On the eve of Human-Robot-Interaction (HRI) becoming customary in our lives, the performance of HRI robotic devices remains strongly conditioned by their gearboxes. In most industrial robots, two relatively unconventional transmission technologies—Harmonic Drives© and Cycloid Drives—are usually found, which are not so broadly used in other industries. Understanding the origin of this singularity provides valuable insights in the search for suitable, future robotic transmission technologies. In this paper we propose an assessment framework strongly conditioned by HRI applications, and we use it to review the performance of conventional and emerging robotic gearbox technologies, for which the design criterion is strongly shifted toward aspects like weight and efficiency. The framework proposes to use virtual power as a suitable way to assess the inherent limitations of a gearbox technologies to achieve high efficiencies. This paper complements the existing research dealing with the complex interaction between gearbox technologies and the actuators, with a new gearbox-centered perspective particularly focused on HRI applications.

In many robotic applications, high torque density and highly efficient actuators are required to handle the high torque motion without hampering mobility through excessive weight and footprint. Traditionally, engineers designing actuators for these types of applications use high-speed electrical motors in combination with high-ratio speed reducers such as harmonic and cycloid drives or lever arms in order to achieve the required torques. However, these systems are not ideal since they suffer from one or more of the following limitations: high cost, load hysteresis, nonlinear behavior, and torque ripple. An alternative to achieving the required torque increase is using an interesting type of compound planetary gear trains (C-PGTs). In this article, a novel type of such a C-PGT, based on the Wolfrom topology, is presented. An extra degree of freedom is created, which can be used to increase the performance of the overall gearbox, i.e., increase its efficiency, reduce its inertia, increase its torque capability, etc. This modified Wolfrom is discussed in detail by deriving a general framework for the design of such a transmission system. This framework consists of listing the specific fitting conditions for such a Wolfrom planetary gear train, the derivation of an efficiency model based on the Rolling Power concept, and a load analysis to determine the weakest gearing element. Finally, a prototype is built to demonstrate the design framework and the feasibility of the Wolfrom variant. Additionally, the trend of the efficiency model is confirmed.

This work gives an overview of all types of constant torque mechanisms (CTMs) based on both function and structure. Based on their architecture, they can be divided into five distinct categories, which all have their specific behavior. It is also shown that these CTMs can be divided into two application types, namely, power assistance or torque stabilization. Constant force mechanisms (CFMs) are more prevalent in the literature; therefore, some discussion is proposed on how CFMs can be transformed into CTMs. It is also shown that some of these CTMs have a very high specific energy, which makes them potentially interesting for use as energy storage/power providers in novel fields like robotics.

Robots often switch from highly dynamic motion to delivering high torques at low speeds. The actuation requirements for these two regimes are very different. As a consequence, the average efficiency of the actuators is typically much lower than the efficiency at the optimal working point. A potential solution is to use multiple motors for a single motor joint. This results in a redundant degree of freedom, which can be exploited to make the system more efficient overall. In this work, we explore the potential of kinematically redundant actuators in dynamic applications. The potential of a kinematically redundant actuator with two motors is evaluated against a single-motor equivalent in terms of operating range, maximum acceleration, and energy consumption. We discuss how the comparison is influenced by the design of the actuator and the way how the power is distributed over the input motors. Our results support the idea that kinematically redundant actuators can resolve the conflicting torque–speed requirements typical of robots.