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Technology and Developments for the Random Positioning Machine, RPM
A. G. Borst
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Jack J. W. A. van Loon
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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.
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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)