L.R.P. Force Feedback Data Glove
Principal
The principle of a force feedback glove is simple. It consists of opposing the movement of the hand in the same way that an object squeezed between the fingers resists the movement of the latter. The glove must therefore be capable, in the absence of a real object, of recreating the forces applied by the object on the human hand with (1) the same intensity and (2) the same direction. These two conditions can be simplified by requiring the glove to apply a torque equal to the interphalangian joint.
The solution that we have chosen uses a mechanical structure with three passive joints which, with the interphalangian joint, make up a flat four-bar closed-link mechanism. This solution use cables placed at the interior of the four-bar mechanism and following a trajectory identical to that used by the extensor tendons which, by nature, oppose the movement of the flexor tendons in order to harmonize the movement of the fingers. Among the advantages of this structure one can cite :
Allows 4 dof for each finger
Adapted to different size of the fingers
Located on the back of the hand
Apply different forces on each phalanx (The possibility of applying a lateral force on the fingertip by motorizing the abduction/adduction joint)
Mesure finger angular flexion ( The measure of the joint angles are independent and can have a good resolution given the important paths traveled by the cables when the finger shut.)
Mechanical structure
The glove is made up of five fingers and has 19 degrees of
freedom 5 of which are passive. Each finger is made up of a
passive abduction joint which links it to the base (palm) and to
9 rotoid joints which, with the three interphalangian joints,
make up 3 closed-link mechanism with four-bar and 1 degree of
freedom.
The structure of the thumb is composed of only two closed-link,
for 3 dof of which one is passive. The segments of the glove are
made of aluminum and can withstand high charges ; their total
weight does not surpass 350 grams. The length of the segments is
proportional to the length of the phalanxes. All of the joints
are mounted on miniature ball bearings in order to reduce
friction.
The mechanical structure offers
two essential advantages : the first is the facility of adapting
to different sizes of the human hand. We have also provided for
lateral adjustment in order to adapt the interval between the
fingers at the palm. The second advantage is the presence of
physical stops in the structure which offer complete security to
the operator.
The force sensor is placed on the inside of a fixed support on
the upper part of the phalanx. The sensor is made up of a steel
strip on which a strain gauge was glued. The position sensor used
to measure the cable displacement are incremental optical
encoders offering an average theoretical resolution equal to 0.1
deg for the finger joints.
Control
The glove is controlled by 14 torque motors with continuous current which can develop a maximal torque equal to 1.4 Nm and a continuous torque equal to 0.12 Nm. On each motor we fixe a pulley with an 8.5 mm radius onto which the cable is wound. The maximal force that the motor can exert on the cable is thus equal to 14.0 N, a value sufficient to ensure opposition to the movement of the finger. The electronic interface of the force feedback data glove is made of PC with several acquisition cards.
The global scheme of the control is given by the next figure. One can distinguish two command loops : an internal loop which corresponds to a classic force control with constant gains and an external loop which integrates the model of distortion of the virtual object in contact with the fingers. In this schema the action of man on the position of the fingers joints is taken into consideration by the two control loops. Man is considered as a displacement generator while the glove is considered as a force generator.
Prototype
Video Demonstraion ( 53.6Mo, avi)
Video Demonstraion ( 9.2Mo, mpeg)
