Biomedical Engineering Reference
In-Depth Information
7.2 Force-Feedback Devices
The increasing demand for enhanced human-computer interaction (HCI) now necessitates
the design of new interfaces that will allow humans and machines to exchange information
using a wider range of modalities than merely vision and voice. As a consequence, ideas
involving new interactions have been introduced, such as machine vision and virtual
reality. Among these new interfaces, force kinesthetic feedback devices are certain to be
favored, since they not only make difficult manipulation tasks possible, or easier, but they
also open the door to a wide range of new applications in the fields of stimulation and
assistance to human operators.
Force-feedback devices are actually parallel [12] or serial [13] robots that function
within specific criteria and come in various types, ranging from gamepads to MIS training
facilities. They provide the user with feedback of contact force and proprioception of the
manipulator hand, or virtual object, during an interaction. In these kinds of robots, the
mechanical system should have high stiffness, low inertia, and friction with no backlash.
Also, force actuators should be capable of back-drivability and be provided with sufficient
force/torque precision. The most precise position sensors should be employed in a high-
frequency local control loop (greater than 1 kHz) to make the vibrations sensible.
Up to now, commercially successful force-feedback products have been mainly centered
on the entertainment industry, since, unfortunately, their use in the medical sector has been
severely curtailed due to the fact they are unable to provide cutaneous cues. Notwith-
standing that, commercial force feedback devices have achieved outstanding results in
displaying hardness within their natural impedance range. Softness displays have proved
that they are able to evoke a reliable softness sensation, enabling better discrimination
than that of similar, but purely kinesthetic, displays. On the other hand, pure softness
displays have limited workspace and softness range, and it is impossible to decouple the
cutaneous and kinesthetic information purely by using tactile displays.
7.3 A Review of Recent and Advanced Tactile Displays
Hitherto, the technologies pertaining to conventional kinesthetic and tactile displays,
together with the conceptual aspects of their integration, were discussed in some detail.
This section introduces some of the more recent and advanced tactile displays.
7.3.1 Electrostatic Tactile Displays for Roughness
Pelrine
et al
. reported on an electrostatic actuator composed of a polymeric elastic dielec-
tric that is sandwiched between compliant electrodes [14]. By applying a voltage difference
between the electrodes, the dielectric contracts in thickness and expands its area due to
the attractive charges on the electrodes [15] (see Figure 7.3). By reducing the voltage,
the dielectric returns to its initial shape and can produce forces due to the stored elastic
energy.
In order achieve this effect, given the applicable voltages, the dielectric can be no more
than a few microns thick and the electrodes must be very compliant in order that they
do not constrain the deformation. Pelrine
et al
. showed more than 30% relative strain in
thickness at electrostatic pressures of over 1 MPa [14] with a silicone elastomer dielectric
