TECHNOLOGY AREAS: Air Platform, Sensors, Weapons

OBJECTIVE: Develop a low-cost force measurement system that will provide high quality information on the interface forces and moments between a weapon and its carriage platform.

DESCRIPTION: Dynamic environments (shock and vibration) experienced by stores during captive carriage on an aircraft, are currently expressed in terms of acceleration response of the store at selected measurement points within the store. Response acceleration can seldom be used to determine the input forces (and moments) to the store at the interface between store and platform. For assurance of store system integrity under dynamic loading, knowledge of interface forces and moments is essential. Accurate measurement of these interface loads in the field will allow accurate input of these loads in the laboratory, thereby satisfying the store boundary condition at the interface. In addition, direct knowledge of the interface forces and moments is essential for material fatigue considerations.

In the past, use of strain gauges on carefully modeled (e.g., detailed finite element models) interface components, such as store hangers, has largely been unsuccessful because of the nonlinear relationship between measured strain on an interface component and the distributed force/moment configuration over the component�s interface points. Likewise it has been demonstrated that the use of "contributing" or "dynamic modal mass" to establish loads at the interface utilizing force as the product of mass and acceleration has been unreliable.

A low-cost measurement system to provide information on the resultant interface forces and moments between a weapon and its carriage platform is sought. Proposed systems will need to make simultaneous measurements along at least two principal axes (preferably along all three axes) with an orthogonal orientation. On occasion multiple measurements in one axis may be desirable in order to estimate the distribution of the force over the component interface surface. The measurements should provide analog or digital outputs available for on-board signal conditioning and recording. At least sixteen bits of dynamic range should be available for recording. Target bandwidth of the measurement system should be from DC to 2500 Hz for laboratory dynamic testing however, a bandwidth to 500 Hz will be sufficient to support material fatigue concerns. Any analog-to-digital sampling should assume a 10:1 oversample ratio providing at least ten digital values for the highest frequency of interest. In addition, video confirmation of store movement under dynamic loading is desired. All measurements in the selected axis system must be of the same bandwidth and phase correlated for multi-axis processing. Typical stores range in weight from 10 to 2000 pounds and may be subject to dynamic loading exceeding 1000 g's acceleration for smaller stores to 20 g�s acceleration for stores above 1000 pounds weight. Typical stores may be carried externally on aircraft pylons or internally on weapon racks. Resultant force and moment measurements will be used in design and design modification of weapon/platform interfaces and in support of the force input to the overall weapons system for dynamic laboratory testing and interface material fatigue purposes. Measurement system application must be in accordance with flight safety standards for military aircraft with captive carry stores.

PHASE I: Determine the feasibility of developing a force/moment measurement system and the ability to creatively apply such technology in a general manner to simple configurations e.g., store hanger and rail.

PHASE II: Design, develop and demonstrate a prototype force/moment measurement system on a typical store/platform interface configuration. A typical store hanger and rail may be provided for demonstration of the technology. Developed systems may be subject to (1) static load/moment test in the laboratory (simulating interface component material fatigue) and (2) dynamic vibration/shock loading on an exciter with the launcher attached to the exciter and a store measurement acceleration profile as controlled input under Time Waveform Replication.

PHASE III: Finalize design and manufacture the interface force/moment measurement system. Provide documentation on installation and measurement system capabilities and limitations. Transition technology to interested parties.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The measurement technology industry e.g., those involved in manufacture of accelerometers, pressure gages, force gages, etc., will benefit through extension of their technology base.

REFERENCES:
1. "Harris' Shock and Vibration Handbook", 5th Edition, Cyril M.Harris and Allan G. Piersol editors, McGraw-Hill, New York, 2002.

2. "Vibration Fundamentals and Practice", 2nd Edition, Clarence W. de Silva, CRC Taylor and Francis, Boca Raton, 2007.

3. "Department of Defense Design Criteria Standard, Airborne Stores, Suspension Equipment and Aircraft-Store Interface (Carriage Phase)", MIL-STD-8591, 12 Dec 2005.

4. "Service Fatigue Loads Monitoring, Simulation, and Analysis", ASTM Special Publication 671, American Society for Testing and Materials, Philadelphia, PA, 1979.

KEYWORDS: Measurement; Force and Moment Loads; Signal Conditioning; Instrumentation; Fatigue; Store Interface