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Comparison of Dexterity Measures
120
100
80
60
40
20
0
0
10
20
30
40
50
Exteriment ID
Dm
Dl
Fig. 6. Comparison of dexterity measures using economy of motion
Yet another measure of dexterity is obtained by using the histogram of displace-
ments. The displacement histogram contains the movements in the multiple of h, the
scale of the movement mentioned in equation (5). Let the movements be distributed
into p buckets m1, m2, …, mp with a separation of h between the buckets of
movement amplitude. Here mi denotes the movement having dimension of (i*h). In
Fig. 5, m0, m1, …, m50 are shown. Peaks of the histogram denotes a process cen-
tered around it. In Fig. 5, we identify three peaks, one corresponding to very small
movements such as instrument noise or segmentation noise, another corresponding to
dexterous movements and another corresponding to the coarse movements.
One measure of dexterity that can be derived from the displacement histogram is
the ratio of dexterous moved distance by non-dexterous moved distance. Let indices
i= j … k be dexterous movements and k+1 … p be the coarse movements.
(8)
For example, we partitioned movements into three scales, displacements (rounded up)
into the buckets with h = 0.01 for a range from 0.01 to 0.1 and with h = 0.1 from
buckets of size 0.1 to 1.0 and buckets with h = 1.0 for 1.0 to 5.0. All measurements
are in mm. We took two of the 40 experiments of a subject and tabulated in Table 1.
In the table count indicates the number of movements (rounded) which had the
same movement size indicated in the size column. Intuitively, the motion presented in
EX 2 is better than the motion presented in EX 1 due to large number of fine move-
ments and lack of very large movements. In the above dexterity measure if we fix the
value of k to be at 1.0 mm, then the dexterity metric Dh will show better quality for
measurements for EX 2. The value of k is dependent on the amount of precision
needed.
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