Page 331 - Concise Encyclopedia of Robotics

Thermocouple
The illustration shows how a laser (dark rectangle) can be used to tell the
difference between a shiny surface (at left) and a rough or matte surface (at
right). A shiny surface, such as the polished hood of a car, tends to reflect
light according to the rule of reflection, which states that the angle of
reflection equals the angle of incidence. A matte surface, such as the
surface of a sheet of drawing paper, scatters the light. The shiny surface
reflects the beam back almost entirely to one of the sensors (circles),
positioned in the path of the beam whose reflection angle equals its incidence
angle. The matte surface reflects the beam back more or less equally to all
of the sensors.
The visible-light texture sensing scheme cannot give an indication of
relative roughness. It can only let a robot know that a surface is either
shiny, or not shiny.A piece of drawing paper reflects the light in much the
same way as a sandy beach or a new-fallen layer of snow. The measure-
ment of relative roughness, or of the extent to which a grain is coarse or
fine, requires more sophisticated techniques.
See also TACTILE SENSING.
THERMISTOR
A thermistor is an electronic sensor designed specifically so that its resist-
ance changes with temperature. The term thermistor is a contraction of
“thermally sensitive resistor.”
Thermistors are made from semiconductor materials. The most
common substances used are oxides of metals. In some thermistors, the
resistance increases as the temperature rises; in others, the resistance
decreases as the temperature rises. In either type of thermistor, the resist-
ance is a precise function of the temperature.
Thermistors are used for temperature sensing and measurement. The
resistance-versus-temperature characteristic makes the thermistor ideal
for use in thermostats and thermal protection circuits. Thermistors are
operated at low current levels, so that the resistance is affected only by the
ambient temperature, and not by heating caused by the applied current
itself. Compare THERMOCOUPLE.
See also TEMPERATURE SENSING.
THERMOCOUPLE
A thermocouple is an electronic sensor designed to facilitate the measure-
ment of temperature differences. The device consists of two wires or strips
of specially chosen dissimilar metals, such as antimony and bismuth,
placed in contact with each other.
When the two metals are at the same temperature, the voltage between
them is zero. However, when the metals are at different temperatures, a
direct-current (DC) voltage appears between them. The magnitude of this

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