Continuous, Limited Sequence,Speed of Response, Control Stability

1. OVERVIEW
 Classification of robots based on path control
 Speed of response and stability
 Control of stability in a robot

2. Classification of Robots based on Path Control
 Commercially available industrial robots can be classified into three categories:
i. Limited – Sequence Robots
ii. Playback robots with point-to-point control
iii. Playback robots with continuous path control

3.  Limited Sequence Robots; do not use feedback to indicate relative positions of the
joints.
 They are controlled by limit switches and/or mechanical stops to establish the
endpoints of travel for each of their joints.
 To establish the positions and sequences, mechanical set up is done rather than robot
programming.
 The individual joints are moved to the extreme positions.
 The main purpose of the controller is to send the signals in a particular sequence.
 Used for pick and place operations.
 The main disadvantage is there is no feedback to indicate desired point is reached.

4.  Playback robots use a more sophisticated control.
 The robot is taught a series of positions or motions, recorded into memory and then
repeated under its own control.
 The procedure of teaching and recording into memory is called as programming the
robot.
 Playback robots have a feedback to ensure the robot has reached the positions that
have been taught.
 Playback robots are classified into two categories: Point-to-Point Control (PTP) and
Continuous Path Control (CP).

5.  Playback robots with point-to-point control; a type of robot programming that makes
the manipulator reach a set point, stop, complete its task, and then move to the next set
point.
 The robot is taught each point, and these points are recorded into the robot’s control
unit.
 Path in between the points is not controlled.
 Used in machine loading and unloading applications and spot welding .

6.  Continuous – path robots are capable of performing motion cycles in which the
path followed by robot is controlled.
 Controller must be capable of storing large number of position locations that
define the curve path.
 Ex : Arc welding

7. Speed of Response and Stability
 They are two important characteristics of dynamic performance related to control system
design.
 The speed of response refers to the capability of robot to move to the next position in a
short amount of time.
 The response time is related to robot’s motion speed and is a function of the control
system.
 Stability is defined as the measure of oscillations which occur in the arm during movement
from one position to next.
 A robot with good stability will exhibit little or no oscillations during the termination of
arm movement.
 Poor stability would be indicated by large amount of oscillations.

8. Control of Stability in a Robot
 The stability of the robot an be controlled to a large extent by adding damping
elements into the robot’s design.
 High damping increases robot’s stability (or less oscillations) but it reduces the speed of
response.
Fig: a. Low damping - fast response b. High damping – slow response
 In the above two diagrams, the robot’s wrist is shown as a function of time with two
cases: a. low damping. b. high damping.
 In low damping, the arm moves to position quickly but exhibits oscillations about the
position.
 In high damping, there is less oscillation but arm movement is slow.