# Underactuated Robotics

Algorithms for Walking, Running, Swimming, Flying, and Manipulation

Russ Tedrake

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Note: These are working notes used for a course being taught at MIT. They will be updated throughout the Spring 2022 semester. Lecture videos are available on YouTube.

# An Optimization Playbook

This chapter is intended to provide a collection of tips and formulations that can make seemingly non-smooth constraints smooth, seemingly non-convex constraints convex, etc. (It's very much a work in progress...)

# Matrices

max eigenvalue. min eigenvalue. rank (nuclear norm, ...)

# Ellipsoids

Imagine an ellipsoid centered at the origin: $E = \{x | x^T S x \leq 1\}$ with $S=S^T\succ0$. The volume of this ellipse is proportional to $\left(\det S^{-1} \right)^\frac{1}{2}$. We know that $\log\det(S)$ is a concave function in the elements of $S$ Boyd04a.

maximizing volume.

minimizing volume.

volume of a semi-algebraic set. (via a contained ellipsoid)

# Polytopes

Sadra's linear encodings of polytopic containment

# Bilinear Matrix Inequalities (BMIs)

PQ trick from LQR Rank minimization

# Geometry (SE(3), Penetration, and Contact)

Hongkai's smooth non-penetration constraints for polygons "Soft" absolute value: $|x|_epsilon \approx \sqrt{x^2 + \epsilon}$. import numpy as np import matplotlib.pyplot as plt x = np.linspace(-2.,2.,101) plt.plot(x, np.sqrt(np.power(x,2) + 0.01))
Barrier function for friction cones in Grandia 2019, Feedback MPC for Torque-Controlled Legged Robots. Linearization of the rotation matrix in Real-time Model Predictive Control for Versatile Dynamic Motions in Quadrupedal Robots Briat15 chapter 1 has some good modeling tricks for LPV systems

# References

1. , "Convex Optimization", Cambridge University Press , 2004.