Previous robotic sample return missions lacked in situ sample verification/quantity measurement instruments. Therefore, the outcome of the mission remained unclear until spacecraft return. In situ sample verification systems such as this Distributed Capacitive (DisC) sensor would enable an unmanned spacecraft system to re-attempt the sample acquisition procedures until the capture of desired sample quantity is positively confirmed, thereby maximizing the prospect for scientific reward.
The DisC device contains a 10-cm-diameter pressure-sensitive elastic membrane placed at the bottom of a sample canister. The membrane deforms under the weight of accumulating planetary sample. The membrane is positioned in close proximity to an opposing rigid substrate with a narrow gap. The deformation of the membrane makes the gap narrower, resulting in increased capacitance between the two parallel plates (elastic membrane and rigid substrate). C-V conversion circuits on a nearby PCB (printed circuit board) provide capacitance readout via LVDS (low-voltage differential signaling) interface. The capacitance method was chosen over other potential approaches such as the piezoelectric method because of its inherent temperature stability advantage. A reference capacitor and temperature sensor are embedded in the system to compensate for temperature effects.
The pressure-sensitive membranes are aluminum 6061, stainless steel (SUS) 403, and metal-coated polyimide plates. The thicknesses of these membranes range from 250 to 500 μm. The rigid substrate is made with a 1- to 2-mm-thick wafer of one of the following materials depending on the application requirements — glass, silicon, polyimide, PCB substrate. The glass substrate is fabricated by a microelectromechanical systems (MEMS) fabrication approach. Several concentric electrode patterns are printed on the substrate. The initial gap between the two plates, 100 μm, is defined by a silicon spacer ring that is anodically bonded to the glass substrate. The fabricated proof-of-concept devices have successfully demonstrated tens to hundreds of picofarads of capacitance change when a simulated sample (100 g to 500 g) is placed on the membrane.
This work was done by Risaku Toda, Colin McKinney, Shannon P. Jackson, Mohammad Mojarradi, Harish Manohara, and Ashitey Trebi- Ollennu of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47690
This Brief includes a Technical Support Package (TSP).
Distributed Capacitive Sensor for Sample Mass Measurement
(reference NPO-47690) is currently available for download from the TSP library.
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Overview
The document presents a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the development of a Distributed Capacitive Sensor (DisC) for measuring the mass of planetary samples in future robotic sample return missions. This technology aims to address the limitations of previous unmanned missions, which lacked in-situ instruments for quantifying sample quantities before returning to Earth.
The core of the system is a proof-of-concept Sample Verification System (SVS) that utilizes a pressure-sensitive elastic membrane, positioned at the bottom of a sample canister. This membrane deforms under the weight of accumulating samples, causing a change in capacitance between the membrane and a rigid substrate placed nearby. The capacitance change is measured using C-V conversion circuits, which provide a readout via a Low-Voltage Differential Signaling (LVDS) interface. This method was chosen for its temperature stability advantages over other techniques, such as piezoelectric sensors.
The document outlines the materials used in the construction of the SVS device, including pressure-sensitive membranes made from Aluminum 6061, Stainless Steel (SUS) 403, and metal-coated polyimide, with thicknesses ranging from 250 μm to 500 μm. The rigid substrate is fabricated from a 1 to 2 mm thick glass wafer using a Micro-Electro-Mechanical Systems (MEMS) fabrication approach. The initial gap between the membrane and substrate is defined by a silicon spacer ring, which is anodically bonded to the glass.
The SVS device has demonstrated significant capacitance changes, ranging from tens to hundreds of pico-farads, when subjected to weights of several hundred grams, indicating its potential effectiveness in measuring sample mass accurately.
The document emphasizes the importance of this technology for maximizing scientific returns from future missions, as it allows for real-time verification of sample quantities, thereby enhancing mission success rates. The research was conducted under NASA's sponsorship, and the findings are intended to support broader technological, scientific, and commercial applications.
Overall, the Technical Support Package highlights the innovative approach taken by JPL in developing a reliable and efficient method for in-situ sample mass measurement, which is crucial for the success of upcoming planetary exploration missions.
