A7.1 - Utilizing CDMA Techniques to Improve Ultrasound Based Distance Measurements

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A7 - Ultrasonic Sensors II
D. Glaser, S. Rupitsch, S. Adelhardt, R. Lerch - Friedrich-Alexander Universität Erlangen-Nürnberg (Germany)
159 - 164


Ultrasound based distance measurements are widely used in industrial (e.g., filling level measurements) as well as in automotive applications. In the majority of cases, one or two ultrasound transducers (transceiver) are used to emit and receive the pressure waves. If the speed of sound in the propagation media is known, the distance can easily be calculated by the time-of-flight of the pressure waves. In this contribution, we present ultrasound based distance measurements with the aid of spread spectrum modulation methods, especially so-called Code Division Multiple Access (CDMA) techniques. In particular, Gold sequences are codes for band-spread modulation offering outstanding properties of the auto- and cross-correlation function.
Our measurement setup mainly consists of a transducer for emitting pressure waves and a hydrophone for the reception of the pressure waves. The distance between transducer and hydrophone is calculated by the crosscorrelation of the excitation signal and outputsignal of the hydrophone.
We present experiments clearly pointing out that CDMA modulation with Gold codes has many advantages for ultrasound distance measurement. In the experiments the deviation of measured and prescribed distances are smaller than 0.3 %. CDMA improves signal-to-noise ratio by magnitudes, allows separation of signal sources and improves accuracy. The improvement in SNR allows to reduce excitation power while still being able to track the distance continuously. However, it must be mentioned that the required processing power is rather high and to track a single signal continuously makes DSP or FPGA indispensable.