ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

End
i
joint angles and length offsets yi ∈ [ θ j, lj]
for each x i . [iv] Compute y i , σ y and
Λ
i
y for output variables.
Compute and plot the statistical uncertainty ellipse ( ek : χ = 5.99 for 95%
x and y ∈ [ θ , l ] .
confidence level) of the distribution of i
7. RESULTS AND DISCUSSION
i j j
The phalangeal joint parameters, extracted by implementing the concept of planar
homography for palmer grasp (Fig. 5) for two healthy adult Indian males, have been
reported in Tables 1, 2 and 3. The localization uncertainty ranges from varies from 1-4
pixels (Table-1) with the associated error ellipse in Fig. 5. The primary source of
localization error ( x ± 3σ ) is partial cross occlusion due to thumb flexion above the
curled fingers at point R where the user has difficulty in selecting point R in Case-2,
evident from associated error ellipse. The maximum uncertainty of estimated joint
_
(a) Case-1 (b) Case-2
Fig. 5 Extracted angles and joint offsets along with error ellipses (a) Palmer view. (b) Lateral View.
angles ( θ ± 3σ
) is ±4.5° (at 95% confidence level) reported in Table-2 with joint limits
[5]. Direct measurement of actual joint angles due dynamic formation of palmer grasp
being unfeasible in this case , the validation has been done by retrieving the right angled
grid in the reference plane which is with in 0.1% - 0.5% error. The uncertainty in
estimation of joint offsets ( li ± 3σ
) has been reported in Table-3, along with actual
directly measured data ( lm ± 3σ
). Maximum estimated uncertainty is ±2.0 mm but the
uncertainty in direct measurement is ±4.0 mm. This is because size of palmer grasps
change with grasping force and varies during repeated grasping by an individual.
Table-1 : Uncertainty in point localization. (pixel)
Selected Points → A C E G I K L N P R T
Case-1
x ± 3σ 178.6
±3.91
169.6
±2.36
200.1
±2.12
173.4
±2.01
214.4
±4.37
152.5
±2.70
171.5
±3.15
42.36
±0.75
56.01
±0.51
55.24
±1.65
84.58
±1.11
y ± 3σ 77.56
±1.67
159.2
±1.33
144.1
±1.94
193.5
±1.13
163.8
±0.95
161.5
±1.89
174.1
±1.5
82.6
±0.84
188.0
±0.72
151.5
±2.4
169.6
±1.34
x ± 3σ 214.4
±0.46
203.6
±0.39
233.9
±0.34
203.1
±0.57
244.4
±0.46
171.8
±0.46
184.5
±0.52
221.0
±0.81
253.5
±0.81
256.7
±10.94 269.4
Case-2
±0.75
y ± 3σ 80.7
±0.51
151.0
±0.25
149.2
±0.72
185.3
±0.63
169.7
±0.47
150.5
±0.61
170.8
±0.78
81.7
±0.86
175.3
±0.86
142.4
±13.91 155.6
±1.17
Y
Z
X