Twisted String Actuator

The Laboratory of Actuation Technology (LAT) is among other things engaged in the investigation and development of the so-called twisted string actuator. This kind of actuator consists mainly of a multi-strand string which is twisted by an electric motor. Such twisting reduces the length of the string resulting in a tensile force against a load. In the simplest case, the string must be held by an axial bearing on the side of the electric motor and guided linearly on the load side as illustrated schematically in Fig. 1 below.


Figure 1: Design of a twisted string actuator

The concept of twisted string actuation was developed and designed within the European project DEXMART (grant agreement FP7 216239) at the Laboratory of Process Automation (LPA) in order to actuate the fingers of a robotic hand. The invention is registered as a German utility patent (DE202009017989, Thomas Würtz et al.). The operating principle and development of twisted string actuation for the DEXMART Hand is described in more detail in the university magazine magazin forschung 1/2011 (in German). A total of 24 actuation units were implemented within the DEXMART Hand to operate the five fingers and the wrist, see Fig. 2.


Figure 2: DEXMART Hand with 24 actuation modules

As this example illustrates, the twisted string actuation system can be suitably integrated within the robotic forearm due to its long and slender design. Moreover, the twisted string actuator exerts high pulling forces due its principle of operation. Each actuation module can generate tendon forces up to 80 N which corresponds to an approximately 1 kg load applied perpendicularly to the tip of an outstretched finger. The manipulating speed is also taken into consideration in this application. The fingers can be moved at a rate of about 2 Hz between the extended and the flexed positions.

The realization of the twisted string actuation system for the DEXMART Hand reveals the potential of this new actuation concept. During construction and optimization of the actuator, some useful knowledge has already been gained. Beyond that, the researchers and designers see further potential which they want to exploit in the field of robotics as well as in other fields of application. The LAT team is engaged, among other things, in characterizing the actuator, in investigating and increasing the lifetime of the string and in conceiving and designing functionally adapted electric motors.


Chris May