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MOBILE ROBOT POSE TRACKING FOR PERFORMANCE ANALYSIS

Alan M. Lytle*, Kamel S. Saidi, William C. Stone

Materials and Construction Research Division

Building and Fire Research Laboratory

Michael Shneier

Intelligent Systems Division

Manufacturing Engineering Laboratory

National Institute of Standards and Technology

*Building 226, Room B144, MS-8611, Gaithersburg, Maryland 20899

alan.lytle@nist.gov, ph. 301-975-6048

ABSTRACT: The NIST Construction Metrology and Automation Group, in cooperation with

the NIST Intelligent Systems Division, is researching robotic structural steel placement as part of

a project to develop an Automated Steel Construction Testbed. The initial phase of this project

centers on tracking a six degree-of-freedom robotic crane with a laser-based site measurement

system to provide position feedback for autonomous steel assembly. Follow-on efforts will use a

high-resolution LADAR scanner co-registered with the site measurement system to provide

world model data. The combination of these two advanced metrology systems provides an

opportunity for testing performance characteristics of mobile intelligent systems.

KEYWORDS: intelligent control, intelligent systems, performance metrics, 3-D coordinate

measurement systems

1.0 INTRODUCTION

The NIST Construction Metrology and

Automation Group (CMAG) is developing a

robotic structural steel placement system for the

testing and validation of advanced tools,

methodologies, and standards for automated steel

construction. This system, the first phase of the

NIST Automated Steel Construction Testbed

(ASCT), will demonstrate autonomous “pick and

place” assembly of structural steel components

using a six degree-of-freedom (DOF) robotic

crane and an external pose estimator [1].

The base platform is the NIST RoboCrane,

which is an inverted Stewart platform parallel

link manipulator. The pose (position and

orientation) estimator, a laser-based site

measurement system (SMS), provides absolute

cartesian position feedback to RoboCrane’s

Real-Time Control System (RCS) for trajectory

planning and dynamic control. A world map of

the robot work volume including target

components and obstacles is created prior to

operations using the SMS.

Future work on the ASCT will include

incorporating a high-resolution LADAR (laser

detection and ranging) system to create and

update the world map, thus eliminating the

requirement for a human operator to digitize the

scene with the SMS prior to operations. The

LADAR scans will be meshed and then

registered to the SMS.
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The use of an independent, external

measurement system to track a mobile robot

within an environment mapped and registered to

the tracking system provides interesting

opportunities for conducting mobile robot

performance analysis. In a future experiment,

the NIST Intelligent Systems Division (ISD), in

cooperation with CMAG, will use this

combination of metrology instruments to test the

navigation and sensing systems on board the

NIST robotic HMMWV test vehicle as part of

the U.S. Army’s Experimental Unmanned

Ground Vehicle System (DEMO III) program.

This paper discusses the development of the

robotic structural steel placement system and a

proposed test methodology for the DEMO III

sensor package performance analysis.

2.0 ASCT

2.1 Operational Concept

Four laser transmitters are positioned on the site

perimeter to illuminate the work volume of

RoboCrane with reference beams (Figure 1). A

digital model of the construction plane including

any obstacles is created using the SMS digitizing

wand.

Figure 1: Illuminating the Work Site with the

SMS.

This world model is then updated with the

positions of the as-built structure and the target

beam through a process of automatic part

identification (barcode), part model database

access, and part fiducial point measurement

using the mobile digitizing wand (Figure 2).

Figure 2: Measuring component locations with

the SMS digitizing wand.

The current pose of RoboCrane is measured

from onboard SMS sensors, and the path planner

calculates the required transformations for beam

pickup and delivery. RoboCrane then executes

the movements (Figure 3).

Figure 3: Steel beam placement with

RoboCrane.

2.2 RoboCrane

RoboCrane is an innovative cable-driven

manipulator invented by the NIST Intelligent

Systems Division and further developed and

adapted for specialized applications over a period

of several years [2,3,4,5]. The basic RoboCrane
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is an inverted Stewart platform parallel link

manipulator with cables and winches serving as

the links and actuators, respectively. The

moveable platform, or “lower triangle,” is

kinematically constrained by maintaining tension

in all six cables that terminate in pairs at the

vertices of the “upper triangle.” This

arrangement provides improved load stability

over traditional lift systems and enables 6 DOF

payload control.

The version of RoboCrane used in the ASCT

project is the Tetrahedral Robotic Apparatus

(TETRA) (Figure 4). In the TETRA

configuration, all winches, amplifiers, and motor

controllers are located on the moveable platform.

The upper triangle only provides the three tie

points for the TETRA cables, allowing the

device to be retrofitted to existing overhead lift

mechanisms.

Figure 4: RoboCrane – TETRA Configuration

2.3 Site Measurement System

The SMS uses commercially available positioning

technology (3D-I) produced by Arc Second, Inc.

in the Constellation and Vulcan product

families. (3D-I, Constellation and Vulcan are

registered trademarks of Arc Second, Inc.)

These systems use stationary, active-beacon

laser transmitters and mobile receivers to provide

millimeter-level position data.

2.3.1 SMS Description

Both Constellation and Vulcan systems use

eye-safe laser transmitters to triangulate the

position of a tuned optical detector. Each

transmitter emits two rotating, fanned laser

beams and a timing pulse. Elevation is

calculated from the time difference between fan

strikes. Azimuth is referenced from the timing

pulse. The field of view of each transmitter is

approximately 290° in azimuth and +/- 30° in

elevation/declination. The recommended

minimum and maximum operating ranges from

each transmitter are 5 m and 50 m, respectively.

Line-of-sight to at least two transmitters must be

maintained to calculate position. The

Constellation receivers each track up to four

transmitters and wirelessly transmit timing

information to a base computer for position

calculation. The Vulcan system is a selfcontained digitizing tool with two optical

receivers on a rigid pole. A vector projection

along the line formed by the two optical

detectors allows 3-D measurement of the tool

tip. Vulcan can track only two transmitters at

one time; however, the transmitter selection can

be manually switched between any of the four

available. Recovery of positional data following

momentary signal blockage takes approximately

one second.

2.3.2 Prior 3D-I / Mobile Robot Integration

Early efforts to use this laser technology for

mobile robot navigation showed that although the

system was capable of guiding a mobile robot

[6], its use was restricted due to loss of track at

relatively low vehicle speeds [7]. Upgrades to
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the positioning technology continued and a

successful combination of indoor 2-D map

creation and autonomous navigation was

demonstrated in a research project at the

Rochester Institute of Technology [8] (Figure 5).

Subsequently, a single receiver Constellation

system was installed on an autonomous lawn

mower at the Carnegie Mellon University Field

Robotics Center and provided positional

reference in a large outdoor setting [9].

Figure 5: SMS Receivers on the RIT/SME

“RC” Robot.

2.3.3 The SMS on RoboCrane

Three SMS receivers are mounted on

RoboCrane at the vertices of the lower triangle

(Figure 6). The receiver locations are registered

to the manipulator during the initial setup process

in the local SMS coordinate frame. For

convenience, all measurements are calculated in

the local SMS coordinate frame, though if

required, mapping to an existing world coordinate

frame could be accomplished. Receiver timing

signals and diagnostic data are wirelessly

transmitted to a base station computer running

Arc Second’s proprietary position calculation

software. Position and SMS diagnostic

information is polled at approximately 7 Hz using

a NIST-developed data communications

application. Position data from the three

receivers are used to calculate RoboCrane’s

pose. Diagnostic data such as number of visible

transmitters, excess signal noise or multipath

reflections are also provided for each position

calculation and is used to assess the quality of

individual position fixes.

Figure 6: RoboCrane with SMS.

3.0 PERFORMANCE METRIC

ANALYSIS OF THE DEMO III SENSOR

SUITE

The NIST Intelligent Systems Division has

provided research services developing control

system architectures, advanced sensor systems,

and standards to achieve autonomous mobility

for various DOD unmanned ground vehicle

programs including the Army Experimental

Unmanned Ground Vehicle System (DEMO III)

[10]. In a future experiment, high-resolution

LADAR scanning and the SMS will be used to

quantify the performance characteristics of the

DEMO III sensor suite.

The test vehicle used by NIST is an Army

HMMWV instrumented with a number of

sensors (Figure 7). These include a LADAR

range sensor that returns a 90° by 20° range

image, a pair of color cameras for stereo

imaging, a color camera, a line scan LADAR,

and a wide-angle panoramic image mosaic

obtained by stitching images from three color

cameras. The vehicle also has an inertial

navigation system (INS) and a GPS receiver.

An artifact field consisting of variable-sized box

structures distributed within a nominal 30 m x 30

m area of clear terrain will be created on the
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NIST grounds. CMAG will create a “groundtruth” digital map of the artifact field and then

track the HMMWV test vehicle within that field.

Figure 7: NIST Robotic HMMWV Test

Vehicle.

The artifact field will be scanned from several

viewing angles using the using a high-resolution

LADAR. The LADAR frame data will then be

post-processed and manually meshed to create a

multi-view map of the entire test area (Figure 8).

The artifact field will also be measured using the

SMS digitizing tool to create control points to

overlay the SMS tracking data on the LADAR

map.

Figure 8: Sample High-Resolution Multi-Scan

LADAR Image (Traffic Intersection).

Three SMS receivers will be mounted on the

HMMWV test vehicle. These receivers will be

mounted high on the vehicle to ensure each

receiver has line-of-sight to at least two of the

four transmitters located on the perimeter of the

test field during tracking. Fiducial points on the

vehicle will be measured to map the SMS

receiver locations to the vehicle coordinate

system. The HMMWV test vehicle will then

traverse the test field at slow speeds and the

track data will be written at  ̃ 7 Hz. Track data

from each receiver will then be post-processed

to provide time-stamped vehicle pose (Figure 9).

Figure 9: Test Vehicle on Artifact Field.

The 6 DOF vehicle track measured by the SMS

will then be overlaid on the LADAR map. This

will then be compared to the world map created

by the DEMO III sensor suite to evaluate its

performance. The performance of individual

sensors and the performance of the system as a

whole will be measured.

There are a number of measurement goals for

the sensor characterization experiment. One is to

see how accurately each of the sensors can

measure the real terrain and the sizes, shapes,

and positions of objects on it. Others are to see

how well the sensors are registered and how

well their positions in the vehicle's coordinate

system can be determined. While the sizes of

the objects within the test range can be

measured accurately, the terrain is not entirely

flat, and the high resolution LADAR will be used

to determine ground truth. The SMS will be used
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to determine how well the INS system works.

Tables 1 through 5 provide manufacturer’s

performance data for the measurement systems

that will be used in the test.

4.0 CONCLUSION

The combination of high-resolution LADAR

imaging with a laser-based SMS provides an

opportunity to create a “ground-truth” digital

model of an autonomous vehicle’s environment

and then track the vehicle as it traverses, senses,

and models that environment. The “groundtruth” map, complete with time-stamped vehicle

pose, can then be compared to the platform’s

world model to quantify the performance of the

on board navigation and sensing suite. A future

experiment using the NIST HMMWV test

vehicle on an artifact site will evaluate this

method of mobile robot performance metric

analysis.

5.0 REFERENCES

1. Lytle, A. M., Saidi, K. S. and Stone, W. C.,

“Development of a Robotic Structural Steel

Placement System.” Proc., 19th

International Symposium on Automation

and Robotics in Construction, Washington,

D.C., September 23-25, 2002.

2. Stewart, D., “A Platform with Six Degrees

of Freedom,” Proc. of the Inst. of

Mechanical Engineering, Volume 180(15),

Part I:371-386, 1965-1966.

3. Albus, J. S., Bostelman, R. V., Dagalakis, N.

G., “The NIST ROBOCRANE, A Robot

Crane”, Journal of Robotic Systems, July

1992.

4. Bostelman, R., Albus, J., Dagalakis, N.,

Jacoff, A., “RoboCrane Project: An

Advanced Concept for Large Scale

Manufacturing,” Proc, Association for

Unmanned Vehicles Systems

International, Orlando, FL, July 1996.

5. Bostelman, R. , Albus, J., Stone, W.,

“Toward Next-Generation Construction

Machines.” Proc. Of American Nuclear

Society 9th International Topical Meeting

on Robotics and Remote Systems, Seattle,

WA, March 4-8, 2001.

6. Beliveau, Y. J., Fithian, J. E., and

Deisenroth, M. P., “Autonomous Vehicle

Navigation with Real-Time 3-D Laser Based

Positioning for Construction.” Automation in

Construction, (5) 1996.

7. Lytle, A. M., “Development of a Remotely

Piloted Vehicle (RPV) with Real-Time

Position Measurement (RtPM) for

Hazardous Waste Site Characterization.”

Master’s Thesis, Virginia Tech, Blacksburg,

VA, 1994.

8. Rochester Institute of Technology, Society

of Manufacturing Engineers (Student

Chapter 18), Robot Construction Project,

2001.

www.rit.edu/~smewww/project/rc/rc.html

9. Batavia P. H., Roth S. A., Singh S.,

“Autonomous Coverage Operations In SemiStructured Outdoor Environments.” Proc.,

IROS, Lausanne, Oct. 2002.

10. Shoemaker, C. M. and Bornstein, J. A.,

“The Demo3 UGV Program: A Testbed for

Autonomous Navigation Research.” Proc.,

IEEE International Symposium on

Intelligent Control. Gaithersburg, MD

1998.

Table 1: GDRS Area-Scan Ladar Specifications (HMMWV Test Vehicle).

Property Specification

8 laser beams, 1 rotating mirror With 8 facets

Scan resolution 32 lines x ?180 pixels

Scan coverage 20 x 90

Angular resolution 0.658 x 0.5

Maximum frame rate 60 scans/s but 30 scans/s
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Range 5 m to 70m (vertical surface)

Range resolution/standard uncertainty ±7.6 cm / 15 cm

Data measurement rate Range: 345,600 measurements/s

Day/Night Operation Range Independent of ambient light

Table 2: Real-Time Performance of Applanix POS LV 420 Intertial Navigation Unit

(HMMWV Test Vehicle).

POS LV 420-RT GPS Outage Duration (minutes)

(using DGPS) 0 min 1 min 3 min 5 min 10 min 20 min

X, Y Position (m) 1.0 1.5 1.75 2.0 2.5 3.5

Z Vertical Position (m) 1.5 to 2.0 2.0 2.0 2.0 2.5 3.0

Roll & Pitch (deg) 0.02 0.02 0.02 0.02 0.02 0.02

True Heading (deg) 0.02 0.02 0.02 0.04 0.06 0.10 0.20

Table 3: Performance of Applanix LV 420 with post-processing (HMMWV Test Vehicle).

POS LV 420 GPS Outage Duration (minutes)

(post processed) 0 min 1 min 3 min 5 min 10 min 20 min

X, Y Position (m) 0.02 0.12 0.40 0.75 1.5 2.5

Z Vertical Position (m) 0.03 0.15 0.50 0.65 1.0 2.0

Roll & Pitch (deg) 0.005 0.005 0.005 0.007 0.007 0.09

True Heading (deg) 0.02 0.02 0.02 0.03 0.035 0.035

Table 4: Manufacturer’s Specifications - Riegl LMS Z210 Ladar.

Property Specification

Scan coverage 80° x 330°

Angular stepwidth 0.072° to 0.36°

Angular readout accuracy 0.036°

Frame scan rate 1 °/sec to 15 °/sec

Minimum Range 2 m

Maximum Range 350 m (25 mm resolution, natural target)

Range resolution 25 mm or 50 mm, selectable

Standard uncertainty Resolution + Distance error of = ±20 ppm

Table 5: Manufacturer’s Specifications - Constellation and Vulcan.

Property Specification

Vulcan Constellation

Transmitter coverage 60° x 290° 60° x 290°

Transmitters required for position calculation 2 2

Maximum number of observable transmitters 2 4

Nominal Laser rotation rate 40 Hz to 50 Hz 40 Hz to 50 Hz

Minimum Range 5 m 5 m

Maximum Range 50 m 35 m

Angular resolution =90 μrad =90 μrad

Data rate (position calculation) 10 Hz 7 Hz
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Standard uncertainty (Instrument x,y,z – U66) ± (1 + D/10000) [mm]

D (mm) - distance between

transmitters

10*RSS (250, D*8) [μm]

RSS- Root Sum Squares

RSS(A,B) = {A2+B2}

1/2

D (m) –distance from farthest

transmitter

No point data averaging

DISCLAIMER: Certain commercial equipment, instruments, or materials are identified in this report in order to specify the

experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are

necessarily the best available for the purpose.