C a m i l l e C a r l o s C a r v a l h o ( N e p t u n c o d e : G F C S Y Q ) D o u g l a s D a n i e l E l i a s S a n t o s ( N e p t u n c o d e : R 6 Y 8 E I ) b u d a U n i v e r s i t y

Gripper study Comparison between Universal Gripper and Three Fingers Gripper

08 Fall

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Summary

Introduction ........................................................................................................................................... 3 Grippers .................................................................................................................................................. 3 Definition ............................................................................................................................................................ 3 Types .................................................................................................................................................................... 4 Impactive ............................................................................................................................................................................. 4 Ingressive ............................................................................................................................................................................. 5 Contigutive .......................................................................................................................................................................... 6 Astrictive .............................................................................................................................................................................. 6 How to choose ................................................................................................................................................... 8

Definition of the problem .................................................................................................................. 8 Universal Gripper ................................................................................................................................ 8 Three Fingers Gripper (Tri Max Gripper) .................................................................................. 12 Comparison ......................................................................................... Error! Bookmark not defined. Conclusion ............................................................................................................................................ 14 Bibliography ........................................................................................................................................ 14

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Introduction The human body is a perfect machine. All the parts are spotlessly connected. Everything works great due to its aim. The world is always trying to construct robots as perfect as the human body to automate process for avoid human mistakes and improve production time. This project work is about one specific part of industrial robots: the gripper. The gripper is the only part of the robot that has direct contact with the workpiece. Its operation is based on the human hand and it is a quite challenge to approximate its perfection to the human hand. There are several types of grippers, according to the aim of it. This project is about two of them, how they work, for what kind of workpiece, and so on.

Grippers

Definition A gripper is a device which enables the holding of an object to be manipulated. The easier way to describe a gripper is to think of the human hand. Just like a hand, a gripper enables holding, tightening, handling and releasing of an object. A gripper is just one component of an automated system. A gripper can be attached to a robot or it can be part of a fixed automation system. Many styles and sizes of grippers exist so that the correct model can be selected for the application. There are two different important forces in the gripping process. One is the grasping, also known as prehension, force. This one is responsible for initial point of prehension, the beginning of the process to grasp the object. The other force is the holding (retention) force, that is responsible to maintain the object in the gripper, until the point of release is achieved. Usually, the retention force is smaller than the prehension. The retention force is determined by the necessary energy for the mechanical motion leads to a static prehension force.

Figure 1 - Gripper that approximates the human hand

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Types

Impactive Impactive gripper is the gripper where the prehension is achieved by impactive force. Impactive gripping requires the motion of solid jaws in order to produce the necessary grasping force. Jaw is the part of the gripper where the fingers are attached. This is the most used gripper form. It is used in rigid objects and some examples are clamps (external fingers, internal fingers, chucks, spring clamps) and tongs (parallel, shear, angle, radial). Object retention of the impactive grippers is based on classical mechanics physical effects (Newtonian). These effects are related with mass points and forces, and requiring more or less extensive mechanisms. These grippers usually have between two and four fingers, normally moving synchronously. The design of this kind of gripper is quite complicated, but this is compensated by their operational reliability, ability to provide adequate force, shape and/or force matched prehension as well as their good adaptability with respect to handling operations.

Figure 2 - Design of a two fingers gripper

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Figure 3 - Two fingers gripper

Ingressive Ingressive gripping results in surface deformation or even penetration of the surface down to some predefined depth. This enables the gripper to hold the workpiece for the necessary time. It can be divided into two types: intrusive and non-intrusive. Pins, needles and hackles are intrusive. Hook and loop non-intrusive. This gripper is used with flexible objects like textiles, carbon and glass fibre. This type of gripper is applied in objects those are not solid in the conventional sense. The properties of these materials do not allow impactive grippers in their manipulation. The intrusive gripper really penetrates through the material, but this has a pre-determined depth, and preferably without damage the material. The non-intrusive usually does not penetrate the material, but pinch it.

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Figure 4 - Design of an ingressive gripper that uses needles to "catch" the workpiece

Contigutive Contigutive prehension implies a direct contact to facilitate gripping. It can be divided into three types. The thermal one is used in flexible objects, as the ingressive gripper, and this can be done by freezing or melting. Chemical contigutive gripper operates with permatack adhesives and are used in carbon fibre with glue impregnation.

Astrictive Astrictive methods are based on binding forces between surfaces. Magnetic and electrostatic adhesion and vacuum suction can lift most objects even without direct initial contact. One of the methods is the vacuum suction which is used in non-porous or rigid materials. The other method is the magnetoadhesion used in ferrous materials, and it can be with permanent magnet or electromagnet. The last method is the electroadhesion with electrostatic field in light sheet materials and microcomponents.

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Figure 5 - Vacuum gripper

Figure 6 - Another vacuum gripper

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How to choose Choose the right robotic gripper is essential to ensure economic advantages and practical success to the given task. Is necessary take into account technical factors of the process and about the part to be manipulated, before define the ideal gripper. The task that will be done with the robot is the main factor that often determine the gripper that should be used. The time that it should be done and how many cycles it will work, defines the acceleration and force that will be required. The main factors to consider about the part to be manipulated are the size, shape, weight and the surface type. Also the precision need and the environmental need are essential factors when choosing the right gripper for the application.

Definition of the problem The given task in this study is take an fragile oval object from one place and put in another compartment. We choose an egg as the part that should be hold by the gripper, in this case will be an "universal gripper" and one "three fingers gripper", after the analysing of both case we should compare the advantages and disadvantages of each one comparing with the other.

Universal Gripper First of all this universal gripper is focused in the problem of the gripping itself, and not in the manipulation, or in complexities like tactile sensing. The main idea of this gripper is replace the individual grippers by a material or interface that upon contact holds itself around the object. A gripper is considered universal if it conforms to arbitrary shapes and is passive in that all shape adaptation is performed autonomously by the contacting material and without sensory feedback. This passive process reduces the number of elements to be controlled and therefore can have advantages in terms of reliability, cost, and gripping speed.

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Figure 7 - Universal gripper

This gripper use the idea of granular material and show that the gripping process is controlled by a reversible jamming transition. The unique properties of a jamming gripper derive from the fact that loose grains in a bag sit at the threshold between flowing and rigid states. This behavior enables the gripper to deform around the target in the unjammed, malleable configuration, then harden when jamming is initiated. Applying a vacuum, enables the gripper to gain remarkable rigidity while almost completely retaining its shape around the target. This system approximates the limit of a robotic hand with infinitely many degrees of freedom, which are actuated passively by contact with the surface of the object to be gripped and are locked in place by a single active element, a pump that evacuates the bag. A wide range of different types of objects are easily handled in pick-and-place operations using a fixed-base robotic arm, without the need to reconfigure the gripper or even position it precisely, as long as it can cover a fraction of a target objects surface. This adaptability includes switching between objects of different shapes, items difficult to pick up with conventional universal grippers, or fragile targets like raw eggs, as well as simple manipulation tasks, such as pouring water from a glass or drawing with a pen.

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Figure 8 - Universal gripper gripping different types of objects

For pick-and-place performance evaluation a CRS A465 robotic arm is used, which includes high-pressure air lines, controlled by an imbedded solenoid valve. Ground coffee is chosen as the grain material for this gripper because of its performance in jamming hardness tests. The relatively low density of ground coffee is also advantageous, as it can be used to fill relatively large grippers without weighing them down and straining the membrane. The bag is filled almost completely but not stretched out so the grains remained loosely packed and the gripper is malleable when no vacuum was applied.

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Figure 9 - Coffee used as the granular material

By establishing a differential jamming pressure Pjam across the bags latex rubber membrane (0.3 mm in thickness) the packing could be jammed. Employing a Venturi aspirator, compressed air was used to generate pressures Pjam around 75 kPa.

Figure 10 - How universal gripper works

The only objects that could not be gripped are those in which the gripper membrane could not reach sufficiently around the sides, e.g., for hemispheres larger than about half the size of the gripper or for thin disks lying flat, or for very soft objects like cotton balls.

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Three Fingers Gripper (Tri Max Gripper) This is study is an analysis of a tree "adaptive" finger gripper. Usually when designing fingers grippers some factors and precautions should be taken into account. The robot can not see and can not feel by itself, so it's impossible for instance determine how much force it should do when grab the object or see if the object is in the right position to be placed somewhere. To solve this, for strong grippers we use sensors, that can "feel" for the robot and send crucial informations to the robot control the actions, but in small situations we can use another "resources" to solve problems, like when solving this problem (grab and an egg). The Tri Max Gripper Project is a simple designed gripper, which can be printed using a 3D printer, that doesn't have sensors but it has three plastic fingers that can conform to the object being gripped.

Figura 11 - Let's make a robot (Tri Max Gripper)

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The system uses three appendages by a simple servo converted into a linear acturtor that is in a basic form. It has a mechanism that it is a simple system, that the centre plunger pushes or pulls the green attachment crown and the appendages follow. The object will be conformed and spreaded by the gripper when it is loaded.

Figura 12 1 Let's Make a Robot (Tri Max Gripper)

The fingers, made of two strips of printed plastic connected by wire linkages, create a leaf spring design. It is attached to a linear actuator at the center point of the gripper. When the linear actuator is pulled in, the fingers pull around the object to be gripped. This create a strong gripper that can hold an egg while shaking it, but not too strong to break it, the expected result.

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Conclusion In this study we had the opportunity to learn different types of gripper, how it works and how decides the best one for a given task. We learn two special types of grippers, universal gripper that can grab different parts (different forms) and a three fingers gripper that can grab oval and fragile objects without sensors. From this study we can start and experiment the development of one of this grippers.

Bibliography 1. Gareth J. Monkman, Stefan Hesse, Ralf Steinmann, Henrik Schunk. (2004). Robot Grippers. Germany: Wiley-VCH. 2. Eric Browna, Nicholas Rodenberga, John Amendb, Annan Mozeikac, Erik Steltzc, Mitchell R. Zakind, Hod Lipsonb, and Heinrich M. Jaegera. (2010). Universal robotic gripper based on the jamming of granular material. PNAS, 1-6. 3. Lets make robots, Tri Max Gripper (conforming robotic gripper) (http://letsmakerobots.com/node/39387) Gareth (2013) 4. Robotiq, How To Choose The Right Robotic Gripper For Your Application. (http://blog.robotiq.com/bid/33127/how-to-choose-the-right-robotic-gripper-for-your-application) Samuel Borchard (2011) 5. Figure 1 - http://robotnor.no/expertise/robotic-systems/robotic-grippers/ - Download 12/12/2014 6. Figure 2 http://grabcad.com/library/robotic-gripper-assy - Download 12/12/2014 7. Figure 3 - http://www.robotsdotcom.com/EOATooling.htm - Download 12/12/2014 8. Figure 4 - http://robotnor.no/research/new-gripper-solutions-for-non-rigid-objects/ - Download 12/12/2014 9. Figure 5 - http://www.schmalz.com/aktuelles/produkte/vakuumgreifsysteme/02102/ - Download 12/12/2014 10. Figure 6 - http://www.directindustry.com/prod/schmalz/vacuum-grippers-7112-1195605.html - Download 12/12/2014 11. Figure 7 - http://www.hizook.com/blog/2010/10/25/jamming-robot-gripper-gets-official-article-published-pnas - Download 12/12/2014

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12. Figure 8 - http://creativemachines.cornell.edu/jamming_gripper?q=jamming_gripper_images - Download 12/12/2014 13. Figure 9 -http://www.hizook.com/blog/2010/10/25/jamming-robot-gripper-gets-official-article-published-pnas - Download 12/12/2014 14. Figure 10 - http://www.kurzweilai.net/low-cost-robotic-gripper-replaces-human-hand-and-fingers - Download 12/12/2014 15. Figure 11 http://letsmakerobots.com/node/39387 - Download 12/12/2014 16. Figure 12 - http://letsmakerobots.com/node/39387 - Download 12/12/2014