project specification

Underactuated robotic hand

A compliant, underactuated robotic hand for dexterous grasping. A compliant and underactuated hand based on soft robotic technology. The hand is capable of dexterous grasping and robust to unanticipated impact. The hand is almost entirely mad of soft, compliant materials or structures, rather than of rigid parts. The hand is capable of enacting 31 out of 33 grasp postures of the human hand. The RBO Hand 2 uses a highly compliant, pneumatic continuum actuator design, called PneuFlex. These actuators can be manufactured within a day and use materials that are cheap and non-toxic. When inflating the contained chamber with air the pressure forces theh ull to elongate along the actuator. The bottomside contains an inelastic fabric to prohibit elongation. This causes a difference in length between the top and bottom side and the actuator bends. Radial fibers stabilize the actuator’s shape and greatly increase the attainable curvature

Specifications

Weight178 g
Payload0.5 kg
Actuator7 PneuFlex actuators
Manufacturing technique3D printing

Overview

A compliant and underactuated hand based on soft robotic technology. The hand is capable of dexterous grasping and robust to unanticipated impact. The hand is almost entirely mad of soft, compliant materials or structures, rather than of rigid parts. The hand is capable of enacting 31 out of 33 grasp postures  of the human hand.

The RBO Hand 2 uses a highly compliant, pneumatic continuum actuator design, called PneuFlex. These actuators can be manufactured within a day and use materials that are cheap and non-toxic. When inflating the contained chamber with air the pressure forces theh ull to elongate along the actuator. The bottomside contains an inelastic fabric to prohibit elongation. This causes a difference in length between the top and bottom side and the actuator bends. Radial fibers stabilize the actuator’s shape and greatly increase the attainable curvature.

Morphology

This hand has an anthropomorphic design in shape and size for three reasons. 

  1. The human hand form enables dexterous grasping in humans. By starting with a human-hand-like morphology, you start with a proven hand design. 
  2. Second, many objects have been built for manipulation by a human hand and match the anthropomorphic form factor. 
  3. Third, you can use well-established grasp taxonomies and compare our de-signs with humans and many other robotic hands

Control

Pneumatic control of the PneuFlex actuators is based on a simple linear forward model forcomputing valve opening times. The model takes intoaccount the regulated supply pressure to achieve a de-sired channel pressure which corresponds to a desiredbending radius or contact force. Alternatives to this digital control are cylinder-based continuous control systems.

Fingers

The five fingers of the hand are single PneuFlex actuators. The index, middle,ring, and little finger are 90 mm long and of identicalshape, the thumb actuator is 70 mm long. All fingers get narrower and flatter towards the finger tip. 

Palm

A key feature of the human hand is the opposable thumb. In this hand this is realized by actuating the palm. The palmar actuator compound consists of two connected actuators. Its base shape is a circular section of 90◦ with 78 mm outer and 25 mm inner radius. The actuator curves perpendicular to the passive layer. The stiffness as well as the actuation ratio remain constant along the curved actuator. They are also designed to be twice as stiff as the fingers to account for the fact that the two actuators in the palm oppose four fingers.

In addition to enabling thumb opposition, the palm also provides a compliant surface that, together with the fingers, is used to enclose objects in various powergrasps. To augment this function, the fingers and the palmar actuator are connected by a thin sheet of fiber reinforced silicone, covering the gap between palma ctuators and fingers. This sheet transmits tensile forces between fingers and palm, and between adjacent fingers. This stabilizes the underlying scaffold during power graspsor for heavy loads.

Thumb

Like the other fingers, the thumb consists of a single PneuFlex actuator. The thumb is shorter and twice as stiff, but also features a linear stiffness profile. A faithful imitation of how humans use their thumb would require a negative curvature close to the tip, and would significantly increase complexity of manufacturing the thumb. Therefore there is a deviation from the human hand. Instead of the inside of the thumb, the backside (dorsal side) is used as the primary contact surface for pincer grasps.

Scaffold

The fingers and the palm are connected to the wrist by individual, flexible struts as part of a 3-D printed polyamide scaffold. The intentionally flat cross section of the struts enables deformation modes, such as arching the palm and spreading the fingers. Space for the respective actuator is provisioned, but not added to the hand described here. The struts decouple displacement between fingers, further increasing passivecompliance of the hand. The flexibility of the strutslimits impact forces, while providing sufficient stiff-ness for heavy payloads without excessive deforma-tion.

The fingers and the palmar actuator compound are bonded to the supporting scaffold. The palm is supported by parts of the scaffold to increase its torsional stiffness during opposition withthe finger.

References

Describes the hand design, actuators, and grasp dexterity.

R. Deimel, O. Brock. - August, 2015.

Wevolver 2022