Simulations on the hexagonal structure showed that the current design had some major flaws which were making the shape change almost impossible to predict. Due to the fact that the hexagon is made up of six triangle elements, the inflatable volume is not similar everywhere which results in the displacement of the hexagon centre either up or down. This means that each hexagon could serve as an actuator that can be easily flipped between these two stages without a huge power consumption. At first it seemed like the ideal solution but by considering an array made up by thousands of these elements, the flipping of one element could trigger the unwanted flipping of hundreds of elements which will result in a chaotic final shape.
The research has been now focused on a new kind of design which is inspired by nature. The design is adapted from the heliotropism of plants. Heliotropism is the growth or movement of an organism, especially a plant, towards the direction of the sunlight. The motion of the plant is performed by motor cells in the flexible segment called pulvinus. The motor cells are pumping potassium ions into nearby tissues and therefore changing its turgor pressure of these cells. The segment flexes due to motor cells elongation at the shadow side due to turgor pressure rise.
By adding the principle of the residual air inflation and the new bio-inspired cell design, a versatile membrane can be obtained. The cells are manufactured by heat welding two thin circular Mylar sheets together. Two rows of inflatable cells with a reflective surface sheet on top and bottom form the membrane structure. In order to obtain the adaptability of the design, piezoelectric micro pumps are added between the two rows of cells to change the pressure between two neighbouring cells. By activating the micro pumps, the volume of the cells can be increased and decreased which results in a shape change of the entire membrane.
Research at the moment is focused on the modelling of the cells as a multibody system to understand the control of the structure. Furthermore, samples have been manufactured to validate the concept of the residual air inflation under vacuum with the new geometry. Additionally, Bartels Mikrotechnik GmbH (Dortmund, Germany) has graciously agreed to support the research and the REXUS sounding rocket with their micro pumps. Thank you very much.