Abstract:
A Foreign Object Damage (FOD) detection system is disclosed for detecting and analyzing ultrasound or stress waves emitted when an object enters the intake of a turbine engine and impacts one or more of the blades in the engine. Upon detection the FOD detection system can immediately inform the operator, inform another electronic device (computer, etc.) and/or latch the event for review by maintenance personnel. The detection system generally consists of one or more stress wave sensors and an electronic assembly to process the stress wave signal received from the sensor(s). The electronic assembly is in communication with the sensor(s) via conventional cabling.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally directed to stress wave analysis and particularly to the use and analysis of stress waves to detect foreign object damage (FOD) in turbine engines; such as, but not limited to, jet aircraft. 
     2. Description of Related Art 
     Foreign object damage (“FOD”) has always been a major concern of turbine engine manufacturers and users. Damaged fan, compressor and turbine blades in an engine can result in catastrophic failure or a loss of operating efficiency. To prevent FOD, some turbine systems use filters. However, this is not practical in all cases, due to size, weight, and performance issues created by filters. In non-filtered systems, periodic manual inspection is used to detect any damage at an unknown time after-the FOD event. This unknown leaves a window for the damage to propagate into a catastrophic failure that would endanger lives and/or equipment. Also, manually inspecting all, the blades is a time consuming and costly process. Accordingly, what is needed in the art is a system for detecting foreign object damage to equipment, such as turbine engines, which detects the damage in a timely and cost effective manner. It is therefore to the effective resolution of the shortcomings of the prior art that the present invention is directed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a FOD detection system consisting of at least one and preferably a plurality of stress wave sensors mounted on an engine at locations that provide transmission path from the fan bearing housing(s), a cable to interface the sensor(s) with the electronics, and an electronic assembly. The electronic assembly conditions the received signal, demodulates the sigrialds and processes the signal to determine/notify if a FOD event occurred. The sensors are preferably externally mounted to the engine. Thus, by using multiple sensors located along the length of the engine, the system can detect the depth that an object has traveled into the engine before the object was destroyed 
     Accordingly, the present invention provides for the detection of a slow energy event in an extremely high energy environment, without the need of a highly trained professional to analyze data, and also gives a real time indication that a FOD event has occurred. The use of the present invention with a turbine engine should increase the safety efficiency and reliability of such engine. 
     Accordingly, it is an object of the present invention to provide a system for detecting and analyzing foreign object damage to a piece of equipment or machinery such as a turbine engine. 
     It is another object of the present invention to provide a system for detecting and analyzing foreign object damage in a relatively timely and cost efficient manner. 
     It is still another object of the invention to use stress wave signals to detect foreign object damage in a turbine engine. 
     It is even still another object of the present invention to reduce damage to turbine engines from ingestion of foreign object(s). 
     It is yet another object of the present invention to improve the reliability of turbine engines. 
     In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a graph illustrating the increase in stress waves to detect foreign object damage at impact; and 
     FIG. 2 is a block diagram for the foreign object damage detection and analysis system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally consists of one or more stress wave sensors  20  and an electronic assembly to process the stress wave signal received from sensor(s)  20 . The electronic assembly is in communication with sensors  20  via conventional cabling. In one embodiment, sensor  20  can include amplification and band pass filtering of the stress wave signal at the sensing element. Alternatively, a non-amplified sensor  20  can also be used, preferably with the use of greater stress wave signal amplification outside the sensing element and a lower noise floor than the preferred amplifying and filtering sensor. 
     Preferably, the stress wave frequency of interest ranges from 20 KHz up. However, other values and ranges can be used and/or analyzed and all are considered within the scope of the invention. To reduce the stress wave signal amplitude range and the signal conditioning electronics&#39; sensitivity sensor  20  may incorporate two features: gain and band pass filtering. In one embodiment, the preferred frequency of interest can be 38 KHz, sensor  20  can have a gain of 72 dB referenced at preferably 100 Hz and band pass filtration can be centered at 38 KHz. Again these values are provided by way of example and are not considered limiting, and other values can be used and are considered within the scope of the invention. 
     Even with the amplification at sensor  20 , the stress wave signal may need additional amplification. This determination is typically a function of the type and power output of the engine being tested. Therefore, an amplifier may be needed in the electronic assembly. The FOD detection system design  10  also preferably includes band pass filtering. Because an impact event resembles an impulse function, a broad frequency band of signals are created by the event. At low frequencies these signals are typically a very low percentage of the overall signal, and would require extreme sensitivity to detect. But at the higher frequencies (i.e. above 20 KHz) the impact signal typically rises out of the background noise by a factor of two or more. This high frequency characteristic is important in selecting the frequency of interest, such as the 38 KHz frequency discussed above by way of example. To further separate the frequency of interest from the background signal, a band pass filter  30  designed around the frequency of interest can be used. The bandwidth can vary depending on the sensor and type of engine. In one embodiment, the filter can be designed to have a 7 KHz pass band. 
     Depending on the stress wave signal strength, an amplifier may be needed in between band pass filter  30  and the demodulator to insure optimum performance. 
     An. amplitude demodulator performs the final stage of the signal conditioning. The resulting signal can be referred to as the stress wave pulse train (“SWPT”), and the area under the resulting curve can be referred to as the stress wave energy (“SWE”). 
     At this point the SWPT must be processed by either analog or digital means to detect the pulse created by the FOD impact event. Detecting the pulse is used for identifying a FOD event. There are many conventional methods that could be used to detect a pulse, which would be obvious to one having ordinary skill in the art. The present invention uses a threshold, preferably provided as software, or alternatively as analog hardware, which is based on (1) the average SWPT, (2) an integrator to determine the SWE above the threshold, and (3) a SWE threshold to eliminate spurious noise from the input signal. The preferred pulse detecting method using a threshold results in a robust peak detection method that allows ample adjustment for various types of engines. The mathematical definition and equation (preferably incorporated into pulse detecting software or alternatively into analog hardware) is the following: 
     
       
           T   peak ={overscore (SWPT)}* T   set   
       
     
               D   peak          {           1   ,             if                 SWPT     ≥     T   peak                 0   ,             if                 SWPT     &lt;     T   peak                                      SWE   peak   =D   peak *∫( SWPT−T   peak ) 
     
       
         
           
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     Where: T set =Threshold set point referenced to average SWPT. 
     T peak =Threshold Voltage. 
     D peak =T peak  exceeded equals 1 otherwise 0. 
     SWE peak =peak energy above T peak    
     The output of the SWE threshold detector is binary and represents a pass or a fail condition. The output signal can be used to trip a variety of indicators, to issue a warning to the air crew, ground crew or other individuals, that a FOD event has been detected. In one embodiment, the present invention uses a latching indicator, however, the output could be monitored real time to detect the transition(s). One embodiment  10  for the present invention FOD Detection System Block Diagram is illustrated in FIG.  2 . 
     The detection and analysis of stress waves can be used to identify that a FOD event has occurred and how deep into the engine the FOD progressed. In the case of aircraft turbine engines this would drastically reduce the risk of engine failure due to a progressive failure initiated by a FOD event. With the use of stress waves, the present invention allows for detection of FOD at impact, not hours, days, weeks or months later (see FIG.  1 ). 
     The present invention has also shown that materials that are considered non-damaging, do not create a signal that would give false alarms. This is due to the fact that the amplitude of the stress waves is proportional to the energy transferred to the fan/turbine blade at impact. This stress wave signal can then be processed through analog or digital means to detect the FOD event. 
     Certain aspects of the present invention include the following: (a) stress waves can be used to detect FOD events in a turbine engine; (b) peak detection methods (digital or analog) can be used to monitor conditioned stress wave signals to detect the FOD event; (c) an integrator (digital or analog) can be used to determine the stress wave energy, followed by a threshold detector to minimize false alarms; (d) a stress wave signal can be averaged then amplified to set a peak detection threshold, via digital or analog means; and (e) a stress wave signal can be amplified, and band pass or high pass filtered to minimize background signals. 
     The present invention is suitable for use in many applications that require the detection of an impact event within operating machinery, and all of such applications are considered within the scope of the invention. Accordingly, the invention is not limited to its above-described use for detection of FOD events within turbine engines, in particular aircraft jet engines. 
     In lieu of conventional cabling, the sensors can communicate with the electronics of the system through wireless technology. 
     The instant invention has been shown and described herein in what is considered to be the most practical and preferred embodiment. It is recognized, however, that departures may be made therefrom within the scope of the invention and that obvious modifications will occur to a person skilled in the art.