In this paper we propose a novel design for a lightweight and compact tunable stiffness actuator capable of stiffness changes up to 20x. The design is based on the concept of a coiled spring, where changes in the number of layers in the spring change the bulk stiffness in a near linear fashion. We present an elastica nested rings model for the deformation of the proposed actuator and empirically verify that the designed stiffness- changing spring abides by this model. Using the resulting model, we design a physical prototype of the tunable-stiffness coiled- spring actuator and discuss the effect of design choices on the resulting achievable stiffness range and resolution. In the future, this actuator design could be useful in a wide variety of soft robotics applications, where fast, controllable, and local stiffness change is required over a large range of stiffnesses.
Fig. 1. (a) 2-D representation of a single elastic ring with model variables indicated; shown in unloaded form (left) and with plate load F (right). (b) Coiled ribbon (left) with diameter of centerline D, thickness H and number of layers n shown in 2-D. We model the ribbon as n nested rings (right). (c) 3-D visualization of n nested cylinders of thickness H and width W under plate loading.
Fig. 2. Non-dimensionalized ring deformation curve with a force ranging from 0 to 400. Displacement (x-axis) goes up to non-dimensional D = 1/π, approximated as 0.2.
Fig. 3. (a) Experimental MTS test for spring (H = 0.127 mm, W = 10 mm, D = 50 mm) over a range of 1-20 coils with a total compression amount of 30 mm. (b) Error plots for MTS experiments run with spring sample: Sample ID 1-4 (from left to right).
Fig. 4. (a) Manipulator in neutral position. (b) Inclined manipulator. (c) Side and (d) top view of the estimated workspace.