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英文原文

THE STRUCTURE DESIGN AND KINEMATICS OF A ROBOT

MANIPULATORml. THEORY

KESHENG WANG and TERJE K . LIEN

Production Engineering Laboratory, NTH-SINTEF, N-7034 Trondheim, Norway

A robot manipulator with six degrees of freedom can be separated into two parts: the arm with the

first three joints for major positioning and the wrist with the last three joints for major orienting. If

we consider theconsecutive links to be parallel or perpendicular, only 12 arm and two wrist

configurations are potentially usefuland different for robot manipulator mechanical design. This

manipulatordesign, the selection of the kinematic chain of therobot manipulator is one of the most

importantdecisions in the mechanical and controller designprocess.

In order to position and orient the end effector ofthe robot manipulator arbitrarily, six degrees

offreedom are required: three degrees of freedom forposition and three degrees of freedom for

orient-ation. Each manipulator joint can provide onedegree of freedom, and thus a manipulator

musthave a minimum of six joints if it is to provide sixorthogonal degrees of freedom in position

andorientation.

The construction of manipulators depends on thedifferent combination of joints. The number

of poss-ible variations of an industrial robot structure can bedetermined as follows:

V =6

examplesix axis 46,656 chains are possible. 6 However, alarge number is not appropriate for

kinematicreasons.

We may divide the six degrees of freedom of arobot manipulator into two parts: the arm

whichconsists of the first three joints and related links; andthe wrist which consists of the last

three joints andrelated links. Then the variations of kinematic chainswill be tremendously reduced.

Lien has developedthe constructions of arm and wrist, i.e. 20 differentconstructions for the arm

and eight for the wrist.2

In this paper, we abbreviate the 20 different armsinto 12 kinds of arms which are useful and

different.We conclude that five kinds of arms and two kinds ofwrists are basic constructions for

commercial indus-trial robot manipulators. This kind of simplificationmay lead to a general

joints andrelated links is called the wrist.

(b) Two links are connected by a lower pair joint.Only revolute and linear joints are used in

robotmanipulators.

(c) The axes of joints are either perpendicular or

According to the authors' knowledge, thisassumption is suitable for most commercially

usedindustrial robot manipulators. We can consider thestructure of arm and wrist separately.

2.1. The structure o f the arm o f robot manipulator

(a) Graphical representation. To draw a robot inside view or in perspective is complicated

and doesnot give a clear picture of how the various segmentsmove in relation to each other. To

draw a robot in aplane sketched diagram is too simple and does notgive a clear construction

combin-ation has four kinds of sub-combination. Thus, 32combinations can be arrived at:

If the second joint is a linear joint and both the otherjoints are perpendicular to it, two choices

in relationto the first and the third joints are considered paral-lel or perpendicular.

In all, there are 36 possible combinations of a simplethree-joint arm.

Nine of 36 possible combinations degenerate intoone or two degrees of freedom.

Seven of the remainder are planar mechanisms.Thus, there are 20 possible spatial simple

arms.

Let us consider R1 [1 L2 I L3 in whichthe first joint permits rotation about the vertical

axis,the second joint is a vertical linear joint (i.e. parallelto the first), and the third joint is a

(joint No. 2). Counting onlyone linkage to represent the group of equivalentswill eliminate eight

(c) Five basic types o f manipulator arm. Althoughthere are 12 useful and different

arm-configurationswhich can be used in the design of a robot man-ipulator arm, in practice only

some of them arepractical and commonly used. We find that mostcommercially available

industrial robots can bebroken down into only five groups according to the.

characteristics of their arm motion and geometricalappearance.The five groups can be

defined as follows and areshown in Fig. 6.

1. Cartesian ( L I L I L)

2. Cylindrical (R II L 1 L)

3. Spherical (R I R I L)