A new, remotely driven, dextrous gripper
could bring great benefits to humankind in addition to its
originally intended purpose, writes Tom Shelley
A fluid power gripper is dextrous like the human hand, feeds back force through fluid pressure and could be made cheap enough to be semi-disposable.
It has been developed to perform complex manipulative tasks underwater, and to allow the grasping and manipulation of soft, fragile object.
One of the first serious commercial applications being considered, however, is on land, to undertake humanitarian de-mining.
The High Dexterity Under water Gripper is the brainchild of Italian consulting engineer, Vincenzo Arrichiello and his small company, Eureka Engineering. His previous experience ranges from the design of underwater water jet propulsion systems for robotic vehicles to automatic garment bagging machines. His workshop is situated in La Spezia.
Eureka Engineering developed the gripper as a subcontractor of DIST-UNIGE (Dipartimento di Informatica, Sistemistica e Telematica, UniversitÓ degli Studi di Genova) working in the European Union AMADEUS (Advanced MAnipulation for Deep Underwater Sampling II project, part of the MAST (MArine Science and Technology) programme.
Its essential principle is to use two flexible bellows coupled by a flexible hose for each of three 'muscles' used for each finger. In each case, the master bellows is coupled to an electric drive, while the other, the slave bellows, is coupled to the device to be operated. The basic idea has been tried before but only with one muscle bellows for each finger and not three. Each finger is made up of two parts connected together by a universal joint. The fixed part is connected to the 'palm' of the gripper and carries the hydraulic lines. The movable part has on its free end an elastomeric, round edged pyramid, with its apex skewed towards the centre of the gripper. Furthermore, the design provides lateral guiding for the bellows, preventing buckling and allowing the bellows to be made very long and out of low stiffness materials. In this way, a large movement and a good force output can be achieved at low operating pressure.
The fingers therefore function very much in the manner of human fingers, and can therefore not only grasp an object, but tilt it in X and Y planes and also twist it, without need for a wrist, giving an overall three degrees of freedom. Because no seals are used in the actuators at either end, the device is almost frictionless, so a very smooth motion can be achieved and fragile objects grasped without damage. A further consequence of the low friction is that grasping force can be accurately calculated or sensed from the pressure of the working fluid, avoiding any need for touch pressure sensors on the finger tips. The bellows can be made out of a wide range materials, from stainless steel to polymers, depending on applications. The system inherently has no backlash and if used for medical or biological work, could be sterilised without suffering damage.
The master bellows are driven by voice coil type motors. Because the working fluid is incompressible, miniature LVDTs at the master bellows are used to accurately measure the positions of the movable ends of the slave bellows. Other kinds of actuators, such as motor driven lead screws could be used to move the ends of the master bellows. Filled with hydraulic oil and used underwater, external pressure is compensated for because the same hydrostatic pressure is exerted on both master and slave bellows. The hydraulic lines in the prototype are 3m long.
The first prototype has been tested by Professor
Giorgio Bartolini and his team at the University of Genoa, and
apparently been found to be satisfactory, although it has yet to
be used in anger underwater. The gripper can grasp spherical
objects up to 140mm in diameter with a force of up to 10N. Power
requirements are 24VDC, 30A. The gripper is 250mm long and 200mm
in diameter. The electrohydraulic drives assembly is 500mm x
380mm x 450mm.
Since then, Arrichiello has built a second prototype using air as a working fluid, which he says, "Works quite well" and has been thinking about ways to get the cost down.
The original Amadeus prototype cost 25,000 euros, An industrialised version, he believes, should cost 20% to 30% less, depending on quantities. "A simpler hand, e.g. one with three fingers having only one degree of freedom for underwater use, and still suited for handling fragile objects, could cost only about one third that of the prototype. The cost of a hand, not for underwater use, with pneumatic actuation could be a lot less, a few thousand euros."
In fact, if Mr. Arrichiello were to incorporate the sorts of pneumatic bellows remote actuation systems developed by Herga Electric and similar companies, we believe it might be possible to develop whole systems costing a few hundred euros or pounds. Herga systems are used for such tasks as switching on and off kitchen waste disposal units and pumps in domestic whirlpool baths, and cost only a few pounds, but have the potential, as revealed in a Eureka cover story in March 1988, to be used in much more sophisticated ways.
Such a development could bring immense benefits, far beyond those imagined by the individuals who originally set the project up. Arichiello says, "I am in talks with a company that is interested in the development of a low cost hand for a telemanipulation system to be used for humanitarian de-mining. The basic idea is to have a semi-disposable device, easy to replace if damaged. In my opinion, my device could be well suited for telemanipulation or robot manipulation of fragile objects in any dangerous or contaminated environment. I also think the device could be used to make a very low cost prosthetic hand, having in mind all the war related mutilations in poor countries."
The development is protected by Italian and European patent applications. The text covers a wide range of possible configurations, in addition to that actually constructed. The company is looking for license agreements.
* Gripper is frictionless, backlash free, and exhibits three degrees of freedom
* It is particularly intended to handle delicate objects.
* The basic principles can also be applied to much lower cost designs which could be regarded as semi disposable
* The basic design is sterilisable
For more technical developments see www.eurekamagazine.co.uk
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