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**ROBOTICS: ROBOT MORPHOLOGY**

Josep Amat and Alícia Casals Automatic Control and Computer Engineering Department

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**User Components of a Robot Environment Net External Sensors**

Control Unit Programming External Sensors Environment Internal Actuators Mechanical Structure Net User

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**Chapter 2. Robot Morphology**

Basic characteristics: - Kinematics chain - Degree of freedom - Maneuvering degree - Architecture - Precision - Working space - Accessibility - Payload

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Degrees of freedom Degrees of freedom correspond to the number of actuators that produce different robot movements D o F A joint adds a degree of freedom to the manipulator structure, if it offers a new movement to the end effector that can not be produced by any other joint or a combination of them.

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**Reference frame origin**

Degrees of freedom Positioning x y z P Reference frame origin Positioning the end effector in the 3D space, requires three DoF, either obtained from rotations or displacements.

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**Reference frame origin**

Degrees of freedom Orientation x y z P Reference frame origin roll pan tilt Orienting the end effector in the 3D space, requires three additional DoF to produce the three rotations.

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**Chapter 2. Robot Morphology**

Basic characteristics: - Kinematics chain - Degree of freedom - Maneuvering degree - Architecture - Precision - Working space - Accessibility - Payload

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Maneuvering degree Maneuvering degree is the number of actuators that although producing new movements do not contribute to new degrees of freedom.

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**Degrees of maneuverability (redundant)**

Multiple access (with redundant DoF) Forced access (without redundancy)

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**Chapter 2. Robot Morphology**

Basic characteristics: - Kinematics chain - Degree of freedom - Maneuvering degree - Architecture - Precision - Working space - Accessibility - Payload

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Robot architecture Robot architecture is the combination and disposition of the different kind of joints that configure the robot kinematical chain.

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Mechanical Structure Kinematics chain: Sequence of rigid elements linked through active joints in order to perform a task efficiently Closed (Parallel): Open:

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Nomenclature: Elbow Arm Wrist Shoulder Trunk Base

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**Characteristics derived from the mechanical structure:**

Degrees of freedom Work Space Accessibility Payload Precision

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**Characteristics derived from the mechanical structure:**

Degrees of freedom Tridimensional positioning: (x,y,z ) Minimum: 3 Degrees of freedom

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**Positioning + orientation: (x,y,z,,,q )**

Characteristics derived from the mechanical structure: Degrees of freedom q Positioning + orientation: (x,y,z,,,q ) Minimum: Degrees of freedom Architecture: Configuration and kind of articulations of the kinematical chain that determine the working volume and accessibility

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**Kind of possible joints:**

In red, those usually used in robotics as they can be motorized without problems

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**Basic characteristics. Definitions**

Degrees of freedom: Number of complementary movements. Movement capability: Working volume, Accessibility and Maneuvering Movement precision: Resolution, Repetitiveness, Precision and Compliance Dynamical characteristics: Payload, Speed and Stability

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**ey ex Movement precision**

Precision (Accuracy) Capacity to place the end effector into a given position and orientation (pose) within the robot working volume, from a random initial position. e increases with the distance to the robot axis. ex ey Precision depends on: Mechanical play (backlash) Sensors offset Sensors resolution Misalignments in the position and size of rigid elements, specially the end-effector E.E. Coordinates of the target Points reached in different tests

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**ey ex Movement precision e increases with the distance**

Precision (Accuracy) Capacity to place the end effector into a given position and orientation (pose) within the robot working volume, from a random initial position. e increases with the distance to the robot axis. ex ey Coordinates of the target e offset Precision + R

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**ey ex Movement precision Repetitiveness**

Capacity to place the end effector into a given position and orientation (pose) within the robot working volume, from a given initial position. Repetitiveness error ex ey Repetitiveness depends on: Mechanical play (backlash) Target position Speed and direction when reaching the target Coordinates of the target Precision + R

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**Movement precision (Statics)**

Resolution: Minimal displacement the EE can achieve and / or the control unit can measure. Determined by mechanical joints and the number of bits of the sensors tied to the robot joints. Error resolution of the sensor = Measurement Rank / 2n

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Mechanical Structure Examples of Joints (movements between articulated bodies) Joint 1 Joint 2 Joint 5 Joint 3 Joint 4 Joint 6 Joint 1 Joint 4 Joint 3 Joint 5 Joint 2

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**Example of a section of a working volume**

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**Classical Architectures: Cartesian Cylindrical Polar Angular**

Architecture: Configuration and kind of articulations of the kinematical chain that determine the working volume and accessibility Classical Architectures: Cartesian Cylindrical Polar Angular

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**Cartesian Work Space (D+D+D)**

Classical Architectures Cartesian Work Space (D+D+D)

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**Classical Architectures**

Example of a Cartesian Work Space Robot (D+D+D)

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**Cylindrical Work Space (R+D+D)**

Classical Architectures Cylindrical Work Space (R+D+D)

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**Classical Architectures**

Example of a Cylindrical Work Space Robot (R+D+D)

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**Polar Work Space (R+R+D)**

Classical Architectures Polar Work Space (R+R+D)

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**Classical Architectures**

Example of a Polar Work Space Robot (R+R+D)

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**Angular Work Space (R+R+D)**

Classical Architectures Angular Work Space (R+R+D)

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**Angular Work Space (R+R+R)**

Classical Architectures Angular Work Space (R+R+R)

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**Classical Architectures**

Example of Angular Work Space Robots (R+R+R)

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**Working space of a robot with angular joints**

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**Inverted robot: Increase the useful working volume**

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**Architecture R-R-D with cylindrical coordinates**

Architecture “SCARA” Architecture R-R-D with cylindrical coordinates ( SCARA: Selective Compliance Assembly Robotic Arm )

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SCARA Robot : examples

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**Resume Cartesian Robot Characteristics**

Joints Observations Cartesian 1a. Linear: X Advantages: : 2a. Linear: Y linear movement in three dimensions simple kinematical model 3a. Linear: Z rigid structure easy to display possibility of using pneumatic actuators, which are cheap, in pick&place operations constant resolution Drawbacks: requires a large working volume the working volume is smaller than the robot volume (crane structure) requires free area between the robot and the object to manipulate guides protection

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**Resume Cylindrical Robot Characteristics**

Joints Observations Cylindrical 1a. Rotation: q Advantages: 2a. Linear: Z simple kinematical model easy to display 3a. Linear: r good accessibility to cavities and open machines large forces when using hydraulic actuators Drawbacks: restricted working volume requires guides protection (linear) the back side can surpass the working volume

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**Resume Polar Robot Characteristics**

Joints Observations Polar 1a. Rotation: q Advantages: 2a. Rotation: j large reach from a central support 3a. Linear: r It can bend to reach objects on the floor motors 1 and 2 close to the base Drawbacks: complex kinematics model difficult to visualize

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**Resume Angular Robot Characteristics**

Joints Observations Angular 1a. rotation q 1 2 3 : Advantages: 2a. rotation maximum flexibility large working volume with respect to the robot size 3a. rotation joints easy to protect (angular) can reach the upper and lower side of an object Drawbacks: complex kinematical model difficult to display linear movements are difficult no rigid structure when stretched

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**Resume SCARA Robot Characteristics**

Joints Observations SCARA 1a. rotation 2a. 3a. q : 1 Advantages: q 2 high speed and precision q 3 Drawbacks: only vertical access

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**Dynamic Characteristics**

Payload: The load (in Kg) the robot is able to transport in a continuous and precise way (stable) to the most distance point The values usually used are the maximum load and nominal at acceleration = 0 The load of the End-Effector is not included. Example of Map of admitted loads, in function of the distance to the main axis

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**Dynamic Characteristics**

Velocity Maximum speed (mm/sec.) to which the robot can move the End-Effector. It has to be considered that more than a joint is involved. If a joint is slow, all the movements in which it takes part will be slowed down. For shorts movements it can be more interesting the measure of acceleration. Vmax time speed Short movements Long movement

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**- Classical Architectures: Cartesian Cylindrical Polar Angular**

- Special configurations

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**Special Configurations. Pendulum Robot GGD**

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**Example of a Pendulum Robot RRD**

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**Special Configurations. Elephant Trunk**

Classical Degrees of freedom Concatenated Degrees of freedom (elephant trunk)

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**Special Configurations. Elephant Trunk**

Classical Degrees of freedom Concatenated Degrees of freedom (elephant trunk)

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**Special Configurations. Elephant Trunk**

Increase of accessibility

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**Special Configurations. Elephant Trunk**

Distributed Degrees of Freedom.

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**Elephant Trunk Examples Applications**

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**Special Configurations. Stewart Platform**

6 Displacements 6 DoF. + X, + Y, + Z + j, + f, + q Special Configurations. Stewart Platform

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**Platform “Stewart” Example**

Workspace

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Example of “Stewart” Robot

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D Robots 6 Rotations 6 DoF.

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**Movement capabilities**

1. Working volume (Workspace): Set of positions reachable by the robot end-effector. Shape is more important than the volume (m3) 2. Accessibility: Capacity to change the orientation at a given position. Strongly depend on the joint limits. 3. Maneuverability Capacity to reach a given position and orientation (pose) from different paths (different configurations). Usually implies the presence of redundant joints (degrees of manipulability or degrees of redundancy).

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Coupled movements Decoupled movements

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Coupled movements The rotation of a link is propagated to the rest of the chain

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Decoupled movements The rotation of a link is not propagated to the rest of the chain

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**Decoupling achieved with a parallelepiped structure**

Mechanical decoupling architectures l2 l1 Ma l1 l2 Decoupling achieved with a parallelepiped structure

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**Example of a decoupled structure with a parallelepiped structure**

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**l2 l1 Ma Mb Mechanical decoupling solutions**

Structure decoupled with connecting rods

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**Decoupling with connecting rods**

Ma By transmitting the movement with connecting rods, the rotation of a joint does not propagates to the following.

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**Ma Mb Decoupling with connecting rods a b**

By transmitting the movement with connecting rods, the rotation of a joint does not propagates to the following.

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**Mb Ma Mb Decoupling with connecting rods Mj Ma Mj**

Transmission with connecting rods through two consecutive joints maintains the orientation of the E.E.

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**Mechanical decoupling solutions**

Structure decoupled with chains

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**Decoupling with chains**

Mj Mj Transmission movements with chains

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**Decoupling with chains**

Mj Mj Transmission systems with chains produce decoupled movements

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**Points potentially weak in mechanical design**

Weak points Mechanical correction Increase rigidness Permanent deformation of the whole structure and the components Weight reduction Counterweight Increase rigidness Reduction of the mass to move Dynamic deformation Weight distribution Reduce gear clearances Backlash Use more rigid transmission elements

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**Points potentially weak in the mechanical design**

Weak points Mechanical correction Axes clearance Use pre stressed axes Improve clearance in axes Friction Increase lubrication Thermal effects Isolate heat source Improve mechanical connection Bad transducers connection Search for a better location Protect the environment

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