**2.1.2 Direct contact interaction**

The analysis of the Direct Contact interaction requires looking with higher level of detail to the physics of the phenomena. The problem of perfectly reproducing the phenomena through artificial stimulus can be seen in a dual way. The first consists in defining the haptic interface as a device able to perfectly reproduce the shape and the spatial distributed mechanical impedance of the contact surface. Or, dually, the device can be seen as a generator of pressure field that perfectly reproduce the interaction pressure distribution that

On the Integration of Tactile and Force Feedback 51

to render several of this characteristics but a device that is able to render the whole set of

A rough classification of the type of device according to the type of details that are able to simulate can be done distinguishing the typical dimension and the scale that describe the





Of course research has been oriented also on other very specific device like (Bicchi et al., 2000) that is able to render the variation of the contact surface area on the fingertip. Actually the spreading of contact surface when contact occur has been demonstrated to be a fundamental input for perceiving the stiffness of deformable objects. The authors also

However we will mainly concentrate our analysis on the integration of tactile, shape and

As previously discussed the complexity of the general problem of creating the perfect illusion of touching a general object is still unsolved in the practice. However there are several device that are able to effectively reproduce a subset of fingertip physical interaction

force. These device are definitely the most popular type of haptic interface.

object properties has not been yet invented.

been developed by Solazzi (Solazzi et al., 2010).

deformation (Wang & Hayward, 2010).

electromagnetic, pneumatic and ultrasonic.

**2.2 Mechanical architecture of integrated systems** 

that generates artificial feeling of certain type of object properties.

integrated the device with a kinesthetic stage.

kinesthetic devices.

geometry of the detail.

We can categorize the devices as:

is generated at the contact surface. These two definitions are dual and functionally equivalent so we will assume this second perspective for defining the ideal haptic interface as a device able to perfectly reproduce the forces of interaction between finger and object i.e. able to generate a continuously distributed pressure field over the contact surface:

$$
\vec{\rho}(\vec{x},t) = \begin{bmatrix}
\rho\_{\vec{x}}(\vec{x},t) \\
\rho\_{\vec{y}}(\vec{x},t) \\
\rho\_{\vec{z}}(\vec{x},t)
\end{bmatrix}.
$$

with: ݔറאሺ௧ሻ

Where ߩԦሺݔറǡ ݐሻ is a pressure field made of three components: ߩ௭ሺݔറǡ ݐሻ is the normal component and ߩ௫ሺݔറǡ ݐሻand ߩ௬ሺݔറǡ ݐሻ the tangent components to the contact surface ሺ௧ሻ. Each component of such pressure distribution together with the contact surface changes independently with time over an infinite bandwidth with an infinite range of magnitude. Moreover the contact surface is also subject to large displacements and deformations that means the device should be able to exert such a pressure distribution in any wanted points of the space, e.g. when a soft object is grasped and lifted the contact area is subject to deformation during the prehension and to large displacement while lifting the object. The design of a system with such performances would require a tactile display with infinite resolution, infinite bandwidth and infinite force exertion capability. This is evidently far beyond what is currently feasible with present technologies.

The problem can be faced only through a simplification. A first reduction of complexity of the problem is achieved by reducing the requirements considering the limitations of the human tactile and kinesthetic senses. This means that the device has not to reproduce completely the physics of interaction but only the subset of components that can be perceived by the human tactile and kinesthetic senses i.e. components that are under perceptual thresholds in terms of intensity, spatial distribution and frequencies are neglected.

Unfortunately also under these hypotheses the problem remains technically unsolvable. Human tactile and kinesthetic senses are actually extremely efficient sensing systems (Jones & Lederman, 2004) with:


The ideal solution is then far from being implemented. However several simplified device have been realized and they are not able to simulate the whole interaction with fingers, but rather they are able to simulate a subset of features of the real interaction.

Basically these devices have been realized for purposely simulate a subset of the stimulus with fingertips that are strongly correlated with certain types of object properties or with certain scale of details.

Some devices are able to render roughness of object, other are able to render small details of the object shape, or global shape, or weight and friction etc. Of course one device can be able to render several of this characteristics but a device that is able to render the whole set of object properties has not been yet invented.

A rough classification of the type of device according to the type of details that are able to simulate can be done distinguishing the typical dimension and the scale that describe the geometry of the detail.

We can categorize the devices as:

50 Haptics Rendering and Applications

is generated at the contact surface. These two definitions are dual and functionally equivalent so we will assume this second perspective for defining the ideal haptic interface as a device able to perfectly reproduce the forces of interaction between finger and object i.e.

> ሻݐ റǡݔ௫ሺߩ ሻݐ റǡݔ௬ሺߩ ሻݐ റǡݔ௭ሺߩ

able to generate a continuously distributed pressure field over the contact surface:

with: ݔറאሺ௧ሻ

currently feasible with present technologies.

& Lederman, 2004) with:

certain scale of details.

intensity, spatial distribution and frequencies are neglected.


ߩԦሺݔറǡ ݐሻ ൌ

Where ߩԦሺݔറǡ ݐሻ is a pressure field made of three components: ߩ௭ሺݔറǡ ݐሻ is the normal component and ߩ௫ሺݔറǡ ݐሻand ߩ௬ሺݔറǡ ݐሻ the tangent components to the contact surface ሺ௧ሻ. Each component of such pressure distribution together with the contact surface changes independently with time over an infinite bandwidth with an infinite range of magnitude. Moreover the contact surface is also subject to large displacements and deformations that means the device should be able to exert such a pressure distribution in any wanted points of the space, e.g. when a soft object is grasped and lifted the contact area is subject to deformation during the prehension and to large displacement while lifting the object. The design of a system with such performances would require a tactile display with infinite resolution, infinite bandwidth and infinite force exertion capability. This is evidently far beyond what is

The problem can be faced only through a simplification. A first reduction of complexity of the problem is achieved by reducing the requirements considering the limitations of the human tactile and kinesthetic senses. This means that the device has not to reproduce completely the physics of interaction but only the subset of components that can be perceived by the human tactile and kinesthetic senses i.e. components that are under perceptual thresholds in terms of

Unfortunately also under these hypotheses the problem remains technically unsolvable. Human tactile and kinesthetic senses are actually extremely efficient sensing systems (Jones


The ideal solution is then far from being implemented. However several simplified device have been realized and they are not able to simulate the whole interaction with fingers, but

Basically these devices have been realized for purposely simulate a subset of the stimulus with fingertips that are strongly correlated with certain types of object properties or with

Some devices are able to render roughness of object, other are able to render small details of the object shape, or global shape, or weight and friction etc. Of course one device can be able



rather they are able to simulate a subset of features of the real interaction.


Of course research has been oriented also on other very specific device like (Bicchi et al., 2000) that is able to render the variation of the contact surface area on the fingertip. Actually the spreading of contact surface when contact occur has been demonstrated to be a fundamental input for perceiving the stiffness of deformable objects. The authors also integrated the device with a kinesthetic stage.

However we will mainly concentrate our analysis on the integration of tactile, shape and kinesthetic devices.
