A New Actuation Concept for Human-Friendly Robot Design

Abstract

ecently, there has been increasing interest in the emerging field of humancentered robotics. This field focuses on applications such as medical robotics and service robotics, which require close interaction between robotic manipulation systems and human beings, including direct human-manipulator contact. As a result, human-centered robotic systems must consider the requirements of safety in addition to the traditional metrics of performance. To achieve safety we must employ multiple strategies involving all aspects of manipulator design, including the mechanical, electrical, and software architectures. Immediate improvement can often be realized with the use of electronic hardware and software safety mechanisms that intelligently monitor and control manipulator operations. Additional improvements can be realized in the mechanical design. The elimination of pinch points and sharp edges can eliminate the potential for laceration or abrasion injuries. However, the most serious hazard present when working in close proximity with robotic manipulators is the potential for large impact loads, which can result in serious injury or death. To evaluate the potential for serious injury due to impact we can make use of an empirical formula developed by the automotive industry to correlate head acceleration to injury severity known as the head injury criteria (HIC). A simple two-degree-offreedom mass-spring model can be used to predict head accelerations that would occur during an uncontrolled impact. In combination with the HIC index, predicted accelerations are used to estimate the likelihood of serious injury occurring during an impact between a robotic manipulator and a human. For the PUMA 560, an impact velocity of 1 m/s produces a maximum HIC greater than 500, more than enough to cause injury (see Figure 1). [The HIC index is correlated with the maximum abbreviated injury scale (MAIS) to provide a mapping from the calculated HIC values to the likelihood of an occurrence of a specific injury severity level. In Figure 1, HIC values and the corresponding likelihood of a concussive injury (or greater) are shown.] As seen in Figure 1, the addition of a compliant covering can reduce impact loading by an order of magnitude or more. However, the amount of compliant material required to reduce impact loads to a safe level can be substantial. (For the PUMA robot, the required thickness of a compliant cover is more than 5 in, assuming an impact velocity of 1 m/s and an allowable maximum HIC index of 100.) Clearly, adding large amounts of compliant covering is impractical and does not address the root cause of high-impact loads; namely, the large A New Actuation Concept for Human-Friendly Robot Design