README

This repository contains the implementation of Dynamic Movement Primitives (DMPs), together with different approaches to obstacle avoidance, in Python 3.5.

In particular, this repository contains all the synthetic tests done for the works:

M. Ginesi, D. Meli, A. Calanca, D. Dall'Alba, N. Sansonetto and P. Fiorini, Dynamic Movement Primitives: Volumetric Obstacle Avoidance, 2019 19th International Conference on Advanced Robotics (ICAR), Belo Horizonte, Brazil, 2019, pp. 234-239.

doi: 10.1109/ICAR46387.2019.8981552

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8981552&isnumber=8981543

bibtex

@INPROCEEDINGS{8981552, author={Ginesi, Michele and Meli, Daniele and Calanca, Andrea and Dall'Alba, Diego and Sansonetto, Nicola and Fiorini, PAolo}, booktitle={2019 19th International Conference on Advanced Robotics (ICAR)}, title={Dynamic Movement Primitives: Volumetric Obstacle Avoidance}, year={2019}, volume={}, number={}, pages={234-239}, keywords={}, doi={10.1109/ICAR46387.2019.8981552}, ISSN={null}, month={Dec}}

and the work, currently under revision,

M. Ginesi, D. Meli, A. Roberti, N. Sansonetto and P. Fiorini, Dynamic Movement Primitives: Volumetric ObstacleAvoidance Using Dynamic Potential Functions

whose preprint can be found at

https://arxiv.org/abs/2007.00518

Install

The package can be installed by running

pip install -e .

pip3 install -e .

Repository Contents

The repository contains two folders: codes and demos.

The codes folder

The codes folder contains all the scripts performing the basis functions needed to implement DMPs (together with obstacle avoidance). In particular:

cs.py implements the so-called "Canonical System";
dmp_cartesian.py is the class that generates and handle the DMP, able to handle both the learning of the weights given a trajectory and to execute a trajectory given the set of weights;
ellipsoid.py implements the method using to extract the minimum volume enclosing ellipsoid algorithm [1];
exponential_integration.py contains the functions needed to perform the numerical integration method "Exponential Euler";
point_obstacle.py is the class which implements obstacle avoidance for point-like obstacles using the methods presented in [2] and [3];
obstacle_superquadric.py is the class which implements volumetric obstacle avoidance for ellipses (2 dimensions) and ellipsoids (3 dimensions) using both static (for our first work) and dynamic (for our second work) potential functions

The demos folder

The demos folder contains all the scripts performing the tests proposed in our works. the folder contains two sub-folders, one for our work with static potentials, and one fo the dynamic potentials. See the comments at the beginning of the code to associate the test to the figure in the paper

Theory: quick recall

Dynamic Movement Primitives are a framework for trajectory learning. It is based on a second order differential equation of spring-mass-damper type: [ \begin{cases} \tau \dot{\mathbf{v}} = \mathbf{K} (\mathbf{g} - \mathbf{x}) - \mathbf{D} \mathbf{v} - \mathbf{K} (\mathbf{g} - \mathbf{x}_0) s + \mathbf{K} \mathbf{f}(s) \ \tau \dot{\mathbf{x}} = \mathbf{v} \end{cases}, ]

where $ \mathbf{x}, \mathbf{v} \in \mathbb{R}^d $ are position and velocity of the system; $\mathbf{x}_0 , \mathbf{g} \in \mathbb{R}^d $ are initial and goal position, respectively; $\mathbf{K}, \mathbf{D} \in \mathbb{R}^{d \times d}$ are diagonal matrices, representing the elastic and damping terms, respectively; $\mathbf{f} (s) \in \mathbb{R}^d $ is the "forcing term"; $\tau \in \mathbb{R}^+$ is a parameter used to make the execution faster or slower. Parameter $s \in \mathbb{R}$ is a re-parametrization of time, governed by the canonical system [ \tau \dot{s} = -\alpha s, \quad \alpha \in \mathbb{R}^+ . ]

Learning Phase

During the learning phase, a desired curve $ \tilde\mathbf{x}(t) $ is shown. This permit to compute the forcing term $\mathbf{f}(s)$. Thus, the forcing term is approximated using Gaussian Radial Basis functions $ { \psi_i(s) }{i=0,1,\ldots, N} $ as [ \mathbf{f}(s) = \frac{ \sum{i=0}^N \omega_i \psi_i(s) }{ \sum_{i=0}^N \psi_i(s) } s , ] with $ \omega_i \in \mathbb{R} $, $ i=0,1,\ldots, N. $

Execution Phase

Once the weights $\omega_i$ have been learned, the forcing term can be computed. The dynamical system can be integrated changing $\mathbf{x}_0, \mathbf{g}$, and $\tau$, thus being able to generalize the trajectory changing initial and final positions, and execution time.

Obstacle Avoidance

To avoid obstacles, a copupling term $\bm{\varphi} (\mathbf{x}, \mathbf{v})$ is added to the first equation of the DMP system. In our approach, we use Superquadric Potential Functions. An isopotential [ C(\mathbf{x}) = \left( \left( \frac{x_1}{f_1(\mathbf{x})} \right) ^ {2n} + \left( \frac{x_2}{f_2(\mathbf{x})} \right) ^ {2n} \right) ^ \frac{2m}{2n} + \left( \frac{x_3}{f_3(\mathbf{x})} \right) ^ {2m} - 1 ] that vanishes on the boundary of the obstacle is created by setting parameters $m,n \in \mathbb{N}$ and functions $f_i(\mathbf{x})$. Then, a potential is created as [ U(\mathbf{x}) = \frac{A \exp (-\eta C(\mathbf{x})) }{ C(\mathbf{x}) }, ] with $ A, \eta \in \mathbb{R}^+ $. Finally, the coupling term is computed as [ \bm{\varphi} (\mathbf{x}, \mathbf{v}) \equiv \bm{\varphi} (\mathbf{x}) = - \nabla_\mathbf{x} U(\mathbf{x}) . ]

References

[1] https://github.com/minillinim/ellipsoid

[2] Hoffmann, H., Pastor, P., Park, D. H., and Schaal, S.. Biologically-inspired dynamical systems for movement generation: automatic real-time goal adaptation and obstacle avoidance. In Robotics and Automation, 2009. ICRA'09. IEEE International Conference on (pp. 2587-2592). IEEE.

tlevendovics/dmp_vol_obst