A Tractor-mounted, Automated Soil Penetrometer¿shearometer Unit for Mapping Soil Mechanical Properties

A tractor-mounted, automated soil penetrometer–shearometer unit was designed and developed for the purpose of simultaneous in situ measurements of soil penetration resistance and shear stress. The automated soil penetrometer–shearometer unit utilises both commercial penetrometer and shearometer. The overall construction of the unit was made up of the main frame to support the moving carriage sub-assemblies, two moving carriage sub-assemblies to support penetrometer and shearometer, the gear driving mechanisms to support the traverse movements of the moving carriages, and the three-point hitch attachment to support the main frame and provide attachment to the tractor. The motion controls for the penetrometer–shearometer unit were performed by the programmable logic controller (PLC) unit and other external electronics and sensing devices. Two high-torque stepper motors were used to drive the penetrometer and shearometer moving carriages in the vertical axis direction while another one low-torque stepper motor was used to drive the rotating spindle of the shearometer. A personal computer data acquisition and differential global positioning system (DGPS) on-board tractor were used to assist in real-time measuring, displaying, and recording the tractor position, soil penetration resistance, and soil shear stress during the field sampling operation. Field test of the automated soil penetrometer–shearometer unit was conducted together with data collection on soil moisture content, terrain slope, and tractor–implement performance parameters. Spatial maps produced from the collected data showed considerable site-specific variation in the measured parameters over the field. Statistical analysis showed significant correlation between soil penetration resistance and shear stress and significant correlation between soil moisture content with penetration resistance and shear stress. Multiple regression equations were formulated using the stepwise analysis method to predict travel speed, implement draught and fuel consumption for mouldboard ploughing on Serdang sandy clay loam soil.