Practical Guide to State-space Control: Graduate-level control theory for high schoolers.
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README.md

Practical Guide to State-space Control

Graduate-level control theory for high schoolers

I originally wrote this as a final project for an undergraduate technical writing class I took at University of California, Santa Cruz in Spring 2017 (CMPE 185). It is intended as a digest of graduate-level control theory aimed at veteran FIRST Robotics Competition (FRC) students who know algebra and a bit of physics and are comfortable with the concept of a PID controller. As I learned the subject of control theory, I found that it wasn't particularly difficult, but very few resources exist outside of academia for learning it. This book is intended to rectify that situation by providing a lower the barrier to entry to the field.

This book reads a lot like a reference manual on control theory and related tools. It teaches the reader how to start designing and implementing control systems for practical systems with an emphasis on pragmatism rather than theory. While the theory is mathematically elegant at times and helps inform what is going on, one shouldn't lose sight of how it behaves when applied to real systems.

Download

A PDF version is available at https://file.tavsys.net/control/state-space-guide.pdf.

Running the examples

Example Python scripts can be obtained from frccontrol's Git repository at https://github.com/calcmogul/frccontrol/tree/master/examples. Furthermore, all scripts in the code directory are runnable by the user. They require Python 3.5+ and frccontrol. frccontrol can be installed with the following command.

pip3 install --user frccontrol

The scripts can be run like follows:

./elevator.py

Some Linux platforms use tk as a backend for matplotlib, so that may need to be installed to see the plots.

Dependencies

To compile the book, the following are required.

  • make (to run the makefile)
  • texlive-core (for latexmk)
  • texlive-latexextra (for bibtex and makeglossaries)
  • texlive-bibtexextra (for additional BibTeX styles and bibliography databases)
  • xelatex (for setting and using custom fonts)
  • biber (for generating bibliography)
  • Python 3.5+
    • frccontrol (to provide FRC wrappers for Python Control)
    • Python Control (to generate plots and state-space results)
      • Installed automatically as a dependency of frccontrol
  • Inkscape (to convert SVGs to PDF)

The following are optional because the book can compile without them.

  • Optional python packages
    • slycot (to generate pole-zero maps for certain state-space model examples)
    • black (to format Python source code)
  • ghostscript (to reduce size of final PDF for publishing)

Compiling the book

After installing the dependencies, just run make. By default, two files will be generated: state-space-guide.pdf, which has uncompressed images (way higher than 300dpi); and state-space-guide-ebook.pdf, which is suitable for online distribution.

The following make targets are supported for compiling the book with various levels of image compression.

Command File Purpose
make book-stamp state-space-guide.pdf Quick content development
make ebook state-space-guide-ebook.pdf Online distribution
make printer state-space-guide-printer.pdf Standard color printing
make prepress state-space-guide-prepress.pdf Book publishing

Style guide

All LaTeX labels, including bibliography entries, should use underscores to represent spaces between words instead of hyphens. Hyphens should only be used where the character being written is already a hyphen. .tex file names should use hyphens and .py file names should use underscores.

Glossary entries in glossary-entries.tex should be lexographically sorted by entry key. The entry key should be the same as the name. The words in the name should be lowercase, and the description should start with a capital letter and end with a period. The first sentence should be a fragment, but sentences after that, if applicable, should be complete.

Bibliography entries in state-space-guide.bib should be sorted lexographically by label. Links to online videos should use the @misc tag. Links to online static resources like PDFs should use the @online tag.

Book content should answer the question why something works the way it does and how and when to use it to solve problems.

Future improvements

Teach topics more thoroughly

The book is still somewhat dense and fast-paced (it covers three classes of feedback control, two of which are for graduate students, in one short book). More examples of concepts should help slow the pace down. I should read through this book as if I was explaining it to my veteran software students and anticipate gaps in their understanding.

Give matrix \mtx{0} and \mtx{I} in equations dimension subscripts

  • m states, n inputs, p outputs

Add exercises at the end of each chapter (with solutions) for reader practice. This book is supposed to be "practical" after all.

  • This should help me see the points I want to teach in each chapter (since the students should be able to complete the problems) so I can make sure the chapter teaches it sufficiently.
  • Basically like implicit learning outcomes?

Clarify what is meant by the controller and observer being mathematical duals of each other.

Add test of P vs PD controller for flywheel

  • Simulate flywheel with stochastic force applied to slow it down (simulates shots)
  • Try P controller, PD controller, P + FF, PD + FF, and P + FF + u_error

Add diagram to Ramsete that shows global and vehicle coordinate frames to help explain what the pose represents.

Finish incomplete topics

Expand the implementation steps section on writing unit tests in Google Test.

Add examples of u_error state-space models to frccontrol and reference them in the book.

The following state-space implementation examples are planned:

  • Rotating claw with independent top/bottom
    • See 971/y2014/control_loops/python/claw.py
    • Use as example of coordinate transformations for states?

Supplementary background

Any other results that are good for background but are unnecessary should be included in an appendix.

  • Add derivations for trapezoidal and S-curve profiles to the derivations appendix.
  • Add section on spline generation to complement motion profiling
    • Mention circular arc approximation
  • Import an LQR derivation based on Lagrange multipliers?
  • Explain N term for LQR
    • It's a cross term between error and control effort. One application for it is making one system behave like some other desired system. This is used on the Blackhawk helicopter at NASA Ames when they want to make it fly like experimental aircraft (within the limits of the helicopter's actuators, of course).
    • Describe implicit model following application
  • Finite horizon, discrete time LQR
  • Add an appendix on Ito calculus to explain where the Wiener process comes from?
  • Add equations for Kalman smoother
  • Add equations for and implementation of MMAE
  • Add EKF for nonlinear pose estimation
  • Derive the two-sensor and three-sensor problems from first principles. The two-sensor problem uses p(x) and p(z_1|x). The three-sensor problem uses p(x), p(z_1|x), and p(z_2|x).
  • Add a section on polytopes for convex optimization? So far, I've seen it used for handling saturated control inputs to prioritize tracking some states over others using the limited control input.
    • See 971/y2017/control_loops/python/polydrivetrain.py

Miscellaneous fixes

  • Makefile should pull frccontrol and rerun all Python scripts if new commits were added. If pull fails due to lack of internet, ignore it.
  • Modify pareto_boundary.py to find and plot real Pareto boundary for LQR instead of using a hand-wavey approximation
  • Fix bugs in dependencies
    • Fix nonsquare system pzmaps in Python Control, then enable non-Slycot pzmaps for remaining examples
  • Eventually convert the book to ConTeXt

Licensing

This project, except for the software, is released under the Creative Commons Attribution-ShareAlike 4.0 International license. The software is released under the 3-clause BSD license.