QDD, Planetary Gears and Strain Wave Gears: Understanding Drive Architectures and Reducers Separately

Distinguishing drive architectures from reducer types helps avoid assumptions about torque, precision and safety based solely on the name of a mechanism.

## QDD uses a low-ratio gearbox QDD (Quasi-Direct Drive) is not synonymous with direct drive, which uses no reducer. A study by Yu et al. describes a QDD actuator as combining a high-torque-density motor with a low-ratio gearbox. The design aims to balance backdrivability, which makes it easier for an external force to move a joint, with control bandwidth. Original paper The paper reports an evaluation using a hip exoskeleton. Its experimental results cannot be generalized to the performance or safety of all QDD products. Claims such as “there is no backlash because there is no gearbox,” “it always produces low torque” or “it is safe for collaborative applications” are not appropriate. ## Planetary gears and strain wave gears are reducer mechanisms Planetary gears transmit rotation through a combination of components such as a sun gear, planet gears and a ring gear. Harmonic Drive's technical explanation describes the meshing of the sun and planet gears, and of the planet gears and ring gear, and presents configurations that combine stages to change the reduction ratio. Manufacturer's technical explanation The strain wave gearing mechanism described by Harmonic Drive uses three components: a wave generator, flexspline and circular spline. It deforms a flexible component to mesh the teeth, achieving a high reduction ratio in a compact configuration. The company cites zero backlash and positioning accuracy as features. This is a manufacturer's description of the mechanism and does not guarantee the rigidity or safety of a completed robot. ## Compare mechanisms using the same classification criteria QDD is a drive design concept that includes a motor and transmission system, while planetary gears and strain wave gears are transmission mechanisms. With that distinction in mind, we recommend comparing specific product specifications and experimental conditions, including required output torque, speed, response to external forces and precision. There is no one-size-fits-all rule that, for example, a hip must use strain wave gearing or an ankle must use QDD.