10g (average: 3-4g) Average lifetime [year] 12

Transcription

1 5. Industrial robots 5.1. Entity of industrial robots Standard EN 775 defines an industrial robot as automatically controlled, programmable, multifunctional, with several degrees of freedom of a machine that is installed in automated manufacturing systems. In short, industrial robots are special purpose machines that are used in manufacturing automation (1) Today s industrial robots allow programmability in different programming languages and programming environments, possibility to determine positions, coordinate systems, paths and repeat the movements with high accuracy, ability of industrial controllers to control inputs and outputs, handle the information, communicate with users and other controllers via different field buses, coordinate robot movements with other devices and be connected into local area networks. The main features of today s industrial robots are summed up in Table 6.1 Table 6.1 Main features of today s industrial robots Feature Value Degrees of freedom 3-7 Speed [m/s] 20 (average 1-5) Repeating accuracy [mm] 0, Payload [kg] Resolution [mm] 0, ,5 Acceleration [m/s 2 ] 10g (average: 3-4g) Average lifetime [year] 12 Today robotics is developing very fast, incorporating mechatronics, electrical engineering and infotechnology. Because of high work quality they are more often implemented in manufacturing systems. An industrial robot (Fig. 1.1) consists of joints, rotating parts, and links that connect them. Joints are actuated with an actuator that can be an electrical motor (stepper motor, brushless DC motor, synchronous motor), pneumatic or hydraulic cylinder etc. Each joint is also equipped with a brake, usually an electromagnetic brake. Brakes hold the joints from moving, when supplied with DC voltage of +24 V the brakes are released. The joints can be roative or linear. Working area of each joint is limited with mechanical brakes or with software. It is necessary for robot not to cross the limit positions. To reduce the motor torque different gear boxes are used in the joints. Using gears allows choosing smaller motors.

2 Industrial robots are also equipped with many sensors that allow them to work safely and precisely. Industrial robot is able to identify a collision with a surrounding object, overspeeding, position divergence, wrong programming parameters etc. In addition many additional protective features can be added to the robot. Fig. 6.1 Fig. 6.1 Manipulator IRB Robot system A robot is not able to work alone. In order to perform any technological operation also other machines and devices are also necessary. A detail must be fed to the robot and information must be given to start work, also the whole process must be monitored during the process. So, each manufacturing unit must be analyzed as a whole system, i.e. a robot system. Robot system is an automatically functioning manufacturing module that consists of one or more manipulators, technological machines and a positioner to hold the detail [10]. The block schema of the robot system is shown in Fig. 6.2 and each block is described below.

3 Teach pendant Interface Mains Controller Manipulator Process Positioner Feedback Fig. 6.2 Block scheme of a robot system Controller Controller is a part of the robot system that controls the manipulator and other devices, gathers and analyzes the information from transducers about the status of the system, also exchanges data between users and other controllers via fieldbuses. Controller controls the manipulator and positioner at one time according to the program and is able to work simulatneously with other controllers. ABB controller IRC 5 is shown in Fig Fig. 6.3 ABB controller IRC Teach pendant A teach pendant is a handheld device that is used for jogging the manipulator, for teaching the targets, programming, testing of a program, configuration. The communication between the

4 user and the controller finds the state through the teach pendant. Teach pendant is not suitable for programming complicated paths. Instead the teach pendant, a special programming environment, (e.g. RobotStudio) must be used. An example of a teach pendant is shown in Fig. 6.4 Fig. 6.4 Teach pendant Manipulator A manipulator is a mechanism that consists of several links, motors and gears. A manipulator is the actuator of the robot system that uses different tools to accomplish the task. A manipulator is able to turn itself in different directions and achieve various positions in the room. The more axes (degrees of freedom) the robot has, the more flexible it is. Each axis is excited with and electrical motor (or other actuator) that is controlled via controller. Each motor is also equipped with a gear and a feedback transducer that is necessary for reliable control of manipulator. The tool (also called end-effector) is connected to the end of the robot arm. Different manipulators are shown in (Fig. 6.5). Fig. 6.5 Examples of some manipulators Positioner Positioner or underframe is a part of a robot system that holds the detail and puts it in appropriate position for the manipulator. Axes of the positioner can also be as an additional degree of freedom of the whole system and be controlled via one controller with manipulator. ABB positioner IRBP 250 D is shown in (Fig. 6.6).

5 Fig. 6.6 ABB positioner IRBP 250 D 5.3. Control of industrial robots Control of an industrial robot is much more complicated than the control of motors or other actuators (cylinders, transistors etc). Control motors required the control of rotational speed, direction and torque. Control of cylinders is based on switching between on and off states. Control of industrial robots must be realized with simultaneous control of many axes a threedimensinal room, also considering the orientation of the tool. An industrial robot moves the end effector so, that it follows the programmed path. For that, the controller must first calculate the path, calculate the characteristic of each motor and monitor the whole process (path tracking). If the path information gathered from feedback sensors deviates from calculated path, the program execution is stopped. A manipulator is always moving from one point to another. Movement between two points can be either linear or non-linear. In order to move between these targets, the controller must calculate the trajectory. When calculating the trajectory, the rotational speed and direction of each motor is determined, so that all axes reach the target at same time. An example of the trajectory between two points is shown in figure Fig. 6.7, where the manipulator with tree axis moves between them, following an unregular path.

6 Θ 3 Θ 2 Θ 3 Θ 1 Θ 2 Fig. 6.7 Path tracking of a manipulator To realize a smooth and accurate path tracking motors on each axis are equipped with transducers (e.g. resolver, absolute encoder) that give a controller the information about motor shaft s position, speed, acceleration and direction. A block schema of a robot controller is shown in Fig Teach Pendant Avansed Sensor Serial User Interface IQ Computer Sensor Interface Fieldbuses Communication Computer Robot Computer Motion Computer Ethernet User Memory Program Memory System Memory USB Fig. 6.8 Control schema of an industrial robot

7 5.4. End effectors Industrial robots are always equipped with an end effector that is used to accomplish a certain task. For example, a gripper can be used for pick and place, holding, supplying. An arc welding gun is used for welding applications. End effectors are also controlled with a controller (also, a separate controller may be used for it). The most common tools are pneumatic or electric grippers, welding guns, borers, milling tools and painting tool. Some end effectors are shown in Fig. 6.9 a b c d Fig. 6.9 Different tools: a arc welding gun; b pneumatic gripper with a camera; c pneumatic gripper; d milling tools 5.5. Industrial robots in manufacturing automation The greatest challenge in today s manufacturing is to achieve good quality with as low price as possible. It means that each company that deals with flexible manufacturing is not competitive without using industrial robots. Industrial robots replace people in the works that are either too hard or dangerous for people, require high accuracy or are too monotonic. The

8 most common works are arc- and spot welding, painting, packaging, die casting, palletizing, gluing and detail handling works. Industrial robots are mainly used in welding applications. Car industry uses welding robots in welding the car body. The welding process harms people with dangerous gases, glaring electric arc and sparks. These factors do not harm a robot. Also, an industrial robot works much faster and more accurately than people. Use of industrial robots in different fields is shown in Fig Fig Use of industrial robots in different industrial Ares 5.6. Classification of industrial robots Different types of applications require specific types of robots that differ by construction, control, power and other technical characteristics. For example, if a process requires moving in three directions it is reasonable to use an industrial robot with 3 degrees of freedom. If a robot with more degrees of freedom is used, then the resources of the robot are not used optimally. Each degree of freedom means adding a new joint to the robot that consists of a motor, mechanical transmission, feedback sensors and cables, which in turn increases a robots price and maintenance costs. Industrial robots can be classified by the design of the manipulator, by the type of joint actuators, by control principles and by the technological process. Classification of industrial robots by the design of the manipulator is shown in (Table 6.2.) Table 6.2 Classification of industrial robots by the design of the manipulator Type Kinematic structure Work range Example

9 Cartesian robot Cylinder robot Sphere robot SCARA robot Articulated robot Parallel robot

10 5.7. Implementation of industrial robots In the following chapters a short description of two technological processes with industrial robots are described: Abgrading of alu wheels and die casting Abgrading of aluminum wheels The manufacturing system consists of one industrial robot that grasps an alu wheel from conveyor. Afterward, using the laser measurement system it puts it in the fixed position, so that an abgrading machine could separate the unlinearities that occur during manufacture. During the process the robot turns the wheel in an appropriate position for the abgrading machine. After the agrading is over, the robot puts the wheel back to conveyor and takes the next detail. Advantages of such a system are continuous manufacturing, good quality because of high accuracy of the robot Die casting Die casting is one of the most dangerous and harmful process for industrial robots, because die casting is made in dirty environment, where the temperature is high and metal sparks get between moving parts of joints (gaskets of motors). In the following example the robot grasps a detail from the rope conveyor with a pneumatic gripper. Using a special camera system the robot positions the detail correctly into the bath with liquid metal (e.g. Zn), holds it for a while and takes out. Afterwards it hangs it back to the rope conveyor Self check 35. What are the most important technical properties of an industrial robots? 1. repeatability precision 2. acceleration 3. number of errors 36. What kind of a device is used to teach the robot the position points? 1. controller 2. personal Computer 3. control unit 37. What is a resolver? 1. feedback sensor rotary electrical transformer 2. sensor for measuring acceleration 3. a Limit switch 38. Which of these manipulator movements is not suitable for this list? 1. cartesian robot 2. SCARA robot 3. articulated robot 4. spherical robot