Mars Science Laboratory Drill
- Wednesday, 01 August 2012
The drill spindle mechanism (DSM), nested within the chuck/spindle subassembly, provides the torque to rotate the bit for drilling and unlocking the fresh bit assemblies from the bit box. The mechanism is actuated by an electrically commutated gear motor that drives the spindle shaft via a spur gear train. The maximum mean contact stress in the bearings is kept low to prevent lube degradation. Mounted to the shaft is a dirt-tolerant torque coupling that transmits torque to the bit. The coupling accommodates axial, radial, and angular motion between the bit and spindle shaft to permit the following functions: the transmission of the hammer blow directly onto the bit, the mating of a fresh bit, and release of the bit both in free space and under load.
The drill chuck mechanism (DCM), also residing within the chuck/spindle subassembly, enables the drill to release worn bits and take hold of fresh ones stored on the rover front panel. The design driver was not just to survive a worst-case load scenario — the complete slip of the rover on a Martian slope — but to release the bit while subjected to it.
The drill percussion mechanism (DPM) generates the impact needed to break the rock and the dynamic (vibration) environment required to move powdered sample through the DBA. The mechanism operates at 1,800 blows-per- minute and has variable impact energy levels that range from 0.05 to 0.8 Joules. The DPM is a functionally simple device consisting primarily of a hammer assembly, energy storage spring, and housing/linear bearing assembly. The DPM is actuated by a long-stroke voice coil that is operated using an open loop voltage drive method. Within the DPM is an array of reed switch sensors that provides coarse hammer position telemetry.
The drill translation mechanism (DTM) provides the linear motion of the bit, spindle, chuck, and percussion drill subassemblies for the following functions: maintaining 120 N weight-on-bit (WOB) during sample acquisition, generating the retraction force to extract the bit from the hole, and mating to a fresh bit in the bit box.
The dual-bridge force sensor is required to sense the low WOB because the nominal axial load is too low to be observed in the actuator current telemetry. The inner diameter of the force sensor is axially clamped to the ball nut. The force sensor outer diameter is axially constrained between two preloaded wave springs. The force sensor and wave springs are housed in a gimbal assembly, which couples the translation mechanism to the translation tube. The gimbal isolates the ball screw and force sensor from radial and bending loads.
This work was done by Avi B. Okon, Kyle M. Brown, Paul L. McGrath, Kerry J. Klein, Ian W. Cady, Justin Y. Lin, and Frank E. Ramirez of Caltech, and Matt Haberland of MIT for NASA’s Jet Propulsion Laboratory. NPO-47523
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