Human-Centered Robotics

In recent years, there has been great interest generated in the emerging fields of service and medical robots. These applications are part of a growing area of human-centered robotics. This area involves the close interaction between robotic manipulation systems and human beings, including direct human-manipulator contact. In such applications, traditional figures of merit such as bandwidth, maximum force and torque capability, and reachable workspace, do not fully encompass the range of metrics which define the requirements of such systems. Specifically, human-centered robotic systems must consider the requirement of safety in addition to the traditional metrics of performance. Thus, it is the challenge of human-centered robotics to successfully blend often competing requirements of safety and performance.

The Stanford Robotics Laboratory has initiated a research effort to design a human-centered, inherently-safe robotic manipulator. While the design and development effort include all aspects of manipulator design, the primary focus has been on addressing the limitations of the mechanical system and its impact on safety and performance. We have focused on efforts to reduce the manipulator weight and inertia to improve its inherent safety characteristics while maintaining performance levels expected of modern manipulators.

DECMMA Actuation Approach

A critical component to this work has been the development of a new actuation approach that seeks to relocate the major source of actuation effort from the joint to the base of the manipulator. This can substantially reduce the effective inertia of the overall manipulator by isolating the reflected inertia of the actuator while greatly reducing the overall weight of the manipulator. Performance is maintained with small actuators collocated with the joints. Our approach partitions the torque generation into low and high frequency components and distributes these components to the arm location where they are most effective.

We refer to the overall approach as Distributed Elastically Coupled Macro Mini Parallel Actuation (DECMMA). The DECMMA approach is analogous to the design of robotic manipulators for use in zero gravity. Under such conditions, gravity induced torques do not exist. Joint actuators provide torques related only to the task, such as trajectory tracking and disturbance rejection, both of which are primarily medium to high frequency in content. We achieve the zero gravity analogy by compensating for gravity torques and low frequency torques using the low frequency actuator located at the base of the manipulator. With the effects of gravity and low frequency torques compensated, joint torque requirements become similar to those encountered by a zero gravity robotic manipulator. However, unlike robotic manipulators designed for space applications, the DECMMA joint actuators do not require a large gear reducer to achieve the required torque and power densities. Thus, the impedance of DECMMA approach, and its resulting safety characteristics, is superior to that of current space robotic manipulators.