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Tham khảo tài liệu 'advances in haptics part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 532 Advances in Haptics Similarly the force JNDs are higher than the JND measured in a VE by Allin et al. 2002 who found a 10 force JND on the index finger with a constant base force at 2.25 N. On the other hand the low velocity JNDs are compareable to the JNDs obtained in a VE by Brewer et al. 2005 who found a 19.7 force JND for a 1.5 N base force. They also reported that their JND is larger than the JND in the literature discussing several reasons such as the difference in the environment and tested joint less subjects training and unfixed background dimensions. In this study the small base force 0.27 N can be the main reason that the JNDs are larger than the JNDs measured by Pang et al. 1991 Jones L. A. 1989 and Allin et al. 2002 . The base force is much smaller than their base forces 1.5-10 N and according to Weber s law the JNDs for low base stimuli are larger than ones for high base stimuli. In the next section Section 5 this difference will be investigated with different base forces to find if the higher JND in this section is due to a small base force. 5. The effect of force intensity on Force Perception in HEVEs In this section the force JNDs of the human haptic system are quantified and the effects of the base force intensity and the force increment decrement on the force JND are investigated. An experiment is conducted for three levels of base force intensity. The Interweaving Staircase IS Bernstein R. S. and Gravel J. S. 1990 method is employed to measure the force JNDs. For the first level 0.15 N force is applied in the same direction as the hand motion to partially cancel the backdrive friction of the haptic device 0.27 N . Therefore the resulting resistive force on the subject s hand is 0.12 N which is the first level of base force. This level is called the low base force. For the second level 0.15 N is applied in opposition to the direction of the hand motion. This force adds to the friction force resulting in a 0.42 N resistive force on the subject
