Contactless speed measurement using the Doppler principle with ultrasound and microwaves
P. Heide, M. Joppich, R. Schubert, SENSOREN - Technologie und Anwendung, Bad Nauheim 1992


Speed and traveled distance can be measured precisely without contact by microwave and ultrasound Doppler sensors. Based on a system theoretical model a simulation environment has been created. It allows to investigate the influence of the geometrical configuration of the Doppler sensor, the influence of the data evaluation algorithms as well as the influence of the statistically distributed scatterers on the ground on the measurement accuracy. Ultrasound Doppler sensor at 80 and 200 kHz have been developed and compared with microwave systems at 10 and 20 GHz. The results of simulations, laboratory tests and measurements on cars agree. The standard deviation of the measured speed for a traveled distance of 1 meter is 1... 2% for microwaves at 24 GHz and 0.5   1.2% for ultrasound at 200 kHz. This error does not depend on the surface properties. It is roughly proportional to the ratio of the wave length to the traveled distance. The evaluation of Doppler signals in the complex plane gives information on the direction of motion. Systematic errors caused by vehicle tilt (changing incidence angle and changing sensor height) are compensated by the so called "Janus principle". Ultrasound sensors can be produced at low costs. Due to the short wavelength, the high directivity of the transducers and due to the limited range caused by the high propagation dampening in air, they allow for precise speed and distance measurements as well as the detection of speed zero. As the ultrasound is blown away by wind its use is limited to low velocities (<30 km/h). Microwave Doppler sensors give limited accuracy for low velocities. The measurement can be perturbed by moving objects in the vicinity. The combination of ultrasound and microwaves increases the accuracy and the reliability of speed and distance measurement considerably.

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