Lukasz Kowalski

Spherical Wind Sensor for the Atmosphere of Mars

A novel wind speed and direction sensor designed for the atmosphere of Mars is described. It is based on a spherical shell divided into four triangular sectors according to the central projection of tetrahedron onto the surface of the unit sphere. Each sector is individually controlled to be heated above the ambient temperature independently of the wind velocity and incidence angle. A convection heat rate model of four hot spherical triangles under forced wind has been built with finite element method thermal-fluidic simulations. The angular sensitivity of the tetrahedral sphere structure has been theoretically determined and compared with the tessellation of the sphere by four biangles. A 9-mm-diameter prototype has been assembled using 3-D printing of the spherical shell housing in the interior commercial platinum resistors connected to an extension of a custom design printed board. Measurements in Martian-like atmosphere demonstrate sensor responsiveness to the flow in the velocity range 1-13 m/s at 10-mBar CO2 pressure. Numerical modelization of the sensor behavior allows to devise an inverse algorithm to retrieve the wind direction data from the raw measurements of the power delivered to each spherical sector. The functionality of the inverse algorithm is also demonstrated.

Heat Flow Dynamics in Thermal Systems Described by Diffusive Representation

The objective of this paper is to analyze the dynamics of heat flow in thermal structures working under constant temperature operation. This analysis is made using the tools of sliding mode controllers. The theory is developed considering that the thermal system can be described using diffusive representation. The experimental corroboration has been made with a prototype of a wind sensor for Mars atmosphere being controlled by a thermal sigma-delta modulator. This sensor structure allows to analyze the time-varying case experimentally since changes in wind conditions imply changes in the corresponding thermal models. The diffusive symbols of the experimental structures have been obtained from open-loop measurements in which pseudorandom binary sequences of heat are injected in the sensor. With the proposed approach, it is possible to predict heat flux transient waveforms in systems described by any arbitrary number of poles. This allows for the first time the analysis of lumped and distributed systems without any limitation on the number of poles describing it.

Active Control of the Surface Potential of Nanostructured Layers

The objective of this letter is to show the first results obtained with a control designed to keep the average surface potential of a nanostructured layerconstant. This condition is equivalent to keeping constant the resistivity of the layer measured at a constant reference temperature. The proposed closed-loop control achieves this objective by adequately changing the average temperature of the temperature waveforms applied to the nanostructured layer. Experiments are shown on which the control is applied to a layer made of Au-functionalized WO3 nanoneedles.

Low pressure spherical thermal anemometer for space missions

A novel spherically shaped thermal anemometer for low pressure, Mars-like conditions, is described. The concept has been designed using finite element multyphysics simulations to find out the thermal conductance to the ambient for varying conditions of wind speed and direction and ambient pressure and temperature. A prototype 1cm diameter suitable to work under these environment in the range 0.25-10m/s speed has been build using 3D printing and tested inside a low pressure chamber. The protopype shown has two separated hemispheres, independently heated above the ambient temperature, providing angle sensitivity in one plane. Measurements of the heating power in both hemispheres required to keep an overheat of 30K are shown as a function of the wind direction showing good sensitivity at 0.5m/s. One improvement of this sensor is the means provided to also heat the core of the sphere where the circuit board is located thereby avoiding most of the conduction losses to the supports. The concept is scalable to other wind speed and pressure conditions and also to full 3D measurements.

Hypobaric chamber for wind sensor testing in Martian conditions

In this paper we will describe the installation of a low pressure stainless steel horizontal chamber to be used in preliminary tests of a Mars atmosphere wind sensor. It includes custom made additional QF ports, electric and pneumatic interfaces together with a vacuum pump system. Absolute pressure, as a function of the pump operational time during the vacuum process and depressurization will be shown as a result of experimental measurements. We will propose the integration of a typical compact wind tunnel with interior of the vacuum chamber. We will also discuss the possibility of the reproduction the same Reynolds number conditions for a given low temperature of a pure carbon dioxide atmosphere just by fine adjustment of either air pressure or air velocity inside the chamber at typical laboratory temperature. Finally, we will recapitulate all pros and cons of this hypobaric chamber.

Using a Second Order Sigma-Delta Control to Improve the Performance of Metal-Oxide Gas Sensors

Controls of surface potential have been proposed to accelerate the time response of MOX gas sensors. These controls use temperature modulations and a feedback loop based on first-order sigma-delta modulators to keep constant the surface potential. Changes in the surrounding gases, therefore, must be compensated by average temperature produced by the control loop, which is the new output signal. The purpose of this paper is to present a second order sigma-delta control of the surface potential for gas sensors. With this new control strategy, it is possible to obtain a second order zero of the quantization noise in the output signal. This provides a less noisy control of the surface potential, while at the same time some undesired effects of first order modulators, such as the presence of plateaus, are avoided. Experiments proving these performance improvements are presented using a gas sensor made of tungsten oxide nanowires. Plateau avoidance and second order noise shaping is shown with ethanol measurements.

Active surface potential control in nanostructured MOX layers

The objective of the paper is to present the implementation of the first sigma-delta control of Surface Potential (SP) for nanostructured layers made of metal oxides in gas sensors. The objective of this type of controls is to improve the time response and reliability of these sensors. This is done by generating adequate temperature waveforms aiming at generating a constant surface potential in the nanostructures. By enforcing the condition constant SP the dynamics of all state variables is altered.