Interfaces with mechanical action

In most tissues of the human body (muscles, skin, joints, tendons, etc.) and in the inner ears, there are sensory receptors that can be mechanically activated. Their sensory stimuli are separated into three parts:

• skin sensitivity;

• muscular proprioception, due to sensory receptors in muscles and tendons;

• kinesthesia, sensitivity to body position and movement in space, due to vestibular receptors in both inner ears, which are sensitive to gravity and acceleration.

Proprioception can be divided into three "physical" domains, from which different virtual reality interfaces are derived in practice. These are: sensitivity to position in space, sensitivity to body movement and sensitivity to forces exerted on muscles. The first two areas (kinesthetic sense) are matched by motion-simulation interfaces, whose aim is to modify the body's orientation in space and subject it to acceleration. The last domain corresponds to force feedback interfaces, whose aim is to apply controllable forces to the body. For skin sensitivity, the interfaces used to stimulate the skin are called tactile feedback interfaces. These three types of interface are presented below.

To create the illusion of being immersed in a virtual world, the subject must be able to perceive it and act on it. If we want to physically immerse the subject's body, either totally or partially, we need to be able to at least detect body movements, thanks to specific body localization interfaces (cf. ). But with these interfaces alone, the user feels nothing in return, and perceives an impalpable virtual world. For the user to experience mechanical sensations in return for his or her actions, it is necessary to exploit at least one of the categories of interfaces with mechanical action: interfaces with tactile feedback, interfaces with motion simulation or interfaces with effort feedback. Skin-sensitive and motion-simulation interfaces are sensory interfaces: they transmit stimuli only from the computer to the user. Force feedback interfaces, on the other hand, are designed to apply forces to the body, but also, again in practice, to measure the position of the part of the body on which they are acting. There are two main reasons for this:

• because force feedback is generally only the consequence of the action (movement) of the user's hand. It is therefore essential to know what this action is, hence the position measurements made on the same interface;

• because the forces to be applied depend, among other things, on the position of the hand, which you need to know.

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