210 lines
7.7 KiB
TeX
210 lines
7.7 KiB
TeX
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% `AltitudeHold.tex' -- describes the FGFS Altitude Hold
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% Written by Curtis Olson. Started December, 1997.
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%
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% $Id$
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%------------------------------------------------------------------------
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\documentclass[12pt]{article}
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\usepackage{anysize}
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\papersize{11in}{8.5in}
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\marginsize{1in}{1in}{1in}{1in}
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\usepackage{amsmath}
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\usepackage{epsfig}
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\usepackage{setspace}
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\onehalfspacing
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\usepackage{url}
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\begin{document}
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\title{
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Flight Gear Autopilot: \\
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Altitude Hold Module
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}
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\author{
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Curtis Olson \\
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(\texttt{curt@me.umn.edu})
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}
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\maketitle
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\section{Introduction}
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Working on scenery creation was becoming stressful and overwhelming.
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I needed to set it aside for a few days to let my mind regroup so I
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could mount a fresh attack.
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As a side diversion I decided to take a stab at writing an altitude
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hold module for the autopilot system and in the process hopefully
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learn a bit about control theory.
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\section{Control Theory 101}
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Before I get too far into this section I should state clearly and up
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front that I am not a ``controls'' expert and have no formal training
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in this field. What I say here is said \textit{to the best of my
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knowledge.} If anything here is mistaken or misleading, I'd
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appreciate being corrected. I'd like to credit my boss, Bob Hain, and
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my coworker, Rich Kaszeta, for explaining this basic theory to me, and
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answering my many questions.
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The altitude hold module I developed is an example of a PID
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controller. PID stands for proportional, integral, and derivative.
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These are three components to the control module that will take our
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input variable (error), and calculate the value of the output variable
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required to drive our error to zero.
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A PID needs an input variable, and an output variable. The input
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variable will be the error, or the difference between where we are,
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and where we want to be. The output variable is the position of our
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control surface.
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The proportional component of the PID drives the output variable in
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direct proportion to the input variable. If your system is such that
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the output variable is zero when the error is zero and things are
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mostly linear, you usually can get by with proportional control only.
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However, if you do not know in advance what the output variable will
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be when error is zero, you will need to add in a measure of integral
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control.
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The integral component drives the output based on the area under the
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curve if we graph our actual position vs. target position over time.
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The derivative component is something I haven't dealt with, but is
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used to drive you towards your target value more quickly. I'm told
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this must be used with caution since it can easily yield an unstable
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system if not tuned correctly.
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Typically you will take the output of each component of your PID and
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combine them in some proportion to produce your final output.
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The proportional component quickly stabilizes the system when used by
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itself, but the system will typically stabilize to an incorrect value.
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The integral component drives you towards the correct value over time,
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but you typically oscillate over and under your target and does not
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stabilize quickly. However, each of these provides something we want.
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When we combine them, they offset each others negatives while
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maintaining their desirable qualities yielding a system that does
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exactly what we want.
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It's actually pretty interesting and amazing when you think about it.
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the proportional control gives us stability, but it introduces an
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error into the system so we stabilize to the wrong value. The
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integral components will continue to increase as the sum of the errors
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over time increases. This pushes us back the direction we want to
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go. When the system stabilizes out, we find that the integral
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component precisely offsets the error introduced by the proportional
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control.
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The concepts are simple and the code to implement this is simple. I
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am still amazed at how such a simple arrangement can so effectively
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control a system.
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\section{Controlling Rate of Climb}
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Before we try to maintain a specific altitude, we need to be able to
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control our rate of climb. Our PID controller does this through the
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use of proportional and integral components. We do not know in
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advance what elevator position will establish the desired rate of
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climb. In fact the precise elevator position could vary as external
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forces in our system change such as atmospheric density, throttle
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settings, aircraft weight, etc. Because an elevator position of zero
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will most likely not yield a zero rate of climb, we will need to add
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in a measure of integral control to offset the error introduced by the
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proportional control.
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The input to our PID controller will be the difference (or error)
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between our current rate of climb and our target rate of climb. The
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output will be the position of the elevator needed to drive us towards
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the target rate of climb.
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The proportional component simply sets the elevator position in direct
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proportion to our error.
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\[ \mathbf{prop} = K \cdot \mathbf{error} \]
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The integral component sets the elevator position based on the sum of
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these errors over time. For a time, $t$
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\[ \mathbf{integral} = K \cdot \int_{0}^{t} { \mathbf{error} \cdot \delta t } \]
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I do nothing with the derivative component so it is always zero and
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can be ignored.
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The output variable is just a combination of the proportional and
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integral components. $w_{\mathit{prop}}$ and $w_{\mathit{int}}$ are
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weighting values. This allows you to control the contribution of each
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component to your final output variable. In this case I found that
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$w_{\mathit{prop}} = 0.9$ and $w_{\mathit{int}} = 0.1$ seemed to work
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quite well. Too much integral control and your system won't
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stabilize. Too little integral control and your system takes
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excessively long to stabilize.
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\[
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\mathbf{output} = w_{\mathit{prop}} \cdot \mathbf{prop} +
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w_{\mathit{int}} \cdot \mathbf{int}
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\]
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We are trying to control rate of climb with elevator position, so the
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output of the above formula is our elevator position. Using this
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formula to set a new elevator position each iteration quickly drives
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our climb rate to the desired value.
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\section{Controlling Altitude}
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Now that we have our rate of climb under control, it is a simple
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matter to leverage this ability to control our absolute altitude.
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The input to our altitude PID controller is the difference (error)
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between our current altitude and our goal altitude. The output is the
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rate of climb needed to drive our altitude error to zero.
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Clearly, our climb rate will be zero when we stabilize on the target
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altitude. Because our output variable will be zero when our error is
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zero, we can get by with only a proportional control component.
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All we need to do is calculate a desired rate of climb that is
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proportional to how far away we are from the target altitude. This is
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a simple proportional altitude controller that sits on top of our
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slightly more complicated rate of climb controller.
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\[
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\mathbf{target\_climb\_rate} = K \cdot ( \mathbf{target\_altitude} -
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\mathbf{current\_altitude} )
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\]
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Thus we use the difference in altitude to determine a climb rate and
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we use the desired climb rate to determine elevator position.
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\section{Parameter Tuning}
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I've explained the basics, but there is one more thing that is
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important to mention. None of the above theory and math is going to
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do you a bit of good for controlling your system if all your
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parameters are out of whack. In fact, parameter tuning is often the
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trickiest part of the whole process. Expect to spend a good chunk of
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your time tweaking function parameters to fine tune the behavior and
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effectiveness of your controller.
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\end{document}
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%------------------------------------------------------------------------
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% $Log$
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% Revision 1.1 1999/03/09 19:09:41 curt
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% Initial revision.
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%
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