Biomedical Engineering Reference
In-Depth Information
to one or many reference values.
The sensor values in a system model are often the base of information that can,
complemented by other sensor information measure the environment or even ac-
tively affect the close proximity by actuators. The sense-control model is a common
process in the scientific field of measurement and control. According to the illus-
tration in Fig. 4.3, the human skin is the main sensor to detect temperature, and
the brain is the decision-making system that decides whether the temperature is
too cold or too warm to have a shower, or if it is the right temperature. The desired
temperature is of course a subjective matter to be decided by the person who takes
the shower. Also, it can be noticed that a fuzzy temperature scale is normally used
when a person is acting as a sensor in a control loop.
Even if we get the most perfect sensor with the best possible performance, we
should bear in mind that it still could give totally erroneous information when not
used properly. The sensor itself usually measures only the extracted parameter
in contact with the sensing element. The parameter performance procedure can
be exemplified by the fever indicator sensor that measures a temperature using
infrared light in the ear of a child. More correctly, the temperature sensor only
measures the temperature in the ear and maybe only in the ear at the moment it
has been in contact with a warm and cosy pillow. The influence of the environ-
ment has to be considered in a measurement process. The environmental or real
world effects have a strong influence on the actual information from the sensor
and should be added to the overall information before a decision is made. There-
Figure 4.3.
The human in a closed control loop where the human body is acting as a sensor
device.
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