The Random Positioning Machine : RPM

The Random Positioning Machine (RPM) or, by some referred to as the 3-D clinostat, is a microweight ('microgravity') simulator that is based on the principle of 'gravity-vector-averaging'. The system may be compared with a classic 2D clinostat although such a clinostat has only a two dimensional averaging of the g vector while the RPM provides a functional volume which is 'exposed' to simulated microweight.
Gravity is a vector, i.e. it has a magnitude and a direction. During an experiment run in this two axis RPM the sample's position with regard to the Earth's gravity vector direction is constantly changing. The sample may experience this as a zero-gravity environment.

As an example we may look at the gravity sensing system of a plant. In the plant there are gravity sensing cells, statocytes, in the stem and the root caps. These statocytes contain heavy particles, amyloplasts, which settle to the lower part of a statocyte under normal 1×g Earth gravity conditions. Without going into more detail, the position of these amyloplasts within the statocyte provides the plant the information it needs to determine the direction of the g-vector. However, these amyloplasts need some time to settle within the cell. When we turn the plant upside down the amyloplasts will settle to the 'top' of the cell. When we would constantly change the direction of the plants with respect to the gravity vector the amyloplasts would never be able to settle. You might say that they are drifting around within the volume of the cell boundaries. Using the RPM we can set the required velocity to prevent the particles from settling. These times / velocities are likely to be different for different systems and have to be determined experimentally.

When needed the RPM could also be programmed to generate partial gravity accelerations (~0.1 - 0.9 × g). The system may also be operated in centrifuge mode, 'classic clinostat' mode, random mode or, since the implementation of the new software, may generate particular reproduceble paths that are generated within the software and executed during the actual experiment.

The core of the full size Random Positioning Machine (RPM) build by Dutch Space (former Fokker Space).

To accommodate various experiments there are two versions or sizes of the machine. The larger of the two (see picture above) is capable to accommodate larger experiments or instruments like microscopes that are needed to monitor or support the actual sample in the center. The smaller 'desktop' version is better suited to accommodate more standardized cell biological or tissue culture experiments using multiwell plates, T25/T75 flasks or the like. While the full size version is accommodated in a dedicated temperature controlled incubator capable of supplying a 5% CO2/air mixture, the desktop RPM can be placed in any regular laboratory incubator and can easily be shiped to another lab for pilot experiments.

The core of the desktop RPM. The footprint of the system is 30×30 cm.

The principle of an RPM is to (randomly) rotate. As with other rotating systems this will generate an acceleration. Since we want to simulate microgravity we are to avoid any additional g forces. The level of simulation within this RPM depends very much on the speed of rotation and the distance of the sample to the center of rotation. In priciple only the exact center of the RPM i.e. the center of rotation provides you the ultimate microgravity simulation. To have a particular level of microgravity simulation you might use the graph as shown here. In this figure a logarithmic scale of speed of rotation w (in rad/s or degr/s) and distance of the sample from the center of rotation (in cm) is depicted. You can see, for example, that when I would place a sample 1 cm off center I need to set the angular velocity to 1 rad/s (or about 57°/s) to have a maximum microgravity simulation level lower than 10^-3g. The simulation level decreases when moving closer to the center of rotaion at the same speed.

Quick-look 'equi-g' contour plot. Adapted from Huijser, Dutch Space (former Fokker Space), 2000. See main text above for details on this graph.

For standard experiments it is preferred to use standard lab disposables as experiment container. When more sophisticated tasks need to be performed that cannot be adopted to standard lab plastics it is possible to use more flight like experiment containers such as LIDIAs or plunger boxes.
Future experiments on the RPM could make use of developments that have taken place within the Technology Development Program at ESA/ESTEC. Various hardware items for gravitation biology, such as small microscopes, video cameras, cell / tissue / plant culture modules, insect modules etc., have been developed. These modules might be used on the Random Positioning Machine to investigate how biological systems will respond to simulated microgravity. See the ESA web site for detailed information on these items, or contact us for their use on the RPM (or the FFM / MidiCAR centrifuge).

All facilities mentioned are open to perform your individual, dedicated, experiments or for collaborative studies. When you are interested to perform a series of (pilot) studies using the RPM you are invited to contact us and discuss various possibilities. You may also directly send an experiment proposal either to DESC or respond to one of the international 'Announcements of Opportunities' (AO) as are in general annually issued by the European Space Agency (ESA) or other space agencies. You may also apply via an unsolicited proposal to ESA via the 'fast track' Continuously Open Research Announcements. You may contact us when you need any support for this. Dutch scientists may also apply nationally via a NSO-NWO proposal. An additional copy of the proposal has to be provided to DESC for technical and operational evaluations.

RPM 'live' in EK-08

(Live data pictures / video of an RPM experiment run are only made available at remote sites when necessary for a particular PI.)


Arabidopsis seedlings grown at 1×g (GR) or in the RPM (RPM). Notice the rather straight growth at 1×g compared to the disoriented growth in the RPM.	EM micrograph of Arabidopsis columella cells grown at 1×g (GR), in the RPM (RPM) or in the Space Shuttle (FL). Notice the similarity in the location of the amyloplasts (m) within the cell. From Kraft et al. Planta. 211(3), 415-422, 2000.