Abstract:
A system for announcing failure of a mechanically actuated arrangement employs a first coupler arrangement that couples an actuator to a structural element that is desired to be controlled, and a first force sensor is coupled to the first coupler arrangement. The first coupler arrangement and the first force sensor constitute a primary load path. Similarly, a second coupler arrangement is coupled to the actuator to the structural element and constitutes a secondary load path. Changes in the forces experienced by one or both of first and second force sensors are monitored by a controller/monitor system. A connector has a screw shaft in the primary load path and a preloaded tie rod in the secondary load path. Axial forces are generated by a drive motor, which can include a ball screw arrangement. Operating impulses generated by the ball screw arrangement are monitored during a predetermined interval of operation.

Description:
RELATIONSHIP TO OTHER APPLICATION 
     This application is a continuation-in-part patent application of U.S. Ser. No. 12/150,754, filed Apr. 29, 2008, now U.S. Pat. No. 7,680,565. U.S. Ser. No. 12/150,754 is a continuation-in-part of U.S. Ser. No. 11/728,214 filed on Mar. 23, 2006, now U.S. Pat. No. 7,366,590, that claims the benefit of U.S. Provisional Patent Application Ser. No. 60/785,080, filed Mar. 23, 2006, the disclosures all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to methods of annunciating impending failure and failure in actuators and drive train components, as well as aircraft control systems. 
     2. Description of the Prior Art 
     There are present in the prior art a variety of vibration test systems that enable vibration analysis of operating mechanical systems to determine machine conditions and operating characteristics. In some of these known systems, a handheld device has an associated probe that is touched to an operating machine. The vibrations of the operating machine cause vibrations sensors in the probe to issue corresponding electrical responses that are analyzed by the handheld monitoring device. Although useful in the determination of the operating characteristics of an operating machine, the known vibration test systems are of only limited utility in determining the operating characteristics of mechanical systems that spend little or no time in steady-state modes of operation. 
     It is a characteristic of actuators, drive train components, vehicle braking systems, that a significant portion of operation time is spent in transient mode. For example, in a screw drive arrangement of the type used for controlling aircraft control surfaces, the operation of the screw drive arrangement is bidirectional over a limited extent. Therefore, this type of drive spends considerable portions of its operating cycles in bidirectional ramp-up and ramp-down transient conditions. Only a small portion of the operating cycle, if any, is spent in a steady-state condition. 
     In the case of drive train components, it is known that these mechanical systems can be tested in steady-state condition on a bench. However, in actual use, these mechanical systems are operated in acceleration, deceleration, and reverse modes of operation. Again, the known vibration test systems are inadequate to analyze the operating characteristics of drive train components in actual use. 
     Vehicle braking systems are mechanical arrangements that clearly operate in transient modes. In conventional use, the vehicle brakes are applied while the vehicle is operating at speed, and almost immediately the rate of operation is reduced by deceleration. The effects of such transients are multiplied in vehicle braking systems that are subjected to the stresses of automatic braking systems (ABS). 
     There is a need in the art for an on-board vibration test system that monitors the operation of a mechanical system of the type that operates principally in transient modes, and develops trend, or historical, data that reflects changes in the operating characteristics of the mechanical system. There is additionally a need for such a system to be useful in the tracing back of the changes in the overall response of the mechanical system to a change in the operating characteristic of a specific component of the mechanical system. 
     In addition to the foregoing, in embodiments where the mechanical system is a ball screw arrangement, there is a need to determine the health of the bearing balls and the bearing balls recirculation system. One mode of failure of the mechanical system occurs when the bearing balls escape. In such a failure mode, the load is transferred to a secondary nut that operates in an acme mode. 
     A further form of failure occurs when the balls are seized within the recirculation system. In effect, the ball screw operates in an acme mode, and the load is not transferred to the secondary load path, which often is an acme nut, until the bearing balls become dangerously worn. 
     A still further form of ball screw failure occurs when the bearing balls become scuffed, which results in accelerated wear. A ball screw arrangement that is operating with scuffed bearing balls is an unhealthy mechanical system that may soon fail. 
     There are numerous aerospace applications in which redundancy is employed in the design of flight critical mechanisms, by duplicating the load path used to transmit motion. This is achieved through individual sets of components connected in series (chained together), forming independent load paths, intertwined to provide fail safe solutions. These load paths are typically known as Primary Load Path and Secondary Load Path. In certain designs used in pilot controls (for example Helicopter pilot controls), there is a third redundant load path provided, in case both primary and secondary load paths fail. 
     Statistically, studies indicate that the likelihood of having consecutive failures (for example shearing/fracture of material due to impurities/inclusions in the material matrix, cracks from wrongful heat treatment, fatigue or improper machining, etc.), leading to loss of primary load path and shortly followed by a failure of the secondary load path within the same flight is small, but nevertheless possible. 
     The mechanisms that are designed with redundant load paths typically control flight critical systems, wherein failure of both load paths is catastrophic. These include, for example, flaps, HSTA, pilot controls linkages, and the like. 
     A significant problem with some critical control systems, such as flaps, is that the forces applied to the control actuators in use can cause the actuators themselves to become damaged, or to become separated from either the support structure or the flap being controlled. 
     Current annunciation systems indirectly deduce whether one of the load paths has failed. These known systems reason that there is present a possibility of load path failure by disconnection (separation), by processing other system parameters such as the flap panel position angles at different locations of the wing, and computing the aero-elasticity of the airframe materials (cables, rods, spars, aircraft skin, etc.), at various air speeds, ambient temperatures, and pressures, and through complex algorithms combined with empirical (experimental) data, to define and declare the failure. 
     The known systems are somewhat reliable, but they use assemblies of expensive components, such as linkages and high precision transducers, and are subject to the engineering design team&#39;s capability to assess subjectively or empirically the contributing factors, such as aero-elasticity, material flexibility, and torsional and linear deflections in response to loads applied at various temperatures and pressure. An empirical formula is created that characterizes the state of the mechanism, and defines the step function for the failure diagnostic criteria that will assist in the determination of a declarable failure. By way of example, this could include a function that calculates and allowable degree of angular asymmetry. 
     There is, therefore, a need in the art for a system and methodology that monitors the health of actuator arrangements that control critical control systems. 
     There is need in the art for a system that signals failure of a primary control system, whereby the aircraft control system being controlled appears to function normally, but is in fact being controlled by a redundant system, and therefore the aircraft control system is significantly nearer to catastrophic failure. 
     There additionally is a need in the art for a system that signals the an actuator or a support element for an aircraft control system is being, or has been, subjected to excessive load forces. 
     There is also a need in the art for a system and methodology that facilitates the vibrational analysis of mechanical systems that operate largely in transient modes. 
     There is additionally a need for a system that facilitates the determination of impending failure of mechanical systems that operate largely in transient modes. 
     There is additionally a need for a system that facilitates the determination of actual failure of mechanical systems that operate largely in transient modes. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a system for announcing failure of a mechanically actuated arrangement. A first coupler arrangement couples an actuator to a structural element that is desired to be controlled. A first force sensor coupled to the first coupler arrangement, and the first coupler arrangement and the first force sensor constitute a primary load path. A second coupler arrangement is also provided for coupling the actuator to the structural element that is desired to be controlled. A second force sensor is coupled to the second coupler arrangement, whereby the second coupler arrangement and the second force sensor constituting a secondary load path. Changes in the forces experienced by said first and second force sensors are monitored by a controller/monitor system. 
     In one embodiment of the invention, the first force sensor is arranged serially with the first coupler arrangement. Similarly, the second force sensor is arranged serially with the second coupler arrangement. In some embodiments, however, the forces experienced by only one of the load paths is monitored. 
     There is provided in an advantageous embodiment of the invention a connector arrangement having a screw shaft portion for inclusion in the primary load path and a tie rod for inclusion in the secondary load path. Preferably, the tie rod is preloaded. In a highly advantageous embodiment, the tie rod is arranged coaxially with respect to said screw shaft portion. 
     Axial forces are generated by the actuation of a drive motor. In one embodiment, the drive motor comprises a ball screw arrangement. In a still further embodiment of the invention, there is provided a system for monitoring operating impulses generated by the ball screw arrangement during a predetermined interval of operation. 
     The controller compares the magnitude of the forces experienced by at least one of the first and second force sensors to a predetermined force value. For example, if a failure should occur in the primary load path, a preload in the secondary load path will be released, and the corresponding change in the forces experienced by the secondary load path will cause a signal to be issued that indicates primary load path failure. 
     The present invention provides reliable real time load path health monitoring, failure detection and annunciation. This is achieved by employing strain-gage transducers that are incorporated in at least one structural element that has been mounted in series with the load paths. 
     More specifically, the present detection technology includes the modification of at least one of the load carrying components in the load path. Instrumentation of the structural elements of the load path with strain-gages that will convert the forces applied through the load path component into electrical signals that are proportional thereto. 
     The force monitoring system of the present invention directly reports information corresponding to the integrity, the health condition, the load carrying capability, and of course failure of a component in a load path under load. This is performed on flight critical mechanisms, such as an aircraft flap system. It is known that during all flight profiles of a flap system (i.e., landing approach flight patterns and take off procedures), the forces developed in the flap actuation mechanism are continuously fluctuating, varying between tension and compression domains, but always at values other than zero. Thus, real time recording and monitoring of flap actuator forces and comparing same with known vital signs, will provide a means to distinguish whether or not the measurements are showing a normal characteristic of a healthy system. 
     A healthy system will show that the absolute values of the forces being propagated through an actuator, over given period of time, will always be greater than a minimum value. The force monitoring system will record in real time data on the load path forces going through an actuator. The last recorded value (e.g. n), will continuously be monitored and analyzed and compared against a base value. The electronic control unit monitoring and analyzing the load path forces can then determine whether there has been separation of a load path using a simple algorithm. For example, if the forces are below a certain minimum threshold (for example 300 lbf absolute value), for a defined period of time (for example 30 consecutive seconds), the load path is declared to be “disconnected” and a corresponding signal is delivered to the pilots. Flap system operation should in some embodiments of the invention be disabled to avoid actuation of a failed system that could then lead to an enhanced, or catastrophic, failures. A cockpit warning light will be illuminated, and a procedure established that the operation of the flaps should not be overridden without ground maintenance intervention after landing. 
     The system of the present invention is simple and reliable, and in some embodiments employs only one load sensing component mounted in the actual load path in the form of a force sensing fastener. Such a force sensing fastener can, in some embodiments of the invention, be a known interface adapter, or fastener in the form of a bolt or a pin, that is redesigned to be instrumented with strain-gage transducers that will report in real time the loads (forces) being applied to the particular structural mechanical element. 
     In some embodiments, the force sensing element is in the form of a bracket, or bolt, or pin, and is designed, built, and calibrated in a laboratory prior to being assembled on the aircraft to levels of accuracy that are appropriate for each application. It is important that measurement of forces be as precise as required by the application, and not necessarily as precise as possible. 
     In the case of outboard flaps panels that are each controlled by two actuators and designed with single load path, the loss of one actuator by separation/fracture of a load carrying component, will not lead to a crash, because the other actuator is designed to be able to keep the flap panel attached to the aircraft wing. Some flap actuator systems require that each actuator be designed to handle the maximum flap load on its own. Thus, depending upon the type of failure that will lead to the loss of the first actuator, it is possible that the second actuator is still operational and will be able to retract (stow), and extend the flap panel. The continuation of flight activity without knowledge of load path failure of the first actuator (through component fracture, ball bearings loss, trunnions shear, etc.), can result in catastrophic failure of the flap system. This can happen particularly if the second actuator that controls the subject flap panel fails by separation, followed by an imminent complete separation (detachment) of the flap panel. The loss of a flap panel can result in a crash due to the asymmetry in lift forces between the aircraft wings. More particularly, the wing that has lost a flap panel will produce a reduced lift force relative to the opposite wing that has its associated flap panel in place. This unbalance in wing lift forces will cause an uncontrolled rolling moment around the longitudinal axis of the airplane. Since the flap systems are typically used at close proximity to the ground, such as to assist in reducing the speed for landing, such uncontrolled rolling motion is typically catastrophic. 
     Advantages of the system of the present invention include:
         The root cause of the failure mode in question is addressed, i.e., whether or not the actuator connected or not.   Simplicity, in that no additional components (fasteners redesigned to host strain gages).   Greater reliability, maintainability, safety than RVDT.   Less Expensive than RVDT, and can easily also be implemented into the inboard flap control system.   Straight forward indication. It is not be necessary to conduct complex tolerance and dimensional analysis study, including aero-elastic deflection of surfaces, winding of flex shafts, etc., in order to determine whether a disconnect has occurred, and to differentiate the disconnect failure condition from normal operating deflections in the surfaces.   Ensures detection through simple means of monitoring the force (load) in the actuator at take-off and/or landing (weight on wheels+flap actuators deployed). Pass criteria: Average Force [Avg (F)&gt;0], over predetermined period of time (10 seconds, 30 seconds, or as determined by system designers). Alternatively, the rate of change of the force over time greater than zero (F/t&gt;0), can be monitored, and when the system becomes quiet or inactive, it is deemed to have ceased to be a load path for load transferring from the spar to the flap panel.   It provides finite indication of failure location, and can precisely identify the attachment point where a pin sheared.   It provides impending failure detection. If one of the two pins that attaches the gimbal to the rear spar fractures (shears) is detected to have failed, the actuator might still be functioning and the load could be transmitted through the second pin.   The load limiter mechanism can accurately be tested with a custom piece of ground equipment (e.g., a turnbuckle), that would lock the actuator, and upon running the test, an exact reading of the load at which the actuator dumped the load (load limiter triggered) can be provided.   If during a flight mission one of the actuators reaches its load limit and the load limiter is triggered, the force sensing pins can provide simple monitoring with precise indication of where the incident occurred and the attendant circumstances (e.g., correlated to altitude, speed, etc.). A conventional pop-out indicator could identify that an actuator was overloaded, but the aircraft operator would not know circumstances of the failure.       

     The foregoing notwithstanding, a disadvantage of the system of the system herein presented is that it does not provide flap position indication, as does the known RVDT arrangement. 
     This aspect of the present invention detects impending significant latent failures and announces same to the crew and/or maintenance personnel. More specifically, disconnection of one of the load paths, either primary or secondary, is achieved while the system is still operational. Appropriate personnel are therefore advised that redundancy is no longer available, and that the airplane is flying on single load path. This is a condition that but is one failure away from a hazardous or catastrophic event. 
     In addition to the foregoing, the invention disclosed herein provides a method of determining variations in operating characteristics of a mechanical system having a rotatory mode of operation. In accordance with a first method aspect of the invention, there are provided the steps of: 
     first monitoring operating impulses generated by the mechanical system during a first interval of operation; 
     first analyzing the operating impulses obtained during the first interval to determine the intensity and frequency of the operating impulses; 
     first correlating the operating impulses obtained during the first interval to corresponding angular positions of the rotatory mode of operation; 
     first producing a first record of the intensity and frequency of the operating impulses obtained during the first interval correlated to the corresponding angular positions of the mechanical system; 
     second monitoring operating impulses generated by the mechanical system during a second interval of operation; 
     second analyzing the operating impulses obtained during the second interval to determine the intensity and frequency of the operating impulses; 
     second correlating the operating impulses obtained during the second interval to the corresponding angular positions of the rotatory mode of operation of the mechanical system; 
     second producing a second record of the intensity and frequency of the operating impulses obtained during the second interval correlated to the corresponding angular positions of the mechanical system; 
     comparing the first and second records to determine differences in the operating impulses obtained during the respective first and second intervals correlated to the corresponding angular positions of the mechanical system; and 
     vibration monitoring of the bearing balls recirculation system. 
     In one embodiment, there are provided the further steps of: 
     further monitoring operating impulses generated by the mechanical system during subsequent intervals of operation of the mechanical system; 
     further analyzing the operating impulses obtained during the subsequent interval to determine the intensity and frequency of the operating impulses of the mechanical system during respective subsequent intervals; 
     further producing a plurality of further records of the intensity and frequency of the operating impulses obtained during respective ones of the subsequent intervals correlated to the corresponding angular positions of the mechanical system; and 
     comparing the first, second, and further records to determine a trend in the differences in the operating impulses obtained during the respective first, second, and subsequent intervals. 
     In a highly advantageous embodiment, there is further provided the step of identifying a cause in the mechanical system for the trend in the differences in the operating impulses obtained during the respective first, second, and subsequent intervals. In mechanical systems that are rotatory, there is provided the step of identifying a cause in the mechanical system for the trend in the differences in the operating impulses obtained during the respective first, second, and subsequent intervals comprises the further step of acquiring rotatory position data of the mechanical system. Such a rotary mechanical system is, in some embodiments, a ball screw system, an acme screw system, a vehicle drive train, a flight control actuator, or a vehicle braking system. 
     In a specific illustrative embodiment of the invention, the bearing balls within the bearing balls recirculation system have respectively discernible characteristics that is identified by a bearing balls recirculation system vibration monitoring system and correlated to specific ones of the bearing balls. In this manner, not only is the health of each of the bearing balls determinable, but also the direction of bearing balls travel within the bearing balls recirculation system. 
     In a still further embodiment of the invention, there is further provided the step of quantifying the trend in the differences in the operating impulses obtained during the respective first, second, and subsequent intervals comprises the further step of acquiring rotatory position data of the mechanical system into respective qualities of operation of the mechanical system. Such quantification may include, for example, good, acceptable, and bad operating conditions that are associated with respective levels of required maintenance. More detailed data responsive to the health of the mechanical system is obtained at an output of a processor. 
     The monitoring that is effected during the practice of the present invention may be any form of monitoring, such as acoustic signal monitoring, vibration signal monitoring; or displacement signal monitoring. The monitored signals, responsive to operating impulses generated by the mechanical system during the respective intervals of operation, are subjected to a Fourier analysis. 
     In accordance with a further method aspect of the invention directed to a flight control actuator, there are provided the steps of: 
     first monitoring operating impulses generated by the flight control actuator during a first interval of operation; 
     first analyzing the operating impulses obtained during the first interval to determine the intensity and frequency of the operating impulses; 
     first correlating the operating impulses obtained during the first interval to corresponding angular positions of the rotatory mode of operation; 
     first producing a first record of the intensity and frequency of the operating impulses obtained during the first interval; 
     second monitoring operating impulses generated by the flight control actuator during a second interval of operation; 
     second analyzing the operating impulses obtained during the second interval to determine the intensity and frequency of the operating impulses; 
     second correlating the operating impulses obtained during the second interval to the corresponding angular positions of the rotatory mode of operation; 
     second producing a second record of the intensity and frequency of the operating impulses obtained during the second interval; 
     comparing the first and second records to determine differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator; 
     determining a vibration signature of a bearing balls recirculation system; and 
     determining a mechanical reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator. 
     In certain embodiments of this further method aspect where the flight control actuator is of the type having bearing elements, one mechanical reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator is deformation in the bearing elements. Additionally, in embodiments where the flight control actuator is of the type having bearing races, mechanical reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator is deformation in the bearing races. Still further, in embodiments where the flight control actuator is of the type having a ball screw containing ball bearings, a mechanical reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator is reduced effectiveness of the ball bearings. In yet further embodiments where the flight control actuator is of the type having an acme screw, a mechanical reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator is increased friction. 
     In embodiments where the flight control actuator is of the type having a screw shaft, a mechanical reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator is damage to the screw shaft. A further reason for differences in the operating impulses obtained during the respective first and second intervals of operation of the flight control actuator is a change in the backlash. 
     The vibration monitoring sensor is installed in some embodiments on the ball nut. Alternatively, however, the vibration sensor can be installed on the screw shaft, or both. In embodiments where the sensor is installed on the screw shaft, there is additionally provided a data port for issuing the impulse data responsive to the steps of first and second monitoring. 
     The impulse data is responsive to the steps of first and second monitoring and is made available to a user at a data display system. Some of the data from the mechanical system is transmitted using a wireless transmission system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Comprehension of the invention is facilitated by the annexed drawing, in which: 
         FIG. 1  is a schematic and function block representation of an annunciation arrangement for a ball screw actuator; 
         FIG. 2  is a schematic and function block representation of an annunciation arrangement for a ball screw actuator having incorporated therein a system for monitoring the health of bearing balls and the bearing ball return system; 
         FIG. 3  is a simplified schematic representation of an aircraft wing arrangement showing a flap and two actuator arrangements associated therewith; 
         FIG. 4  is an enlarged simplified schematic representation of an actuator that is useful in the wing arrangement of  FIG. 3 ; and 
         FIG. 5  is a simplified schematic representation of a tie rod arrangement useful in the practice of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic and function block representation of an annunciation arrangement  100  for a ball screw actuator. As shown in this figure, a screw shaft  110  is configured to engage a ball nut  115 , and there is provided a ball recirculating system  117  that provides a multiplication of bearing balls (not specifically designated) in the interface between the screw shaft and the ball nut. As screw shaft  110  is rotated, illustratively in the direction of torque arrow  120 , ball nut  115  is urged in the linear direction in accordance with bidirectional arrow  122 . 
     In this embodiment of the invention, screw shaft  110  is coupled to a rotary encoder  130  that issues electrical signals that correspond to the angular position of the screw shaft. There is additionally provided in this specific illustrative embodiment of the invention a vibration transducer  132  that is shown to be directly coupled to the ball nut. Vibration transducer  132  receives acoustic or vibrational or displacement information from the ball nut, and produces corresponding signal that is propagated to a Fast Fourier Processor (FFT)  134 . The output of the FFT  134  is propagated to a synchronizer  136  that receives the angular position signals from rotary encoder  134 . Thus, FFT “snapshots” are correlated to the angular position of the screw shaft and stored in a correlating memory system  140 . It is to be noted that in some embodiments of the invention the data from vibration transducer  132  is propagated to FFT  134  by means of a wireless transmission system. This, as noted herein, provides significant advantages in applications where the ball screw arrangement is inaccessibly disposed on the mechanical system (not shown). In applications where the present arrangement is installed in an aircraft (not shown), other items of data may wirelessly be transmitted. However, in some aircraft applications the rotary encoder data is available from the aircraft&#39;s system. 
     A processor  142  receives information from correlating memory system  140 , which may, in certain embodiments, contain data corresponding to historical FFT snapshots obtained during prior test intervals. Processor  142  additionally will, in certain embodiments, maintain a count of the angular position data and correlate same to linear displacement of the ball nut along the screw shaft (in the direction of bidirectional arrow  122 ). Thus, trend data responsive to angular position of the screw shaft, which corresponds to possible deflection of bending thereof, and trend data responsive to linear displacement, which corresponds to thread damage, is obtained. In some embodiments, the recirculation cycle of the bearing balls (not specifically designated) is counted to form a data cycle that will reveal damage to one or more of the bearing balls. All of the foregoing data is made available to a user (not shown) at a display  146 , in response to data and mode requests entered at an input  144 . 
     1. Methods of Annunciating Failure in Flight Controls and Aerospace Jack Screw Actuators (Ball Screws, Roller Screws, and ACME Screws), and Rotary Actuators. 
     a. Using System Vibration Signature Monitoring 
     A compact micro-processor controlled on-line data collection vibration signature monitoring system equipped with vibration and/or velocity and/or accelerometers allows early failure detection in jack screws. 
     Failure related to damage in the moving components of flight controls and aerospace electromechanical actuators, such as ball screws, roller screws, ACME screws, threads, bearing&#39;s outer races, inner races, ball bearings, rollers, cage, return tubes or return circuits, anti-backdrive devices, gear train components, clutches, motor assembly components can be diagnosed on-line by monitoring the impulses caused by material damage in these areas (features/components), measuring the intensity and frequency of the impulses, and observing the trend of these values over a period of time. 
     The following types of failures, without limitation, can be diagnosed using vibration monitoring:
         i. deformation in the bearing elements or races;   ii. increased friction (scuffing, skidding);   iii. distortion or breakage of the screw shaft (shaft bending will cause unbalance in the system, therefore periodic vibrations, and the fracture of the shaft, if the actuator is still operational, would have a different resonance frequency due to the different length which would generate a different vibration signature (response) when excited by the system during actuation).   iv. separation/loss of ball bearings from the ball nut (due to any reasons, especially due to failure of the ball recirculation system).   v. distortion or breakage of the anti-backdrive (no back) components such as the pawl, the pawl shaft, the cage or the thrust plate.       

     The angular position of the mechanical system is determined from on-board encoders and resolvers that are coupled to a system shaft. In some embodiments, it is advantageous to know the RPM of the system so that the outside noise can be filtered out and a clean signal extracted for analysis. 
     If the failure monitoring pertains to a flight control actuator, and because vibration monitoring testing requires higher RPMs, the tests are, in certain embodiments, conducted on the ground, at predetermined maintenance cycles. In a preferred embodiment, however, the microprocessor unit is an integral part of the actuator system and, coupled with angular position determining circuitry, can be used to analyze the mechanical system during actual use in transient modes. However, depending on the test frequency, it can also be designed as aircraft ground maintenance equipment. 
     b. Using Strain Gages 
     The secondary load path elements in actuators are sometimes preloaded (e.g. tie rod mounted through the screw shaft of an actuator). The preload is necessary in some applications to increase the column buckling stability of the screw shaft under compressive stresses. The secondary load path in the case of a tie rod serves the purpose of carrying the total load in case of failure of the primary load path (e.g. screw shaft). 
     Detecting dormant failures of such elements is critical. The strain gage is, in some embodiments, permanently attached to a component operating under constant compressive or tension stress, and appropriate ports for effecting electrical communication can, in certain embodiments, be disposed in an accessible external socket of the actuator system. After the assembly operation is finalized and the subject component that needs to be monitored has been preloaded, an “initial condition” (“birth certificate”) reading as supplied, for example, by OEM, can be taken and recorded (electronically or manually) by connecting a data reading or data-acquisition instrument to the external socket where the strain gage is connected. 
     At further maintenance cycles, or during on-line live monitoring, the preload variance can be monitored, studied for trends, and used for failure diagnostic and warning indicators triggering (e.g. a fracture in a tie rod operating in tension, would indicate zero stresses after fracture and the system would announce failure after comparing this new reading with the initial calibration reading recorded at the time the system was manufactured or placed in service after maintenance). 
     c. ACME Screw (Square Thread, Castle Thread, Etc.) Actuators—Using Sliding Friction Threads Backlash Indicator 
     A probe is attached to the end face, in lieu of a sector of thread inside the of the ACME nut, or in lieu of a sector of thread on the screw. The maximum allowable backlash in the unit resulting either due to wear of the nut threads or screw threads, depending on the geometry of the thread, will determine (using basic geometry and trigonometry math formulas), a set distance to the flanks of the threads. 
     Any excessive wear in the threads will cause the probe to engage with the threads and the failure to be detected. 
     Many faults and types of damage in linear and rotary actuator systems lead to mechanical vibrations with frequencies directly related to the rotational speed of their components or rotor. Of special interests are e.g., unbalance, alignment errors, ball bearing-pass frequencies, gear mesh frequencies in gearboxes, that occur as rotor-synchronous vibrations or harmonics (orders) of the rotor&#39;s rotational frequency. 
     The vibration behavior that the actuator system exhibits over the entire speed range will provide important information about the resonance profile of the actuator system, which later can be used for diagnostic purposes. The FFT spectrum can be used for the faults and damage analysis of an actuator system. In a highly advantageous embodiment po the invention, FFT snapshots are correlated to angular position of the mechanical system. In addition to enabling assessment of the operation of a mechanical system that operates in transient modes, such correlation to angular position facilitates identification of a mechanical system component that is about to fail, as evidenced by changes in the angular position correlated FFT snapshots over time. By employing a precise and accurate tracking analysis, an extremely fast and selective narrowband measurement of rotor synchronous vibrations can be measured. Thus the most significant vibration signals from the actuator system can be analyzed in any operating phase: stationary operation, run-up, coast-down, reverse, or over a longer time, to account for the thermal mechanical events of the actuator system. 
     The operation of the present invention is enhanced in certain embodiments by archiving the data locally (in a memory chip attached to the actuator), whether while performing online monitoring or off-line measurements, the overall readings, the current value, the previous value and the relationship with the alarm value shall always be available. Based on a pass or fail criteria an alarm signal is, in certain embodiments, provided when the vibration measurement instrumentation detects out-of-range or otherwise unacceptable values, or a modified profile of the readings compared to the baseline configuration and vibration signature model stored at the beginning of life of the actuator. Various failure modes (such as ball bearing damage, ball bearing escape and separation from the assembly, gear tooth breakage, skewered roller damage, ratchet or pawl fracture, radial bearings failure, slip clutch components failure, tie rod or torsion rod failure, etc.), can be simulated and induced into the system during qualification testing of the actuator, to ensure proper calibration and understanding of the impact produced by the failure of each of the different components onto the overall vibration signature of the assembly. 
     The use of such vibration diagnostic annunciation system allows early detection of impending failure, before its magnitude becomes critical or catastrophic. The vibration annunciation method to the present invention is useful to detect numerous failure types such as, but not limited to, ball escape, bearing race pitting or spalling, cracking or fracture of rotary or stationary components. Problems arising from unbalance, misalignment, gear damage, bearing damage, can be recognized at an early stage. Impulses caused by damage to the outer race or inner race of a bearing surface, to the roller(s) or ball bearing(s), or corresponding cages are a good indicator of the bearing condition. Reliable monitoring of the actuator assembly condition is possible by measuring the intensity and frequency of the impulses, and observation of the trend of these values over a period of time allows accurate diagnostic of the integrity and operation readiness of the jackscrew actuator. 
     Flight control actuator system damage and losses related to abnormal aircraft operations (unscheduled repairs or accidents), as a result of failed actuator can be successfully avoided by monitoring the “health” (structural integrity) of the actuator system by using the vibration monitoring failure annunciation. 
     In general aerospace applications, and specifically in flight controls actuation, weight control and reduction is very important. The advantage of using active vibration monitoring failure annunciation consists in minimum weight increase to the actuator system, consisting of two to three pickups (accelerometers), and wiring. The rest of the diagnostic logic can be supplied in a separate enclosure dedicated to data collection and monitoring the vibration signature of the actuator system, or it can easily be integrated into the existing flight control computers of the aircraft as an additional subroutine in the complex software programs that already govern the functionality of the flight control systems with today&#39;s modern aircraft. 
     The accelerometer probes can be mounted on the actuator assembly housing or attached directly to various subcomponents of the system. In some cases for the evaluation of the actuator system condition, simultaneous measurements from two points on this actuator system must be considered, i.e., a simultaneous acquisition through two channels of the instrument would be required for comprehensive diagnosis of the actuator system. A dual channel operation approach will be more accurate in providing reliable measurements on the system. 
     For systematic acquisition evaluation of all measurements types for predictive actuator maintenance, the following types of readings can be used: amplitude phase versus speed check amplitude phase versus time. 
     For efficient fault detection multiband pass space filters in the frequency range of measurement can be applied. The computer instrumentation software can be tailored to various operating ranges of the actuator system, and a database archiving system can provide information for predictive actuator system diagnostic and maintenance. 
     The vibration diagnostic method will allow predictive actuator system maintenance, therefore higher levels of aircraft availability for flight missions (dispatchability) prevention of unscheduled repairs, limitation of flight control system components damage or flight incidents, by early fault diagnosis, lengthening of intervals between inspections and timely planning to have optimally scheduled repair actions are the basis for the cost-effective significance of this strategy for an entire company. 
     The prerequisite for this is continuous knowledge of the current actuator system condition during operation. Mechanical vibrations, bearing condition values, speeds and process values are authoritative indicators with which the actuator system condition can be assessed and diagnosed. Which characteristic parameters should be acquired and how often, depends not only on the complexity and absolute value of the actuator system but also on the criticality of the aircraft system that is monitored. 
     The characteristic parameters will be calibrated on the monitoring system and initial vibration signature certificates will be recorded at entrance into service, as mounted on the aircraft. This represents a reference data set that will be used as a baseline, and will be therefore considered a normal operating condition data set. 
     The periodic readings, whether online with active sensors mounted on the actuator housing or attached directly to specific components, or off-line at predetermined maintenance checks intervals, can be archived via computer software and a common database for all measured data. 
     Standard acceleration, velocity and displacement sensors can be used. 
     The measuring functions that can be employed in the vibration diagnostic, are: 
     Absolute bearing vibrations relative shaft vibrations bearing condition speed measurement tempted to measurement damage to the internal and external lead screw threads, failure of the ratchet pawls of the anti-backlash brake (No Back), damage to the skewed roller clutch components (rollers, cage). 
     It is advisable to employ instrumentation that allows flexible configurations for various setups of the high and low pass filters for broadband measurement. This guarantees optimum adaptation to the individual measurement task. It is advisable to employ an averaging function for noise influence. 
     Damaging the internal components of an actuator system, such as material separation from the screw or nut threads, pitting and spalling of the rollers and ball bearings, breakage of the ratchet-pawls in a slip clutch, breakage in the cage of the skewed roller clutch, would result in a beat effect which will be easily detectable when recording frequency versus time. 
     If unacceptably high overall vibrations, intermittent beatings or bearing conditions, are found in the process of monitoring and diagnosing actuator system health, the causes can be identified by using frequency analysis (FFT) and envelope analysis. The anomalies (increased vibrations), can be traced to unbalance, misalignment, a bearing or gear fault or some other source. 
     The control and failure-detection in torsion, compression, or tension stressed beams using strain gage measurement can effectively be used in preventing catastrophic failures by early detection of dormant secondary load path failures, and is mostly beneficial in difficult to access areas of the airplane, where visual or direct access inspection can be accomplished only by disassembly of a multitude of components. As example, but not limited to, the hard to access internal tie rod or torsional spring assemblies are good candidates for this monitoring method. 
     The early detection of upcoming failure with minimum invasive disassembly and labor, is critical in flight controls actuators. The secondary load path failure through cracking, and ultimately through fracture of the tie rod or torsion rod inside the screw shaft of an actuator can be identified and diagnosed by employing strain gauges mounted directly on the broad shaft, either by using a single probe or multiple (odd number) probes setup for a logical voting decision making process within the monitoring or diagnostic instrumentation. 
     The strain gauges can be attached directly to the rod that is operating under tension, compression or torsion stresses. The corresponding wiring from these strain gauges shall be then routed in a specifically designed gap between the inner diameter of the screw shaft and the outside profile of the tie rod, or in a different manner (channels on the outside surface of the rod, drilled holes in the rod, etc.), towards the end face of the rod that protrudes outside of the screw shaft, and outside of the actuator where possible. In this location the wiring is routed to a connector that is available to be used by either the maintenance personnel, equipped with an off-line piece of instrumentation, or connected on-line directly to the flight control computers of the airplane for active monitoring of the secondary load path integrity inside the screw shaft of the actuator. 
     The use of noninvasive methods that allow continuous on-line remote monitoring and a highly reliable operation readiness of the internal components in a flight control actuator, is beneficial because it also minimizes the probability of errors that may occur in case maintenance personnel would have to disassemble many components to expose and inspect internal features that are provided for the safety of the actuator system. The probability of errors in reassembling the components in a prescribed order to ensure proper functionality of the actuator system is reduced or eliminated by using this method (strain gauges wired to an external connector or by using strain gauges that will report wirelessly the status of the stresses at their location), therefore the system safety will be increased when employing this type of monitoring and diagnostic method. 
       FIG. 2  is a schematic and function block representation of an annunciation arrangement  200  for a ball screw actuator having incorporated therein a system for monitoring the health of bearing balls and the bearing ball return system. Elements of structure that have previously been discussed are similarly designated in this figure. 
     There is shown in this figure adjacent to ball recirculating system  117  a bearing ball sensor  230  that is configured to sense the passage of bearing balls. In particular, ball recirculating system  117 , in this specific illustrative embodiment of the invention, distinguishes between the conventional bearing balls (not specifically designated) and bearing balls  225  (marked with an “X”). Bearing balls  225  are possessed of a discernable characteristic, such as a magnetic characteristic, that is observable by ball recirculating system  117  through the bearing balls return conduit. In addition, bearing balls  225  are arranged within the train of bearing balls in accordance with a predetermined sequence, whereby the number of bearing balls and the direction of travel within ball recirculating system  117  can be determined. For example, bearing balls  225  can be staggered within the bearing balls train as two such bearing balls  225  (a first pair of bearing balls) being disposed adjacent to one another and an additional bearing ball  225  separated from the first pair. In this manner, the total number of bearing balls and their direction of travel within ball recirculating system  117  can be determined. In addition, as will be discussed below, the health of respective ones of the bearing balls can be determined by analyzing the acoustic characteristic associated with each such bearing ball. 
     The output of bearing ball sensor  230  is delivered to a bearing balls counter  236  that maintains a sequential count of the bearing balls. This data then is delivered to a correlating memory system  240  that, as described below, correlates each of the bearing balls to an associated acoustic signature. 
     The acoustic signature is derived from a signal provided by an acoustic or vibration sensor  232 . The vibration signal is propagated, illustratively wirelessly to, subjected to a frequency analysis at, an FFT  234 . The output of FFT  234  is correlated to the associated ones of the bearing balls at correlating memory system  240 , the output of which is delivered to processor  142 . In some embodiments of the invention the correlating memory systems and the FFT systems are incorporated with the processor in a single device, which may be an ASIC (not shown). In other embodiments, bearing ball sensor  230  and vibration sensor  232  are combined as a single sensor. In such embodiments where the sensors are combined, bearing balls  235  can be configured to produce a predetermined acoustic signature that is distinguishable over the acoustic signatures of the other bearing balls. 
     The resulting data is displayed at display  146 , which in some embodiments of the invention is a computer display, or may simply be one or more indicator lamps. In addition, a data output port may be provided for communicating the health of the ball screw and its associated bearing balls and ball recirculating system to a maintenance system (not shown). 
     In a still further embodiment of the invention, the health of the recirculation system is determined by analyzing a vibration characteristic of the system, and subtracting therefrom in the processor the vibration characteristic of the ball nut; 
       FIG. 3  is a simplified schematic representation of an aircraft wing arrangement  300  showing a flap  302  and two actuator arrangements  310  and  320  associated therewith. Actuator arrangements  310  and  320  are interconnected in this specific illustrative embodiment of the invention by a data/control cable  330  that delivers data and control signals to a controller/recorder  332 . Actuator arrangements  310  and  320  are connected to flap  302  at respective connections  312  and  322 . Connection  312 , which in this embodiment of the invention is substantially identical to connection  322 , will be described in greater detail below in relation to  FIG. 4 . 
       FIG. 4  is an enlarged simplified schematic representation of a portion of actuator arrangement  310  that is useful in the wing arrangement of  FIG. 3 . Elements of structure that have previously been discussed are similarly designated. As shown in this figure, actuator arrangement  310  is coupled to the flap at connection  312 . Connection  312 , however is comprised of a primary load path and a secondary load path. With reference to the figure, the primary load path include, in this specific illustrative embodiment of the invention, a pair of force transducers  335  and  337 . The secondary load path includes a further force transducer  340 . 
     Referring once again to  FIG. 3 , the primary load path, in addition to incorporating force transducers  335  and  337  (not specifically designated in this figure, see,  FIG. 4 ), includes a further pair of force transducers  344  and  346 . Force transducers  344  and  346  serve to couple a drive motor  350  to a support spar  352 . In response to drive signals delivered by data/control cable  330 , drive motor  350  is urged to travel in the directions of arrow  354  along a screw shaft/tie rod  356 . In embodiments of the invention where drive motor  350  is a ball screw arrangement, ball screw monitoring arrangements of the type discussed hereinabove in relation to  FIGS. 1 and 2  can be employed. 
       FIG. 5  is a simplified schematic representation of a screw shaft/tie rod  356  useful in the practice of the invention. As shown in this figure, screw shaft/tie rod  356  contains a preloaded tie rod  360  that forms a secondary load path. The tie rod is contained, in this specific illustrative embodiment of the invention, within a screw  364  that forms a primary load path. A trunion  370  is installed on screw  364  and induces a nut  366  to produce axial loading in the directions of arrow  372 . 
     A load sensing element  380  is installed at the end of the tie rod, and a strain gauge transducer  382  is shown in this embodiment to be installed thereon. The strain gage transducer has an electrical connector  384  associated therewith. 
     Although the invention has been described in terms of specific embodiments and applications, persons skilled in the art may, in light of this teaching, generate additional embodiments without exceeding the scope or departing from the spirit of the invention described herein. Accordingly, it is to be understood that the drawing and description in this disclosure are proffered to facilitate comprehension of the invention, and should not be construed to limit the scope thereof.