Robot Architectures

8/2/2019 Robot Architectures

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ROBOTICS: ROBOT MORPHOLOGY

Josep Amat and Alcia Casals

Automatic Control and Computer Engineering Department


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User

Components of a Robot

Control Unit

Programming

ExternalSensorsEnvironment

InternalSensors

Actuators

Mechanical Structure

Net


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Chapter 2. Robot Morphology

Basic characteristics:- Kinematics chain

- Degree of freedom

- Maneuvering degree

- Accessibility

- Precision

- Working space

- Payload

- Architecture


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Degrees of freedom correspond to the number ofactuators 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.

Degrees of freedom


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Degrees of freedom

Positioning

x

y

z

P

Referenceframe origin

Positioning the endeffector in the 3D space,

requires three DoF, eitherobtained from rotations ordisplacements.


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Degrees of freedom

Orientation

Orienting the end effector inthe 3D space, requires three

additional DoF to producethe three rotations.

x

y

z

P

Referenceframe origin

roll

pan

tilt


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- Degree of freedom


- Accessibility

- Precision

- Working space

- Payload

- Architecture


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Maneuvering degree is the number of

actuators that although producing new

movements do not contribute to new degrees of

freedom.

Maneuvering degree


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Degrees of maneuverability (redundant)

Forced access(without redundancy)

Multiple access(with redundant DoF)


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- Degree of freedom


- Accessibility

- Precision

- Working space

- Payload

- Architecture


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Robot architecture is the combination anddisposition of the different kind of joints that

configure the robot kinematical chain.

Robot architecture


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Mechanical Structure

Open: Closed (Parallel):

Kinematics chain: Sequence of rigid elements linked

through active joints in order to perform a task efficiently


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Nomenclature:

Arm

Wrist

Elbow

Shoulder

Trunk

Base


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Payload

Characteristics derived from the mechanical structure:

Degrees of freedom

Work Space

Accessibility

Precision


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Tridimensional positioning: (x,y,z )

Minimum: 3 Degrees of freedom


Degrees of freedom



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

Minimum: 3 + 3 Degrees of freedom

q

Architecture: Configuration and kind of

articulations of the kinematical chain thatdetermine the working volume and accessibility


Degrees of freedom


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Kind of possible joints:

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

Basic characteristics Definitions


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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

Basic characteristics. Definitions


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ex

ey

Movement precisionPrecision (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.Precision depends on:

Mechanical play (backlash)

Sensors offset

Sensors resolution

Misalignments in the positionand size of rigid elements,

specially the end-effector E.E.Points reached in different tests

Coordinatesof the target


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ex

ey


Precision+ R

e offset

Movement precisionPrecision (Accuracy)


(pose) within the robot working volume, from a random initial

position.

e increases with the distanceto the robot axis.


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ex

eyRepetitiveness depends on:

Mechanical play (backlash)

Target position

Speed and direction when

reaching the target

Repetitivenesserror

Precision+ R

Movement precisionRepetitiveness


(pose) within the robot working volume, from a given initial position.


S


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Movement precision (Statics)

Resolution:

Minimal displacement the EE can achieve and / or the control unit canmeasure.

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 StructureJoint 1

Joint 2

Joint 4

Joint 5

Examples of Joints (movements between articulated bodies)

Joint 1

Joint 2

Joint 5

Joint 3 Joint 4

Joint 6

Joint 3


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Example of a section of a working volume


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Architectures

Architecture: Configuration and kind ofarticulations of the kinematicalchain that determine the working

volume and accessibility

Classical Architectures: Cartesian

CylindricalPolar

Angular

Cl i l A hit t


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Classical Architectures

Cartesian Work Space (D+D+D)



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




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Cylindrical Work Space (R+D+D)




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




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Polar Work Space (R+R+D)




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




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Angular Work Space (R+R+D)




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Angular Work Space (R+R+R)




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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 SCARAArchitecture R-R-D with cylindrical coordinates

( SCARA: Selective Compliance Assembly Robotic Arm )


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SCARA Robot :examples

Resume Cartesian Robot Characteristics


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Resume Cartesian Robot CharacteristicsRobot Joints Observations

Cartesian 1a. Linear: X

2a.

3a.

Advantages::

linear movement in three dimensions simple kinematical model rigid structure easy to display possibility of using pneumatic actuators,

which are cheap, in pick&placeoperations

Linear: Y

Linear: Z

constant resolution

Drawbacks:

requires a large working volume the working volume is smaller thanthe robot volume (crane structure) requires free area between the robot

and the object to manipulate

guides protection

Resume Cylindrical Robot Characteristics


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Resume Cylindrical Robot Characteristics

Cylindrical1a. Rotation: q2a.

3a.

good accessibility to cavities andopen machines

large forces when using hydraulic

actuators

restricted working volume requires guides protection (linear)

the back side can surpass theworking volume

Robot Joints Observations

Linear: Z

Linear: rAdvantages:

simple kinematical model

easy to display

Drawbacks:

Resume Polar Robot Characteristics


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Resume Polar Robot Characteristics

Polar 1a. Rotation: q2a. Rotation: j3a. Linear: r

large reach from a central support

It can bend to reach objects on the floor

motors 1 and 2 close to the base

complex kinematics model difficult to visualize


Drawbacks:

Advantages:

Resume Angular Robot Characteristics


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Angular 1a. rotation

2a. rotation

3a. rotation

q1q2q3

:

maximum flexibility large working volume with respect

to the robot sizejoints easy to protect (angular)

can reach the upper and lower side of an object

complex kinematical model difficult to display linear movements are difficult

no rigid structure when stretched

g


Advantages:

Drawbacks:

Resume SCARA Robot Characteristics


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high speed and precision

only vertical access

:SCARA 1a. rotation

2a. rotation

3a. rotation

q1q2q3


Advantages:

Drawbacks:

Dynamic Characteristics


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Example of Map of admitted

loads, in function of the

distance to the main axis

y

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.

Dynamic Characteristics


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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

Longmovement

Architectures


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Architectures

- Classical Architectures: Cartesian

Cylindrical

Polar

Angular

- Special configurations

Special Configurations. Pendulum Robot GGD


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Example of a Pendulum Robot RRD

Special Configurations. Elephant Trunk


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Classical Degrees of freedom Concatenated Degrees of

freedom (elephant trunk)



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Classical Degrees of freedom Concatenated Degrees offreedom (elephant trunk)



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Increase of accessibility



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Distributed Degrees of Freedom.

Elephant Trunk Examples Applications


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Special Configurations. Stewart Platform


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6 Displacements 6 DoF.

+ X, + Y, + Z

+j, + f, + q

Platform Stewart Example


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Workspace

Example of


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Example of

Stewart Robot

Robots


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D Robots

6 Rotations 6 DoF.

Movement capabilities1 Working volume (Workspace):


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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

Coupled movements


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

Decoupled movements


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

Mechanical decoupling architectures


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l1 l2

l2

M l1

Decoupling achieved with aparallelepiped structure


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Example of a decoupled structurewith a parallelepiped structure

Mechanical decoupling solutions


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M

Structure decoupled with connecting rods

l1l1

l2

l2M

Decoupling with connecting rods


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M

p g g

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



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MM

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

p g g



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MMj

M MMj

M

Transmission with connecting rods through two

consecutive joints maintains the orientation of the E.E.

Mechanical decoupling solutions


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Structure decoupled with chains

Decoupling with chains


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Mj Mj

Transmission movements with chains

Decoupling with chains


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Mj Mj

Transmission systems with chainsproduce decoupled movements

Points potentially weak in mechanical design


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Weak points Mechanical correction

Permanent deformationof the whole structureand the components

Increase rigidness

Weight reduction

Counterweight

Dynamic deformation

Reduction of the mass

to move Weight distribution

Backlash Reduce gear clearances

Use more rigidtransmission elements

Increase rigidness

Points potentially weak in the mechanical design

W k i M h i l i


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Axes clearance Use pre stressed axes

Friction Improve clearance in axes

Increase lubrication

Thermal effects Isolate heat source

Bad transducersconnection

Improve mechanicalconnection

Search for a better location

Protect the environment

Weak points Mechanical correction

Robot Architectures

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