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/Robot crane technology •'Jnal report QC100 .U5753 N0.1267 1989 V198 C.1 NISTNIST

PUBLICATIONS

1 States uepartment of Commerce

j| Institute of Standards and Technology

NIST Technical Note 1267

Robot Crane Technology

(Final Report)

Nicholas G. Dagalakis, James S. Albus, Kenneth R. Goodwin, James D. Lee,

Tsung-Ming Tsai, Hoosh Abrishamian, Roger Bostelman, and Charies Yancey

QC

100

.U5755

#1267

1989

C.2
rheNational Institute of Standards and Technology^ was established by an act of Congress on March 3, 1901. The Institute's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit To this end, the Institute conducts research to assure international competitiveness and leadership of U.S. industry, science and technology. NIST work involves development

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' Located at Boulder, CO, with some elements at Gaithersburg, MD.
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NIST Technical Note 1267

Robot Crane Technology

(Final Report)

Nicholas G. Dagalakis, James S. A&xs. Kenneth R. Goodwin, James D. Lee,

Tsuig-Ming Tsai. Hoosh Abrishamian, Roger BastsJman

Roixit SystsmB Dwision

and Charles Yancey

SfeuduBS Diwisnn Oarter for BiAino Tadmology

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National Insauto of Standards and Tfldvwlogy

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Sponsored by: Detanse Advanced Research Proiects Agency pARP/j

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AlineMti VA 22209-230B

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NOTE: As of 23 August 1988, the National Bueau d

Slandards (NBS) became the Nadond insttubB of

SbndanJs and Technology ^<UST) wtwn Presidant

Reagan si^ied into law the Onv4xB Trade and

Comp^tlveness Act

nobert^ Mosbechar. Secretaiy

Natfonaf kKShJte of Standards and Tectmdogy

Rbynond Q. Kanmar, AcfinQ Dinactor
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent

and Technology Washington: 1989 of Documents

Technical Note 1267 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Washington, DC 20402

Tech. Note 1267

60 pages (July 1989)

CODEN: NTNOEF
TABLE OF CONTENTS

Executive Summary 1

1 . Introduction 5

1.1 Objective 5

1.2 Approach 5

1.3 Background 5

2 . The Proposed Crane Suspension Mechanism 7

3. NIST Work Prior to April 1, 1988 12

4. Report of the Progress in the Period From April 1 to November 31, 1988 14

4.1 Task I 14

4.2 Task II 15

4.3 Task III 2 5

4.4 Task IV 35

5 . Summary and Conclusions 4

1

6. Acknowledgments 44

7. References 45

Appendix I 49

111

Robot Crane Technology

Executive Summary

The objective of the Robot Crane Technology Program is to develop kinematically constrained,

dynamically stabilized, robot cranes capable of lifting, moving, and positioning heavy loads over

large volumes, capable of supporting fabrication tools and the inspection of large size and difficult

to reach structures.

Among the significant technologies pursued are:

* Achieve a better theoretical understanding of the properties of the proposed crane suspension mechanism, such as stiffness, dynamics, and stability.

* Demonstrate the use of the robot crane mechanism for factory automation, such as loading and unloading of machines, and moving heavy fixtures.

* Demonstrate the use of the robot crane mechanism for the fabrication and maintenance of

large and difficult to reach structures, such as airplanes, ships, and submarines.

* Demonstrate the use of the robot crane mechanism for the accurate and efficient

positioning of heavy payloads for the construction of airplanes, ships, submarines,

buildings, dams, and bridges.

The approach taken builds on previous work at the NIST Robot Systems Division which has

analyzed and measured the stiffness of a small model six-cable suspension system. This system is a modified Stewart platform.

Under DARPA sponsorship, we have;

1. Extended this work to measure and optimize the stiffness of full-size models.

2. Actively damped oscillations in a small scale six-cable suspension platform.

3. Constructed an intermediate sized six-cable suspension platform for an industrial robot.
Progress in the period from April 1 to November 31, 1988:

* A theoretical model of the proposed robot crane suspension mechanism has been

developed and tested with a full-size robot crane model. The stiffness of the crane suspension mechanism for a full-size model was measured using two different types of

extemal loads and compared with the theoretical model's predicted values. The full-size

model suspension consisted of six wireropes of 3/8-in diameter, which supported a total load of approximately 19,000 lb, from a height of approximately 29 ft. The stiffness to a horizontal extemal force and the stiffness to a moment about the vertical axis applied to

the lower suspended platform were measured. In the first case the theoretical model

predicted stiffness averaged 14.6% less than that measured by the experiment. In the

second case the theoretical model predicted stiffness averaged 33.8% less than that

measured by the experiment. At the present, the wireropes have been replaced with new

ones of 3/4-in diameter and a more thorough investigation of this difference is planned.

* The proposed crane suspension stiffness optimization problem was formulated. The

optimization problem was studied for four different extemal loads. These were an extemal horizontal force, an extemal vertical force, a moment about the vertical axis, and

a moment about a horizontal axis. Each of these loads was applied to the lower

suspended platform. A global and a local maximum analysis study were conducted and

general design rules were formulated. The selection of the maximum stiffness design

depends on the type of extemal forces and moments which are expected to dominate the

loading of the lower crane platform. The most convenient way to proceed with the

selection of a design is probably to pick a few different sizes of the upper crane platform

and then follow the mles described in section 4.2 for the selection of the lower platform

size. The combination of the upper and lower platform with the highest stiffness is

the best choice. The results of the optimization search have been tabulated and can be

used to facilitate the design process.

* A three degree-of-freedom, robot crane, payload vibration compensation device was

constmcted and tested on a small crane model. This device consisted of a servo-driven

horizontal X-Y table and rotary joint about the vertical axis, which was suspended from

the model crane lower platform. The oscillations of the payload in the horizontal plane

and its rotations about the vertical axis were detected by extemal sensors and a digital

controller determined the appropriate command signals to the servo drives in order to
compensate them. A simple proportional gain digital controller algorithm was used for

this stage of the investigation. The gains were selected experimentally to minimize the

number and amplitude of the oscillations of the payload in the corresponding direction.

The payload response to force disturbances along the horizontal plane showed a

significant improvement when the vibration compensation control was on. The payload

response to moment disturbances about the vertical axis did not show any noticeable

improvement when the vibration compensation control was on.

* An intermediate-size model robot crane was constructed for factory automation

applications. Its upper platform was an equilateral triangle with a side length of 6 ft. Based on the stiffness optimization study, the lower platform was selected to be one-half

the size of the upper platform. A PUMA-5601 robot arm was suspended from the lower

platform of this model. A robot tracking laser interferometer was used to monitor the

movements of the arm in three-dimensional space and software was written to process

and display the path tracking data. The robot arm tumed out to be quite stiff to any type

of external disturbances. Above a certain level, though, the wireropes would buckle,

making the arm feel very soft [ see fig. 12 of Dagalakis, N. G., et al., 1988]. This

breakaway feature could be very desirable for applications where safety requires that the

forces or moments appUed by the robot arm do not exceed certain limits.

Proposed future work:

* Theoretical study and measurement of the buckling characteristics of the robot crane suspension mechanism.

* Design and test active damping and the micro-positioning control systems of an

intermediate scale crane platform.

* Add vision, touch, and force sensors and demonstrate the ability of a robot crane to load

and unload machine tools in a factory environment.

* Develop the proper sensors and grippers and demonstrate the ability of a robot crane to pickup and deliver heavy fixtures in a factory environment.

1 Products named in this report are listed for purposes of information only. There is no

implied endoresement of any product or impUcation that they are the best available for the

purpose.
* Develop die prc^)er sensors and grippers and demcMistrate the ability of a robot crane to

pidciq) and deliver heavy fixtures in a factory environmem.

* Develc^ die proper sensors and grippers and dem<mstrate the abili^ of a robot crane to

provide latraial suppoat to do drilling and (kbumng of large and inaccessible sheet metal

structures.

* Retrofit a tower crane, d&yelop the premier sensors and grqjpers and demonstrate die

ability of a robot crane to pickup and deliver heavy paylc^ds of different shapes and

sizes for (X)nstructioQ wosk.
Robot Crane Technology

1 . Introduction

1.1 Objective

The objective of the Robot Crane Technology Program is to develop kinematically constrained,

dynamically stabilized, robot cranes capable of lifting, moving, and positioning heavy loads over

large volumes, capable of supporting fabrication tools and the inspection of large size difficult to reach structures.

1.2 Approach

Our approach built on previous work at the NIST Robot Systems Division to analyze and measure

the stiffness of small model six-cable suspension systems. These systems are modified Stewart

platforms.

Extend this work to measure and optimize the stif&iess of full-size models, and actively dan^

osdlladons in tiie six-cable suspension platforms.

Design and construct a six-cable suspension platform and control system for an industrial robot so

the robot can move and work anywhere on or above a factory floor suspended by six cables from a

bridge crane.

1.3 Background

TTie last 10 years have seen a tremendous growth in the use of robots in the manufacturing industry

with more than 20,000 units installed in the United States, most of them in the automotive or

automotive-related industries [U.S. Department of Commerce, 1987]. Amidst this astounding

technological development, the application of robots in the large-scale fabrication, assembly and

heavy construction industry is virtually nonexistent in the United States. The reasons for delay are probably because these are made-to-order industries, requiring precision in the construction of

components and blocks, requiring an enormous number of structural members and machines most

of which are heavy and bulky. In the case of heavy construction equipment, delays occur because

they operate in a more complex and unconstrained environment - that of the construction site.
Airplane manufacturing and shipbuilding, which are heavy assembly and construction industries,

being labor intensive, are becoming more expensive every year while robotic automation is becoming less expensive and more capable every year.

In Japan, the Japanese Shipbuilding Society started a 5-year research and development plan of

"Modemization of Production Technology in Japan" in 1982 [Kubo, M., 1987]. The program is sponsored by a consortium of seven major Japanese shipbuilders, and funded by the Japan

Foundation for Shipbuilding Advancement. As a result of this effort, prototypes of large size

gantry-type robots for welding, surface preparation, and painting of ship structures have been built

by Ishikawajima-Harima Heavy Industries Co. and are now being tested by Sumitomo Heavy

Industries, Ltd.

Currently, ordinary cranes are stable only in the vertical direction. The load is free to rotate in all

directions and sway in the horizontal plane under the slightest side pressure, similar to a pendulum.

Under these conditions it would be very difficult for the crane to support any robotic operations

due to the excessive compliance and low damping of its end-effector. Automatic crane antisway

control devices have been proposed and tested by several people [ Kogure, H. et al., 1978,

Carbon, L., 1976, Gercke, U.S. Patent No. 2,916,162]. Although these devices tend to suppress the pendulum motions in the horizontal directions they fail to suppress any pitch, roll or yaw rotations of the load. Other systems have been developed which try to solve the sway

problem by employing several wires and winches [ Noly, U.S. Patent No. 4,350,254]. These

systems add considerable complexity and cost to the load-handling system and have not found

practical application thusfar.

Conventionally designed robots have been scaled up and used as cranes, but they are largely

impractical for handling heavy loads. Due to the low payload-to-manipulator arm/weight ratio of

these robots, they have to be constructed of gigantic dimensions, occupy a large floor area, and

consume large amounts of power.

NIST has proposed a new crane design, which despite its simplicity, results in a very stiff load

platform which can be used as a robot base or end-effector for heavy loads.
2. The Proposed Crane Suspension Mechanism

Considering the requirements for a robot crane, which should provide superior stiffness to load

roll and sway, have a large work volume without occupying any significant floor space, and have a reasonable size, we have proposed the mechanism concept shown in figure 1 [ Albus, J. S.,

1987]. It consists of an equilateral triangular platform suspended by six wireropes, two at each

vertex of the triangle, from an overhead carriage. The carriage can be attached to either an overhead (see fig. 2) or a boom crane, depending on the application. The carriage includes a

single winch onto which all six wireropes attach as shown in figure 3, and rope guides which

guide the six wireropes away from the winch in three pairs equidistantly spaced [ Albus, J. S., et

al., 1987]. If desired, it is possible to control the lengths of the individual wireropes with

actuators or brakes properly positioned either at the lower platform or the carriage.

The suspended platform behaves as if it is supported by an extendable single solid beam with its

elastic properties dependent on the weight of the load and the height of the crane for a given crane geometry and wirerope type and size. This is a significant improvement in stiffness over a conventional crane. It enables the load to be accurately positioned and provides a stable platform

which can be used to exert torques and side forces on objects being positioned. The suspended

platform can be used as a stabihzed base for the direct mounting of conventional manipulator arms

as shown in figure 2, or it can be used for the support of special substructures for specific crane

applications. These robot crane designs can also be used for factory automation where they offer

several advantages as compared with the usual current practice of having a manipulator dedicated to serve each machine tool. These advantages are higher payload and fewer manipulators, which

hang from overhead rails and thus do not occupy valuable factory floor space.

The suspension mechanism of the proposed robot crane platform shown in figure 1 imitates the

behavior of a parallel Hnk manipulator arm. The arm mechanism is called parallel because the links

are positioned side by side, "approximately" parallel to each other and each link serves a role

"approximately" equal to that of its neighbor. This is different from the design of the more

common serial link manipulators built of a chain of links connected end-to-end in a serial manner.

Parallel link manipulators are, in general, known for the simplicity of their mechanical design, and

their high strength and stiffness-to- weight ratios, because their actuators bear no moment loads but

act in simple tension or compression. They are also known for their high force and moment

capacity, since their actuators all act in parallel. Such manipulators with solid adjustable length

beams in the place of the wireropes were first used for the design of tire test machines [Gough, V.

E. et al., 1957, 1962]; later they were used for the design of flight simulators [Stewart, D., 1965].
Controlled

X

Trolley Motion

Modified Bridge

Crane Trolley

Load

V James S. Albus

1/15/85

Figure 1. Mechanism concept.

8
7

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TV Camera

Figure 2. Overhead crane and robot manipulator combination.

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10
With the increasing interest in robotic arm manipulators, studies have been conducted for their use

as a mechanical wrist [Bennett, W. M., 1968], a compliant device [McCallion, H., at.al., 1979], a force/moment or position sensor [Koliskor, A. S., 1982], a robot arm [Fichter, E. F., et al., 1980,

1984, 1987, Powell, I. L., 1982, Landsberger, S. E., et al., 1985, Sheridan, T. B., 1986,

Konstantinov, M. S., et al., 1985], and an industrial manipulator for assembly [Gadfly, 1983] and

for grinding[Multicraft, 1987].

The design discussed here takes advantage of the suspended crane load to maintain the wireropes

in tension and thus form six flexible wires which, with their elastic deformation, oppose any

displacement of the payload. The stiffness created by this elastic deformation is superimposed

onto the pendulum-effect-created-stiffness of ordinary cranes. Individual rope length control of the

position and orientation of the platfomi is also possible.

11
3. NIST Work Prior to April 1, 1988

A small model of the proposed robot crane suspension mechanism was constructed- The model

consisted of two aluminum triangular plates of equal side length 228.6 mm (9.0 in), see figure 1. The lower platfonn was suspended by six steel wires. The height, which ranged finom 3 to 4.5 ft, and the suspended weight W, which ranged from 100 to 350 lb, varied depending on the test conditions. During testing, external loads (force or moment) of various amplitudes and

orientations were applied throu^ a multiaxis load-ceU. The resulting displacement in the direction

of load application was measured.

A full-size DKxiel of the crane suspension mechanism with a lateral translation end-load outfitting

subplatform structure, also was built, as shown in figure 4. It consists of a support frame

mounted on the concrete reaction wall of a seismic test facility and two I beams connected at a 90

degree angle. These two beams are suspended by six wires from the support frame at approximately a 29-ft height and form the model of the lower platform and the subplatform

structure. Each wire is connected to these beams through a load-cell for monitoring its tension and

a tum-buckle for adjusting its length. The load, approximately 5,000 lb and the counter balance

weight, approximately 10,000 lb, are simulated by lead bricks placed in two baskets which hang

from the two ends of the long boom beam. This model was instrumented and used for the testing

reported in this work.

A computer model of the proposed robot crane stiffness based on the nonlinear equations of the

quasi-static motion of the lower platform was developed. Assuming small motions of the lower

robot crane platform, a linearized equations model of the stiffiiess was developed. The small robot crane model test results were used to calculate the stif&iess to extemal loads and to compare with

the predicted values from the theoretical models. This work was described in [Dagalakis, N. G.,

et al., 1988].

12
40'-0" (12.2 m)

IX>NG BOOM

6'-9" (2.0 m)

LONG

Figure 4. Schematic of the full-size model.

13
4. Report of the Progress in the Period From April 1 to November 31, 1988

4.1 Task I

The objective of this task was to measure the stiffness of a full-size model of the proposed robot

crane suspension mechanism subjected to certain types of external loads and to compare the results

with those predicted by the theoretical model. The theoretical model was previously developed and

is described in [Dagalakis, N. G., et al., 1988].

A full-size model with a lateral translation end-load outfitting subplatform structure was completed

and used for the experimental work required for this task. Figure 4 shows a schematic drawing of

this model. The upper and lower platforms were constructed from steel I beams. The upper

platform was mounted on a specially designed steel frame which was then attached to the side of a

seismic reaction wall. The two platforms were constructed as equal equilateral triangles with a side

length of 6 ft. The vertical distance between the platforms was 28.6 ft. The lower platform was

suspended from the wireropes through six turn-buckles and six load-cells. The turn-buckles

allowed compensation for small differences of the wirerope lengths and made possible the leveling

of the lower platform. The load-cells were used to monitor the tensions of the wireropes. The

length of four of the wireropes was 25.8 ft, while the length of the other two was 24.8 ft. The

difference in lengths was due to the I beam construction of the upper platform. The total weight

suspended from the lower platform was 19,100 lb.

Due to the large magnitude of the external load forces and moments involved special heavy fixtures

had to be constructed to perform the stiffness measurement tests. A heavy optical bench was used

for the application of horizontal side forces and two large stanchions for the application of pure

moments. The translation and rotation of the lower platform was measured with a pair of

displacement transducers.

The wireropes used to suspend the lower platform were 3/8-in diameter, 6x19 classification, with

fiber core. In order to measure their Young's modulus of elasticity and stiffness it was decided to obtain their stress strain curve. Three small specimens, each 3-ft in length were prepared and

tested on a materials testing machine. They were loaded and unloaded twice during each test. The

stiffness of the wirerope at 3,000 lb tension force of the second loading cycle was used to calculate

the modulus of elasticity. Its average value was found to be 13, 170, 208. Ib/in^.

14
The measurements which have been conducted so far were the stiffness for a horizontal force

applied to the center of gravity of the lower platform suspension plane and the stiffness for a moment about the vertical axis applied to the same platform. Figure 5 shows the force versus displacement plot from the first case superimposed to the theoretical model predicted plot. Figure 6

shows the moment versus angular displacement plot from the second case superimposed to the

theoretical model predicted plot.

In the first case the theoretical model predicted stiffness averaged 14.6% less than that measured by

the experiment. In the second case the theoretical model predicted stiffness averaged 33.8% less

than that measured by the experiment. At present, the wireropes have been replaced with new ones of 3/4-in diameter and a more thorough investigation of this discrepancy is planned.

4.2 Task II

The objective of this task was the formulation and study of the proposed robot crane suspension

stiffness optimization problem and to determine whether there are design rules for the maximization

of the stiffness to external loading.

The optimization problem studied was the following. Determine the optimum combination of the

dimensions of the upper and lower platform of the proposed robot crane suspension mechanism,

which maximize its stiffness for practical values of the total suspended weight, crane height, and

diameter of the steel wireropes. In other words, assuming that a given payload has to be delivered

to a location of a given height, determine the robot crane design which will maximize its stiffness.

The stiffness matrix of the suspension mechanism for small displacements of the lower platform is given by eqs (2) and (3) of [Unger, J., et al.,1988]. This matrix relates the three-dimensional

space forces and moments applied to the center of gravity of the lower platform suspension plane

(formed by the three suspension points) with the resulting translations and rotations.

The stiffness functions are as follows:

1. The stiffness K5 to a single external moment about the vertical axis, given by eqs (3) and (4)

of [Unger, J., et al.,1988].

2. The stiffness Kq to a single external moment about a horizontal axis, given by eq (6) of

[Unger, J., et al.,1988].

3

.

The stiffness K^ to a single external force about a horizontal axis, given by eq (5) of [Unger,

J., et al.,1988].

15
g 40.0000 "

•D- Test #1 •- Test #2

•O- Test #3 •- Theoretical model

0.0000

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000

Displacement x [in]

Figure 5. Robot crane force-displacement test results.

16
350.00 •

300.00 •

5 250.00 •

•- Test#1

•o- Test#2

"

Theoretical model

0.00 1 I I I I I 0.000 0.100 0.200 0.300 0.400

Angular Displacement [Degrees]

Figure 6. Robot crane moment-angular displacement test results.

17
4. The stiffness K3 to a single external force about the vertical axis, given by eqs (3) and (4) of

[Unger, J., et al.,1988].

The optimization properties of these functions were studied using optimization searching

techniques software programs and exhaustive search computer simulation and plotting programs.

The first type of programs used function gradient information to search for optima. The second

type of programs were used to gain a better understanding of the behavior of the functions and to check the results of the first. Two different types of analyses were performed, the global

maximum analysis and the local maximum analysis. In the first case the relative maxima of the

previously mentioned stiffness functions were located for a wide range of payloads and heights.

In the second case the size of the upper platform was fixed and the size of the lower platform

which maximizes the same stiffness functions was determined (for more details refer to [Unger, J.,

et al.,1988]). The stiffness functions considered were linear with respect to the wirerope diameter,

and increasing diameter would increase the stiffness without reaching any maximum. The

wirerope diameter then was treated as an input parameter during the optimization search and not as an optimization variable.

The results of the optimization search are as follows:

1 . The global analysis search of the stiffness K5 function did not locate any relative maximum in

the range of payloads (10,000 to 100,000 lb) and heights (10 to 100 ft) searched.

The local analysis search (for specific sizes of the upper platform) did identify local maxima and

minima. Figure 7 shows a typical plot of the stiffness function versus the size of the lower

platform, from the local analysis search. Where A and B are equal to one-half the side length of

the lower and upper equilateral triangle crane platforms respectively. The Young's modulus E and

area refer to the modulus of elasticity and effective cross sectional area of the selected wirerope.

An area of 0.057 in ^ corresponds to a wirerope diameter of 3/8 in. From this figure it can be seen

that the stiffness K5 is equal to zero when A is equal to zero. As A increases the stiffness

increases reaching first a local maximum and then a local minimum; beyond that the stiffness keeps

increasing as A increases. For practical applications where A cannot exceed 10 to 20 ft this plot can be used to select A, which maximizes the suspension stiffness to a single external moment

about the vertical axis.

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2. The global analysis search of the stiffness Kq function did locate a relative maximum in the

range of payloads (10,000 to 100,000 lb) and heights (10 to 100 ft) searched. This maximum is achieved when the ratios A/H and B/H take a specific value which ranges from approximately 1.2

to 1.5 for a payload range from 10,000 to 100,000 lb and wirerope diameter from 3/8 to 3/4 in,

where H is the crane height. Although the value of the stiffness at that maximum is quite high the

resulting design is not very practical for crane applications since it would require very large size

upper and lower platforms (large A and B) for any H of practical significance.

The local analysis search (for specific sizes of the upper platform) did identify local maxima and

minima. Figure 8 shows a typical plot of the stiffness function versus the size of the lower

platform, from the local analysis search. From this figure it can be seen that the stiffness Kq is

equal to zero when A is equal to zero. As A increases the stiffness increases reaching first a local

maximum and then a local minimum beyond that the stiffness keeps increasing as A increases. For

practical applications, where A cannot exceed 10 to 20 ft, this plot can be used to select A which

maximizes the suspension stiffness to a single extemal moment about a horizontal axis.

3. The global analysis search of the stiffness Kx function did locate a relative maximum in the

range of payloads (10,000 to 100,000 lb) and heights (10 to 100 ft) searched. This maximum is achieved when the ratios A/H = 0.707 and B/H = 1.414 and the corresponding value of the

stiffness is

where A is the effective cross sectional area of the wirerope, E its Young's modulus of elasticity,

and W the total suspended weight. Unfortunately the resulting design is, again, not very practical

for crane applications since it would require very large size upper and lower platforms (large A and

B) for any H of practical significance.

The local analysis search (for specific sizes of the upper platform) did identify local maxima and

minima. From eq (1 1) of [Unger, J., et al.,1988] it can be seen that Kx has always an optimum,

either a maximum or a minimum, when the lower platform is one-half the size of the upper

platform (A = B/2), and the corresponding value of the stiffness is

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Figure 9 shows a typical plot of the stiffness function versus the size of the lower platform, from

the local analysis search. From this figure it can be seen that the stiffness Kx reaches a maximum

when A is equal to zero. This corresponds to the case of an inverted pyramid suspended from its base, which is very stiff to a single external force about a horizontal axis applied at its peak.

Unfortunately this is not a realistic design because the stiffness to any moment applied to the lower

platform would be zero as we saw previously. The maximum at A = on figure 9 follows a local

minimum and a local maximum at A = B/2. By varying the values of the payload, height and

wirerope diameter it is possible to result in the movement of the maximum towards higher values

of A and its replacement by a minimum at A = B/2 as can be seen from figures 2 and 3 of [Unger,

J., et al.,1988]. To select the proper A then the value of Kx for the smallest possible A will have

to be compared with that of the local maximum and the largest of the two used for the selection of

A, which will be used for the actual design.

4. The global analysis search of the stiffness K3 function did locate a relative maximum in the

range of payloads (10,000 to 100,000 lb) and heights (10 to 100 ft) searched. This maximum is achieved when A = B = and it is not a realistic design because the stiffness to any moment

applied to the lower platform would be zero as we saw previously.

The local analysis search (for specific sizes of the upper platform) did identify a local maximum

when the lower platform is one-half the size of the upper platform (A = B/2), and the

corresponding value of the stiffness is

^ * 6AEH2 W

Kb =(H2+B:i)3/2 + HFigure 10 shows a typical plot of the stiffness function versus the size of the lower platform, from

the local analysis search.

From the optimization results discussed so far it is obvious that the selection of the maximum

stiffness design depends on the type of external forces and moments which are expected to dominate the loading of the lower crane platform during actual operation. To help with the

selection of the proper design we have tabulated the results of the computer optimization searches

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for three different wirerope diameters (3/8, 9/16, 3/4 in), for a range of payloads from 10,000 to 100,000 lb in increments of 10,000 lb and heights from 10 to 100 ft in increments of 10 ft

4.3 Task III

The objective of this task was the construction and preliminary testing of a vibration compensation

device to damp the oscillations of the payload suspended from the lower platform of the crane.

A small model of the proposed robot crane, shown in figure 11, was constructed for this task. Its upper and lower platforms were two triangular aluminum plates which had the same size. The

length of the side of the equilateral triangle platforms was 9 in (A = B = 4.5 in). Six steel wires of

0.042-in diameter were used to suspend the lower platform from a height of approximately 4 ft. This model was equipped with a lateral translation end-load outfitting subplatform model structure

(a more detail description of this design is given in [Albus, J., et al., in the press] and figs. 6 and

7). The subplatform structure was modelled with a 2 x 1 in cross section, 6-ft long steel beam.

Two baskets were suspended from its ends for the placement of lead bricks. The front-end basket

simulated the payload and the rear end basket the counter weight.

The vibration compensation device used for these tests consisted of a servo-driven x-y table and

rotary joint. This device was suspended from the lower platform of the small model robot crane so

that it was positioned between the platform and the steel beam (see schematic in fig. 11). In a

sense this was a three degrees-of-freedom serial micro-positioner manipulator suspended from the

lower platform. With this device it is possible to translate the payload on a plane parallel to that of

the lower platform and to rotate it about an axis orthogonal to the same plane. Although these are only three of the degrees-of-freedom that the payload has, they represent the ones with the largest

amplitude of oscillations for this design.

The translation and rotation of the payload was measured by three Linear Voltage Differential

Transformers (LVDTs), which were fixed horizontal on the floor and were positioned as shown in

figure 11. The LVDT transducers measure the displacement of the moving core of their

electromagnet and produce an analog voltage signal which corresponds to its absolute position.

The tip of the moving core of each of these LVDT sensors was in contact with the steel beam so

that it could follow the horizontal component of the low fr-equency oscillations of the beam.

Considering only small amplitude oscillations of the lower platform it was assumed that the motion

25
Figure 1 1. Schematic of the vibration compensation system.

26
of the steel beam can be approximated by a translation on a horizontal plane and a rotation about a

vertical axis.

The outputs of the three LVDT displacement transducers were digitized and sent to an IBM-AT/PC

computer which served as a digital controller. The digitized signals were processed by the digital

controller to determine the horizontal translation and the vertical axis angular rotation of the steel

beam with respect to a reference frame fixed on the floor. Based on that information the necessary

amount of translation and rotation of the vibration compensation device is determined, which will

bring the payload back to its original position. This information is then processed by a control

algorithm which determines the proper command signals to be sent to the three servo drives. A

detailed description of these algorithms is provided in Appendix I.

After the electrical and electronic circuitry of the vibration compensation device was completed (see

schematic of fig. 12) and tested, the three servo drives were tuned for optimum response to simple

step input command signals from the digital controller. The first digital controller algorithm to be

tested was a simple proportional controller. Each of the required displacements of the three axes of

the vibration compensator was multiplied with a constant and sent to the servo-drive controllers.

The value of that constant was determined experimentally so that each axis has the fastest response

possible to an impulse excitation, with no more than two oscillations before reaching steady state.

Figure 13 shows the response of the vibration compensator to an impulse excitation along the

longitudinal axis (x-axis) of the steel beam with no digital controller action. Figure 14 shows the

response to a similar excitation with digital control action. Figure 15 shows the response of the

vibration compensator to an impulse excitation orthogonal to the longitudinal axis (y-axis) of the

steel beam with no digital controller action. Figure 16 shows the response to a similar excitation

with digital control action. Figure 17 shows the response of the vibration compensator to an impulse moment excitation along the vertical axis (z-axis) of the steel beam with no digital

controller action. Figure 18 shows the response to a similar excitation with digital control action.

As can be seen from these test results there is a significant improvement in the response of the

payload of the robot crane model to horizontal force excitation inputs, but not in the response to moments about the vertical axis. An increase in the digital controller gain of that axis leads to more

oscillatory response and eventually instability. This points to the need to use more sophisticated

controller algorithms, at least for that last axis.

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4.4 Task IV

The objective of this task was to construct an intermediate-size robot crane model for factory

automation use. Suspend a robot arm from the lower crane platform, as shown in figure 2, and

demonstrate the capability to monitor the position of the robot arm end-effector, as it moves in

three-dimensional space, with noncontact sensors.

The largest size upper platform which could be suspended from the ceiling of our lab, without the

wireropes interfering with the Ught fixtures, was an equilateral triangle of 6-ft long side (B = 3 ft). The diameter of the wireropes was selected to be 3/8 in. The height 12 ft 8 in was selected to allow

enough clearance between the robot arm end-effector and the floor. A PUMA-560 robot arm

which became available for this task weighs approximately 150 lb with its end-effector. This

allows us to suspend several hundred pounds of payload from the lower platform in addition to the

robot arm.

Since the robot arm is relatively short and has low manipulation power, no significant moments

were expected to be developed on the lower crane platform during its operation. Thus it was

decided to build a crane suspension system which is stiff to external forces rather than moments.

Based on the results of the optimization analysis that design could be achieved with a lower

platform which is one-half that of the upper platform (A =B/2 =1.5 ft). Figure 19 shows a picture

of the intermediate-size model with the robot arm hanging undemeath.

This type of robot crane design can be used for the automation of industrial operations. If the

upper platform is mounted on the carriage of a gantry crane the robot arm can be used to load and

unload several machine tools. Compared to the current practice of using dedicated robot arms to load and unload machine tools this design could reduce the number of needed robots and free

valuable factory floor space for other use. By hanging a hook undemeath the lower platform, as shown in figure 2, heavy parts and fixtures can be moved, and with the help of the robot arm be

accurately positioned. By mounting drills, deburring tools, etc., from the lower platform, the

lateral stiffness capability of the crane suspension can be used to do drilling, deburring, etc., on

sheet metal structures while the robot arm could hold vision or touch sensors to assist with the

machining operation. Similarly, vision or other sensors could be used for the inspection of large

structures, such as airplanes, ships, etc.

35
Figure 19. Intermediate size model.

36
To demonstrate the capability to monitor the movement of the robot arm in three-dimensional space

while hanging underneath the crane lower platform we used the newly completed Robot Laser

Tracker interferometer which was developed at NIST jointly by our Division and the Precision

Engineering Division [Lau, K., et. al., 1985]. This instrument measures the spherical coordinates

of a retroreflecting optical target which is mounted on the moving body to be monitored. It shoots a laser beam to the target which is then used by an interferometer to measure the change in the

distance between the target and a mirror at the head of the instrument. When the target moves the

mirror is servoed to follow it. Two precision encoders measure the azimuth and zenith angles of

the mirror. The distance and angle information is then combined to determine the x, y, z position of

the opdcal center of the target with respect to the coordinate system of the Robot Laser Tracker

interferometer. Figure 20 shows a picture of the Robot Laser Tracker following the optical target

mounted at the end-plate of the PUMA-560 arm. Figure 21 shows a three-dimensional plot of the

optical target trajectory as the robot is commanded to follow the standard test path proposed to the

American National Standards Institute (ANSI) by the Robotic Industries Association (RIA).

Figure 22 shows the same trajectory with respect to a coordinate frame located at the centroid of the

trajectory which allows a better observation of details.

37
oot-l o

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Kl

«-> ooo

38
Figure 21. Target trajectory with respect to the Laser Tracker coordinate system.

39
Figure 22. Target trajectory with respect to a centroid coordinate system.

40
5. Summary and Conclusions

The robot crane technology program has made good progress toward its objective of developing

kinematically constrained, dynamically stabilized, robot cranes capable of lifting, moving, and

positioning heavy loads, capable of supporting fabrication and construction operations.

Progress in the period from April 1 to November 31, 1988:

* A theoretical model of the proposed robot crane suspension mechanism has been

developed and tested with a full-size robot crane model. The stiffness of the crane suspension mechanism of a full-size model was measured for two different types of

external loads and compared with the theoretical model predicted values. The full-size

model suspension consisted of six wireropes of 3/8-in diameter, which supported a total load of approximately 19,000 lb, from a height of approximately 29 ft. The stiffness to a horizontal extemal force and the stiffness to a moment about the vertical axis applied to

the lower suspended platform were measured. In the first case, the theoretical model

predicted stiffness is on the average 14.6% less than that measured by the experiment.

In the second case, the theoretical model's predicted stiffness is on the average 33.8%

less than that measured by the experiment. At the present the wireropes have been

replaced with new ones of 3/4-in diameter and a more thorough investigation of this

difference is planned.

* The proposed crane suspension stiffness optimization problem was formulated. The

optimization problem was studied for four different extemal loads. These were an extemal horizontal force, an extemal vertical force, a moment about the vertical axis, a moment about a horizontal axis, each of them applied to the lower suspended platform.

A global and a local maximum analysis study were conducted and general design mles were formulated. The selection of the maximum stiffness design depends on the type of

extemal forces and moments which are expected to dominate the loading of the lower

crane platform. The most convenient way to proceed with the selection of a design is probably to pick a few different sizes of the upper crane platform and then follow the

mles described in section 4.2 for the selection of the lower platform size. The

combination of the upper and lower platform with the highest stiffness should be the best

choice. The resuhs of the optimization search have been tabulated and can be used to

facilitate the design process.

41
* A three degrees-of-freedom robot crane payload vibration compensation device was

constructed and tested on a small crane model. This device consisted of a servo-driven

horizontal X-Y table and rotary joint about the vertical axis, which was suspended from

the model crane lower platform. The oscillations of the payload in the horizontal plane

and its rotations about the vertical axis were detected by extemal sensors and a digital

controller determined the appropriate command signals to the servo drives in order to compensate them. A simple proportional gain digital controller algorithm was used for

this stage of the investigation. The gains were selected experimentally in order to minimize the number and amplitude of the oscillations of the payload in the

corresponding direction. The payload response to force disturbances along the horizontal

plane showed a significant improvement when the vibration compensation control was

on. The payload response to moment disturbances about the vertical axis did not show

any noticeable improvement when the vibration compensation control was on.

* An intermediate-size model robot crane was constructed for factory automation

applications. Its upper platform was an equilateral triangle with a side length of 6 ft. Based on the stiffness optimization study, the lower platform was selected to be one-half

the size of the upper platform. A PUMA-560 robot arm was suspended from the lower

platform of this model. A robot three-dimensional tracking laser interferometer was used

to monitor the movements of the arm in space and software was written to process

and display the path tracking data. The robot arm tumed out to be quite stiff to any type

of extemal disturbances. Above a certain level though the wireropes would buckle

making the arm yield ( see fig. 12 of [Dagalakis, N.G., et al., 1988]). This could be a very desirable feature for applications where safety requires that the forces or moments

applied by the robot arm do not exceed certain limits.

Proposed future work:

* Theoretical study and measurement of the buckling characteristics of the robot crane suspension mechanism.

* Design and testing of active damping and the micro-positioning control systems of an

intermediate scale crane platform.

* Add vision, touch, and force sensors and demonstrate the ability of a robot crane to load

and unload machine tools in a factory environment.

42
* Develop the proper sensors and grippers and demonstrate the ability of a robot crane to pickup and deliver heavy fixtures in a factory environment.

* Develop the proper sensors and grippers and demonstrate the ability of a robot crane to provide lateral support to do drilling, deburring, etc., of large and inaccessible sheet

metal structures.

* Retrofit a tower crane, develop the proper sensors and grippers and demonstrate the

ability of a robot crane to pickup and deliver heavy payloads of different shapes and

sizes for construction work.

43
6 . Acknowledgments

The robot crane technology program was funded by the Defense Advanced Research Projects

Agency, Information Science and Technology Office, ARPA order 6380. Dr. Robert L. Resenfeld

was the project sponsor.

We would like to acknowledge the help of the following people. Mr. Mitch Tarica for the design

and construction of most of the mechanical components used for this work and his help with some

of the experiments. Mr. Wendell Wallace for the construction of the vibration compensation

electrical system. Mr. Frank Rankin for his help with the construction of the full-size robot crane model.

44
7 . References

Albus, J. S. A stabilized lifting platform. Internal Nat Bur. Stand (U.S.) Memo; 1987.

Aibus, J. S.; Dagalakis, N. G.; Wang, B-L; linger, J.; Lee, J. D.; Yancey, C. W. Available

robotics technology for applications in heavy industry. To be published by the Iron and Steel

Engineer; 1988.

Bennett, W. M. Mechanical wrist for a robot arm. Mech. Eng. DepL Mass. Inst of Tech. B.S.

Thesis; 1968.

Carbon, L. Automation of grab cranes. Siemens Review 43(2): 80-85; 1976 February.

Dagalakis, N. G.; Albus, J. S.; Wang, B-L; linger, J.; Lee, J. D. Stiffness study of a parallel link

robot crane for shipbuilding applications. Proc. Offshore Mechanics and Arctic Eng, Conf.; 1988

February; Houston, TX.

Fichter, E. F.; McDowell, E.D. A novel design for a robot arm. Proc. InL Computer Tech. Conf.:

250-256; 1980 August; San Fransisco, CA.

Fichter, E. F. Kinematics of a parallel connection manipulator. ASME Paper 84-DET-45 delivered

at the Design Eng. Tech. Conf.; 1984 October, Cambridge, MA.

Fichter, E. F. A Stewart platform based manipulator: General theory and practical construction.

The Kinematics of Robot Manipulators, MTT Press: 165-190; 1987.

"Gadfly- the Answer to Electronic Component Assembly," Assembly Automation: 20-22; 1983

February.

Gough, V.E., "Contribution to Discussion of Papers on Research in Auto Stability and Control

and in True Performance by Cornell Staff, Proc. Auto Div. Inst Mech. Eng., pp.392-403, 1956-

1957.

Gough, V. E.; Whitehall, S. G. Universal tyre test machine. Proc. 9th Int. Automobile Technical

Congress: 117-135; 1962.

45
Kogure, H.; Tojo, M. Recent developments in crane control. Hitachi Review 27(6): 315-320;

1978.

Koliskor, A. S. Development and investigation of industrial robots based on specification by 1-

coordinates. Soviet Engineering Research 2(12): 75-78; 1982.

Konstantinov, M. S.; Sotirov, Z. M.; Zamanov, V. B.; Nenchev, D. N. Force Feedback Control

of Parallel Topology Manipulating Systems. I*roc. 15th Int. Symp. on Industrial Robots: 181-188;

1985 September; Tokyo, Japan.

Kubo, M. Robots and computers to automate Japanese shipbuilding. Keynote address II, 6th Int. Symp. on Offshore Mechanics and Arctic Engineering: 1987 March; Houston, TX.

Landsberger, S. E.; Sheridan, T. B. A new design for parallel link manipulators. Proc. Systems

Man and Cybernetics Conf.: 812-814; 1985 November, Tucson, AZ.

Lau, K.; Hocken, R.; Haynes, L. Robot end point sensing using a laser tracking system.

Proceedings of the Workshop on Robot Standards: 104-1 11; 1985 June; Detroit, MI.

McCallion, H.; Johnson, G. R.; Pham, D. T. A compliant device for inserting a peg in a hole. The

Industrial Robot : 81-87; 1979 June.

Multicraft A/S; Oslo, Norway.

Powell, I. L. The kinematic analysis and simulation of the parallel topology manipulator. The

Marconi Review XLV (226): 121-138; 3rd quarter 1982.

Sheridan, T. B. Human supervisory control of robot systems. Proc. Int. Conf. on Robotics and

Automation: 808-812; 1986 April; San Fransisco, CA.

Stewart, D. A platform with six degrees of freedom. Proc. of the Inst, of Mech. Eng. 180(15) Part

I: 371-386; 1965-1966.

U. S. Department of Commerce, International Trade Administration. 1987 U.S. Industrial

Outlook.

46
Unger, J.; Dagalakis, N. G.; Tsai, T. M.; Lee, J. D. Optimum stiffness study for a parallel link

robot crane under horizontal force. Proceedings of the 2nd Int. Symp. on Robotics and

Manufacturing: 1037-1046; 1988 November, Albuquerque, NM.

47

Appendix I

Development of the Vibration Compensation Signal Processing and Control

Algorithms

IV. 1 The Signal Processing Algorithms

The translation and rotation of the payload was measured by three Linear Voltage Differential

Transformers (LVDTs), which were fixed parallel to the floor and were positioned as shown in

figure 11 of the main report. The LVDT transducers measure the displacement of the moving core of their electromagnet and produce an analog voltage signal which corresponds to its absolute

position.

Figure IV. 1 shows the steel beam, which supports the payload and the counterweight (for more

details refer to sec. 4.3 of the main report), and the three LVDTs. LVDTs 1 and 2 are positioned

parallel to each other with their sensor probe in contact with the same side surface of the beam,

while LVDT 3 is oriented with its axis orthogonal to the other two, so that its probe is in contact

with another side surface of the beam. To reduce friction the surface of the beam which comes in

contact with the LVDT probes was covered with Teflon tape. At the beginning of each test the

LVDT sensors were carefully positioned so that they are all at the same horizontal level and the axes of their moving cores are orthogonal to the corresponding side of the beam.

Let (x\y, y\y, zb ) be a coordinate system fixed on the beam with origin located at (x^^, y^b* Zcb-^- The location of the origin (x^\y, y^^, z^^) is measured from the absolute reference coordinate

system ( x , y , z ) fixed on the floor.

Any flat surface of the beam can be described by eq

%^b + ^byb + %^b = ^b ' (^)

where a^, b^, c^ are the directional cosines of the surface normal and d^j is the distance of the

surface from the origin x^^j, y^^, z^,^.

Each LVDT sensor will give the coordinate ( x , y , z ) of the contact point between the sensor probe and the beam side surface, measured with respect to the absolute reference coordinate frame.

49
3 (LVDT)

2 (LVDT)

Figure IV.l. The steel beam and sensor locations.

50
The transfonnation between the absolute coordinate ( x , y , z ) and the beam coordinate (x^, y^,

z\y) is given by

Xb

yb

^b J

•^xx ^xy 'xz

^yx "^yy ^yz

L '^zx '^zy '^zz

y-ycb

z-^cb

(2)

where n^x. ^zz ^® ^® directional cosines between the two sets of coordinate axes, x y z and Xb yb ZbThe coordinates (xb, yb. ^b) of the contact point between the sensor probe and the beam surface

must satisfy eq (1), i.e.

[ ^b ^b ]

'^b

yb

^b J

= db- (3)

Considering only small amplitude oscillations of the lower crane platform it was assumed that the

motion of the beam can be approximated by a translation on a horizontal plane and a rotation about

a vertical axis. LVDT sensor 1 and 2 were in contact with surface AB of the beam and could measure its angular displacement. Sensor 3 was in contact with surface AD of the beam and in

conjunction with sensors 1 and 2 can measure its translation.

Determination of the Beam Angular Displacement

Let the normals to surfaces AB and AD with respect to the beam coordinate frame be (aj, bj, 0)

and (a3, b3, 0), respectively. The unit vector (nj^b^ Oyb, 0) of the line segment connecting the

contact points between sensor probes 1, 2 and surface AB, with respect to the beam coordinate

frame, is perpendicular to the surface normal (ai,bi,0). Therefore,

(nxb,nyb,0) = (-bi,ai,0). (4)

51
As the beam with the payload moves, its motion can be described by rotation matrices and

translation vectors. At any time, die unit vector of the line connecting sensor probes 1 and 2, widt

respect to the refei^mce coordinate frame, can be vqnesented by

"y

J

cosOj

- sinOj

sin6|

cosO|

1 J

n t y

L

(5)

Where 6lis die angular displacement of the beam far a rotation about the vertical axis. Provided

that sensor probes 1 and 2 always maintain contact with surface AB the unit vector (n^, Uy, 0) is

direcdy related to the positions measured by the sensor probes 1 and 2. The relations are given by

% =

ny =

X2-X1

^(x2-xi)2 + (y2-yi)^

y2-yi

^(x2-xi)2 + (y2-yi)^

(6.a)

(6.b)

From eqs (4) and (5),

- -bi ai

. ai bi J

cos6i

sin6

1 J

n.

L

^

J

(7)

ThcrcfoK, the angle of the beam rotation can be obtained diiecdy by solving eq (7).

cos6i =

sinOj =

-bi n 1 "x + ai n,

(ai

bj n,

+ bi2)

(8)

(ai2 + bi2)

52
DeterminatioD of the Beam Translation

Let (x^, y^) be 1}^ cooidiiialBS of apoint located on die beam, with lespect to the beam coordinate

firame. The cooidiiiates of diat same point with respect to die reference cooidinate firame are given

by

X

y J

cosOj sinO}

- sinO| cosO} [:]*[:;] (9)

Where 6| rqnesents the angular displac^nent of the beam cocnlinate frame with respect to the

refer^ice coordinate frame, and (uj, V|) the vecter of translation between die same frames.

To detenmne (ax, vj) , die eqs repres«iting the soi&ces to which the soisor probes are in ccmtact

must be used. Again, assuming a two-dimensicmal motion case, the relevant surfaces are T^nesQited by

Smfece AB: aj X12'' + biyi2^ = dj

Surface AD: a3 13^ + b3 y3^ = d3

(10.a)

(lO.b)

Where (x12^, y12'') is a beam point located on surface AB and (X3'', y3'') is a beam point located

on surface AD. Substituting eq (9) into eqs (10.a) and (lO.b), gives

aiI(^12-ai)«Kei - (yi2-vi)sin8i] + bj [ (xi2 - ^1) sinOj + (yi2-vi)

cosBi ]= dj (ll.a)

a3 [ (x3 - uj) cos8i - (y 3- vj) sinSj ] + b3 [ (X3 - uj) sinGj + (ys - vj) cosGj ] = d3 (ll.b)

fr^me and (X3, y3) are ihe oxHdinates of a point located csa surface AD with respect to the

refeu^ice frame.

53
After rearranging terms, eqs (11a and b) become

(aj cosGj + bj sinBj) u^ + (-a^ sinGj + bj cosSj) vj = (aj cosGj + bj sinGj) X12 + (-aj sinGj + bj cos9 j) yi2 + dj (12.a)

(a3 cosG^ + b3 sin6j) uj + (-sl^ sinOj + b3 cosGj) vj = (a3 cos9 J + b3 sin6j) X3 + (-a3 sin9j + b3 cos6j) y3 + d3 (12.b)

The sensor readings must satisfy eqs (12a and b).

Let (xj, yj, 0), (x2, y2» 0), (X3, y3, 0), be the sensor readings of probe 1, 2, 3, respectively.

Then

^2^1 + ^2^1 = §2

^3 "1 + ^3 ^1 = ^

(13)

where

^2 = ^1 = aj cosGj + bj sinGj

T|2 = T|i = - a^ sinGj + bj cosG i ^3 = a3 cosGj + bj cosGj

TI3 = - 33 sinGj + b3 cosGj

^1 = ^1 ^1 + T|i Yi + dj

^2 = fe X2 + Ti2 y2 + d2

53 = ^3 ^3 + ^3 ys + ^3

Rewriting eq (13), gives

^1 ^1 "

81

^2 ^2

" "1 -

. ^1 . = 82

h ^3 - - 83 -

(14)

54
Multiplying both sides of eq (14) by the transpose of the coefficient matrix, gives

'Pi qi

"

. qi ^1 .

=

«i

. pi - where

Pi = ^1^ + ^2^ + fe^

11 = ^1^1 + ^2^2 + ^3^3

ri = Tli^ + ^^ + 1132

«1 = ^1 ^1 + ki h + ^3 S3

Pi = ^1^1 + TI2S2 + 113 S3

Solving eq (15), gives the translation vector of the beam as

(15)

ui = «in - Pi qi

Pl ri - qi^

Vl =

Pl Pl - qi ^\

Pl n - qi^

(16)

IV. 2 The Control Algorithms

Let the coordinates of an arbitrary point on the beam be (x^, yb)^ with respect to the beam

coordinate frame. Equation (9) gives the coordinates ( x , y ) of the same point with respect to the

reference frame. If a target position ( x^, yt ) is specified, the displacement vector required for the

point to reach the target position from its current position is

Ax = x^ Ay = y

vt .

The displacement vector ( Ax, Ay ) is related to the beam increments of the angle of rotation and

the translation as follows:

Ax

= AGj

- sinGj cos9j xb - +

Auj

Ay J

- cosG]^ - sinGj [ yb \ Av^

(17)

55
Defining

X =

y =

- xt> sin 0j + ybcos0j

- x^ cos9j - yt> sinGj

eq (17) can be rewritten as

Auj + (AGj) X = Ax

Avj + (AGj) y* = Ay

(18)

(19)

Another eq needed to solve for AGj, Auj and Avj is obtained by minimizing the following

quadratic functional

I = a(Aui)2 + P(Avi)2 + y(AGi)2

resulting in

aAu^ + pAvj + yAG^ =

(20)

(21)

where a, P, y are constants. Therefore, the required amounts of beam displacement are obtained

by solving eqs (18), (19) and (21):

Au 1

Av

AG 1 J

n-1 r- 1 X

J. Ax

1 * y Ay

a P y _

-

(22)

An IBM-AT/PC computer was programmed to solve eq (22). The program was written in C

language. The feedback signals came from the three LVDT position sensors. Their outputs were

filtered by antialiasing filters and then passed through voltage amplifiers to adjust their voltage

range (see fig. 12). The signals were then digitized by an A/D data acquisition system which was

connected and controlled by the computer. At the end of each control cycle the digital command

signals were sent to a D/A converter, which was connected and controlled by the computer. The

D/A output signals were passed through voltage amplifiers to adjust their range before reaching the

three servo-amplifier controllers responsible for driving the three torque motors.

56
Several square pulse excitation tests were conducted to tune the servo amplifiers and the digital

controller. The rise and fall times of the x-y table servo joints for a square pulse amplitude which

was 40% of the maximum command signal amplitude were approximately 0.8 seconds. The

corresponding times for a square pulse amplitude which was 100% of the maximum command

signal amplitude for the rotary joint was 2.5 seconds.

<JU . S . GOVERNMENT PRINTING F F I C E i 1 9 8 9- 2 4 2 - 2 / 4

3

57

NBS-IMA (REV. 2-80) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

NIST/TN-1267

2. Performing Organ. Report No. 3. Publication Date July 1989

4. TITLE AND SUBTITLE

ROBOT CRANE TECHNOLOGY PROGRAM

(FINAL REPORT)

5. AUTHOR(S) Nicholas Dagalakis, James S. Albus. Kenneth R. Goodwin, James D. Lee,

Tsung-Ming Tsai, Hoosh Abrishamian, Roger Bostelman, Charles Yancey

6. PERFORMING ORGANIZATION (If joint or otiier than NBS. see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURQ, MO 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Defense Advanced Research Projects Agency (DARPA)

1400 Wilson Blvd.

Arlington, VA 22209-2308

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) , , -i • , . The effort to develop kinematically constrained, dynamically stabilized, robot

cranes capable of lifting, moving and positioning heavy loads over large volumes,

capable of supporting fabrication tools and the inspection of large size and

difficult to reach structures, is described in this report.

The approach taken was to build on previous work at the NIST Robot Systems Division

which has analyzed and measured the stiffness of a small model six-cable suspension

system. The system is a modified Stewart platform.

Under DARPA sponsorship, we have:

1. Extended this work to measure and optimize the stiffness of full-size models.

2. Actively damped oscillations in a small scale six-cable suspension platform.

3. Constructed an intermediate sized six-cable suspension platform for an industrial robot.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

automation; construction; crane; factory; model; robot; stiffness; testing

13. AVAILABILITY

rj(] Unlimited

I I For Official Distribution. Do Not Release to NTIS

K~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

12Z\ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

60

15. Price

USCOMM-DC 6043-PBO

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