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Electric Steering That Feels Right
Danaher's steer-by-wire system relies on a unique torquefeedback device that simulates the feel of conventional steering
Joseph Ogando, Senior Editor -- Design News, September 22, 2008
FEEDBACK DEVICESTORQUE FEEDBACK DEVICE,
DANAHER MOTION
As good as they've become, steer-by-wire systems can still suffer from a perception problem. Drivers accustomed to conventional hydraulic steering don't necessarily appreciate the “feel” of electric steering. Danaher Motion, however, has come up with a new way to satisfy even the touchiest of drivers.
The company's Torque Feedback Device (TFD) interfaces with a steering wheel to provide tactile feedback for the vehicle operator, as well as position or velocity feedback to control steering actuators.
For the steering response, the TFD isn't that unusual in that it provides position or velocity input to a controller that commands an actuation mechanism. Usually, it would apply to steering, though other types of by-wire actuation systems would work, too.
What sets the TFD apart is the tactile response it gives back to the driver. According to Geoff Rondeau, engineering department manager, the TFD features a new twist on electromechanical braking. “Traditionally brakes have had a crude, on-off torque input,” he says. The TFD, by contrast, has a variable torque output proportional to a dc input. “Unlike a motor, which generates motion, the TFD generates resistance,” Rondeau says.
To generate that variable resistance, which is what simulates the familiar feel of hydraulic steering, the TFD relies on a patent-pending design from inventor John Hehl. That design uses an electromagnetic coil to drive a moving armature. The armature, in turn, compresses a set of friction discs attached to a central shaft that's ultimately integrated into the steering column or some other kind of human tactile interface. The higher the current applied to the coil, the greater the compressive force exerted by the armature.
Because it's used primarily for steering, the TFD also incorporates redundant sensors for fail-safe shaft feedback. All components and the finish assembly met EU Directive 2002/95/EC (RoHS) and the TFD is designed for use in EN 1175- and IEC 61508-compliant systems.
Initial applications for the TFD include electric, recreational, marine and off-highway vehicles. Gaming and consumer applications requiring a haptic interface are a possibility, too.
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