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 robotic actuators, low speeds and high torques are usually required. Small electric motors, which are more efficient at high speeds and low torques, do not fit the requirements directly. In order to transform the motor characteristics into the desired output characteristics, a transmission system is needed. Ideally, it should be optimally designed and adapted to the desired characteristics and the available space. Scaling laws can provide a way to design these desired output requirements as a function of the size parameters. These are however not yet available for transmission systems. To fill this gap, several scaling laws are developed throughout this paper for some of the most important robotic characteristics, i.e. maximum continuous output torque and reflected inertia, in function of the number of stages, the transmission ratio and the size parameters of different types of transmissions. These laws show that diameter has a much bigger influence on the characteristics of transmissions than length. All derived laws show good comparison with catalog data of manufacturers like Maxon, Moog, Neugart, Harmonic Drive , Sumitomo and SKF.

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