MDP646: ROBOTICS ENGINEERING. Mechanical Design & Production Department Faculty of Engineering Cairo University Egypt. Prof. Said M.

Transcription

1 MDP646: ROBOTICS ENGINEERING Mechanical Design & Production Department Faculty of Engineering Cairo University Egypt Prof. Said M. Megahed 1

2 ROBOT MECHANICAL STRUCTURE AND TOOLING INTRODUCTION The mechanical structure of an industrial robot consists of two main parts: the robot base and the arm. The robot base is usually stationary but in some workstations, it can be moved to different workstations via a track or a rail. In general, the robot arm consists of a number of moving links forming a kinematic chain connected to a fixed or movable base. The links are joined together by revolute R and/or prismatic P joints driven through transmission systems by the robot drivers. The arm usually consists of two basic elements: a basic structure and a wrist. The robot basic structure consists of the first three links (bodies) of the robot arm and the other links is called its wrist or hand. The robot is equipped by a device called the end-effector to perform the prescribed task which may be a gripper or a tool. This chapter is organized in the following main sections: Robot mechanical structure Robot tooling and end effectors These items are important for the selection of a suitable robot for a specified application. 2.2 ROBOT MECHANICAL STRUCTURE Topological Structure of a Robot Arm A robot manipulator consists of links connected by joints driven by separate motors. A robot usually includes two types of joints, connecting its links. These are commonly of revolute R, and/or prismatic P types. Unlike the joints in the human arm, the joints in a robot manipulator are normally restricted to one degree of freedom, to simplify its mechanics, kinematics, and control. Figures 2.1(a & b) present two industrial robots and their basic movements while Fig. 2.1c presents a parallel robot. The robot arm under study consists of n moving links forming one simple kinematic chain connected to a fixed base. The links are joined together by revolute and/or prismatic joints. The links and joints are numbered from 1 to n starting from the base side which has the number ι 0. A schematic representation of a robot arm is shown in Fig. 2.2 while Fig. 2.3 presents the symbols used for presenting robot joints and links. A robot link is a solid mechanical structure which connects two joints. The main purpose of robot links is to maintain a fixed relationship between the joints at its ends. The last link of a manipulator has only one joint, located at the proximal end of the link ι n (the end closest to the base). At the distal end of this link (the end furthest away from the base) instead of a joint, there is usually a place to attach a gripper or an end effector: a tool plate. The common link configurations are shown in Fig

3 (a) Example of an industrial robot (b) FANUC robot (c) Parallel robot Fig. 2.1: Robot mechanical structures 3

4 Fig. 2.2: Schematic representation of an industrial robot (a) Revolute joint R (b) Prismatic joint P (c) Revolute connections (d) Prismatic connections (e) Robot links Fig. 2.3: Robot joints and links 4

5 2.2.2 Robot Basic Structures and Geometric Configurations The robot basic structure consists of the waist, the shoulder, and the forearm, which respectively gives the waist rotation, shoulder action, and elbow action (see Fig. 2.4). The robot wrist consists of the last three movements of the forearm rotation, and the wrist rotation and action (Roll, Pitch, and Yaw). Fig. 2.4: T3 Robot There are 8 basic structures of robots depending on the type of the first three joints (R or P). These basic structures are RRR, RRP, RPR, PRR, RPP, PRP, RPP, and PPP. These basic structures give 4 basic configurations of robot arms shown in Fig These are: a- Scara and polar coordinate for basic structures RRP, RPR, and PRR b- Cylindrical coordinate for basic structures PPR, PRP, and RPP c- Jointed configuration for basic structure RRR d- Cartesian coordinate for the basic structure PPP The main function of the robot basic structure is the positioning of its wrist Robot Wrist The robot wrist is designed for orienting the end-effector to do a task or to grasp an object. In general, the wrist consists of three intersecting revolute joints RRR and is called spherical wrist (see Fig. 2.4). This property facilitates the robot arm modeling and control. Some robots have non-spherical wrists (see Fig. 2.1b). A robot wrist consisting of only two joints RR is shown in Fig. 2.5 Robot Type: The robot type may be defined by a succession of letters R and/or P which defines the order and type of its joints starting from the base side and ending by last joint of its wrist. For example, the robots given in Fig. 2.1a & b are of type RRRRRR. 5

6 (a) Cartesian robot (b) Cylindrical robot θ1 θ2 (c) Scara robot (d) Polar robot (e) Spherical robot (f) Jointed arm Fig. 2.4: Robot configurations Fig. 2.5: Example of a robot wrist 6

7 2.3 ROBOT TOOLING AND END EFFECTORS Robots may be equipped by either a gripper to handle a working piece or a tool to do a specific task. The purpose of the robot end effector is to perform the robot's operating functions in the different applications. These include gripping, positioning, drilling, spray painting, welding, gluing, grinding, and many others. The robot end effector may be a gripper or a device to pick up and hold an object. It may also be a tool, a laser or welding gun, a paint sprayer, or other device that is attached to the wrist of the robot. The tool may be either held by a gripper or mounted directly on the wrist-mounting surface. The end effector may also be a sensing or a measuring device. In other words, the end effector is the "business end" of the robot. Using robot end effectors is better than human hands in dealing with heavy objects, corrosive substances, hot objects, or sharp and dangerous objects. They are not as good at handling complex shapes and fragile items. Also, since the current hands do not have good tactile sensing capability, they cannot do the many complex tasks that human body touch alone. End effector prices are generally ranged between 3% and 10% of the robot price. Sometimes, end effector prices are as low as 0.1% or as high as 20% for smarter or more specialized grippers. Most of robot end effectors used in industry will require an input power for their operation. Power transmission lines that dangle from the end effector can easily catch on equipment, be served, and present severe hazards. Each power line must terminate in some sort of connector at the wrist socket. Connector options for various types of power are: electrical, pneumatic, hydraulic, optical, and mechanical. Transmitting the shaft rotation through a flexible cable from a motor that is mounted farther back along the manipulator can improve performance by reducing the mass and weight at the wrist. End effector motions are: go to a point, follow a contour, and follow a contour at a specified speed. The following sub-sections outline some of the available robot grippers and tooling Robot Grippers Robot grippers or end effectors can roughly be classified into five categories: Mechanical: Two or more fingers or jaws for external/internal grasping. Vacuum: One or more vacuum cups for handling nearly flat work pieces that are reasonably smooth and clean. Magnetic: One or more electromagnets or permanent magnets for handling ferrous materials. Expandable: Inflatable bag or cuff to handle odd shapes and fragile materials, also inflatable prehensile fingers. Adhesive: For soft materials (like cloth) that are of lightweight and not suitable for vacuum techniques. Robot grippers generally use one of four methods for holding an object: friction, physical constraints, attraction, or support. Figure 2.6 presents some kinds of these grippers, which are used in many robotics applications. These grippers include the mechanical 7

8 type where friction or the physical position of the gripper itself retains the object. The suction cups are used for flat objects and the magnetized gripper devices for ferrous objects. Hooks are used to lift parts off conveyors and scoops or ladles for fluids, powder or granular substances. Friction and constraining grippers are usually linkage mechanisms operated by one or more actuators, some of which may be served. Friction grippers exert pressure on a work piece, either by expanding within it or by closing on it from outside. The work piece can be pulled away from such a gripper with sufficient forces. Friction grippers generally rely on soft materials at the point of contact with an object in order to give sufficient force of friction for a secure grasp. Physically constraining grippers may or may not exert pressure on a work piece. Instead, these grippers grasp the work piece by placing solid material around it in order to prevent it from moving. Most of these grippers hold a work piece rigidly, but in one popular design suitable for light-duty use. (a) Mechanical grippers Fig. 2.6: Common types of grippers 8

9 (b) Vacuum grippers with applications (c) Magnetic gripper (d) Heavy load grippers (f) Flexible grippers (g) Adjustable gripper for delicate parts (h) Prehensile fingers Fig. 2.6 (Continued): Common types of grippers 9

10 Attraction grippers use their magnetic force or suction to hold an object. Suction cups will probably be more useful to aerospace manufactures since most of the material they handle is nonmagnetic and in sheet form. Adhesion has not been used much in grippers to date but could well be. The most widely used form of support gripper is a hook, and this gripper is usually found only on crane-type manipulators. A hook can be a useful accessory on a manipulator when the hook is being operated under remote control during the equipment set up for a batch production run Robot Tooling The tools which are grasped by mechanical grippers include the spot or arc welding gun, spray painting gun, drilling spindle, routers, grinder wire brushes, and heating torches. Some examples are given after: Routers, sanders and grinders: A routing head, grinder, belt sander, or disc sander can be mounted readily on the wrist of an industrial robot. Thus equipped, the robot can rout work piece edges, remove flash from plastic parts, and do rough snagging of castings. For finer work, in which a specific path must be followed, the tool must be guided by a template. The template is a substitute for the visual and sometimes tactile control that a human worker would exercise. In such a case, the overall accuracy achieved depends upon how accurately the work piece is positioned relative to the template. Usually, the part is automatically delivered to a holding fixture on which the template is mounted. Figure 2.7 shows a deburring process using an air grinder, a screw/nut driver, and a vibratory bowl feeder. This feeder is used for small parts to orient and position for automatic assembly machines and robots. (a) Deburring with an air grinder (b) Screw/nut driver Fig. 2.7: Robot Tooling (c) Vibratory Bowl Feeder Spray gun (Fig. 2.8): Ability of the industrial robot to do multi-pass spraying with controlled velocity fits it for automated application of primers, paints, and ceramic or glass frits, as well as application of masking agents used before plating. For short or medium-length production runs, the industrial robot would often be a better choice than a special-purpose setup requiring a lengthy changeover procedure for each different part. Also, the robot can spray parts with compound curvatures and multiple surfaces. The initial investment in an industrial robot is higher than for most conventional automatic spraying systems. When the cost of frequent changeovers is considered, the 10

11 initial investment assumes less importance. Industrial robots can be furnished to meet intrinsically safe standards for installation in solvent-laden, explosive atmospheres. Fig. 2.8: Spray painting robot Spot welding gun (Fig. 2.9): A general purpose industrial robot can maneuver and operate a spot welding gun to place a series of spot welds on flat, simple-curved, or compound-curved surfaces. In production line operations on appliances or auto bodies, stop-and-go rather than continuous line motion is preferred. Otherwise, weld placement accuracy suffers because the robot must track a moving target as well as place the welds. When the time available is too short for one robot to make all the welds within its reach, the number of welds can be divided among two or more robots, as is done in the automotive industry. Similarly, if all of the welds are not of the same type, there must be a different gun and so a different robot for each. The robot can position welds within in., but the line must position the work accurately. Fig. 2.9: MIG welding torches Stud-welding head: Equipping an industrial robot with a stud-welding head is also practical. Studs are fed to the head from a tubular feeder suspended from overhead. One caution concerns accuracy with which welded studs can be located. An industrial robot can position a stud within in., but on-the-line work positioning must be exact. The weight of the head is rarely a significant limitation. Stud-welding heads are well within the 100-lb capacity of standard robots. 11

12 Inert gas arc welding torch: Arc welding with a robot-held torch is another application in which an industrial robot can take over from a man. The welds can be single or multiple-pass. The most effective use is for running simple and compound curved joints, as well as running multiple short welds at different angles and on various planes. Maximum work piece size is limited by the robot's reach, unless the robot is mounted on rails. Where the angle at which the gun is held must change continuously or intermittently, the industrial robot is a good solution. But long welds on large, flat plates or sheets are best handled by a welding machine designed for that purpose. In addition to welding for fabrication purposes, wear-resistant surfaces and edges can be prepared by laying down a weld bead of tough, durable alloy. And the robot will handle a flame cutting torch with equal facility. Ladle: Ladling hot materials such as molten metal is a hot and hazardous job for which industrial robots are well-suited. In piston casting, permanent mold die casting, and related applications, the robot can be programmed to scoop up and transfer the molten metal from the pot to the mold, and then do the pouring. In cases where dross will form, dipping techniques will often keep it out of the mold. However, other solutions such as vacuum pouring tubes may be preferable. Heat torch (Fig. 2.10): The industrial robot can also manipulate a heating torch to bake out foundry molds by playing the torch over the surface, letting the flame linger where more heat input is needed. Fuel is saved because heat is applied directly, and the bake out is faster than it would be if the molds were conveyed through a gas-fired oven. Fig. 2.10: Heat torch Pneumatic nut-runners, drills, and impact wrenches: General purpose industrial robots are especially well suited for performing nut-running and similar operations in hazardous environments. Drilling and countersinking with the aid of a positioning guide is another application. Mechanical guides will increase the locating accuracy of the robot and also help shorten positioning time. Without such guides, both accuracy and positioning time suffer, and a human worker is often faster. In the application illustrated, the positioning guide surrounds the impact wrench that unscrews a lifting lug from the nose of a projectile in a munitions plant. 12

13 Tool changing: A single industrial robot can also handle several tools sequentially, with an automatic tool-changing operation programmed into the robot's memory. The tools can be of different types or sizes, permitting multiple operations on the same work piece. To remove a tool, the robot lowers the tool into a cradle that retains the snap-in tool as the robot pulls its wrist away. The process is reversed to pick up another tool. RDS End Effectors (Fig. 2.11): To maximize production de-paint rates, the robot de-paint system (RDS) end-effectors utilized three parallel blast nozzles. The operator was given a view of the blast process by means of a camera. Dual aircooled lamps were provided for lighting. A crash hoop was included to prevent inadvertent contact between the robot and the aircraft. Pressure sensors were provided for the blast nozzles as a controllable process parameter. SwRI designed, developed, and implemented a patented sensing system that uses electromagnetic detectors to feedback the aircraft surface condition. The system determined the degree of paint removal, which was used to adjust the end-effectors velocity accordingly. Fig. 2.11: RDS end effector Wire harness assembly end effectors (Fig. 2.12): This end-effector was developed to incorporate gripping, sensing, and transfer mechanisms for automated wire harness assembly. The device was designed to handle multiple wire sizes and types, including twisted-pair combinations. Two stepper motors, numerous pneumatic actuators, and multiple compliance axes are used to drive its internal mechanisms. 13

14 Fig. 2.12: Wire harness assembly end effector: Canopy polisher end effector (Fig. 2.13): The canopy polishing end effector uses a pneumatic random-orbital sanding motor with quick disconnect abrasive pads. Four stages of abrasive are used to achieve material removal and optical clarity. Water is introduced through the end effector and abrasive pad to promote more efficient cutting and remove debris. The end effector can be programmed to exert a specific force upon a canopy. The end effector has built in yaw-pitch compliance and is generally conformed to the compound surfaces of canopies. The pad has pitchyaw compliance and is conformable to localized surface details on the compound curved surfaces of the canopies. Deriveter end effector (Fig. 2.14): The end-effector for the robotic deriveter was developed and implemented for the repair and rebuild of Navy aircraft wing equipment. The deriveter was used in precise drilling, location of the drilling center, punching, and hole inspection. Nondestructive evaluation was conducted using an eddy current probe. A vision system recognized, identified, and classified the types of fasteners encountered. Sensors were included to detect dull and damaged tooling and a tool change/calibration station was incorporated in the system to exchange and calibrate drills and punches. An installation tool to reinstall rivets was completed. The system was able to locate rivets within inch and ±2 degrees of the angle of the rivet centerline. The system could also discriminate between aluminum and steel flat head rivets, jo-bolt fasteners, Philips, or slotted screws and bolts. 14

15 Fig. 2.13: Canopy polisher end effector Fig. 2.14: Deriveter end effector Robot end effectors fix alignment problems (Fig. 2.15): CV vertical compliance devices detect axial misalignment between two parts during assembly process or loading application. CH horizontal compliance devices work when lateral misalignment is present during assembly or loading application. Units provide lateral and rotational compliance to reduce jamming and misalignment. AU series emergency stop modules protect robots or other equipment from loads caused by collisions, excessive moment loads, and programming errors. 15

16 Machine tool as an end effector: Gear manufacturing process can be done by a robot as shown in Fig Fig. 2.15: Robot end effectors fix alignment problems Fig. 2.16: Machining Tools 16

17 2.4 SUMMARY Part I-Module#3: 2- Robot Mechanical Structure and Tooling The choice of a robot mechanical structure depends on the task to be performed. The robot mechanical structure must satisfy the task positioning and orienting within the desired accuracy. This is essential for studying the kinematics, dynamics, and control of robot arms. This chapter is concerned with the study of robot mechanical structure, geometric configuration, and end effector tooling. REFERENCES ceeps.colostate-pueblo.edu/met/met451/lectures/endeffectors.ppt- Supplemental Result lecture%205%20automation%20yr%202.ppt 17