End Effector Design Issues

Good end effector design is in many ways the same as good design of any mechanical device. Foremost, it requires:

• A formal understanding of the functional specifications and relevant constraints. In the authors, experience, most design “failures” occurred not through faulty engineering, but through incompletely articulated requirements and constraints. In other words, the end effector solved the wrong problem.

• A “concurrent engineering” approach in which such issues as ease of maintenance, as well as related problems in fixturing, robot programming, etc., are addressed in parallel with end effector design.

• 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. 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.

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 thermocouples. For these sensors, a dedicated microprocessor as well as analog instrumentation is typically required to interpret the signals and communicate with the

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).