Engr Haider-e-Karar

AUTOMATION & ROBOTICS

Gripper Selection & DesignEngr Haider-e-KararTeaching Assistant

Mehran University of Engineering and Technology

Gripper's Design

• Requirements for The design of gripper should satisfy the following requirements :

1. To match the abilities of the arm and controller. 2. To grasp and hold the object securely.3. To complete the task.

• Different grippers are needed for different applications, which are of highly varied types. These may be : • To hold complicated shapes which may be fragile, assembling parts and fitting

them into clamps/fixtures etc. (Once a part is picked up it must be securely held in such a way that its position and orientation are as desired).

Tips for Designing a Gripper

• Following are some tips for designing a gripper : A. Grippers with passive fingers :

1. Select the grip choice, based on the task to be performed and the geometry and characteristics of parts to be grasped (for example, parts way be fragile, slippery, irregularly spaced, may require contact forces to be distributed etc.; each needs different suitable gripper).

2. Consider the local environment, working space, conveyor position etc. which also influence the design.

3. Determine the number of joint links and their configuration geometry by the part geometry and task movements.

4. Decide material of construction based on type of duty, corrosion resistance, or heat resistance required.

5. Select proper type of actuator as follows : • Hydraulic for high forces;• Electric motors for best control;• Pneumatic for less cost and ease etc.

6. Ensure compliance and proper safety sensors for assembly work and for good part handling.

7. Examine whether the design is serviceable, robust, economical, and whether it is possible to use less moving parts.

8. Examine whether it is desirable to incorporate two or three grippers at the end of arm.

• Grippers with active feedback : 1. Use sensor based robots to handle heavy parts. 2. Use active wrist with passive fingers to handle forces greater than 1

to 2-5 kg.

Consideration of selection of gripper

1. Actuation selectioni. Mechanical or friction gripping methodsii. Magnetic grasping iii. Vacuum actuation

2. Drive Selection i. Pneumatic drive systems ii. Hydraulic drive systems for heavy duty operation iii. Electrical drive for light duty applicationsiv. Speed reduction of mechanical transmission

3. Protection Selectioni. Heat shield for sensors and actuators ii. Forced cooling by air or water cooling to take away the heatiii. Selection of heat resistance material for fingers and components of gripper.iv. Shield from hazardous chemicals.

4. Process Selectioni. Accurate processing methods for fingersii. Interchangeability standards for the fingersiii. Ease of assembly of fingers and linkagesiv. Leak prevention for pneumatic / hydraulic actuatorsv. Shape compatible processing methods for fingers.

Force Analysis of Grippers Mechanism

• The primary aim of the gripper mechanism is to convert input power into required motion and force to grasp and retain an object.• Following are the two ways of constraining an object in the gripper :

1. The gripper fingers may enclose the object partly, thereby constraining its motion. This can be accomplished by designing the contacting surfaces of the fingers as shown in Fig.

2. To hold the object between fingers and the object by friction. This approach demands that the finger must apply a force that must be adequate to retain the object against gravity, acceleration and any other force that might arise during holding operation.

Gripping Mechanism consisting of fingers, linkage frame and a pneumatic cylinder

Gripping Force Pg

• Gripping force to retain the part• + and – signs relate to acceleration (downward) and acceleration

(upward)

• Actuating force required to obtain gripping force

• Pressure needed to operate the piston p

(

Power Required

• Power required to produce air pressure p to operate gripper

Example

Commonly used gripper (end-effector) mechanisms• Commonly used gripper (end-effector) mechanisms are described

below : 1. linkage actuation :

— Opening width and the speed of gripping depends on the linkage configuration. — Higher gripping force at less actuating force due to linkage. — Many design possibilities using linkages.

Linkage Actuation Rack-pinion actuation

2. Rack-pinion actuation:

— Two pivoted pinions are operated by a rack which is operated by a hydraulic pneumatic cylinder— Pinions oscillate to move the racks (two). — Two fingers attached to the racks open and close to grab and release the object.

3. Cam actuation :

— A cam attached to a cylinder moves to and fro.— Spring-locked followers move on the cam profile.— The finger pads (attached to the pivoted lever, connected to the followers) open and close to release and grab the object.

4. Screw actuation

— A threaded block engages with a screw and moves forward and backward; the fingers attached to levers connected to the threaded block by a hinged joint, open and close providing releasing and gripping action. — The screw is rotated by a motor through reduction box.