Page 497 - Biomedical Engineering and Design Handbook Volume 2, Applications

APPLIED UNIVERSAL DESIGN 475

For simplification of this example, let us assume that the peak force on the head during a crash
is a function of height only and follows the following hypothetical equation:
4
F = 2h + 7 when h < 60 in
3
= 2h + 6h when 60 ≤ h < 72 in
= 2h 5 when 72 ≤ h in
where F is the peak force on the head and h is the subject’s height when standing.

Steps
1. Determine the range of heights.
2. Select initial sample size n.
3. Using a random-number generator that can produce a uniform random distribution, select n values
of h.
4. Calculate a total of n peak forces by calculating the force for each height.
5. Apply all relevant statistical measures to these peak forces. They might include mean, standard
deviation, correlation coefficients, etc.
6. Repeat steps 2 to 5 with a larger sample size.
7. Compare the statistical measures from each sample size. TABLE 16.6 Random Heights
Repeat steps 2 to 5 with an ever-increasing sample size until
Height, in
the results of these statistical measures converge. Use these
results in your design. 58.25513474
69.04791406
Suppose that h is in the range from 58 to 77 in and n is initially 71.38007752
selected to be 10. A random-number generator has generated the sam- 65.59837642
ple heights using the random number seed 123 shown in Table 16.6. 76.76573992
Figure 16.9 shows the mean, standard deviation, and maximum 59.89321574
for the peak force for n = 10 above, where n = 15 and the random 75.82985931
seed is arbitrarily chosen as 7,638, n = 20 with seed 418, n = 50 76.9483932
63.6593524
with seed 6,185, n = 100 with seed 331, and n = 200 with seeds
43 and 3,339. Note the range of values at n = 200 when two different 74.27295755
seeds were selected. The engineer would then decide if he or she is
satisfied with the results. Remember, in an actual computer simulation, the CPU time may be
significant, and a sample of even n = 10 may be expensive.

16.3.2 Weight Is Critical

Many products that a person with disabilities may use are carried, worn, or are mounted on a
wheelchair. The heavier the product, the more energy that a person must exert to carry it. When a
device is mounted on a self-propelled wheelchair, the user must also expend additional energy to
account for this weight. On an electric wheelchair, the added load will shorten the interval between
recharges and reduce available power. The reduction of weight as a critical factor is one that is
shared with the aerospace industry. Additional engineering efforts to reduce the product’s weight
will yield significant benefits to the consumer.

EXAMPLE Lisa, a 15-year-old high school student, wears an ankle-foot orthosis (AFO). This AFO,
worn around her calf and under her foot, supports her ankle. To the casual observer, the AFO seems
light. Through Lisa’s daily activities, the extra weight is noticed. Fig. 16.10 shows her AFO. An engi-
neer may initially check that the peak stresses do not exceed their limit. But as an aerospace engineer
would ask, why do we allow all these areas of low stress? If the material is thinned or removed from
the low-stress regions, the AFO will not fail, and its weight will be reduced. Figure 16.11 shows a