• By Daniel M Wells, AgustaWestland; Tiziano Frattini, AgustaWestland; David L Churchill, LORD-MicroStrain Sensing Systems; Stephen DiStasi, LORD-MicroStrain Sensing Systems
  • Posted Friday, July 24, 2015 - 10:00

Energy Harvesting Helicopter Rotor HUM System Development
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Development of a helicopter on-rotor HUM system powered by vibration energy harvesting

ABSTRACT
Helicopter transmission health and usage monitoring is mature and operating on many helicopters worldwide. Attention is now being given to improving the monitoring of rotor systems, in order to 1) further enhance safety by the early detection of incipient failures, and 2) reduce the maintenance burden by minimising or ultimately replacing high-frequency on-aircraft rotor component inspections. To realise these aims, rotor monitoring needs to go beyond the traditional track and balance management based on airframe vibration measurement and one approach is more localised sensing on rotor components. AgustaWestland is currently evaluating self-powered wireless sensing technology in a two-phase research effort under its Rotorcraft Technology Validation Programme. Firstly, AgustaWestland has conducted trials on an AW139 helicopter of a single wireless sensor node developed by LORD-MicroStrain Sensing Systems, which was located on the main rotor rotating swashplate – this has proven the key technology enablers of vibration energy harvesting and low-power/low-range radio-frequency data transmission from the rotor to the airframe. The second phase will be the development of a multi-node main rotor monitoring system with sensors mounted on selected rotor controls components, with embedded data processing in the nodes to characterise backlash in non-rotating bearings, and to reduce the volume of data transmission. The programme to take the technology forward to service implementations will focus on continuing development of wear feature detection and condition indicators, design studies for integration of the sensor nodes within the rotor system, and validation/verification.

         

1. GLOSSARY
ADC Analogue-to-Digital Converter, AW AgustaWestland, CAA (UK) Civil Aviation Authority, CI Condition Indicator, EASA European Aviation Safety Agency, GPS Global Positioning Systems, HM Health Monitoring, HUM/S Health & Usage Monitoring / System, IEEE Institute of Electrical and Electronic Engineers, kts Knots, L-MS LORD-MicroStrain Sensing Systems, mW milli-Watt, PC Personal Computer, PCL Pitch Control Link, RAM Random Access Memory, R&D Research & Development, RF Radio Frequency, RSSI Radio Signal Strength Indicator, RTD Resistance Thermometer Detector, RTVP Rotorcraft Technology Validation Programme, SCV Super-Capacitor Voltage, STA Synchronous Time Average, VDC Volts-Direct Current, VEH Vibration Energy Harvester, WSDA Wireless Sensor Data Aggregator, WSN Wireless Sensor Node

 2. INTRODUCTION
The primary focus in helicopter Health and Usage Monitoring (HUM) has been on transmission systems, to the point where transmission vibration monitoring is operating on many helicopter types in service today to enhance safety and assist maintainers.
To date, rotor HUM, or more specifically condition monitoring of the rotor, has largely been limited to post-maintenance and periodic assessment/control of track and balance, through measurements on the airframe of rotor 1/rev vibration and blade position. Over time, relationships between higher harmonics of 1/rev and the condition of some rotor components have been derived through service experience. Maintenance manuals typically contain component checklists for when airframe vibration exceeds empirically-based limits.

Of course rotor systems are also subject to routine on- and off-aircraft condition checking by visual inspection and wear measurement. The on-aircraft regime is usually at relatively high periodicity compared to other aircraft systems and often with disproportionate impact on aircraft availability.

Attention is now being given to improving rotor system HUM, both for enhanced safety and reduced maintenance burden. In 2008, the Civil Aviation Authority (CAA) in the UK reported a review of Rotor HUM[1] and concluded, inter alia, that:
  - Although the rate of [helicopter] accidents is declining, a few recent high profile cases have demonstrated that there is still significant safety benefit in main and tail rotor fault detection.
  - For many rotor faults, improved detection is unlikely to come from existing fixed-frame vibration measurements, and rotating-frame technologies should be investigated.

In 2010, the UK Air Accidents Investigation Branch report[2] of a main rotor blade spindle failure included Safety Recommendation 2010-027 that “EASA, with the assistance of the CAA, should conduct a review of options for extending the scope of HUMS detection into the rotating systems of helicopters”. EASA subsequently awarded a research contract to Cranfield University in the UK to review technology for improving the monitoring of both internal gearbox components and rotors.

In the same timeframe, AgustaWestland (AW) identified the possible benefits of focussed rotor condition monitoring in providing early warning of degrading condition at the component level, and initial cost/benefit studies indicated operating cost and availability benefits by reducing or ultimately replacing some of the relatively high-frequency and disruptive on-aircraft maintenance regimes for rotors. AW also identified that the sensing technology base had developed to the point where self-powered on-rotor HUMS was worth evaluating.

This paper describes an AgustaWestland research and development programme for the application of self-powered wireless sensor node (WSN) technology for on-rotor HUM, for which LORD-MicroStrain Sensing Systems (L-MS) is providing the experimental system. The paper first outlines AW’s programme and approach to on-rotor HUM, then describes the development of the WSN technology and a risk reduction flight trial of the core technology elements. Next, the plan for a follow-on flight demonstration of a multi-node experimental system is outlined, and the paper concludes with thoughts on the anticipated programme for taking the technology forward to in-service application.

 

3. AW ON-ROTOR HUM PROGRAMME AND APPROACH

3.1 On-Rotor HUM R&D Programme Outline
On-Rotor HUM is being pursued by AW in its Rotorcraft Technology Validation Programme (RTVP), a four-year research and demonstration programme for rotor technologies supported by the UK Government’s Technology Strategy Board, which commenced in 2010.

The overall aim of the On-Rotor HUM programme is to demonstrate generic technology that can be taken forward to product-specific application. The programme is structured as follows:
  - Requirements capture
  - Technology review and selection
  - Rotating controls wear feature development
  - Lab based system evaluation
  - Phase 1: risk reduction trial of single wireless sensor node.
  - Phase 2: development/trial of representative multi-node system.
  - Routemap for application-specific development.

AW selected the Department of Aerospace Engineering at the University of Bristol, UK as their academic partner, and they led the technology review, lab-based demonstrator and bearing wear detection testing and analysis activities.
AW selected L-MS as the technology provider for the experimental flight trials, due to their sector-leading expertise and experience in helicopter wireless sensing and energy harvesting.

3.2 On-Rotor HUM Approach
AW’s approach for On-Rotor HUM was driven by the requirements capture and technology review activities.

The requirements capture activity was conducted by AW and involved reviewing international civil accident/incident statistics and internal workshops. The conclusions of this activity were that:
  - Most rotor failures occur in main rotors rather than tail rotors and in the rotor hub and controls rather than blades.
  - Most benefit from on-rotor HUM will come from condition monitoring of mechanical components subject to wear, in particular bearings, eg swashplate duplex bearing, Pitch Control Rod bearings, rotating scissors bearings, damper spherical bearings.
  - Acceleration and strain will be the primary parameters for condition monitoring, but temperature and stiffness also need to be considered.
  - Initially, the main rotor should be addressed since this will show most benefit and is easier to do, compared to the tail rotor.
  - Usage monitoring by component level sensing is not required, mainly because other ongoing AW programmes on the logging of the as-flown operational aircraft usage spectrum are expected to provide improvements in component life tracking.
  - On-rotor monitoring needs to be self-powered rather than powered by the aircraft via an electrical slip-ring or batteries.

As usage monitoring is not being pursued, from this point the subject system is referred to as On-Rotor Health Monitoring (HM).

The technology review was conducted through literature and product searches, and determined that:
  - Vibration or strain energy harvesting is the most promising means of local power provision for helicopters, but application-specific implementations will be required.
  - However, the power available is not likely to exceed a few 10s of milli-Watts (mW) for a practicable size/mass of harvester mounted on a rotor, this therefore drives for a system with very low power requirements.
  - The IEEE 802.15.4 radio type is arguably the most applicable for wireless data transmission, since its base form is predicated on low-power/low-range operations and it is in widespread use. Nevertheless, the radio data transmission is likely to be the most power-hungry process in the WSN.
  - There are several existing low power microcontrollers available that are suitable.
  - Novel forms of low-power sensing technology need to be evaluated, particularly for strain.

AW’s derived approach for an On-Rotor HM system is therefore as follows:
  - It will consist of multiple independent wireless sensor nodes (WSNs) operating in a star network via an IEEE 802.15.4-based radio protocol with a data collector in the airframe (ultimately the central HUMS). A self-configuring mesh network is not deemed  
     necessary and in any case would add complexity and increased power requirements.
  - The WSN will be powered by an internal vibration energy harvester (VEH), acquire data from 3-4 sensors at most, conduct initial data processing for Condition Indicator (CI) generation and routinely transmit only the CIs so as to reduce the volume/power of
     data transmission.
  - Data acquisition will follow the windowed ‘snapshot’ approach of typical transmission vibration monitoring, since power constraints preclude continuous real-time acquisition (note that this also effectively precludes usage
     monitoring which generally requires continuous data acquisition).
  - CI threshold checking will be performed by the central HUMS and will command the WSN to transmit raw acquired data when thresholds are exceeded, and on an occasional scheduled basis for sensor checking and data-basing.

3.3 RTVP On-Rotor HM Evaluation Approach
The RTVP On-Rotor HM activity aims to evaluate/demonstrate the key ‘generic’ technology elements, as a risk reduction step before the decision to pursue a fully-realised type-specific on-rotor HM implementation.

To ease the design and implementation of the evaluation system, it was decided that:
  - The system would be independent from any other aircraft systems, including the central HUMS.
  - The WSNs would be mounted on the rotating swashplate, since this provides the most free space and ease of attachment via existing bolts.
  - Sensors would be external to the WSNs to allow flexibility in locating them on target rotor components; the need to wire the sensors to the WSNs is seen as an acceptable compromise.
  - The system would be autonomous requiring no intervention from aircrew.

AW is following a two stage approach to the evaluation.

Phase 1 is the key risk reduction step aimed at proving the vibration energy harvesting and rotating-to-fixed frame RF data communications, using a single WSN and sensors mounted on the rotating swashplate. Two accelerometers and a Resistance Thermometer Detector (RTD) temperature sensor were chosen as representative types, however their purpose and locations were selected only so as to allow the WSN’s functionality to be demonstrated.

Phase 2 will build a multi-node system to demonstrate all the essential elements of an On-Rotor HM system, with candidate rotor components monitored using relevant sensors mounted on the components, and data processing/reduction added to the WSNs.

Flight trials in both Phase 1 and 2 are being conducted on one of AW’s AW139 prototype aircraft designated to the RTVP. This has a typical helicopter main rotor and rotor controls configuration, so it provides a ‘generic’ test-bed and therefore the On-Rotor HM technology being investigated should eventually be applicable to other helicopter types.

4. EXPERIMENTAL WIRELESS SENSOR NETWORK

4.1 Requirements
AW contracted L-MS to provide flight-worthy experimental-standard wireless sensor network technology as the heart of the evaluation On-Rotor HM system. AW specified a set of operational, functional, and design requirements, including:
  - Energy harvesting performance
  - Low-power wireless data transfer with means to prevent/minimise loss of data
  - Multi-node synchronization
  - Network scalability
  - Configurable operational settings.
  - Minimal weight and volume
  - Durability/reliability at the outset given the eventual need for long-term ‘fit-and-forget’ service use

AW also specified experimental safety-of-flight requirements, selected from DO160F and MIL-STD-810F.

L-MS developed a new vibration energy harvesting device for the programme, as their existing technology base was not available. However, L-MS tailored and re-packaged their existing wireless data network and communications technology for the other elements of the wireless On-Rotor HM system.

The remainder of this section describes the wireless system. The installation onto the AW139 main rotor is described in sections 5 and 6.

4.2 Wireless Sensor Network
The system concept is a ‘star’ network of WSNs communicating independently with a data collector. Data collected by the nodes are transmitted wirelessly to the data collector located in the airframe, which stores the data for download after the flight. For the technology evaluation, the network is completely independent of other aircraft systems, to ease embodiment and safety-of-flight approval. Ultimately, for any final embodiment, a decision would be needed as to whether the additional functionality would be integrated into existing central HUMS for the weight and data management advantages, or left as standalone to reduce the complexity/cost of implementation.

4.3 Wireless Sensor Node
The WSN architecture is shown in Figure 1. The WSN interfaces with two piezoelectric charge-mode accelerometers and one RTD temperature sensor. The accelerometer inputs pass through separate charge-amplifiers and low pass anti-aliasing filters, before being read by a high-speed 16-bit ADC. The RTD input goes through an amplification stage before being sampled by a 12-bit ADC. An IEEE 802.15.4 compliant, 2.4 GHz radio provides the node bi-directional wireless communication with the data collector. Data is sampled and transmitted as described in section 4.6.

Figure 1. Energy-harvesting wireless sensor node for vibration
and temperature monitoring block diagram.

The electronics are powered by a super-capacitor charged by a resonant magneto-inductive vibration energy harvester (VEH) tuned to the blade passing frequency of the target helicopter. The super-capacitor was chosen as the energy storage device rather than a re-chargeable battery, since although it does not retain charge when not powered, this is outweighed by its higher power density, longer life and ability to sustain more charge/discharge cycles, quicker charge time, better capability to meet rapid changes in demanded current, operates across a wider temperature range and no flight safety issues.

The wireless sensor electronics, VEH, and super-capacitor are mounted within a single ruggedized enclosure, shown in Figure 2. This also features two sealed microdot connectors for the accelerometer inputs and a Glenair connector for the platinum RTD input, as well as a separate Glenair connector for connection of backup 9V DC power and data download. The antenna is embedded into a moulding located on the top of the enclosure.

Figure 2. Energy-harvesting wireless sensor node for vibration and temperature
monitoring.

4.4 Vibration Energy Harvester (VEH)
Vibration centred on the blade passing frequency (rotor 1/rev multiplied by the number of blades) is evident on the helicopter main rotor and had been quantified previously by AW. This vibration provides an opportunity to power low to medium duty cycle WSNs mounted on the rotor.

L-MS designed a VEH utilising a resonant spring-mass architecture with a magneto-inductive energy generating element (Figure 4).

Figure 4: vibration energy harvester (VEH) shown during preliminary
testing on shaker.

The VEH is tuned to the main rotor blade passing frequency (5R) of the AW139 helicopter. The dynamic mass moves in parallel with the rotor axis and includes the coils, while the magnets are fixed to avoid damping due to nearby ferrous materials and to lessen damping due to eddy currents. Current is conducted from the coils through the springs to the stationary PC board; the voltage is rectified, stepped up or down with a DC-DC buck boost converter and then used to charge a super-capacitor. Charging is controlled to stop when the super-capacitor achieves 3.6VDC to protect the WSN electronics, and to restart when the voltage drops to 3.5VDC (to provide some hysteresis).

Vibration data for the steady-level speed range of the AW139 helicopter provided by AW were used to design and size the VEH, and for preliminary testing of the harvester on a vibration shaker table. The VEH exhibited an output power ranging from 2.0mW for 80kts steady level forward speed, to 8.5mW for the 150kts, a bandwidth of ±1 Hz (Figure 5) and resonance shift of only 2% across the specified operational temperature range (Figure 6).

Figure 5: vibration energy harvester output power vs. input vibration frequency.

Figure 6: Effects of temperature on the resonant frequency of the structural vibration energy harvester.

4.5 Wireless Sensor Data Aggregator
A ‘wireless sensor data aggregator’ (WSDA) from L-MS’ existing product range is used as the data collector for this application (Figure 3).