Strain Gauge Utilization for Aerial Vehicle Dynamic Load Measurement
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The strain gauge is a commonly used tool for dynamic load and strain measurement of a system. The work presented in this thesis describes the development and evaluation of strain gauges applied to both an aerodynamic decelerator system and an unmanned aerial vehicle. This thesis has three main objectives: (1)develop and evaluate test a circular parachute strain gauge-based load distribution measurement system, (2) develop and evaluate a strain gauge thrust estimation system for a quadrotor unmanned aircraft, and (3)compare the developed strain gauge-based thrust estimation technique with an indirect real time parameter estimation technique for motor fault detection. In pursuit of the ﬁrst thesis objective, a load distribution measurement system for the suspension lines of circular parachutes was developed. The motivation to create a load distribution measurement system stems from parachute system design traditionally requiring an extensive ﬂight testing regimen. Numerical solution-based design is difﬁcult due to the highly nonlinear deformation behavior of the parachute canopy. Traditionally, circular parachutes are assumed to have symmetric canopy loading upon inﬂation and during terminal descent. Asymmetric canopy loading can have a signiﬁcant impact on circular parachute suspension line loads, but is typically neglected. The developed strain gauge-based load distribution measurement system for circular parachutes has wireless capabilities and can be readily applied to a wide variety of aerodynamic declarator systems. The developed system can be used to observe asymmetric behaviors in order to help determine the signiﬁcance of asymmetric canopy loading. Custom strain gauge load cells with mounted custom circuitry to calibrate, amplify, and transmit the load data were ﬁxed to canopy suspension lines. Parachute drop testing was performed to evaluate the effectiveness to identify any signiﬁcant asymmetric canopy loading behavior. Drop testing was performed with a 1.2m (4.0ft) quarter-spherical cross based canopy with a payload of 2.0kg (4.4lbs). A 12m (39ft) guide-line based drop rig was implemented to prevent canopy rotational movement that could hinder testing repeatability. Load distribution data was ﬁrst veriﬁed via both static calibration and in-ﬂight total canopy load measurements. Drop testing was then conducted to identify loading asymmetry during both inﬂation and terminal descent. Results demonstrated the use of the strain gauge-based load distribution measurement system for measuring signiﬁcant asymmetric canopy loading patterns. In pursuit of the second thesis objective, strain gauges were used to aid in the development of a thrust estimation system for individual motors/propellers of a small quadrotor unmanned aerial vehicle (UAV). Small UAVs have become increasingly utilized for a wide range of applications; however, such aircraft typically do not undergo the same rigorous safety protocols as their larger human-piloted counterparts. A thrust estimation technique for a quadrotor unmanned aircraft was developed and evaluated that could potentially improve ﬂight control design by increasing sensory feedback information. Strain gauges were integrated into the quadrotor frame to provide total force measurements on each arm of the aircraft. A dynamic model coupled with state information from motion capture and on-board measurement data was implemented to compensate for inertial forces caused by rotational and translational acceleration. Testing was conducted to evaluate the accuracy of the individual load cells, inertial compensation,and free-ﬂight motor thrust estimates. Results demonstrate inertial force compensation during high frequency aircraft motion, which could potentially be useful for detecting an in-ﬂight failure. The measurement system therefore has the potential to quickly detect an in-ﬂight failure. The focus of the third thesis objective is to expand on the development of the thrust estimation system by performing an evaluation of the fault detection capabilities. A comparative study was conducted of the thrust estimation system along with a real time parameter estimation in the frequency domain during two motor failure scenarios of a small quadrotor UAV. Detecting and mitigating disturbances caused by in-ﬂight mo tor/propeller failures is an important aspect of a robust ﬂight controller for multirotor aircraft. The comparative study was performed in an attempt to determine whether direct thrust estimation (strain gauge-based) or indirect thrust estimation (parameter estimation using on-board measurement) more accurately and quickly capture an in-ﬂight failure. Flight test results were post-processed to mimic real-time parameter estimation and strain gauged-based fault detection. Results show the strain gauge-based parameter estimation exhibits noisy estimates, but does have faster response to the failure. The parameter estimation using onboard data does not respond to failures as quickly as the strain-gauge based technique, but does produce better parameter estimate stability. Although both estimation techniques display strengths and weaknesses, neither technique is optimal for real time failure detection individually. A combination of the real-time parameter estimation in the frequency domain and the strain gauge-based thrust estimation techniques may yield a fast yet stable fault detection system. The evaluation of the fault detection capabilities of the thrust estimation system did not prove unsuccessful, however it has warranted further investigation into the overall effectiveness of the system for fault detection.
Table of Contents
Introduction -- Literature review -- Validation and flight testing of a wireless load distribution measurement system -- Feasibility of in-flight quadrotor individual motor thrust measurements -- Conclusion