Phoenix_Robotic_Arm.pdf

The Phoenix Mars Lander Robotic Arm

Robert Bonitz, Lori Shiraishi, Matthew Robinson, Joseph Carsten, Richard Volpe, Ashitey Trebi-Ollennu

Jet Propulsion Laboratory

California Institute of Technology

4800 Oak Grove Drive

Pasadena, CA 91109-8099

Robert.G.Bonitz,Lori.R.Shiraishi,Matthew.L.Robinson,Joseph.Carsten,volpe,ashitey@jpl.nasa.gov

Raymond E. Arvidson

Department of Earth and Planetary Sciences

Washington University in Saint Louis

Saint Louis, MO 63130

arvidson@rsmail.wustl.edu

P. C. Chu, J. J. Wilson, K. R. Davis

Honeybee Robotics Spacecraft Mechanisms Corporation

460 W. 34th Street

New York, NY 10001

chu,wilson,davis@honeybeerobotics.com

Abstract —The Phoenix Mars Lander Robotic Arm (RA)

has operated for 149 sols since the Lander touched down on

the north polar region of Mars on May 25, 2008. During its

mission it has dug numerous trenches in the Martian

regolith, acquired samples of Martian dry and icy soil, and

delivered them to the Thermal Evolved Gas Analyzer

(TEGA) and the Microscopy, Electrochemistry, and

Conductivity Analyzer (MECA). The RA inserted the

Thermal and Electrical Conductivity Probe (TECP) into the

Martian regolith and positioned it at various heights above

the surface for relative humidity measurements. The RA

was used to point the Robotic Arm Camera to take images

of the surface, trenches, samples within the scoop, and other

objects of scientific interest within its workspace. Data

from the RA sensors during trenching, scraping, and trench

cave-in experiments have been used to infer mechanical

properties of the Martian soil. This paper describes the

design and operations of the RA as a critical component of

the Phoenix Mars Lander necessary to achieve the scientific

goals of the mission. 12

primary 90-sol (sol = 1 Martian day) mission of scientific

exploration in the vicinity of the landing site [10]. Favorable

conditions permitted extending major operations through

149 sols after which limited energy permitted only restricted

operations consisting primarily of collecting weather data.

The RA was an essential system for achieving the scientific

goals of the Phoenix mission by providing support to the

other science instruments as well as conducting specific soil

mechanics experiments [11]. One of the primary mission

goals was to analyze soil samples in the TEGA [1], and

MECA [2][3]. The RA supported these goals by acquiring

both surface and sub-surface dry and icy soil samples from

the area in the vicinity of the Lander and dumping the

samples into the TEGA and MECA inlet ports. Subsurface

samples were acquired at varying depths from within

trenches excavated by the RA. Very strong icy soil was

encountered at shallow depths (~2 to 7 cm).

In addition to delivering samples, the RA positioned the

RAC to take images of the surface, trench floor and end

walls, rocks, and dumped soil piles. During soil sample

acquisition, the scoop was positioned for the RAC to take

close up images of the soil samples in the scoop prior to

delivery to the TEGA or MECA. There is a specially-

designed depression in the scoop blade to contain small

samples that allowed for very close imaging by the RAC at

high spatial resolution. The arm positioned the RAC for

imaging of the TEGA and MECA entry ports for pre- and

post-delivery assessment of the delivery process. The RAC

was also used for stereo imaging of the surface to generate

digital elevation maps (DEMs) for use by the RA operations

team in building RA command sequences.

T ABLE OF C ONTENTS

1. I NTRODUCTION .................................................................1

2. S YSTEM D ESCRIPTION .....................................................2

3. S URFACE O PERATIONS .....................................................5

4. C ONCLUSION ..................................................................10

A CKNOWLEDGEMENTS ......................................................10

R EFERENCES ......................................................................10

B IOGRAPHY ........................................................................11

1. I NTRODUCTION

The Phoenix Mars Lander (Figure 1) touched down on the

north-polar region of Mars on May 25, 2008 and began its

The RA inserted the TECP [9] probes into the soil to

characterize thermal and electrical conductivity. The RA

also positioned the TECP to measure atmospheric relative

humidity from 0.15 m to 1.8 m above the surface.

2 IEEEAC paper #1695, Version 5, Updated December 15, 2008

The RA performed its required activities successfully

enabling the science instruments to analyze samples in- situ

from which to infer physical properties, geomorphology,

chemistry, and mineralogy of the north-polar region.

The joint actuators consist of brushed DC motors with

multi-stage speed reduction consisting of planetary gears

and a harmonic drive (except the wrist, which has a bevel

gear at the output of the planetary gears). Braking is

achieved by actively shorting the motor leads to slow the

motor until magnetic detents capture the rotor. The detents

provide sufficient holding torque to assure no slippage

while power is off. Position sensing is accomplished via

quadrature encoders at the motor shaft and potentiometers at

the joint output. The encoder counters are initialized based

on potentiometer data or by running each joint up against a

known mechanical hardstop located at the end of each

joint’s travel. The encoder counts are stored in flash

memory at the end of each day for use during initialization

the following day. Each joint is equipped with a heater and

temperature sensor to assure that the motor operation is

conducted at or above minimum operating temperature.

The RA workspace is located on the north side of the

Lander and is depicted in Figure 6.

Figure 1 ─ Phoenix Mars Lander depicted as winter

approaches

2. S YSTEM D ESCRIPTION

Mechanical Overview

The RA (Figure 2) is a 4-degree-of-freedom manipulator

with a back-hoe design that provides motion about shoulder

yaw (azimuth), shoulder elevation, elbow, and wrist pitch

[4]. The arm was designed to withstand the harsh

environment at the landing site: diurnal temperature

excursions from -90

to -20

The scoop is divided into a front chamber that collects

materials excavated by the front blade and a rear chamber

that houses the rasp and collection area for samples

produced by rasp. The two chambers are separated by a

labyrinth through which material is transferred from rear to

front by rotating the entire scoop about the RA wrist axis.

See Figure 5.

Figure 2 ─ Engineering model of the 2.4m-long Phoenix

RA in the Payload Interoperability Testbed at the

University of Arizona

The front chamber is used to collect regolith during digging

and sample acquisition. This chamber includes a funnel

feature which channels acquired material into a slot,

improving the accuracy of sample delivery (Figure 4). The

scoop and funnel are made of aluminum with a clear

anodize for abrasion and wear resistance. This coating on

the interior surfaces also reduces glare when viewing

acquired samples with the RAC. A tungsten carbide

secondary blade on the bottom side of the scoop provides a

means of penetrating harder materials and is used primarily

for scraping on indurated material.

C, CO 2 atmosphere, pressure as

low as 5 Torr. The arm links are made of aluminum with

titanium end fittings. The end effector consists of the

forearm-mounted RAC, and the scoop and TECP mounted

on the wrist (Figure 3).

Figure 3 ─ RA end effector tool suite: scoop with

primary and secondary blades for scraping and a rasp

for acquiring icy soils; RAC for imaging soil and rock

targets, instrument ports, and the scoop interior; and

TECP for measuring temperature and electrical

conductivity

Figure 6 ─ Area reachable by the scoop is shown in

green and represents the digging workspace on the

Martian surface

Rasp

Figure 4 ─ Scoop interior showing the channel used to

guide the material sample when delivering to the TEGA

and the MECA

The rasp is high-speed cutting tool mounted on the back of

the scoop and is the primary tool for acquiring icy soil

samples. The bit is made of tungsten carbide and is driven

by a brushed DC motor. Torque is transferred from the

motor shaft to the rasp cutting bit through a set of spur gears

and a pair of miter gears, yielding an overall speed

reduction of 1.25:1 from the motor to the bit. The motor is

current up to the limit to the motor during operation. The

device includes resistive strip heaters that warm the

mechanism to allowable operational temperatures. A

temperature sensor on the motor provides temperature

feedback. All components used to power the rasp cutting bit

are thermally isolated from the main scoop body by an

insulating material. This thermal isolation helps ensure that

the icy-soil samples are kept cold, thus, minimizing loss due

to sublimation.

Figure 5 ─ Cut-away view of the scoop showing the rasp,

rear and front scoop chambers, and primary and

secondary blades

Icy soil acquisition is accomplished by first preloading the

scoop against the surface so that the spring-loaded rasp

cutting bit retracts into the scoop. A knurled contact plate

grips into the icy soil to prevent motion of the scoop during

rasp operations. The rasp cutting bit is then energized,

resulting in rapid penetration of the surface, causing a

plume of cuttings to ballistically enter the rear chamber of

the scoop. The rasp cutting bit is mounted within a pivoting

housing, allowing the bit to protrude out of the scoop during

sample acquisition as well as to retract fully into the scoop

during preloading. The pivot housing is spring loaded using

a torsion spring. The torsion spring acts to force the cutting

bit into the material, providing the necessary load on the bit

for penetration into icy soil.

The rasp is also used to provide vibration to help move

samples from the rear chamber of the scoop to the front

chamber and down the front funnel to the instrument inlet

ports. The vibration is provided by a cam feature attached to

the rasp cutting bit that engages a low-friction surface,

which is grounded with respect to the scoop. As the rasp bit

rotates at high speed, the cam feature causes the pivot

housing to oscillate such that the spring-loaded housing

impacts its lower hard stop once per revolution, causing

high-frequency vibration.

(C&DH) computer over a serial link. Firmware running on

the RAE microprocessor provides for low-level motor

command execution to move the joints to the specified

positions, heater and rasp command execution, analog-to-

digital calibration, and sensor monitoring.

Algorithms and Software

The RA flight software resides on the Lander Command

and Data Handling computer and provides the following

functions:

•

Bio-barrier

To prevent contamination of the Martian sub-surface with

Earth organisms per NASA planetary protection policy, the

RA meets Category IV-B bio-burden requirement [5]. In

order to the meet this requirement, the RA was sterilized

prior to final integration onto the Lander and encased in a

bio-barrier [13] (Figure 7). The bio-barrier maintained

sterilization during the journey to Mars and was deployed

shortly after landing on the Martian surface.

Initialization (load parameter table, collision object data

base, and state files; request power on);

Expansion of high-level task commands (e.g., dig);

•

Generation of arm movement trajectories;

•

Validation of collision-free motion paths;

•

Control of arm motion, joint heaters, and rasp;

•

Setting parameters (e.g., motor current limits) in the

RAE;

Reading sensor data and monitoring the arm status;

•

Fault detection and recovery;

•

Sending arm sensor data to telemetry.

Figure 7 ─ RA shown encased in the bio-barrier that

maintained sterility to prevent contamination of the

Martian sub-surface with Earth organisms. The bio-

barrier was held in place by a series of latches that were

released shortly after landing by a pyro-activated pin

puller. Torsion springs at each end then rotated the ribs

(right to left in the figure) to open the bag.

The RA has a full suite of arm motion commands that

provide for coordinated joint motion as well as Cartesian

motion of the end effector [8]. Joint moves are specified as

either absolute moves or relative moves to the current

position. Cartesian moves are specified as absolute or

relative moves with respect to the payload coordinate frame

located at the base of the RA. The operator specifies

Cartesian motion in the local frame of the currently-selected

tool (scoop, scraping blade, TECP, rasp load plate, or

RAC). The four degrees of freedom for Cartesian position

are specified as the three translation coordinates plus the

angle that the currently selected tool approach vector makes

with the plane of the Lander deck (except for the RAC

whose orientation cannot be controlled separately). Each

motion command is broken up into a series of via points

that are sent sequentially to the RAE for execution by the

firmware.

The arm is also commanded to perform more complicated

tasks such as digging a trench or acquiring a sample using

the scoop by a single command. The software expands the

high-level command into the appropriate set of motion

commands which are executed sequentially. This approach

saves uplink bandwidth and eases the burden on the

operator in developing complicated command sequences.

The software also tracks time and energy resources used

during command execution and will gracefully terminate

operations when allocations are exceeded. This feature was

most useful when digging a trench due to the uncertainty of

the soil properties that affect the execution of the dig trench

command.

Control Electronics

The RA Electronics (RAE) consists of two printed-circuit

boards located in the lower Payload Electronics Box (PEB)

and provides power conditioning; motor voltage control and

drivers; heater drivers; joint encoder counting; and analog-

to-digital conversion of potentiometer voltages, temperature

sensor voltages, motor currents, and heater current. A

board mounted externally on the PEB limits the current to

the rasp to prevent overheating. The RAE provides the

interface to the Lander Command and Data Handling

•

In addition to providing for control of the free-space arm

motions, the software is also capable of executing guarded

moves where the arm moves towards its commanded

position until contact is made. This is accomplished by

monitoring motor currents and computed joint torques

versus preset thresholds. Guarded moves are employed

when inserting the TECP into the ground, preloading the

rasp load plate on a hard icy surface, acquiring samples, and

digging trenches. Thus, RA operation is robust with respect

to surface location uncertainty.

which was then imported into the RSVP. An example is

shown in Figure 8.

RA command sequences were then generated using RSVP’s

built-in sequence editor. The RA operator visually verified

that the commanded RA motion in each sequence was as

planned using RSVP’s motion simulator prior to uplink to

the Lander.

Image data were used to verify the results of each terrain-

interacting RA activity including assessing trench profiles,

rasp placement and ejecta, post-scraping surface

topography, TECP insertion impressions, quantity of sample

acquired, and the sample delivery process. RA sensor data

was imported into RSVP and arm motions played back to

verify completion of arm activities. The playbacks were

very useful in determining where indurated layers (e.g., ice

table) were encountered during trenching since they visually

showed where the arm went into its surface accommodating

mode and when hard material impeded arm motion (Figure

9).

To aid in safety and increase autonomy, the RA software is

capable of detecting and recovering from faults and

anomalous events. Faults and events are defined as follows:

Fault - inability to complete a command due to failure of

hardware (sensor, actuator, electronics, etc.);

Event - inability to complete a command due to anything

other than a fault (e.g., arm motion impeded by a rock in

the digging path).

If a fault or event is detected, the fault or event type is

reported in telemetry. Depending on the fault or event

detected, the RA software either attempts to recover from

the fault or event or place the arm in a safe configuration.

When the RA encounters conditions that impeded its motion

during digging (a rock in the soil, encountering the icy soil

table, etc.), the software engages a built-in accommodation

algorithm, similar to the one reported in [7] to compensate

for this condition by adjusting the scoop trajectory and, if

necessary, dumping the scoop contents and re-executing the

digging motion.

The primary operations tool for commanding the Phoenix

RA is the Rover Sequencing and Visualization Program

(RSVP) [6] used on Pathfinder and the Mars Exploration

Rover projects and adapted for Phoenix. RSVP provides

target designation from panorama image data, generates

command subsequences via programmed macros, simulates

arm motion, checks for collisions, estimates command

durations, and outputs a complete command sequence file

for uplink to the Lander.

Figure 8 ─ RSVP with DEM of the terrain from SSI

images acquired from sols 0 through 84

3. S URFACE O PERATIONS

Overview

This section includes a brief description of the process used

to operate the arm and the results of some example

activities. RA operations depended heavily on imaging data

from the Surface Stereo Imager (SSI) and the RAC. The

SSI and RAC were used to generate digital elevation

models (DEMs) of the RA workspace for use in defining

targets for RA activities (digging, scraping, sample

acquisition, rock pushing, and TECP insertion into the

surface). Each RA activity that involved interaction with

the terrain was preceded by imaging and DEM generation

•

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