Biomimetic and Biologically Inspired Control - Biomimetics: Biologically Inspired Technologies - page 413

a b c

Hand force
+
Muscle force

Experiment Muscle force

10 (N)

Figure 16.9 Comparison of the interaction force vectors between the human hand and the crank in experiment
(a), and numerical simulations of (b) that use the minimum muscle force change criterion, (c) the combination of the
hand interaction force change and the muscle force change criterion.

Compared with the experimental result of the measured force shown in Figure 16.9(a), the
computational results for the cases of the minimum muscle force change criterion and the combin-
ation of the hand interaction force change and muscle force change criterion are, respectively,
shown in Figure 16.9(b) and (c) (Ohta et al., 2004).
From Figure 16.9, it is observed that the predicted numerical result of the contact force vectors
when using the minimum muscle force change criterion (which was proposed for P.T.P. motion in
the free motion space) is inadequate here. Instead, the human arm tends to minimize

J ¼ ð

T f

( F T F þ w f T

f)d t

0

the combination of the hand interaction force F change and the muscle force f change as in
Figure 16.9(c) and Figure 16.10. Therefore, we strongly suggest that human arm movement is
realized by different optimal criterions according to different task conditions as well as task
requirements.
The combined criterion also captures well the muscle activities in the constrained multi-joint
motions. It covers both the motions in the free motion space and the constrained motion space, since
in the free motion space the interaction force at the end-effector is zero. Therefore, the combined
criterion reduces merely to the minimum muscle force change criterion in the free motion space.
How can the central nervous system measure the hand contact force and how can it solve the
optimal constraint dynamic motion control are left as open questions.

16.4 MECHANICAL INTERACTION AND ENVIRONMENTAL ADAPTATION

This section further goes to describe the motor control functions on the mechanical interaction with
dynamic environment. It is well known that human can perform physical interactions with uncertain
dynamic environment skillfully. In fact, through force feedback from tendon and the co-activation
of antagonist muscles, human can control the arm's mechanical impedance adaptively with respect
to the environmental dynamics so as to realize the desired time response of the motion as well as the
contact force (Hogan, 1984).
In order to realize such adaptive motor functions by a robot, we should not only search for the
soft artificial actuators such as biological muscles, but also discover the control principles of the
motor functions. Technically, according to the task requirements, the contact tasks can be specified