MECHANICAL ENGINEERING-Introduction to robotics

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					         MECHANICAL ENGINEERING
                                     Introduction to robotics
        Module 1 : Introduction to robotics

Objectives :
In this course you will learn the following


Automation



Robot

Lecture 1 : Automation
    Automation
    There are several examples of automation one comes across daily, simple examples being sewing
    machines, packaging machines. Such machines are generally equipped to perform in a specific way or
    to execute specific tasks. A sewing machine is designed to produce specific stitch lengths and likewise
    a packaging machine is designed to wrap a specific size of the product. When product sizes change
    some parts of the machine are to be manually changed to accommodate the new size. Such machines
    are essentially designed to package millions of products of a specific size and are hence special
    purpose machines. The cost of such a machine is distributed over large sales volumes.

    More recently there is a demand for variety. A good example is shirts of a size suiting a given
    individual. Likewise there is a demand for soaps of various sizes and shapes. This requires machines
    that can handle various shapes and sizes and every time such a requirement arises one has to stop the
    machine and readjust or reset some of the links or components to handle the new product. This is a
    task that is not only time consuming but also requires skill.

    Human beings, unlike machines can not only handle tools and products of different sizes and shapes
    but are also capable of executing a variety of tasks. Engineers have often sought similar capabilities in
    machines and this has been possible now with the availability of inexpensive microprocessors.

   Used in conjunction with special servo-motors, actuators and sensors, the microprocessor has
   revolutionized automation. It is now possible to build automation devices that can be operated under
   the guidance of a program. A familiar example is a printer that can be programmed to print the
   alphabet. A few key strokes would enable the user to change over to a program that enables one to
   draw diagrams. This capability is extended further through the use of sensors. For example a sensor in
   the printer does not permit the printing to begin unless a paper is present. These capabilities are
   extended further and when the machine is able to change its activity to suit a given situation it is called
   “autonomous”.
Robots

An important part of the automation scene is the area of “Robotics” a multidisciplinary field that involves
mechanical, electronics and several other engineering disciplines. Though the ultimate aim is to attempt
emulate human activities, something which is extremely difficult to attain, these attempts have resulted in
development of robots. These are beneficial in handling hazardous tasks and for operating in hazardous
areas like chemical or nuclear plants. Examples of such tasks include plates being x-rayed for inspection of
internal cracks and flaws, a routine but hazardous operation.

Where complex movements are involved as in welding along a 3D profile, robots can be used for assuring
quality and consistency. In assembly operation of precision and tiny parts, like in watches, robots perform
with accuracy and repeatability. (The SCARA robot developed in Japan is one such robot specifically
suit5able for precision assembly tasks.) Painting is hazardous to humans and also complex movements are
involved (for example in painting a car body) and in such applications robots may replace human beings.

Robots have certain inherent capabilities and limitations, just as any other machine or human being does,
and these should be borne in mind when attempting to use them in a given application. A lathe is best used
for generating cylindrical objects and milling machines are ideal for producing prismatic parts. One would
not attempt to use a lathe for manufacture of prismatic parts or a milling machine to produce cylinders. Thus
manufacturing processes are chosen to suit the product and conversely, products should be designed to suit
the manufacturing process. This philosophy applies to robotics also. One cannot expect a given robot to
execute any arbitrary task or handle any product. Some times it may be beneficial to redesign the product to
enable robots to handle them with ease. A wellknown example of designing a product to suit robots is the
SONY “Walkman” which has been designed for ease of assembly by robots.

Today robot finds applications in industries, medical and other fields. For example, in eye surgery
(replacement of retina), where a cylindrical portion needs to be replaced, the operation is best done by
robots. Mobile robots like walking machines, hopping machines are examples of robots, and so also are
robotic aircraft and ships. Nuclear and power plants uses fish like robots which move inside pipes for
purpose of inspection

Computers are required for higher level control of such complex systems. Computers convert higher level
commands to lower level commands for purpose of interpreting sensor outputs and controlling motors in
these machines. In autonomous robots, operating at remote locations, endurance of power supply (batteries)
may be an issue.

Lecture 2 : Anatomy of Robots
    Objectives
      In this lecture you will learn about

    Anatomy and SubSystem of robots


    Manipulators

    Robot control




    Anatomy of industrial robots

There are several classes of robots: robotic aircraft, robotic ships, mobile robots and others. An important
application of robots is in industry – for machine tending, welding, painting, assembly and etc. These
“industrial robots” can be viewed as consisting of a mechanical portion “the manipulator” controlled by a
microprocessor.

   Subsystems of industrial robots include:

Actuators

Transmission systems

Power supplies & power storage system


Sensors


Microprocessors & controllers

Algorithms & softwares (higher level & lower level)

Actuators:

Actuators are basically prime movers providing both force and motion. Pneumatic cylinders, hydraulics,
permanent magnet motors, stepper motors, linear motors are some conventional actuators. More advanced
ones are based on hi-tech polymers, shape memory alloys, piezo patches, and pneumatic muscles. Brushless
servo motors also exist for low noise levels, and printed armature motors are used for quick response.



Transmission systems:

The transmission system used in robot to transmit power and motion consists of chains, timing belts, metal
belts, cables and pulleys and linkages. Gear boxes and harmonic drives serve to provide speed reduction.
Ball screws are used with suitable mechanisms to convert rotary motion to linear motion and if needed back
to oscillatory motion. Drive stiffness is an important consideration in robotics and so also is backlash.




Power supplies:

Hydraulic and Pneumatic power packs: These consist of a motor driving a positive displacement pump or
compressor to generate the high pressure fluid flow. In using hydraulic systems the necessity of having an
oil tank increases the weight of the system, additionally the issue of ensuring that the oil is free of
contaminants is to be handled. In pneumatics power pack dry air is desired.

Electric motors use what ate known as PWM (pulse width modulation) amplifiers. These are electronic
devices, consisting of transistors used as switches to rapidly switch on and off the supply in a controlled
manner to control motor speeds. Such drives have higher efficiency.



Sensors and other electronics:
The sensors for feedback in robots consists of tachometers and encoders and potentiometers to sense motor
motions, simple switches, force sensors, acceleration sensors, optical systems, special cameras and vision
systems.




Electronics:

There are a host of electronic circuits, motor controllers, analog to digital converters and digital to analogue
converters, frame grabbers and so on utilized to handle sensors and vision systems and convert the inputs
from them into a form usable by the processor for control of the entire system in conjunction with the
algorithms and software developed specifically for the purpose.



Software:



The software used consists of several levels. Motor control software consists of algorithms which help the
servo to move smoothly utilizing the data from feed-back units. At the next level there is software to plan
the trajectory of the end effector and translate the same into commands to individual motor controllers. The
output of sensors is also to be interpreted and decisions made. At the highest level there is software which
accepts commands from the user of the robot and translates it into appropriate actions at the lower level.



Thus for control of the robot we have several levels:



Control of individual motors and actuators.



Planning trajectory & individual actuators in motion.



Planning trajectories of end effector.



Acting upon sensors input



Planning task

Lecture 3 : Industrial Manipulators & AGVs
    Objectives
      In this course you will learn the following
    History of development of robots.
    Main body types of manipulators with examples.
    Typical end effectors.
    Power transmission systems in robots.
    Tasks executed by robots/ manipulators.
    Part presentation.




History of robots :


1954- Devol & Engleburger – establish Unimation Incorporation.


1961- Robots are used in die casting application.



1968- AGVs (automated guided vehicles) implemented.



1970- Stanford arm developed.



1979- SCARA robot for assembly developed in Japan .

   Main bodies and wrists

Fig. 3.1.1 shows a typical industrial robot with a main body and a wrist.
                                                 Figure 3.1.1 .

Figure 3.1.1 shows PUMA robot (the manipulator). A total of 6 variables are required, for specifying the
position and orientation of a rigid body in space. Therefore PUMA has 6 axis of rotation with 1 DOF
(degree of freedom) per axis.

The functioning of this robot is like a human arm. Each DOF has an actuator for motion.

Types of Main bodies type



One generalization is that the main body of the robot is used to position an object (or tool) while the wrist is
used to orient it. Grippers are used to grasp objects.




                                                  Figure 3.2.1

Cartesian Robot (see figure 3.2.1). On several shop floors “Gantry” type of Cartesian robots (consisting of
overhead rails) are used for operations over large spaces.
                                                 Figure 3.2.2

Cylindrical main body. PPR (See figure 3.2.2). Such motions are found typically in drilling machines. A
similar main body is used in robots to access points in a cylindrical volume. (Essentially R- q motion in a
plane – which in turn translates along the Z axis.)




                                     Figure 3.2.3 -Spherical Main Body


Spherical main body (RRP - Figure 3.2.3) There is a base rotation and a portion of the arm moves in and out
(a telescopic motion). The work volume is a portion of a hollow sphere. (Essen




                                        Figure 3.2.4 Articulated Arm


Articulated type main body robot(typical human arm) (RRR Type )(See Figure 3.2.4)
                                         Figure 3.2.5 -SCARA Robot.


SCARA robot – This also has a cylindrical work space. RRP main body. Such robots were used to assemble
the SONY walkman. The “P” is for raising and lowering the end effector. Otherwise all the motion is in a
horizontal plane. (See Figures 3.2.5)

Wrists




                                                                          Figure 3.3.1

WRISTS : Wrist roll, yaw, and pitch (Figure 3.3.1). There are 3 motions and 3 actuators are required for
motion.



End Effectors (Figure3.3.2): Welding head, riveter, spot welder.



Grippers in manipulators

Grippers are used to grip, pick, place, and release the object.

There may be single gripper and / or multiple grippers. Many a time grippers are actuated by pneumatic
systems.
                                                 Figure 3.3.2


EE types (Figure3.3.2): spot welding gun for different position weld on automotive assembly line.
Following figure shows typical End Effectors used on assembly, machining line.




                                                 Figure 3.3.3



Gripper (figure 3.3.3) This pneumatic gripper (balloon shaped) is being used to pickup hollow cylindrical
objects by gripping them on the inside surface.




                                                 Figure 3.3.4



In this gripper the gripper faces move parallel to each other using a parallel bar mechanism. Ultrasonic
waves detect whether the object is present and then the fingers close to pick the object. (Figure 3.3.4)

Transmission
Ball screw drive (motor at base) Ball Screws reduce friction and preloading them reduces backlash (Figure
3.4.1.1)




                                                  Figure 3.4.1.1

Motor rotation is converted into linear motion of a nut engaging a screw and this in turn is converted into
oscillation of output.




                                               Figure 3.4.2.1
        Linkages for transmission. The actuator (mounted at the base, drives the output through linkage
        mechanisms.       Tasks Planning for robots
                                                       1.
Point to point tasks (PTP): This requires the robot to carry an object from one position to another. The end
locations (position and orientation) are known. A simple manipulator for such tasks is the pneumatic
manipulator.
                                                       2.
Continuous Path Motion - Painting application are an example where the end effector has to move over a
desired curve in space. Painting, being hazardous for manual operation servo controlled electric robots (with
fire proof motors) are employed.
                                                       3.

Palletizing (soft drink bottles to be placed in a crate). This is a special type of Point to Point task – this
occurs when bottles are placed in a crate. See Figure 3.5.1
                                                  Figure 3.5.1

                                                     4.
Assembly tasks are typically those which involve insertion of a peg into a hole. See Figure 3.5.2




                                        Figure 3.5.2: Assembly of parts


Stiffness and work space are among the parameters for selecting a robot manipulator. The question as to
whether one can position and orient a rigid body in any way in the work space is of importance. In some
portion of the workspace called the dexterous work space wherein a high degree of orientation is possible,
elsewhere the range of orientation is far less.

     Part Presentation Most industrial manipulators do not posses adequate number of sensors to
determine whether the part it has to handle is in the right position and orientation. So part presentation
systems are used to present parts in correct orientation.




                                                  Figure 3.6.1
                                                 Figure 3.6.2

 In Fig 3.6.1 and 3.6.2 parts are inspected by a camera and if they are wrongly oriented, rejected for
example by blowing a jet of air at them as in Fig. 3.6.1.