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AN2350

An overview on robotics

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An overview on robotics

1.1

Structure of a manipulator

A manipulator, from a mechanical point of view, can be seen as an open kinematic chain

constituted of rigid bodies (links) connected in cascade by revolute or prismatic joints, which

represent the degrees of mobility of the structure. These manipulators are also known as

serial manipulators.

Only relatively few commercial robots are composed of a closed kinematic chain (parallel)

structure. In this case there is a sequence of links that realize a loop. From this point on, we

refer only to serial manipulators.

In the chain mentioned above, it is possible to identify two end-points: one end-point is

referred to as the base, and it is normally fixed to ground, the other end-point of the chain is

named the end-effector and is the functional part of the robot. The structure of an end

effector, and the nature of the programming and hardware that drives it, depends on the

intended task.

The overall motion of the structure is realized through a composition of elementary motions

of each link respect to previous one.

A revolute joint allows a relative rotation about a single axis, and a prismatic joint permits a

linear motion along a single axis, namely an extension or retraction.

It is assumed throughout that all joints have only a single degree-of-freedom: the angle of

rotation in the case of a revolute joint, and the amount of linear displacement in the case of

a prismatic joint.

The degrees of mobility must be suitably distributed along the mechanical structure in order

to furnish the needed degrees of freedom (DOF) to execute a task. If there are more

degrees of mobility than degrees of freedom the manipulator is said to be redundant.

The workspace of a point H of the end-effector is the set of all points which H occupies as

the joint variables are varied through their entire ranges. The point H is usually chosen as

either the center of the end-effector, or the tip of a finger, or the end of the manipulator itself.

The workspace is also called work volume or work envelope.

Size and shape of the workspace depend on the coordinate geometry of the robot arm, and

also on the number of degrees of freedom.

The workspace of a robot is a fundamental criterion in evaluating manipulator geometries.

Manipulator workspace may be described in terms of the dexterous workspace and the

accessible workspace. Dexterous workspace is the volume of space which the robot can

reach with all orientations. That is, at each point in the dexterous workspace, the end-

effector can be arbitrarily oriented.

The accessible workspace is the volume of space which the robot can reach in at least one

orientation. In the dexterous workspace the robot has complete manipulative capability.

However, in the accessible workspace, the manipulator's operational capacity is limited

because of the terminal device can only be placed in a restricted range of orientations.

In other words, the dexterous workspace is a subset of the accessible workspace.

Table 1 shows a classification of the manipulators accordingly to the type and sequence of

the degrees of mobility of the structure, and of their workspaces.

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