**4. Experiment to validate sketching skills for fixed templates**

To check that the users learn sketching faster when they are provided with force feedback, we carried out the following test: The image used in the experiment is shown in the Figure 17b, which is based on the original image shown in the Figure 17a. The test image was chosen due it has the basic components of more complex designs (lines, curves, circles, squares, etc).

Fig. 17. Test Image.

We requested to 10 users to trace the test image seven times as precisely as possible in terms of position. Since this task should model free drawing, no restriction on completion time or effector velocity was imposed. The users were divided in two groups of 5 members. At the beginning of each experiment, the users had five minutes for drawing on the test image in order to become familiar with the HDS. The users of the first group were stimulated only with visual feedback throughout the experiment. The users of the second group were stimulated with visual feedback on every trial, and they received the force feedback only in the second, fourth and sixth trials.

We used the performance parameter described by (6) to measure if user learning of drawing skills was indeed accelerated. The PSD parameter involves the time, velocity, distance and error (between the HEE Position and the point on the trajectory nearest to the HEE) when the user is drawing.

$$P\_{SD} = \frac{\int\_{o}^{t} |\Delta| dt}{L = \int\_{0}^{t} v dt} \tag{6}$$
