robot architectures.ppt

8/11/2019 Robot Architectures.ppt

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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 freedomcorrespond 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

Posi t ion ing

x

y

z

P

Reference

frame origin

Positioning the end

effector in the 3D space,

requires three DoF, eitherobtained from rotations or

displacements.


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Degrees of freedom

Orientat ion

Orienting the end effector in

the 3D space, requires three

additional DoF to producethe three rotations.

x

y

z

P

Reference

frame origin

roll

pan

tilt


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- Degree of freedom


- Accessibility

- Precision

- Working space

- Payload

- Architecture


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Maneuvering degreeis the number of

actuators that although producing new

movements do not contribute to new degrees of

freedom.

Maneuvering degree


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Degrees o f maneuverabi l i ty (redundan t)

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 archi tectu re


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Mechanical Struc ture

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 that

determine the working volume and accessibility


Degrees of freedom


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Kind of poss ib le jo in ts :

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

Basic character ist ics Defini t ions


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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 character ist ics . Defini t ions


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ex

ey

Movement precis ion

Precision (Accuracy)

Capacity to place the end effector into a given position and orientation

(pose) within the robot working volume, from a randominitial

position.

eincreases 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

Coordinates

of the target


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ex

ey

Coordinates

of the target

Precision

+ R

eoffset

Movement precis ion

Precision (Accuracy)


(pose) within the robot working volume, from a randominitial

position.

eincreases with the distance

to the robot axis.


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ex

eyRepetitiveness depends on:

Mechanical play (backlash)

Target position

Speed and direction when

reaching the target

Repetitiveness

error

Precision

+ R

Movement precis ion

Repetitiveness


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

Coordinates

of the target


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Movement precis ion (Stat ics)

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 Struc ture

Joint 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 of

articulations of the kinematical

chain 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)

Cl i l A hit t


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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 robo t : Inc rease the useful wo rking vo lume


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Architecture SCARAArchi tecture R-R-D w ith cy l indr ical coord inates

( SCARA: Selective Compliance Assembly Robotic Arm )


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SCARA Robot :

examples

Resume Cartesian Robo t Character ist ics


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Resume Cartesian Robo t Character ist ics

Robot 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, inpick&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 Cyl ind r ical Robot Character ist ics


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Resume Cyl ind r ical Robot Character ist ics

Cylindrical1a. Rotation: q2a.

3a.

good accessibility to cavities and

open machines large forces when using hydraulic

actuators

restricted working volume requires guides protection (linear)

the back side can surpass theworking volume


Linear: Z

Linear: r

Advantages:simple kinematical model

easy to display

Drawbacks:

Resume Polar Robo t Character ist ics


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Resume Polar Robo t Character ist ics

Polar 1a. Rotation: q2a. Rotation: j

3a. 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:


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Resume SCARA Robot Character ist ics


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high speed and precision

only vertical access

:SCARA 1a. rotation

2a. rotation

3a. rotation

q1

q2

q3


Advantages:

Drawbacks:

Dynam ic Character ist ics


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Example of Map of adm it ted

loads, in fun ct ion o f the

distance to the main axis

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.

Dynam ic Character ist ics


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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

Long

movement

Architectures


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Architectures

- Classical Architectures: Cartesian

Cylindrical

Polar

Angular

- Special configurations

Special Con f igurat ion s. Pendulum Robo t GGD


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Example of a Pendulum Robot RRD

Special Con figu rat ion s. Elephant Trun k


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Classical Degrees of freedom Concatenated Degrees of

freedom (elephant trunk)



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Classical Degrees of freedom Concatenated Degrees of

freedom (elephant trunk)



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Increase of accessibility



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Distributed Degrees of Freedom.

Elephant Trunk Examples App l ications


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Special Con figu rat ions . Stewart Platform


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6 Disp lacements 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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Robots

6 Rotatio ns 6 DoF.

Movement capabi l i ties

1 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 m ovements

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 decoup l ing archi tectures


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12

2

M

1

Decoupling achieved with a

parallelepiped structure


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Example of a decoupled structure

with a parallelepiped structure

Mechanical decoup l ing solut ions


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M

Structure decoupled with connecting rods

1

1

2

2

M

Decoupl ing w i th connect ing rods


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M

p g g

By transmitting the movement with connecting rods, the

rotation of a joint does not propagates to the following.



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M

M

By transmitting the movement with connecting rods, the

rotation of a joint does not propagates to the following.

p g g



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M

M

j

M

M

M

j

M

Transmission with connecting rods through two

consecutive joints maintains the orientation of the E.E.

Mechanical decoup l ing solut ions


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Structure decoupled with chains

Decoupl ing w i th chains


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M

j

M

j

Transmission movements with chains

Decoupl ing w i th chains


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M

j

M

j

Transmission systems with chains

produce decoupled movements

Points potent ial ly weak in mechanical design


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Weak points Mechanical correction

Permanent deformationof the whole structure

and the components

Increase rigidness

Weight reduction

Counterweight

Dynamic deformation

Reduction of the mass

to move Weight distribution

Backlash Reduce gear clearances

Use more rigid

transmission elements

Increase rigidness

Points potent ial ly 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 transducers

connection

Improve mechanicalconnection

Search for a better location

Protect the environment

Weak points Mechanical correction

robot architectures.ppt

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