480 Requirements and their respective impact on the design of the product were discussed. Capabilities of  different Har monic Drive® gear types and configurations could be involved due to the early project phase.  In general , the primary basis for fulfillment of requirements is the suitable choice of an existing gear  configuration. But with increased requirements and/ or contrary gear characteristics it may be necessary to  introduce targeted modifications to existing Harmonic Drive® g ear types or even to develop a new Harmonic  Drive® gear type to fulfill the  requirements. Early collaboration with Harmonic Drive AG enabled tar geted  introduction of modifications to strengthen the actuator performance/ features. Solutions prepared by  Harmonic Drive AG were exchanged and discussed on a regular basis.  During the concept definition the  concept was sent back  and forth between the com panies and modifications were implemented from both  companies. With this iterative approach,  the concept improved quickly and at the end, a very  unconventional, highly integrated solution was found.     The collaboration took place under a very open minded collaboration and both sides tried to maximize the  product. This was only possible with due to a trustful environment  between the two companies which is also  a matter of the team setting and the personalities involved. In the end it resulted in a Harmonic Drive® g ear  concept  that met the requirements and furthermore exceeded the expected actuator performance. The  concepts, which were elaborated during joint discussions in early project phase, were designed in detail and assessed with respect to the operation scenario during design/ development phase by Harmonic Drive  AG. 

Development at Harmonic Drive AG   

Harsh environmental conditions in space present challenges to configurations, base materials and lubricants and cause the need for space- rated gears to differ from gears for industrial applications in items  such as  material and documentation. With a development team for gears related to non- industrial  applications, Harmonic Drive AG is active on the field of space- related development and testing within  several projects to support users and to strengthen space applications using strain wave gears [2, 3].    Strain wave gears with ratios up to 160 are common, but also higher ratios, such as  320, are possible and  already realized by Harmonic Drive AG. For higher ratios, strain wave gears can be com bined with a prestage. C onfigurations to combine a pre- stage gear  with a Harmonic Drive® gear  are conceivable for space  applications especially as high integration is important . Thus one can benefit of the advantages of a gear  based on strain wave gear principle while realizing higher ratios than one stage could provide.    The final concepts combined two  strain wave gears resulting in a possible ratio range from 2500 up to  approximately 18000.  A gear ratio of approximately 5000 was selected for the F uture Actuator.  A rotatory  input motion is transmitted by 2 stages to a rotatory output motion. Due to restrictions regarding installation space, weight and robustness the intention was to highly integrate the 1st stage in the 2nd stage. Designs  were elaborated and evaluated regardi ng characteristics such as their assembly effort, compac tness and  operability. A full finite element  model of the double- stage strain wave gear was created to assess the  operability and to dimension the parts according the operation scenario.  Several optim izing loops were  performed during which the design was adapted to the operation scenario.  This resulted in the design of a  very compact , back -drivable component set with high accuracy and a high gear ratio.  

Testing at  Harmonic Drive AG   To characterize the component set at Harmonic Drive AG , test equipment was designed, documented and  manufactured. During an acceptance test , the no load starting torque, no load back driving starting torque,  transmission accuracy and torsional stiffness w ere measured as ref erence. Furthermore,  the parts were  visually inspected before delivery. Three component sets were manufactured and characterized at  Harmonic Drive AG. The results of the transmission accuracy and stiffness measurements are presented  in the following.