The result of a trial might look like Figure 9, with the line representing the stability boundary for that particular device configuration, user and virtual environment. The shaded region indicates the range of impedances that can be implemented stably. The value of this type of plot is that we can use it to quantitatively compare different configurations of the device (or even other devices altogether).
Preliminary experiments showed that results varied greatly with the type of grip used by the subject (again recall Figure 3, Section 2.2). Generally, it is much easier to destabilize a virtual wall when pushing against the handle with one finger than when enveloping the handle in a full-fingered grip. However, one-finger interaction is extremely tiring if used for more than just a few minutes. Since we had several hours worth of virtual walls to go through, subjects were required to keep four fingers on the handle at all times.
Subject learning was embraced rather than avoided. Since normal interaction with a haptic interface would include learning, we felt it important to keep that aspect intact. Subjects were allowed to familiarize themselves with the device before the experiment began. Once they indicated they were comfortable interacting with and destabilizing walls, we started collecting data. With each new configuration, the subjects were allowed practice time to adjust to the new settings. Subjects were given as much time and as many attempts as they desired to generate instability, so that a given trial was ended only when the subject labeled the interaction as stable or unstable.
To eliminate fatigue, subjects were not allowed to work with the device for more than one hour at a time, with several hours rest before starting again. They were allowed to progress through the trials at what they considered a comfortable pace. To ensure consistency of results, some parameter values were repeated at the end of each configuration, as were certain configurations at the end of the experiment. Three subjects were used to gauge how parameters varied from person to person.
For each of the four factors discussed above, two conditions were examined:
| Damper | engaged | disengaged |
| Sampling rate | high (1 KHz) | low (100 Hz) |
| Encoder resolution | high (900K cpr) | low (8K cpr) |
| Velocity filter | first order, 30 Hz cutoff | none |
Sixteen "configurations" (combinations of the above four conditions) were possible. All of these configurations were studied for each subject. Within each configuration, the maximum achievable stiffness was found for the entire range of achievable damping. These data lead to plots of maximum virtual stiffness (K) vs. maximum virtual damping (B), representing the stability boundary for each configuration (as in Figure 9 discussed earlier). By comparing these plots, we can see the effect of each of the four factors on the Z-width of the device.
