Glamorgan University, School of Technology
Module: Manufacturing Systems
Submission Date: 24th May 2005

Robotic Application

by: Laura Germer

Table of Contents

1. Introduction 3

2. Scenario Description 3

3. Robot  4

3.1. Drive System  5
3.2. Robot Geometry  5
3.3. Motion Control  7
3.4. Teaching 7
3.5. Payload 8
3.6. Repeatability + Accuracy  9
3.7. Robot Selection  10
3.8. Work Envelope and Space Requirement 11

4. End-Effector  12

5. Sensing System 14

5.1. Proximity Sensing 15
5.2 Tactile Sensing  16
5.3 Actuator Sensing  16
5.4 Future Development  18

6 Safety Equipment and Procedures  18

7 Maintenance 19

8 Proposed Shop Floor Layout  19

8.1 Ancillary Equipment 19
8.2 Layout 20

9 Evaluation of Robot System 21

9.1. Possible Benefits  21

10 Alternative Production Methods 23

10.1 Manual Spraying 23
10.2 Rotary Disk Atomization  23
10.3 Spraying Nozzles 24
10.4 Immersion Bath 25
10.5 Dyed Plastics 25

11 Conclusion 26

12. Bibliography 27

1. Introduction

This assignment is meant to enable the student to demonstrate her ability to design a shop floor production scenario. To do so the complete configuration of a robot manufacturing operation had to be planned. This included selection of robot, drive system, teaching method, end effectors, sensing systems, actuators, ancillary and safety equipment. Furthermore economic considerations like maintenance and benefits are discussed. To round up the assignment up reflections about alternative production methods and a conclusion are incorporated. Due to the complexity of the subject and the number of issues required in the assignment outline the word count had to be exceeded to give reasonable explanations.

2. Scenario Description

The manufacturing scenario is the spray painting of bumpers. The bumpers are moved with a special conveyor system. The conveyor consists of holder racks on which two bumpers can be secured at the same time, one below the other, while being supported on the back side for painting (see Illustration 1) The movement of the conveyor is linear. The conveyor is moving slowly to allow the robot enough time to perform the painting operation, but it does not stop. After the paint spraying the conveyor crosses a fast drying device.

Illustration 1: Paint Spraying Booth (ABB) [illustration only in downloadfile]

3. Robot

To develop a robot application it is useful to define first what distinguishes a robot from other manufacturing instruments. According to Todd (1986, p.9) a robot has the following characteristics. It must be:

o produced by manufacture rather than by biology
o able to move physical objects or be mobile itself
o a power or force source or amplifier
o capable of some sustained action without intervention by an external agent
o able to modify its behaviour in response to sensed properties of its environment, and therefore be equipped with sensors.

Furthermore we can tell the difference between manipulators and vehicles. The first are connected to the ground to transmit forces and torques, the second propel themselves about on a surface (Todd, 1986, p.15). In case of the given scenario we therefore talk about a manipulator. In the following table the options for each given specification categories are listed. The options picked for the scenario are highlighted. Explanation for the reasons follows in the remainder of the chapter.

Table 1 [table only in downloadfile]

3.1. Drive System

Even though a pneumatic actuator drive is the least expensive option it is not suitable since they normally require a mechanical stop to their movement (Asfahl, 1992, p.144). This means only the end point of their movement is defined, which is fit for machine servicing, but for the painting operation a control of the path is crucial. A hydraulic drive would be the most expensive option and the forces that the hydraulic drive could muster are not needed for the painting operation. Because the hydraulic drive operates with incompressible fluids (mostly oil), it requires a pumping mechanism and a fluid tank which adds additional bulk and maintenance requirements.

Electric drives comply with the demand of precision movement and are affordable. To prevent sparks that may cause a fireproof version needs to be implemented. An AC servo motor was chosen, because servo motors are used for operations of controlled motion and where closed loop situations exist (Asfahl, 1992, p.47). This is the case in the spray painting scenario since the programme requires feedback from the robot to produce good quality. The AC servo motor does not contain the threat of sparks in contrast to the DC version.

3.2. Robot Geometry

Polar and cylindrical configurations are designed for the handling of parts which require orientation maintenance. Therefore their design is not ideal for the given scenario.

According to Asfahl (1992, p.140) Cartesian configuration robots are normally mounted to the ceiling with a kind of gantry. This gantry would be clogged with paint particles over time and impair proper movement of the robot. Due to these reasons an articulated configuration was selected. This means that the robot can basically perform the movements of a human arm (see illustration 4). Especially the flexibility of the "wrist" component is important for the painting operation, because it allows fine adjustment to the surface. The choice of an articulated robot implies six degrees of freedom (see Illustration 2), one for each joint.

[...]