Aerodynamic Multisensor

It is well known, dust and electrical charge play an important role in the Martian environment. Planet-wide windstorms have been observed to last for several months and dust devils up to 10 kilometers high are seen with daily frequency. The electrostatic interactions between dust particles carried by the wind create several difficulties for landing missions thus a prototype of an aerodynamic electrometer to measure the electrostatic and triboelectric properties of Mars atmospheric dust has been constructed (left). The instrument uses the embedded sensor technology to evaluate materials exposed to dust laden Martian winds and this knowledge provides the necessary input parameters for design of landers, rovers, and habitation facilities. This instrument consists of an array of insulating materials with each material backed by a miniature electrometer. It was designed to be exposed to mineral particles typical of the Martian environment. The overall sensor array has an aerodynamic configuration to minimize turbulent flow. The electrometer sensor uses a simple reference capacitor design as was used on the Mars Environmental Compatibility Assessment (MECA) Electrometer, a flight-ready instrument that included five sensors in a line array with a resolution of 5 million elementary charges. The probe consists of a field-sensor electrode that is enclosed by a guard electrode, which in turn is enclosed by an electrically grounded shield. The probe is embedded in a cylinder to within 2.5 mm of the surface The overall gain of the electronic circuit is 0.25 pC/mV. The current version of the instrument contains six sensors to measure the electric field induced by any net charge on six different insulator surfaces. The charge develops through frictional contact between the cylindrically shaped insulators and incident granular material.

The schematic of the Aerodynamic Multisensor showing a cutaway of the shield, guard, and the sensing electrode embedded in the polymer material. The dust laden flow deposits charge onto the surface that induces charge across a known reference capacitor (not shown). The voltage read across the capacitor is directly corrolated to the charge deposited onto the surface of the cylinder. The output voltage of the electrometer is a measure of the local electric field that is induced on the electrometer's probe sensor electrode. The amount of charge that develops on the insulator surface can be determined from the output voltage using the circuit gain 0.25 pC/mV.
A prototype of the aerodynamic electrometer and its associated electronic housing is shown with a single cylindrical sensor/guard probe embedded in a Lucite cylinder (0.75 inch diameter). A similarly sized Teflon cylinder is also shown with a coating of Martian soil simulant dust on its front surface that was produced at 5 mbar (air) using a prototype dust impeller system. This prototype system is used to simulate a wind storm and mineral particles were propelled toward the instrument at speeds reaching 30 m/s.
Windborne dust particles were generated using a dust particle impeller (left) that was developed at Kennedy Space Center to simulate a Martian dust storm in a vacuum chamber. Constant wind speeds of 20 m/s have been measured with the dust impeller operating at gas pressures ranging from 5 mbar to 1 bar. The dust impeller is placed in the same small vacuum chamber that contains the prototype electrometer. After the impeller is turned on, a small amount of granular material is propelled towards the cylindrical electrometer. Data is presented below for Teflon and Fiberglass cylinders that were struck in separate experiments by JSC Mars-1 simulant, SiO2, and Fe2O3.

Fiberglass and Teflon were chosen due to their separation on opposite sides of the triboelectric series. Most materials when rubbed onto Fiberglass will charge it positive, whereas Teflon would most likely charge negative. The cylinders were exposed to windborne dust particles in separate experiments. Data were taken in a vacuum chamber containing a room temperature CO2 atmosphere at 9 mbar. The vacuum chamber is backfilled twice with CO2 to 133 mbar, and then pumped back down to 9 mbar before data were taken. The testing granular materials were JSC's Mars-1 simulant, SiO2, and Fe2O3.

Each graph contains labels indicating the onset of the charging process that occurs after the dust agitator is turned on, and the subsequent discharging process. Sensors reveal the magnitude and sign of the electrostatic charging due to the interaction between airborne particles and the surfaces of the materials, as well as the charge developed when the materials are rubbed against the soils.

As proposed, this work involves many commercial as well as space exploration applications. Examples of commercial applications include measurement and control of triboelectric charging in xerographic toners, pharmaceutical powder handling, and semiconductor manufacturing.