Page 366 - Mechatronics with Experiments

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direction perpendicular to both the current and the magnetic field vector. If the direction
of the magnetic field or the current changes, so does the direction of the induced voltage.
The induced voltage magnitude is also function of the material type. It is very small for
conductors, but large enough for semiconductors such that this effect is widely used in
sensor designs. The sensor output voltage is function of the magnetic field strength (B), the
current (i) and the material of the sensor,

V out = V out (B, i, material) (6.68)
In a sensor application, the current (i) is fixed by the sensor power supply voltage (V ) and
s
resistor (R ). A Hall effect sensor requires an external power supply for the current i and
s
magnetic field density (B) for it to work. The material of the sensor is also fixed. Then, the
output voltage varies as a function of the magnetic field strength for a given Hall sensor.
In the sensor by design, the measured variable must be arranged such that it changes the
magnetic field strength over the Hall sensor. For instance, the measured variable may be
the position of a magnet relative to the sensor. Similarly, the magnet may be part of the
sensor, and a ferrous metal object entering the field changes the magnetic permeability
(magnetic conductivity) around the sensor, and hence changes the magnetic field density
(B). As a result the output voltage is changed (i.e., Figure 6.24). This principle is also used
in presence sensors which have only two state ON/OFF outputs. Typical voltage levels
produced by the Hall transducer are at the millivolts level, hence must be amplified by an
op-amp for processing in a measurement and control system. The op-amp may amplifiy
the Hall effect sensor voltage linearly to measure analog distance or can convert it to an
ON/OFF output signal for presence sensing. Since the sensing element of the Hall sensor
can be a semiconductor, both the sensing element and digital signal processing circuit
can be integrated into a single silicon chip. For instance AD22151 by Analog Devices is
one such integrated Hall effect sensor. It operates from a 5 VDC power supply and can
◦
operate in an environment with a temperature range of −40 to +150 C. Such integrated

Hall output #1
Hall output #2
Hall output #3
+5 VDC
Ground
N
S

S
Stator
N

Rotor

FIGURE 6.24: Three Hall effect sensors used to sense commutation positions in a brushless
DC motor.