Page 41 - Control Theory in Biomedical Engineering

28 Control theory in biomedical engineering

the corpus luteum. The corpus luteum secretes estrogen and progesterone.
This buildup of progesterone and estrogen further increases the formation of
the endometrial lining. These hormones work together and send a second
negative feedback to inhibit the release of FSH and LH. This causes the cor-
pus luteum to deteriorate, slowing the production of estrogen and proges-
terone. The drop in these hormones signals menstruation.
As is claimed in Houk (1988), feed-forward controllers generally make
persistent errors unless they are adjusted via adaptive controllers. Fig. 25 pre-
sents the model reference adaptive strategy. The reference model compares
the desired and the actual outputs in order to compute the error signal. Based
on the dynamic evolution of this error signal, the evaluator adjusts the char-
acteristics of the feed-forward controller. In fact, this type of controller does
not produce change in outputs in real time, but rather modifies the way the
system reacts to future inputs. These properties characterize adaptive control
from conventional feedback. In Houk and Rymer (2011), an adaptive strat-
egy is explained for adaptive control of muscle length and tension. In fact,
the adaptive control system contains different elements to estimate the
dynamics behaviors of the controlled system. Therefore controller parame-
ters are accordingly modified and adapted.
In conclusion, three basic strategies exist in the control process in
homeostasis, namely negative feedback, feed-forward and adaptive control,
as summarized in Fig. 26. This figure illustrates the structural features of these
three control strategies and their combination in the same case. In fact, a
feedback controller generates forcing functions by comparing desired per-
formance with actual performance as monitored by a feedback (closed) loop.
In contrast, a feed-forward controller generates commands without using
continuous negative feedback (open loop). Therefore, an adaptive

Fig. 25 Model reference of adaptive control system. (The figure has been adopted from
Houk, J.C., 1988. Control strategies in physiological systems. FASEB J. 2(2), 97–107.)