CONTROL STRATEGIES FOR APACTS MICROMACRO-MANIPULATOR INTEGRATION

for

Naval Surface Warfare Center

Carderock Division

9500 MacArthur Blvd.

West Bethesda, MD 20817-5700

Richard J. Norcross

Intelligent Systems Division

National Institute of Standards and

Technology

Gaithersburg, MD 20899-8230

NISTIR 6873

2002
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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Disclaimer

No approval or endorsement of any commercial product by the National

Institute of Standards and Technology is intended or implied. Certain

commercial equipment, instruments, or materials are identified in this report to

facilitate understanding. Such identification does not imply a recommendation or

endorsement by the National Institute of Standards and Technology, nor does it

imply that the materials or equipment identified are necessarily the best available

for the purpose.

Copyright

This publication was prepared by United States Government employees as part

of their official duties and is, therefore, a work of the U.S. Government and not

subject to copyright.

Acknowledgement

This report is partial fulfillment of sub-contract AM 02-9802001 with AmDyne

Corporation of Millersville, Maryland.
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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Abstract

The Carderock Division of the Naval Surface Warfare Center is developing the

Automated Paint Application, Containment, and Treatment System (APACTS).

APACTS will apply anti-corrosive and anti-fouling paints to ship hulls in an

environmentally sound manner. To provide accurate motion over very large

surfaces, the APACTS motion system employs a self-propelled base carrying a

long reach macro-manipulator, which in turn carries a quick response micromanipulator to maneuver the paint nozzle and containment device. The

manipulators run separate but coordinated trajectories whose combination is the

path of the paint nozzle. Based on feedback from sensors and the operator’s

observations, the controller shifts the path to keep the paint nozzle at the

appropriate position relative to the surface being painted. The micro- and macromanipulator trajectories must adapt to the ship hull and ensure the micromanipulator does not exceed its operating volume. This report compares

strategies for correcting the paint nozzle position. These strategies include; direct

modification of the macro-manipulator trajectory, modification of the micromanipulator trajectory during various portions of its trajectory cycle, and

alterations to the micro-manipulator trajectory. The experiments discussed in this

report show that the best results occurred when the micro-manipulator

trajectory shifts immediately upon error detection during the trajectory upstroke

and the macro-manipulator trajectory modifies to keep the micro-manipulator

centered. The results further indicate an extended period on the upstroke prior

to painting enhances overall system accuracy.
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Table of Contents

1 Summary..............................................................................................5

2 Introduction.........................................................................................7

3 Methods, Assumptions, and Procedures.........................................8

3.1 Manipulators..................................................................................9

3.2 Target Surface..............................................................................11

3.3 Trajectory.....................................................................................11

3.3.1 Micro-Manipulator Trajectory.............................................12

3.3.2 Macro-Manipulator Trajectory............................................14

3.4 Operator Interface.......................................................................15

3.5 Test Procedure.............................................................................16

4 Results and Discussions....................................................................18

5 Conclusions........................................................................................21

6 Recommendations............................................................................21

7 References..........................................................................................22

List of Figures

Figure 1. APACTS System Concept.......................................................5

Figure 2. Modified ATR-60 AWP............................................................9

Figure 3. Micro-Manipulator.................................................................10

Figure 4. DDG-51 Frame 420 Outline...................................................11

Figure 5. Sample Micro-manipulator Trajectory................................13

Figure 6. Rotation Correction...............................................................14

Figure 7. Sample Monitor Screen..........................................................16

Figure 8. Results of Runs on Vertical Surface.....................................18

Figure 9. Results of Runs on Hull Mock-up........................................19

Figure 10. Dot Plot of Micro Correction during Paint Stroke Run..20
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1 Summary

 The Carderock Division of the Naval Surface Warfare Center, Naval Sea

Systems Command is developing the Automated Paint Application,

Containment, and Treatment System (APACTS) to apply anti-corrosive and antifouling paints onto Navy ship hulls in an environmentally sound manner.

APACTS (Figure 1) employs three motion components: (1) a self-propelled

repositionable base, (2) a long reach macro-manipulator, and (3) a quick

response micro-manipulator to move the paint nozzle and containment device

along the hull surface. The combined motion trajectories of the macro- and

micro-manipulators produce the paint application trajectory. Feedback from

sensors mounted on the containment device modify the trajectories in real-time

to keep the nozzle at appropriate positions relative to the surface.

Figure 1. APACTS System Concept
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The sensors indicate the speed, the position and the orientation of the paint

nozzle relative to the surface. The sensors can be fully automatic or can include

interpretation by an operator. The controller uses the sensor data to adjust the

nozzle path to correctly apply the paint.

While the macro-manipulator trajectory follows the surface at a slow speed, the

micro-manipulator moves to keep the nozzle on sequential horizontal stripes. A

portion of the micro-manipulator trajectory is equal in magnitude but opposite

in direction to the macro trajectory. During these movements, the combined

motions hold the nozzle vertically steady over the surface. APACTS applies paint

during the steady period then steps down to align for the next stripe.

While the nozzle is steady, the difference between the nozzle position on the

micro-manipulator and any mark on the surface reveals the system’s vertical

position error. The controller can remove the error by shifting the micro

trajectory and, when the shifts accumulate, by modifying the macro trajectory.

In some instances and in some directions, the controller can correct the error by

directly manipulating the macro-manipulator’s trajectory. However,

experiments indicate that direct macro correction is inferior to micromanipulator based corrections that remove error by shifting the micromanipulator trajectory.

Previous experiments tested the system’s ability to coordinate macro and micro

motions in response to corrective inputs [ 9]. The experiments verified that the

interfaces were sufficient to coordinate and stabilize the two manipulators.

However, the system suffered periodic position errors that could cause

unacceptable gaps in the paint coverage.

The controller in [ 9] corrected the position errors only during the paint

application portion of the micro-manipulator trajectory. The initial experiments

reported here confirmed gaps in the nozzle position. The next experiments

attempted to correct the position error throughout the micro trajectory. The

controller generated a reference mark to show the proper tool position at all

times. The operator observed the position of the surface mark to the reference

mark and commanded trajectory shifts to close the observed position errors. The

system performance showed no improvement with these modifications. The

relative motion was too quick and the manipulator and display components

were too slow to permit accurate following.

Subsequent enhancements modified the trajectory definition. Most position

errors occurred during the initial portion of the paint stripe. The trajectory was

therefore modified to include a period that held the nozzle vertically steady prior

to the time for paint application. Thus the controller had the opportunity to

correct the position errors prior to paint application and the overall system

performance showed significant improvement.
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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2 Introduction

To guard against the harshness of the sea, ships are covered with anti-corrosive

and anti-fouling paints that must be periodically replaced to maintain their

effectiveness. During replacement, hazardous airborne particles (HAPs) are

inadvertently discharged into the environment diminishing the air quality and

endangering shipyard personnel and the surrounding harbor. The Naval Sea

Systems Command, Naval Surface Warfare Center, Carderock Division,

(NAVSEA NSWCCD), Environmental Quality Department conducts research

and development leading to fleet implementation of pollution-control materials,

processes, and equipment that enable Navy ships to be environmentally

responsible. NSWCCD is responsible for providing the Navy with the technical

expertise to solve existing and emerging waste management problems. Pursuant

to that responsibility, NSWCCD is developing the Automated Paint Application,

Containment, and Treatment System (APACTS) to significantly reduce HAP

discharge from the painting operation [1].

The primary components of APACTS are the delivery, containment, treatment,

and manipulation systems. The delivery system consists of a paint mixer,

strainer, sprayer, nozzle, and associated equipment. The containment system

surrounds the paint sprayer and includes a capture shroud, recovery vacuum,

hoses, and controls. The treatment system includes waste transport, waste

isolation, filter elements, and their support equipment. The manipulation system

consists of those devices that move the sprayer and containment shroud. The

components complement each other to produce an effective, economic, and

environmentally sound system.

The Intelligent Systems Division of the National Institute of Standards and

Technology (NIST-ISD) supports APACTS development through the

investigation of new and existing technologies to carry, maneuver, and

manipulate the APACTS sprayer and containment system. Because a single

manipulator would be unable to achieve all of the Navy’s performance

requirements at an acceptable cost, APACTS uses a series of three manipulators

to position the system about the dry dock, to reach along the hull, and to

maintain proper standoff and motion. After the mobile base positions APACTS

in or around the drydock, a long reach but slow response macro-manipulator

carries a high accuracy, fast response micro-manipulator to simultaneously

provide sufficient reach and accuracy.

The combination of dissimilar manipulators has several names including macromicro, macro/micro, maxi-mini, and major-minor. Many researchers have

investigated macro-micro control [2] - [8]. These approaches rely primarily on

either a well-defined trajectory or a well modeled pair of manipulators. While

none of these approaches adequately address the problems of working

throughout a very large volume in a poorly defined environment, several micromacro control strategies may be extended to the APACTS problem.
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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This report reviews the interaction between the sensors and the manipulator

trajectories, and between the micro-manipulator and macro-manipulator

trajectories. These experiments indicate superior results occur when: the micro

trajectory shifts immediately upon error detection, the micro-manipulator

trajectory includes an extended upstroke, and the macro-manipulator trajectory

changes to keep the micro-manipulator centered.

3 Methods, Assumptions, and Procedures

APACTS will paint wide vertical swaths on any hull surface by spraying

sequential horizontal stripes. The experiments discussed below, tested methods

to follow complex surfaces while periodically maintaining the manipulator end

point over evenly spaced surface positions. The experiment’s trajectories are the

vertical component of the APACTS trajectory.

The experiment’s trajectory periodically held the manipulator end point over a

series of marks on the target surface. APACTS used two manipulators connected

in series. The end point was held in place by driving one of the manipulators in

an equal but opposite direction of the other. The second manipulator (the micromanipulator) periodically moved in the same direction as the first (the macromanipulator) to step the end point to the next surface position. Given the size

and mass of the manipulator, the manipulator controller was unable to maintain

reasonably accurate positions, and position errors became evident during the

hover.

The experiment methodology assumed that the end-point errors were primarily

attributable to errors in the macro-manipulator trajectory. Different runs of the

experiment compared several error correction methods. One method permitted

the operator to directly adjust the macro-manipulator’s speed. In the other

methods, an operator observed the effect of the error on the tool position

through a camera on the micro-manipulator and, via a joystick, offset the micromanipulator’s trajectory as necessary to remove the error. The micromanipulator communicated the offsets through the micro-macro interface and

the macro-manipulator adjusted its trajectory to re-center the micromanipulator.

Each experiment began with an operator maneuvering the manipulator to a

random point at the high end of one of two test surfaces and initiating the

combined trajectories. The manipulator trajectories were similar for each

experiment and contained no a priori knowledge of the surface. A camera,

mounted at the end point of the second manipulator, fed the tool view of the

surface to a computer that logged the run. An evaluator replayed the log and

recorded the vertical position error at regular intervals during each stripe. The

evaluation was based on the percentage of times the end point was within some

tolerance within each stripe.
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3.1 Manipulators

The experiment used two serially-connected manipulators to carry a collection of

sensors along a target surface. A larger, slower manipulator (the macromanipulator) carried a smaller, faster manipulator (the micro-manipulator)

which carried a rod containing the sensors.

The macro-manipulator is a modified ATR-60 aerial work platform (AWP) from

Snorkel, Inc. of St. Joseph, MO. The standard AWP has digital proportional

valves on several of its actuators. The experiment’s AWP has similar digital

proportional valves on all actuators and each actuator is fitted with absolute

position and relative motion sensors. Servo control modules monitor the

actuator motion and adjust the oil flow through the valves to cause the actuator

to follow a motion path. A supervisory controller coordinates the actions of the

servo modules such that the AWP's basket can follow Cartesian paths or a

surface as shown in Figure 2. The macro-manipulator's supervisory controller [

10] updates the goal position at 8 Hz. The actuator controllers close the actuator

servo loop at 30 Hz.

Figure 2. Modified ATR-60 AWP

The experiment’s micro-manipulator has three serially-linked actuators that

position and orient a 60 cm (2 ft) bar within a vertical plane (Figure 3). The

manipulator uses a Dell Inspiron 3800 laptop computer running National

Instruments LabView under Windows NT to compute position goals for each of

the actuators every 15ms. An Inside Out Edgeport USB Converter delivers the

goal points from the laptop to the actuators via a serial line. The actuators are

Smart Motors (Anamatics, Inc. of Carlsbad, CA) operating in Anamatics'
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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“Extended Cam Mode” that smooths discreet point goals at the motor. A microcontroller (Little Star from Z-World, Inc) generates a constant 8533 Hz

quadrature encoder signal to drive a virtual input cam. The position goals vary

the output cam to produce the desired motion of the three actuators.

The manipulator carries three Migatron RPS-401A ultrasonic sensors to measure

the surface distance at three points in front of the manipulator. The microcontroller, that generated the simulated cam from the previous paragraph, reads

the sensor outputs and returns their values to the coordinating controller via a

serial line and the USB converter. The controller uses the data to compute the

distance, orientation, and curvature of the surface.

Figure 3. Micro-Manipulator

The micro-manipulator carries a small color video camera that provides position

feedback to the operator during the experiment. A paper target with regularly

spaced sets of colored lines cover the target surface. The position of those lines

on the video screen comprises the position feedback. A Macintosh 7600/120

computer running Avid VideoShop records the video images for subsequent

evaluation.

Color Video Camera

Ultrasonic Sensors

Target Surface

Macro-manipulator Basket

Micro-manipulator
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3.2 Target Surface

The manipulators followed two surfaces in the experiments. The first was a 12 m

(40 ft) high vertical wall. The second was a mock-up of frame 420 of a DDG-51

(Figure 4 and Figure 3). The frame had constantly changing curvature and a

minimum radius of curvature of 1.8 m (6 ft) which matched the original APACTS

goal [1].

Figure 4. DDG-51 Frame 420 Outline

0

2

4

6

8

10

12

-10 -5 0 5 10

Athwartship Position /m

Mock-up Portion

Targets, with evenly spaced sets of colored horizontal lines at 15 cm (6 in)

intervals, covered both test surfaces. The lines provided a visual reference for the

operator to monitor and correct the vertical position of the manipulator’s tool

point (i.e., the camera). During expected APACTS operations, the previous paint

line would provide the visual reference to the operator. Thus the surfaces were

reasonable representations of hull surfaces that APACTS is expected to

encounter.

3.3 Trajectory

The experiment uses the vertical component of the vertical compensation

trajectory discussed in [11]. Under the vertical compensation trajectory, APACTS

paints in vertical swaths, where a swath is a set of horizontal stripes painted
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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sequentially from top to bottom. The needs of the paint, the shipyard, and the

APACTS machinery determine the trajectory. When requirements permit, extra

cycle time can be used to enhance overall performance. The test trajectory

represents typical parameters with specific compensation for limits to the

available sensors.

In an APACTS application, the desired production rate, the paint spray width, the

paint application speed, the stripe width, and the acceleration limits determine

the micro-manipulator’s trajectory. Independent system and application

constraints limit each factor. For example, the micro-manipulator’s accelerations

generate forces on the macro-manipulator that can excite undesirable

oscillations. Thus the macro-manipulator’s stiffness limits the micromanipulator’s accelerations. Similarly, the micro-manipulator reach restricts the

stripe width. The characteristics of the specific paint batch dictate the paint

application speed and the spray width. The shipyard’s requirements set the

production rate.

Under most scenarios, more time is available for the cycle than required for the

vertical motion. The trajectory consumes the extra time: by increasing the

production rate, by reducing the acceleration limits, by providing an idle period

prior to the upstroke, or by including an extra up stoke prior to painting. Each

alternative has specific advantages. Increased production is always desirable.

Reduced accelerations improve the macro-manipulator handling. An idle period

minimizes the total vertical motion. And, as will be shown below, an extra up

stroke period improves system accuracy.

The test trajectory imitates an APACTS application with a 15 cm (6in) effective

spray width (aligned vertically), a 72 cm/s (32 in/s) nozzle speed (applied

horizontally), and 240 m2/h (2600 ft2/h) production rate. The nominal macromanipulator trajectory moves the basket vertically down a wall at 3 cm/s (1

in/s). The 72 cm/s nozzle speed and the 15 cm spray width are atypical.

However, limitations of the available vision sensor require the narrow spray

width and the increased nozzle speed compensates so other parameters are

representative.

3.3.1 Micro-Manipulator Trajectory

The micro-manipulator’s trajectory is a six segment quadratic spline with a

continuous first derivative. The paint application portion and the total cycle time

of the trajectory is the same for all tests of all of the control strategies. Likewise,

none of the trajectories include surface specific information and execute without

regard to the surface being followed or the starting point on that surface. During

execution, sensors on the manipulator detect the surface’s relative position and

orientation. The controller and operator monitor the surface position and shift

the trajectory to keep the tool point (i.e., the paint nozzle) over the proper

position of the surface.
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The micro-manipulator trajectory is a repetitive sequence of positions that

traverses only a portion of the manipulator’s range in any direction. In most of

the control strategies, the controller removes nozzle position errors by shifting

the micro-manipulator’s trajectory into the unused portion of the manipulator’s

range. For example, if the micro-manipulator can access between 10 and 30 cm in

a given direction, and the trajectory requires 10 cm of motion in that direction,

then operating between 18 and 28 cm rather than between 15 and 25 cm

removes a 3 cm nozzle position error.

Accumulated shifts can exceed the micro-manipulator’s operating volume.

Therefore the micro-manipulator periodically sends the accumulated shifts

(a.k.a., the offset) to the macro-manipulator. The offset triggers a change in the

macro-manipulator trajectory that reverses the position errors and re-centers the

micro-manipulator.

Figure 5. Sample Micro-manipulator Trajectory

-10

-5

0

5

10

0 5 10

Time /s

Paint

Stroke Relocation

Stroke

Added Upstroke

The experiment’s micro-manipulator has three degrees-of-freedom. Thus

trajectory shifts are available in three directions; towards and away from the

surface, up and down along the surface, and rotation about the axis formed by

the cross product of these axes. In the experiments, the operator commands the

correction along the surface while the controller generates the perpendicular and

rotation corrections.

Corrections must keep the manipulator in a position from which other

corrections may be made. The corrections along and parallel to the surface form

a rectangular box that extends from the manipulator’s fundamental trajectory.

From within this box, the controller may safely make subsequent corrections.

However, extensive rotations within the box brings subsequent motions into the
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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manipulator’s structure. For example, a ninety-degree rotation positions the

operating volume over the manipulator’s body. Therefore, instead of rotating

within the box, the controller rotates the rectangular box around a central point

(Figure 6) such that the correction box remains tangential to the surface and clear

of the manipulator’s structure. The rotation corrections are small and induce

offsets in the other directions that are removed by changes to the macromanipulator trajectory. The distance from the rotation point to center of the

corrections box is a significant factor when determining the Macro-Manipulator’s

trajectory.

Figure 6. Rotation Correction on Micro-manipulator

j3

j2

j1

r

3.3.2 Macro-Manipulator Trajectory

The macro-manipulator trajectory was the same for all runs of the experiment

on both surfaces. The macro-manipulator moved the micro-manipulator down

the surface at a slow, and slowly changing, speed. The controller modified the

macro-manipulator’s direction and speed based on sensor data.

The basic macro trajectory is a constant velocity jog. The macro-manipulator’s

nominal speed is the quotient of the Production Rate and the Swath width. For

these experiments, the trajectory begins at 3 cm/s (1 in/s). The jog direction is a

vector parallel to the surface. The controller determines the direction from the

sensors on the micro-manipulator. Both the direction and speed vary during the

run.

The sensors constantly measured the distance to the surface. From the sensor

distances, the sensor spacing, and the micro-manipulator kinematics, the

controller computed the slope of the surface and commanded the macromanipulator along that direction.

The macro-manipulator’s commanded speed is a function of the radius of

curvature of the surface, the radius of the orientation correction, and the nominal

speed.

Corrections Box
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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Speedmacro = Speednominal

R + r

R

(1)

Where

r is the distance used in the rotation correction.

R is the surface radius defined by:

 

R=

i

− i−1

W

(2)

i

is the mean surface angle of the i’th stripe.

W is the width of the stripe.

The controller computed the surface radius from the average surface angle

during the previous two paint strokes. Although, the surface angle was constant

during the paint stroke, the controller measured the angle through a

combination of the manipulator’s actuator positions and the sensor readings. The

combination generated noise in the measured surface angle that caused

unacceptable errors in the surface radius calculation. The mean of all of the

readings from the paint stroke provided a simple filter for the calculation of the

surface radius.

Modification of the macro-manipulator speed requires a similar modification to

the micro-manipulator speed. For the nozzle point to remain over a point on the

surface, the micro-manipulator must move at the same speed but opposite

direction from the macro-manipulator. Therefore, when the surface curvature

requires a change of the macro speed, the micro trajectory must be similarly

distorted. This distortion may be a change of the micro-manipulator speed or

multiple small shifts of the micro-manipulator trajectory. These experiments use

the latter approach.

The trajectories for the micro and macro-manipulators are independent but

intertwined. Neither trajectory requires a priori information of the surface.

Instead, the controller shifts and distorts nominal trajectories in response to the

real-time sensor readings of the surface. To keep the nozzle point stationary

over the surface, the upward motion of the micro-manipulator must be equal to

the downward motion of the macro-manipulator. The controller sets the macromanipulator’s speed as required by the surface and shifts the micromanipulator’s trajectory to generate the proper speed.

3.4 Operator Interface

The macro-micro control concept relies on observations of the task to determine

the motions of the manipulators. The macro-manipulator components do not

have the tight tolerances commonly found in servo controlled manipulators.

Thus the position based on actuator sensors and the kinematics calculation is

unreliable and other sensors must detect the relative pose of the tool point to the
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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task. While the full APACTS application may use sensors to automatically

generate the information, an operator generates the appropriate feedback

signals for this experiment.

The experiment's test surface was marked at 15 cm (6 in) intervals with

horizontal lines that represented the lower edges of the paint stripes. The micromanipulator carried a small camera, whose video signal was displayed on a

monitor at the control station. Markings, superimposed on the video signal, gave

the operator a reference to assess the position of the camera relative to the

surface. Figure 7 shows the reference box used during the control strategies that

modified the micro-manipulator’s position. The monitor displayed additional

marks to assist the operator in the control of motions away from the center.

Figure 7. Sample Monitor Screen

In the upper left corner of the operator’s monitor, the controller displayed the

distance from the sensors to the surface. The controller maintained the standoff

during the test runs. The monitor displayed the value as a check of the

controller’s performance for the operator.

The operator generated corrections via either a joystick connected to the

controller or a virtual dial on the controller interface. Through the joystick the

operator added up to ±7.5 cm/s (±3in/s) to the velocity of the micromanipulator tool point. The operator returned the joystick to zero once the

desired shift was observed on the monitor. Via the dial, the operator changed

the macro-manipulator speed. The dial included buttons to apply small changes

to the macro speed. Depending of the test, the operator used either the dial or

the joystick.

3.5 Test Procedure

These experiments compared four control techniques for maintaining nozzle

position over a large surface. The operator began each test run by maneuvering

the manipulator to a target line near the upper end of the test surface and

initiating each manipulator’s trajectory. As the manipulator followed the surface,

the controller collected distance data from the sensors and maintained the tool

point’s standoff and surface orientation. A camera provided the tool view of the
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

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surface to the operator who corrected the vertical position over the surface. For

each control technique the controller provided the operator with indicators that

assisted the operator’s corrections. A separate computer recorded the tool view

for subsequent evaluation.

For tests along the flat surface, the controller introduced an initial speed error to

the macro-manipulator. The error was between 30% and 50% of the nominal

speed. The operator was aware of neither the magnitude nor the direction of the

error. The tests along the hull mock-up did not include an intentional speed

error.

The “Micro Correct during Painting” (μCp) control approach allowed the

operator to adjust the micro-manipulator position during the micro trajectory’s

paint stroke. The controller displayed a reference box, and the operator

commanded small position corrections to the micro-manipulator. As the target

line rose or fell in the reference box, the operator commanded an up or down

shift in the micro-manipulator’s position. Through these corrections, the

operator kept the nozzle over the target line on the surface. Since the nozzle

point hovered only during the paint stroke, the controller provided feedback

and accepted the corrections only during the paint stroke.

Under the “Macro Correct” (MC) control approach, the operator took direct

control of the macro-manipulator speed. The macro-manipulator’s response was

too slow to allow the nozzle position to be returned to a specific surface target

line within a single paint stroke. Therefore, the controller displayed multiple

marks on the monitor. The operator attempted to keep the surface mark at the

same grid mark from paint stroke to paint stroke. For all other approaches, the

controller provided only a single centered reference mark on the monitor and

the operator performed the more difficult task of centering the target line. In

addition, video distortion caused the grid mark to cover a greater distance at the

edges of the monitor than at the center. These distinctions biased the results

slightly in favor of the MC control approach.

“Micro Correct during full cycle” (μCf) included a floating reference mark on the

tool view and allowed the operator to constantly make position corrections.

When the trajectory was in the relocation stroke, the controller computed the

location on the monitor the line should be during each control cycle and

displayed a moving mark for the operator to follow.

The “Micro Correct with Extended Upstroke” (μCu) approach added a section to

the micro trajectory prior to the paint stroke that matched the velocity of the

paint stroke. The operator used this portion to remove errors accumulated

during the relocation stroke and returned the nozzle position to within the

desired tolerance before the paint application.

To score a test, an evaluator replays a video of the operator’s monitor that had

been recorded during the test run. The evaluator records the vertical position

error at regular intervals during each stripe. The score is the percentage of times
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

18

the micro-manipulator end point is within ±0.8 cm (0.3 inch) of the surface

marking within each stripe. While the necessary accuracy requirement for

proper application of marine paints is unknown, the evaluation tolerance exceeds

the assumed requirements of the task. The evaluator collects the scores for each

stripe and computes the median, quartile, and extreme values to produce a net

evaluation for the control approach. The net evaluations provide a qualitative

comparison between the control alternatives.

4 Results and Discussions

These experiments compared different control strategies for maintaining the

APACTS nozzle on a horizontal stripe. One set of experiments were conducted

along a vertical surface, and another set were conducted on a mock-up of a ship

hull. The results compared the control technique’s ability to handle system and

surface induced errors.

This report compares control strategies via box plots. A box plot displays the

median value as a circle, the extreme values as vertical lines, and the upper and

lower quartile (the middle half of the data) as a box. The box plot provides a

rapid, qualitative comparison between alternatives[12].

Figure 8 shows the results of tests along the vertical surface. For these tests, the

experimenter adds a small random error to the macro-manipulator speed. This

speed generates position errors that the operator removes by modifying the

macro-manipulator speed (for the MC approach) or the micro-manipulator

position (for other approaches). The results compare the ability of the control

techniques to handle system errors.

Figure 8. Results of Runs on Vertical Surface

100

75

50

25

0

Points within ±8mm

/(%)

μ Cu μ Cf μ Cp MC None

Correction Technique

All of the micro-manipulator correction techniques performed well on the

vertical surface. Despite a favorable bias in the evaluation criteria, the MC
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

19

technique had difficulties removing the initial speed error. The macromanipulator responded slowly and the operator was required to observe the

effect of the correction before making subsequent corrections. When the

operator attempted to revise the correction too early the system became

unstable, running off one limit, then running off the other. However, once the

operator closed on the correct speed, the MC system performed well at

maintaining the position and several stripes were completely within the

tolerance.

The operator conducted a second set of experiments along the hull mock-up

(Figure 9). For these tests, the experimenter did not add an error to the macromanipulator speed. The test ran from a flat surface, around a 2 m (6 ft) radius

curve, and onto another flat surface. The controller adjusted the macromanipulator speed and direction for the varying surface radii. Various modeling

and system inaccuracies generated position errors throughout the run. For

example, the controller had no a priori information on the shape of the surface

and computed the surface radius (and macro speed) based on the previous, not

current, surface. Also, due to the increased inertia, the macro-manipulator’s

horizontal motion was less accurate then its vertical motion. The operator

removed errors by modifying the macro-manipulator speed (for the MC

approach) or the micro-manipulator position (for other approaches). The tests

compared the ability of the different control techniques to handle surface

induced errors.

Figure 9. Results of Runs on Hull Mock-up

100

75

50

25

0

Points within ±8mm

(%)

μ Cu μ Cf μ Cp MC None

Correction Technique

Although the evaluation requirement is less stringent for the Macro Correction

technique, this technique was again inadequate. Because there was no initial

speed error, there was no large initial position error to overcome. The operator

achieved the proper nozzle position during some of almost all of the stripes.

However, the speed error varied throughout the run and the operator was
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

20

unable to determine the proper speed before the proper speed changed. Thus

the nozzle position was incorrect for much of each stripe.

Many of the micro-manipulator correction techniques also had difficulties on the

sharply curved surface. To investigate why, the operator conducted a run of the

Micro Correction during Paint (μCp) technique while displaying the grid

normally used to evaluate the MC technique (Figure 10). The results showed the

difficulty derived from the accumulation of errors during the relocation stroke.

At the beginning of the paint stroke, the nozzle was often a full centimeter

outside of the tolerance. The operator removed the error quickly, but, due to the

magnitude of the correction, overshot the desired goal. The result was a number

of stripes where the nozzle was temporarily outside of the tolerance.

Figure 10. Dot Plot of Micro Correction during Paint Stroke Run

0.5

1.0

-0.5

Position Error /cm

-1.0

Beginning Middle End

0.0

-1.5

-2.0

Location within Stripe

The operator was unable to improve the system performance by making

additional micro position corrections during the relocation stroke. The Micro

Correct during Full Stroke (μCf) technique permitted the operator to correct the

micro-manipulator position throughout the micro trajectory, However during

the relocation stroke, the relative speed between the camera and the surface rose

rapidly from 0 cm/s to 12 cm/s ( 5 in/s). Furthermore, the camera did not

maintain perpendicularity to the surface during the relocation stroke and the

computed line position was incorrect. Thus the relocation was too quick and

computed position too inaccurate for the operator to respond adequately. The

results for μCf showed no improvement over correcting during the paint stroke

alone.

The Micro Correct with Extended Upstroke (μCu) technique displayed adequate

performance on both surfaces. The Extended Upstroke technique had several
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

21

disadvantages. The extended upstroke trajectory was taller but had the same

duration. Thus the relocation velocities and accelerations were greater then the

other techniques. The increased accelerations were expected to cause increased

position errors. However, the errors were removed prior to painting in almost

all of the stripes.

5 Conclusions

Many techniques are available to coordinate the control of the APACTS nozzle

position in a macro-micro system. These techniques are either macromanipulator based or micro-manipulator based. On flat surfaces, all of the

techniques generate sufficient accuracy for APACTS. On irregular surfaces with

unidentified disturbances the micro-manipulator based techniques are superior.

The control techniques vary the method to correct the macro-manipulator

trajectory. The macro-manipulator techniques change the speed of the macromanipulator directly. The micro-manipulator techniques shift the micromanipulator position to maintain the nozzle position and adjust the macromanipulator speed slowly to keep the micro-manipulator within its operating

volume. The micro-manipulator based control techniques generate better

accuracy.

On flat or regular surfaces, where there are no system perturbations, all of the

control techniques adequately maintain the nozzle in the proper position.

However, on irregular surfaces, such as common to ship hulls, or in situations

with unidentified and unmodeled perturbations, small velocity errors accumulate

into significant position errors. A control strategy that allows an operator or

controller to remove position errors immediately prior to paint application while

hovering over the surface is able to maintain a more accurate position during the

subsequent paint application stroke.

The tolerance used in these experiments may exceed the requirements to apply

marine paints to ship hulls. Furthermore, changes to the APACTS manipulators,

such as more responsive actuators or stiffer links, may improve the system

performance of any of techniques to the tolerance used here. However, a

position control strategy that includes a short period immediately prior to the

paint application will provide superior end point control regardless of the

tolerance desired or the specifics of the structure.

6 Recommendations

There are two recommendations based on the results of these experiments.

Primarily, the APACTS control system should provide a period of hover prior to

the painting stroke of the micro-manipulator’s trajectory and allow micromanipulator position corrections there. Second, experiments should be

conducted to determine the actual position tolerance requirements for APACTS’

targeted paint systems.
Control Strategies for APACTS Micro-Macro-manipulator Integration August 21, 2000

22

7 References

[1] Carderock Division, Naval Surface Warfare Center, SOL N00167-97-SS-R1,

“Mechanical Ship Hull Paint Application System For Use in Drydock”, Commerce

Business Daily, April 10, 1997.

[2] T. Yoshikawa, K. Hosoda, T. Doi, H. Murakami, "Dynamic Trajectory Tracking Control of

Flexible Manipulator by Macro-Micro-manipulator System", Proc of ICRA, pp. 1804-1809,

1994.

[3] T. Yoshikawa, K. Harada, A. Matsumoto, "Hybrid Position/Force Control of FlexibleMacro/Rigid-Micro-manipulator System", IEEE Transactions on Robotics and

Automation, Vol. 12, No. 4, Aug 1996.

[4] O. Khatib, "Reduced Effective Inertia in Macro/Mini Manipulator Systems", Proceedings

of ACC, pp. 2140-2147 (1988).

[5] K. Nagai, T. Yoshikawa, "Impedance Control of Redundant Macro-Micro-manipulators",

Proc. of Int'l Conf on Intelligent Robots and Systems, pp. 1438-1445, 1994.

[6] A. Sharon, N. Hogan, D. Hawitt, "High Bandwidth Force Regulation and Inertia

Reduction Using a Macro/Micro-manipulator System", IEEE ICRA, PP. 126-132, 1988.

[7] K. Nagai, Y. Nakagawa, S. IWASA, K. Ohno, "Development of a Redundant MacroMicro-manipulator and Contour Tasks Utilizing its Compliant Motion", Proc. of Int'l Conf

on Intelligent Robots and Systems, vol. 1, pp. 279-284, 1997.

[8] T. Narikiyo, H. Nakane, T. Akuta, N. Mohri, N. Saito, "Control System Design, for

Macro/Micro-manipulator with Application to Electrodischarge Machining", Intelligent

Robots and Systems, pp 1454-1460, 1994.

[ 9] R. Norcross, "One Degree Micro-Macro-manipulator Integration Test", NIST Technical Report 6562,

Gaithersburg, MD 2000.

[ 10] J. Albus, H. McCain, R. Lumia, “NASA/NBS Standard Reference Model for Telerobot Control

System Architecture (NASREM)”, NIST Technical Report 1235, Gaithersburg, MD 1989.

[11] R. Norcross, "Trajectory Considerations for the Automated Paint Application,

Containment, and Treatment System (APACTS)", NIST Technical Report 6326,

Gaithersburg, MD 1999.

[12] J. Neter, M Kutner, C Nachtshiem, W. Wasserman, Applied Linear Statistical Models ,

Richard D. Irwin, Inc., Chicago, IL, 1996.