A sound-based positioning system with centimeter accuracy for mobile robots in a greenhouse using frequency shift compensation

Small distributed robots have the potential to be commercialized in greenhouse operations due to their low cost, high efficiency and light weight compared to conventional greenhouse robots. In order to guide the path of these distributed dynamic robots, a positioning system with a centimetre accuracy is needed. While a GPS navigation system does not perform well in such an environment, the passive Spread Spectrum Sound-based Local Positioning System (SSSLPS) using FDMA (Frequency Division Multiple Access) does have the potential to accurately localize robots in such an environment if certain limitations can be overcome. One such issue is the Doppler effect. In this paper a Doppler shift compensation (DSC) algorithm, which applies a carrier frequency detection method, was used for passive SSSLPS (FDMA), and evaluated in terms of its ability to compensate for the frequency shift of the sound signals when measuring distance and position. The results show the applied DSC algorithm can measure distance using a compensated reference signal at 0.1–0.8 m/s constant velocity and 0.06–1.06 m/s2 acceleration. The positioning error of the passive SSSLPS (FDMA) mounted on the crawler robot was 50.3 ± 22.3 mm accuracy, while the failed detection rate was 2.6%. These results suggest the applied DSC algorithm can effectively compensate for Doppler shift when using passive SSSLPS with FDMA for dynamic robot positioning.