Technology and Developments for the Random Positioning Machine, RPM

Microgravity Science and Technology, Sep 2008

A Random Positioning Machine (RPM) is a laboratory instrument to provide continuous random change in orientation relative to the gravity vector of an accommodated (biological) experiment. The use of the RPM can generate effects comparable to the effects of true microgravity when the changes in direction are faster than the object’s response time to gravity. Thus, relatively responsive living objects, like plants but also other systems, are excellent candidates to be studied on RPMs. In this paper the working principle, technology and control modes will be explained and an overview of the previously used and available experiment systems will be presented. Current and future developments like a microscope facility or fluid handling systems on the RPM and the option to provide partial gravity control modes simulating for instance Mars or Moon gravity will be discussed.

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Technology and Developments for the Random Positioning Machine, RPM

A. G. Borst 0 Jack J. W. A. van Loon 0 0 J. J. W. A. van Loon Dutch Experiment Support Center (DESC), Dept. Oral Cell Biology, ACTA Vrije Universiteit , Van de Boechorststraat 7, 1081BT Amsterdam, The Netherlands A Random Positioning Machine (RPM) is a laboratory instrument to provide continuous random change in orientation relative to the gravity vector of an accommodated (biological) experiment. The use of the RPM can generate effects comparable to the effects of true microgravity when the changes in direction are faster than the object's response time to gravity. Thus, relatively responsive living objects, like plants but also other systems, are excellent candidates to be studied on RPMs. In this paper the working principle, technology and control modes will be explained and an overview of the previously used and available experiment systems will be presented. Current and future developments like a microscope facility or fluid handling systems on the RPM and the option to provide partial gravity control modes simulating for instance Mars or Moon gravity will be discussed. - Microgravity simulation by means of continuous change of orientation of objects relative to the gravitys vector can generate effects comparable to the effects of true microgravity when the changes are faster than the objects response time to gravity. Thus, relatively responsive living objects, like plants are excellent candidates to be studied on clinostats, 3D clinostats and the Random Positioning Machine (RPM). The clinostat provides rotation around one horizontal axis. The objects in such a clinostat experience gravity continuously rotating in one vertical plane. Especially for larger objects it was recognized that the 3D random rotation could provide a better simulation of the weightlessness conditions as compared to the classical 2D clinostat (Kraft et al. 2000). To describe the working principle of an RPM we can use the analog of a sphere fountain, a round marble stone supported by a concave water bearing. We assume the marble perfectly symmetrical around two axis and the water stream providing no rotation forces to the marble. Imagine walking on top of the marble and think of the effect to the geometrical centre of the spherical item. Now the gravity vector points to the opposite direction with respect to the walker. If walking in a straight line the marble will rotate around one axis (clinostat mode). The walker, and therefore also the imaginary gravity vector, is always crossing one meridian. We can define walk paths over the ball continuously changing the direction of the gravity vector with respect to the spheres center. But by just varying the orientation we are not sure that this motion is effective in averaging gravity. To effectively simulate microgravity, symmetry in the walk path over the sphere is required in order to average the orientation in two axes the walker has to spend the same time on each surface section of the marble. This concept of a rotating sphere for microgravity simulation was breadboarded in 2001 by a student team at Dutch Space (Fig. 1). Access to the spheres interior where the sample could be suspended in the centre of the sphere was possible by detaching the two halves. Although the system did work and it was easily possible to create random rotation with the drive system, this project did not lead to a product since it was difficult to produce a segmented sphere with perfectly smooth outer surface. A perfect sphere is required to allow a rotation of the sample without vibrations and linear accelerations. Within this concept it was also difficult to provide power and data connections to the inside of the sphere. A system that comes close to the rotational freedom of the sphere is the use of two perpendicular and independently driven frames (Fig. 2). Application of this mechanism was first introduced by von Sachs (1879). This concept has been adopted by all the manufacturers of 3D clinostats and RPMs see also the history and the development of the RPM that has been described by van Loon (2007). The discrete rotation axes allow the implementation of slip rings to provide power and exchange data with the experiment that can be mounted onto the inner frame. The control of the two rotating frames is, however, not straightforward. There are a number of control modes that can be selected. We will Fig. 1 Ball RPM. The ball RPM is supported on two freely rotating contact points. The third contact point is a wheel connected to a motor that drives the rotation of the large sphere. The orientation of this motor and drive wheel can be varied by a second motor. The combined control of these motors allows a free walk path over the sphere, resulting in a continuously change in orientation of the sphere with its suspended sample system Fig. 2 RPM with two independently driven perpendicular frames also describe a number of experiment systems which have been developed in order to allow the execution of aut (...truncated)


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A. G. Borst, Jack J. W. A. van Loon. Technology and Developments for the Random Positioning Machine, RPM, Microgravity Science and Technology, 2008, pp. 287, Volume 21, Issue 4, DOI: 10.1007/s12217-008-9043-2