ROBOTICSUnit 1

Robot classification(Session-6)

Mechanical Engineering DepartmentK L University

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Learning Objectives

  In this lecture you will learn about • Robot classification based on control strategies.• Robot classification based on DOF.

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Classification of robots based on control methods.

  Robots are broadly categorized into following four types based on control methods 1.Limited sequence robots2.Playback robots with point-to-point control3.Playback robots with continuous path control4.Intelligent robots.

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Non servo control robots Basically all robots are controlled either servo control or non servo control.Non servo control

1. implemented by setting limits or mechanical stops for each joint and sequencing the actuation of each joint to accomplish the cycle.

2. No control over the motion at the intermediate points, only end points are known

3. Ex: end point robot, limited sequence robot, bang-bang robot.

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Non servo control robots4. Programming accomplished by

(a) setting desired sequence of moves (b) adjusting end stops for each axis accordingly (c) the sequence of moves is controlled by a

“sequencer”, which uses feedback received from the end stops to index to next step in the program

5. Low cost and easy to maintain, reliable relatively high speed

6. repeatability of up to 0.01 inch7. limited flexibility8. typically hydraulic, pneumatic drives

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Servo control robotsServo Control

Point to point ControlContinuous Path Control

Closed Loop control used to monitor position, velocity (other variables) of each joint

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Limited sequence control robots Limited sequence robots do not use servo-control to

indicate relative positions of the joints. They are controlled by setting limit switches and/or

mechanical stops to establish the endpoints of travel for each of their joints.

With this method of control, the individual joints canonly be moved to their extreme limits of travel.

lowest level of control. Robot programming is severely limited in this case.

such robots are used for simple pick and place type

tasks.

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Limited sequence control robots

Pneumatic drive is most suitable in these applications.

There is no feed back in this control Any of three drive systems can be used with this type

of control system. However , pneumatic drive system most commonly employed.

Applications for this type of robot generally involve simple motions, such as pick-and-place operations.

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Playback control robots Playback robots use a more sophisticated control unit

in which a series of positions or motions are “taught” to the robot, recorded into memory, and then repeated by the robot under its own control.

The procedure of teaching and recording into memory is referred to as the programming the robot.

Playback robots usually have some form of servo-control to ensure that the positions achieved by the robot are the positions that have been taught.

Playback robots can be classified into two categories: point-to-point(PTP) robots and continuous path(CP) robots.

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PTP control robots PTP robots are capable of performing motion cycles

that consist of a series of desired point locations and related actions.

Only the end points are programmed, the path used to connect the end points are computed by the controller

User can control velocity, and may permit linear or piece wise linear motion

Feedback control is used during motion to ascertain that individual joints have achieved desired location

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PTP control robots Often used hydraulic drives, recent trend towards servomotors. loads up to 500lb and large reachApplications

• pick and place type operations• palletizing• machine loading and unloading• spot welding

Pick and Place• Moves items from one point to

another• Does not need to follow a specific

path between points• Uses include loading and

unloading machines, placing components on circuit boards, and moving parts off conveyor belts.

A cartesian robot for picking and placing circuits on circuit-boards

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CP control robots CP robots are capable of performing motion cycles in which the path followed by the robot is controlled. In addition to the control over the endpoints, the path taken by the end effector can be controlled Path is controlled by manipulating the joints throughout the entire motion, via closed loop controlStraight line motion is a common form of continuous path control for industrial robots.The programmer specifies the starting point and the end point of the path, and the control unit calculates the sequence of individual points that permit the robot to follow a straight line trajectory.

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CP control robotsSome robots have the capability to follow a smooth, curved path that has been defined by a programmer who manually moves the arm through the desired motion cycle.

Applications:CP control is required for certain types of industrial applications such as spray painting, polishing, grinding, and arc welding.

Continuous path control

• Moves along a specific path

• Uses include welding, cutting, machining parts.

Robotic seam welding

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Intelligent robotsIntelligent robot constitutes a growing class of industrial

robot that possesses the capability not only to play back a

programmed motion cycle but to also interact with its

environment in a way that seems intelligent.

Intelligent robots can alter their programmed cycle in

response to conditions that occur in the workplace.

They can make logical decisions based on sensor data

received from the operation.Typical applications for intelligent robots are assembly tasks and arc welding operations.

Classification based on Degrees of Freedom (DOF)

• The number of independent movements that an object can perform in a 3-D space is called the number of degrees of freedom. • A rigid body free in space has six degrees of freedom-three for position and three for orientation• It is required to find DOF for a manipulator which in turn decides the number of actuators required.

Degrees of Freedom (DOF)

• T1, T2, and T3 are three translational motions• R1, R2, and R3 are three rotational motions

Degrees of Freedom (DOF)

The figure shows the open Kinematic chain manipulator with two DOF has two links and two joints.

Degrees of Freedom of different joints

Degrees of Freedom (DOF)

The manipulator having 6-DOF is called a spatial manipulator. It has three joints for positioning and three for orienting the end-effector. Spatial manipulators with more than 6-DOF have surplus joints and are known as redundant manipulators. A redundant robot provides more freedom to move around obstacles and operate in a tightly confined workspace.

Derivation for DOFF= Degrees of freedom of a mechanism Ci= Number of Constraints imposed by joint i

fi= Degrees of relative motion permitted by joint ij = Number of joints in a mechanism, assuming that all the joints are binary ji= Number of joints with i degrees of freedomn= Number of links in a mechanism, including the fixed linkλ= Degrees of freedom of the space in which a mechanism is intended to function (λ= 6 for spatial mechanisms and λ= 3 for planar mechanisms)

Derivation for DOFIf the links are all free of constraints, the DOF of an n-link mechanism F= λnFor a mechanism at least one link is fixed, then the DOF is equal to F= λ(n-1)The total number of constraints imposed by the joints is equal to Hence the DOF value of a mechanism is generally given by F= λ(n-1) - The number of constraints imposed by a joint and the degrees of freedom permitted by the joint are equal to the motion parameter λ

1

j

iCi

1

j

iCi

i iC f

Derivation for DOFHence the total number of constraints imposed by the joints is

The DOF of an mechanism is equal to

1 1 1( )j j j

i i ii i iC f j f

1( 1) j

iiF n j f

Find the number of DOF associated with the four bar linkage as shown in figure.

Solution:λ= 3n= 4j= 4

F= 3(4-4-1)+4×1DOF is F= 1 Ans

Find the number of DOF associated with the planar five bar linkage as shown in figure.

Solution:λ= 3n= 5j= 5

F= 3(5-5-1)+5×1DOF is F= 2 Ans

1( 1) j

iiF n j f

Find the number of DOF associated with the planar pantograph mechanism as shown in figure.

Solution:λ= 3n= 7j= 6+2 = 8

F= 3(7-8-1)+8×1DOF is F= 2 Ans

1( 1) j

iiF n j f

Figure shows a spatial mechanism in which a moving platform is connected to a fixed base by three extensible limbs with a spherical joint at the upper end and a revolute joint at the lower end. Further more each limb is made up of two binary links connected by prismatic joint. Find the number of DOF of the mechanism

Solution:λ= 6n= 8j= 9The degrees of relative motion permitted by all the joints = 3 Revolute joints+ 3Prismatic joints+3

Spherical joints = 3×1+3×1+3×3 = 15

F= 6(8-9-1)+15DOF of the mechanism F= 3 Ans

1( 1) j

iiF n j f

1

jii

f

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Recap  In this lecture one has to learnt about •Robot classification based on control strategies. • Robot classification based on DOF