Non-contacting sensor measures steering angle and force

Chicago-At the National Design Engineering Show this past March, engineers visiting BEI Duncan Electronics Div. (Tustin, CA) could see a whole new class of steering and position sensing technology. Called NCAPS(TM) (Non-Contacting Angular Position Sensor), the patented technology has the potential to significantly reduce packaging problems, as well as weight and power consumption requirements in rotary position sensing applications.

NCAPS uses multi-channel transceiver (transmitter and receiver) signals that are interrupted in a sinusoidal fashion by a coupler (the only moving part). Advanced DSP (Digital Signal Processing) techniques sum the receiver outputs and reference the composite-signal phase to the angular position. The technology adapts to reliably sense the driver-applied torque for feedback in electric power steering assist systems, and features mechanical memory at start-up without using electronic memory or a reset function.

By adapting principles used in radar and RF communication systems, rather than magnetic or optical techniques, NCAPS provides a non-contacting sensor that combines the economy of low-cost potentiometers with accuracy and repeatability approaching that of more expensive optical encoders. In fact, the design achieves better than 2% linearity over -40 to +85C temperatures. It's highly repeatable, less than 0.1% error, and accuracy (absolute linearity) is less than 1%, typically plus or minus 0.5%.

"When we set out to design NCAPS," explains BEI President and COO Asad Madni, "we wanted something that was really advanced, yet extremely simple in order to reduce hardware cost. We wanted to get away from the limits of magnetic based feedback, and difficult-to-manufacture optics," Madni explains. "We didn't want to operate on amplitude due to misalignment issues, so we came up with a signal processing technique that is based on the single mixer down conversion process used in radar and RF communications systems."

Essentially, NCAPS consists of three disks: a transmitter, receiver, and coupler. The transmitter and receiver disks are fixed in position. The coupler moves with the item being measured.

"The entire sensor consists of three PC cards and electronics that can be reduced to a CMOS ASIC in order to cut costs," Madni says. "Its phase detection technique provides a robust, fault tolerant device that is not dependent on amplitude variation, and the mechanical design uses standard PCB material and manufacturing techniques that lend themselves to economical high-volume production." By combining well proven and durable materials with transceiver and digital signal processing techniques, the design delivers a package size less than 12-mm thick. The technology has some unique and desirable features that add to the sensor's performance. For example, the sensor's selectable internal operating frequency can be any desired value from a few kHz to many MHz. "This is particularly valuable as a frequency can be chosen that is least likely to

interfere with adjacent devices, such as a radio receiver," says Madni. "And it allows the cross talk immunity between two adjacent NCAPS for close stacking in a very thin package." The basic sensor element size ranges from 25-mm diameter to as large as needed. For any specific applications and sensor size, the electronics portion is common-only the rotor (or slider for the linear sensor) and the stator design and size change to accommodate specific packaging and installation requirements.

Although BEI developed the technology to meet stringent automotive and heavy equipment industry standards for vibration and shock (1.5g, 20-2,000 Hz vibration, and 1m drop-to-concrete shock), NCAPS is also suitable for position and force sensing applications. Moreover, since its original development, a linear version of NCAPS has evolved that provides built-in feedback in voice-coil actuators for motion control systems.