A Double Claw Robotic End-Effector Design

A Double Claw Robotic End-Effector Design
Today faster and cheaper computer processors make robots smarter and less expensive. Researchers are working to make robots move and ‘think’ more efficiently. Although most robots in use today are designed for specific tasks, the goal is to make universal which can to do anything a human can do.

      The definition of an industrial robot given by the Robotics Industries Association (R.I.A.) is, ‘A reprogrammable, multifunctional manipulator designed to move material parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks’. ISO defines Robot as: ‘Robot is an automatically controlled, re programmable, multi-purpose machine with several reprogram able axes which is either fixed in place or mobile for use in industrial automation application’.

End effector is a device or tool that’s connected to the end of a robot arm. The end effector is a part of robot arm that is designed to inter act with the environment.  The exact nature of this device depends on the application of the robot. The structure of an end effector and the nature of the programming and hardware that drives it depend on the task the robot will be performing. Robotic end effectors are a device that attaches to the robot arm and enables the general purpose robot to perform a specific task. End-effectors function as robotic hands. These tools are typically connected to robot flanges, such as wrists, to perform applications.

End effectors are also known as:
  • Robotic accessories
  • Robotic peripherals
  • Robotic tools
  • End of arm tooling (EOAT).

Gripper design considerations are crucial in producing a functional and cost effective product for rehabilitation applications. This gripper is designed to be mounted to any robotic arm, and in particular, wheelchair mounted robotic arms that are used to enhance the manipulation capabilities of individuals with disabilities that are using power wheelchairs. Most grippers of similar objectives use two fingers for grasping, but the dexterity of these fingers limit the use of the gripper. This project attempts to provide a new design with enough dexterity to widen the range of grasping tasks that are used in the Activities of Daily Living (ADL) in an effort to improve performance and usability.
This work focuses on people who have limited or no upper extremity mobility due to spinal cord injury or dysfunction, or genetic predispositions. Robotic aides used in these applications vary from advanced limb orthosis to robotic arms. Persons that can benefit from these devices are those with severe physical disabilities, which limit their ability to grasp and manipulate objects. These devices increase self-sufficiency, and reduce dependence on caregivers.
The main objective of this work is to design and fabricate a gripper that is capable of grasping various door handles and knobs, cylindrical and spherical objects, tapered and conical objects, rectangular and odd-shaped objects, sheets of paper, light switches and buttons, and other larger objects up to four inches in width that are commonly used for activities of daily living. As a criterion, the gripping force objective was set to roughly ten pounds of force. The gripper was to be mounted on the end of a robotic arm which was

connected to a wheelchair. This factor limited the gripper’s size and weight, so design ideas were kept simple but effective. 3-D models of the considered designs were created using Pro/E and then later printed out to scale using a Rapid Prototype printer for design adjustments before building the actual gripper.

In designing a gripper, functionality is very important, and it remains one of the main factors considered in most robotics applications. If the design has good functionality, minimal cost, high durability, and the aesthetic characteristics are met, a good product is likely to be produced. In order to decide on a good design for a gripper, several aspects have to be inspected, such as the tasks required by the mechanism, size and weight limitations, environment to be used in as well as material selection. Some of
the ADL tasks that will be performed using the gripper  are opening doors, grasping a glass to drink from, flipping on a light switch, pushing and turning buttons and knobs, holding books and similar objects, handling tiny objects such as a CD or loose sheets
of paper, or holding a small ball.

Specific considerations were taken in the attempt to optimize the functionality of the gripper. It was decided early on that the gripper would utilize parallel motion generated from a dual four bar mechanism attached to each side of the two fingers creating 8 links between the gripper surfaces and the driving mechanism itself. As a start, the gripper’s fingers (paddles) were first put into consideration. Through the required tasks expected out of the overall device the gripper’s surfaces were designed to be varied for the adequate handling and use of household objects mentioned. For those items, the profile was decided initially to be angled as shown in figure 3.1. The angled surface was designed to constrain and secure handled objects, and the middle opening was meant to secure spherical objects. When opening a spring-loaded door, the two teeth at the tip of the paddles can secure the handle during the robot motion, and make the grasping force independent of the closing force of the gripper.

A rounded surface was later implemented in the place of the angled paddle surfaces as shown in figure 3.2, which would give the gripper a softer look as well as better function while grasping objects. A spherically channeled surface was decided to be placed in the center of the paddle surface with the intention to contour to spherical door knobs. Small protrusions were added to the end of each paddle at the tip of the gripper for grasping smaller objects allowing added dexterity and the operations of press buttons and toggle switches. The tips were specifically made narrow for precision operations and rounded off to prevent the marring of surfaces that they would come in contact with. Optional protrusions extending toward the center of the grip at the tip of one of the paddles was added to allow objects such as door handles and door knobs to be pulled open with more security, rather than relying on friction and the locking of the mechanisms grip alone. The other paddle would have a small opening for the protrusions to go through when closing the gripper is required as seen at the bottom of Figure 3.2.

An extra flat surface placed closer to the driver mechanism would be beneficial in grasping larger rectangular objects such as boxes or books. By relying on the finger tips of the gripper alone to grasp larger objects, a greater moment would be generated on the driving mechanism and higher stresses induced in the links to achieve the same amount of gripping force attainable from a location closer to the driving mechanism itself. Figure3.3 shows these changes to the paddles.

As a final modification to the paddles, a spring hinge was added to the back of the flat paddle surface, near the hinge location, to allow for a small amount of torsional rotation. The thought behind this modification was for an added degree of freedom in the paddles to allow for a better grasp on tapered objects such as cups and for self-adjustment. Four main contact surfaces were intended for this gripper: The spherical area at the center of the paddles for spherical objects, the two round surfaces on both sides of the paddle for handling cylindrical and tapered objects, the two flat surfaces at the bottom and top of the paddles for handling rectangular and large objects, and the paddles’ tips for handling small objects, switches, knobs and sheets of paper.

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