186 Mechanism Advantages  

The STIG mechanism has several features that make it well -suited for its function within the coring drill.  

Simple and Robust Shifting Algorithm   Initiating a gear shift with STIG is essentially two steps. The STIG lead screw is pos itioned according to the  desired dual-output actuato r ratio. Then the spindle actuator is rotated until a stall is sensed. Because a  spring is used in series between the lead screw and the selector, there is no need for alignment of the  spindle actuator out puts before initiating a shift. In other words, it is not necessary to ensure that the selector  can shift out of one gear and directly into the other. Instead, the spring preload force pulls the selector out  of one interface  and into a neutral zone, and li ghtly loads the selector against the desired actuator output  until alignment occurs at a later time. Without the spring, loss of position knowledge during a rover fault  would be extremely problematic, as no force sensor is in place to allow a hunt -and-peck  style of shifting .  

No Need for a Dedicated Gear Selection Actuator   Because of the connection between the STIG and CBLO lead screws, the STIG gear cannot be chosen  independently of the corer’s CBLO position and independently of operations related to the sample tube.  This interconnectiv ity allows for STIG to be incorporated into the corer without the need for a dedicated  gear selection actuator. Although it may initially appear that this would limit the number of corer  configurations  possible (one CBLO pos ition while STIG interfaces with the high speed spindle output, one  CBLO position while STIG interfaces with the high torque spindle output), two STIG configurations map to  four CBLO positions. The STIG spring allows this mapping.  

Full Functionality in B oth Output Rotational Directions   STIG retains the ability to provide motion in either rotational direction to the bit at either torque level by  avoiding the use of ratchets or impact drivers . This ability greatly benefits sampling operation algorithms  and fault recoveries. For example, the bit can be rotated in the non- drilling direction while being retracted  from a borehole, reducing the chance of becoming stuck. Additionally, if the bit becomes stuck during high  torque breakoff, the bit can  be rotated with high torque in the opposi te direction to alleviate a jam.  

No Sacrificial Components   A pressure of single digit MPa is all that is sustained by the axial faces of the selector and actuator during  a shift . Contact geometry at the interface is planar. Because of the low contact pressure and presence of  wet lubrication (Braycote 600EF), there is no need for a clutch. Mechanism life is not expected to be driven  by any sliding interface. As mentioned earlier, an actuator stall indicates the completion of a gear shift. So  during a shift and prior to said stall, spindle speeds may be kept very low. Because of these low speeds  and the low axial pressure applied to the parts, no debris generation is expected. The slow spindle speeds  used while seeking a stall are necessary before commanding high speed motion that could otherwise  damage the splines without full engagement .  

Dual -Output Spindle Actuator  

The dual -output actuator consists  of a four-stage planetary gearbox, with a unique concentric output shaft  arrangement. A cross section can be seen in Figure 9. Several challenging requirements included a bit - seizure fault load case of 68  Nm on the high speed shaft, which is approximately 10X the n ominal operating  torque load , while minimizing mass, overall envelope size and maintaining low overall drag torque over a  wide operational temperature range of - 70°C to +70°C.  

A typical gearbox design would use ball bearings to support the planet gears in the first stage and increment  up to higher capacity bearings as the stages progress to the output stage. The spindle gearbox used a ball  bearing first stage, while the second  and fourth  stages required a larger size ceramic hybrid needle roller  bearing to handle the bit -seizure case and the high loads of the low -speed output shaft. The third stage  utilized a smaller sized roller bearing to minimize mass and overall size. Traditional approaches to support