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RealTime Monitoring Power Reduction Gearbox

Final Report Summary - RTMGEAR (RealTime Monitoring Power Reduction Gearbox)

Executive Summary:
Towards the Advisory Council for Aeronautical Research in Europe (ACARE) goals, which intends to reduce aircraft CO2 emissions and fuel consumption by 50% within the next decade, the Clean Sky program has focused its intervention in several concurrent developments, such as aircraft aerodynamics and weight, and highly efficient engine concepts. In this framework, Snecma is leading the Clean Sky SAGE2 activities, to study the development of an innovative concept based in a geared open rotor engine that will end up as a breakthrough for the next 15 years, thanks to the improvements in the propulsive efficiency, without the penalties of any duct drag.
As an innovative high efficiency power gearbox is being developed by the Topic Manager, it is very important to keep the an early tracking of as many physical parameters as possible, in order to provide useful feedback data for design processes optimization; during operation, similar parameters will allow the aircraft to identify potential structural health problems, such as unexpected vibration, temperature and pressure variations, and metal debris in the oil.
Long term RTMGear' consortium developments aim at providing such capability for real-time continuous in situ monitoring of critical parameters for evaluating the health of critical components, such as shafts, gear teeth, structural components, and bearings. Although the current low TRL RTMGear activity was focused in lab applications, where monitoring physical parameters directly in rotating elements, and in harsh environments, were not possible with currently COTS technology.
The goal of this activity was to develop a telemetric system, composed by three independent Rotating Telemetry Units (RTU), to monitor the rotating elements of the gearbox. A Ring Gear Telemetry Unit (RGTU) and a Sun Gear Telemetry Unit (SGTU), both based on traditional temperature and strain sensors, and respective electronics circuits. A third, Surface Acoustic Wave Sensing Unit (SSU), exploring SAW passive sensors.
In the scope of RTMGear activity, the consortium developed/studied two different telemetry concepts, embracing several technical challenges requested by the Topic Manager, such as:
- Modularity: capability to adapt to different arrangements of strain and temperature channels, and to be applied in different components of the power gearbox, such as sun shaft, sun gears, and ring gear;
- Reduced dimensions: allowing for integration inside power gearbox;
- High speed sensor signals: acquire multiple strain gages with bandwidth in the order of 60 kHz;
- Resist to harsh environment:
> Rotating speed (up to 10,000 rpm);
> High temperature (up to 200 ºC);
> Oil mist.
The RTMGear project was developed in two parallel branches, exploring different approaches to the same problem. A complete new development of an innovative concept for a telemetric system, consisting in new designs for electronics, firmware and software, mechanical, and structural simulation, capable of satisfying the needs of both Ring Gear Telemetry Unit (RGTU) and a Sun Gear Telemetry Unit (SGTU). And, a parallel study on a Surface Acoustic Wave Sensing Unit (SSU) solution was carried out, selecting the best available SAW technologies and testing them applied to the power gearbox scenario.

Project Context and Objectives:
Acquiring signals from a broad range of sensor types in lab environment is a common need among most technological companies, and many manufacturers provide top quality equipment that can do the job very efficiently. The problem comes up when the need does not fit inside the major manufacturers mass market range, or the environment is not suitable for these equipment, such as limited volume for installation, high temperature, high acceleration, or the specific situation of RTMGear, which intends to measure physical parameters directly in the rotating parts of the power gearbox. The RTMGear project intends to provide viable solutions for these type of applications, and to effectively tackle the technical challenges, it was split into three major sequential activities, namely specifications, system design and testing.

The specifications work package was a very intensive development and conceptual validation task, allowing the project team to clearly understand the technical challenges of RTMGear system, on both technologies under development and evaluation, namely the telemetric system based in electronics for harsh environment, and SAW sensors.
At the very beginning stage of the project, Active Space Technologies developed and manufactured a simplified test rig to perform unitary tests on the different technologies under evaluation, such as temperature sensor based in SAW technology, inductive power transfer and bi-directional RF communications. The most relevant outcomes of this testing phase were the following:
- Available SAW sensors with common RF dipole antennas work properly while in line-of-sight, but when installed in the rotating shaft, the measurements were not very accurate, and at a certain rotating speed (yet far from the required top speed), the communication between the interrogator and the sensor could not be established at all;
- The wireless power transmission concept was tested with large diameter coils, surrounded by steel enclosures, but the efficiency was very low. So, the concept evolved to a solution improved with ferrite beds, to help guiding the magnetic field from the static to the rotating coils, which increased the efficiency to a figure in the order of 50%;
- The first concept of the 360º ring antennas were tested with a portable RF transmitter, at the rotating side, and a spectrum analyser in the static side. This test clearly demonstrated that the concept was not the ideal solution for RTMGear, because the angular movement modulates a variable undesired pattern in the RF signal.
After the first tests, the specifications document was baselined, and the initial concepts evolved to the preliminary design, implementing the improvements required to correct the identified limitations, such as:
- Design and simulate the behaviour of a 360º ring antenna to be used with COTS SAW sensors;
- Design and test a new electronic circuit to stimulate the inductive power coils, and redesign of the coils bracket, in order to improve the energy transmission, without the need for the ferrite beds;
- Redesign the RF couplers and corresponding RF signal transmitter, to eliminate the modulation effect observed in the tests performed with the initial concept.
The preliminary design also consisted of an extensive SOTA analysis of harsh environment technologies, crossing the assessment of solutions for high temperature, components in bare die format, ceramic and Rogers PCBs.

Starting from the system architecture defined and validated in the preliminary design phase, all the electronic and mechanical components were re-evaluated according to the specific requirements of RTMGear final prototype, such as dimensions, mass, availability/lead time, performance, bare die format, and footprint. This was a long iterative task, starting by the selection of the best technical solution, followed by the supplier identification and inquiry, and ending up choosing the component that best satisfies the trade-off between performance, price and availability.
Having all the electronic circuits and mechanical design stable, reviewed and approved, the PCBs layouts were designed in Altium Designer, considering the usage of a Rogers for the telemetric system PCBs. The manufacturing of the PCBs was sub-contracted, but all the assembly was performed in-house, namely die and wire bonding. This was a process with several unexpected problems, having had a large impact in the project schedule. However, the final results were very good, and the wire pull tests results showed that the process is beyond usual parameters defined in military standards.
The assembly process was performed in parallel with the unit testing activities, in order to ensure a stable progress in the first prototype. Instead of assembling everything at once, which would increase the likelihood of ruining the very expensive prototype. This approach showed to be a good strategy for the sake of progressing steadily, but had a large impact in the project schedule.

SAW sensing design took into consideration the outcomes of the preliminary design phase. The torque and temperature sensors were purchased and the 360º ring antennas were designed and manufactured internally. Major effort during the implementation phase was related to the design of the static and dynamical test set-ups, which included mechanic system design and implementation. Towards this end, IT team established a cooperation with the Mechanical Engineering Department in Aveiro University, which specified and build the required set ups, and supported the integration of the sensors in the load cell testing unit.

The telemetry unit testing has been split in three phases, Phase 1: Static Functional Testing, Phase 2: Rotating Functional Testing, and Phase 3: Environmental Testing. Phase 1 testing has been completed and Phase 2 is about to start. It is worth noting that the test results are very promising and that there were no hardware failures, nor fixes or patches to the electronic prototype. All critical subsystems were tested successfully. These include wireless power transmission, data communications, both uplink and downlink, and signal acquisition. The rotating unit is able to be powered via the wireless power link while receiving commands from the static unit. It can send data through the RF link at a considerable data rate while digitizing a large possible variety of sensors with its analogue acquisition capability.
Due to time constraints, testing phases 2 and 3 were not performed in the framework of RTMGear. These final testing stages will be done after CSJU official RTMGear activity closure, with recourse to Active Space Technologies internal funding. The final results are extremely important to Active Space Technologies and critical for the success of the RTMGear system, and therefore they will be shared amongst CSJU and the Topic Leader.

SAW system tests were carried out using universal test machines, taking advantage of the fruitful cooperation with Mechanical Engineering Department of Aveiro University, able to provide a viable reference for the quantities under measurement. The test results included the calibration of the SAW torque sensing system under static conditions. These results allowed to remove errors due to the assemblage of the sensors and measure the static calibration function from strain (µm/m), to relative frequency.

Project Results:
Due to the nature of the RTMGear activity, which consisted of the evaluation of two different technologies to perform similar functionalities, it makes sense to present the outcomes separately.
The most relevant S&T outcomes of the telemetric system, developed under the scope of the RTMGear activity are:
1. Innovative small dimension wireless slip ring concept for low power (up to 10 W) and high speed data transmission (up to 10 Mbps), ready to operate at harsh environments characterized by high temperature (up to 150 ºC) and high acceleration (up to 40,000 g);
2. Development of electronic devices for high temperature, using high quality bare die components, supporting up to 200 ºC;
3. Measurement and transmission of information from several different sensors, such as: thermocouples, PT100 or PT1000, accelerometers, strain gages, torque sensors, and angular position;
4. Acquisition bandwidth up to 60 kHz, with 24 bits resolution;
5. Optimized mechanical design to allow for in-situ monitoring of high speed rotating shafts (up to 50,000 rpm);
6. Closed loop control of inductive wireless power transmission, for improved efficiency and power quality in the receiver side;
7. High efficiency design of the inductive coupler, reaching efficiencies above 80%;
8. Capability to adapt the design to measure parameters in several parts of gearboxes, such as: sun shaft, ring gear, and teeth of power gears.

The most relevant S&T outcomes of the SAW sensors study, developed in the scope of the RTMGear activity are:
1. SSUs can be used to measure temperature and torque in the shaft of a power gear box, however the full potential of the technology cannot be explored with COTS solutions. In particular, with the purchased solution:
a. The limited configuration options of the interrogation unit have a negative impact in the real acquisition rate;
b. Regarding the strain sensors, the design is inadequate for measurements under rotation: the U-FL connectors are glued in one extremity of the piezoelectric slab; under rotation the connector and cables apply the centrifugal force on the sensor, which works as a cantilever;
2. In order to use SAW sensors to measure temperature and strain/torque inside a gear box, custom-made solutions which meet the needs and constraints of the application scenario need to be designed and fabricated.

Potential Impact:
Due to the technical and challenging similarities between TSA and RTMGear projects (both supported by Clean Sky), the technical design and outcomes were shared between the two projects, which improved the overall quality and quantity of develop work. The synergies between the two projects enhanced the potential impact, the dissemination activities, and exploitation of results.

The results of the project advance the development and understanding of wireless power and wireless signal transmission for the aircraft industry, in particular for systems with high rotating speeds and high temperature. The benefits for noise reduction, reduced fuel burn and reduced associated environmental footprint are indirectly attained by measuring in-loco parameters of the most relevant components, such as power gearboxes, thus providing better characterization accuracy of the internal components, enabling the increase of the intervals between maintenance preventive interventions.
In fact, the goals of the Clean Sky JTI, as set by ACARE - Advisory Council for Aeronautics Research in Europe, are to demonstrate and to validate "the technology breakthroughs that are necessary to make major steps towards the environmental goals sets the European Technology Platform for Aeronautics & Air Transport and to be reached in 2020":
- 50% reduction of CO2 emissions through drastic reduction of fuel consumption;
- 80% reduction of NOx (nitrogen oxide) emissions;
- 50% reduction of external noise;
- A green product life cycle: design, manufacturing, maintenance and disposal / recycling.
Within the framework of Clean Sky, the results of the project have the potential to indirectly support the above mentioned objectives since it allows studying and understanding power gearboxes behaviour. This knowledge will enable optimization of aircraft gearboxes by measuring in-loco parameters, enabling the extension of maintenance preventive intervals, without compromising the safety of the aircrafts.

The dissemination activities of the project included presentation of the results in fairs (namely Le Bourget), press releases which resulted in several technical papers and presentation in conferences.
Active Space Technologies exhibited the running Clean Sky projects at the Le Bourget 2015 air show, where the objectives of the RTMGear project were presented to potential customers, and relevant industrial players.

The RTMGear project, which was included in Clean Sky's SAGE has a clear bridge to the programs Engine ITD and Fast Rotorcraft IADP in the framework of Clean Sky 2. Actually, Active Space Technologies is currently implementing i-Bearing (FRC) and was selected for negotiating i-Gear (FRC) within Clean Sky 2 contracts.
The long term goals of the above mentioned projects, among others, are to provide a strong contribution to the definition and development of a future end-to-end maintenance operation concept, to the condition monitoring business model, and to real-time operational analysis. Ultimately, the next generation fast rotorcraft is designed for 20-25 people or 2,500 kg payload, with a recurring cost below 1.5 times a conventional helicopter. These projects, leveraging on the RTMGear technologies, will enhance condition monitoring capabilities and directly contribute to reducing recurrent cost, namely maximizing the periods between maintenance and overhauls. Hence, enhancing condition based maintenance strategies.
From a technical point of view, the project is expected to contribute to the development of knowledge in the design and manufacturing of sensing, data acquisition, and wireless power transmission which are required by the European aeronautical industry to improve its competitiveness regarding sensing, deeper understanding and measurement of engines, gearboxes, and propellers behaviour. As a consequence, an optimization of engine and gearbox components and of propellers can be expected.

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