**2.1 Design guidelines**

Two types of design guidelines have been employed during the preliminary definition of the system: qualitative and performance guidelines. According to qualitative guidelines the device had to show ergonomic features, which make the use of the system very comfortable. These guidelines have regarded: the workplace, the quality of the visual feedback, the aesthetics of the system, and the modality of interaction. Another very important guideline is the reduction of mental load during the use of the device.

The system design and control should allow the user to feel and control haptic interaction just below his fingertips, while directly viewing the effects of his actions on the computer screen (co-location). Specific state of the art analyses [Jansson & Öström, 2004] have verified that co-location greatly enhances the user performances in HCI while reducing the mental load of the interaction. The haptic device has therefore to be calibrated in order to collimate the position of the usage tool (end-effector), with the pointer within the computer screen. The accuracy provided in design, should ensure that the sensitivity of the system is far beyond the pixel resolution of the screen.

Training Motor Skills Using Haptic Interfaces 201

An LCD monitor of the proper size and resolution 34x27 cm (1024x768 pixels) was chosen and integrated within a desktop plane. The haptic device is a 2DOF system which employs and hybrid serial parallel kinematic structure. This solution behaves an end-effector, which slides over the monitor and desk plane in correspondence with the computer pointer. The specific kind of materials adopted has chosen in order to reduce the friction factor among

In order to improve the performances of the haptic device (transparency, manipulability, workspaces, reflected mass), the following solutions to the design guidelines mentioned

 Both actuators have been grounded and attached to the base link. As in parallel manipulators, the grounding of the motors allows to reduce the amount of movable masses and to increase the overall stiffness. The specific serial parallel design was used for transmitting the actuation to the end effector and to solve the typical workspace

 Brushed DC motors have been selected: the amount of target force in this kind of applications is limited to few newtons; therefore the iron-less construction of the rotor

 Capstans and metallic in tension tendons have been used as means of transmission of forces from the actuators to the joints. This solution allows completely avoiding geared transmission for the reduction of the transmission ratio, and therefore to avoid the

The structural parts have been realized with light materials (aluminium, plastic) and the specific design used allows the system to correctly operate even in presence of medium

The whole system has been designed to be integrated with the work-plane of a desk: the computing unit, the power supply, the motors and the electronics for the control of the haptic interface have been placed under the desktop so that the desk plane is completely

free and the operator has direct access only to the visual and haptic systems.

limitations of parallel devices (Frisoli, Prisco, Salsedo, & Bergamasco, 1999);

**2.2 Design of the haptic device** 

previously have been adopted:

allows the best compromise of performances.

Fig. 4. Assonometric view of the haptic interface.

tolerance manufacturing.

backlash and friction issues related to this kind of solutions.

surfaces.

These guidelines have steered the main design choices of the system: the presence of the haptic interface should minimally interfere with the visual feedback and therefore it was decided to make use of transparent materials for building; all cables and connection should be hidden to the user; the device should offer the possibility of using a common pen as an interaction tool or the possibility of changing the end part with different tools; the device should be able in any case to replace and substitute the mouse in all its basic functions (point, select, click,…); the kinematic of the device should be designed in order to not interfere with the user limbs, it should preferably move in the opposite space with respect the user; the device, whenever unused, has to be closable in order to left the desktop free.

According to performance guidelines the device should have the following characteristics: a comfortable workspace wide enough to allow user to interact in writing operation: the workspace estimated had to cover at least the same dimensions of a notebook (270 x 360 mm); for design and writing related applications a typical position resolution well below 1 mm was identified. Backlashes were not allowed in kinematic design. A set of preliminary experiments, carried out with pens in writing-and-contour-following-tests suggested a target continuos force of about 1-1.5N. Similar tests outlined a maximum residual mass of 0.2 Kg.

A high isotropy of the mechanical parameters (manipulability ellipsoid ~1) was required all over the workspace in order to reduce the distortion effects and to maximize the exploitation of the motors. In order to achieve high control bandwidth and facilitate the system maintenance and development a high degree of integration with the hosting OS was required.

Finally, in order to reduce the cost related to the system manufacturing two features were required to the system design: simplicity and possibility of manufacturing with low precision (low cost) technologies such as the laser cutting..

Fig. 3. The Concept and Implementation of the Haptic Desktop.
