Pneumatic robotic arm controlled by on-off valves for automatic harvesting based on vision localisation

Previous works about robotic arm for the automatic harvesting of an agricultural crop of quality small plants (red radicchio), were mainly concerned about the study of the control system for pneumatic actuators moving the robotic arm. The choice of pneumatic actuators was due to their physical favourable characteristics for positioning applications. The main problem of this choice was linked to non-linear behaviour of pneumatic systems. This non-linearity makes accurate position control of a pneumatic actuator difficult to achieve. A recent trend developed the use of inexpensive on/off solenoid valves to control pneumatic actuator systems. Furthermore, the use of pulse width modulated signals (PWM) across the power devices reduces control problems such as dead band and stiction. The results ensure the possibility of following a desired trajectory using potentiometer feedback and a PI controller with parameters tuned in an iterative way. This paper shows experimental results carried out considering a vision feedback obtained with a cheap cam mounted on the top of end-effector. This vision feedback is processed in order to localise the plant position along the movement plane of the robotic arm end-effector. A kinematical model of the robotic arm defines each actuator trajectory to lead end-effector to grasp the plant in the taking plane according to the vision feedback. All the control and localisation system was realised using a single commercial PC.