Declic: Linux 2.6 on the International Space Station

The Declic experiment program on the International Space Station needs Linux to integrate reliably with a variety of microcontrollers with minimal intervention from the crew. Here's how the development is going.

In October 2001, three French scientists defined a new project for the study of phase transitions of fluids under microgravity conditions. Declic (Dispositif pour l'Etude de la Croissance et des Liquide Critiques) permits a wide experimental program, operated from the French USOC control centre in Toulouse in close relationship with the other control centers located at NASA and the European Space Agency (ESA). Scientists can do telescience experiments with real-time data sent from the Declic facility to ground, with almost no help from astronauts.

The only astronaut help needed is some exchanges of experiment boxes, the so-called inserts. ALI, one of the inserts, stands for Alice-like insert and refers to the previous experiments, Alice and Alice-2, from the Mir Space Station. Alice stands for Analyse des Liquides Critiques dans l'Espace (analyses of critical fluids in space). A critical fluid is a fluid at a specific temperature and density where the transition between fluid and gas behaves differently compared with the same fluid on Earth.

The French governmental space organization CNES is developing Declic, and it awarded the contract to the European aerospace organization EADS, a joint venture of the German Daimler-Chrysler Aerospace AG, the French Aerospatiale Matra and Spanish CASA. EADS is using four subcontractors for the actual development and is doing the integration tests and project control in Bordeaux. The University of Amsterdam in the Netherlands had experience with several of the previous critical point programmes, therefore we are developing a substantial part of Declic: two thermostat boxes where the experiments take place, the electronics, software for thermal regulation and parts of the data acquisition for scientific research. Two other subcontractors are working on optics, data processing electronics, software for video cameras, data storage and the ISS interface. The fourth subcontractor is developing a complete experiment insert for solidification experiments. Electronics and software for this experiment also are being developed at our institute.

Figure 1 offers a simple overview of the several parts of the Declic facility, which basically contains two large boxes. The first box holds the experiment insert, which is surrounded by optics, video cameras and different sorts of sensors for observing the scientific phenomena. The fluids enclosed in a safe containment inside the insert are stabilized at a high-precision temperature. It's no simple house thermostat; it's a high-accuracy thermal control system that can keep fluids within 10 micro Kelvin of a specific temperature.

The second box (see also Figures 8 and 9) contains the electronics for data handling and temperature control. The electronics and software situated in this box is what I describe in this article. Two important subsystems are located in this second box, the power and data handling system (PDHS) and the central regulation electronics (CRE). The PDHS consists of a CompactPCI industrial Pentium PC running Linux, some microcontrollers and commercial PCI cards. It collects data coming from video cameras and the CRE, stores it on hard disk and interfaces with the ISS computers. Although a real-time link to ground exists, most of the data needs to be stored on hard disk. A removable hard disk will travel by space shuttle to give the scientists their valuable measurement data.

Figure 1. Block diagram of the Declic facility. The power and data handling system is a Pentium-based system running Linux.

Figure 2. The sample cell unit of the ALI insert at the University of Amsterdam. The sample cell contains the liquid to be studied at a critical temperature. The blue box behind the SCU is the insert box.

Figure 3. The ALI insert for study of liquids at critical point at low temperatures. The right side contains the microcontrollers and electronics for thermal control and scientific data acquisition.

Figure 4. Bart van Deenen Doing Thermal Testing of the ALI Insert

Figure 5. ALI Insert Box with Sample Cell Unit

Figure 6. The DSI insert for solidification experiments at COMAT in Toulouse (France). The hole at the top is for accessing video cameras once located inside the Declic facility.

Figure 7. CRE Electronics Box to Be Located near the PDHS

Figure 8. CRE Electronics Box during Testing

Figure 9. The HTI (High Temperature Insert) during manufacturing at the University of Amsterdam; this insert will be used to study water at 373°C.

Temperature control is handled by the CRE. The regulation electronics and software are able to control different types of thermostats inserted in the first box. In the previous experiments, one used fluids with a critical point of about 45°C; this will be done again in the first insert.

Another insert will study the critical point of water, which is near 373°C. At this temperature, water shows an unexpected aggressive behavior, which is scientifically very interesting. Currently, five different experiment inserts are being produced that will be situated inside Declic, all having different characteristics. For the critical point inserts developed in Amsterdam, we have chosen to use platinum resistors for temperature measurements, because it's the only sensor still functional at high temperatures that can maintain the required accuracy. All these sensors are directed to microcontroller boards with analog-to-digital converters of 24 bits. Still, these 24 bits are not enough to reach the high accuracy expected, so all values of the A/D converter first are filtered digitally by an FPGA, a programmable chip. The microcontroller sends the data to the CRE Pentium PC; the CRE CPU gathers all data from different microcontrollers and sends the collected data on a TCP stream to the PDHS.