Tank patrol

Automotive Design & Production, May, 2004 by Christopher A. Sawyer

So what is it this time?

How about a new way to measure fluid levels in a vehicle? It's 3% more accurate than conventional fuel tank sensors, and eliminates the need for a float and float arm (similar to the one in a toilet's water tank). A fully electronic unit, it consists of a wide, flat strip that extends into the tank, topped by a cap housing the electronics. Unlike a conventional design, you can tell if it's working without pulling the tank, and it should last for the life of the vehicle.

Sounds expensive. How does it work?

According to David Nyce, owner of Revolution Sensor Co. (Apex, NC), "An oscillator circuit energizes a conductor pattern in the sensing probe and makes a standing wave pattern on the sensing element." The strip measures the difference in permitivity--basically the difference in the dielectric constant--between the liquid and the air by looking at the changes that occur in the electromagnetic field. Which is why Nyce calls it an "SEF", or Shaped Electromagnetic Field, sensor.

But float-type sensors are a known quantity. Why would I switch?

The battle for real estate around the vehicle has led to the use of oddly shaped fuel tanks. Not only are they non-rectangular, an increasing number are shaped like saddlebags. That is, the tank is split into two volumes joined by an inverted "U" that clears the exhaust system or driveline. This limits the space needed for the float arm to arc (it's not unusual for them to get hung up in a saddlebag tank) it forces the use of two sensors, and a wide arc is needed to provide enough force to operate the resistive element. Depending on the design of the tank, these sensing elements may be totally different. An SEF-equipped fuel tank would still need two sensing units, but the room formerly used to package the float arms can be used for other things, like fuel. And, unless there is a large difference in size or shape between the two sides of the tank, a single sensor design can be used. Angling the sensor from the front of the tank toward the deepest part of each half, for example, not only measures the depths of the tank, but cuts down on the sudden drop in fuel level often seen with float-type units.

[ILLUSTRATION OMITTED]

Does this mean the SEF unit gives more accurate readings?

"Certainly," says Nyce, "it's easier to make an SEF unit more linear than a float type." That's because engineers would have to map float changes as each gallon of fuel is consumed, and do that for each vehicle. This is difficult when the float arm doesn't reach into the deepest part of the tank. "The conductive pattern for the SEF element can be printed any way you want it," says Nyce. "It's up to the car companies as to how accurate they want the gauge to be." But Nyce insists that an SEF-equipped vehicle won't have a fuel gauge that lingers over the first half of the tank, while the second half slips away faster than a weekend. "I'd expect," says Nyce, "that most companies would choose basic sensor patterns that could be used over a wide variety of tank designs. It may not provide the ultimate linearity, but it would offer the best trade-off between cost and linearity."

How do I find out more?

Check out Revolution's web site at: www.rev.bz. Though the company opened in October 2003, Nyce has been working on the technology--originally with MTS Systems--for a number of years. He currently has two automotive clients--one Japanese, one domestic--and hopes to have the first units in production vehicles in the 2007 model year.

By Christopher A. Sawyer, Executive Editor