Thursday, 23 February 2012

Micro pumps arrived + StrathSat-R's PDR

Yesterday, the micro pumps from Bartels Mikrotechnik (; Dortmund, Germany) arrived at the University of Strathclyde. The pumps have the purpose to act as an actuator element for the Self-inflating Adaptive Membrane (SAM) for the REXUS sounding rocket experiment StrathSat-R. SAM consists of two layers of inflatable spheres. By changing the pressure between two adjacent cells, the shape of the entire membrane can be changed.

Next week the team will travel to Kiruna (Sweden) for the Preliminary Design Review (PDR) of the sounding rocket experiment. The workshop and review is held by the German Aerospace Center (DLR), the Swedish National Space Board (SNSB) and the European Space Agency (ESA).

Description of the REXUS 13/14 sounding rocket experiment SAM

In the following the layout of the REXUS13/14 sounding rocket experiment SAM will be described. The deployable structure of SAM consists of two rows of spherical cells that are deployed by using the expansion of trapped air in the spheres when subjected to vacuum (space) conditions. The two layers of spheres are manufactured out of 30micron reflectively coated Mylar. Each cell is manufactured by heat welding circular sheets of Mylar together. The diameter of each cell is 14 cm; the diameter of the entire structure is 98 cm. The connection between the ejectable module and the deployable structure is obtained with an adhesive.

In order to change the shape of the structure, micro pumps are added to vary the trapped air pressure between two neighbouring cells. The pressure change in these cells will then change the volume of the two spheres and therefore change the shape of the entire structure. This bio-inspired structure originates from the heliotropism of plants (movement of plants towards the sun).

Mp6 micro pumps from Bartels Mikrotechnik GmbH (Dortmund, Germany) are used to change the pressure between two cells. There will be two pumps mounted to the inside wall of the deployment storage. The rows of spheres closest to the sides of the cube are joined to form one big actuator. The micro pumps will pump the residual air from the bottom to the top layer of Actuator 1 to lift up the up side and respectively from the bottom to the top layer of Actuator 2 to lift up the down side of SAM. The control software will alternate between the actuation of Actuator 1 and Actuator 2. A more precise actuation timeline needs to be validated by tests of the pumps and the structure in the vacuum chamber.

Monday, 13 February 2012

New bio-inspired design

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.