There are three main categories of industrial robots based on path control: limited sequence robots, playback robots with point-to-point control, and playback robots with continuous path control. Limited sequence robots use limit switches to control joint positions and are best for pick and place operations. Playback robots can be taught paths and positions which are stored and repeated; point-to-point robots move between defined points while continuous path robots can precisely follow curved paths like those needed for arc welding. The stability and speed of a robot's movements are important and can be controlled through damping elements, with higher damping providing more stability at the cost of slower speeds.

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.