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The project gives a brief introduction to the history of robotics while going on to explain the various types of robots that are built and their classification. A detailed description of the various mechanical platforms and driving mechanisms has been provided. The commonly used robotic designs have also been looked into and their characteristics have been explained. Finally, the fabrication process of the robotic gripper has been studied and explained. Grippers are key components in robotized assembly system.
The design and construction of highly dexterous robot hands has been a major research and development objective for at least the past two decades. Many of the above robot hands have the general objective of achieving a high degree of dexterity in a wide variety of situations, and this generality in their objective may sometimes lessen their effectiveness in specific classes of applications. This project focuses on the development of a universal robot gripper.
The gripper utilizes a2minimal amount of hardware, and can be employed in a wide variety of pick-and-place applications with minimal changes to the mechanical and control program configurations. The gripper is the mechanical interface between the robot and its environment. The robot performs the pick-and-place functions needed for assembly tasks. As with other peripheral equipment, grippers should have sufficient versatility to deal with the variety of parts an assembly robot has to handle. This project focuses on strategies for fabrication of an effective gripping device. The main section of the project covers the introduction for fabrication of grippers.
Machines and mechanization are the ancestors of today's robots. The ancients started with things like water clocks and irrigation equipment. Later, windmills and water wheels turned gears and equipment to help produce a product. These ancient machines did tasks with or without human help. Industrialization made use of heavy mechanization to mass produce merchandise. In the 20th century, machines took some form of "intelligence." They were able to work independently, solve problems and execute solutions. Cybernetics involved improving robot intelligence. Today, robots explore sea floors, wander inside caves, explore and study other planets and build cars.
Leonardo da Vinci created many robot-like sketches and designs in the 1500's.
The word robot first appeared in print in the 1920 play R.U.R. (Rossum's Universal Robots) by Karl Kapek, a Czechoslovakian playwright. Robota is Czechoslovakian for worker or serf (peasant). Typical of early science fiction, the robots take over and exterminate the human race.
1954: The first programmable robot is designed by George Devol, who coins the term Universal Automation. He later shortens this to Unimation, which becomes the name of the first robot company (1962).
Isaac Asimov popularized the term robotics through many science-fiction novels and short stories. Asimov is a visionary who envisioned in the 1930's the positronic brain for controlling robots; this pre-dated digital computers by a couple of decades. Unlike earlier robots in science fiction, robots do not threaten humans since Asimov invented the three laws of robotics:
- A robot may not harm a human or, through inaction, allow a human to come to harm.
- A robot must obey the orders given by human beings, except when such orders conflict with the First Law.
- A robot must protect its own existence as long as it does not conflict with the First or Second
- Joseph Engleberger and George Devoe were the fathers of industrial robots. Their company, Unimation, built the first industrial robot, the PUMA (Programmable Universal Manipulator Arm, a later version shown below), in 1961.
1980s: The robot industry enters a phase of rapid growth. Many institutions introduce programs and courses in robotics. Robotics courses are spread across mechanical engineering, electrical engineering, and computer science departments.
3. Types and classification of robots.
Industrial robots are available commercially in a wide range of sizes, shapes, and configurations. They are designed and fabricated with different design configurations and a different number of axes or degrees of freedom. These factors of a robot's design influence its working envelope
4. Common Robot Designs
Robots which have three linear (prismatic joints P, as opposed to rotational R joints) axes of movement (X, Y, Z). Used for pick and place tasks and to move heavy loads. They can trace out rectangular volumes in 3D space.
The positions of these robots are controlled by a height, an angle, and a radius (that is, two P joints and one R joint). These robots are commonly used in assembly tasks and can trace out concentric cylinders in 3D space.
Spherical robots have two rotational R axes and one translational P (radius) axis. The robots' end-effectors can trace out concentric spheres in 3D space.
The positions of articulated robots are controlled by three angles, via R joints. These robots resemble the human arm (they are anthropomorphic). They are the most versatile robots, but also the most difficult to program.
4.5 SCARA (Selective Compliance Articulated Robot Arm)
SCARA robots are a blend of the articulated and cylindrical robots, providing the benefits of each. The robot arm unit can move up and down, and at an angle around the axis of the cylinder just as in a cylindrical robot, but the arm itself is jointed like a revolute coordinate robot to allow precise and rapid positioning. The robot consists of three R and one P joints; an example is shown below.
We will mostly deal with robotic arms; some other interesting types of robots are mobile robots, humanoid robots, and parallel robots.
4.6. Mobile robots
Mobile robots have wheels, legs, or other means to navigate around the workspace under control. Mobile robots are applied as hospital helpmates and lawn mowers, among other possibilities. These robots require good sensors to "see" the workspace, avoid collisions, and get the job done.
4.7. Parallel robots
Most of the robots discussed so far are serial robots, where joints and links are constructed in a serial fashion from the base, with one path leading out to the end-effector. In contrast, parallel robots have many "legs" with active and passive joints and links, supporting the load in parallel. Parallel robots can handle higher loads with greater accuracy, higher speeds, and lighter robot weight; however, a major drawback is that the workspace of parallel robots is severely restricted compared to equivalent serial robots. Parallel robots are used in expensive flight simulators, as machining tools, and can be used for high-accuracy, high-repeatability, high-precision robotic surgery.
5. Mechanical platforms -- the hardware base
A robot consists of two main parts: the robot body and some form of artificial intelligence (AI) system. Many different body parts can be called a robot. Articulated arms are used in welding and painting; gantry and conveyor systems move parts in factories; and giant robotic machines move earth deep inside mines. One of the most interesting aspects of robots in general is their behavior, which requires a form of intelligence. The simplest behavior of a robot is locomotion. Typically, wheels are used as the underlying mechanism to make a robot move from one point to the next. And some force such as electricity is required to make the wheels turn under command.
A variety of electric motors provide power to robots, allowing them to move material, parts, tools, or specialized devices with various programmed motions. The efficiency rating of a motor describes how much of the electricity consumed is converted to mechanical energy. Let's take a look at some of the mechanical devices that are currently being used in modern robotics technology.
Permanent-magnet, direct-current (PMDC) motors require only two leads, and use an arrangement of fixed- and electro-magnets (stator and rotor) and switches. These form a commutator to create motion through a spinning magnetic field.
AC motors cycle the power at the input-leads, to continuously move the field. Given a signal, AC and DC motors perform their action to the best of their ability.
Stepper motors are like a brushless DC or AC motor. They move the rotor by applying power to different magnets in the motor in sequence (stepped). Steppers are designed for fine control and will not only spin on command, but can spin at any number of steps-per-second (up to their maximum speed).
Servomotors are closed-loop devices. Given a signal, they adjust themselves until they match the signal. Servos are used in radio control airplanes and cars. They are simple DC motors with gearing and a feedback control system.
5.2 Driving mechanisms
Gears and chains:
Gears and chains are mechanical platforms that provide a strong and accurate way to transmit rotary motion from one place to another, possibly changing it along the way. The speed change between two gears depends upon the number of teeth on each gear. When a powered gear goes through a full rotation, it pulls the chain by the number of teeth on that gear.
Gears are most often used in transmissions to convert an electric motor's or in this case the drive shaft's high speed and low torque to a shaft's requirements for low speed high torque.
Gears essentially allow positive engagement between teeth so high forces can be transmitted while still undergoing essentially rolling contact.
The basic law of gearing says that a common normal (the line of action) to the tooth profiles at their point of contact must in all positions of the contacting teeth; pass through a fixed point on the line-of-centers called the pitch point. As such any two curves or profiles engaging each other and satisfying the law of gearing are conjugate curves, and the relative rotation speed of the gears will be constant.
A gear train is a set or system of gears arranged to transfer rotational torque from one part of a mechanical system to another.
Gear trains consists
- Driving gears - it is attached to the input shaft
- Driven gears or Motor gears - it is attached to the output shaft
- Idler gears - it is interposed between the driving and driven gear in order to maintain the direction of the output shaft the same as the input shaft or to increase the distance between the drive and driven gears. A compound gear train refers to two or more gears that are used to transmit motion.
Alternatively pinion is the smaller of the two gears (typically on the motor) drives a gear on the output shaft. A gear or wheel is the larger of the two gears.
Gears are generally used for one of four different reasons:
- To reverse the direction of rotation
- To increase or decrease the speed of rotation
- To move rotational motion to a different axis
- To keep the rotation of two axis synchronized
Pulleys and belts:
Pulleys and belts, two other types of mechanical platforms used in robots, work the same way as gears and chains. Pulleys are wheels with a groove around the edge, and belts are the rubber loops that fit in that groove.
A gearbox operates on the same principles as the gear and chain, without the chain. Gearboxes require closer tolerances, since instead of using a large loose chain to transfer force and adjust for misalignments, the gears mesh directly with each other. Examples of gearboxes can be found on the transmission in a car, the timing mechanism in a grandfather clock, and the paper-feed of your printer.
Power supplies are generally provided by two types of battery. Primary batteries are used once and then discarded; secondary batteries operate from a (mostly) reversible chemical reaction and can be recharged several times. Primary batteries have higher density and a lower self-discharge rate. Secondary (rechargeable) batteries have less energy than primary batteries, but can be recharged up to a thousand times depending on their chemistry and environment. Typically the first use of a rechargeable battery gives 4 hours of continuous operation in an application or robot.
There are literally hundreds of types and styles of batteries available for use in robots. Batteries are categorized by their chemistry and size, and rated by their voltage and capacity. The voltage of a battery is determined by the chemistry of the cell, and the capacity by both the chemistry and size.
6. Degrees of freedom
The term degree of freedom relates to locating or positioning of a body in space. A body in space has six degree of freedom since it can translate linearly along three mutually perpendicular axis and rotational movements about the same three axes. Three linear movements allow the body on the end effectors of the robot to move a desired position in space and three rotational movements allow the body to be oriented about that position.
The term degree of movements relates to the number of axis in which the robot may move in one particular robot configuration.
Regardless of the configuration of a robot, movement along each axis will result in either a rotational or a translational movement. The number of axes of movement (degrees of freedom) and their arrangement, along with their sequence of operation and structure, will permit movement of the robot to any point within its envelope. Robots have three arm movements (up-down, in-out, side-to-side). In addition, they can have as many as three additional wrist movements on the end of the robot's arm: yaw (side to side), pitch (up and down), and rotational (clockwise and counterclockwise).
7. Mechanical design of the Gripper
7.1. General Design Description
The mechanical design of the robotic gripper needed to address the required interaction between the robot and the environment in order to grasp and hold the object securely and to execute the operation. When objects to be grasped are of different shape and size the friction method is normally used whereby the part is restricted from moving by the friction present between the fingers and the object. In this way the fingers exert sufficient force to hold the part against gravity, acceleration and any other force that might arise during the holding portion of the work cycle.
This is achieved through a mechanical design that incorporates multiple fingers and multiple joints per finger, through the installation of proximity and force sensors on the gripper, and through the employment of innovative and practical control system architecture for the gripper components. The gripper is installed on a standard six degree-of-freedom industrial robot, and the gripper and robot control programs are integrated in a manner that allows easy application of the gripper in an industrial pick-and-place operation.
The gripper or the end effector constitutes the end of the kinematic chain of an industrial robot and makes possible the interactions with the work environment. Although universal grippers with wide clamping ranges can be used for varied object shapes, in many cases they must be adapted to specific work-pieces shapes.
A robotic end effector is the "hand" of the robot's arm. By attaching a tool to the robot flange (wrist), the robotic arm can then perform designated tasks. Examples of robotic end-effector include robotic grippers, robotic tool changers, robotic collision sensors etc.
In many case, the robotic end effector requires additional power supplies to operate. It depends on the type of functions the end-effector perform, the popular one is the pneumatic, because it is easier to supply air to the end of a robot arm and. The only disadvantages of pneumatics are that it has a slightly lower power to weight ratio than hydraulics and it is not as controllable or easy to feed as electricity.
For certain applications some degree of sensory feedback from the gripper is necessary. For examples, the insertion or gripping forces measurement, proximity sensor to detect the presence of objects between the jaws of the gripper, collision detection unit which attaches between the robot flange and the end effector so that if excessive force is applied to the tool the robot arm will stop.
7.2 Robot -End Effectors:
End Effectors is the part that is connected to the last joint of a manipulator which generally handles objects, makes connection to other machines or performs the required tasks. Robot manufacturer generally do not design or sell end effectors. The hand of the robot has provision for connecting special end effectors that are specifically designed for a purpose.
The robot end-effector or end-of-arm tooling is the bridge between the robot arm and the environment around it. Depending on the task, the actions of the gripper vary. A robotic end-effector which is attached to the wrist of the robot arm is a device that enables the general-purpose robot to grip materials, parts and tools to perform a specific task. The end-effectors are also called the grippers.
There are various types of end-effectors to perform the different work functions. The various types of grippers can be divided into the following major categories.
- Mechanical grippers
- Hooking or lifting grippers
- Grippers for scooping or ladling powders or molten metal or plastics
- Vacuum cups
- Magnetic grippers
- Others: Adhesive or Electrostatic Grippers
The grippers can be classified into,
- Part handling grippers
- Tools handling grippers
- Special grippers
The part handling grippers are used to grasp and hold objects that are required to be transported from one point to another placed for some assembly operations. The part handling applications include machine loading and unloading, picking parts from a conveyor and moving parts, etc.
There are grippers to hold tools like welding gun or spray painting gun to perform a specific task. The robot hand may hold a deburring tool.
The grippers of the robot may be specialized device like remote center compliance (RCC) to insert an external mating component into an internal member, viz. inserting a plug into a hole.
The other type of end-effectors employs some physical principal like magnetism or vacuum technology to hold the object securely.
7.2.1 Classification of End-effectors:
An end effector of a robot can be designated to have several fingers, joints and degrees of freedom. Any combination of these factors gives different grasping modalities to the end-effector.
The general end-effectors can be grouped according to the type of grasping modality as follows,
- Mechanical fingers
- Special tools
- Universal fingers
184.108.40.206 Mechanical Fingers:
They are used to perform some special tasks. Gripping by mechanical type fingers is less versatile and less dexterous than holding by universal fingers as the grippers with mechanical fingers have fewer numbers of joints and lesser flexibility.
The grippers can be sub grouped according to finger classifications like two, three and five-finger types. The two-finger gripper is the most popular.
Robot end-effectors can be classified on the basis of the mode of gripping as external and internal gripping. The internal gripping system grips the internal surface of objects with open fingers whereas the external gripper grips the exterior surface of the object with closed fingers.
Robot end-effectors are also classified according to the number of degrees of freedom (DOF) incorporated in the gripper structures. Typical mechanical grippers belong to the class of 1 DOF. A few grippers can be found with more than 2 DOF.
Using some special tooling action, robot grippers can be designed to retain objects by electromagnetic action or under the action of vacuum. Electromagnets and vacuum cups are typical devices in this class. Usually, if the objects to be handled are too large and ferromagnetic in nature, electromagnetic grippers may be employed. In some applications where the objects are too thin to be handled, they can be held by vacuum grippers.
220.127.116.11 Universal Fingers:
Usually comprise multipurpose grippers of more than three fingers and or more than one joint on each finger which provide the capacity to perform a wide variety of grasping and manipulating assignments.
18.104.22.168 Mechanical Gripper:
A mechanical gripper is an end-effector that uses mechanical fingers actuated by a mechanism to grip an object. The fingers are the appendages of the gripper that actually makes contact with the object. The fingers are either attached to the mechanism or an integral part of mechanism.
7.3. Types Of Grippers
7.3.1. The Clapper
The Clapper can be built using metal, plastic or wood. It consists of a wrist joint. Connected to the wrists are 2 plastic plates. The bottom plate is secured to the wrist and the top plate is hinged. A small spring-loaded solenoid is positioned between the two plates. When solenoid is active, the gripper is closed and when solenoid is not active, the gripper is open.
The choice of solenoid is important. It must fit between the 2 flaps and should have a flat bottom to facilitate mounting. It must operate within the voltage used in your robot (usually 6V or 12 V). If solenoid doesn't have mounting flanges opposite the plunger, mount it in the center of the bottom flap using household cement
7.3.2. The Two Pincher Gripper
The two-pincher gripper consists of two movable fingers, somewhat like the claw of a lobster.
In today's industry the two-finger mechanical grippers with a single degree of freedom are the most usual used device. The fingers have symmetrical motions with respect to the gripper axis. A particular category of grippers for industrial robots has two degrees of freedom and a single driving motor. The relative positions of the component elements depend on the frictional coefficients between work piece and fingers and on the initial position of the work piece with respect to the gripper's frame.
7.4 Development and Fabrication of the Two Pincher Gripper
8.Scope For Further Work
The Robotic Gripper is essentially a vital part of robot design. In its history it was simple and sometimes ineffective but day by day modern advances have been inputted to such robotic systems which have proved to be highly efficient, effective and versatile.
A flurry of innovations and developments is on the agenda in context of robotics designs of the future. Major manufacturers are constantly striving to improve existing technology as R&D divisions focus on figuring out ways and means to conjure up better and simpler forms of robots.
Other such technologies that have been significantly improved in robotic designs are in:
- Health care: hospitals, patient-care, surgery, research, etc.
- Laboratories: science, engineering, etc.
- Law enforcement: surveillance, patrol, etc.
- Military: demining, surveillance, attack, etc.1`
- Mining, excavation, and exploration
- Transportation: air, ground, rail, space, etc.
- Utilities: gas, water, and electricity
With such advances in technology the future of robotics design seems promising.
- Stan Gibilisco, Concise encyclopedia of Robotics
- Klafter D Richard; Robotic Engineering An Integrated Approach, 1st Edition, 1989.
- Craig J John, Introduction to Robotics Mechanics and Control, 3rd Edition, Pearson Education, Inc, 2005.
- Schilling J Robert, Fundamentals of Robotics Analysis and Control, 1st Edition, Prentice Hall, 1990.