framework and implementation of two stage alignment for large components based on p o and f t

Peer-review under responsibility of the scientific committee of the 5th CIRP Global Web Conference Research and Innovation for Future Production doi: 10.1016/j.procir.2016.10.019 Science

Procedia CIRP 56 ( 2016 ) 73 – 78

2212-8271 © 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer-review under responsibility of the scientific committee of the 5th CIRP Global Web Conference Research and Innovation for Future Production

doi: 10.1016/j.procir.2016.10.019

ScienceDirect

9th International Conference on Digital Enterprise Technology - DET 2016 – “Intelligent Manufacturing in

the Knowledge Economy Era Framework and implementation of two-stage alignment for large

components based on P&O and F/T

a School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China

* Corresponding author Tel.: +86-10-8231-6795; fax: +86-10-8233-8165 E-mail address: du_fuzhou@163.com

Abstract

A two-stage alignment framework of large components based on position and orientation (P&O) and force/torque (F/T) is put forward to solve

the adjustment difficulties of accurate P&O during the alignment process, considering the geometric and physical characteristics of large

components The basic environment and enabling technologies of the framework are introduced The P&O-guided alignment and the

F/T-driven alignment of the framework are set up Then, a mathematical model of P&O and assembly relationship model of P&O for P&O-guided

alignment are proposed Moreover, a six-dimensional F/T analytical algorithm based on screw theory for F/T -driven alignment is given The

geometric and mechanical models of components are presented Based on the obtained models, the compliance assembly strategies are

analyzed The alignment experiment, using aerospace products, was performed on the self-designed alignment system, and the experimental

results proved the proposed the framework based on P&O and F/T is correct and effective

© 2016 The Authors Published by Elsevier B.V

Peer-review under responsibility of the Scientific Committee of the “9th International Conference on Digital Enterprise Technology - DET

2016

Keywords: P&O-guided; F/T-driven; Compliance assembly; Large components alignment

1 Introduction

In recent years, with the rapid development of the

assembly technology toward being digital, flexible and

intelligent, Germany and the United States as the

representative of the world's leading industrial countries are

moving towards a new round of industrial revolution which

uses intelligent equipment and information communication

Various information sensing technologies and data collection

methods are the premises of industrial automation and

information technology integration in the intelligent assembly

Measurement Aided Assembly (MAA) [1]-[2] is one of its

forms of implementation, also is the inevitable trend of large

component intelligent assembly technology development The

large components such as airframes, satellites and rockets

typically have similar characteristics i.e., large size, easily

deformed, complex coordination relationship, etc In the

conventional assembly process, large scale fixtures, which

consist of large steel structures configured for a special

component or structure, are used to position the components,

with manual operation to realize the geometrical relationships and constraints between components, and to ensure their variation within acceptable tolerances With such an assembly process it is difficult to accurately adjust the position and orientation (P&O) Moreover, it needs a large work space and

a lot of manpower So, the conventional low accuracy and inefficient assembling process cannot meet the demand of advanced, flexible, accurate and high-efficient large components assembly [3]-[4]

The development of large scale metrology technologies, which are based on the high-precision and efficient digital measurement systems, have become the key technologies during assembly for process control and quality assurance To improve the efficiency and precision of assembly, a novel assembly system based on MAA is proposed by manufacturers and researchers which has been employed in level docking system in large component assembly of spacecraft [5], digital alignment system for large component assembly of aircraft [6]-[7], vertical docking system in large component assembly of satellite[8], etc

© 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the scientifi c committee of the 5th CIRP Global Web Conference Research and Innovation for Future Production

The MAA technology uses the digital measurement system

(such as electronic theodolite, laser tracker, indoor GPS,

camera systems) to measure the P&O of components and

assist the adjustment of P&O [9]-[10] However, the

electronic theodolite and laser tracker are by-point

measurement way Indoor GPS can be used in a fixed space,

and be susceptible to signal interference Camera systems are

more sources of error With the improvement of the

manufacturing accuracy, the accuracy of the measurement

system will be lower than that of assembly design, which will

fails the assembly In robotic assembly applications (such as

peg-in-hole, surface grinding), when the assembly objects are

in contact during assembling, the compliant control methods

that make the interaction forces as a constraint condition are

used to recognize and change the contact state, and finish the

assembly [11] These methods provide a new assembly idea

for the MAA technology The force measurement and control

technology will depend on two important parts: sensors and

force control

configuration of the six-dimensional F/T sensor based on SP,

the task-oriented design method of the six-dimensional F/T

sensor and a six-dimensional F/T sensor to complete

peg-in-hole assembly task have been introduced in [12]-[13] A

six-beam sensor based on SP and an idea of “joint less” structure

and beam sensors to improve the precision and sensitivity in

measuring a small F/T have been proposed in [14] A

six-dimensional heavy F/T sensor with high stiffness and good

linearity based on SP has been presented in [15]

Experimental results verified the feasibility and validity of the

sensor by the established calibration platform [15]

algorithm based on a six-dimensional F/T sensor and a hole

detection algorithm have been reported in [16] The

six-dimensional F/T sensor to estimate the contact phases and

designed the assembling strategy to achieve the force-guided

characteristics for a force-guided robotic assembly have been

studied and analytical derivations for different contact states

are presented in [20] A modified control scheme for SP with

compensation for interaction force control and positional error

recovery is introduced in [21].A novel strategy of the

high-precision chamferless peg-hole insertion with a

six-dimensional F/T sensor is introduced in [22]

The MAA technology for large component assembly so far

presented only considers the geometrical characteristics and

doesn't give any attention to the physical characteristics

Focusing on the geometric and physical characteristics of

large components, this paper proposes a two-stage alignment

paper is organized as follows: Section 1 introduces the

development and application of MAA technology, highlights

assembling Section 2 provides the two-stage alignment

framework of large components based on P&O and F/T in

model.Section 4 provides the F/T-driven alignment based on

six-dimensional F/T feedback A practical alignment system is

and the obtained results are discussed in section 5 Section 6 concludes the paper and assesses the presented framework

2 Framework of large components two-stage alignment based on P&O and F/T

As shown in Fig 1, the two-stage alignment framework of large components based on P&O and F/T can be divided into two sections:

(1)The base environment for aligning large components:

two-stage alignment of large components based on P&O and F/T are shown in Fig 1 The software system includes Product Data Management (PDM) system, database, integrated control platform, etc The hardware system includes a digital measurement system, force sensors, P&O adjustment platform, fixed platform, control cabinet, assembly fixtures, etc The software and hardware will communicate with each other

Fig 1 The two-stage alignment framework of large components based on

P&O and F/T (2)The enabling technologies for aligning large component: The alignment based on P&O and F/T has two stages as illustrated in Fig.1 ķ The P&O-guided alignment The measurement process model is integrated with the assembly planning to instruct the deployment and planning of digital measurement systems, enabling automation Through processing and analyzing measurement data in the alignment process, the geometrical information of components viz., P&O, dimension, variation and others are calculated and compared with the design requirements Then, the feedback from the analysis is used to adjust the P&O adjustment platform ĸ The F/T-driven alignment The compliant assembly model is integrated with the assembly planning to follow the six-dimensional F/T feedback, enabling automation Through processing and analyzing measurement data in the

components is calculated Then, the feedback from the

analysis is used to adjust the P&O adjustment platform The

two-stage alignment of large component based on P&O and F/T

in this paper can be described by the flowchart shown in Fig 2

Fig 2 The two-stage alignment of large component

3 P&O-guided alignment based on model

3.1 Mathematical model of P&O

Schematic diagram of the P&O adjustment platform is

shown in Fig 3 It consists of a moving platform and a

stationary platform, which are connected using six stretchable

limbs through spherical joints Such arrangement offers

6-Degrees-Of-Freedom (DOF) motion due to the movements of

six limbs as a whole The cartesian coordinate system o 0 -x 0 y 0 z 0

is located in the center of the top surface of the stationary

platform and the cartesian coordinate system o 1 -x 1 y 1 z 1 is located

in the center of the bottom surface of the moving platform The

centers of the spherical joints are denoted as A i and B i (i=1,…,6)

Fig 3 Schematic diagram of

the P&O adjustment platform Fig 4 The relationship between P&Os

The P&O of the moving platform with respect to o 0 -x 0 y 0 z 0

{x,y,z,Į,ȕ,Ȗ}, which includes the amount of rotation and

displacement of o 1 -x 1 y 1 z 1 with respect to o 0 -x 0 y 0 z 0 Where Į,ȕ,Ȗ

are the rotational angles of each axis of o 1 -x 1 y 1 z 1 ; x,y,z are the

displacements of the original point of o 1 -x 1 y 1 z 1

position of A i in o 0 -x 0 y 0 z 0 and o 1 -x 1 y 1 z 1 can be expressed in the

form of vectors as P i0 = [A i0,1]T , P i1 = [A i1,1]T Where A i0 =

(x i0 ,y i0 ,z i0 ) is the coordinate in o 0 -x 0 y 0 z 0 A i1 = (x i1 ,y i1 ,z i1) is the

descriptions of the same point; hence they can be related

using a linear transformation as follows:

3 3 3 1

1 0 1 0, 1 0 [ ]

i i

R M

T P P T u u

where R3 h 3 is called the rotation matrix, and M3 h 1 is called the

displacement vector If P i1 is known, and P i0 is obtained

though measurement, according to (1), the x,y,z,Į,ȕ,Ȗ can be

calculated by measuring the coordinates of at least three points which are mutually non-collinear

3.2 Assembly relationship model of P&O

The position of A i in o 2 -x 2 y 2 z 2 can be expressed in the form

of a vector as P i2 =[A i2 ,1] T , where A i2 =(x i2 ,y i2 ,z i2 ) is the coordinate in o 2 -x 2 y 2 z 2 As shown in Fig 4, the P&O of the

moving platform with respect to o 0 -x 0 y 0 z 0 can be expressed by

relation to o 0 -x 0 y 0 z 0 can be expressed by T 2-0 and the P&O of

the moving platform with respect to o 2 -x 2 y 2 z 2 of measurement

coordinate system is expressed by T 1-2

T 1-0 , T 2-0 and T 1-2 are related as T 1-0gP i1 = P i0 , T 2-0gP i2

=P i0 , T 1-2gP i1 =P i2 The relationship between these P&Os is

as follows: T 1-0 = T 2-0gT 1-2 When there are multiple P&Os in the alignment process, their relationship can express the final assembly relationship and determine the assembly parameters The relationship between P&Os can be obtained following the method so far discussed

4 F/T-driven alignment based on six-dimensional F/T feedback

4.1 Analytical algorithm of six-dimensional F/T

According to screw theory, the external load [F s M s]T on the

moving platform in o 1 -x 1 y 1 z 1 as shown in Fig 3, can be calculated by applying the force equilibrium equation as follows

[ ]

F G f (2) where F=[F s M s]T =[F x F y F z M x M y M z]T , f=[f 1 f 2 f 3 f 4 f 5 f 6]T

> @

6 6

1 1

01 02 03 04 05 06

A B

A B





G

(3)

where A i and B i are coordinates in o 1 -x 1 y 1 z 1 f i is the measured

calculated using (2)

4.2 Geometric and mechanical models of components

The components which are studied in this paper have certain rigidity, and their P&Os are adjusted for alignment at low speeds Therefore, the alignment process for the large component can be described as a typical peg-in-hole assembly The contact state of three points on the upper circle surrounding the hole is determined by the P&O-guided alignment process, which is the beginning of the F/T-driven alignment process The geometric and mechanical models of

components are analysed as shown in Fig.5 In (b), F and M are calculated by using the analytical algorithm f1 and f2 are

the supporting forces and ȝ is the friction coefficient

Following equations can be established

l d D

d s l



­

¯

(4)

2 1

yp

zp

xp

E P

°

¦

®

¯

(5)

Fig 5 (a) Geometric analysis; (b) Mechanics analysis

4.3 Compliance assembly strategies

When the three contact points on the upper circle

surrounds the hole, the P&O of peg should be adjusted Say,

F h and M x are applied (as in fig.6)

Fig 6 Mechanics analysis of P&O adjustment process

°

®

¯

(6)

When the peg is in uniform motion and a yh =0, a zh =0, Į xp=0,

f 1 > 0, f 2> 0, then

cos sin

yh

zh

F

F



!

 (7)

2 2

2 2

2

2

cos 1

sin 1

xp

h

d d

M

F

P

M P

P

M P







(8)

The P&O of peg can be adjusted until the next contact state satisfying the above relationship The same analytical method

is used for adjusting other contact states as well From the change of contact state, F/T-driven alignment process is described in Fig 7

Fig 7 F/T-driven alignment process from the change of contact state

5 Experimental results and Discussion

As shown in Fig 8 and Fig.9, the designed experimental system includes laser tracker, force sensors, P&O adjustment platform, fixed platform, control cabinet, assembly fixtures and an integrated control platform As shown in Fig 10 and Fig.11, the GUI for integrated control platform includes functional areas, navigation tree, a graphical display for status monitoring and functional dialogs Experimental process is divided into two stages:

Fig.8 Experimental environment of P&O-guided alignment process

Fig 9 Experimental environment of F/T-driven alignment process (1) P&O-guided alignment process: Firstly, measurement plan is carried out that includes laser tracker configuration and station planning, P&O measurement characteristics planning (planning results as in Fig 8) etc Then, the measurement field is constructed Secondly, Laser tracker automatically measures the P&O measurement characteristics to fit the P&O

of components Then the assembly coordination is determined The automatic measurement dialog and the assembly

coordination dialog are shown in Fig 10 Thirdly, the

calculation results are used to adjust the P&O adjustment

platform These steps are repeated until the P&O adjustment platform reaches the target P&O

Fig 10 GUI of Integrate control platform for P&O-guided alignment process

Fig 11 GUI of Integrate control platform for F/T-driven alignment process (2) F/T-driven alignment process Firstly, the experimental

setup has six force sensors which are placed in each limb of

P&O adjustment platform to measure the forces on each limbs and then the six-dimensional F/T is calculated based on the

analytical algorithm (as shown in Fig 9) Secondly, the

gravity of the moving platform, assembly fixtures and

components are compensated The gravity compensation

dialog in GUI of integrated control platform is shown in Fig

11 Thirdly, the compliance assembly is started (as in Fig 11)

The calculation results are used to adjust the P&O adjustment

platform The F/T-driven alignment process is repeated until

the interaction force meets the threshold value

In the above experiments, the alignment process is

different stages In the P&O-guided alignment process, the

precision of the system depends on the precision of digital

measurement system and the positioning accuracy of P&O

adjustment platform In the F/T-driven alignment process, the

precision of the system depends on the precision of force

sensors The precision analysis is out of scope of this paper as

this document is drafted to introduce the idea

6 Conclusions

Considering the geometry and physical characteristics of

the alignment of large component, a two-stage alignment

framework for large components based on P&O and F/T

methods is presented in this paper It can evaluate the target

alignment quickly from the geometry and mechanical

parameters Two stages of framework are presented, which

are the P&O-guided stage and the F/T-driven stage The

measurement process model and the compliance assembly

model are presented to design an intelligent alignment process

The implemented framework contains functionality that

supports planning and automatic measurements, P&O fitting,

MAA automation, six-dimensional F/T measurement and

compliance assembly automation The alignment experiment

was performed on the self-designed alignment system using

aerospace products and the relevant experimental results

proved that the proposed two-stage alignment framework for

large components is effective Future research will focus on

the accuracy analysis of six-dimensional F/T measurement

and more applications of artificial intelligence technology in

the F/T-driven stage

The proposed alignment approach can be applied to the

sleeve connection of large components, just like peg-in-hole

assembly It can also be applied to the alignment of surface

and plurality of holes for large components But the large

components should have enough rigidity

Acknowledgements

This work is under the support of National Defense Basic

Scientific Research (No A2120132007) and Fund of National

Engineering and Research Center for Commercial Air-craft

Manufacturing (No SAMC14-JS-15-055)

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