End Effectors: Grippers and Tools

        END EFFECTORS

In our discussion of robot configurations , we mentioned that an end effector is usually attached to the robot's wrist. The end effector enables the robot to accomplish a specific task. Because of the wide variety of tasks performed by industrial robots, the end effector must usually be custom-engineered and fabricated for each different application. The two categories of end effectors are grippers and tools.

          Grippers

Grippers are end effectors used to grasp and manipulate objects during the work cycle. The objects are usually workparts that are moved from one location to another in the cell. Machine loading and unloading applications fall into this category (Section 7.5.1). Owing to the variety of part shapes, sizes, and weights, grippers must usually be custom designed. Types of grippers used in industrial robot applications include the following:

   mechanical grippers, consisting of two or more fingers that can be actuated by the robot controller to open and dose to grasp the workpart; Figure 7.10 shows a two-finger gripper

   vacuum  grippers,  in which suction  cups  are used to hold flat objects

   magnetized  devices,  for holding  ferrous  parts

Mechanical grippers are the most common gripper type. Some of the innovations und advances in mechanical gripper technology include:

   Dual grippers, consisting of two gripper devices in one end effector, which are useful for machine loading and unloading. With a single gripper, the robot must reach into the production machine twice. once to unload the finished part from the machine, and the second time to load the next part into the machine. With a dual gripper, the robot picks up the next workpart while the machine is still processing the preceding part: when the machine finishes, the robot reaches into the machine once to remove the finished part and load the next part. This reduces the cycle time per part.

   interchangeable fingers that can be used on one gripper mechanism. To accommodate different parts, different fingers are attached to the gripper.

   Sensory  feedback  in the fingers  that  provide  the gripper  with capabilities   such  as:

            sensing the presence of the workpart or (2) applying a specified limited force to the workpart during gripping (for fragile workparts).

    Multiple fingered  grippers  that  possess  the general  anatomy  of a human  hand.

   Standard gripper products that are commercially available, thus reducing the need to custom-design a gripper for each separate robot application.

          Tools

Tools are used in applications where the robot must perform some processing operation on the workpart. The robot therefore manipulates the tool relative to a stationary or slowly moving object (e.g., work part or subassembly). Examples of the tools used as end effectors by robots to perform processing applications include:

   spot welding  gun

   arc welding  tool

   spray painting  gun

   rotating  spindle  for drilling,  routing.  grinding,  and so forth

    assembly  tool (e.g., automatic  screwdriver)

   heating  torch

   water jet cutting  tool.

In each case, the robot must not only control the relative position of the tool with respect to the work as a function of time, it must also control the operation of the tool. For this purpose. the robot must be able to transmit control signals to the tool for starting, stopping, and otherwise regulating its actions.

   In some applications, multiple tools must be used by the robot during the work cycle, For example. several sizes of routing or drilling bits must be applied to the workpart. Thus, a means of rapidly changing the tools must be provided. The end effector in this case takes the form of a fast-change tool holder for quickly fastening and unfastening the various tools used during the work cycle.