The Centaur 2 (C2) platform is a compact vehicle with four independently steered and actuated wheel pods, allowing the vehicle to pivot in place and tilt in two directions. It is designed to interface with and carry the anthropomorphic robot torso of Robonaut 2. There are two nearly identical interface mounting locations on opposite sides of the vehicle body; each provides both power and data channel access. To explore soil-moving capabilities of this versatile platform, an articulated excavator was required for transporting raw material (soil) to an analog volatile extraction processor.
The NASA Glenn Digger is the excavator built specifically for the C2. The Digger, when mounted to C2, was designed to shovel existing loose regolith from the terrain, similar to a traditional tractor “front loader,” then raise the loaded bucket up and deposit the load into a hopper of a predefined height. The hopper would prospectively be the input to a machine that would produce oxygen from lunar regolith that is needed for long-term lunar habitation. The final design of the Digger consists of two parallel arms coupled together and supporting a bucket on one end. In addition to the bucket rotating, the arms are rotationally actuated at their “shoulder,” giving it two degrees of freedom. Multiple strain gauges were implemented as an embedded force measurement system in the excavator’s arms. These strain gauges can accurately measure and resolve multi-axial forces on the excavator.
The shoulder is powered by two drive motors, each attached to a 10:1 planetary and 160:1 harmonic gearbox for a final ratio of 1600:1. The motors are each controlled by an AMC motor driver, and operate in a master slave configuration where the slave motor follows the position of the master. A power-off enabled brake is installed on the arm master drive to provide a holding torque when power is turned off. The transmission for the bucket also consists of a 10:1 planetary and smaller 160:1 harmonic gearbox. A chain attaches the output of the harmonic drive to the bucket. This configuration allows for the motors and drives to be contained within the arm and protected. The bucket has curved edges where the sides join the main length panels. This helps assure evacuation of the bucket and makes the structure stronger. Teeth were added to both leading edges of the bucket to mitigate excavation forces. The final mass of the Digger is 94 kg, 56 kg less than the allowable limit.
The main controller is attached to the base plate between the shoulder drive motors. The purpose of the main controller is to coordinate the movements of the arm and bucket with C2’s requests while reading the sensors and providing feedback to C2. The sensors on the Digger are two absolute position sensors on the arm and bucket, a relative position encoder on each motor, and four full bridge strain gauges, two on each arm. All sensor data is converted to serial data and transmitted to the main controller.
In order to measure the forces exerted on the arms of the excavator, four full bridge strain gauges were attached to the arms, two on each side. In normal operation, the gravitational force of a load will introduce forces in Z and X directions. When the arms of the excavator are moved upwards and positioned at some angle relative to the horizontal line, the gravitational load force will be decomposed to both directions relative to the arm.
Adding strain gauges to the Digger provides a good estimate of reaction forces at the C2 vehicle. Also from these measurements, other reaction forces can be established, such as the load carried in the bucket. The location of the strain gauges is important for this design. The highest force resolution is capable from attaching the strain gauges at the highest strain location. Also, since two strain bridges are used on each arm, forces can be resolved in any direction.
By knowing the load in the bucket, efficient and automated digging processes can be developed.