**4.1 Introduction**

60 Haptics Rendering and Applications

while a finer representation localized around the contact point with the user is computed at faster rates. The complexity of this approach is in the transfer of the effects from the fast model to the slow one. An example of application of this approach to textile simulation is

Timing is very important in real-time interaction and in particular it is interesting to discuss how time behaves in simulation. The simulation takes some real computational time to perform an integration step, and if the simulation is based on iterative methods then this computation can take a variable amount of time. The desired behavior of the simulation is to be synchronous with the real timing allowing presenting a realistic behavior. Due to the computational time required by the integration step this means that the simulation has to perform a larger time step than the simulation, eventually estimating in advance the final computational time. There is anyway an issue in the selection of the integration time step, that depends on the integration method and the material parameters: a too large time step is not able to express the propagation of deformation waves inside the material, and, at the same time, a too large time step can produce numerical issues when part of the matrix depends on time and others are constant. This issue is well represented by the Courant condition that, for implicit integration, states how the squared maximum integration step should be of the order of a ratio between the mass of each element and the stiffness factors. This condition together with the computational time function can express how a given material and a simulation

The integrated rendering deals with the combination of kinesthetic and tactile rendering based on the overall interaction of the proxy with the virtual environment. Such integration is realized by the communication between the kinesthetic haptic rendering module and the tactile rendering module running at different rates. In such communication the tactile rendering should receive sufficient information for actuating the haptic interface. Although there is not a reference approach for tactile devices, such information can be identified as a distribution of contact points over the fingertip expressing for each of them the amount of pressure and the relative velocity against the contacting object. The surface properties of the material together with this piece of information can be then used for generating the vibrations that allow simulating the tactile feedback. An example of approach is discussed in Böttcher et al. (Böttcher, D

In this section we discuss an integrated system for kinesthetic and tactile simulation applied to the interaction with virtual textiles. In the first part we introduce the scope of the system and its main characteristics. Then we proceed with the presentation of the integrated haptic interfaces. The section is closed by a discussion about the haptic rendering strategy that supports the discussed haptic interface, taking into account the models and the approaches

provided by Bottcher et al. (Böttcher, Dennis Allerkamp, & F.E. Wolter, 2010).

implementation are not suitable for real-time computation.

**4. Case study: Haptic display of textile properties** 

**3.3 Integrated rendering** 

Allerkamp, & F. E. Wolter, 2010).

presented in the previous sections.

Textiles are deformable objects characterized by very fine surface and bulk physical properties, indicated with terms such as stiffness, smoothness, softness, fullness, crispness, thickness, weight, etc. Taken as a whole they constitute the so called Fabric Hand (Behery, 2005) of a specific fabric, which is the basis for assessing its quality in relation to a given use. These properties can be well distinguished and quantitatively evaluated by the human haptic sensorial system, with an important contribution given by the sense of sight. There is experimental evidence that the highly sophisticated mechanoreceptors located in the human skin are combined in the brain with those generated by the kinesthetic sensors located in the physiological articulations and in the muscles providing the so-called Tactile Picture of the fabric. For example, when gently stroking the fingertip on a fabric to evaluate its smoothness, the kinesthetic sensors give to the brain information about the fingertip speed and the global force exerted on the fabric while the mechanoreceptors sense the small local fluctuation of the tangential force due to friction.

Due to the limitations of the present technology, since the beginning it has been decided to focus the system simulation capability on the interactions that can be attained using only two fingertips: the ones of the index and the thumb (see Fig. 2).

Fig. 2. Scenario of the interaction in which the user can use thumb and index finger for rubbing and stretching a standing piece of virtual textile.

Taking into account the above considerations, the reference configuration for the development of the device responsible for generating the artificial mechanical stimuli to be delivered on the fingertips, has been conceived as the combination of two independent force-controlled manipulators (Force Feedback Device, FFD), and two arrays of independently actuated pins (Tactile Actuator, TA).

Each FFD is able to track the movements of the index and thumb fingertips and to convey the global force of arbitrary direction on it, and each TA mounted on the end-effector of the corresponding FFD is able to deliver to the surface of the fingertip skin specified spatial and temporal sensory input patterns, (see Figure 2).

On the Integration of Tactile and Force Feedback 63

Fig. 4. PERCRO Dual finger Exoskeleton with 3 DoF for each finger able to deliver 5N on the fingertip of index and thumb. The device was purposely developed for the haptic

> HAND EXOSKELETON Mechanical Performances

finger <sup>3</sup>

The transducers were arranged in an array of 5x6 with a spacing of 2.4 mm with 4mm of

Symbol QUANTITY Value

*Fmax* Maximum continuous force 5N *W* Weight of the whole device 1.1 kg *wa* Weight of one finger mechanism 0.51 kg *Bw* Mechanical Bandwidth (expected) 25 Hz

Fig. 5. Scheme of the integration of tactile display with the hand exoskeleton

interaction with textiles and for the integration with a tactile display.

DoF Degrees of Freedom for each

Table 1. Mechanical performances of the hand-exoskelton

thickness (Fig. 5).

Fig. 3. General architecture of the mechanical interface
