# Decentralized Multi-Agent Control of a Manipulator in Continuous Task Learning

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## Abstract

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## 1. Introduction

#### 1.1. Context

#### 1.2. State of the Art and Related Works

#### 1.3. Paper Contribution

#### 1.4. Paper Layout

## 2. Problem Description

- Reaching: the agents receive a reward if the robot’s end-effector approaches the cube;
- Grasping: a grasping reward is given if both of the fingers of the gripper touch the cube, so that the grasping phase is fulfilled;
- Lifting: when the cube is lifted off the surface, the lifting reward is provided, and this constitutes a successful episode.

## 3. Methodology

#### 3.1. Preliminaries

- Value-based methods, which learn a value function that is then used to derive a policy;
- Policy-based methods, which have a parameterized policy and directly search for the optimal policy;
- Actor-critic methods, which learn both the value function and the policy. The role of the critic is to judge the actions taken by the actor on the basis of the learned value function. The actor then updates its policy parameters based on the critic’s feedback [35].

#### 3.2. Modular Framework for Decentralized Learning

## 4. Implementation of Proposed Approach

#### 4.1. Learning Environment

#### 4.2. State-Space

- Robot information: the data about the robot and its state is a 36-dimensional vector that contains the joint positions, velocities, gripper position, and the end-effector position, velocity, and orientation. The position of the joints is divided and represented by trigonometric functions, i.e., by sine and cosine representations. The robot has 7 DoFs, which gives a vector of 14 values for the position of each joint represented by a sin and cos function. The gripper is defined by a 2-dimensional vector that specifies the position of the left and right fingers. The joint velocities are represented by a single value for each joint, making a 7-dimensional vector. The end-effector data is represented by a 7-dimensional vector, where the position is represented in cartesian coordinates followed by a quaternion to represent the orientation. The end-effector’s state also includes a 6-dimensional vector specifying the linear and angular velocity.
- Object information: the data of the object that needs to be picked up is represented by the position and orientation in the world reference frame. In order to facilitate the reaching of the cube, a relative position vector between the object and the robot’s gripper is added. The object state, thus, makes a 10-dimensional vector.

#### 4.3. Action Space

#### 4.4. Training Details

#### 4.5. Reward Shaping

- ${r}_{dist}$: the distance reward which is computed using the relative position of the gripper w.r.t. the object;
- ${r}_{vel}$: the velocity reward that takes into account the end-effector velocity vector when approaching the object;
- ${r}_{grip}$: the gripper open reward depending upon the action of the gripper.

#### 4.6. Software Implementation

## 5. Results

- One meta-PPO agent and two low level SAC agents;
- Two PPO agents;
- Two SAC agents;
- One PPO agent and one SAC agent.

- One SAC agent;
- One PPO agent.

#### 5.1. Decentralized Multi-Agent Approach

#### 5.1.1. Meta-PPO Agent with Two Low Level SAC Agents

#### 5.1.2. Two PPO Agents

#### 5.1.3. Two SAC Agents

#### 5.1.4. One SAC Agent One PPO Agent

#### 5.2. Decentralized Single-Agent

#### 5.2.1. Decentralized Single-Agent SAC

#### 5.2.2. Decentralized Single-Agent PPO

#### 5.3. Comparison of Centralized and Decentralized Approach

## 6. Discussion

## 7. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Main conceptual scheme of the proposed approach using a multi-agent and decentralized reinforcement learning approach.

**Figure 5.**Schematic representation of: (

**top**) policy network with 46-dim input (environment states) and 8-dim output (actions); (

**bottom**) value network with considered 46-dim input and 1-dim output.

**Figure 6.**Progression of mean accumulated reward for the meta-agent with two low-level SAC agents. Training results are reported for three experiments with different random seeds. The standard deviation of three runs is shown in the plot.

**Figure 7.**Progression of mean accumulated reward for the two PPO agents.Training results are reported for four experiments with different random seeds. The standard deviation of four runs is shown in the plot.

**Figure 8.**Progression of mean accumulated reward for the two SAC agents. Training results are reported for four experiments with different random seeds. The standard deviation of four runs is shown in the plot.

**Figure 12.**Progression of the mean accumulated reward for the decentralized single-agent PPO. Training results are reported for three experiments with different random seeds. The standard deviation of three runs is shown in the plot.

Approach | Maximum Accumulated Reward |
---|---|

One meta-PPO agent and two low-level SAC agents | 346 |

Two PPO agents | 580 |

Two SAC agents | 607 |

One PPO agent and one SAC agent | 130 |

Decentralized single SAC agent | 1203 |

Decentralized single PPO agent | 480 |

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**MDPI and ACS Style**

Shahid, A.A.; Sesin, J.S.V.; Pecioski, D.; Braghin, F.; Piga, D.; Roveda, L.
Decentralized Multi-Agent Control of a Manipulator in Continuous Task Learning. *Appl. Sci.* **2021**, *11*, 10227.
https://doi.org/10.3390/app112110227

**AMA Style**

Shahid AA, Sesin JSV, Pecioski D, Braghin F, Piga D, Roveda L.
Decentralized Multi-Agent Control of a Manipulator in Continuous Task Learning. *Applied Sciences*. 2021; 11(21):10227.
https://doi.org/10.3390/app112110227

**Chicago/Turabian Style**

Shahid, Asad Ali, Jorge Said Vidal Sesin, Damjan Pecioski, Francesco Braghin, Dario Piga, and Loris Roveda.
2021. "Decentralized Multi-Agent Control of a Manipulator in Continuous Task Learning" *Applied Sciences* 11, no. 21: 10227.
https://doi.org/10.3390/app112110227