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Current sensing with zero-flux technology

Feature

By Loic Moreau, Danisense

Historically, engineers have been very creative when it comes to measuring current, and today a there’s a wide a large diversity of techniques used. However, when the application requires very high accuracy – such as power analysers, MRI scanners and other medical equipment, and power supplies used in particle accelerators in large scientific research establishments such as CERN – zero-flux technology, also called ‘fluxgate’, is accepted as the best solution.

Figure 1

Commonly used technologies

Out of the many most commonly used current-sensing technologies, there are two main methods: one based on Ohm’s Law, which uses a precision resistor called a “shunt” in parallel with the meter. This is a relatively simple approach, but doesn’t provide very high levels of accuracy, and is subject to temperature-drift errors.

The second approach uses the magnetic field generated by the current’s circulation. This method can be used to measure AC only – as with a current transformer, or AC and DC. Zero-flux technology is a part of this second family of test methods.

Zero-flux current transducer

Looking at Figure 1, a pick-up coil is constructed using a magnetic bar surrounded by a pick-up coil, shown on top left of the figure. On the top right is the equivalent electric circuit with a resistor and inductance. When a voltage is applied, the current in the circuit is shown in the red curve. The current grows progressively, following the slope according to the inductance’s value, up to the point where the inductance saturates. At this time, the equivalent electric circuit can be considered to be resistive only.

When creating a zero-flux current transducer, the magnetic material used for the pick-up bar has specific magnetic properties, so the current is shown in the blue curve. Initially, the current grows slowly due to a high impedance value. Then, suddenly, the inductance saturates and, like a switch, the current increases very rapidly to achieve the end point as before.

If a square voltage signal is applied, the profile of the current is a succession of positive and negative saturation and de-saturation cycles. Placing a conductor close to the fluxgate element causes the circulation of the current to create an additional magnetic field that will influence the signal by shifting the zero position.

Finally, by using a signal processing calculation called second harmonic, it is possible to recreate the primary current signal as shown by the purple curve in Figure 2a.

Figure 2

Improving the technology

To improve the technology even further, transducer manufacturers often combine zero-flux technology with the closed loop principle, shown in Figure 2b. The fluxgate element is placed in the air gap and when it measures the magnetic field, the current output is re-injected through the secondary winding which then generates a magnetic field on the opposite direction. Using this method, the magnetic field experienced by the fluxgate is always zero, eliminating offset and linearity issues.

Four main zero-flux topologies currently exist on the market; see Figure 3. The first (Figure 3a) is based on a magnetic core with an air gap, plus the secondary winding. It resembles a closed-loop Hall Effect current transducer, where the Hall element in the air gap has been replaced by the fluxgate. The main benefit is a good offset drift.

The second topology (Figure 3b) is a single core which takes the role of the fluxgate element. As there is no air gap, one of the key benefits is its EMC robustness and also its high resolution. But as the saturation of the core occurs quickly, the bandwidth is limited to a few Hertz.

The third topology solves this issue by adding a winding core (Figure 3c) which only measures the AC signal, as with a current transformer, offering all the benefits. However, sometimes the performance is still not enough, so there is one last topology, called “balanced core” (Figure 3d). Here there are two fluxgate elements placed in opposition, as in a Wheatstone bridge. Therefore, whatever the external environment conditions – such as EMC or temperature variations – there is a natural passive compensation between the two sensing elements. Thanks to this, it is possible to achieve a measurement accuracy of 1ppm – even in harsh environments.

Figure 3

Danisense solutions

By combining complex magnetic performance with advanced electronics Danisense provides efficient and precise solutions that match the requirements of worldwide customers in demanding industries. Danisense was founded in 2012 and today is based in Denmark and Japan. The company’s founders and key employees are highly-experienced, with specialised knowledge about high-precision current transducers, enabling Danisense to create solutions that enable its customers to quickly and easily measure AC and DC currents with accuracies to 1ppm. Its products are of the highest quality and have an extremely flat frequency response and outstanding DC stability.

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