**4.5 Haptic rendering**

In this case study the target task is the simulation of interaction with a textile of a relatively small size, namely 20x20 cm, whose physical parameters have been taken from Kawabata measurements (Kawabata & Niwa, 1993). This textile is simulated by means of a multi-rate approach that decouples simulation, haptic rendering and tactile rendering. The discussion starts with the description of the simulation, then the haptic rendering and finally the integration part. The overall architecture of the system is shown in Fig. 10.

Fig. 10. Architecture of the software highlighting rates and components

#### **4.5.1 Simulation**

68 Haptics Rendering and Applications

(a) (b) Fig. 9. Coil current of a matrix with 5 row and with capacitance *C* of 100µF (a) and 33µF (b).

In this case study the target task is the simulation of interaction with a textile of a relatively small size, namely 20x20 cm, whose physical parameters have been taken from Kawabata measurements (Kawabata & Niwa, 1993). This textile is simulated by means of a multi-rate approach that decouples simulation, haptic rendering and tactile rendering. The discussion starts with the description of the simulation, then the haptic rendering and finally the

GUI Textile

Collision

Tactile Device UDP

Response

Tactile Renderer

300Hz

Haptics

Haptic Proxy

Haptic Device

UDP

1kHz

Simulation

200Hz

Config

To all modules

Material Sim Collision

State

integration part. The overall architecture of the system is shown in Fig. 10.

Visualization

Fig. 10. Architecture of the software highlighting rates and components

Perf Monitor

From all modules

**4.5 Haptic rendering** 

Display

30/60Hz

Log

Example States

The geometrical model adopted is based on FEM because they have been proved to be able to map and represent physical properties of material in a better way. In particular the textile is represented by thin triangular shells along the application to cloth simulation by Etzmuss (Etzmuss, Keckeisen, & Strasser, 2003). The physical forces employed are the stretch and bend as expressed by the elastic tensor. These forces are computed from the deformation of the triangle against its original condition but such computation is quite expensive. For this reason we employed co-rotated triangles in which the triangle deformation is expressed by a combination of a rotation and a deformation of the planar version of the triangle. The overall equation of motion of the nodes of the triangles takes into account these forces in addition to external forces like gravity. The node equation is then integrated using implicit Euler that provides more precise simulation at the cost of resolution of a system of equations. This system of equation is solved using a preconditioned conjugate gradient in which the preconditioning is the block diagonal.

#### **4.5.2 Collision**

The collision method employed is based on the contact between two spherical proxies of the subject fingers and the vertices of the textile. Each collision makes the textile instantly to move to avoid the collision and then the collision is implemented as a constraint that blocks the motion of the node toward the proxy sphere. This constraint on the node is taken into account in the system resolution by means of a modified preconditioned conjugate model using the filtering approach introduced by Baraff (Baraff & Witkin, 1998). This representation of constraints allows implementing also generic constrained nodes to hang the textile in a given position.

#### **4.5.3 Proxy and haptic feedback**

The haptic feedback module runs in a separate thread and it has two objectives: provide feedback of interaction among the finger proxies, and the interaction with the textiles. The finger feedback is provided by means of a damped virtual spring that is activated when the two proxies are below a given threshold. This spring is necessary to compensate the effective size of the gimble that surround the fingers. When each proxy sphere collides with the nodes of the textile it moves the node outside the sphere. The overall force on the proxy is made by two components one impulsive that accounts for the instantaneous motion of the textile, while the other is the pressure applied by the node on the sphere. This pressure of the node on the proxy can be measured as the action performed by the node over the constraint that is effectively the residual of the iterative resolution of the system. This approach is different with respect the previous work (Fontana et al., 2007) the haptic force was based on a penalty method.

The model described so far does not take into account two related aspects of the interaction: friction and textile compression. Without introducing textile compression friction emerges from the forces exchanged between the proxy and the nodes due to the work of the constraints of the contacting nodes. Effectively this model produces small exchanged forces due to the small work of these constraints. An additional contribution to the normal component is caused by the compression of the textile. The FEM model does not take into

On the Integration of Tactile and Force Feedback 71

These parameters have been selected for balancing frame-rate with precision of the system. Fig. 11 shows a snapshot of the execution of the execution of the system: the two fingers are the big spheres. On the top line there are the constrained vertices. The small light circles

Haptic interface technology has started in the late fifties with first teleoperation applications. After sixty years of research there have been strong improvements each of the field of Machine Haptics, Computer Haptics and Human Haptics knowledge. The haptic systems that are now available have been employed in many fields like medicine, industry and education. However, while current technology has demonstrated to be quite effective for the simulation of Mediated Contact scenarios, there is still a lack in the simulation of Direct Contact. At the same time, the implementation of a system able to effectively simulated interaction with bare finger could be a real breakthrough. Many researches are currently working on integration of tactile and

In this chapter we introduced the main issues that concern with integration of tactile and kinesthetic feedback considering both Machine Haptics and Computer Haptics aspects.

An example of case study that includes the integration of a hand-exoskeleton with a pinarray device is presented. A possible approach for the real-time rendering of both tactile and

Part of the work described in this chapter was partially funded by the RTD national project

Fig. 11. Photo of the Haptic device and textile simulation

kinesthetic devices and rendering toward this objective.

kinesthetic components is shown.

MANTES financed by the Regione Toscana.

**6. Acknowledgments** 

near the fingers are the contacting points.

**5. Conclusions** 

account these forces because it is a thin shell, but we can use them for the haptic computation. In particular when two proxies are both contacting the same region of the textile they can squeeze the material increasing the normal force on the contacting node, hence raising the effect of friction. The effect of the friction force on the nodes of the textile is integrated in the simulation with a stick slip model. A node subject to static friction is sticked to the surface of the sphere by means of a full constraint.
