls let drivers fine-tune seats so they can be comfortable on short or long drives. One of the keys to design is to make it easy for these drivers to set the controls and forget about them.

“We’re looking for ways to let the customer get to a comfortable position quickly,” said Gary Braddock, Group Chief Designer at Ford. “All the controls for the seat must work together.”

That requires a fair number of microcontrollers. On high-end seats, the main controller is often augmented by three or four subcontrollers. However, some have as many as eight, with the expectation that more may come.

“There’s a demand to put more features in a seat, which means adding motors, managing power, and adding sensors so you can see where the seat is,” said Jens Eltze, New Business Development Manager for NEC Electronics America’s Automotive Strategic Business Unit.

Those chips do not require the computing power or I/O (input/output) count used in engine controls and infotainment. Seat controllers typically handle only a few tasks that are performed occasionally.

“The main controllers are going up in pin count,” said Rick Matz, Segment Marketing Manager for Renesas Technology America’s Automotive Business Unit. "The main controllers, often 16-bit processors, have 80 pins. That’s going to 100 as they add more LIN and more I/O."

The chips located around the seat, managing the specifics of lumbar support or seat back movement, are quite small. Compact size makes it easier to package them in ways that do not hinder comfort.

“Most applications are still addressed with 8-bit controllers,” said Willie Fitzgerald, Marketing Director at Microchip Technology’s Automotive Products Group. "From a size point of view, we’re looking at small packages with less than 14 pins."

The chips don’t need much computing power. “For the main controllers, 32 MHz is usually sufficient, because they offload a lot of controls to the subcontrollers," Matz said. "Those subcontrollers concentrate on one task, so 20 MHz is sufficient."

Unless designers consolidate functions, computing power probably will not change much. But memory requirements are growing rapidly, even on mainstream vehicles.

“Several years ago, 2 Kbytes of memory was enough,” Fitzgerald said. “Now people want to store memory positions for two people, which has pushed that up to 4 to 8 Kbytes.”

On higher-end seats, positions for three or four drivers may be stored. Those positions may have more parameters such as lumbar supports, so memory on the main controller is moving up significantly.

“Right now, memory runs from 128 to 256 Kbytes on higher end vehicles; soon it will go to 512 Kbytes to 1 MByte,” Matz said.

Storage size is also being driven upward by diagnostics, which are becoming more important. Memory sizes are also being impacted by OEM requirements for standardization. Many ask for AUTOSAR, which makes it easier for them to buy control modules from different vendors.

“No matter whose processor a module uses, they’re all running the same code,” Matz said. “Adding a layer of abstraction takes more software, which pushes memory sizes up.”

While processors are the main component, they rarely function without additional devices. A growing number of sensors are providing input.

“Along with temperature sensors, you need current sensors to protect the motors,” said Nathan John, System LSI Manager for NEC Electronics America’s Automotive Strategic Business Unit.

Yet another factor changing seat design is the growing use of networks, which keep cabling requirements down. “Over the last few years, the biggest change has been the addition of LIN,” Fitzgerald said.

LIN (local interconnect network) is not the only network in modern seats. Designers must increasingly account for cabling of safety and entertainment systems that are not typically designed by electronic engineers who work with seats. Occupant detection, rear-seat entertainment, and other features are becoming much more common.

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