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Autonomous Wireless Current Sensor for Aircraft Power Lines

Periodic Reporting for period 2 - AMPWISE (Autonomous Wireless Current Sensor for Aircraft Power Lines)

Reporting period: 2019-08-01 to 2021-07-31

AMPWISE is a Cleansky 2 project.

The objective of the project is to develop an energy autonomous wireless smart and low-cost current sensor for monitoring of electric lines in the context of the coming generation of aircraft, which relies on composite materials for the fuselage. A consequence is that the fuselage becomes non-conductive electrically, thus it cannot be used to carry the return current. Rather, structural elements such as beam rails which form the skeleton of the aircraft are conductive and therefore are candidates to carry the return current. However, the effect of electrical current flowing through these elements is not well known, which brings the need to be able to measure it during the aircraft lifetime, without having an adverse impact on weight and complexity.

AMPWISE completed in July 2021. The resulting wireless current sensor prototype has reached TRL4 while some of its components have achieved high level of maturity. For instance, the AC and DC sensor technology is integrated in an ASIC developed and produced by project partner SENIS. Due to lack of 4.2 GHz transceivers on the marketplace, the AMPWISE wireless sensor communicates on the 2.4 GHz ISM band. Thanks to its adaptive TDMA protocol and optimal implementation, the communication system consumes less than 25 micro-amperes in average while taking and transmitting one AC and DC measurement every five minutes during flight. The accuracy of the electrical current sensor is 1%. The system is self-powered by an inductive energy harvester which generates 0.4 mW from a 25 A RMS 360 Hz current flowing in the aircraft structure.
The system architecture was done in WP1.

The communication system was provided by WP2. AT the project start, partner CSEM built a demonstrator of a WAIC band 4.2-4.4 GHz transceiver using a frequency shifter coupled with an Atmel AT86RF231 radio compliant with IEEE802.15.4. The tests show that the demonstrator exhibits a packet success rate about 5% better than the radio operating without the frequency shifter. This is probably due to the higher interferences in the 2.4 GHz band. The drawback of the demonstrator is that it is bulky, thus it was decided to switch to the 2.4 GHz ISM band for the final prototype. The result is a 25 x 25 mm PCB which core component is a Nordic Semiconductor nRF52840 micro-controller plus 2.4 GHz transceiver system-on-chip. The PCB includes power management electronics which controls the charge of a super-capacitor from the low-power and irregular supply coming from the energy harvester realized in WP3. The network topology is a multi-star topology, with each star controlled by a Wireless Data Concentrator (WDC). The communication protocol is a TDMA mastered by the WDC. The protocol adapts its duty cycle automatically depending on the demands from the application. The implementation on the wireless sensor is optimal thanks to a real-time clock which minimizes deviations and allows for the smallest possible guard time when waking-up the radio to communicate. The wireless sensor node consumes only 25 micro-amperes in average when acquiring one AC-DC measurement every 5 minutes. The communication PCB also has its own secure elements to store cryptographic keys. A security concept was established and proposed several operational solutions for a secure and practical distribution of the cryptographic keys.

The autonomous energy supply was the objective of WP3. The power supply design and development delivered an inductive energy harvesting coil and core structure as well as a new, advanced design of the power management circuit, with novel techniques in cold-starting, rectification and storage. This circuit was implemented on WP2 communication PCB and on the interface sensor node board developed in WP5. In integration tests, the performance of the power supply was demonstrated to be adequate for supporting the use case specifications, for bus bar currents with flow as low as 10 A RMS, at 360 Hz.

WP4 objective was the design of the electrical current sensor. The total current consumption at 5VDC power supply is only 7mA when active, almost zero when inactive. In the second project period, the design was adapted to cater for lower currents (25 A, down from 200 A) after a revision of the estimates done by the topic manager. DC measurements are taken from Hall elements. AC measurement are taken from pick-up coils. The linear characteristic and 1% precision of the sensor prototype were demonstrated and reported in project deliverable D4.3. The sensor technology was exploited industrially its designer SENIS who developed an ASIC and out of it. Data pre-processing was implemented by CSEM on the micro-controller (sensor switches control and averaging for DC, RMS computation for AC).

A control and monitoring application was delivered by WP6. It is written in Python and allows the easy configuration and control of the wireless sensor network as well as measurement data visualization and logging.

Partner SERMA developed an interface PCB in WP5 to integrate the communication system, the sensor and the energy harvester. The integration PCB handles the energy harvester output, caries the antenna and the 5V current sensor power supply. It relays the control and analogue signals between the communication system and the sensor. SERMA also performed the functionality and qualification tests of the prototype at the project end. The resulting assessment is that the feaasibility of the technology is proven, and that the prototype reached TRL4. A path for TRL increase is established, with the possibility to integrate the sensor technology of SENIS, the communication system and the energy management into a unique package.

Dissemination: at least 4 scientific publications, one industrial conference and presentation and demonstrations for high-school and undergraduate students. Exploitation: partner SENIS created an ASIC. The consortium will promote the results beyond aviation, in train transportation and energy.
Progress along the following 3 axes:

Energy harvesting
- state of the art: power line harvesting proof-of-concept demonstrated
- AMPWISE progress: harvesting from return current structure (bus bars) and advanced design of the power management circuit, with novel techniques in cold-starting, rectification and storage.

Current sensors:
- state of the art: current transformer, Rogowski Coil and through voltage measurement on a resistor.
- AMPWISE progress: open-loop, Hall-based current sensor with a slotted ferromagnetic ring. Compact AC and DC sensor suitable for bus bars. The AMPWISE current sensor is the enabling technology of partner SENIS ANYCS product implemented on an ASIC.

Wireless communications:
- state of the art: adaptive TDMA, coexistence and robustness based on channel hopping, security based on pre-distribution of keys
- AMPWISE progress: WAIC band demonstrator, dedicated optimal antennas. Optimized TDMA implementation on a compact 2.4 GHz transceiver with hardware/software co-design and smart management of the duty cycle by the concentrators. Minimal energy consumption.

The programme of which AMPWISE takes part (Clean Sky 2) aims at further reducing energy consumption and pollution emissions. Furthermore, the technology developed by AMPWISE has a large application potential, well beyond aeronautics: train, electrical vehicles, energy supply, etc, with possible positive impact on safety and efficiency.
Inductive energy harvester
2.4 - 4.3 GHz frequency converter implementation
Wireless sensor node architecure
Sensor analogue part AC measurements tests results
AMPWISE network architecture
block diagram of the inductive energy harvester
assembly of the current sensing boards. The small PCB with the Hall effect sensors is plugged onto t
Test setup with two wireless sensors mounted on the bus bar
Sensor analogue part DC measurements tests results
Harvested voltage and power vs. current flow in aircraft structure
Wireless sensor network server user interface
The concept of inductive harvesting from a current-carrying structure. Coil winding and core shapes