Design and experiment of the contact-style ridge-following navigation system for a tea garden self-propelled trimming machine

Currently, self-propelled trimming machines for tea gardens have problems of low automation level and poor maneuverability. In this work, a contact-style automatic ridge-following navigation system and its corresponding control algorithm for a small-scale, wheeled trimming machine in tea gardens were proposed considering the structural features of tea ridge boundaries. A contact-style automatic ridge detection mechanism based on low-cost laser ranging sensors was designed.The working principle of the detection mechanism was introduced and analyzed. The position and heading information of the trimming machine with respect to the tea ridge was derived through the opening and closing of two side baffles of the detection mechanism caused by the changing profiles of the tea ridge boundaries. The navigation system hardware included a power supply unit, remote control unit, sensing unit, steering unit, driving unit , and control unit. The sensing unit consisted of two low-cost laser ranging sensors for measuring the changing distances of the baffles of the ridge following the detection mechanism and an absolute optical encoder for determining the real-time steering angle of the steering wheel. The steering unit replaced the original manual steering mechanism of the trimming machine with a stepper motor to realize electrical control. By analyzing the four pose states of the trimming machine with respect to the tea ridge, the kinematic navigation model of the trimming machine was derived, and the lateral and heading deviations of the trimming machine with respect to the tea ridge centerline were obtained. A navigation controller was designed based on an LQR (linear quadratic regulator) algorithm derived from the kinematic model of the trimming machine. The position and heading information of the machine were taken as control inputs. After calculation by the control algorithm, proper steering signals were sent to the stepper motor for deviation correction. To validate the feasibility of the contact-style navigation system and evaluate the performance of the control algorithm, field experiments in a tea garden were conducted. The trimming machine was controlled to automatically follow a tea ridge at three different speeds, 0.2, 0.4 , and 0.6 m/s. The experiment for each speed was repeated three times. Results showed that under three speeds, the maximum absolute deviation of the trimming machine was no larger than 0.134 m, the absolute deviation maximum was no larger than 0.107 m, and the standard deviation maximum was no larger than 0.105 m. This navigation performance was comparable to mainstream navigation systems developed based on RTK-GNSS, vision and laser sensors. The mean absolute deviation and standard deviation increased slightly as the running speed of the trimming machine increased. However, they were kept in the 0.15 m limit. This indicated that the navigation method based on the contact-style automatic ridge-following mechanism was feasible, and the LQR algorithm derived based on the kinematic model of the trimming machine could meet the accuracy requirement of tea ridge following under a practical tea garden environment. The low-cost, contact-style navigation system and its corresponding algorithm proposed in this work can provide a reference for the development of automatic navigation systems for self-propelled operation machines in tea gardens.