Crc Press - Mechanical Engineering Handbook - Robotics 1 Part 4 doc

14-30 Section 14 © 1999 by CRC Press LLC •An attention to details in which issues such as power requirements, impact resistance, and sensor signal routing are not left as an afterthought. Some of the main considerations are briefly discussed below. Sensing Sensors are vital for some manufacturing applications and useful in many others for detecting error conditions. Virtually every end effector design can benefit from the addition of limit switches, proximity sensors, and force overload switches for detecting improperly grasped parts, dropped parts, excessive assembly forces, etc. These binary sensors are inexpensive and easy to connect to most industrial controllers. The next level of sophistication includes analog sensors such as strain gages and thermo- couples. For these sensors, a dedicated microprocessor as well as analog instrumentation is typically required to interpret the signals and communicate with the robot controller. The most complex class of sensors includes cameras and tactile arrays. A number of commercial solutions for visual and tactile imaging are available, and may include dedicated microprocessors and software. Although vision systems are usually thought of as separate from end effector design, it is sometimes desirable to build a camera into the end effector; this approach can reduce cycle times because the robot does not have to deposit parts under a separate station for inspecting them. Actuation The actuation of industrial end effectors is most commonly pneumatic, due to the availability of compressed air in most applications and the high power-to-weight ratio that can be obtained. The grasp force is controlled by regulating air pressure. The chief drawbacks of pneumatic actuation are the difficulties in achieving precise position control for active hands (due primarily to the compressibility of air) and the need to run air lines down what is otherwise an all-electric robot arm. Electric motors are also common. In these, the grasp force is regulated via the motor current. A variety of drive mechanisms can be employed between the motor or cylinder and the gripper jaws, including worm gears, rack and pinion, toggle linkages, and cams to achieve either uniform grasping forces or a self-locking effect. For a comparison of different actuation technologies, with emphasis on servo-controlled appli- cations, see Hollerbach et al. (1992). Versatility Figure 14.4.7 shows a how/why diagram for a hypothetical design problem in which the designer has been asked to redesign an end effector so that it can grasp a wide range of part shapes or types. Designing a versatile end effector or hand might be the most obvious solution, but it is rarely the most economical. A good starting point in such an exercise is to examine the end effector taxonomy in conjunction with the guidelines in Tables 14.4.1 and 14.4.2 to identify promising classes of solutions for the desired range of parts and tasks. The next step is to consider how best to provide the desired range of solutions. Some combination of the following approaches is likely to be effective. Interchangeable End Effectors. These are perhaps the most common solution for grasping a wider array of part sizes and shapes. The usual approach is to provide a magazine of end effectors and a quick- change wrist so the robot can easily mount and dismount them as required. A similar strategy, and a simpler one if sensory information is to be routed from the end effector down the robot arm, is to provide changeable fingertips for a single end effector. Compound End Effectors. These are a “Swiss army knife” approach that consists of putting a combi- nation of end effectors on a single arm, or a combination of fingertips on a single end effector. As long as the end effectors or fingertips do not interfere with each other and the ensemble does not weigh too much for the robot arm, this solution combines the advantage of not having to pause to change end effectors with the advantages of custom-designed tooling. Figure 14.4.8 shows a compound end effector with tools for feeding, measuring, cutting, and laying down wires in a cable harness. . 1 4- 3 0 Section 14 © 19 99 by CRC Press LLC •An attention to details in which issues such as power requirements, impact. taxonomy in conjunction with the guidelines in Tables 14 .4 .1 and 14 .4. 2 to identify promising classes of solutions for the desired range of parts and tasks. The next step is to consider how best. or a self-locking effect. For a comparison of different actuation technologies, with emphasis on servo-controlled appli- cations, see Hollerbach et al. (19 92). Versatility Figure 14 .4. 7 shows