In this study, an additional electrohydraulic circuit was designed and assembled to a conventional hydraulic steering circuit of a small farm tractor (Kubota L4708). It is not only flexible to merge with a GPS or a visual navigation module, but the system also has a manual/auto mode, easy for replacement and maintenance. The design that is suitable for farming comprises 4/3 electrohydraulic proportional valve with infinite position as a main steering actuator. The proposed hydraulic circuit is able to connect with a microcontroller which was programmed to command the steering system by using a pre-calculated pathway. By using the Autopilot Navigation, the constructed path was calculated, and the driving signal was sent to drive a virtual servo motor. The driving PWM signal was transformed to an analog signal and sent to the tuning block in the micro-controller. Mathematical modelling included cylinder modelling, solenoid valve modelling, flow forces and the orifice area. By using logging input/output data from the valve driver and the Linear Variable Differential Transformer; LVDT, a transfer function of the system is set up. Referring to analysis of stability and frequency response based on simulation and actuator test results, the system was stable but had an imbalance when comparing left and right turns due to a complication of hydraulic line connections. Field experiments were done. The PID tuning block worked quite well and the steering angle relates linearly to the longitudinal displacement of the piston.
展开▼
