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flowing through it, contracts by 0.508mm, which approximately corresponds to 3%,
exerting a force of 70 g. After calculation, the stimulation force for an element is
estimated at 20 g with a maximum stroke of 2.1mm and a current varying between
0.24 to 0.28A. The response time is approx. 0.1 sec, while the relaxation time is
approx. one second (at ambient temperature) (Johnson, 1990).
An additional achievement in the R.D. Howe team concerns an interface made up
of a line of 10 pins that are individually actuated and directed towards the inside of
the fingertip (Peine, 1997; Wellman et al., 1997). During the heating stage of the wire
by an electric current, the wire undergoes a transformation (it contracts), which makes
it possible to lift the pin. This conception makes it possible for each pin to move by
3mm and produce more than 1N force. The major problem with the AMF technology
is the response time (which is too slow). This problem has been overcome by using
water cooling and position control for each pin by using optical sensors.
Again, in the R.D. Howe team at Harvard University (Kontarinis &Howe, 1993),
researchers have proposed a prototype of a matrix tactile interface based on AMF type
wire-based actuators. This interface contains 3
3 actuators with a space of 2.2mm
between each of them. These actuators could produce displacements of 3mm and a
force of 1.2N per element. A controller and a forced air cooling have been used to
overcome the bandwidth limitations of the AMF. Response times obtained in the two
ways (heating and cooling) are 62ms for a displacement of 2.5mm.
A collaboration between the University of Newcastle, Bosch and Schlumberger
has made it possible for Taylor et al. (1998) to make a tactile interface by using AMF
actuators. Each actuator is made up of a wire with a length of 120mm and a diameter
of 0.1mm. The NiTi wire is attached to an adjustable spring on one side and is fixed
to the body of the interface on the other. The interface consists of 8 modules with
8 actuators each. The spacing between each actuator is 2.54mm. The rise and fall
times are approximately 350msec for amplitudes of 1.7mm.
Vincent Hayward and his team, at the McGill University, have developed tactile
interfaces, working on a third 5 stimulation direction and which results in cross or
lateral deformations of skin (Hayward, 2000; Hayward & Cruz-Hernandez, 2000;
Hayward, 2001).
The actuators are manufactured in lead zirconate titanate ceramic plates made up
of four 0.25mm layers and covered by electrodes. The arrangement of the electrodes
makes it possible to apply a voltage between the two ends. They provide a displacement
200V. A matrix of 100 blades with a density of
1 actuator/mm 2 was made in the last proposed version (called STReSS). The blades
are cut in the form of a comb (10 blades per comb). The matrix consists of 10 combs.
Thus, a large temporal and spatial resolution is obtained (Pasquero & Hayward,
With the aim of reproducing the sensation of touching a cloth surface, Konyo
and Tadokoro (2000) have developed a Nafion-Platinum 6 electroactive polymer-based
tactile interface. The interface has been designed with a number of polymer actuator
blades operating in flexion, where the cilium is 2mm wide and 5mm long.
m for an applied voltage of
5 The other two being vertical indentation and vibrotactile.
6 Actuators known as IPMC: Ionomeric Polymer Metal Composite.
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