Fluxgate sensors are precise vector sensors of magnetic fields (can measure both magnitude and direction).
A fluxgate sensor uses a soft magnetic material (with a low coercive force/field and high permeability) as its core material and operates under magnetic saturation state. In saturation, the permeability of the core drops, causing the flux associated with the magnetic field decreases.
Fluxgate Sensor Principle
The term “fluxgate” comes from this “gating” or limiting of the flux at saturation point. The core is magnetically saturated alternatively in opposing directions, normally by means of an excitation coil driven by a sine or square waveform.
Thus, an alternating cycle of magnetic saturation in both direction is formed: magnetized (to the saturation point) → unmagnetized → inversely magnetized (to the saturation point in the opposite direction) → unmagnetized → magnetized (to the saturation point).
This constantly changing field induces a current in the second (sensing) coil. Note that the peak of the excitation current should be big enough to ensure the core to be saturated. In the absence of an external magnetic field, the input and output currents will match.
However, when the core is exposed to an external magnetic field, it will be more easily saturated in alignment with that field direction and less easily saturated in the opposite direction. Hence, the alternating magnetic field, and the induced output current, will be out of phase with the input current.
Some fluxgate sensors operate with current sensing in the pickup (secondary) coil, and others with voltage sensing in the pickup (secondary) coil. No matter which mechanism is used, the magnitude of the output signal should be proportional to the strength of the external magnetic field to be measured.
Features of Fluxgate Sensors
For low magnetic field sensing, the fluxgate sensors provide the best trade-off between cost and performance. It can measure both magnitude and direction of a DC (static) or low-frequency AC magnetic field in the range of 0.1 nT – 0.1 mT with the achievable resolution of 0.0001 μT.
Many fluxgate sensors have a bandwidth of a few hertz to kilohertz frequencies.
Fluxgates can operate over a wide temperature range. Typical temperature stabilities drift less than 0.1 nT ⋅ °C−1 with a temperature coefficient around 30 ppm ⋅ °C−1. Some fluxgates can be compensated to 1 ppm ⋅ °C−1.
In terms of dynamic range and resolution, fluxgate sensors perform better than the Hall effect sensors and are preferable to superconducting quantum interference devices (SQUIDs) because of their lower cost and size. If the fluxgate operates in feedback mode, the linearity error may be as low as 10−5.
Fluxgate sensors, using soft magnetic materials as their core materials (with low coercitive field Hc), have the following characteristics:
Low losses at the excitation frequency (usually in the tens of kilohertz range)
A low saturation induction value (implies a low power consumption)
A minimal magnetostriction effect
Low magnetic noise due to easy reversibility of the magnetization