04-08-2012, 12:38 PM
Manipulator Arms
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The common industrial manipulator is often referred to as a robot arm, with links and joints
described in similar terms. Manipulators which emulate the characteristics of a human arm are
called articulated arms. All their joints are rotary (or revolute). A representative articulated
manipulators is the ASEA robot.
The motion of articulated robot arms differs from the motion of the human arm. While robot
joints have fewer degrees of freedom, they can move through greater angles. For example, the
elbow of an articulated robot can bend up or down whereas a person can only bend their elbow in
one direction with respect to the straight arm position.
Many applications do not require arms with articulated (or revolute) geometries. Simpler
geometries involving prismatic or sliding joints are often adequate. Prismatic and revolute joints
represent the opposite extremes of a universal screw. In a revolute joint, the screw pitch is zero,
constraining the joint to pure rotation. In a prismatic joint, the pitch is infinite, constraining the
joint to pure sliding motion. Revolute joints are often preferred because of the strength, low
friction and reliability of ball bearings. Joints that allow a combination of translation and
rotation (such as lead screws) are not normally used to join the links of robot arms.
Wrists
The kinematic structure of the robot arm allows to postion its end point at any (x,y,z) location in
the 3D space (…. within the robot's working space)
In order to provide for the proper orientation of the hand/end-effector the robot arm should have
a wrist. Typically a robot wrist provides the same 3D rotations as a human hand: roll, pitch, and
yaw. A wrist where the three axes of rotation intersect is called a spherical wrist. These have
the advantage that the mathematical model used to calculate the wrist joint angles from their
position and orientation in space is soluble.
One problem in achieving spherical wrist design is the physical difficulty of fitting all the
components into the available space. The size of the human wrist is small because the muscles
which power it are located in the forearm, not in the wrist.