Wolfram Teppan, LEM, Switzerland, WTe@lem.com,
Dominik Schläfli, LEM, Switzerland, DSc@lem.com
Abstract
A high temperature current transducer with enhanced rejection of external magnetic fields is
presented. Working principle, design, and application (aircraft actuators) are explained. The
main features are small size, enhanced rejection of external magnetic fields and − most important
for the application − an operating temperature in the range between -65 °C and
225 °C. Results of FEM simulations are compared with measurement results. The performance
at static and dynamic operating conditions over the temperature range is presented.
The research lead in this project has received funding from the European Union Seventh
Framework Programme (FP7/2007-2013) under grant agreement ACP8-GA-2009-243119
"CREAM".
1. Introduction
The current political, environmental and economic trends in the field of air transport are leading
to the All Electric Aircraft (AEA) of the future. The goal of this concept is to eliminate as
many hydraulic power sources and complicated circuits of high-pressure hydraulic lines as
possible and to replace them by electrical actuators. Reliable electric actuators are one of the
technical bottlenecks for realizing this ambitious technological vision of the AEA. Those electrically
powered electro-mechanical actuators (EMAs) − actuators for the control surfaces, for
the braking system, for the landing gear, for thrust reversal or various pumps close to the engine
−are located in harsh environments. They contain an electrical servo drive that needs a
precise torque control and therefore it also needs current transducers (Fig. 1).
Fig. 1. Overall View of the Measuring Head: a CAD View on the Left, an Early Sample on the Right
The typical ambient temperatures can be as low as −65 °C in the wings and as high as
200 °C on the outside of the actuator if the application is close to the engine. No standard
current transducers are available for such a temperature range. In a research effort that has
been started in the context of a European project called CREAM [1], the design of such a
transducer has been included as one of the goals. This work is concentrating on the design
of a robust enhanced measuring head; the electronics used is similar to the ones used in
commercial products which have been shown to work at 200 °C for short periods. For a reli-
able long-term function in a future product, an ASIC manufactured using a high-temperature
technology would be needed.
2. Measuring Head
Because of the high temperature, special materials like LCP resins for the plastic parts and
polyimide insulated winding wire are used.
2.1. Parts
Like in any closed-loop current transducer, the main parts are made from highly permeable
alloys and plastic parts as insulators and for structural purposes.
Main Magnetic Core
The main magnetic core (Fig. 2) is designed as a stamped and folded part made from sheets
of a high permeability alloy with a thickness of 0.5 mm. As not too many parts are needed for
the project, EDM (electric discharge machining) had been chosen to obtain the flat shape;
the final form is obtained with prototyping tools. The parts have been annealed to obtain their
best magnetic properties.
Fig. 2. Magnetic Core, Left: CAD Model, Right: Prototype
The lower hollow portion of the core receives the fluxgate and the secondary winding is
wound around it; the flat upper portion closes the magnetic circuit together with the lateral flat
portions
The shape of the core has been designed with some distinguishing features with respect to
known structures: as the whole magnetic core is made from only one piece, there is no parasitic
air-gap that reduces the shielding effect of the core with respect to external magnetic
fields; the cavity for the fluxgate is closed with the exception of small slots and the openings
near the lateral portions. As those openings are rather small, they do not affect the shielding
effect too much.
Fluxgate Core, Fluxgate Coil Former, Coil Support and Coil Former
The fluxgate core is a simple rectangle made from a very soft magnetic amorphous alloy. Its
dimensions are approximately 10 mm × 1.5 mm.
It is shown in the next paragraph, positioned on the fluxgate coil former (Fig. 3, left). This
small coil former is an injected plastic part. A prototype tool for this part and the other plastic
parts (a multi-cavity tool for cost reasons) has been used.
Fig. 3. Fluxgate Coil Former with Core, Coil Support and Coil Former Half
For series production, the pins would be inserted by an automatic tool added to the injection
machine; for the prototype quantities they are inserted by hand before the mold is closed for
an injection cycle. The pins of the final version are not bent so that the part can be soldered
in plated through holes of a PCB.
The coil support (Fig. 3, center) has been added to facilitate the assembly of the measuring
head. Because the coil former needs to rotate in the aperture of the magnetic core, no pins
can be added to this part (see below). In addition, the part enables the mechanical fixing of
all the parts of the measuring head until their final assembly on its substrate. For the pins of
this part, the same remarks apply as for the fluxgate coil former.
The coil former needs to be assembled around the fluxgate side of main magnetic core.
Therefore, it is composed of two identical halves (Fig. 3, right) that are kept together by clips
and finally by the tension of the winding wire.
For the winding process of the secondary coil, the coil former is rotated with the help of the
molded teeth by a sprocket of the winding tool.
2.2. Assembly
Fluxgate Winding
The fluxgate winding (Fig. 4) is a standard linear coil that can be produced at low cost.
Fig. 4. Wound Fluxgate
For high volume production, the interconnection of winding wire (polyimide insulated) and
pins would be realized fully automated by arc welding, for the prototypes, it is made by wrapping
by hand and soldering with high temperature solder.
Assembly of Magnetic Core, Coil Support and Coil Former Halves
The assembly of the measuring head is done mostly manually, only the winding is done by a
special semi-automatic tool. The coil support is assembled with the main magnetic core, then
the first and the second coil former half is added (Fig. 5).
Fig. 5. Assembly of Magnetic Core, Coil Support and Coil Former Halves
Care must be taken in order not to bend the magnetic core during this and the following
steps; otherwise the magnetic properties of the core may be degraded.
Secondary Winding, Final Assembly
The secondary winding has to be applied by a special process that is known for so called
common-mode chokes, which are standard components for EMC filters. This winding technology
is well proven for lower turn counts; the challenge here is to use it at the defined turns
count of 1500.
Fig. 6. Partial Measuring Head Assembly with Secondary Winding
After this step, the fluxgate is inserted. It is held in place by the clips on its tip, and it blocks
the rotation of the geared coil former by notches that engage between its teeth.
At last, the measuring head is mounted in a power electronics module and the primary conductor
is added. Its actual shape is slightly different and it is insulated.
Winding Tool
This small machine consists of two main parts, a driving mechanism with holders to turn the
fluxgate coil former and the secondary coil former, and a guide for the enamelled copper wire
that translates according to the position where the wire touches the partially completed winding.
Both parts are driven by small servo motors with gear boxes (Fig. 7, left).
Fig. 7. Winding Tool, Fluxgate Holder
The holder for the fluxgate is shown in more detail on the right side of Fig. 7.
The holder for the secondary coil former is more complicated because this part rotates in the
aperture of the magnetic core (Fig. 8).
Fig. 8. Parts of the Winding Tool for the Secondary Winding
2.3. Electronics, Specification
The electronic circuit that provides the functions needed to operate the two coils of the
measuring head and to provide an output signal is based on a standard schematic provided
in the datasheet of the ASIC used [2].
This is the specification of the complete transducer:
Nominal current (rms): 14 A
Measurable peak current: 25 A
Common mode voltage transients: 40 kV/µs
Power supply voltage (limits: ±5 %): 5 V
Supply current, idle (max., occurs at min. ambient temperature): 20 mA
Supply current with primary current: 20 mA +IP/1500
Maximum supply current: 37 mA
Working ambient temperature range: −55 °C to 220 °C (short term)
Sensitivity: 58.5 mV/A
3. Measurements
3.1. Static Error Over Temperature
With a first prototype, the static error has been measured at 25 °C only. The static error is
measured in the usual way, a primary current is applied and the corresponding output voltage
as well as the output of a reference transducer is measured.
The different values for the primary currents are applied in a cyclic manner with the highest
amplitudes at the beginning of the test. By this test method, the static behavior of a transducer
can be assessed in a short measurement time.
Fig. 9. Static Error at 25 °C, Early Prototype
The outer lines shown in Fig. 9 are from cycles with peak currents of ±70 A (five times the
maximum current, this shows the influence of overloads)
If the errors would be referred to the maximum values of the corresponding cycles, the maximum
values would be below 0.1 %, a value that is typically achieved by high performance
industrial transducers.
A test with a sample made with high temperature materials shows the sensitivity variation up
to 220 °C (Fig. 10).
The visible slope change corresponds to a sensitivity change of roughly 0.35 % between
room temperature and 210 °C. Still, the temperature of the electronics was 25 °C for all
measurements.
Fig. 10. Static Error at Temperatures Between 35 °C and 210 °C
3.2. Transient response
The transient response with a slope of the primary current 60 A/µs from 0 to 25 A is shown in
the figure below.
Fig. 11. CREAM Transducer – Transient Response
The distance between the envelopes is due to the output ripple of the transducer that will be
3.3. Frequency response
The Bode plot below (Fig. 12) shows the frequency response of the CREAM current transducer
3.3. Frequency response
The Bode plot below (Fig. 12) shows the frequency response of the CREAM current transducer
suppressed by the filter in the CREAM application.
Fig. 12. Frequency Response
Those values are typical for a closed-loop current transducer with a magnetic coupling like in
this case. Because of the limited bandwidth of the application, the decreasing amplitude will
be masked by the frequency response of the low-pass filter in the A-D converter circuit.
3.4. Immunity to external magnetic fields
The measured values for the immunity to external magnetic fields are roughly 3 times better
than the values that can be reached by similar compensating current transducers on the
market. At 100 Hz and 1 mT, the measured error was roughly 130 mA/mT (referred to the
primary current). Simulations predicted even better results; the differences have still to be investigated.
4. Conclusion
A design concept for a current transducer for the use in harsh environments is presented and
validated. Such transducers will be needed for actuators in future "all electric aircrafts" (AEA)
5. Acknowledgement
The research lead in this project has received funding from the European Union Seventh
Framework Programme (FP7/2007-2013) under grant agreement ACP8-GA-2009-243119
"CREAM". The CREAM project addressing the area of "Aeronautics and Air Transport" (ATT)
started on the 1st of September 2009 and will last 36 months.
6. References
[1] CREAM project website: http://www.creamproject.eu/
[2] Texas Instruments datasheet of the DRV401: http://www.ti.com/lit/ds/symlink/drv401.pdf
