Modelling and Characterisation of Twisted String Actuation for Usage in Active Knee Orthoses∗

In active knee orthoses, quiet, powerful, and light-weight actuators are required to support the user during challenging tasks like sit-to-stand and stair climbing. The twisted string actuation concept, based on parallel arranged strings twisted by a small DC motor, is a promising approach to cover the defined requirements. For characterisation of the parallel arranged strings, a test bench is introduced to investigate the transmission and the failure behaviour of the twisted string transmission. The application of different test conditions for characterisation requires a mechanical load which is generated by an additional DC motor and a rope winch. This programmable mechanical load is applied to the strings and measured by a force sensor. A force controller is used to generate a desired mechanical load profile. This paper outlines the mechanical model and the implementation of the mechanical load test bench for the twisted string actuator, and the transmission characterisation.