1. Design for Risk Reduction

We have designed both hardware and software to minimize risks, while retaining the power of an open platform. These aspects of the design are often described as inherent safety features. For example, the PR2's arms are back-drivable. This means that when an arm encounters an object, be it a table or a person, the interaction will drive the motors back and bring the arm to a stop. The PR2's arms can't "punch through" an object the way traditional industrial robots can.

We have further designed the PR2's arms using relatively small motors with respect to their payload. This is possible due to a spring counterbalance offsetting the gravity forces acting on the arms. That is, the arms do not need to hold their own weight against gravity, so the motors need only be strong enough to hold the payload. The arms simply can't push very hard.

In software, we have incorporated low-level checking to limit the current in a motor, the velocity of a motor, and the range through which a joint should travel. We obviously discourage users and developers from changing these configurations. High-level applications also avoid obstacles in navigation and movement using the various on-board sensors.

These design choices also help make the PR2 robust. However, a robot with the PR2's capabilities can never be absolutely safe. Your safety as well as the safety of others critically depends on your constant attention. You must be aware of the potential dangers of using a PR2, and learn to anticipate and prevent problems.