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<h1><a href="index.html" style="text-decoration:none;">Underactuated Robotics</a></h1>
<p data-type="subtitle">Algorithms for Walking, Running, Swimming, Flying, and Manipulation</p>
<p style="font-size: 18px;"><a href="http://people.csail.mit.edu/russt/">Russ Tedrake</a></p>
<p style="font-size: 14px; text-align: right;">
© Russ Tedrake, 2020<br/>
<a href="tocite.html">How to cite these notes</a> |
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<p><b>Note:</b> These are working notes used for <a
href="http://underactuated.csail.mit.edu/Spring2020/">a course being taught
at MIT</a>. They will be updated throughout the Spring 2020 semester. <a
href="https://www.youtube.com/channel/UChfUOAhz7ynELF-s_1LPpWg">Lecture videos are available on YouTube</a>.</p>
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<chapter style="counter-reset: chapter 15"><h1>Algorithms
for Limit Cycles</h1>
<p>The discussion of walking and running robots in Chapter 4 motivated the
notion of limit cycle stability. Linear systems are not capable of producing
stable limit cycle behavior, so this rich topic is unique to nonlinear systems
design and analysis. Furthermore, the tools that are required to design,
stabilize, and verify limit cycles will have applicability beyond simple
periodic motions.</p>
<p>The first natural question we must ask is, given a system $\dot{\bx} =
f(\bx)$, or a control system $\dot{x} = f(\bx,\bu)$, how do we go about
finding periodic solutions which may be passively stable, open-loop stable, or
stabilizable via closed-loop feedback? It turns out that the trajectory
optimization tools that we developed already are very well suited to this
task.</p>
<section id="trajopt"><h1>Trajectory optimization</h1>
<p>I introduced the trajectory optimization tools as means for optimizing a
control trajectory starting from a particular known initial condition. But
the fundamental idea of optimizing over individual trajectories of the
system is useful more broadly. Even for a passive system, we can formulate
the search for a periodic solution as an optimization over trajectories that
satisfy the dynamic constraints and <em>periodicity constraints</em>,
$\bx[0] = \bx[N]$: \begin{align*} \find_{\bx[\cdot]} \quad \subjto \quad &
\bx[n+1] = f(\bx[n]), \quad \forall n\in[0, N-1] \\ & \bx[0] = \bx[N].
\end{align*} Certainly we can add control inputs back into the formulation,
too, but let's start with this simple case. Take a moment to think about
the feasible solutions to this problem formulation. Certainly a fixed point
$\bx[n] = \bx^*$ will satisfy the constraints; if we don't want these
solutions to come out of the solver we might need to exclude them with
constraints or add an objective that guides the solver towards the desired
solutions. The other possible solutions are trajectories that are periodic
in exactly $N$ steps. That's pretty restrictive.
</p>
<p>We can do better if we use the continuous-time formulations. For
instance, in our introduction of <a
href="trajopt.html#direct_collocation">direct collocation</a>, we wrote
\begin{align*} \min_{\bx[\cdot],\bu[\cdot]} \quad & \ell_f(\bx[N]) +
\sum_{n_0}^{N-1} h_n \ell(\bx[n],\bu[n]) \\ \subjto \quad & \dot\bx(t_{c,n})
= f(\bx(t_{c,n}), \bu(t_{c,n})), & \forall n \in [0,N-1] \\ & \bx[0] = \bx_0
\\ & + \text{additional constraints}. \end{align*} But we can also add
$h_n$ as decision variables in the optimization (reminder: I recommend
setting a lower-bound $h_n \ge h_{min} > 0$). This allows our $N$-step
trajectory optimization to scale and shrink time in order to satisfy the
periodicity constraint. The result is simple and powerful.</p>
<example><h1>Finding the limit cycle of the
Van der Pol oscillator</h1>
<p>Recall the dynamics of the Van der Pol oscillator given by $$\ddot{q} +
\mu (q^2 - 1) \dot{q} + q = 0, \quad \mu>0,$$ which exhibited a stable
limit cycle.</p>
<p>Formulate the direct collocation optimization: \begin{align*}
\find_{\bx[\cdot],h} \quad \subjto \quad & q[0] = 0, \quad \dot{q}[0] > 0,
\\ & \bx[N] = \bx[0], \text{(periodicity constraint)}\\ &
\text{collocation dynamic constraints} \\ & 0.01 \le h \le 0.5
\end{align*}
</p>
<p>Try it yourself:</p>
<jupyter>examples/limit_cycles.ipynb</jupyter>
<p>As always, make sure that you take a look at the code. Poke around.
Try changing some things.</p>
<p>One of the things that you should notice in the code is that I provide
an initial guess for the solver. In most of the examples so far I've been
able to avoid doing that--the solver takes small random numbers as a
default initial guess and solves from there. But for this problem, I
found that it was getting stuck in a local minima. Adding the initial
guess that the solution moves around a circle in state space was enough.
</p>
</example>
<todo> example: simplest flapping robot </todo>
</section> <!-- end trajectory optimization -->
<section id="lyapunov"><h1>Lyapunov analysis</h1>
<p>Recall the important distinction between stability of a trajectory in
time and stability of a limit cycle was that the limit cycle does not
converge in phase -- trajectories near the cycle converge to the cycle,
but trajectories on the cycle may not converge with each other. This is
type of stability, also known as <em>orbital stability</em> can be written
as stability to the manifold described by cycle $\bx^*(t)$, \[ \min_\tau
|| x(t) - x^*(\tau) || \rightarrow 0.\] In the case of limit cycles, this
manifold is a periodic solution with $\bx^*(t+t_{period}) = \bx^*(t)$.
Depending on exactly how that convergence happens, we can define orbital
stability in the sense of Lyapunov, asymptotic orbital stability,
exponential orbital stability, or even finite-time orbital stability.</p>
<p>In order to prove that a system is orbitally stable (locally, over a
region, or globally), or to analyze the region of attraction of a limit
cycle, we can use a Lyapunov function. In particular, we would like to
consider Lyapunov functions which have the form cartooned below; they
vanish to zero everywhere along the cycle, and are strictly positive
everywhere away from the cycle.</p>
<figure><img
src="figures/limitCycleLyapunovCartoon.svg"/><figcaption>Cartoon of a
Lyapunov function which vanishes on a limit cycle, and is strictly positive
everywhere else. (a small segment has been removed solely for the purposes
of visualization).</figcaption></figure>
<subsection><h1>Transverse coordinates</h1>
<p>How can we parameterize this class of functions? For arbitrary cycles
this could be very difficult in the original coordinates. For simple
cycles like in the cartoon, one could imagine using polar coordinates.
More generally, we will define a new coordinate system relative to the
orbit, with coordinates</p> <ul> <li>$\tau$ - the phase along the
orbit</li> <li>$\bx_\perp(\tau)$ - the remaining coordinates, linearly
independent from $\tau$.</li> </ul> <p>Given a state $\bx$ in the original
coordinates, we must define a smooth mapping $\bx \rightarrow (\tau,
\bx_\perp)$ to this new coordinate system. For example, for a simple ring
oscillator we might have:</p> <figure><img width="80%"
src="figures/transverseCoordinates.svg"/><figcaption>A moving
coordinate system along the limit cycle.</figcaption></figure> In general,
for an $n$-dimensional state space, $\tau$ will always be a scalar, and
$\bx_\perp$ will be an $(n-1)$-dimensional vector defining the remaining
coordinates relative to $\bx^*(\tau)$. In fact, although we use the
notation $\bx_\perp$ the coordinate system need not be strictly orthogonal
to the orbit, but must simply be <em>transversal</em> (not parallel).
Having defined the smooth mapping $\bx \rightarrow (\tau,\bx_\perp)$, we
can always rewrite the dynamics in this new coordinate system:
\begin{gather*} \dot{\tau} = f_1(\bx_\perp,\tau) \\ \dot\bx_\perp =
f_2(\bx_\perp,\tau). \end{gather*} <p>The value of this construction for
Lyapunov analysis was proposed in <elib>Hauser94a</elib> and has been
extended nicely to control design in <elib>Shiriaev08</elib> and for
region of attraction estimation in <elib>Manchester10b</elib>. A
particular numerical strategy for defining the transversal coordinates is
given in <elib>Manchester10a</elib>.</p>
<theorem><h1>A Lyapunov theorem for orbital stability</h1>
<p>For a dynamical system $\dot\bx = f(\bx)$ with $\bx \in \Re^n$, $f$
continuous, a continuous periodic solution $\bx^*(\tau)$, and a smooth
mapping $\bx \rightarrow (\tau,\bx_\perp)$ where $\bx_\perp$ vanishes on
$\bx^*$, then for some $n-1$ dimensional ball ${\cal B}$ around the
origin, if I can produce a $V(\bx_\perp,\tau)$ such that \begin{gather*}
\forall \tau, V(0,\tau) = 0, \\ \forall \tau, \forall \bx_\perp \in
{\cal B}, \bx_\perp \ne 0, V(\bx_\perp,\tau) > 0, \end{gather*} with
\begin{gather*} \forall \tau, \dot{V}(0,\tau) = 0, \\ \forall \tau,
\forall \bx_\perp \in {\cal B}, \bx_\perp \ne 0, \dot{V}(\bx_\perp,\tau)
< 0, \end{gather*} then the solution $\bx^*(t)$ is <em>locally orbitally
asymptotically stable</em>.</p>
</theorem>
<example><h1>Simple ring oscillator</h1>
<p> Perhaps the simplest oscillator is the first-order system which
converges to the unit circle. In cartesian coordinates, the dynamics
are \begin{align*} \dot{x}_1 =& x_2 -\alpha x_1 \left( 1 -
\frac{1}{\sqrt{x_1^2+x_2^2}}\right) \\ \dot{x}_2 =& -x_1 -\alpha x_2
\left( 1 - \frac{1}{\sqrt{x_1^2+x_2^2}}\right), \end{align*} where
$\alpha$ is a positive scalar gain.</p>
<figure><img width=80%
src="figures/ringOscillator.svg"/><figcaption>Vector field of the
ring oscillator</figcaption></figure>
<p>If we take the transverse coordinates to be the polar coordinates,
shifted so that $x_\perp$ is zero on the unit circle, \begin{align*}\tau
=& \text{atan2}(-x_2,x_1) \\ x_\perp =& \sqrt{x_1^2+x_2^2}-1,
\end{align*} which is valid when $x_\perp>-1$, then the simple
transverse dynamics are revealed: \begin{align*} \dot\tau =& 1 \\
\dot{x}_\perp =& -\alpha x_\perp. \end{align*} Taking a Lyapunov
candidate $V(x_\perp,\tau) = x_\perp^2$, we can verify that $$\dot{V} =
-2 \alpha x_\perp^2 \prec 0, \quad \forall x_\perp > -1.$$ This
demonstrates that the limit cycle is locally asymptotically stable, and
furthermore that the invariant open-set $V < 1$ is inside the region of
attraction of that limit cycle. In fact, we know that all $x_\perp > -1$
are in the region of attraction that limit cycle, but this is not proven
by the Lyapunov argument above. </p>
</example>
<p>Let's compare this approach with the approach that we used in the
chapter on walking robots, where we used a Poincaré map analysis to
investigate limit cycle stability. In transverse coordinates approach,
there is an additional burden to construct the coordinate system along the
entire trajectory, instead of only at a single surface of section. In
fact, the transverse coordinates approach is sometimes referred to as a
"moving Poincaré section". But the reward for this extra work is
that we can check a condition that only involves the instantaneous
dynamics, $f(\bx)$, as opposed to having to integrate the dynamics over an
entire cycle to generate the discrete Poincaré map, $\bx_p[n+1] =
P(\bx_p[n])$. While computing $P$ in closed-form happened to be possible
for the simple rimless wheel model, it is rarely possible for more complex
models. As we will see below, this approach will also be more compatible
with designing continuous feedback controller that stabilize the limit
cycle.</p>
</subsection> <!-- end transverse -->
<subsection><h1>Transverse linearization</h1>
<p>In the case of Lyapunov analysis around a fixed point, there was an
important special case: for stable linear systems, we actually have a
recipe for constructing a Lyapunov function. As a result, for nonlinear
systems, we often found it convenient to begin our search by linearizing
around the fixed point and using the Lyapunov candidate for the linear
system as an initial guess. That same approach can be extended to limit
cycle analysis.</p>
</subsection>
<subsection><h1>Region of attraction estimation using sums-of-squares</h1>
<p>Coming soon. If you are interested, see <elib>Manchester10a+Manchester10b</elib>.</p>
</subsection>
</section> <!-- end lyapunov -->
<section><h1>Feedback design</h1>
<subsection><h1>For underactuation degree one.</h1>
<p>It turns out many of our simple walking models -- particulary ones with
a point foot that are derived in the minimal coordinates -- are only short
one actuator (between the foot and the ground). One can represent even
fairly complex robots this way; much of the theory that I'll elude to here
was originally developed by Jessy Grizzle and his group in the context of
the bipedal robot <a
href="https://web.eecs.umich.edu/~grizzle/papers/RABBITExperiments.html">RABBIT</a>.
Jessy's key observation was that limit cycle stability is effectively
stability in $n-1$ degrees of freedom, and you can often achieve it
easily with $n-1$ actuators -- he called this line of work "Hybrid
Zero Dynamics" (HZD). We'll deal with the "hybrid" part of that in the
next chapter, but here is a simple example to illustrate the "zero
dynamics" concept. [Coming sooon...] </p>
<todo>Add a simple example here.</todo>
<p>Stabilizing the zero dynamics is synonymous with achieving orbital
stability. Interestingly, it doesn't actually guarantee that you will go
around the limit cycle. One "feature" of the HZD walkers is that you can
actually stand in front of them and stop their forward progress with your
hand -- the control system will continue to act, maintaining itself on the
cycle, but progress along the cycle has stopped. In some of the robots
you could even push them backwards and they would move through the same
walking cycle in reverse. If one wants to certify that forward progress
will be achieved, then the tasks is reduced to inspecting the
one-dimensional dynamics of the phase variable along the cycle.</p>
<p>The notion of "zero dynamics" is certainly not restricted to systems
with underactuation of degree one. In general, we can easily stabilize a
manifold of dimension $m$ with $m$ actuators (we saw this in the section
on <a href="acrobot.html#task_space_pfl"> task-space partial feedback
linearization</a>), and if being on that manifold is sufficient to achieve
our task, or if we can certify that the resulting dynamics on the manifold
are sufficient for our task, then life is good. But in the
"underactuation degree one" case, the manifold under study is a
trajectory/orbit, and the tools from this chapter are immediately
applicable.</p>
</subsection>
<subsection><h1>Transverse LQR</h1>
</subsection>
<subsection><h1>Orbital stabilization for non-periodic trajectories</h1>
</subsection>
</section> <!-- end feedback -->
</chapter>
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