ACCURATELY ME

Interface Inc., discuss why choosing the right force sensor parameters is key to complex aerospace force measurement testing

Aerospace design codes do not usually give design methods for measuring force loads except for some relatively simple mechanical layouts. Today, aerospace force measurements push the limits of the force sensors, requiring high accuracy, fatigue rated, and well designed force sensors. The design engineer must make careful considerations when choosing a force sensor that will run 100K load cycles or more, and more importantly, recognise how small changes in lateral forces, temperature shifts, and reversals in compression tension forces apply to a complex system load measurement. There are several key factors to consider in the selection of a high quality force sensor for aerospace force measurement: • Fatigue rating • Temperature range & sensitivity • Moment sensitivity • Creep • Materials • Symmetry

Force Sensor Fatigue Rating

Fatigue rating for a given force sensor, should guarantee a service life of ≥100 million fully reversed loading cycles at full rated capacity. Fatigue rating is a key design parameter that should be considered when force sensors are subject to failure as a result of repetitive loadings – meaning that much lower applied forces will create failure simply because of the repetitive, reverse loading.

It is generally acknowledged that a structural fatigue failure develops in three stages: 1. Repeated cycling builds up local plastic deformation, and a microscopic crack is initiated.

2. The crack propagates and a larger section becomes weakened.

3. Stress concentration in the section of cracking increases rapidly, and continued cycling enlarges the crack until sudden fracture occurs.

The stress which a metal can withstand under

34 AEROSPACE TESTING CATALOGUE 2010

cyclic loading usually becomes less and less as the number of cyclic loadings is increased, resulting in fatigue failure within a force sensor. Additionally, strain failures can occur because of the grain boundary slip in metals from this type of loading as well. Typically, force sensor manufacturers will test fatigue rated force sensors at 130 – 150 per cent of rated capacity, allowing for shorter, but stringent, tests to ensure product reliability and specification conformity. A fatigue rated force sensor is designed to keep stresses and strains in a safe range. Its ratings are chosen to present a test designer with a force sensor that has been designed to minimise and eliminate the failure potential from repeated and reversed loads (compression / tension).

Most strain gages are made from copper-nickel alloys which can lose approximately 10 per cent of their natural output to temperature compensation circuitry. This loss is not present with custom manufactured, self compensated gages. Interface Inc. designed and utilises a specialised alloy that was originally designed to meet the strict specifications of the aerospace industry’s fatigue testing requirements. The result is that special alloy gages have approximately 130 per cent of the fatigue resistance of the typical copper-nickel alloy gages.

Force Sensor Moment Sensitivity

Hydraulic actuators are used to apply a force (both tension and compression) along the line of force in most aerospace fatigue testing applications. However, often because of design constraints, staying exactly within the axis of line of force is not possible due mostly to mechanical misalignments within the force measurement system. Thus, off-axis or lateral forces can be readily introduced into the test system – being realised as extraneous errors, and resulting in invalid or speculative data spikes and errors.

A force sensor that is “moment

compensated” has been specifically design to account for off-axis or lateral forces that are not

directly in line with the intended line of force. The cell is designed to eliminate the lateral forces from the measurement results. This is usually accomplished by a force sensor’s radial design and its symmetric use of well placed strain gages.

While the radial design creates a ‘cancelling out’ of the forces (left – right, top – bottom, etc.) the strain gage placements (typically ≥8 strain gages) help minimise error within the system. Good force sensor design therefore incorporates both mechanical and electrical design considerations, without added complexity resulting in nearly eliminating (≤0.1 per cent) ‘lateral moments’ from the test results. For the designer of force testing it means that choosing a force sensor with moment compensation effectively eliminates laterally induced force errors from the system’s design.

Force Sensor Temperature Compensation

Low temperature sensitivity and variation within a test system is extremely important. In fact, it’s not acceptable to know only how the temperature of the environment fluctuates, but exactly how the force sensor will operate and compensate for these changes relative to the a changing environment.

In fuselage testing, ground temperature and higher air (25m elevation) within the testing shed can vary by 10- 30 degrees C over the height difference. A key design value is to choose a force sensor with a temperature effect on Zero of ≤0.0015 per cent RO per degree C. While many manufacturers compensate the force sensor at low and high temperatures, the lower the zero balance vs. temperature slope, the better. Manufacturers design the force sensors for a ‘flat temperature response’ through the complexity of their design, the quality of materials and placement of their strain gages.

Temperature effect on output is another important consideration since it is desirable to keep the output under load at different temperatures to stay the same. A good Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72