To better design and evaluate simulator control loading systems, humans' JND in the haptic perception of manipulator stiffness was investigated. To assess changes in such a mechanical property, humans must infer the change through changes in perceived contact force and/or manipulator displacement. To study the relative roles of these two perceptions, we measured stiffness JNDs of seven participants through a forced-choice stiffness discrimination procedure, on three conditions which we referred to as the force, displacement and free conditions. In the force condition, the manipulator displacement was fixed and subjects could only base their stiffness discrimination on changes in the contact force, and likewise in the displacement condition, discrimination was on the basis of changes in the manipulator displacement while the contact force was fixed. In the free condition subjects could uses either or both force and displacement cues to discriminate between different stiffness settings. The lowest stiffness JND was observed in the force condition, followed by a similar results from the free condition. The highest JND level and a poor consistency of results among individuals were obtained in the displacement condition. Subjects indicated that their stiffness discrimination in the free condition was based on the force comparison. To validate this finding, we used two models, one assumed force differences as a criterion for detection and the other assumed the displacement, to estimate the experimental results. Model comparisons indicated that subject's stiffness discrimination is largely based on detection of force differences.
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