Ultrasonic Doppler Vector Speed Sensor for Autonomous Tractors

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The purpose of this study is developing a speed sensor for autonomous tractors. Ultrasonic or microwave Doppler speed sensors are used in autonomous driving systems. However, those sensors were originally intended for tractor performance monitoring. The accuracy of those sensors is not enough for position detection. The authors studied on the error factors of ultrasonic Doppler speed sensor, and developed an accurate speed sensor. Results of the study are summarized as follows,1. Improvements of sensitivity and resolution: For most conventional sensors, it was difficult to measure very low speed or reverse motion. The problem was solved by frequency multiplication of the received signal and mixing with a reference signal in the developed sensor. The sensitivity was also improved using a preamplifier and a tuned amplifier.2. Reduction of the error from the change in reflection angle on the ground: An offset parabolic reflector was designed and attached to the sensor to keep the transmission and received waves in the constant direction. An ultrasonic transmitter and a receiver of relatively low frequency were used for the sensor. Accuracy of the measurement on smooth ground surfaces was improved.3. Comparative test: Test equipment for speed sensors was constructed using a 4m long linear actuator. High accuracy of the developed sensors was confirmed by comparison with commercial sensors.4.Experiments of an autonomous tractor: An autonomous tractor was developed using internal sensors and a GPS. Experiments of autonomous driving were carried out on a horizontal forage farm. In the case of 270m driving at a speed of 0.8m/s, the maximum displacement from the reference line was about 0.5m.