Bistable actuators provide "parking positions" at the ends of the stroke of the actuator that do not require current to remain within. Studies suggest that less than 100 binary DOF will provide sufficient resolution for many practical tasks, and that the associated computing requirements for solving their inverse kinematics are reasonable [ref].
The paper explores the use of binary actuation for space applications: "Solving the forward and inverse kinematics, in order to follow trajectories for a binary robot, is a very different problem than that for a continuous robot. Because the actuators can achieve only finite displacements, a binary robot can reach only finite locations and orientations in space. The workspace of a binary robot is thus a point cloud and the inverse kinematics problem becomes simply a search through this cloud."
This multitude of wires adds unacceptable weight and complexity. In this study, a more effective control architecture has been designed (see Figure 7b). A common power line and ground runs between all of the actuators. The power signal is encoded with a high-frequency component that instructs which actuators to turn on or off. Each actuator has a small decoder chip that can be triggered into either binary state by the carrier signal piggybacked on the power line. The carrier signal is a sequence of pulses that identifies a unique address in the form of a binary word. Once triggered, the decoder chip stays latched to that state until triggered otherwise. This power bus architecture reduces the wiring of the entire system to only two wires (power and ground) and can easily be implemented in the form of conducting paint/tape (on the non-conducting polyethylene substrate) to minimize the structural disturbance forces due to wiring.
The paper focuses on the use of Shape Memory Alloys (SMAs). They describe a locking mechanism to hold the SMA in each position. It is comprised of bi-stable mechanisms sandwiched by flexure beams. The bi-stable mechanisms use detents to passively lock the joint into discrete states, while the flexures add out-of plane rigidity.
Moustapha was one of the authors on the paper above. The thesis describes the application of dielectric elastomer actuators to mechanical systems, incorporating the dielectric elastomer into a flexible frame that provides an elastic restoring force.
Steven Dubowsky is a professor in mechanical engineering and aero-astro. He was the advisor for the theses above. In 2015 he along with his students got a patent for a compliant substrate with discrete actuators on the reverse side to establish different surface shapes. Their paper details analysis for a 1D and 2D system and experimental results for a 3D system.
Points of interest include, by randomly selecting the elasticity of the elastic elements that form the backing structure, it is possible to dispearse the point cloud of accessible locations. Futhermore, their 3D demonstration setup utilized continuous linear actuators from Firgelli in a binary manor due to "force limitations of current binary actuators". This therefore demonstrates that whilst there is a great deal of literature for promising MEMs scale binary actuators, there doesn't seem to be a good one for meso scale applications. There's an interesting part to the anaylsis that demonstrates the increase in access to the workspace when taking the system from 6 actuators in a one-tier system to 17 actuators in a two-tier system (arrange along a 2D triangulated truss). The 17 actuator system is stated as having a near-continuous point cloud workspace.
So my resulting questions are:
This paper models, simulates and fabricates bistable mechanisms like the one below, which generates two linearly separated states. This kind of design would lend itself very well to injection moulding.
This paper describes a micro-feeding device composed of translation and positioning systems. This work is part of a larger popular topic in MEMs research into micro bistable devices. The device utilizes a bistable mechanism for vertical displacement of the tethers and a Scratch Drive Actuator (SDA) attached to a spring for horizontal displacement.
