**2.2 Design of the haptic device**

200 Haptics Rendering and Applications

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

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

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

force of about 1-1.5N. Similar tests outlined a maximum residual mass of 0.2 Kg.

precision (low cost) technologies such as the laser cutting..

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

required.

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 surfaces.

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


Fig. 4. Assonometric view of the haptic interface.

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 tolerance manufacturing.

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.

Training Motor Skills Using Haptic Interfaces 203

The kinematics of the haptic interface has been determined according to the chosen link lengths and describe end-effector's coordinates referred to the central pivoting joint (Figure 7).

2 21 1 sin( ) sin( ) *Y L qL q ee* (2)

From (1) and (2) *L*1 and *L*2 are the lengths of link 1 and link 2 while 1 *q* and 2 *q* are the angles referred to the joints. On the other hand, the end-effector's coordinates referred to the

> 1 1 1 1 *r <sup>q</sup> <sup>R</sup>*

2 2 2 2 *r <sup>q</sup> <sup>R</sup>*

From (3) and (4) are the rotations of the two actuators, 1*r* and 2*r* are the radii of motor pulleys and *R*1 and *R*2 are the radii of the capstans. Using the above relationships it was possible to identify the commercial components to be used within the haptic device (motor type and size, sensors, etc). The homogeneity of the conditioning number has allowed us to size out the design by using the workspace centre as reference point. Maxon motor 3557024CR was used for actuation. With this solution the Haptic Interface can generate on user's hand up to 3 N of continuous forces and 5 N of peak forces. To detect device motion 1024 counter per cycle optical encoders where adopted with a 4X decoding. Such a choice leads to a spatial sensitivity of about 30m at workspace centre (pixel size is about 300m).

2 21 1 cos( ) cos( ) *X L qL q ee* (1)

(3)

(4)

Fig. 6. Workspace of the haptic interface.

motors, 1 *q* and 2 *q* can be computed as follows:

**2.4 Components choice** 

Figure 4 shows an assonometric view of the design prototype: in this system, on the desk plane only the required mobile parts of the interface are present; a new design of the prototype allows moving event the capstans and motor pulleys below desk plane. Three buttons have been left near the haptic base in order to control the computing-unit power, to enable force feedback, and to switch on/off the desk light.
