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Iterative Linear Quadratic Regulator

.. image:: https://travis-ci.org/anassinator/ilqr.svg?branch=master :target: https://travis-ci.org/anassinator/ilqr

This is an implementation of the Iterative Linear Quadratic Regulator (iLQR) for non-linear trajectory optimization based on Yuval Tassa's paper <https://homes.cs.washington.edu/~todorov/papers/TassaIROS12.pdf>_.

It is compatible with both Python 2 and 3 and has built-in support for auto-differentiating both the dynamics model and the cost function using Theano <http://deeplearning.net/software/theano/>_.

Install

To install, clone and run:

.. code-block:: bash

python setup.py install

You may also install the dependencies with pipenv as follows:

.. code-block:: bash

pipenv install

Usage

After installing, :code:import as follows:

.. code-block:: python

from ilqr import iLQR

You can see the examples <examples/>_ directory for Jupyter <https://jupyter.org>_ notebooks to see how common control problems can be solved through iLQR.

Dynamics model ^^^^^^^^^^^^^^

You can set up your own dynamics model by either extending the :code:Dynamics class and hard-coding it and its partial derivatives. Alternatively, you can write it up as a Theano expression and use the :code:AutoDiffDynamics class for it to be auto-differentiated. Finally, if all you have is a function, you can use the :code:FiniteDiffDynamics class to approximate the derivatives with finite difference approximation.

This section demonstrates how to implement the following dynamics model:

.. math::

m \dot{v} = F - \alpha v

where :math:m is the object's mass in :math:kg, :math:alpha is the friction coefficient, :math:v is the object's velocity in :math:m/s, :math:\dot{v} is the object's acceleration in :math:m/s^2, and :math:F is the control (or force) you're applying to the object in :math:N.

Automatic differentiation """""""""""""""""""""""""

.. code-block:: python

import theano.tensor as T from ilqr.dynamics import AutoDiffDynamics

x = T.dscalar("x") # Position. x_dot = T.dscalar("x_dot") # Velocity. F = T.dscalar("F") # Force.

dt = 0.01 # Discrete time-step in seconds. m = 1.0 # Mass in kg. alpha = 0.1 # Friction coefficient.

Acceleration.

x_dot_dot = x_dot * (1 - alpha * dt / m) + F * dt / m

Discrete dynamics model definition.

f = T.stack([ x + x_dot * dt, x_dot + x_dot_dot * dt, ])

x_inputs = [x, x_dot] # State vector. u_inputs = [F] # Control vector.

Compile the dynamics.

NOTE: This can be slow as it's computing and compiling the derivatives.

But that's okay since it's only a one-time cost on startup.

dynamics = AutoDiffDynamics(f, x_inputs, u_inputs)

Note: If you want to be able to use the Hessians (:code:f_xx, :code:f_ux, and :code:f_uu), you need to pass the :code:hessians=True argument to the constructor. This will increase compilation time. Note that :code:iLQR does not require second-order derivatives to function.

Batch automatic differentiation """""""""""""""""""""""""""""""

.. code-block:: python

import theano.tensor as T from ilqr.dynamics import BatchAutoDiffDynamics

state_size = 2 # [position, velocity] action_size = 1 # [force]

dt = 0.01 # Discrete time-step in seconds. m = 1.0 # Mass in kg. alpha = 0.1 # Friction coefficient.

def f(x, u, i): """Batched implementation of the dynamics model.

  Args:
      x: State vector [*, state_size].
      u: Control vector [*, action_size].
      i: Current time step [*, 1].

  Returns:
      Next state vector [*, state_size].
  """
  x_ = x[..., 0]
  x_dot = x[..., 1]
  F = u[..., 0]

  # Acceleration.
  x_dot_dot = x_dot * (1 - alpha * dt / m) + F * dt / m

  # Discrete dynamics model definition.
  return T.stack([
      x_ + x_dot * dt,
      x_dot + x_dot_dot * dt,
  ]).T

Compile the dynamics.

NOTE: This can be slow as it's computing and compiling the derivatives.

But that's okay since it's only a one-time cost on startup.

dynamics = BatchAutoDiffDynamics(f, state_size, action_size)

Note: This is a faster version of :code:AutoDiffDynamics that doesn't support Hessians.

Finite difference approximation """""""""""""""""""""""""""""""

.. code-block:: python

from ilqr.dynamics import FiniteDiffDynamics

state_size = 2 # [position, velocity] action_size = 1 # [force]

dt = 0.01 # Discrete time-step in seconds. m = 1.0 # Mass in kg. alpha = 0.1 # Friction coefficient.

def f(x, u, i): """Dynamics model function.

  Args:
      x: State vector [state_size].
      u: Control vector [action_size].
      i: Current time step.

  Returns:
      Next state vector [state_size].
  """
  [x, x_dot] = x
  [F] = u

  # Acceleration.
  x_dot_dot = x_dot * (1 - alpha * dt / m) + F * dt / m

  return np.array([
      x + x_dot * dt,
      x_dot + x_dot_dot * dt,
  ])

NOTE: Unlike with AutoDiffDynamics, this is instantaneous, but will not be

as accurate.

dynamics = FiniteDiffDynamics(f, state_size, action_size)

Note: It is possible you might need to play with the epsilon values (:code:x_eps and :code:u_eps) used when computing the approximation if you run into numerical instability issues.

Usage """""

Regardless of the method used for constructing your dynamics model, you can use them as follows:

.. code-block:: python

curr_x = np.array([1.0, 2.0]) curr_u = np.array([0.0]) i = 0 # This dynamics model is not time-varying, so this doesn't matter.

dynamics.f(curr_x, curr_u, i) ... array([ 1.02 , 2.01998]) dynamics.f_x(curr_x, curr_u, i) ... array([[ 1. , 0.01 ], [ 0. , 1.00999]]) dynamics.f_u(curr_x, curr_u, i) ... array([[ 0. ], [ 0.0001]])

Comparing the output of the :code:AutoDiffDynamics and the :code:FiniteDiffDynamics models should generally yield consistent results, but the auto-differentiated method will always be more accurate. Generally, the finite difference approximation will be faster unless you're also computing the Hessians: in which case, Theano's compiled derivatives are more optimized.

Cost function ^^^^^^^^^^^^^

Similarly, you can set up your own cost function by either extending the :code:Cost class and hard-coding it and its partial derivatives. Alternatively, you can write it up as a Theano expression and use the :code:AutoDiffCost class for it to be auto-differentiated. Finally, if all you have are a loss functions, you can use the :code:FiniteDiffCost class to approximate the derivatives with finite difference approximation.

The most common cost function is the quadratic format used by Linear Quadratic Regulators:

.. math::

(x - x_{goal})^T Q (x - x_{goal}) + (u - u_{goal})^T R (u - u_{goal})

where :math:Q and :math:R are matrices defining your quadratic state error and quadratic control errors and :math:x_{goal} is your target state. For convenience, an implementation of this cost function is made available as the :code:QRCost class.

:code:QRCost class """"""""""""""""""""

.. code-block:: python

import numpy as np from ilqr.cost import QRCost

state_size = 2 # [position, velocity] action_size = 1 # [force]

The coefficients weigh how much your state error is worth to you vs

the size of your controls. You can favor a solution that uses smaller

controls by increasing R's coefficient.

Q = 100 * np.eye(state_size) R = 0.01 * np.eye(action_size)

This is optional if you want your cost to be computed differently at a

terminal state.

Q_terminal = np.array([[100.0, 0.0], [0.0, 0.1]])

State goal is set to a position of 1 m with no velocity.

x_goal = np.array([1.0, 0.0])

NOTE: This is instantaneous and completely accurate.

cost = QRCost(Q, R, Q_terminal=Q_terminal, x_goal=x_goal)

Automatic differentiation """""""""""""""""""""""""

.. code-block:: python

import theano.tensor as T from ilqr.cost import AutoDiffCost

x_inputs = [T.dscalar("x"), T.dscalar("x_dot")] u_inputs = [T.dscalar("F")]

x = T.stack(x_inputs) u = T.stack(u_inputs)

x_diff = x - x_goal l = x_diff.T.dot(Q).dot(x_diff) + u.T.dot(R).dot(u) l_terminal = x_diff.T.dot(Q_terminal).dot(x_diff)

Compile the cost.

NOTE: This can be slow as it's computing and compiling the derivatives.

But that's okay since it's only a one-time cost on startup.

cost = AutoDiffCost(l, l_terminal, x_inputs, u_inputs)

Batch automatic differentiation """""""""""""""""""""""""""""""

.. code-block:: python

import theano.tensor as T from ilqr.cost import BatchAutoDiffCost

def cost_function(x, u, i, terminal): """Batched implementation of the quadratic cost function.

  Args:
      x: State vector [*, state_size].
      u: Control vector [*, action_size].
      i: Current time step [*, 1].
      terminal: Whether to compute the terminal cost.

  Returns:
      Instantaneous cost [*].
  """
  Q_ = Q_terminal if terminal else Q
  l = x.dot(Q_).dot(x.T)
  if l.ndim == 2:
      l = T.diag(l)

  if not terminal:
      l_u = u.dot(R).dot(u.T)
      if l_u.ndim == 2:
          l_u = T.diag(l_u)
      l += l_u

  return l

Compile the cost.

NOTE: This can be slow as it's computing and compiling the derivatives.

But that's okay since it's only a one-time cost on startup.

cost = BatchAutoDiffCost(cost_function, state_size, action_size)

Finite difference approximation """""""""""""""""""""""""""""""

.. code-block:: python

from ilqr.cost import FiniteDiffCost

def l(x, u, i): """Instantaneous cost function.

  Args:
      x: State vector [state_size].
      u: Control vector [action_size].
      i: Current time step.

  Returns:
      Instantaneous cost [scalar].
  """
  x_diff = x - x_goal
  return x_diff.T.dot(Q).dot(x_diff) + u.T.dot(R).dot(u)

def l_terminal(x, i): """Terminal cost function.

  Args:
      x: State vector [state_size].
      i: Current time step.

  Returns:
      Terminal cost [scalar].
  """
  x_diff = x - x_goal
  return x_diff.T.dot(Q_terminal).dot(x_diff)

NOTE: Unlike with AutoDiffCost, this is instantaneous, but will not be as

accurate.

cost = FiniteDiffCost(l, l_terminal, state_size, action_size)

Note: It is possible you might need to play with the epsilon values (:code:x_eps and :code:u_eps) used when computing the approximation if you run into numerical instability issues.

Usage """""

Regardless of the method used for constructing your cost function, you can use them as follows:

.. code-block:: python

cost.l(curr_x, curr_u, i) ... 400.0 cost.l_x(curr_x, curr_u, i) ... array([ 0., 400.]) cost.l_u(curr_x, curr_u, i) ... array([ 0.]) cost.l_xx(curr_x, curr_u, i) ... array([[ 200., 0.], [ 0., 200.]]) cost.l_ux(curr_x, curr_u, i) ... array([[ 0., 0.]]) cost.l_uu(curr_x, curr_u, i) ... array([[ 0.02]])

Putting it all together ^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: python

N = 1000 # Number of time-steps in trajectory. x0 = np.array([0.0, -0.1]) # Initial state. us_init = np.random.uniform(-1, 1, (N, 1)) # Random initial action path.

ilqr = iLQR(dynamics, cost, N) xs, us = ilqr.fit(x0, us_init)

:code:xs and :code:us now hold the optimal state and control trajectory that reaches the desired goal state with minimum cost.

Finally, a :code:RecedingHorizonController is also bundled with this package to use the :code:iLQR controller in Model Predictive Control.

Important notes ^^^^^^^^^^^^^^^

To quote from Tassa's paper: "Two important parameters which have a direct impact on performance are the simulation time-step :code:dt and the horizon length :code:N. Since speed is of the essence, the goal is to choose those values which minimize the number of steps in the trajectory, i.e. the largest possible time-step and the shortest possible horizon. The size of :code:dt is limited by our use of Euler integration; beyond some value the simulation becomes unstable. The minimum length of the horizon :code:N is a problem-dependent quantity which must be found by trial-and-error."

Contributing

Contributions are welcome. Simply open an issue or pull request on the matter.

Linting

We use YAPF <https://github.com/google/yapf>_ for all Python formatting needs. You can auto-format your changes with the following command:

.. code-block:: bash

yapf --recursive --in-place --parallel .

You may install the linter as follows:

.. code-block:: bash

pipenv install --dev

License

See LICENSE <LICENSE>_.

Credits

This implementation was partially based on Yuval Tassa's :code:MATLAB implementation <https://www.mathworks.com/matlabcentral/fileexchange/52069>, and navigator8972 <https://github.com/navigator8972>'s implementation <https://github.com/navigator8972/pylqr>_.


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