Most of this chapter has been concerned with grippers rather than tools as end effectors. As indicated in before section tools are used for spot welding, arc welding, rotating spindle operations, and other processing applications.
In this section, let us summarize our discussion of grippers by enumerating some of the considerations in their selection and design.
Certainly one of the considerations deals with determining the grasping requirements for the gripper. Engelberger defines many of the factors that should be considered in assessing gripping requirements.
The following list is based on Engelberger’s discussion of these factors:
The part surface to be grasped must be reachable. For example, it must not be enclosed within a chuck or other holding fixture.
The size variation of the part must be accounted for, and how this might influence the accuracy of locating the part. For example, there might be a problem in placing a rough casting or forging into a chuck for machining operations.
The gripper design must accommodate the change in size that occurs between part loading and unloading. For example, the part size is reduced in machining and forging operations.
Consideration must be given to the potential problem of scratching and distorting the part during gripping, if the part is fragile or has delicate surfaces.
If there is a choice between two different dimensions on a part, the larger dimension should be selected for grasping. Holding the part by its larger surface will provide better control and stability of the part in positioning.
Gripper fingers can be designed to conform to the part shape by using resilient pads or self-aligning fingers. The reason for using sell-aligning fingers is to ensure that each finger makes contact with the part in more than one place.
This provides better part control and physical stability. Use of replaceable fingers will allow for wear and also for interchangeability for different part models.
A related issue is the problem of determining the magnitude of the grasping force that can be applied to the object by the gripper. The important factors that determine the required grasping force are:
The weight of the object. Consideration of whether the part can be grasped consistently about its center of mass. If not, an analysis of the possible moments from off-center grasping should be considered.
The speed and acceleration with which the robot arm moves (acceleration and deceleration forces), and the orientational relationship between the direction of movement and the position of the fingers on the object (whether the movement is parallel or perpendicular to the finger surface contacting the part).
- Whether physical constriction or friction is used to hold the part.
- Coefficient of friction between the object and the gripper fingers.