Abstract:
A system and method are provided for monitoring the structural integrity of one or more blades in a blade-based device, such as a wind turbine. Physical and electrical (e.g., lightening) sources of damage, wear and the like are considered. Generation of power for sensor and communication circuitry may be integrated into the system. Timely and cost-effective repair of any structural or weather-related damage or other issues may be provided, thereby improving operating efficiency and safety of blade-based devices.

Description:
BACKGROUND 
     The disclosure is directed to rotating structures where monitoring the structure status can improve reliability and reduce down-time and maintenance cost. The disclosure is directed particularly to structures that generate energy from the flow of gases or liquids, such as wind turbines. The disclosed technology can be applied to other rotating members that may need to be monitored such as aircraft propeller blades and turbine fan blades, or centrifuge arms. The disclosed technology can also be applied to non-rotating structures or bodies under stress or load such as a tall building or the mast of a boat. 
     Wind turbines form an important part of a balanced energy strategy. Many wind turbine blades are made by hand out of fiberglass. If not properly manufactured, after few months of operation they may begin to develop cracks at weak spots in the blade. These cracks are initially small, and if detected can be repaired on site with little cost, with the blade still mounted on the turbine. If the crack is not repaired promptly, the crack becomes larger until the blade ultimately fails (e.g., breaks). Once a blade fails, the turbine must be taken out of service, until a new blade can be transported to the site and installed, which is very expensive. Prompt detection of turbine blade cracks is therefore important for economical operation of wind turbine energy facilities. 
     Blade balance is also very important when operating wind turbines. Typically on each turbine there are multiple (e.g., three) blades, which should be balanced prior to installation to prevent any excessive unwanted loads or forces on the main shaft and the gearbox. Damage to the gearbox due to blade imbalance is gradual and will result in excessive gearbox wear within a few months of operation. Wind turbulence and turbine yaw also will have an adverse effect on the gearbox. Blade imbalance can also occur due to local weather conditions, for example due to imbalanced ice or snow accumulation on one or more blades. As turbine down time and gearbox repair is very expensive, early detection of blade imbalance and the application of corrective action to balance the blades are important to prevent problems with the wind turbine. 
     Lightning strikes also form a hazard for wind turbine blades, and detection of lightning strikes also allows for more efficient and cost-effective turbine blade maintenance. Shock to turbine blades, for example from striking an errant bird, wind-borne debris, and so on are also a risk. 
     Early identification of risks such as blade imbalance, lightning strikes, and mechanical shocks is crucial to safe, efficient, and cost-effective operation. However, blades rotate around a pivot point, which makes it difficult to provide power to a sensor and receive sensor signals over a wired connection. Effective monitoring of the turbine blades should sense strain on the rotating turbine blades, synchronize measurements with the blade rotation to account for strain effects due to gravitational forces, send data wirelessly to a central location for analysis, and provide a means to remotely power the sensor. Such a monitoring system has not been disclosed in the prior art. 
     SUMMARY 
     Accordingly, the present disclosure is directed to systems and methods for addressing the aforementioned shortcomings. One aspect of the present disclosure is directed to monitoring, identifying, and facilitating action to minimize risks associated with latent blade damage. Wind turbine blades may be formed from large pieces of fiberglass, which are expensive to transport and to install due to their large size, and expensive to repair if they break. Stress-induced cracks can form in the turbine blade, which leads over time to extensive blade damage. However, these cracks are relatively inexpensive to repair if caught early and while they are still small. 
     According to an aspect of the present disclosure, a low cost, real-time blade damage monitoring system and method is disclosed. The system and method can detect problems such as cracks, imbalance, shock, lightning strikes, and so on in or to turbine blades, and allow these problems to be addressed quickly before more extensive damage occurs to the turbine blade. It is essential that the monitoring system be able to detect cracks and excessive blade imbalance before significant damage is done to the turbine system. 
     According to another aspect of the present disclosure, methods and systems are disclosed for turbine blade characterization. The characterization may be based on one or more of: detecting and measuring blade micro strains, blade torsion, blade shocks, lightning strikes, and blade position. 
     According to a still further aspect of the present disclosure, blade characterization may be synchronized with blade position to modulate sensor and transmitter power. Increased sensor and transmitter power source lifespan may be provided. 
     According to yet another aspect of the present disclosure, power for operating blade condition sensors, transmitters/receivers, and processing devices may be locally generated. Such an arrangement reduces maintenance requirements for such components, thereby reducing operating cost both of the monitoring devices and of the turbine system as a whole. 
     The above is a summary of a number of the unique aspects, features, and advantages of the present disclosure. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present disclosure will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present disclosure are described and will be apparent to those skilled in art from the following detailed description taken together with the accompanying figures, in which like reference numerals in the various figures denote like elements. The figures are not to scale. 
         FIG. 1  is an illustration of a wind turbine device and related components in which the systems and methods of the present disclosure may be utilized. 
         FIG. 2  is an illustration of a blade position sensor for synchronizing data collection with turbine blade position according to an embodiment of the present disclosure. 
         FIGS. 3A and 3B  are illustrations of operation of the blade position sensor in two distinct turbine blade positions, respectively, according to an embodiment of the present disclosure. 
         FIG. 4  is a block diagram of a blade sensor system according to an embodiment of the present disclosure. 
         FIG. 5  is a schematic diagram of a power generator used to power a blade sensor according to an embodiment of the present disclosure. 
         FIG. 6  is a schematic diagram of a regulated power supply which conditions the power received from a power generator according to an embodiment of the present disclosure. 
         FIG. 7  is a schematic diagram of a combination of blade position detector and timer used to initiate and terminate a strain measurement according to an embodiment of the present disclosure. 
         FIG. 8  is a schematic diagram illustrating the operation of lightning suppression and lightning detection according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present disclosure will now be described in detail with reference to examples thereof. However, such examples are merely illustrative, and should not be read as limiting the scope of the present disclosure, or the embodiments thereof, within the boundaries of the claims appended hereto. 
     We initially point out that description of well known starting materials, processing techniques, components, equipment and other well-known details may merely be summarized or are omitted so as not to unnecessarily obscure the details of the present disclosure. Thus, where details are otherwise well known, we leave it to the application of the present disclosure to suggest or dictate choices relating to those details. 
     One embodiment of a wind turbine assembly  100  in which the systems and methods of the present disclosure may be utilized is illustrated in  FIG. 1 . In the exemplary embodiment, two or more wind turbine rotor blades  110  (two blades being shown in  FIG. 1 ) rotate around hub  125  on shaft  130 , which is mounted to nacelle  140 , which in turn is supported by wind turbine tower  170 . Blades  110  are shaped such that the force of the wind causes the blades  110  and shaft  130  to rotate. Shaft  130  drives gearbox  145  and generator  150 , producing electrical power. Electrical power is sent down tower  170  in the form of direct current electricity to inverter  180 , which converts direct current electricity to alternating current electricity and sends the power to a substation power facility via power lines  185 . Tower  170  is mounted to the ground  175 , which supports the tower as well as acts as the grounding point for lightning strikes. 
     One or more sensor circuits  115  are mounted to each one or more of blades  110 , and monitor the status and health of the blades of wind turbine  100 . Details of sensor circuit  115  and other elements are shown only in situ for one of the blades  110  in  FIG. 1 , but it will be understood that each blade  110  may be similarly equipped with sensor circuit  115  and other elements disclosed herein. In the exemplary embodiment, sensor circuit  115  includes a thin piezo-electric film sensor, which measures micro strains and torsional strain, which are the strain resulting from bending and twisting force applied to blades  110 . This piezo-electric film is very flat, and measures micro strains when it bends, and measures torsion when it twists. 
     Each sensor circuit  115  is coupled to a transmitter for wirelessly transmitting sensed strain data to a processing unit for determining blade condition. According to one embodiment of the present disclosure, sensor circuits  115  transmit sensor data using a wireless link (e.g., radio-frequency, Bluetooth, etc.) via antennae  120 ,  160 , and receiver  154  to turbine control unit  155  located in nacelle  140  for analysis. The elements comprising sensor circuit  115  are discussed below with reference to  FIG. 4 . 
     Typical applications will provide between one and three sensor circuits  115  per turbine blade  110 . A larger number of sensors per blade provide more comprehensive strain data for analysis, while a smaller number of sensors minimize the cost of the monitoring system. 
     According to another embodiment of the present disclosure, sensor circuit  115  is connected to an electrical power generator to enable the measurement and transmission of sensor data. Alternatively, each sensor circuit  115  may be connected to a replaceable power source in other embodiments, or sensor power may be provided from nacelle  140 . 
     Turbine blades  110  have conductive end caps  118  at their outermost ends, connected to a lightning rod that in turn is connected to ground to minimize the damage to the blade from a lightning strike, as is known in the existing art. 
     Turbine control unit  155  housed inside nacelle  140  uses antenna  160  and receiver  154  to receive information about each blade  110  from its attached blade sensor circuit  115 . In the preferred embodiment, receiver  154  is a commercial component such as the RX-RM-AUDIO superheterodyne receiver module from ABACOM Technologies (Ontario, Canada). 
     Information received by turbine control unit  155  includes the analog voltage corresponding to the micro strains and torsion measured on each blade  110  by the attached sensor  115 , and the position of the blade  110  where the data was taken. This information is used to analyze the condition of each turbine rotor blade  110 . The strain data from blade  110  is converted to a frequency domain representation, for example by Fast Fourier Transform (FFT), and is sent, again for example, according to a known internet protocol (IP) address via a standard TCP/IP data format to central computer  190  for analysis, along with other analog signals from accelerometers and other data collection devices connected on the gearbox and the generator. In an alternate embodiment, data from sensor circuits  115  are analyzed by the turbine control unit, and alarm data sent to central computer  190  when measured data from sensor circuits  115  exceed parameters expected for safe operation. 
     Central computer  190  analyzes the frequency-domain Fourier-transform representation of the strain spectrum from sensor  115 . This spectral analysis allows static strain effects that have a low-frequency component to be distinguished from high-frequency energy resulting from environmental and other external source such as the thunder following a lightning strike. The collected data can be averaged over many rotations of the blade  110  to filter out transient noise signals such as wind noise or blade vibration. 
     To simplify measurements and understand the conditions of measurement, it is best to take the measurements under certain specified conditions. In one embodiment, the conditions are defined for the blade position to be pointing upward (0 deg angle) and pointing downward (180 degree angle). In a three-blade system under normal conditions, the three identical, healthy and balance blades  110  rotate at about 8 to 15 revolutions per minute, so the time of measurement between each blade is less than a second. If the blades are identical, their micro strain measurements and corresponding Fourier transforms of this data will be very similar. If one blade is damaged, it will display a difference compared to other two blades, triggering an alarm at central computer  190 . 
     According to one embodiment of the present disclosure, the position detection is done by proximity condition of a reed relay and a magnet. With reference to  FIG. 2 , when magnet  240  passes by the reed relay (not shown), it energizes the relay and contacts are closed to command the measurements. The relay rotates with the blade. The magnet  240  is attached to one end of a bar  220 . A bearing  230  is secured to bar  220  opposite magnet  240 . The bar and the ball bearing act like a pendulum or plumb bob and magnet  240  always points downward (due to gravity) as the blade rotates. 
       FIGS. 3A and 3B  show two positions, respectively, of two turbine blades  310   a ,  310   b . Blade  310   a  is shown in the straight up (0 deg) position, and blade  310   b  in the straight down (180 deg) position. While shown separately, each of blades  310   a ,  310   b  rotate around hub  325 . Blade position detector  320   a  is shown in association with blade  310   a  in the straight up position and blade position detector  320   b  is shown in association with blade  310   b  in the straight down position, but again in practice these are the same, single blade position detector  320 . 
     With blade  310   a  in the straight up position, the magnet of blade position detector  320   a  overlaps with sensor  340   a  but not sensor  330   a , signaling turbine control unit  115  that blade  110  is straight up. Conversely, with blade  310   b  in the straight down position, the magnet of blade position detector  320   b  overlaps with sensor  330   b  but not sensor  340   b , signaling turbine control unit  155  that blade  110  is straight down. Sensors  340   a  and  330   b  are used to trigger data acquisition of sensor circuit  115  ( FIG. 1 ), and to power down sensor circuit  115  after data acquisition. While a gravity and magnet system has been described above for blade position determination, many other methods of sensing orientation, such as optical sensors, and the like, are also contemplated hereby. 
       FIG. 4  shows a block diagram of a blade sensor system  400  according to one embodiment of the present disclosure, which includes sensor circuit  115  and autonomous power supply  410 . In a preferred embodiment, blade sensor  480  is part number DT4-028 K from Measurement specialties (www.meas-spec.com), a 0.86″×6.72″ strain sensor that is 40 micro meter thick. 
     According to an embodiment of the present disclosure, autonomous power supply  410  supplies power locally to sensor circuit  115 , and is comprised of power generator  420  and power regulator  430 , with power output  435 . The rotation of blade  110  ( FIG. 1 ) will cause the movement of a magnet inside a coil and thus generates power from output  435  to the rest of blade sensor circuit  115 , as discussed further below. 
     Blade position detector  220  detects the position of the rotating turbine blades in the vertically up (zero degrees) and vertically down (180 degrees) position. 
     Adjustable timer  450  sets the measurement interval following a trigger signal from blade position detector  220 . When a blade  110  is in the vertically up or vertically down position it will trigger timer  450  to turn transmitter  460  on for a certain duration of time. This duration of time is important for calculation of number of samples and maximum frequency of the signal for the FFT transformation into frequency domain. 
     Multiple blade sensor circuits  115  transmit status information to turbine control unit  155 . To identify which blade is sending the information, blade designator  470  provides a unique digital signature enabling turbine control unit  155  to determine the source of each signal. An oscillator at a known frequency representing the number of the blade sensor circuit  115  (e.g., blade 1→1 KHz, blade 2→2 KHz, etc.) is connected to the transmitter  460  for identification of signals from each sensor circuit  115 . 
     Piezo-electric sensor  480  converts the micro strain and torsional strain into voltage and supplies the strain data to transmitter  460 , which sends the data via antenna  120  to turbine control unit  155 . Since sensors  470  are close to one another, in the case of independent blade pitch control, multiple strain sensors  480  can be connected to one transmitter and power supply. Alternately, piezo-electric strain sensor  480  may be replaced or augmented with a Bragg grating or other fiber optic sensor, strain gauge sensor, accelerometer, velocity sensor, velometer and proximity probes or other known means of sensing physical integrity of the blade. 
     Lightning sensor  485  senses current flow from conductive end cap  118  which results from a lightning strike to blade  110  through conductive rod  820  (shown in  FIG. 8 ). 
     In the exemplary embodiment, transmitter  460  is a TX-FM-RADIO commercial integrated circuit by ABACOM Technologies (Ontario, Canada). In the exemplary embodiment transmitter antenna  120  is a helical antenna consisting of enameled copper wire with a diameter of 0.5 mm closely wound on a 3.2 mm diameter form. Transmitter antenna  120  is implemented with 26 turns of wire if transmitting at a center frequency of 418 MHz center frequency, or 24 turns of wire if transmitting at a center frequency of 433 MHz. Alternately, transmitter  460  may be implemented using an infrared optical source or other known means of data transmission. Communication from blade sensor  115  to turbine control unit  115  may or may not be limited to wireless, hard wired, Ethernet, or other known communication means. The sensor signal could be routed from blade  110  through a slip ring to nacelle  140 , or use other known signal routing means. 
     Details of power generator  420  are shown in  FIG. 5 . Power generator  420  is made of a plastic tube  510  about 15 cm long with a permanent magnet  520  that can easily slide up and down inside the tube. The outside of the tube  510  is a wire coil  530  wound with many turns of small gauge wire. This tube  510  is installed tangential to the direction of the rotation of blade  110 . 
     The size and weight of the various components of power generator  420  are selected such that as blade  110  rotates, permanent magnet  520  moves up and down in the tube  510  by the pull of gravity (overcoming centrifugal force from the blade rotation). This up and down motion of magnet  520  within coil  530  generates electrical current to supply sensor  115  and to charge capacitor  580  for energy storage. Alternately, power generator  420  may be implemented using motion driven power, battery power, solar power, nuclear power, controller/scada driven power, thermal power, or other known means of power generation. Alternately power may be supplied externally via a hard-wired external power source, for example via nacelle  140 . Capacitor  580  may be augmented or replaced with a battery or other known means of power storage. 
     When the wind turbine is not operating, current sensor  550  will not detect any charging current and will turn off transmitter  460  to preserve energy stored in capacitor  580 . This simple, small, and lightweight autonomous and independent power generation inside the blade will supply the transmitter without any need for any other source of power. 
     The non-regulated output  560  of power generator  420  is an input  610  to a regulated power supply  430 , shown in more detail in  FIG. 6 . Transistor  620  clamps the output voltage at a fixed value using a bias network comprising two diodes  646  and  648 , a resistor  644  and a capacitor  642 . The bias current for transistor  620  is provided by output resistor  654 , and filtering of output power  630  is provided by capacitor  652 . Power regulator  430  also may be implemented with an integrated circuit, or other known method of voltage regulation. 
     The combination of blade position detector  440  and timer  450  are shown in  FIG. 7 . Blade position detector  440  consists of two switches  710  and  715 , each of which is formed by the contacts of a reed relay that temporarily closes once during a full rotation of blade  110 . The reed relay was included in the  FIG. 2  description, but not shown. Switch  710  is closed when blade  310   a  is at 0 degree position, and magnet  240  overlaps sensor  340   a  as shown in  FIG. 3 . Switch  715  is closed when blade  310   b  is at 180 degree position, and magnet  240  overlaps sensor  330   b  as shown in  FIG. 3 . 
     Each time blade detection switch  710  or  715  closes, it triggers timer  720  constructed using an LM555 integrated circuit (or similar), which generates a 500 ms timing pulse at output  730 , during which time data is collected from sensor  115 . Timer  720  has a power source  740  and pull-up resistor  762  for bias, and a combination of resistor  764  and capacitor  766  that sets the duration of the timing pulse. Timer  720  is also connected to power voltage  740  and ground  750 . Alternately, blade position detector  440  may be implemented using a shaft encoder, tachometer, or other known means of rotation measurement. A trigger may or may not be used to activate measurements for the purpose of comparable phasing and possibly pitch. This trigger may be laser, optic, magnetic, electronic, or scada based. 
     The lightning protection operation of conductive end cap  118  and lightning sensor  485  is further illustrated in  FIG. 8 . Under ideal circumstances, a lightning strike to blade  110  will occur at conductive end cap  118 . The lightning strike produces electrical current, which flows down rod  820  through the center of blade  110 , to hub  125  to ground  850 . As a result of the current through rod  820 , an electrical voltage is produced in coil  840 , which is wrapped around rod  820 . This resulting voltage is sensed by lightning sensor  485 , and a signal is sent to central computer  190  that a lightning strike occurred in blade  110 . Data analysis is performed, and a repair operator is notified if a potentially unsafe condition exists. 
     While a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. 
     Furthermore, various of the above-disclosed and other features and functions, or alternative thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications variations, or improvements therein or thereon may be subsequently made by those skilled in the art which are also intended to be encompassed by the claims, below. 
     Therefore, the foregoing description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.