Small‐Scale Magnetic Actuators with Optimal Six Degrees‐of‐Freedom

Abstract

Magnetic miniature robots (MMRs) are small‐scale, untethered actuators which can be controlled by magnetic fields. As these actuators can non‐invasively access highly confined and enclosed spaces; they have great potential to revolutionize numerous applications in robotics, materials science, and biomedicine. While the creation of MMRs with six‐degrees‐of‐freedom (six‐DOF) represents a major advancement for this class of actuators, these robots are not widely adopted due to two critical limitations: i) under precise orientation control, these MMRs have slow sixth‐DOF angular velocities (4 degree s−1) and it is difficult to apply desired magnetic forces on them; ii) such MMRs cannot perform soft‐bodied functionalities. Here a fabrication method that can magnetize optimal MMRs to produce 51–297‐fold larger sixth‐DOF torque than existing small‐scale, magnetic actuators is introduced. A universal actuation method that is applicable for rigid and soft MMRs with six‐DOF is also proposed. Under precise orientation control, the optimal MMRs can execute full six‐DOF motions reliably and achieve sixth‐DOF angular velocities of 173 degree s−1. The soft MMRs can display unprecedented functionalities; the six‐DOF jellyfish‐like robot can swim across barriers impassable by existing similar devices and the six‐DOF gripper is 20‐folds quicker than its five‐DOF predecessor in completing a complicated, small‐scale assembly.

Magnetic Miniature Actuators with Six‐Degrees‐of‐Freedom Multimodal Soft‐Bodied Locomotion

“…Based on our local reference frame assignment, the MMR's rotation about the ‐axis is therefore its sixth‐DOF motion. [ 43,54 ] This frame assignment also indicates that the MMR's will be along the negative ‐axis when the actuator deforms to its inverted “V”‐shaped configuration. To fully describe the orientation of the proposed MMR, we also introduce an intermediate reference frame and a global reference frame (Figure 1C(i)‐(ii)), and the axes and vectors expressed in these frames are denoted by the subscripts and , respectively.…”

“…This reference frame represents the local reference frame of the actuator, and the axes and vectors expressed in this frame are denoted by a subscript . Because it will be easier to analyze the proposed MMR if its sixth‐DOF axis is parallel to one of the principal axes in the local reference frame, [ 43,54 ] here we assign ‐axis to be aligned with the MMR's in its “U”‐shaped configuration (Figure 1C(iii)). Based on our local reference frame assignment, the MMR's rotation about the ‐axis is therefore its sixth‐DOF motion.…”

“…[ 10,13,53 ] In addition, unlike actuation signals such as pressure, heat, and chemicals, magnetic fields can be specified not only in magnitude but also in their direction and spatial gradients. [ 42,43,54 ] By having more independent control parameters, MMRs have also shown to be the most functional among all the classes of small‐scale actuators. [ 10,30,55–62 ]…”

“…While soft MMRs with six‐DOF do exist, such actuators have so far only realized one mode of swimming locomotion. [ 43 ] This is because existing six‐DOF actuators are currently limited to having a 1D magnetization profile for their soft components, [ 43 ] so that their changes in geometry and magnetization profile are easy to account for during deformation. Although it is easy to implement six‐DOF control on soft MMRs with 1D magnetization profile, such simple profiles have also severely constrained the dexterity of these actuators.…”

“…As the proposed six‐DOF MMR can perform seven modes of soft‐bodied locomotion, its dexterity will also be significantly higher than existing six‐DOF soft MMRs that can only execute one type of swimming gait. [ 43 ] Our proposed actuator is made up of a central magnetic beam with two identical buoyant components attached to its free ends ( Figure A(i) and see Section S1, Supporting Information, for the fabrication process and material properties of the MMR). The buoyant components are used to make the proposed soft MMR more neutrally buoyant so that it can easily swim against gravity in aquatic environments.…”