High Dexterity Underwater Gripper

 1. Foreword
 2. Description of the actuation technique
 3. Gripper detailed description
 3.1. Articulated gripper
 3.2. Electrohydraulic drives assembly
 4. Specifications
Patent info
Downloadable files
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1. Foreword

The High Dexterity Underwater Gripper is an advanced underwater gripper specially developed to be able to perform complex manipulative tasks and to allow the grasping and manipulation of soft, fragile objects.

The High Dexterity Underwater Gripper is based on a electrohydraulic actuation technique (the Direct-Electric-Drive Fixed-Volume Closed-Circuit Hydraulic System), proprietary of Eureka Engineering.

A prototype of the High Dexterity Underwater Gripper has been developed by Eureka Engineering as a subcontractor of DIST-UNIGE in the AMADEUS II project (MAST programme)

The AMADEUSII prototype features three two-Degree-Of-Freedom fingers, anyway the technique lends optimally itself to the implementation of simpler (e.g., 3 x 1DOF fingers) or more sophisticated designs.

2. Description of the actuation technique

The Direct-Electric-Drive Fixed-Volume Closed-Circuit Hydraulic System is based on the use of a hydraulic circuit formed by two flexible bellows coupled by a flexible line.

One the bellows (master) is coupled to the electric drive while the other (slave) is coupled to the device to be operated.

The circuit formed by the two bellows and the connecting line, is completely filled with hydraulic fluid; being this fluid virtually non-compressible, the circuit results not only a closed-circuit but also a fixed-volume one.

Thanks to this, the system shows the following main features:

To achieve a wide bandwidth of the actuation system, the master bellows is driven by an electric linear motor, this is directly coupled to the bellows.

Thanks to the DEDFVCCHS actuation technique, the gripper shows some special features:

3. Gripper detailed description

The gripper system is formed of two main sub-systems:

3.1. Articulated gripper

The articulated gripper is formed of:

3.1.1. Palm

The palm provides the mounting base for the three fingers, these are positioned equally spaced at 120o to each other.

The mechanical interfacing to different robotic arms is easily achieved by the use of a customized mechanical adapter.

A miniaturized underwater video camera can be housed into the mechanical adapter to provide a close-up monitoring of the manipulator operation.

3.1.2. Finger

Each finger is actuated by three slave bellows housed inside its structure.

The finger is formed of two parts, a fixed part and a moveable one; these parts are joined by a cardan (universal) joint, allowing a 2 degree-of-freedom motion.

The moveable part is, at rest, aligned with the fixed one and, thanks to the joint, is free to move inside a cone having the axis coincident with the axis of the fixed part, the vertex located at the center of the joint and a half-width of about 25 degrees.

The fixed part carries the connectors of the hydraulic lines and is mounted to the palm.

The moveable one carries on its free end, the tip, an elastomer termination. The tip's shape, a rounded-edges pyramid, skewed toward the center of the gripper and topped by a hemisphere, is optimized for the manipulation and grasping of objects having different size and shape; moreover the characteristics of the elastomer help to increase the low-contact-force grasping capability.

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3.2. Electrohydraulic drives assembly

The electrohydraulic drives assembly is formed of nine identical units, one for each of the bellows in the gripper, and of the mounting cage

3.2.1. Electrohydraulic unit

The enclosure of the electrohydraulic unit is cylindrical in shape; one of the two end- plates carries the connector of the hydraulic line, while the other one carries the liquid- tight electrical cable connector and the inlet of the pressure-compensating device.

The enclosure houses: the pressure-compensating device, the electrohydraulic drive, the position sensor and an electrical board carrying the cabling interconnection terminals.

The pressure-compensating device is formed by a flexible bellows mounted liquid-tight to the inner side of the end plate.

The electrohydraulic drive is formed by the master bellows, the linear electric motor and the position sensor.

The linear electric motor is of the voice coil type, more specifically is of the moving-coil type; that is, the motor stator houses the permanent magnets and forms the magnetic circuit, while the coil moves inside the gap of the circuit.

The master bellows and the linear motor are placed on the same axis, and the moveable end of the bellows is directly coupled to the coil assembly, so friction and backlash phemomena are avoided.

A miniature LVDT (Linear Variable Differential Transformer) linear position sensor is used to accurately measure the motion of the moveable end of the bellows.

3.2.2. Cage

The cage allows the tightly packed mounting of the electrohydraulic drives assembly; the nine drives are disposed in a 3 X 3 array configuration.

The cage is provided with handles for transportation, and makes it easy to mount and fasten  the assembly on board of an underwater vehicle.

The hydraulic lines bundle, 3m in length, allows the installation of the gripper on many of the currently available underwater robotic arms.

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4. Specifications

Grasping range (spherical object):    up to 140mm diameter
Grasping force:                                    0 to 10N

Power requirements:                           24 VDC, 30 A

  gripper:                                               250 (l) x 200 mm (dia) [approx.]
  electrohydraulic drives assembly:    500 x 380 x 450 mm [approx]

Weight:                                                 35 kg [approx]

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First page of Patent   (.pdf, 26kB)

Full text of Patent   (.pdf, 85kB)

Download a PDF version of this page  (197kB)

Here are some gripper's movies
DIST1  (.avi, 1.6MB)

DIST2  (.avi, 8MB)
DIST3  (.avi, 8MB)
DIST4  (.avi, 8.6MB)
DIST5  (.avi, 8MB)
EGG  (.avi, 4.5MB)

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