Flux-Gate

The Danisense flux gate current sensor technology is based on a closed loop system, powered by a flux gate as the magnetic field detector.

All Danisense current transducer products are based upon the fluxgate principles. Our parameters are the following:

  • Fluxgate principle
  • Excellent linearity: 1 to 3ppm
  • Ultra-stability: offset vs. time < 1 ppm/year
  • “ppm level” accuracy
  • Current or voltage output types
  • Very large and flat bandwidth
  • Recommended as current probes for power meters
  • Suitable for use in MRI, accelerators
  • Extended operating temperature range from -40ºC to +85ºC
  • Recommended for current measurements in renewable energies or automotive

 

The magnetic field in the toroid generated by the primary current (Ip) is counteracted by the compensating secondary current (Is) generated by the integrator.

The flux gate detects magnetic fields in the toroid from DC to less than 100 Hz at sub ppm levels and tells the integrator to compensate them out.

At higher frequencies, the feedback winding (Nfb) detects magnetic fields in the toroid at ppm levels and tells the integrator to compensate them out as well.

The secondary current (Is) is therefore proportional to the primary current (Ip) with the ratio Np:Ns

Achieving very precise measurement

There are many different types of current measurement technologies, from basic shunt and Hall.

Effect devices to more complex systems. The determining factor is usually the accuracy required, with emerging eV, solar, traction and power grid applications demanding accuracy levels to 100s of ppm, while medical MRI equipment and physics institute accelerators can require single figure ppm performance. At these higher levels, simple devices cannot deliver and Danisense is offering DC and AC current sensors based on its fluxgate technology which provides measurements down to 1ppm.

diagram comparing conventional pick-up coil to flux gate

Danisense proprietary fluxgate is a closed loop compensated technology with fixed excitation frequency and second harmonic zero flux detection. The magnetic field in the toroid generated by the primary current is counteracted by the compensating secondary current generated by the integrator. The Fluxgate detects magnetic fields in the toroid from DC to less than 100 Hz at sub ppm levels and tells the integrator to compensate for them. At higher frequencies, the feedback winding detects magnetic fields in the toroid at ppm levels and again tells the integrator to compensate for them. Figure 1 compares how the current behaves normally (red line) and then through the Fluxgate element (blue line). Through a pick up coil wound on standard magnetic materials, the current is linear until saturation. However, if magnetic materials for the core are chosen for their deep saturation and sensitivity, the graph changes to show a very definite and identifiable step change.

diagram showing two fluxgate elements compared

Figure 2 is a simplified plot showing the effect of applying a square voltage (left) resulting in sharp positive and negative signals, then if a primary DC current Ip through a wire conductor is introduced, a DC magnetic field is added which shifts the signal (right). Finally, advanced signal processing is applied and by using second harmonics the values for the new signals can be extracted to provide a measurement of the current in the conductors and its DC current value. This is the basic FluxGate (or zero-flux) technology. (This can be supplemented with an additional AC feedback winding to extend the frequency range of AC current measurements.)

A simple, single Fluxgate structure and magnetic core will provide accurate DC and low-frequency AC measurements but has very low bandwidth so is not suitable for full-bandwidth AC measurement.

Furthermore, the effects of temperature and other environmental conditions mean that the magnetic field can drift. Some manufacturers use electronic compensation circuitry, which adds cost and complexity and can also be prone to inaccuracy. By contrast, Danisense employs a dual balanced

Figure 3a

fluxgate block diagram

Figure 3b

fluxgate simplified signal plot diagram

Fluxgate structure which employs two magnetic cores in opposition, similar in concept to a Wheatstone Bridge. This provides natural compensation, eliminating the effect of any drift. The block diagram is shown in Figure 3a and the simplified signal plot in Figure 3b. However, in order for the two Fluxgate elements to balance each other, they must be perfectly matched.

flux gate frequency characteristics diagram

Danisense’s major product line is the DS series units, and products span 200-10,000A (DS200 – DR10000). Units feature excellent linearity (0 to FS) and have an offset stability with time of less than 0.1ppm/month and a flat bandwidth (DC to ~ several hundred kHz). Phase shift for AC measurement is industry-leading – see Figure 4 – and both current output and voltage output models are available. Aluminum housings provide immunity to electro-magnetic and common mode noise (dv/dt).

New PCB-mount current transducer versions

Most recently, Danisense has launched its ultra-stable, high-precision DP series of PCB-mount current transducers for isolated DC and AC current measurement up to 72A. With a footprint of 64.9 x 60mm, a height of 32mm and lightweight of 250g, this compact device is suitable for 1U power supplies and other space-constrained applications.

Benefitting from Danisense’s closed-loop, compensated Fluxgate technology which provides a fixed excitation frequency and second harmonic zero flux detection for best in class accuracy and stability, DP series PCB-mount current transducers feature a maximum linearity error of 10 ppm and a measurement resistance of up to 100Ω at full scale. Devices are programmable for 12,5A, 25A, or 50A usage via the PCB layout. Applications include MPS for particle accelerators, stable power supplies, precision drives, battery-testing and evaluation systems, power measurement, and power analysis.

Summary

By combining complex magnetic performance with advanced electronics Danisense provides efficient and precise solutions that match the requirements of worldwide customers in demanding industries. Perhaps the most demanding application for any current sense transducer is within the power supply for particle accelerators, so the company is especially proud that it has an active program with CERN.

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