| The
Universal Current Sensor |
INTROUCTION to the Universal Current Sensor
The measurement of electric
current strength is not always easy, especially when the measured signal requires
further electronic conditioning. Simply connecting an ammeter to an electrical
circuit and reading out the value is no longer enough. The current signal must
be fed into a computer in which sensors convert current into a proportional voltage
with minimal influence on the measured circuit. The basic sensor requirements
are galvanic isolation and a high bandwidth, usually from DC up to at least 100
kHz. Conventional current measurement systems therefore tend to be physically
large and technically complex. MAGNETORESISIVE
SENSORS Conventional
sensors are physically large and technically complex; also they have disadvantages
as stated above. Hence they are replaced by magnetoresistive current sensors.
The magnetic field sensors are based on the magnetoresistive effect. These sensors
can be easily fabricated by means of thin film technologies wit widths and lengths
in the micrometer range. To reduce temperature dependence, they are usually configured
as a half bridge or a full bridge. In one arm of the bridge, the barber poles
are placed in opposite directions above the two magnetoresistors, so that in the
presence of a magnetic field the value of the first resistor increases and the
value of the second decreases.
MAGNETORESISTIVE
EFFECT The
anisotropic magnetoresistive effect is known to be present in a whole family of
ferromagnetic alloys. Most of these alloys are composed of iron, nickel, and chromium,
and may be primary or ternary. They have in common a more or less strong anisotropy
in their magnetic properties. Whenever these materials are exposed to a magnetic
field during crystal formation, a preferred orientation in magnetization will
result. The same happens when the materials are forced into shape that is a mechanical
anisotropy is imposed.
It is found that changing the orientation of the magnetic moment in the wire caused
a current passing through it to change correspondingly. The orientation could
be changed by apply in an external magnetic field, and generally an increase in
current was observed. This phenomenon is called anisotropic magnetoresistive effect.
The ferromagnetic materials can be deposited as thin films and structured into
small strips that are typically 40mm thick,10mm wide, and 100mm long. In most
general case, the electrical resistance of AMR material depends on the angle between
the direction of the magnetization, and the direction of the current going through
it. When the current and magnetic moment are parallel, the resistance of the strip
is greatest; when they are at a 90 degree angle to each bother, it is smallest.
Magnetoresistive
field sensors are usually configures as a half or full bridge. The barber poles
are positioned such that in the presence of magnetic field the value of first
resistor increases and that of second decreases.
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