Biomedical Engineering Reference
In-Depth Information
CHAPTER
4
Feedback and Control Systems
Chapter Outline
4.1
Introduction ................................................................... 159
4.2
Biological Feedback Mechanisms.............................................. 160
4.3
Biomechatronic Feedback Mechanisms ....................................... 160
4.4
System Representation ........................................................ 162
4.5
System Models ................................................................ 164
4.6
System Response ............................................................. 174
4.7
System Stability ............................................................... 181
4.8
Controllers ..................................................................... 188
4.9
Controller Implementation ..................................................... 201
4.10
References .................................................................... 205
4.1
INTRODUCTION
A
system
can be defined as the artificial boundary surrounding a collection of interacting
components, which are considered a “black box” consisting of a limited number of inputs
and outputs. In the natural world, this could be a complete ecosystem, a single human
being, or a physiological system (e.g., cardiovascular, respiratory) within a human being.
From a mechatronic perspective, systems can be as complex as atomic power stations or
as simple as a pneumatic actuator or a single electronic component.
From a biomechatronic perspective, it is often important that a system's characteristics
be well enough understood to be controlled. The control can occur in an open-loop manner,
which implies that the system is provided with an input, and this will determine the output
state. For example, a skin patch that provides a constant supply of insulin to a diabetic is
an open-loop system. Unfortunately, open-loop control does not cater to changes in the
system's characteristics, so if the diabetic drank a large glass of fruit juice, his blood sugar
level would increase substantially, with potentially serious consequences. If, however, the
skin patch was replaced with a more sophisticated device that measured the blood sugar
level and compensated for changes by adjusting the rate at which insulin was infused, the
blood sugar level would remain reasonably stable. This is known as closed-loop control,
and it relies on feedback of an output parameter to help regulate the system.
This controlled process may be applied on a macroscopic scale (i.e., feedback from
the output to the input of a complex system) or on a much smaller scale, such as with a
simple filter that applies a proportion of the output voltage to the input.
159
