Modelling and Control of a Bubble Actuators System for Preventing Pressure Ulcers Generation

An intense and/or prolonged pressure on the same body area can occlude the blood vessels, causing the concentration of the toxic substances and the cell death. The Pressure Ulcers (PUs) are a frequent complication in bedridden patients having limited activity and mobility, often affected by loss of sensory perception and metabolic/circulatory changes (elderly people, surgical patients, intensive care admissions, spinal cord injury patients). In order to prevent and to stop the PU progression the current strategies employ passive (or static) and active (or dynamic) support surfaces. The passive solutions (soft materials and foams, gels, water mattress, air filled surfaces) reduce the pressure magnitude by maximizing the contact surface area. On the contrary the active solutions - mattresses or cushions composed by a dense array of soft actuators - can modify the contact pressure of circumscribed regions by inflating or deflating specific air cells. The dynamic solutions provide a pressure redistribution, also ensuring an adequate and equal support and a comfortable sitting position. The aim of this project is the study of an active automated seat cushion. The system, equipped of a real-time pressure mapping with closed-loop control, can identify the critical points, offload specific air cells and redistribute the resulting pressure in order to eliminate the pressure peaks and promote the reperfusion. In particular, this project is focused on the design, the modelling and the control of the soft bubble actuators which constitute the contact surface of the cushion. In order to define the shape and the size of the soft actuators, different finite element simulations are carried out. After defining a hyperplastic (rubber like) material, the air cells are pressed down vertically and their mechanical behaviour is recorded. Different external loads are applied for several initial inner pressure, analysing the internal volume reduction and the variable stiffness of the bubble actuators. Finally, the data obtained by the finite element analysis are used to simulate the inflation and the deflation of the soft actuator. After comparing the desired pressure level and the current inner pressure of the air cell, the system control can modify the second one in order to reduce the difference between them.