Overview
Today, wired sensors are used for monitoring aircraft engines, structures, gearboxes, etc. Wireless Sensor Network (WSN), i.e., smart sensors with radio interfaces, promise unprecedented operational benefits; such as, reduced airplane sensor wiring costs and weight as cabling is limited to specific scenarios and the flexibility to be deployed on-board aircraft without requiring a redesign of the data wiring layout.
To this end, the StrainWISE project developed an advanced wireless sensor platform to which a strain gauge can be connected, being able to operate in the wings of the aircraft for a long time, thanks to the use of embedded energy harvesters. The technical strategy was to build an integrated autonomous platform from existing, already proofed hardware and software, except for the energy harvesting part, which are specifically tailored to meet the objectives of the call.
The resulting platform clearly advanced the state of the art in a number of aspects:
- The WSN operations will always be tuned to their maximum efficiency resulting in lower energy needs, thus smaller form factor;
- Efficient energy scavenging will permit further reductions in size and weight;
- Communication reliability will be improved by using all nodes as potential relays;
- Management will be reduced to a minimum by automatic configuration and the use of standard protocols (SNMP).
A number of specific tests were performed in order to meet the requirements, limitations and constraints imposed by operation in airborne environments.
The consortium was made of three highly qualified complementary entities. CSEM brings its expertise in WSN and ultra-low power electronics. SERMA, an experienced actor in the aeronautics technology, will bring expertise in the aeronautics environment and constraints, as well as production and test facilities. Imperial College London, one of the world leading laboratories, provides the scavenging and energy management expertise.
Funding
Results
Executive summary:
The STRAINWISE project realised a wireless and energy autonomous strain monitoring system for commercial aircraft. It was the result of the integration of state-of-the-art low power electronics with innovative solutions in the fields of ultra-low power wireless communications and thermo-electric energy harvesting.
The system aimed at providing the aircraft operators with a new tool for the predictive maintenance of airframes, with minimal weight impact and low usage costs. The main benefit of such tools was a significant reduction of maintenance costs by replacing regular checks that imply the automatic substitution of elements that have gone through their quota of flight cycles or hours by the analysis of the load endured by the aircraft structure during the flights. Then, maintenance actions can be decided for the aircraft parts that are indicated as potentially weakened or damaged by the analysis. The minimal weight impact stems from reduced cabling due to the wireless nature of the system. The usage and maintenance costs of the load monitoring system are low thanks to the combination of energy scavenging and ultra-low power communications as this eliminates the need for batteries and their regular replacement and assures a long system life.
The project targeted two specific use cases. The first use case addresses load monitoring in widely distributed pilot points of the Vertical tail plane (VTP) of commercially exploited aircraft. Measurements sessions are triggered by the Aircraft avionics when a flight sequence that may involve heavy loads on the VTP is detected from the observation of other flight parameters. One expects up to 10 measurements sessions of 30 s per flight. The second use case describes the monitoring of strain in the landing gear. In this case, measurements are acquired continuously during the landing phase from landing gear extension until the aircraft has reached a low speed on the ground. The specified maximal data acquisition duration is 500 s.
For both use cases, specified measurements sampling frequencies are between 120 and 500 Hz. All data samples have to be stamped with the aircraft time with a synchronisation error of at most 1 ms.
A specific Time division multiple access (TDMA) protocol had been developed to fulfil the requirements for the data acquisition chain. It automatically adapts its duty cycle to the traffic needs in order to minimise energy consumption, so that the autonomous power supply is sufficient. During flight, the maximal wake-up delay is 0.5 s. The protocol was reliable because it supports repetitions. Moreover, it integrates a synchronisation mechanism that is very light in terms of protocol overhead. Experiments made with the current implementation have shown 100 % packet arrival success rate and a maximal synchronisation error of less than 600 µs on the data timestamps within a single cell. If several cells are used the synchronisation error is still below 1 ms using a simple synchronisation algorithm for the network wired part.
A power supply based on thermoelectric energy harvesting that utilises a Heat storage unit (HSU) partially filled with a Phase change material (PCM) was designed and tested. This method, initially proposed by the European Aeronautic Defence and Space (EADS), was previously described in deliverables D1.1 and D5.1. Various types of PCMs were tested based on the flight temperature profile provided by Airbus. The temperature gradient was converted into usable electrical power using Thermoelectric generator (TEG)s. In a flight cycle, phase change occurs twice which causes the generated voltage from the TEGs to switch polarity. The TEGs were observed to generate approximately 1 V peak voltage for a temperature gradient of 20 degrees of Celsius. Rectification of the TEG voltage was performed using a new and custom-designed rectifier topology. The power management electronics developed for this application enabled the harvested energy to be used to recharge an internal battery and also to supply electrical power to transmit strain gauge data wirelessly.
The system built within the STRAINWISE project comprised the following entities: an energy autonomous sensor node, a Wireless data concentrator (WDC) and a Wireless sensor network (WSN) server. All entities have been fully implemented in hardware/software and successfully went through functional and qualification tests. Specifically, the qualification tests showed that the devices can sustain heavy vibrations (DO-160 Cat. R Curve C1) and extreme temperatures (+85 to -55 degrees of Celsius) whilst respecting a challenging maximal electro-magnetic field emissions constraint (DO-160 Cat. H Curve C1). A complete STRAINWISE system has been delivered to Airbus for in-flight tests.