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.

Human-Centered Robotics aims to use robotic devices to improve our life. In Europe alone, around 650.000 people live with limb amputations, 40 Mio. have jobs with high of lumbar injuries, and 40 Mio. are 80+ years. Worldwide, active prostheses, exoskeletons, and service robots could help improve these people’s lives. However, their adoption is unfortunately strongly hindered by technological limitations in the actuators powering these devices.Backdrivability characterizes an actuator’s ability to be driven from the load side, and it is a crucial property to enabling capable human-centered robotic devices. In this paper, we describe the underlying factors that determine actuator backdrivability in robotics and investigate suitable scaling laws to understand how these factors are conditioned by the motor and gearbox selection and the specific operational cycle of a robotic device. This analysis unveils the complexity and challenges faced to accurately model and predict this complex phenomenon, contradicting an extended hypothesis in the robotics community that sees low-ratio transmissions as the best strategy to build backdrivable, lightweight actuators.