The Universal Current Sensor
Abstract
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.
MAIN FEATURES
EASY FABRICATION
The ferromagnetic materials can be formed into thin films and can be structured into small strips that are typically 40mm thick, 10mm wide and 100mm long. This makes the fabrication of the sensor very easyTEMPERATURE INDEPENDENCE
To reduce temperature dependence, they are configured as half bridge or as full bridge.LINEARITY
Measured quantity is directly proportional to the output. The
current flowing through Permalloy conductor generates a magnetic field
that exactly compensates the magnetic field generated in the conductor
that is to be measured. Hence the device is linear.
NO MAGNETIC SHIELDING IS REQUIRED
Magnetoresistive sensors are not affected by the external magnetic
field. This is achieved by the full bridge configuration of four
magneto resistors. Barber poles have the same orientation in the two
arms, so no external field will affect the system.
COMPACT AND CHEAP
Permalloy can be drawn into thin sheets or thin films or thin strands. Hence they are compact and easy to fabricate and cheap.WORKING
Magnetic field sensors based on the magnetoresistive effect can be
easily fabricated by means of thin film technologies with widths and
lengths in micrometer range. For best performance, these sensors must
have a very good linearity between the measured quantity and the output
signal. Even when improved by the barber poles, the linearity
magnetoresistive sensor is not very high, so the compensation principle
used on hall sensors is also applied here. An electrically isolated
aluminum compensation conductor is integrated in the same substrate
above the Permalloy resistors. The current flowing through this conductor generates a magnetic field
exactly compensates that of the conductor to be unmeasured. In this way
the MR element always work at the same operating point; their
nonlinearity therefore becomes irrelevant. The temperature dependence is
also almost completely eliminated. The current in the compensation
conductor is strictly proportional to the measured amplitude of the
field; the voltage drop across a resistor forms the electrical output
signal.
Magnetoresistive sensors, as are hall elements are very well suited
or the measurement of electric currents. In such applications it is
important that external magnetic fields do not distort the measurement.
This achieved by forming a full bridge are specially separated. The
barber poles have the same orientation in the two arms, so that only a
field difference between the two positions is sensed. This configuration
is insensitive to external homogenous perturbation fields. The primary
conductor is U shaped under the substrate, so that the magnetic fields
acting on the two arms of the bridge have the same amplitude but
opposite directions. This way the voltage signals of the two
half-bridges are added.
The sensors require neither a core nor a magnetic shielding, and
can therefore be assembled in a very compact and cheap way. The output
is calibrated by a laser trimming process or by a digital calibration.
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