Robot Path Control

Robot Path Control

• 1.
  OVERVIEW  Classification ofrobots based on path control  Speed of response and stability  Control of stability in a robot
• 2.
  Classification of Robotsbased 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 SequenceRobots; 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 robotsuse 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 robotswith 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 Responseand 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 Stabilityin 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.