# Design and Testing of Two Haptic Devices Based on Reconfigurable 2R Joints
## 3 Methodology of the Experiment
### Experimental Task Planning  
Testing haptic devices involves a large number of operators as the performance of the device depends on human skill. In different studies, the number of participants has ranged from 10 [31] to more than 50 [32]. The tasks that are usually performed during the tests can be divided into objective and subjective tasks. The objective tests results are normally numerical and can be easily subjected to any kind of statistical analysis without further preparation. The subjective test results are based on human opinion and in order to put them into a form suitable for numerical assessment, special questionnaires are used. The National Aeronautics and Space Administration Task Load Index Assessment (NASA TLX) [33] and After-Scenario Questionnaire--International Business Machines  
Figure 2: The experimental setup: (**a**) schematic representation (view from the top): 1 operator, 2 haptic device, 3 parallel manipulator, 4 projector; (**b**) real implementation (projector is turned off). (ASQ IBM) [34] are the questionnaires that are normally used in haptic device research. In these questionnaires, the operator is asked to grade several aspects of the test. In NASA TLX seven questions with a scale from 0 to 20 are used, and in ASQ IBM three questions with a scale from 1 to 7 are used. The obtained grade can be used in statistical tests. Among the most widespread tests in studies of haptic devices are the analysis of variance (ANOVA) [35; 36; 37], the Mann-Whitney U test [38; 39; 40; 41] and the Friedman test [42; 43]. Considering the information provided, the requirements for the experiment were established. The group of testers should consist of 15-25 participants. The 2-RFR mechanism should be controlled in two modes: velocity and position. In order to obtain testers' feedback, the NASA TLX questionnaire should be used, as it can provide more information. In both working modes, a set of four tasks should be performed:  
1. Defining the workspace boundaries/singular positions without force feedback. 2. Defining the workspace boundaries/singular positions with force feedback. 3. Execution of the proposed trajectory. 4. Following the leading object within a certain tolerance. Of these tasks, the third and the fourth are the most important as they are intended to evaluate the general performance of all the haptic device parts and systems. In addition, the reconfigurability should be tested by blocking one of the DOFs of the proposed joints. It was expected that this feature could provide precise straight motion in a horizontal or vertical direction, depending on the experimental setup. As this test produces a positive or negative result, it was decided that it would not be included in the tasks performed by the group of testers. As the tasks require the visualization of different images (trajectory line, moving object) on the background of the controlled parallel mechanism, a digital projector was used. A schematic representation of the experimental setup is presented in Figure 2a. The operator 1 holds the handle of the haptic device 2 and looks at the 2-RFR parallel manipulator 3, where the image corresponding to the current task is projected with the projector 4. In the Figure 2b the real experimental arrangement can be seen. The main goal of tasks one and two was to estimate the performance of the force feedback system. It was necessary to draw conclusions on how helpful this system is for the operator. The second objective of these tasks was to introduce the testers to the haptic device controls. In the first test, the operator was asked to find the workspace limits intuitively, based only on visual feedback. In the second test the goal was the same; however, the force feedback system was turned on. As the workspace of the mechanism had a high amount of positions that were close to boundaries, several directions of end-effector motion were proposed for the operator: two vertical and two horizontal lines. This measure was also an attempt to save time. The operator was asked to choose.