Reinforcement Learning

(Part 1)

MIT 6.421:

Robotic Manipulation

Fall 2023, Lecture 19

Follow live at https://slides.com/d/HoT1aag/live

(or later at https://slides.com/russtedrake/fall23-lec19)

Levine*, Finn*, Darrel, Abbeel, JMLR 2016

Last time: Visuomotor policies

perception network

(often pre-trained)

policy network

other robot sensors

learned state representation

actions

x history

OpenAI - Learning Dexterity

Recipe:

Make the simulator
Write cost function
Deep policy gradient

Ex: OpenAI Gym \(\Rightarrow\) Gymnasium

import gymnasium as gym

class FooEnv(gym.Env):
  metadata = {'render.modes': ['human']}

  def __init__(self):
    ...
  def step(self, action):
    ...
  def reset(self):
    ...
  def render(self, mode='human'):
    ...
  def close(self):
    ...

https://gymnasium.farama.org/

import pydrake.all


builder = DiagramBuilder()
....
diagram = builder.Build()
simulator = Simulator(diagram)


simulator.AdvanceTo(...)
observation = sensor_output_port->Eval(context)
reward = reward_output_port->Eval(context)


context = diagram.CreateDefaultContext()


meshcat.Publish(context)

DrakeGymEnv

from pydrake.gym import DrakeGymEnv

Deep RL + Teacher-Student

Lee et al., Learning quadrupedal locomotion over challenging terrain, Science Robotics, 2020

Deep RL + Teacher-student (Tao Chen, MIT at TRI)

OpenAI - Learning Dexterity

"PPO has become the default reinforcement learning algorithm at OpenAI because of its ease of use and good performance."

https://openai.com/blog/openai-baselines-ppo/

model = PPO('MlpPolicy', env, verbose=1, tensorboard_log=log)

stable_baselines3/common/policies.py#L435-L440

 # Default network architecture, from stable-baselines
net_arch = [dict(pi=[64, 64], vf=[64, 64])]

Policy Architecture

Actions

Observations

builder.ExportOutput(inv_dynamics.get_desired_position(), "actions")

Network

builder.ExportOutput(plant.get_state_output_port(), "observations")

approximately:

Cost Function

angle_from_vertical = (box_state[2] % np.pi) - np.pi / 2
cost = 2 * angle_from_vertical**2  # box angle
cost += 0.1 * box_state[5]**2  # box velocity
effort = actions - finger_state[:2]
cost += 0.1 * effort.dot(effort)  # effort
# finger velocity
cost += 0.1 * finger_state[2:].dot(finger_state[2:])
# Add 10 to make rewards positive (to avoid rewarding simulator
# crashes).
output[0] = 10 - cost