Figure 1.
Apparatus for producing compliance and vibration stimuli.
Subjects stepped from one side of the platform a onto the vibrating plate b, and onto the opposite platform a'. They then turned, stepped on b again, and returned to a. In Experiment 2, the plate also displaced up to 2 cm in the vertical direction, compressing a volume of EVA foam that was controlled by the linear servomechanism c, to produce the commanded compliance (see Fig. 3). Subjects entered their responses after each trial at the keyboard d and received instructions from the large-screen video monitor e.
Figure 2.
The thin lines (rows 1, 3, and 5) show force profiles from footsteps of one participant onto the plate, and the darker waveforms are the corresponding vibration feedback stimuli. Vibrations could be felt only during foot-plate contact. Stimuli are labeled with a 2-letter string, with the first encoding waveform type (S, W, T = Sinusoidal, White noise, Textured noise), and the second encoding envelope type (C, P, D = Constant, Proportional, Dynamic). The vibration amplitude range was normalized for display purposes.
Table 1.
Experiments 1 and 2: Peak and RMS amplitudes of plate acceleration, for the high amplitude
vibration stimuli.
Figure 3.
View of the variable compliance mechanism.
A. Users stepped onto vibrating plate a, which was driven by vibration actuator b and mounted on suspension c. The plate displaced in the vertical direction, guided by low-noise ball bearing slides d, and compressing a pair of foam inserts e. To produce the commanded compliance, the foam inserts e were positioned by the linear servomechanisms f before each stimulus presentation, while the plate assembly was lifted by servos g. Participant-applied forces were measured by load cells h under four corners of the plate assembly. B. Image of the apparatus and shoe as used in the experiment. Opaque panels k and fabric (not shown) hid the device configuration from subjects' view. Four optical motion capture markers m tracked the displacement of the plate with high precision.
Figure 4.
Force-displacement profiles and fits of stiffness vs. foam position used to calibrate the apparatus.
A. Examples of three compression profiles are shown (overlaid) for each of three stiffness values. Each calibration was based on sixty such profiles, with more than 3000 data points each. B. Calibration curve fit of stiffness vs. foam position based on measurements at each position and stiffness.
Figure 5.
Experiment 1: Subjective compliance ratings for all 19 stimuli, averaged across subjects.
A higher value means more compliant (less stiff). “X” labels the no-vibration stimulus. Others are labeled with a 3-letter string encoding waveform, envelope, and amplitude level (0, 1 = Linear amplitude 0.5, 1.0). Error bars = 1 standard error of the mean (SEM). All vibrating stimuli were significantly more compliant than the no-vibration stimulus.
Figure 6.
Experiment 1: Average compliance ratings for the three stimulus factors “waveform type”, “envelope type”, and “amplitude”.
Stimulus labels are as given in Fig. 5. Error bars: SEM.
Figure 7.
Experiment 2: Mean proportion of comparison stimuli judged stiffer than the standard.
A higher proportion implies a judgment of “stiffer”. The standard had a stiffness of 90 N/mm, as indicated by the dashed line, and did not present any vibration feedback. Results shown are averaged between all 20 subjects. Error bars: SEM.
Figure 8.
Experiment 2: Median values of the PSE and slope from per-subject psychometric fits at each vibration amplitude level.
Error bars: SEM (outliers excluded).
Table 2.
Experiment 2: Mean proportions of comparison stimuli that were judged to be stiffer than the standard, as a function of stiffness, with different levels of vibration.
Table 3.
Experiment 2: Median values of the PSE, slope, and intercept from per-subject psychometric curve fits at each vibration level.
Figure 9.
Experiment 2: Psychophysical amplitude detection thresholds and response proportions at stiffness level 90 N/mm.
Top: Response proportions for all 20 subjects at two lowest vibration levels, and stiffness level 90 N/mm. Bottom: Amplitude threshold levels, displayed in dB referenced to . Subjects are sorted in order of increasing threshold (same ordering top and bottom). The dashed lines indicate the amplitudes of the vibrating stimuli used in Experiment 2. Error bars:
2 SEM.