**2.3.1 Scenario dependency**

The global problem of artificial creation of touch experience is too complex and a simplification can be made developing specific devices that are able to simulate only certain types of scenario. Basically, according to scenario, the device can be simplified in order to be optimized for a certain kind of application. Scenarios can be classified according to the complexity of the interaction starting from one finger interaction on a planar surface getting to spatial interaction with multiple fingers of the same hand:


The user can interact with an artificial surface moving on planar trajectories. According to the type of virtual surface, the speed and the forces exerted by the finger, the tactile display generates distributed stimulus. The kinaesthetic device can apply forces only on directions that are parallel to the plane and can simulate different macroscopic properties of the surface like friction or stiffness. Govindaraj in 2002 realize on planar system presenting a device able to simulate the interaction with a piece of fabric that is lying on a hard planar surface (Govindaraj et al., 2002). Another example of planar device has been realized by Yang(Yang & Zhang, 2009) that choose a cable-driven actuation system for the kinesthetic device.

In the case of planar devices the requirements for the integration of tactile and kinesthetic displays are less demanding. The weight and the encumbrance of the tactile display are less

On the Integration of Tactile and Force Feedback 55

The haptic rendering for object manipulation and exploration has the characteristic of combining the techniques from physics simulation with the principles of control and knowledge about human perception. Without losing generality haptic interaction scenarios can be represented by a physics simulation in which one or more virtual entities have the special role of exchanging haptic information with the user. These entities are called *proxies*. In some scenarios these virtual entities represent a user body part that directly interacts with other objects in the environment. A different type of representation is the one happening when the user interacts by means of a virtual tool as a stylus or a more complex tool. Examples of applications in the former type are exploration of surfaces, grasping, haptic rehabilitation and scenarios in which an avatar is employed. Examples of the latter type are drilling, simulation and even driving simulation. Clearly, the type of proxy depends on the type of haptic interface and the specific attachment to the user body part. For direct contact, like virtual finger exploration, the robotic endeffector applies forces on the fingers of the user using a rigid attachment to the user, or, following an encountered haptic approach. Instead devices in which the user holds a

The type of proxy has effects, for the purpose of our discussion, in terms of the geometry and physical modeling of contact having to simulate, in one case, the contact between the finger or a body part with virtual objects, while in the other the contact of a virtual rigid

The general aim of the proxy is to be respectful of the physical properties of the virtual world, meaning that its behavior will avoid penetration with other objects. There is anyway a caveat. In real world the user moves the haptic interface end-effector specifying in this way a requested virtual position for the proxy. This requested position and orientation is a virtual entity called *haptic handle*. When the proxy touches an object like a virtual wall, the haptic interface produces a force to represent the opposition of the wall to penetration but this force is not always able to prevent the motion of the user in real space into the space where the virtual object is located. The discrepancy between proxy position and haptic handle position is a key element of haptic rendering. The common to rendering approach is to simulate the presence of a virtual spring between the proxy and the handle with a factor proportional to the stiffness of the object in contact. This approach is called penalty based because force is proportional to the penalty of entering with the handle into the object. This spring can introduce some instability and for this reason it can be replaced by a damped spring. Please note how, in this discussion, we have not taken into account the effect of the

Moreover, we have not made distinctions between impedance and admittance interfaces. In the formers the low level controller receives forces to be applied to the user at the endeffector, and in most of the cases such force is computed on the side of the computer performing simulation and collision. In the latters the low level controller specifies a

Contact is a fundamental element of haptic interaction with virtual objects. At large it can be considered as the result of two modeling aspects, first the geometrical aspect then the

position to be held with a given stiffness, up to the maximal force of the device.

stylus or a handle are more suited for tool based proxies.

object that transmits the contact to the user.

force produced by the spring on the proxy.

**3.1 Geometric and physical modeling of contact** 

critical since there is no need of gravity compensation. Moreover the single finger interaction puts no strict limits to vertical encumbrance of the display.


One finger device has a superior level of complexity since the weights of the display has to be compensated by the kinaesthetic device. There are various implemented solutions that differ from the type of tactile display that is employed.

Wagner in (Wagner, 2005) proposed the integration of kinaesthetic feedback with a shape display. Kheddar introduced the concept of a multilevel device for the different spatial level of details (Kheddar, 2004).


In the case of multi-finger interaction a major problem has to be considered. For allowing the simulation of grasping of reduced thickness the fingers have to be free to get close each other. This results in very though requirements for the thickness of the tactile display in the palmar area of the fingertip. The display has to be as flat as possible in order to allow the finger of user to get close together during grasps of small virtual objects. There are no experiments that exactly establish what is the exact tolerable thickness of a display for supporting multi-finger interaction, but simple tests can demonstrate that it must be kept in the range of few millimetres.

One possible way to get around this issue is to employ an electro-tactile display that can reach a very flat shape like in (Sato et al.,2007). However, electro-tactile displays show several disadvantages like large variation of perceived stimulus from subject to subject, dependence from wetness of the finger surface, employment of high voltage etc. Those unwanted feature made them hardly usable in the context of a virtual reality simulation.

Other attempts has been done for obtaining mechanical tactile displays with flat shape.

Benali-Khoudja in (Benali-Khoudja et al., 2003) developed a pin-array display based on a multilayer approach that has been conceived for integration with kinesthetic feedback. In the European Project HAPTEX a system composed by a desktop haptic device and a piezo-electric pin-array display have been adapted for the integration and tested (Fontana et al., 2007).

Recently a novel pin-array display based on a customized solenoid transducer has been purposely developed by authors for the integration with a hand exoskeleton (Salsedo et al.,2011).
