Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 17

2.4.7 Hall Effect Sensors

A Hall Effect sensor is a transducer that varies its output voltage in response to
changes in magnetic field.
In its simplest form, the sensor operates as an analogue transducer, directly
returning a voltage. They contain a Hall element constructed from a thin sheet
of conductive material with output connections perpendicular to the direction of
current flow. With a known magnetic field, its distance from the Hall plate can
be determined. Using groups of sensors, the relative position of the magnet can be
deduced.
Electricity carried through a conductor will produce a magnetic field that varies
with current, and a Hall sensor can be used to measure the current without inter-
rupting the circuit. Typically, the sensor is integrated with a wound core or perma-
nent magnet that surrounds the conductor to be measured.

2.4.8 Magnetoresistive Sensors

Magnetoresistance is the property of a material to change the value of its electrical
resistance when an external magnetic field is applied to it.
MR sensors measure both linear and angular position and displacement in the
Earth's magnetic field (below 1 gauss). They are an excellent solution for locating
objects in motion. By affixing a magnet or sensor element to an angular or linear
moving object with its complementary sensor or magnet stationary, the relative direc-
tion of the resulting magnetic field can be quantified electronically.

2.4.9 Ultrasonic Position Sensors

Ultrasonic sensors work on a principle similar to radar or sonar which evaluates
attributes of a target by interpreting the echoes from radio or sound waves, respec-
tively. Ultrasonic sensors generate high-frequency sound waves and evaluate the
echo which is received back by the sensor. Sensors calculate the time interval
between sending the signal and receiving the echo to determine the distance to an
object.

2.4.10 Accelerometers

Accelerometers are sensors that measure acceleration they are subjected to. Most
are based on resistive or capcitative and piezoelectric methods.