Fly-by-wire technology, the more expensive but reliable electronic control method which is replacing mechanical systems in aircraft, is finding its way into motor cars.
Cambridge-based Absolute Sensors Limited (ASL) has developed the simple, non-contact Spiral Sensor for detecting rotary and linear movement in car instruments where reliability is crucial.
The electromagnetic device, based on standard printed circuit board design, is said to be unaffected by heat, vibration, dirt or grease. Unlike the Hall effect sensors commonly used in cars, it produces a smooth, continuous output signal.
Applications include throttle position sensing, engine timing, fuel performance and emissions control, but costs make the sensor prohibitive for use in less vital functions such as electronic window winding.
Spiral, which gets its name from the spiralling shape formed by the overlapping waveforms of its printed circuitry, will appear in US cars in 2002. ASL has a licence with North Carolina-based BTR subsidiary Fasco Controls to produce the sensors.
A 5V battery is used to power Spiral and it is connected to the vehicle’s central computer. Positional changes such as throttle movement are monitored without any mechanical connection between throttle and position sensor. Instead, the sensor is instead coupled electromagnetically, through a field induced in two coils on the sensor board.
The sensor employs the theory of electrical resonance, in which a circuit oscillates at its natural frequency depending on the circuit components. Spiral’s sinusoidal tracks represent the natural frequency of the circuit, against which any voltage changes can be measured.
When movement occurs, a resonator device on the instrument oscillates at a different frequency from the system’s natural frequency. This induces a change in voltage amplitude in two coils on the sensor board, which constitute circuit resistance. One coil interprets the sine, the other the cosine, of the angle that measures the change in voltage resistance in the sinusoidal waveform.
The angle of movement is calculated theoretically by the sensor’s ASIC computer chip, which plots the sine reading against the cosine reading.
Spiral sensor technology operates successfully within a wide manufacturing tolerance, helping to cut costs. Technical director David Ely expects Spiral to be cheaper and more robust than other transducers, such as the optical encoders and resolvers used to determine linear position through a geared drive system.
ASL, a start-up company of UK technology consultancy Scientific Generics, is exploring wider non-automotive applications for monitoring shaft speed and position sensing in electric motor control. Servomotor drive systems used in robotics would be typical of the possible applications.