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36 m@ Robotics Simplified Objectives The main objective of this chapter is to familiarize the readers of the significance of end-effectors and their various modes of operation. This chapter explains in detail the working of various types of grippers like mechanical grippers, magnetic grippers, vacuum grippers, adhesive grippers, etc., as well as various types of tools used for various applications like welding, spray painting, etc. This chapter aims to give an understanding to the readers about the basics of gripping, comparing the advantages and disadvantages of various types of grippers. It also explains the various factors to be considered for the selection of grippers and Engelberger criteria for its selection. End-effectors Robots are designed for various purposes, and their size as well as payload capacities will vary for various operations. Therefore, they need some tools called end-effectors, which are the tools at the end of robotic arm. These robots use specific tools as end-effectors. The end-effector is decided based on the specific purpose, and its orientation and control are the major design considerations. The end-effector has a major role in the robot system. In the initial design phase, a flow chartis prepared to describe the tasks to be performed in each step. For example, it describes the steps on how to move the end-effector to handle the object, how to grab it, hold it, where to place it, etc. All these details are required to design an end- effector. For designing an end-effector specific to a particular process, it is required to know the dimension of the object to be grasped, the surface texture of objects to be handled, their orientation, position, surrounding environment, etc. From all these data, the specifications needed for an end-effector for the particular purpose are prepared, and a suitable end-effector is designed. Another important factor is the environment where it operates. For working in a clean environment like semiconductor manufacturing factories, end-effectors should be planned and designed such that they will not generate any particles during the operation. End-effectors should be made of selected materials like stainless steel or polymers or coated with materials like baked-on powder coating. End-effectors can make use of polymer washers and bearings to avoid the use of oil and surfaces that are not in direct contact with each other to avoid generating particles and wearing out of its parts. Air bearings are also employed in some applications where extreme clean environment is required. In some cases, the end-effector needs to be protected when it is employed to work in a hazardous environment where the operation involves the presence of highly reactive chemicals, and a protective covering may be provided to avoid chemical reactions. The material of the end-effector can also be chosen such that it will not react with the chemicals present in the working environment. End-effectors made End-Effectors Wi 37 i inum and ni with ene nickel alloys are preferred for working with chlorine, fluorine, In robotic applications related to food processing, cleanliness is most important. Such an equipment would be manufactured using non-corrosive materials with well-insulated and sealed electrical connections. Sterilization of the equipment must be performed periodically to prevent contamination by bacteria, germs, and other pathogens. The end-effectors are mainly classified as grippers and tools. End-effectors or tools connected at the end of robot wrist should have the following characteristics: . They must be capable of gripping, lifting, and releasing the part or work piece required for the process. ¢ Itshould be possible to sense the presence of a part in the gripper by utilizing sensors placed either on the tooling or at a particular position in the work area. e The weight of the end-effector should be at a minimum possible value as it adds up to the weight of the part for calculating the maximum payload. e Itshould be ensured that the part to be operated is properly gripped under all possible conditions of acceleration. End-of-arm tooling is defined as the subsystem included in an industrial robot that links the mechanical option of the robot (manipulator) to the art being handled or worked on. An industrial robot simply takes the form of a mechanical arm with a flat tool-mounting plate at its end, which can be moved to any location within its reach accurately. The robot is linked to the work piece or part through the tools or grippers attached to the tool-mounting plate. It is the end-effector that enables a robot for production by making its arm usable for operation. Types of end-effectors An industrial robot is essentially a mechanical arm with a flat tool-mounting plate at its end that can be moved to any spatial point within its reach. End-of-arm tooling in the form of specialized devices to pick up parts or hold tools to work on parts is physically attached to the robot's tool-mounting plate to link the robot to the work piece. A robot can become a production machine only if an end-effector has been attached to its mechanical arm by means of the tool-mounting plate. End-effectors are normally classified as grippets and tools. Grippers are specifically used to grip or hold the objects. They grab or grip the objects to be taken from one position to another. It is mainly used in loading and unloading processes, picking parts from a case or conveyor belt, arranging parts in a pallet, etc. Grippers are mechanical hands Made of suction devices, magnets, or hooks. End-Effectors @ 39 tc. Thus, mechanical gri race ee ¢, magnetic ari 8 ad sii a be pneumatic gripper, vacuum gripper, adhesive gripe ’ Mechani z Pper, etc. The figure 2.2 shows a simple model of a mechanical gripper. Miechanical' grippers are also classified on the basis of how they grip an object. External grippers grip the object on its exteri . . : 7 ° grippers grip it on the internal surface: mer Saracens nena Figure 2.2: Mechanical gripper A common form of gripper is made with two fingers, which can grasp the object properly. To overcome the gravity in order to grasp an object, two methods are commonly employed. One method is the physical constriction method by applying an opposite force that is to be lifted (refer to figure 2.3). It is implemented by making the contact surfaces of the gripper fingers to match with the shape of the part: Object to be gripped Figure 2.3: Mechanical gripper (physical constriction method with pads) 40 W Robotics Simplified Another method is to utilize the friction, where the fingers apply a frictional force on the work piece that is strong enough to counter the gravitational force acting on it. If we consider a simple case when only gravity is acting on the object to be gripped: nF = mg where wis the coefficient of friction nis the number of fingers to grip the object Fis the gripping force mis the mass of the object gis acceleration due to gravity Mechanical grippers are provided with a polyurethane pad for getting greater friction and thus better gripping. These pads are made of materials that are relatively soft. They have higher values of coefficient of friction and protect the object from scratch or damage. Pads will also protect the part from damage while gripping. This method is more economical because of the low complexity design of the fingers of the gripper. Mechanical grippers can be designed and developed on the basis of their specific application and based on the size of object to be grasped. Some robots are provided with dual grippers to increase productivity and in cases in which a robot needs to load two objects in a single task. Some grippers are provided with three fingers useful for handling cylindrical pieces. Example 2.1: Find out the weight of an object that can be gripped by a parallel- fingered gripper if the coefficient of friction between the object and gripper fingers is 0.23 and the force on the gripper finger is 2,800 N. Ans: When frictional force is utilized for gripping, where the fingers apply a frictional force that is strong enough to keep the part against gravitational force: pnF = mg where 1 is the coefficient of friction, n is the number of fingers to grip the object, F is the gripping force, m is the mass of the object, and g is acceleration due to gravity: ERE — 0,23x 2x 2800 g 9.81 = 131.29 kg Magnetic grippers Magnetic grippers are utilized in applications where the work piece to be handled is made up of ferromagnetic materials, Because of their magnetic character, these grippers can easily handle the parts with holes. The gripping effect will be maximized when the magnet has complete contact with the surface of the metal part. Flat metal parts are best suited for magnetic grippers as there will not be air gaps between the materials, which can reduce the strength of magnetic force. Stronger magnets are 42. @ Robotics Simplified i th magnetic grippers is that while grasping a metal sheet from Ate a acts they ean take Pritiple sheets because of the force of magnetic attraction. One method to solve this is to separate the sheets by giving provision for the same in the stacking device for proper picking by the magnetic gripper. For taking sheet at the top of the stack, a charge is induced in the iron sheets using a magnetic field, due to which the top sheet repels the rest of the sheets because those same charges repel each other. Such a stacking arrangement device is called fanner. Another method is to design the magnetic gripper such that the effective depth of ation of the magnetic force corresponds to the thickness of the sheet so that the other sheets below it is not displaced by this magnetic force. Vacuum cups Vacuum cups are a simple form of grippers that can be used for gripping objects with smooth, flat, and clean surfaces (refer to figure 2.5). But they cannot be used for gripping all types of objects and surfaces with holes. Their major advantage is that they do not require electric power, and thus, the issue of slipping of the object during power failure does not come in vacuum grippers. The gripper based on vacuum is light in weight and needs only one surface of the object to grip. Its drawback is that its application is limited to smooth, clean, and flat surfaces. They are manufactured from silicone, neoprene, etc. similar to rubber or soft plastic material. The number of suction cups is decided based on the dimensions of the object to be grasped: Figure 2.5: Vacuum cups These types of grippers a vi re pr _f i i « pa the ai pro ided with a vacuum pump or venturi to create —— r betwee i + chi F gripping capability of the vacuum cup ; ieee the object to be gripped. The FaPxa End-Effectors Mi 43 Where P is negative air pressure betw: een the F i the area of vacuum cup with effective vacuu vacuum cup and the object, and A is Adhesive grippers Some grippers can utilize adhesive materials for gripping lightweight materials like fabrics. Such application does not require high power and force. This gripping technique is utilized when the object must grip on one surface and when vacuum or magnetic gripping is not possible. The drawback of this mechanism is that its efficiency is easily affected as the adhesive used for gripping will lose its adhesive property when used many times. After each use, its adhesiveness, and hence, the reliability of gripper, decreases. Therefore, provision should be made to load the adhesive continuously like the ribbon of a typewriter so that adhesion is ensured during the gripping process. Tools Robots require grippers in applications where they must grasp a work part or hold or pick and place an object. In many applications, robot needs to manipulate a tool instead of gripper for a particular operation. Robots require certain devices called tools for performing certain tasks like drilling, welding, spray painting, etc. Tools are designed for these specific tasks and are designed such that they can be held or fitted to the end of the robotic arm. Programming can be done for changing, selecting, and operating them without human involvement. For an end-effector, the path control is of utmost importance; hence, it needs to be steadily and continuously controlled during every instant of its motion. For the effective operation of the tool, it should be able to move in continuous path for a careful and steady motion for its operation. When a spray gun is used as a tool by the robot, the paint will be too thin if it moves fast and will be too thick if it moves slowly. Some of the tools used in robotics are as follows: * Tools for arc welding ¢ Tools for spray painting * Tools for spot welding * Rotating spindles © Deburring tools End-Effectors Mi 45 Tools for spray painting Painting is done as the final step in : metallic material from corrosion, Roblts sherarnee fini fumes may be carcinogenic and can affe painting task if done manually. In robotic s the paints on a metal. It has a spray gunn " face. I . any surface. It should be done consistently with uniform spraying so that the whole rea is painted wi ini i “ : neon _ with beinera wastage of paint. It is clearly shown in figure 2.7. For with a continuous path control capacity is employed: shing and protecting the used for spray painting as the ct the health of person performing the Pray painting, a spray gun is used to coat 0zzle through which paint is disbursed to Area to be painted Figure 2.7: Spray painting tool - spray gun The advantages of robotic spray painting are the ability to remove the operators from hazardous environment filled with fumes, carcinogenic components, and fire hazards. It also enables smooth movement of the spray-painting device for better finish. A spray gun is controlled by the spray-painting operation, the major facto and OFF, rate of paint flow, fluid pressure, interlock functions of the robot controller through programming. The spray gun needs to be cleaned periodically, which is done effectively by including a cleaning cycle in the program at regular intervals, without much loss of production time. The spray nozzle is placed in cleaning jets, which spray the solvent on the nozzle for cleaning. robot by proper program execution. During the rs like turning nozzle of spray gun ON etc. are controlled and maintained using 46 WW Novercs pimpiyiea Tools for spot welding ing i i ique i ich metal i istance spot welding is the welding technique in whic paleo joined together by applying heat and pressure from the electric current to the area to be welded. In this process, copper alloy electrodes are kept in contact with the metal sheets to be welded at localized points to be welded. -welding process, pressure and heat are applied to the weld area using Shaped elloy copper electrodes, which allows an electrical current to pass through the weld pieces. The material melts with this heat, and this molten material solidifies and makes the joint for the two sheets. The current passed should be sufficient to reach its melting point by means of resistance heating. The end-effector commonly used in spot welding robots is a spot-welding gun at the end of a robot wrist. The robot performs the spot-welding operation based on the program. The parts to be welded are held together, and the electrodes are positioned at the points to be welded. Then, these electrodes are squeezed together against the parts, and the current is passed through when it gets welded because of the heat. The electrodes are removed and allowed to cool before next welding. The electrode cooling process is speeded up using the water circulation system. Specific spot-welding needs just a second, while more time is required for properly positioning the parts and electrodes. Also, wear and tear of the tip of electrodes of spot-welding guns occur quickly due to the heat used in welding. Therefore, the tops of electrodes are periodically dressed for removing the residues and deposits of previous welding operations, and the shape is restored for maintaining the quality of welding. Thus, robotic spot welding improves the quality of the process, provides better safety for the operators, minimizes labor cost, and gives a better control for the operation. The welding gun consists of a frame to open and close the Pair of electrodes (refer to figure 2.4). The current is delivered to the electrodes by means of large electric cables. A powerful robot is needed to manage the spot-welding gun, causing it to be very heavy, weighing almost 100 kg, including the pair of electrodes and large cables attached. The most common application is spot welding of car bodies in motor industries. Spot welding guns are available as pneumatic and hydraulic guns. As pneumatic guns are faster and apply a uniform force on the electrode, they are preferred more. Hydraulic guns are employed in welding applications where limited space is available or higher forces need to be applied on the electrodes. Based on the type of action, spot welding guns are classified as C-type and X-type guns. In a C-type gun, the operating cylinder is connected directly to the moving electrode, and in the X-type guns or scissors, the operating cylinder is remotely located with respect to the moving electrode, and the force is being applied with a lever arm. C guns are generally the cheapest and the most commonly used. 48 @ Robotics Simplified used for milling applications like engraving or routing, which involves material removing at high speeds and finishing surface materials with high precision. Some spindles are used for grinding using a grinding wheel to grind away materials Like metal, glass, ceramics, or other materials to the desired shape or finish. Spindles used for deburring should be operated with high speed and low torque. Spindles used for finishing and polishing should allow the tool to glide gently over the finished work piece with high RPMs and low horsepower. Thus, in all these applications, spindles and their performance are responsible for the proper execution of each operation. Deburring tools A burr is defined as an undesirable effect like a raised edge or small piece of the material that is protruded at the edge of a work piece after the machining or modification process. It can also occur during casting, forging, welding, cutting, plating, painting, etc. Deburring is a finishing process to remove these burrs from the work piece as they create hindrance in assembly work, and its sharp edges can injure 2 worker's hands. Also, it affects the finishing of the product. Deburring tools are used in robotics for this purpose. They have straight, angular, or multi-positioned head and are actuated by electric, pneumatic, or hydraulic power. While spinning at high speeds, a rotary cutting deburring tool moves on an air cushion that provides a reliable field of compliance, as well as maintaining a constant force. It provides more stiffness for the tool in the path direction and a lower stiffness in the direction of contact. Robotic deburring tools are mostly radially compliant or dally compliant. Some tools offer resistance in a direction in line with the axis of the tool and are radially compliant. They can be utilized for removing parting lines or flash from work parts. Flash refers to the excess unwanted material attached to a final product, which must be removed. This normally occurs due to leakage of the material between the two surfaces of a mold. Some other tools are stiff in the radial direction, perpendicular to the axis of the tool, and are axially compliant. These axially compliant tools operate with a motor and floating rotary file for cutting. A robot deburring system comprises of a spindle motor, a deburring tool, and a tool holder. The deburring tool is rotated and oscillated in the required direction by the spindle motor and rotates the work piece when required. This is possible only when the work piece is small and lightweight. Robot deburring tools include flexible tools, rotating files, oscillating files, and internal deburring tools. Flexible tools include belts used for straight line edges and brushes utilized for the corners. Brushes are made of aluminum oxide, ceramic, or silicon carbide filaments on nylon or steel material with a rigid base (refer to figure 2.10 A). Belts are manufactured with coated abrasives with cloth or paper backing. Solid deburring tools are rotary files that are widely used for various applications (refer to figure 2.10 B). They are available in different shapes and geometry and made of different End-Effectors @ 49 materials. Oscillating files are used for Temoving small and slender b urs. Figure 2.10: Tools for deburring (A) Deburring brush (B) Deburring tool Design and selection criteria for grippers The selection of a gripper is usually made by examining the geometry of the part, its orientation, the space available, and the manufacturing treatment to be performed. External gripping is the most widely used type, where the closing force is utilized to clench the part. An internal grip makes way for unobstructed access to the outside surface of the part, which is necessary for polishing /buffing, grinding, or painting applications. The opening force of the gripper is used to hold the part. There are numerous types of grippers both in style and power source. Determining which is the best type to use is an important issue that robotics users must face. Selection of the gripper is based upon several factors that may need consideration: ¢ Source of power: Based on the source of power for its operation, they are classified as: pneumatic, hydraulic, and electric grippers: Pneumatic grippers operate with the power obtained by introducing compressed air into a chamber of the device and powering a piston attached to a rod. The figure 2.11 shows a simple diagram of a pneumatic gripper. This energy is utilized for the mechanical motion of the gripper. In pneumatic grippers, jaws operate either in parallel or angular manner. The force required for its actuation is managed by the air pressure by adjusting the valve. The drawback is that it cannot be controlled fully. Mostly the jaws would mostly be in either open or close state: ° End-Effectors @ 51 Hold ports 2 1 5 \ \ \ \ 4 \o\ ZA —— SN Figure 2.12: Hydraulic gripper « Gripping force: The force required for gripping an object depends on its mass and its acceleration due to robotic motion. Gripping force also depends on the coefficient of friction between the gripping part and the gripper. The required gripping force can be reduced by designing fingers exactly for the gripping part. Even though it increases the weight-carrying capacity, it reduces the flexibility of handling. « Style of gripping: Gripping can be done in an angular or parallel manner. In parallel grippers, the jaws move parallel to the body of the gripper, and in angular grippers, the jaws of the gripper move in the form of an arc by opening and closing around a central pivot point. This is implemented based on some linkage mechanisms. Parallel grippers (refer to figure 2.13) are made with guide rails along which the base of fingers move parallel-to each other. OPEN CLOSED -_- — > + Figure 2.13: Parallel gripper The type of gripping is determined based on the specific needs of an application. ough angular grippers are more economical, they cannot be used for all applications due to their sweeping action. The figure 2.14 shows a simple diagram of angular grippers. Parallel grippers are employed more in applications with space ae 52 & Robotics Simplified constraints. Their fingers fit into small areas in a better way and are commonly employed for pulling a part out of a machine laying deep inside. The most popular configuration is two-jaw parallel grippers as they are easy to design and program as it works only in one axis of motion: In f/f} © ° ° o M S uy —_—_ — —_— =< oven aos Figure 2.14: Angular gripper A gripper can also be classified as external or internal gripper. External grippers grasp the exterior surface of the handling part with their closed fingers, whereas internal grippers grip the internal surface of the handling part with their open fingers. Internal grippers are used in cases in which some objects need to be held from inside like a coil of wire or when the external part of the object must be accessed and must be kept free. The figure 2.15 shows an internal gripper where the object is gripped internally like holding a bottle by gripping on the internal surface or through the opening of the bottle. Thus, the object is gripped by the opening force of the gripper: GZ Object gripped Ue p | | 1} ah ee | 1T\\ | | | \, / lo Q | \ \ | | Figure 2.15: Internal gripper 54 i Robotics Simplified The following are the major points pointed out by Engelberger: It should be possible for the gripper to reach the part to be gripped. The variation in the size of the part to be worked has to be accounted for positioning the part with maximum accuracy. Design of a gripper should be done by considering the part size variation during the loading and unloading of part. The gripper should not create any problem like scratching or distortion of the part to be gripped. If possible, grasping of part with larger dimension should be selected for better stability. Self-aligning fingers called resilient pads can be designed such that each finger makes multiple contacts with the part in different positions. Thus, the following factors should be considered while selecting the gripper to be employed for a specific task: Part to be gripped or handled: The dimension and weight, shape of the part, shape modifications during handling, condition of the surface, etc. should be considered. Method of actuation: It should be taken into consideration which method of actuation to be implemented in this system. It can be mechanical grasping, using magnet, adhesive gripping, vacuum cups, etc. (Refer last sections for detailed explanation of each.) Source of power and control signals: End-effectors require power to operate and control signals to control its operation. The main types that can be opted are pneumatic, hydraulic, electric, or mechanical. Th i é , , q - These h. in the last section of this chapter. ave been explained 
