Robotic Arm (RA)

Instrument Overview

The Phoenix Robotic Arm (RA) is a 2.4-m long arm mounted on the Phoenix Mars lander. It has three joints. The shoulder joint, where the RA is attached to the lander deck, has two degrees of freedom: motion in azimuth and elevation. The elbow and wrist joints each have one degree of freedom allowing further motions in elevation. Attached to the RA are: a scoop, a Thermal and Electrical Conductivity Probe (TECP), and the Robotic Arm Camera (RAC). The RA was designed to acquire samples of martian soil, dig trenches to uncover subsurface ice, modify the terrain close to the lander, and serve as a means to insert the TECP into the soil. The scoop has a drill bit on its back; after rasping icy soil, a series of RA motions was often used to send the material to the front of the scoop. Both the scoop and TECP were mounted on the wrist joint of the arm. Motor currents, link lengths, and joint positions of the RA were used to determine the force the RA exerted during its motions and the position of the scoop during these motions. This information was useful in monitoring the safety of the instrument and has the science return of allowing analysis of soil properties at the landing site. For more details, see Bonitz et al. 2008 [BONITZETAL2008], Arvidson et al. 2009 [ARVIDSONETAL2009], and Shaw et al. 2009 [SHAWETAL2009].

Scientific Objectives

The science objectives of the RA are the following: (1) Collect soil samples from various depths and locations relative to polygonal landforms. Deliver these samples to onboard instruments. (2) Uncover subsurface ice. (3) Obtain vertical (and in some cases horizontal) profiles of data relevant to scoop - soil interactions (e.g. during scraping and excavation). This data can be used to determine physical properties of the soil (e.g. soil cohesion). (4) Expose trench walls to aid in determining the presence/absence of fine-scale soil/ice layering. (5) Position the RAC for viewing of various targets in the workspace and below the lander deck. (6)Place the TECP in soil targets of interest; also place it at various heights in the air for relative humidity measurement. (7) Support any science investigations that require force to be applied (e.g. moving a rock, pressing down on soil surfaces). (8) Rasp icy soil for delivery to on-deck instruments.

Calibration

The RA was calibrated both in the field (Death Valley, January 2000) and in the Payload Interoperability Testbed (PIT). Calibration of the RA's ability to position itself accurately and then measure its position was conducted by commanding a series of movements and using a laser tracker for a second position measurement. Calibration of rasp cutting torque was conducted with a dynamometer for measurement of output torque and force; this was done for the range of expected temperatures. See Bonitz et al. 2008 [BONITZETAL2008]. Position was repeatedly calibrated during mission operations. Maximum force limits were also reset frequently during the mission.

Operational Considerations

The quality of the data was affected by the characteristics of the surface. The data was collected at higher temporal resolution on sols when the spacecraft was expected to be able to send higher data volume to Earth.

Electronics

The RA is controlled by two circuit boards located in the lander's Payload Electronics Box (PEB). These circuit boards are responsible for power conditioning, motor and heater drivers, motor and heater current, motor and temperature sensor voltages, potentiometer voltage conversion, and joint encoder counting. Another board on the PEB controls rasp current. The PEB also provides sensor monitoring and command execution for joint movement and heater and rasp function. The PEB is responsible for communication with the lander's Command and Data Handling computer. See Bonitz et al. 2008 [BONITZETAL2008] for more information.

Location

The instrument operated in the northern plains of Mars (68.22N, 234.25E areocentric; [HEETETAL2009]). The test instrument operated at the University of Arizona, Tucson, Arizona, USA.

Operational Modes

The RA employed variable frequencies of data collection. Maximum force limits were also variable.

Subsystems

The RA consists of the following subsystems: (1) links (e.g. upper and lower arm), (2) joint actuators, potentiometers, and encoders (3) scoop with rasp and two blades, (4) temperature sensors, (5) TECP, and (6) RAC.

Measured Parameters

Measured parameters include: time of observation, position of scoop at time of observation, and forces exerted at time of observation.