MSSTM Gripper and Grasping Fixture

Needs for a gripper in the Mars Sample Return Mission

The study examined the following elements of the Surface Sample Handling System:

• Sample discharge from a drill into a sample vessel (SV).
• Positioning of multiple vessels under the drill, by manipulating the Sample Container (SC).
• Packaging the samples in vessels and un-capping/capping the vessels to hermetically contain the samples, 
• Transfer of the Sample Container, which contained multiple sample vessels, to the Mars Ascent Vehicle
• either from a rover (mobile case) or from the lander (static case). 
• An End-effector (gripper), for the robotic-arm; this is used for grasping the sample container (full of sample vessels), locking the two halves of the container together and securing it in the Mars Ascent Vehicle.

Possible realisations of the end-effector

A key element of the sample transfers is the tool on the end of the robotic arm, used to grasp the sample container and to secure the two halves together. Three concepts were investigated, each with a common screwdriver gear train for tightening the bolt that secures two halves of the sample container, but using a different gripping technique. The Bayonet end-effector (locking jaws for gripping) emerged as the best solution because of its high operational reliability, simplistic design, lower mass, volume and power requirements. The mechanism can provide a tightening/un-tightening torque up to 100Nm. Alternative designs were the ‘three-finger’ design and the ‘inner-jaw’ design.

The design chosen for the MSSTM end-effector

The End–Effector (EE) is designed to ensure that, the robotic arm can mate it with the corresponding grapple fixture (GF) placed at the SC bottom-half, even in presence of initial important misalignments. The "nose" of the EE is shaped so that it can enter the GF and "guide" the robot motion to achieve at the end very tight fitting. Then, a motor within the EE activates three locking blades, which rotate into recesses of the GF, locking the EE to the the sample-container bottom-half. Finally another powerful motor within the EE, actuates the central HEX key (which in the process of grasping has engaged into the corresponding allen bolt in the GF). The rotation of the key can attach or detach the SC bottom-half to its location into the the drill or into the MAV.

The MSSTM EE has proven itself in a thorough programme of testing

The EE underwent a programme of test to confirm the quality of the implementatio. The programme included:

• Alignment tests: to guarantee the matching of the EE interface to the GF against the possible misalignment ranges.
• Locking tests: to verify the proper locking of the EE to the GF
• Tightening tests: to verify engagement of the HEX drive in the allen bolt
• Torque capacity tests: to verify that EE was able to tighten a screw with a torque of 40 Nm.
• Thermal tests: to guarantee the functionality of the EE at the very low Martian tempera- tures
• Dust tests: to verify that the EE would operate even in presence of dust accumulation
• Locking cycling tests: to verify that the EE would maintain its performance even after several operation cycles

Key Results

The Following key results were achieved in the design, manufacturing assembly and testing of the end effector:

• Self-alignment, utilising the spherical shaped nose, met or exceeded the design target (+/- 5 mm, +/- 5°), including in the presence of dust.
• Locking and tightening motor currents were well within expected values, but increased significantly when dust is present.
• Operation from -55 to +20C was successfully demonstrated.
• Locking cycling tests were successful over a ‘life-cycle’ of over 35 cycles. with no un-intended release of the sample container.
• Some further detailed design work is needed for the hex-key, to prevent it jamming in the presence of dust. Further dust testing would be also be required, to quantify the ingress of dust into bearings.