The main types of Fluxgate sensors are:
Generally, Fluxgate technologies provide low offset & offset drift, because the magnetic core is cycled throughout its B-H loop suppressing any magnetic offset in the Fluxgate core (not avoiding offset or offset drift of the processing electronics or, for the "standard" Fluxgate, the magnetic offset created by the main toroid).
It is also providing excellent accuracy due to the quasi absence of offset. Compared to Hall effect based technologies, this advantage is more noticeable for small currents measurements, where the relative effect of the offset is more significant.
Fluxgate technology benefits from excellent over-current recovery, again because any permanent magnetization of the field sensing element is reset with subsequent B-H cycles (and again, not affecting the main toroid of the "standard" Fluxgate).
It has much higher sensitivity than other technologies, allowing the measurement of very low Ampere-turns (determined by design and dependent on the required magnitudes of BEXT and BSI).
With its large dynamic range, Fluxgate current sensors allows for the measurement of both small and large currents with the same transducer.
The sensors also benefits from very high resolution, provided by the low offset, as well as a large temperature range, as the low offset drift make Fluxgate technologies suitable for broader operating temperature ranges (still limited by the transducer materials and component limits)
Last but not least, Fluxgate sensors come with high bandwidth – fast response time, provided by the current transformer effect (when relevant) which is further enhanced on C-type and IT-type Fluxgates.
On the other hand the technology has some limitations.
To start with, Fluxgate technology does provide limited bandwidth for the simpler designs.
It is common to notice a large noise level at the excitation frequency, present at the output and possibly coupled into the primary. Voltage noise injection into the primary lines is also one of the drawback, it is yet acceptable for the vast majority of applications.
Fluxgate current sensors have a relatively high secondary power consumption, similar to that of closed loop Hall effect transducers.
Finally, the design of Fluxgate transducers is relatively complex and makes for more difficult customization. The complex, yet efficient designs, are also more expensive to produce.
No use of Hall generators. The magnetic flux created by the primary residual current IPR (sum between IL and IN) is compensated by a secondary current. The zero-flux detector is a symmetry detector using a wound core connected to a square-wave generator. The secondary compensating current is an exact representation of the primary current.
In a voltage output transducer, the compensating current is converted to a voltage through a precision resistor, and made available at the output of a buffer amplifier. The magnetic core is actually made up of a pair of 2 magnetic shells inside which the detector is located.
Closed loop Fluxgate CTSR type current sensors are mainly used in the industrial domain. That technology can also be used in the following applications:
The operating principle is that of a current transformer, equipped with a magnetic sensing element, which senses the flux density in the core. The output of the field sensing element is used as the error signal in a control loop driving a compensating current through the secondary winding of the transformer.
At low frequencies, the control loop maintains the flux through the core near zero. As the frequency rises, an increasingly large fraction of the compensating current is due to the operation in transformer mode. The secondary current is therefore the image of the primary current.
In a voltage output sensor, the compensating current is converted to a voltage through a precision resistor, and made available at the output of a buffer amplifier.
Closed loop Fluxgate CAS-CASR-CKSR current sensors have been designed mainly for industrial applications:
ITC current sensors are high accuracy sensors using fluxgate technology. This high sensitivity zero-flux detector uses a second wound core (D’) for noise reduction. A difference between primary and secondary ampere turns creates an asymmetry in the fluxgate current.
This difference is detected by a microcontroller that controls the secondary current that compensates the primary ampere turns (IP x NP).
This results in a very good accuracy and a very low temperature drift.
The secondary compensating current is an exact representation of the primary current.
Closed loop Fluxgate ITC type current sensors have been specially designed for the railway environment. They are available for Traction and Industry domains, among other applications areas:
IT current sensors are high accuracy, large bandwidth sensors using fluxgate technology with no Hall generators. The magnetic flux created by the primary current IP is compensated by a secondary current.
The zero-flux detector is a symmetry detector using two wound cores connected to a square-wave generator. The secondary compensating current is an exact representation of the primary current.
Closed loop Fluxgate IT type sensors are used in the industrial and medical environment. Its ultra-high precision measurement of current provides very high accuracy and excellent linearity in these domains.
Typical applications include:
C-type closed loop Fluxgate sensors are a significant part of the LEM voltage product portfolio. This technology was developed in co-operation with the University of Auckland - New Zealand (Prof. Dan Otto) and provides very high performance in terms of accuracy, temperature drift, bandwidth and response time. This high performance is the result of a patented design used for the compensation of Ampere-turns.
This technology uses two toroidal cores and two secondary windings and operates on a fluxgate principle of Ampere-turns compensation.
For the voltage type a small (few mA) current is taken from the voltage line to be measured and is driven through the primary coil and the primary resistor.
Closed loop Fluxgate C type current sensors are used in industrial applications requiring very high accuracy, for example calibration units, diagnosis systems, test platforms and laboratory equipment. It is also appropriate when the application needs an absolute robustness of performance with temperature changes.
Typical applications include: