The Mars Electrostatics Chamber (MEC) is an environmental chamber designed primarily to create atmospheric conditions like those at the surface of Mars to support experiments on electrostatic effects in the Martian environment. The chamber is equipped with a vacuum system, a cryogenic cooling system, an atmospheric-gas replenishing and analysis system, and a computerized control system that can be programmed by the user and that provides both automation and options for manual control. The control system can be set to maintain steady Mars-like conditions or to impose temperature and pressure variations of a Mars diurnal cycle at any given season and latitude. In addition, the MEC can be used in other areas of research because it can create steady or varying atmospheric conditions anywhere within the wide temperature, pressure, and composition ranges between the extremes of Mars-like and Earth-like conditions.
The MEC (see figure) includes access ports for installation and removal of experimental devices, and vacuum-feedthrough ports for connecting to the devices from the outside. Also included are feed-through ports for pressure sensors, thermocouples, and gas-supply tubes that are permanent parts of the apparatus. There also are access ports for visual monitoring of experimental devices.
The temperature in the chamber can range from a minimum of 150 K to a maximum of 473 K. The temperature at 48 different locations within the chamber is monitored by use of thermocouples. Temperature is controlled mainly by balancing (1) the inward leakage of heat from ambient temperature against (2) the removal of heat by circulation of a mixture of warm gaseous nitrogen and cold vaporized liquid nitrogen through a cooling shroud inside the chamber. The rates of flow of the warm and cold nitrogen are monitored by flowmeters and regulated by controllable valves. Additional heating is provided by tape heaters outside the chamber and additional cooling by a liquid- nitrogen cold plate.
Following initial evacuation, the chamber is backfilled with an atmospheric gas mixture (e.g., CO2 with small amounts of N2, Ar, O2, and H2O to the Martian atmosphere) at low pressure [typically between 6 and 9 millibars (between 600 and 900 Pa) for the Martian atmosphere]. Thereafter, pressure is brought to and maintained at the required value by use of a feedback control system that balances the rate of flow of atmospheric gas into the system against the rates of leakage and of vacuum pumping. The feedback control system includes a pressure sensor and a gasfeed throttle valve.
The composition of the gas is monitored by use of a separately operated residual-gas analyzer, the output of which is sent to the computerized control system. A mass flow controller maintains the desired relative concentrations of the gases in the atmospheric gas mixture.
A programmable logic controller (PLC) is the heart of the computerized control system. The PLC accepts inputs from a manual control panel, capacitance manometers, flowmeters, pressure controllers, and thermocouples. The PLC provides outputs to indicators on the manual control panel, and to the vacuum, heating, cooling, pressure, and gas-composition systems described above. Numerous outputs are sent to a graphical user interface (GUI) that features "soft" controls and indicators that emulate those of the manual control panel with the addition of elaborate graphical management capabilities. The GUI notifies the PLC when it is ready to accept or provide information relative to the control process. Optionally, the operation of the MEC can be controlled by use of the manual control panel alone, or partly by use of the manual control panel and partly by use of the GUI. This option affords flexibility for manually performing tests while maintaining safe operation by use of automatic control.
This work was done by Carlos Calle and Dean C. Lewis of Kennedy Space Center, and Randy K. Buchanan and Aubri Buchanan of VirCon Engineering. For further information, access http://technology.ksc.nasa.gov/WWWaccess/ techreports/2001report/200/207.html. KSC-12590