Frequency-Hopping Pseudo-Random Pulse Width Modulation to Eliminate Crosstalk of Sonar Sensors in Mobile Robots

To eliminate crosstalk problem among multiple sonar sensors widely used in mobile robots, a novel frequency-hopping pseudo-random pulse width modulation (FPPWM) sequence is put forward. To ensure sharp autocorrelation characteristic, which is necessary for a sonar sensor to avoid crosstalk based on correlation technique, each FPPWM sequence is designed to include multiple variable-width and fixed-amplitude pulses, where the duration between two neighboring pulses is random and the frequency range is within bandwidth of the ultrasonic transducer. To adapt to the intrinsic physical characteristic of sonar transducers, i.e. the transmitted ultrasonic frequency can not follow that of the suddenly changed triggering sequence, the proposed sequence covers several frequency bands, which makes sure that the frequency difference between two neighbor triggering pulses is small enough for the transducer to follow. On the other hand, different combination of frequency bands has also enhanced the randomness of the proposed sequence. The FPPWM sequence is a digital signal generated by a DSP processor and no D/A converter is needed. To facilitate the correlation processing, the reconstructed echo sequence is used by compressing the real echo sequence. Real experiments using two Polaroid 600 series instrument grade electrostatic transducers validate the suitability of the FPPWM sequence