Sensors And Actuators; Switches As Digital Sensors; Light Detection Using The Phototransistor; Signal Output With The Incandescent Bulb - fischertechnik Mobile Robots II Manual De Instrucciones

3 Sensors and Actuators

You now know that our most important unit, the interface, "plays" together
with the PC. Now it is a question of how the interface can detect the
signals from our environment.
Let's start with the inputs. In technical language, input signals are often
simply called inputs. There is only one possibility for a computer, and this
includes our Intelligent Interface, to detect and process signals. We have to
make the environmental stimuli "computer-compatible." Consequently, all
sensors are converters, which convert the desired "sense" into an electric
signal. Because we do not want to follow the construction instructions
"blindly," it makes sense to look into the basic properties of the available
sensors.
This is even more important for expanding the interface later to handle
new applications that you define yourself.

3.1 Switches as Digital Sensors

The simple, logical levels "0" and "1" can
be depicted using a switch. Precision snap
switches are used in the system of fischer-
technik construction kits. The special property
of snap switches lies in their switching be-
havior. If you press the red button carefully
and slowly, you feel a clear pressure point
when the switch contact switches over with a
slight clicking sound. If we release the switch
lever slowly, we have to let the lever go substantially further than the
original switch on point to switch it back. This difference between
mechanical switching on and off positions is called hysteresis. The switching
hysteresis of contacts or other electronic switches is an important property.
If they did not exist, i.e., the switching-on point would be the same as the
switching-off point, substantial programs in signal assessment would result.
Tiny interferences such as very slight jittering in the switching time point
would result in several unintentional contact activations; it would not be
possible to count events precisely. The switch is designed as a change over
switch. Consequently, you can assess both imaginable starting positions,
i.e., closed and open when idle, in your experiments.
3.2 Light Detection using the Photo-
transistor
The phototransistor is a semi-conductor element, the electric properties of
which are dependent on light. A normal transistor is a component with three
connections. These connections are called
emitter, base and collector. Its main task is
to amplify weak signals. Weak current, which
flows from a signal into the base of the
transistor, results in much stronger current at
the collector of the transistor.
The current amplification can reach factors
of more than 1,000. But the phototransistor
from the construction kit only has two connections. What happened to the
third connection?
We want to detect light with our transistor. Everybody is familiar with solar
cells, with which power is generated using sunlight. The phototransistor
should be understood as a combination of mini solar cell and transistor. The
base connection is not to the outside (consequently, it appears as a dotted
line in the drawing). In its place, light pulses (photons) generate a very
small photocurrent, which is then available amplified from the transistor at
the collector for assessment. In order for this to function as described here,
the phototransistor requires additional external wiring. Because this is con-
tained in the interface, this is not important for us.
The phototransistor can be used both as a digital sensor and an analog
sensor. In the first case, it serves for detecting clear light-dark transitions,
e.g., a marked line. But it can also differentiate the strength of light; then
the phototransistor operates as an analog sensor.
3.3 Signal Output using an Incandescent Bulb
An incandescent bulb serves for outputting simple light signals. To put this
in technical language, we will call the incandescent bulb an optic actuator.
The structure of an incandescent bulb is very simple. A filament made of
thin tungsten is mounted between two connection pins in a glass bulb, in
which there is a vacuum. If current flows through the filament, the tungsten
filament heats until it glows white. Because there is no oxygen in the glass
bulb, the filament does not burn and consequently the lamp has a long
operating life. Due to the strong thermal stress on the coiled filament, the
wire filament expands each time the light is switched on and contracts
when it is switched off. These minimal movements
due to material wear result in "burning out" of the
incandescent bulb at some time.
One possible use of incandescent bulbs is for
displaying switching states. Warning messages can
also be generated by the programming of a
blinking lamp.
We also need the bulb in another case. A special
sensor is created together with two phototransistors, with which lines can be
detected. The bulb works as a light source, so that the phototransistor can
detect color marking based on light reflected with different strengths.
A special feature of the incandescent bulb, which is used in fischertechnik
construction kits, is the optic line contained in the glass bulb. This improves
focusing of the light rays, and markings are detected more reliably as a re-
sult, for example.

3.4 Direct Current Motors as Power Source

Direct current motors are important actuators for mobile systems. Two
different types of motors are contained in the "Mobile Robots II"
construction kit. Although they differ greatly in a mechanical sense, their
electric structures are identical.
Direct current motors are made of a rotating "rotor" and a fixed "stator."
The rotor should be understood as a conductor loop in principle, which is in
the magnetic field of the stator. If current flows through the conductor loop,
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