Abstract
The German Aerospace Center (DLR) operates the ACT/FHS research helicopter with a full model-based control system applicable from hover to 120 knots forward flight, including take-off and landing. The feedforward controller used in this system is based on linear model inversion of identified local models. Therefore, the overall performance of the feedforward controller is highly dependent on the accuracy of the used inverted models. While several approaches exist to analyze model accuracy in time and frequency domains, a validation method for inverse models is rarely considered in the literature. In this paper, a flight test campaign that has been conducted to analyze model validity of the feedforward controller is presented and evaluated. A new model validation method using the feedforward controller to follow a reference command is discussed. Measured deviations from the commanded references are evaluated in the time domain to highlight the model subsystems that are not precisely represented by the linear models. The obtained results are compared with a validation method from system identification. Model extensions are proposed to increase the model accuracy for feedforward controller usage.
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Abbreviations
- \({\mathbf {A}}\), \({\mathbf {B}}\), \({\mathbf {C}}\), \({\mathbf {D}}\) :
-
State space matrices
- \(\delta\) :
-
Pilot inputs
- \(\delta _{\rm FF}\) :
-
Feedforward controller outputs
- \(\delta _{x}\), \(\delta _{y}\) :
-
Longitudinal, lateral cyclic pilot controls
- \(\delta _{p}\), \(\delta _0\) :
-
Pedal and collective pilot controls
- \(x\), \(x_{\rm c}\), \(x_{\rm meas}\), \(x_{\rm s}\) :
-
System, command model, measured, simulated states
- \(u\), \(v\), \(w\) :
-
Longitudinal, lateral and vertical airspeed components, aircraft-fixed system
- \(p\), \(q\), \(r\) :
-
Roll, pitch and yaw rates
- \(\phi\), \(\theta\), \(\psi\) :
-
Roll, pitch and yaw attitude angles
- \(l_{a1}\), \(l_{a2}\), \(l_{b1}\), \(l_{b2}\) :
-
Lead-lag system states
- \(\nu\) :
-
Dynamic inflow
- \(p_0\), \(p_1\), \(p_2\) :
-
Offset and drift parameters
- \(J_{p, {\rm rms}}\) :
-
Total root mean square error of \(p\) responses
- \(J_{p, {\rm rms},\delta _y}\) :
-
Root mean square error of \(p\) response to \(\delta _y\) input
- \(J_{p, {\rm rms},p_{\rm c}}\) :
-
Root mean square error of \(p\) response to roll command \(p_{\rm c}\)
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Wartmann, J. Model validation and analysis using feedforward control flight test data. CEAS Aeronaut J 6, 429–439 (2015). https://doi.org/10.1007/s13272-015-0152-5
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DOI: https://doi.org/10.1007/s13272-015-0152-5