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Nano and Micro Systems

Readout electronics on the bottom side of the proof-of-concept DisC sensor.

In Situ MEMS Sample Verification Sensor System Developed for Future Sample Return Missions

A MEMS sample verification sensor system (SVS) has been developed for in situ mass measurement of planetary rock and soil samples during future robotic sample return missions. Past robotic sample return missions such as the Russian Luna, NASA’s Genesis and Stardust, and JAXA’s Hayabusa missions did not have an in situ sample acquisition verification system. As a result, confirmation of successful sample acquisition could only be done after the return of the sample capsule to Earth. The in situ SVS would enable the unmanned spacecraft system to positively confirm the acquisition of the desired sample quantity prior to its return.

Step-by-step assembly of process of the DisC sensor inside a sample canister part.

The SVS contains a 10-cm-diameter distributed capacitive sensor or the DisC sensor, comprising of a pressure-sensitive elastic membrane at the top fixed in close proximity to an electrode-patterned bottom substrate. This forms a parallel plate capacitor with multiple segmented electrodes. When placed at the bottom of a sample canister, the top membrane deforms under the weight of an accumulating planetary sample, thus changing the capacitance of the sensor proportional to the sample mass (below). The concentric distributed capacitor design divides the overall sample weight measurement range into discrete steps that can be then be identified as baseline, minimum, mission target, and maximum. The sensor is designed to be inherently robust against shock as a rigid bottom substrate prevents the top membrane from overstressing. This sensor stack forms the false bottom of a sample canister. It is fastened to the canister bottom using pressure-loaded fasteners, which also help mitigate thermal stresses by accommodating relative dimensional changes of the assembly due to CTE mismatch under large temperature variation. At the back of the sensor substrate, a capacitance readout electronics PCB is integrated that transmits change in capacitance data via a low-voltage differential signaling (LVDS) interface. The sensor is designed to prevent sample contamination. The proof-of-concept sensor has shown high sensitivity of 1pF/gram at low mass range while maintaining capability to measure large mass, in excess of 1,500 grams. This development is a joint effort between MDL and the Robotics section within JPL.

Graph showing the variation of capacitance of segmented electrodes with respect to changing load.

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