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Sensors
Just like a human being a robot has to sense its environment to
take decisions. A simple example is paper detection by a
printer. There is a wide range of sensors for different purposes
and precision levels. We shall limit our exposition to some basic
sensors that are useful and yet easily constructed. We shall
divide these into two categories:
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Binary sensors: These provide yes-no answers.
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Continuous sensors: These are for measuring light
intensity or pressure or temperature.
Binary sensors
All the binary sensors that we shall discuss here are based on an
absolutely simple circuit called a voltage divider.
The voltage at the point P is
V R1 / ( R1 + R2 )
What happens if we replace it by a switch? If the switch is
closed then the voltage at P is "high", else it is "low".
Even this simple circuit finds great application in robotics as
limit switches. These are tiny push button switches strewn at
various extremities of a robot. Every time the robot hits
something a nearby push button gets pressed.
Many interesting variants can be created by replacing the
push button with different types of switches. One example is a
reed switch.
It is a switch that is open by default, but is closed in
proximity of a magnet. This can be used to great advantage when
making a proximity sensor which works even inside muddy water!
ANother variant replaces the reed switch by a phototransistor. A
phototransitor is a semiconductor device. A semiconductor is a
material that behaves as non-conductor under ordinary condition,
but becomes a conductor under special condition. The special
condition triggers its conductivity depends on the type of the
semiconductor material. For a phototransitor the special
condition is illumination. It starts conducting when light falls
on it. Thus it is somewhat like a light-operated switch that is open by
default, but is closed by light. However, it is not a
particularly good switch if the light is not very bright. In dim
light the phototransitor conducts, but not much. Actually this
can be used to measure the intensity of light, as we shall see
later. But in order to make a binary sensor, we shall employ
a comparator.
While this looks like a satisfactory circuit, we shall soon learn
about a serious flaw in it, and also learn how to rectify it.
Continuous sensors
All continuous sensors depend on analog-to-digital conversion
(ADC). A simple but useful example may be constructed using the
phototransitor circuit above if we replace the comparator by an
ADC chip. Such sensors are useful for measuring the intensity of
light enabling a robot to steer towards or away from a light
source.
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