In this paper we investigate a series of candidate regular planar mesh geometries in terms of their suitability for utilization as mesh primitives for constructing compliant robotic structures. In prior work, we established a framework of compliant, articulate robotic meshes, termed Modular Active Cell Robots (MACROs), created from contractile Shape Memory Alloy linear-actuators (Active Cells). In this paper, we examine how to utilize these MACROs and other mesh-like robots in large regular or semi-regular topologies. We evaluate axial strains and stiffness characteristics for a range of MACRO meshes created using different mesh-primitives, which are drawn from known regular space-filling lattice geometries. We then describe the implications of these results on the design of MACRO structures, including the tradeoffs between different primitives for various structural performance properties.

In this paper, we present the design of a shape-memory-alloy (SMA)-based compliant linear actuator [active cell (AC)] and the use of these in designing and modeling articulated meshes, which form the mechanical subsystem of a class of proposed modular active-cell robots (MACROs). The ACs are capable of undergoing ~25% strain and groups of cells are connected via passively compliant nodes to produce articulated mesh networks. The deformation of compliant meshes of ACs is modeled by scale-invariant parametric equations derived from the physics of SMA deformations and a reduced-order model of the cells. Parameters of the implemented system were used to develop a simulation platform that predicts the mechanical deformation of the networked robot given electrical inputs at arbitrary nodes of the network. We provide results of several experimental trials used to validate and establish the accuracy of this deformation model. The error in predicting deformations in small meshes is shown to be under 10% over both time-varying inputs and at steady states.

We present the design of simple, centimeter-scale modular actuation units (“Active Cells”) and passive compliant nodes that are electromechanically networked to create macroscopically deformable Modular Active Cell-based Structures (MACROs). Each Active Cell is a single degree-of-freedom linear actuator (a “muscle unit”), consisting of fiberglass end-pieces connecting two strands of Nitinol shape-memory alloy and a passive biasing spring. The Nitinol strands are coiled into a tight spring to increase deformations when activated through resistive heating. In-depth examination of the optimization of Nitinol coils with an antagonistic spring is presented, resulting in large repeatable axial cell strains of up to 25%. The design of these cellular muscle units to obtain maximal repeatable stroke is presented, allowing for the construction of larger networks of cells (MACRO modules, akin to a biological “tissue”) that can be customized to a target application. Finally, experimental demonstration of the construction and actuation of some simple MACRO modules is described.

The proposed research effort explores the development of active cells - simple contractile electromechanical units that can be used as the material basis for larger articulable structures. Each cell, which might be considered a “muscle unit”, consists of a contractile Nitinol SMA core with conductive terminals. Large numbers of these cells might be combined and externally powered to change phase, contracting to either articulate with a large strain or increase the stiffness of the ensemble, depending on the cell design. Unlike traditional work in modular robotics, the approach presented here focuses on cells that have a simplistic design and function, are inexpensive to fabricate, and are eventually scalable to sub-millimeter sizes, working towards our vision of robot structures that can be custom-fabricated from large numbers of general cell units, similar to biological structures.