Unit 2 - Robot Manipulators
Components of a Robotic System
Mechanical system
- Locomotion apparatus (wheels, crawlers, mechanical legs)
- Manipulation apparatus (mechanical arms, end-effectors, artificial hands)
Actuation system
- Animates the mechanical components of the robot
- Motion control (servomotors, drives, transmissions
Sensory system
- Proprioceptive sensors (internal information on system)
- Exteroceptive sensors (external information on environment)
Control system
- Execution of action set by task planning coping with robot and environment’s constraints
- Adoption of feedback principle
- Use of system models
Components of a robotic system
Robot Mechanical Structure
Mechanical structure of robot manipulator: sequence of rigid bodies (links) interconnected by means of articulations (joints)
- Arm ensuring mobility
- Wrist conferring dexterity
- End-effector performing the task required of robot
Mechanical structure
- Open vs. closed kinematic chain
Mobility
- Prismatic vs. revolute joints
Degrees of freedom
- 3 for position + 3 for orientation
Workspace
- Portion of environment the manipulator’s end-effector can access
Cartesian Manipulator
- Three prismatic joints
- Very good mechanical stiffness
- Wrist positioning accuracy constant everywhere in the workspace
- Low dexterity (all joints prismatic)
- Electric (seldom hydraulic) actuation
- Employed for transportation and assembly
Manipulation of objects of large dimensions and heavy weight gantry structure
Cartesian manipulator and its workspace
Gantry manipulator
Cylindrical Manipulator
- One revolute joint and two prismatic joints
- Good mechanical stiffness
- Wrist positioning accuracy decreases as horizontal stroke increases
- Horizontal prismatic joint makes wrist suitable to access horizontal cavities
- Employed for carrying objects even of large dimensions
- Hydraulic actuation preferred to electric actuation
Cylindrical manipulator and its workspace
Spherical Manipulator
- Two revolute joints and one prismatic joint
- Fair mechanical stiffness
- Wrist positioning accuracy decreases as radial stroke increases
- Employed for machining
- Electric actuation
Spherical manipulator and its workspace
SCARA Manipulator
- Two revolute joints and one prismatic joint (all axes of motion are parallel)
- High stiffness to vertical loads and compliance to horizontal loads Selective Compliance Assembly Robot Arm
- Positioning accuracy decreases as distance of wrist from first joint axis increases
- Employed for manipulation of small objects (vertical assembly tasks)
- Electric actuation
SCARA manipulator and its workspace
Anthropomorphic Manipulator
- Three revolute joints
- Most dexterous structure (similarity with human arm)
- Wrist positioning accuracy varies inside workspace
- Wide range of industrial applications
- Electric actuation
Manipulation of larger payloads closed kinematic chain with higher stiffness to guarantee comparable positioning accuracy (parallelogram geometry between shoulder and elbow joints)
Anthropomorphic manipulator and its workspace
Manipulator with parallelogram
Parallel Manipulator
- Multiple kinematic chains connecting base to end-effector
- High structural stiffness
- High operational speeds
- Reduced workspace
Hybrid structure employed for execution of manipulation tasks requiring large values of force along vertical direction
- Parallel arm
- Serial kinematic chain
Hybrid parallel-serial manipulator
Parallel manipulator
Wrist and End-effector
Spherical Wrist
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Three revolute joints determining end-effector orientation
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High compactness and dexterity
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Position and orientation decoupling
End-effector
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Specified according to task the robot should execute
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For material handling tasks gripper of proper shape and dimensions determined by object to be grasped
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For machining and assembly tasks tool or specialized device (welding torch, spray gun, mill, drill, screwdriver, etc.)
Spherical wrist