Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-11-26
2002-03-12
Chapman, John E. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S597000, C073S802000
Reexamination Certificate
active
06354152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Structural stress measurements are critical to aircraft and many other structures. Overstressed members require expensive inspections at best and can fail in flight at worst. Uncertainty about actual loads borne by structural members forces aircraft designers to increase the mass of critical parts as a safety margin. That increased mass reduces the aircraft's performance while increasing stress on other structural members. Typically static structures such as bridges, dams and buildings must be designed for external loads imparted by earthquake, wind and even sunlight. Severe external loads, such as those imparted by turbulence on an aircraft or earthquake on a bridge, require expensive and time consuming inspections before the machine or structure can be returned to service. The art of determining loads has attempted to reduce that uncertainty with a myriad of external sensors overlaid on structures. Those external sensors are separate, expensive systems which require precision attachments and calibration. None of them is capable of actually measuring internal loads in structural members so each measures an external strain, position or even optical characteristic in order to estimate internal stress.
2. Description of the Related Art
One of the many attempts to measure stress or loads in structures is illustrated in U.S. Pat. No. 5,381,005 by Chazelas and Turpin. Their Optical Fiber Stress Detector Using Amplitude Modulation is a variation on fiber optic detectors which are overlaid on or embedded in structural elements. While functional, such overlaid systems all share the limitations of any system which adds complexity, cost and weight while not contributing to a structure's function. The present invention avoids most of those problems by using functioning fasteners as sensors where possible.
Optical sensors are common in this art for their precision and non-metallic composition. The present invention's primary function is determining dynamic loads in aircraft and other structures while in operation. That function is met for ships with the optical sensors described in U.S. Pat. No. 5,942,750 Method and Device for Continuous Monitoring of Dynamic Loads by Sannerhaugen and Hellvik. As is always the case in the art, '750 teaches an externally applied apparatus of sensors overlaid on the structure. Optical strain sensors are rigidly attached to a ships hull and connected by fiber optic cable to the processing means. The entire cost of this system is added to the structure's cost while many potential failure points are evident, such as the security of each strain sensor's attachment.
Another variation on this theme is seen in U.S. Pat. No. 5,569,857 Vehicle Stress Detecting and Measuring Method and Stress Detecting Device Using said Method by Miyazaki. That inventor isolated the shear vector from various forces within a structural member by, again, using external sensors applied with the typical drawbacks.
An example of the extremes reached in attempting to determine stress in structural members is seen in U.S. Pat. No. 5,699,159 Loadmeter Employing Birefringence to Measure Mechanical Loads and Stresses by Mason. His invention is interesting in that loads are calculated without physical contact with the member. In addition to the other problems inherent in applying an external measurement system, it requires the added complexity of a “birefringent coating” on the member with cameras and computers equipped to evaluate the “spectral demodulation”.
Various strain gages are also common in the art. The present invention is capable of calculating the external loads on a member or part through knowledge of the loads or stresses resulting within that part. Moses and Hogan meet the same need using strain gages in U.S. Pat. No. 5,905,212 Load and Deflection Measurement System for Elastomeric Bearings. Once again, conventional devices such as the “metal foil strain gages” are applied to the outside of the member in order to indirectly determine the relevant forces. Those external sensors require precision application and maintenance while only estimating the internal loads.
There are several interesting variations on the external sensor theme. One objective of the present invention is detecting cracks. That need is addressed by Belk and White in U.S. Pat. No. 5,969,260 Remotely Interrogatable Apparatus and Method for Detecting Defects in Structural Members. As with the rest of the art, the Belk et al invention requires the overlay of a sensing system on the structural member in question; in this case a wire is incorporated in composite structures. While functional, it includes further limits such as one-time operation (since breaking the detection circuit renders it inoperative), no warning before an overload causes the member to fail or crack, and intermittent operation available only when a technician connects more hardware to the detection device. The present invention meets the same need with none of these limitations.
Position sensing is also used to estimate loads. Another objective of the present invention is determining the payload weight and distribution of a vehicle. This objective is also attempted in U.S. Pat. No. 5,973,273 Method for Determining Weight of a Vehicle in Motion by Tal and Elad. Once again, limitations of the art are severe. '273 teaches measuring the deflection of vehicle axles with conventional, external sensors in order to infer the weight borne by them. That method only works when the vehicle is in motion and only if the characteristics of the springs are know with precision. The present invention would serve the same purpose by measuring the load inside relevant structures at rest or in motion and with no need to know the characteristics of the vehicle's suspension.
The advantages over all such externally applied conventional sensors are many.
An art previously unrelated to this subject of measuring internal structural stresses is that of setting the torque on a fastener. Safe and reliable joints require fasteners installed to the proper torque. This art includes mechanical devices such as torque wrenches or indicating pins mounted on fasteners which move as torque is applied. More precision is possible through he use of ultrasonic transceivers to measure the time of flight of a sound pulse, i.e. a bolt's length, as it elongates during tightening. That technique of setting torque on a bolt by measuring elongation with ultrasonics is well known and, while certainly possible with the present invention, is not an object of this invention. An important segment of this unrelated torquing art is the development of fasteners containing piezoelectric crystals permanently embedded within.
The torquing art with the aid of ultrasonics is summarized in Fastener Incorporating Ultrasonic Transducer U.S. Pat. No. 4,294,122 by Couchman. He teaches providing fasteners modified with permanent transducers for actively measuring preloads on those fasteners using ultrasonic signals. His modified fastener incorporates an acoustic transducer permanently anchored in an opening formed in its head. Fasteners such as that described by Couchman '122 are required for the present invention. His stated objective is, “. . . to obtain more accurate preloading measurements and also to provide for improved quality control inspection of the fastener and to allow acoustic monitoring of critical fasteners.” with the sensing tool temporarily pressed against the surface of the fastener. In other words, he developed a method for measuring the preload torque in fasteners and then rechecking that torque on occasion. Here again, the inventor's sole emphasis is on the binding force exerted by the fastener on the materials which it binds.
In the related patent Fastener Incorporating Removable Ultrasonic Transducer U.S. Pat. No. 4,295,377, Couchman repeats his points from '122 while incorporating a “transducer (which) may be removed for repair or replacement purposes.”. Improved acoustic conta
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