Rock Abrasion Tool (RAT)
The Rock Abrasion Tool (RAT) is an integral part of the Athena Science payload; it was produced to act as the payload's geologic hammer and more. The RAT exposes fresh surfaces of martian rocks to other instruments on the payload. The RAT also brushes dust and debris from an excavated hole or an unaltered rock target in addition to its rock hammer equivalency role. To accomplish these tasks, the RAT is a sophisticated 3-axis precision-controlled device that will allow it to act also as a rock physical properties science instrument. The returned RAT data will be inverted and compared to a library of Earth rocks.
The RAT is 128 mm long and is contained within a circle 85mm in diameter and has a mass of 687 grams.
Information in this instrument description is taken from the Rock Abrasion Tool Mars Exploration Rover mission paper [GOREVANETAL2003]. See this paper for more details.
The chief scientific objectives of the RAT are:
- to remove the outer, exposed surface of rocks,
- to provide data such as motor currents so torque can be directly calculated and empirically correlated to the density and hardness of the rock and any coatings that have formed on the rock, and
- to collect information on the mineralogic and magnetic properties of the abraded rock by collecting magnetic dust on four magnets mounted on the lower part of the RAT housing.
It is necessary to calibrate the RAT prior to each grinding operation in order to compensate for certain time-variant or temperature-dependent parameters. Calibration begins with the initialization for the RAT Z-axis. This is required to correct for undetectable migration of the Z-axis that may be caused by vibration experienced during Rover traverses. To initialize the Z-axis, the motor is 'homed' using its hard stop, thus establishing an absolute reference frame for subsequent position moves. Next, the Rotate motor 'no load current' is calibrated. This is necessary to account for changes in drive train friction, lubricant viscosity and dust buildup over time and temperature range. To calibrate the Rotate motor 'no load' current, both the Rotate and Revolve motors are energized at operational levels for a preset period of time. Sensor values are averaged over this period and updated parameters are stored for subsequent use. Likewise, the Rotate motor 'seek-scan voltage' is calibrated in the same manner. It is preferred, but not required, that the RAT be calibrated in free space, before being placed against a rock surface. This minimizes the chance of encountering obstructions during calibration.
The RAT is required to be able to grind 5 mm below the surface of a rock after it has been properly positioned against an appropriate rock surface. An appropriate rock surface is considered a surface with local deviations no greater than +/-5 mm within a 50 mm radius from the point where the RAT Z-axis intersects the surface plane. It is the responsibility of the control of the Instrument Deployment Device (IDD) to position the RAT against an appropriate rock surface such that the RAT's Z-axis is within +/-15% from the rock's surface normal. The IDD must preload the RAT against the rock surface with a force no less than 10N and no greater than 100N. Once the RAT has been positioned on the rock, the grinding process is handled completely by the RAT with no additional manipulation required by the IDD or Rover.
Several important operational constraints were levied upon the RAT grinding operation. First, the pose of the (IDD) must not be disturbed during the grinding operation. Second, the grinding operation must not expend more than an allotted amount of energy. Third, the grinding operation should take no longer than a preset time limit. The RAT control approach was driven primarily by these operational requirements and constraints. An emphasis was placed on flexibility and modularity, so that an operator can easily tune the grinding operation by changing key parameters.
From an actuator/sensor perspective, the RAT is an electrical interface similar to other devices (IDD, mobility) on the Rover. The RAT has three DC brush motors, and each motor has an incremental quadrature encoder for position feedback. The RAT also has redundant discrete switches to sense contact with the rock. Finally, the RAT has a temperature sensor and motor heaters to monitor the mechanism's temperature and ensure that the RAT is within operational temperature limits prior to beginning operations.
The RAT relies on the Rover control electronics for low-level servo control and sensor processing. The Rover motor controllers allow the motors to be commanded in one of three modes: servo to position, servo to velocity and open-loop voltage mode. The low-level servo sample rate is on the order of 1 kHz. Motor current sensing is also provided by the Rover control electronics. Current limits are enforced at multiple levels (controller and application software) to prevent damage to the actuators, grinding wheels and IDD.
The RAT is mounted on the end of the IDD.
The Rock Abrasion Tool's main function on Mars is to remove the outer, exposed surface of rocks. In order to accomplish this task, the RAT is designed to cut approximately 5 mm into the rock and expose the fresh, unweathered surface. During the grinding operations, the RAT records data such as motor currents, motor positions, temperature, and instrument state. Torque can be directly calculated from motor current data and empirically correlated to the density and hardness of the rock. Combining these parameters with a close-up view of the fresh surface from the Microscopic Imager (MI), the scientist should be able to assess the compositional and the depositional histories of the rock.