Hardware Systems

Sensors

The PR2 ships with sensors in the head, arms, and base. The head contains two stereo camera pairs coupled with an LED pattern projector, a 5MP camera, a tilting laser range finder, and an IMU. The forearms each contain an Ethernet-based, wide-angle camera, while the grippers have three-axis accelerometers and pressure sensor arrays on the fingertips. The base has a fixed laser range finder.

With a nominal sensor payload, the PR2 is designed for sensor modularity and extensibility; additional sensors can be mounted easily. The PR2 comes with a variety of sensor mounting points, including the top of the pan-tilt head, on the shoulders, and along the forearms.

Computation

Battery System

The PR2 robot has two eight-core i7 Xeon system servers on-board, each with 24 GB of RAM, a 500 GB internal hard drive, and a 1.5 TB external removable log drive.  The computers and most of the sensors communicate over a 16-port gigabit Ethernet hub with a 32-gigabit backplane. The robot also has an on-board, dual-radio router that can be bridged into a WLAN, as well as a secondary, stand-alone access point for laptop or smart phone access.

The PR2 has a 1.2 kWh battery pack with on-board chargers. The system supports hot plugging, with transparent transitions between charging mode (plugged in) and battery power mode (unplugged). The PR2 has the capacity for two hours of run-time, depending on usage.

Mobility

The PR2 design employs four steered and driven casters to achieve quasi-holonomic performance. Although fully holonomic/omni-directional drivetrains exist, these systems do not allow for the robust performance required of a personal robot (e.g., driving over doorway thresholds, curbs, and extension cords).The PR2 base provides fast, accurate response that is capable of easily traversing 1-2 cm obstacles.

Manipulation

To manipulate common household and workplace objects, the PR2 has two 7-DOF arms with a range of motion similar to that of a human. Each arm carries a simple, single-DOF gripper capable of grasping everyday objects.

Forgiving, Back-drivable Arms

The arm's four main DOFs are back-drivable. When the arm encounters an object, the force of the interaction drives the motors back; the arm gives way. This back-drivability ensures that the PR2's arms can bump into obstacles without attempting to push through them, and that the robot can operate in unstructured environments.

Each arm has 7 DOFs in total: pan and tilt shoulder joints, upper arm roll joints, elbow flexion joints, forearm roll joints, wrist flexion joints, and gripper roll joints. The arms have a payload of 1.8 Kg, adequate for holding and manipulating typical household objects (i.e., kitchen implements, doors, chairs). The generous wrist torque enables the PR2 to complete real-world tasks, such as holding a frying pan by the handle or sweeping with a broom. 

Picking up a Wide Range of Objects

The PR2 has a simple, yet capable gripper outfitted with a pressure sensor array on the gripper tips. The simple gripper design is capable of grasping a diverse set of objects—from pens and silverware to frying pans and door knobs.

Extending the Reach

The spine of the PR2 can telescope to extend its reach from objects on the ground to objects on countertops.

Real-time control on all DOFs

Real-time controllers for all joints and degrees of freedom are implemented on a single computer using real-time Linux extensions. Executing at 1 kHz, the controllers communicate joint angular positions, motor current commands, and diagnostic information with the actuators and sensors via an EtherCAT network. EtherCAT is a 100-Megabit, real-time, deterministic communications framework. Local PWM amplifiers regulate the current for each brushed DC motor.

An Open Hardware Platform

In order to develop cutting-edge robotic technologies, it is important that both hardware and software platforms be open to users. We have designed the PR2 from the ground up to be open.

Hardware

Whether you are adding a new sensor, or designing your own gripper, we have designed the PR2 for you to modify and extend at multiple levels. For extensive modifications, such as replacing hardware components, we provide the electrical and mechanical specifications for the PR2's interfaces. This enables all users to fully access the PR2's modular capabilities.

For quick modifications, the PR2 has bolt-patterns on the head and shoulders for easy mounting. The PR2 uses standard technologies, like Ethernet, wherever possible, and we provide extra 12V power, Ethernet ports, and USB ports.

Software

The software we have written for the PR2 is open-source. Whether you are modifying low-level hardware drivers, or creating new high-level behaviors, it is important that you be able to freely access and modify the source code. We also give administrators root access to the computers so that all the software systems can be accessed

Arm Modularity

Sensor Modularity