Classification of Robot Arms:by Control Method

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Classification of Robot Armsby Control Method

• The Control unit is the brain of the robot. It
  contains the instructions that direct the
  manipulator to perform various movements in the
  proper manner and at the proper time to
  accomplish a particular task.
• Feedback is the process of receiving information
  on how well a task or positioning of the
  end-effector is progressing.
• Closed-loop servo-controlled systems use
  continuous feedback information.
• Open-loop nonservo-controlled systems do not have
  feedback information.
• A Servo mechanism (simply referred to as Servo)
  is a technique for knowing the exact position of
  a moving object at any time. This is
  accomplished by feedback information.

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Classification of Robot Arms by Control Method

• Open-loop nonservo-controlled Systems
• No way of knowing whether the arm has moved as
  indicated
• Controlled entirely by on/off switches or by
  simple speed adjustments, i.e. no feedback
  information.
• Techniques have been developed to have adequate
  feedback information to function adequately,
  e.g.
• Have a physical stop (or switch) to indicate the
  end of a move or a limit is reached or
• Use a very reliable drive system to ensure the
  movement is as indicated.
• Relative high speed due to small size of arm
  full power can be applied to the axis actuators
• Low cost and easy to maintain and operate
• Extremely reliable devices, gt good
  repeatability
• Have limited flexibility w.r.t. positioning

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Classification of Robot Arms by Control Method

• Closed-loop servo-controlled Systems
• Information about the position and velocity of
  the links are continuously monitored and feedback
  to the control system
• Allow the arm to move and stop anywhere within
  the limits of travel of the individual links
• The controller (computer) has large memory
  capacity
• Have at least 6 components
• A sensing device to provide the feedback e.g.
  force, touch.
• A command or input signal specifies a desired
  position, speed, etc.
• A feedback signal indicates how well the arm is
  carrying out the command (from the sensing
  device)
• A comparer compares the input signal and the
  feedback signal.
• An amplifier to raise the comparer signal to a
  power level capable of activating the output
  device
• An output device usually the motor to be
  controlled

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Classification of Robot Arms by Control Method

• The Control unit can also be classified according
  to the operating method
• Pick-and-Place Control Units
• generally small and pneumatic powered
• no positional information feedback gt open-loop
  nonservo-controlled
• Point-to-Point Control Units
• able to reach any point within its work envelope
• can have as many points in its work sequence as a
  particular task may require (limited only by the
  size of its memory)
• Continuous-path Control Units
• able to reach any point within its work envelope,
  (similar to point-to-point)
• can map the exact path to take between points
• able to perform semiskilled tasks, e.g.
  arc-welding, spray-painting.

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Classification of Robot Armsby Specification of
other Characteristics

• Number of Axes
• Payload
• Speed
• Reach and Stroke
• Repeatability, Precision and Accuracy

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Classification of Robot Armsby Number of Axes

• Degrees of freedom
• 3 major axes, 1-3, (from the base) to determine
  the configuration of the wrist gt gross work
  envelope
• 3 minor axes, 4-6, (the wrist)
• Additional ones (7-n) are called redundant ones
  to provide extra flexibility to reach obscure
  space, get around corners, avoid obastacles,
  grasp special types of objects

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Classification of Robot Arms by Number of Axes

• The Wrist
• 3 degrees of freedom Yaw, Pitch and Roll (YPR)
• determines the possible orientations that the
  tool (hand) can assume.

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Example of a Welding Tool attached to the end of
a single axis wrist
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Classification of Robot Arms

• Payload (kg)
• Load carrying capacity, i.e. the maximum weight
  of the load that the robot can carry
• Speed (mm/sec)
• Closely related to payload
• Maximum speed no load
• Cycle time defined as the time required to
  perform a periodic motion similar to a simple
  pick-and-place operation.
• Examples
• Westinghouse robot - 92 mm/sec (no load)
• Adept robot - 9000 mm/sec (no load)
• Adept robot carrying a 2.2 kg along a 700 mm
  path consisting of 6 straight line segment has a
  cycle time of 0.9 sec.
• gt average speed is approx. 778 mm/sec (ltlt 9000
  mm/sec)

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Classification of Robot Arms by Reach and Stroke

• A rough measure of the size of the work envelope
• Horizontal Reach is defined as the maximum radial
  distance the wrist can be positioned from the
  vertical axis about which the robot rotates.
• Horizontal Stroke is defined as the total radial
  distance that the wrist can travel
• Reach gt Stroke
• Minimum distance the wrist can be positioned from
  the base axis ?
• gt (Reach - Stroke)
• Vertical Reach/Stroke

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Classification of Robot Arms Repeatability,
Precision Accuracy

• Repeatability vs Accuracy
• Precision is a measure of spatial resolution with
  which the end-effector can be positioned within
  the work envelope.