ASURING FORCE
specification for aerospace design is to choose a force sensor with ≤0.0015 per cent per degree C specification. Again, mechanical design and strain gage materials make a significant difference.
Force Sensor Creep Compensation
Force sensor creep is defined as “the change in the force sensor signal that takes place over time and under a constant load. For aerospace applications, creep becomes an extremely important design parameter, especially in static tests. While initial measurements taken at time 1 are the same taken at time 2, the difference in real time can be months or years in a fuselage test. Accuracy over time is a key parameter in the test design.
While there are several methods and procedures, the “creep measurement of the force sensor system at 20 minutes under load” is a standard with many companies. Not all force sensors are measured for creep, and many force sensor manufacturers can’t meet the tight specifications needed for aerospace or ‘critical mission’ testing designs.
Force Sensor Loading Symmetry
Force sensor symmetry refers to whether a force sensor exhibits the same output sensitivity for both compression and tension use. While most tension and compression force sensors are typically and primarily used in only one direction, applications in fuselage testing symmetry, where fatigue loading from a hydraulic actuator happens repeatedly, becomes a very important parameter. All aerospace force sensors should be calibrated in both compression and tension so that the force sensor exhibits the same sensitivity in both directions (compression and tension). However, sensitivity curves for compression and tension may have a different slope. While it is a very small error when the cell is made correctly, it is vital that the force sensor effectively functions symmetrically about its central (neutral) load position. For aerospace designs a force sensor manufacturer should
AEROSPACE TESTING CATALOGUE 2010 35
guarantee a symmetry error of ±0.1 per cent. Fortunately, simply applied engineering principles are able to ensure that complicated aerospace test structures and loading systems yield valid results. Accurate results are achieved when force sensors, with properly rated fatigue specifications, moment compensation, temperature compensation,
symmetry, and creep specifications are understood and designed into the force sensors. These force sensors are made from specific materials for both the mechanical structure as well as the strain gages, ensuring that the overall force system meets the stringent parameters of the aerospace industry’s design codes.
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