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1.10 ROBOT END-EFFECTORS An end-effector is attached to the wrist of a manipulator in the form of either a device used for grasping and holding materials, parts etc. or a tool to perform a specific task. The former is known as a gripper and the latter is one designed in the form of spot welding electrode, spray cai ee painting gun, and others. This section describes the working principles of some of the grippers used in manipulators, 1.10.1 Classification of Grippers Grippers are classified into several groups as described below. 1. Internal and External Grippers: Let us take the example of a steel pipe to be gripped as shown in Fig. 1.16. This pipe can be gripped either internally (refer to Fig. 1.16 (a)) or externally (refer to Fig. 1.16 (b)), and accordingly, the gripper is called either internal or external, respectively. one LLLLLLL, LLLLLL, L Af LLILLLTT, LLELTTTL LT. (a) (b) Figure 1.16: Schematic views showing (a) internal gripper, (b) external gripper. 2. Single Gripper and Double Gripper: Depending on the re- quirement to perform some specific tasks, either one or two in- dependent gripping device(s) is (are) attached to the wrist of a manipulator, and it is known as either single or double gripper. respectively. 3. Hard Gripper and Soft Gripper: A gripper is called hard, if a point contact exists between its finger and an object. On the other hand, there may be an area contact. between a soft gripper and the object to be grasped. 4. Active Gripper and Passive Gripper: An active gripper is equipped with sensor(s), whereas a gripper without a sensor is known as a passive one. For example, human hand is an active universal gripper but Remote Center Compliance (RCC) is a pas sive gripper. 1.10.2 A Few Robotic Grippers . Mechanical Grippers [5] — : In mechanical grippers, objects are grasped and held using mecha! i schani ; less versatile ical fingers actuated by some mechanisms. They are wie saad rs costly. Some of the mechanical grippers are describ below. piston moves towards the left-hand side (shown by solid ar- row), two followers come closer to one another coed to the na- ture of cam profiles) resulting into a closing o gripper pads, The conditions related to opening of the gripping pads are displayed using the dotted arrows. 2. Vacuum Gripper A vacuum gripper developed in the form of an elastic cup can be used to grasp thin parts (say, plates). This grasping becomes possi- ble due to a difference in pressures of inside and outside the elastic cup. A vacuum can be maintained inside the cup using either a vacuum pump or a venturi. A vacuum pump is capable of cre- ating a higher vacuum compared to that developed by a venturi but the latter is simpler and less costly in comparison with the former. Fig. 1.18 (a) shows a vacuum gripper designed using a rn “ro venturi TS =F _» elastic cup object (b) Figure 1.18: Schematic views showing (a) vacuum gripper, (b) an object grasped by vacuum gripper. ' its ve- venturi. Air passes through the venturi and consequently, i locity increases and pressure decreases, A vacuum is created, W “ the pressure inside the elastic cup becomes less than atmospher pressure. Fig. 1.18 () displays a thin plate grasped by the vacuum gripper. If the flow of air is stopped, the pressure inside the elastic cup becomes equal to that of the atmosphere, and consequently, the object will be released from the gripper due to its self-weight. 3. Magnetic Gripper It can be used for grasping different parts made of magnetic ma- terials. For example, it will be able to handle various steel (except stainless steel) components. It is to be noted that either a perma- nent magnet or an electro-magnet can be used for the said purpose. In order to separate the parts from the electro-magnet, its polarity is to be reversed. However, a stripping device is to be used for the said purpose in case of permanent magnets as explained below. Fig. 1.19 shows an application, in which a permanent magnet is / permanent magnet oO ‘ ; y Steel pin ‘ ‘ 7 Steel plate Figure 1.19: A schematic view of stripping device of a permanent magnet. utilized to grasp a steel plate. The magnet is able to grasp the plate, as its lines of force pass through it. To detach the steel plate from the permanent magnet, a steel pin is inserted through two opposite holes made on it, as shown in the figure. Some of the magnetic lines of force will pass through the steel pin and conse- quently, the lines of force passing through the steel plate become weaker. The steel plate may, thus, be detached from the magnet. 4. Adhesive Gripper It is used to grasp and handle lightweight objects only. Here, an adhesive substance is used to perform the gripping action. Its applications are limited and it is used only when other grippers like magnetic gripper or vacuum gripper cannot be utilized. 5. Passive Gripper Let us consider a task related to insertion of a peg into a hole. It may be a difficult task for a manipulator, as it may suffer from lateral error shown in Fig. 1.20 (a). To overcome this lateral er- ror, chamfering is provided to the hole, as displayed in Fig. 1.20 (b). However, it may suffer from another error, namely angular error. Thus, this problem of inserting a peg into the hole becomes a difficult one for a manipulator. The said problem can be solved successfully using a Remote Center Compliance (RCC), as shown in Fig. 1.21. The RCC may be : sie the hole through some trials. Y be able to insert the peg into 
