Design and experiment of cotton topping device for height profiling based on canopy point cloud fitting

Mechanical cotton topping can acquire canopy height to selectively remove terminal buds under nonuniform field. However, rigid cutting devices often cause excessive stalk injury, whereas visible-light vision can dominated by illumination variation, canopy gaps, and leaf occlusion. In this study, a rolling-grid cotton topping device was developed using canopy point-cloud height fitting. A systematic evaluation was also conducted to achieve real-time height profiling, flexible terminal-bud removal, and field performance for mechanical production. A prototype was designed to integrate a high-clearance tracked chassis, an infrared binocular camera, a height-profiling mechanism, a rolling-grid topping mechanism, and a control system with programmable logic controller (PLC). Three-dimensional canopy point clouds were acquired by active infrared structured light. The point clouds were processed using spatial filtering, outlier removal, down-sampling, surface interpolation, median height extraction, and smoothing. The canopy height was fitted to transmit into the controller for real-time adjustment of topping mechanism. The rolling grid screened large leaves outside the working area and then guided terminal buds into the grid during operation, where a nitrile rubber hob removed them after striking, bending, and pulling. Field tests were conducted in Changji City, Xinjiang, China. An optimization was then made to take rubber thickness, compensation depth, cutter rotational speed, and vehicle speed as test factors. The test showed that the canopy height followed the fluctuation trend, as plant height manually measured in the field. The mean absolute error was 33.03 mm and the root mean square error was 37.86 mm, compared with manual measurements. The plant height was generally lower than the actual ones, because canopy gaps and surface fitting were reduced the profile during fitting. However, the detected value was close to the terminal-bud region suitable for height-profiling control. The point clouds provided a stable height input to reduce the dependence on visible-light image recognition. Field topping tests showed that the topping quality depended on four factors: rubber thickness, compensation depth, cutter rotational speed, and vehicle speed. Among them, rubber thickness was enhanced to remove terminal buds, because the more rigid hob transmitted greater impact force, but excessive rigidity also increased injury to tissues below the terminal bud. Compensation depth allowed more terminal buds to enter the effective working area for the high topping rate, but excessive downward compensation caused further stalk insertion for over topping. Rotational speed of the cutter strengthened the striking and breaking behavior of rubber hob, whereas excessive speed aggravated mechanical damage. Vehicle speed reduced the topping rate, because the profiling mechanism had less time to respond to plant-to-plant height differences. Response surface analysis showed that rubber thickness had the greatest influence on both topping rate and over-topping rate, followed by vehicle speed, cutter rotational speed, and compensation depth. The optimal combination of parameters was obtained, with a rubber thickness of 8 mm, compensation depth of 15 mm, cutter rotational speed of 700 r/min, and vehicle speed of 0.2 m/s. Tests were verified as an average topping rate of 83.66% and an average over-topping rate of 20.00%. The relative errors between measured and predicted values were 1.909% and 3.928%, respectively, indicating reliable regression and parameters. The results demonstrated that the rolling-grid structure separated leaves from terminal buds. The rubber hob achieved flexible removal instead of rigid cutting. The rolling-grid cotton topping device was realized to coordinate canopy perception, profiling control, terminal-bud screening, and flexible removal. The canopy point-cloud height fitting can provide reliable height information for field profiling. The rolling-grid and rubber-hob mechanism was feasible for mechanical topping. The high over-topping rate was attributed to fixed compensation depth, profiling response delay, and natural variation in terminal-bud morphology. Topping accuracy and stability can be expected for adaptive compensation, faster profiling response, lighter mechanisms, optimal rubber materials, and field validation with multiple varieties.