Lecture 2 - Arm configuration PDF

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Summary

In these notes the Arm configuration of Robot is explained. How to take the degree of freedom with the examples are explained. Also the wrist configuration is explained....

Description

Lecture 2 Arm Configuration Human Arm Characteristics 2 dof

1 dof

3 dof

1 dof

Joints and Joint Notation Scheme Types of Robot Joints

Required Degrees of Freedom    

For moving objects placed on a line or a circle - 1 dof For moving objects anywhere on a flat surface - 2 dof Picking objects anywhere on a 3D space - 3 dof Reaching in to complex profiles - > 3 dof

Arm Configurations

Arm Configurations

Robot Configurations and Work Envelope  Cartesian

Work Volume or Work Envelope is a term that refers to the space within which a robot can manipulate its wrist’s end

 Cylindrical

 Spherical

 SCARA (Selective compliance assembly robotic arm) A variation of the cylindrical configuration involving, 1 linear joint and 2 rotary joints

 Arm Configurations

 Polar

 Jointed Arm Configuration

 Robot Configurations Selective compliance assembly robotic arm o SCARA () o A variation of the cylindrical configuration, involving o 1 linear joint and 2 rotary joints

 Wrist Movements

 Degrees of Freedom Degrees of freedom is often used to describe the number of directions that a robot can pivot or move a joint.

 Further classification of Joints

 Joint Notation Scheme

TRR

 Sketch the following arm configurations For the following arm and body designations describe the robots using sketches LLR, RLR, LRR, LVR, LL - TRL  Required Degrees of Freedom o For moving objects placed on a line or a circle - 1 dof o For moving objects anywhere on a flat surface - 2 dof o Picking objects anywhere on a 3D space- 3 dof

o Reaching into complex profiles - > 3 dof

 Examples Ex 1

2 DOF

Ex 2

3 DOF

Ex 3

3 DOF

Ex 4

4 DOF

Ex 5

5 DOF

Ex 6.

6 DOF

Wrist Configurations  Arm configurations carry and position the wrist, the second major part of a Manipulator, Wrist is attached to the end point of the arm  Wrist subassembly enables the manipulator to orient the end-effector to perform the desired task properly.  The end effector, if gripper, must be oriented at an appropriate angle to pick and grasp a work piece.  For an arbitrary orientation of wrist in 3-D space, the wrist must possess at least 3-DOF.  This 3-DOF gives three rotations about the three principal axes.  Less than 3-DOF wrists are also used depending on the requirement.  Wrist must be compact and at the same time must not affect the performance of the arm.

Wrist with Roll, Pitch and Yaw motions is shown below.

 Design and Control Issues o Robots require higher mobility and dexterity than conventional machine tools. o The mechanical structure of a robot consists of rigid cantilever beams connected by hinged joints. o This is inherently poor in stiffness, accuracy and load carrying capacity.

 Design and Control Issues o The errors accumulate because the joints are in series. o The position and motion of each joint is affected by the position and motion of other joints. o The weight and inertial load of each link is carried by the previous link and the links undergo rotary and linear motion, making centrifugal and Coriolis accelerations significant.

 Manipulation and Control

o In the analysis of spatial mechanisms (manipulators), the location of links, joints and end effector in 3-D space is continuously required. o These need to be calculated using mathematical methods. o To describe the position and orientation of a body in space a frame or a coordinate system is attached to the body o The position and orientation of this frame with respect to a reference frame, mathematically describes the location of the body.

 Robot Specifications o Drive System Electrical Hydraulic Pneumatic o Speed of Motion o Load-Carrying Capacity (Pay – Load) o Control Systems o Precision of Movement Spatial Resolution Accuracy Repeatability o Work Volume

 Drive System o Hydraulic drive: gives a robot great speed and strength. These systems can be designed to actuate linear or rotational joints. The main disadvantage of a hydraulic system is that it occupies floor space in addition to that required by the robot. o Electric drive: compared with a hydraulic system, an electric system provides a robot with less speed and strength. Accordingly, electric drive systems are adopted for smaller robots. However, robots supported by electric drive systems are more accurate, exhibit better repeatability, and are cleaner to use. o Pneumatic drive: are generally used for smaller robots. These robots, with fewer degrees of freedom, carry out simple pick-and-place material handling operations.  Speed of Motion o Maximum speed of 500 degrees/second can be achieved. o Highest speed can be achieved by large robots with arm extended fully. o Hydraulic robots tend to be faster than electric drive robots. o Robots with electric drives have better control. o Selection of the most desirable speed also depends on - Accuracy of positioning of the object - Weight of the object to be handled - Distances to be moved.

 Spatial Resolution, Accuracy, Precision

o Spatial Resolution is the smallest increment of movement into which the robot can divide its work volume. o Accuracy refers to a robot’s ability to position its wrist at a target point within its work volume.  Spatial Resolution, Accuracy, Repeatability

Repeatability is the ability of the robot to position its wrist at a point in space that had previously been taught to the robot.

 Load Carrying Capacity o The load the robot can carry at its weakest position without including the weight of the end effector, while maintaining its specified accuracy. o Effective/Net load carrying Capacity = (Load carrying capacity – weight of the end effector)@ the weakest position...