AMars “sample caching rover” mission designed to collect, document, and package samples for future collection and return to Earth was recommended as the highest-priority mission for 2013–2022 by the 2011 Planetary Decadal Survey. A key premise of the Mars 2020 rover, which will gather samples for potential future return, is that it should be possible for the samples to be packaged and left on Mars for an extended period of time (at least five Mars years) without loss of scientific value. Two particularly important characteristics of a sample are the structure and relative positioning of the rock fragments and grains. These characteristics can be affected by shock and vibration that could fracture the sample and create relative movement among the fragments and grains, and therefore cause loss of valuable scientific information.
The Mars 2020 mission is proposing to store samples in individually sealed, thin-wall sample tubes. To preserve the structure and relative positions of the rock fragments and grains in these samples, a method to cage the samples mechanically in the sample tube is desired that could mitigate fracturing during shock events, as well as reduce the potential movement of rock fragments and grains. The caging method is desired to be compatible with a smooth, thin-wall tube (no internal features to assist with retention or caging); passive activation; and robust to variations in tube diameter, surface roughness, dust, temperature, vibration, and shock.
A cam plug design exhibiting low insertion force and high removal force was developed to seal the tube and cage the sample. A spring-loaded plug holder was also designed to house and dispense the plug in the tube.
The cam plug design consists of a pair of cams to provide retention, a housing to hold the cams and maintain alignment inside the tube, an axle for the cams to rotate upon, a torsion spring to deploy the cams and provide initial surface contact force on the tube surface, wipers to clear off dust particles from the tube wall to both create a better sealing surface and provide a means to retain/cage small particles, spacers to space the wipers to allow for deflection of the fingers during plug insertion, and retaining rings to retain the wipers and spacers on the ends of the plug.
The cams are designed with a logarithmic spiral profile that allows the cams to maintain a desired camming angle (and therefore be designed for a particular coefficient of friction between the cam and tube) that can adapt to a range of tube diameters (the tube diameter could vary due to machining tolerances, abrasion and deforming during coring operations, and dust accumulation). When the cam plug is inserted into a tube, the cams push inward, allowing low insertion force. When the cam plug is attempted to be pulled or pushed back out of the tube (e.g., through forces generated by acceleration on the rock mass), the cams engage onto the tube wall and lock the plug in place. The torsion spring helps maintain continuous contact against the tube wall, as well as reengage contact if slipping occurs (providing resiliency). Additionally, the cam design allows the plug to travel further down the tube while maintaining caging if samples later compact within the sample tube due to settling, vibration, or shock events.
The plug holder mechanism holds onto the plug with a ball lock that can be released by pressing up against a spring. A kickoff spring provides positive pressure to help release the plug from the plug holder into the tube.