Actuator modeling

I've added actuator modeling to JSBSim. To use an actuator, stick an
actuator component in there where the control surface position is calculated
in the <flight_control> section.

The actuator component adds the following mechanisms:

hysteresis
first order lag
rate limiting
position limiting
position bias
malfunctions: fail-to-zero, fail-hardover, fail-stuck

Here is an example of an actuator for a rocket gimbal:

  <actuator name="fcs/pitch-actuator-pos">
    <input>fcs/pitch-actuator-cmd</input>
    <lag> 60 </lag>
    <bias> 0.0017 </bias>
    <hysteresis_width> 0.0017 </hysteresis_width>
    <rate_limit> 0.085 </rate_limit>
    <clipto>
      <min> -0.17 </min>
      <max>  0.17 </max>
    </clipto>
    <output>propulsion/engine/pitch-angle-rad</output>
  </actuator>

In this case, the actuator output is referenced by the property name,
"fcs/pitch-actuator-pos". The input is the value of the property,
"fcs/pitch-actuator-cmd". The input is lagged using the first order transfer
function [60/(s + 60)]. The final output has a bias of 0.0017 (since the
input is in radians, the output is also in radians). The bias simulates a
mounting misalignment. The hysteresis is 0.0017. The actuator is constrained
to move no faster than 0.085 rad/sec (again, since the input is in radians,
the rate limiting is in radians/sec). This equates to 5 degrees/sec. The
final position is limited at +/- 0.17 radians (10 deg). This component is
the last in the series, so the <output> is sent to the property
"propulsion/engine/pitch-angle-rad".

Jon
Jon S. Berndt
Development Coordinator
JSBSim Project
www.JSBSim.org


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