Background: The RTR Hands

THE RTR1 HAND

The RTR1 hand is composed of a palm (realized in epoxy resin through STL) and three fingers prototypes (realized in ABS, acrylonitrile/butadiene/styrene through FDM): 2 equal fingers, composed by three phalanxes, and the third finger, which is in opposition, formed by two phalanges. The RTR1 hand weights ~250 grams.

  
The RTR1 Hand

This model of mechatronic hand has 6 independent degrees of freedom and 2 passive d.o.f. The sensory system is composed of:

• 6 position sensors for detecting the angular displacement of the active joints;
• 3 force sensor on the fingertips.

additional resources

THE RTR2 HAND In the RTR2 hand, the actuation system is based on the concept of underactuation. In other words the fingers can self-adapt to the object shape with a simple movement of a slider linked to the finger through a steel wire. Two DC actuators (MINIMOTOR, CH), integrated in the palm, actuate the hand: the first (6 V, diameter: 10mm) is used only for the thumb positioning: thanks to a four linked mechanism the thumb can abduct and adduct; the second (6 V, diameter: 17mm) moves the slider and pulls the wires, in such a way that all the 3 fingers close together towards the object. The RTR2 hand weights ~350 grams. The control system is embedded in the palm and its sensory system is composed of proprioceptive and exteroceptive sensors.

The RTR2 Hand

An example of cylindrical palmar prehension An example of prehension by subtermino-lateral opposition An example of prehension by terminal opposition An example showing the grasp adaptability

additional resourcesRTR2 Hand in action - pouring water in a glass: AVI movie, 2.8MB - download
RTR2 Hand in action - grasp stability: AVI movie, 2.12MB - download
RTR2 Hand in action - grasping a candy: AVI movie, 0.8MB - download

THE SPRING HAND

Also in the SPRING Hand, the actuation system is based on the concept of underactuation. One DC actuator (6 V, diameter: 17mm, MINIMOTOR CH), integrated in the palm, actuates the hand. The movement of the slider, bringing the wires in tension, causes the flexion of all the fingers. The SPRING hand weights ~400 grams. Its sensory system is composed of a tension sensor fixed to the slider in order to continuously monitor the cable tension applied by the motors.

The SPRING Hand

An example of cylindrical palmar prehension

An example showing the grasp adaptability The SPRING Hand applied on an anthropomorphic arm

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CYBERHAND Sensory System

The artificial sensory system is the core of the hand control system, and it has a twofold function: first, it provides input signals for the low-level control loop of the grasping phase, thus enabling local and autonomous control of the grasp without requiring user's attention and reaction to incipient slippage. Moreover, the ultimate function of the artificial sensory system is to generate sensory signals to be transmitted to the user through an appropriate interface (high-level control loop).

The Cyberhand sensory system is divided in two modules: a proprioceptive and an exteroceptive sensory subsystems.

The proprioceptive sensory system

The proprioception on the first prototype of the cybernetic hand has been designed in order to provide the required information on all the phalanges of the hand. The solution consists of fifteen joint position sensors (Hall effect sensor based) embedded in all the joints of each finger, an incremental encoder on each motor (Commercial magnetic encoders by Minimotor SA, CH), and five tension sensors on the cables acting as the tensiometer developed for the CyberHand. As the Golgi tendon organs give information on the tendon stretches, five tensiometers measure the tension on the cables controlling the fingers flexion.

The 3D CAD model of the Force Tension cable/tendon sensors and its working concept outline
The results of the CAD simulation for the Cable tension sensor performed before the fabrication of the sensor
The joint position sensors are embedded in the fingers	The joint position sensors developed for the CyberHand
Measurements of self repetibility of the MP joint angle sensor. The developed sensor presents good linearity and dinamics.   The joint sensor integration

The exteroceptive sensory system

The main required output of the tactile sensory system is the force vector at contact point between the hand and the grasped object. The control system should extract from the sensory outputs the following information:

• contact making and breaking between object and fingertips;
• contact making and breaking between hand-held object and environment;
• the slip friction between object and fingertips;
• the local shape at contact points;
• the overall object shape;
• the force vector at the contact point (tangential and normal force components).

The exteroceptive information are essentially tactile information. The idea developed was to distribute tactile sensors over and inside the hand. It was feasible using two types of sensors, on-off touch sensors and 3-component force sensors.

The prototype of the on-off touch sensor and its application on the finger of the optimised hand. The sensors distribution has been thought to replicate the fovea where the sensor density decreases starting from the most receptive area (fingertip) to the periphery (other phalanges and palm). The matrix of on/off contact sensors has 32 sensitive areas for each finger (15 on the distal phalange, 8 on the intermediate phalange, 9 on the proximal phalange), 10 sensitive areas on the palm and 4 sensitive areas on the dorsum.
The 3D CAD model of the first version of the 3 components force sensor integrated in the fingertip of the CyberHand first prototype.
The structure of the 3 components force sensor fabricated at SSSA (MICRO MILLING MACHINING KERN CNC HSPC)	Integration of the 3 components force sensor inside the finger structure.

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Cosmetic Glove

To reduce the energy absorption caused by the articulated flexion of the cosmetic glove, due to the anthropomorphic design adopted for the mechanical hand, a new type of silicone glove (with reduced thickness at the joint level) has been ideated.

Section view of a finger of the cosmetic glove for the CyberHand.

The first prototype of the cosmetic glove for the Cyberhand.