Smart Braids

“Smart Braids” are conductive reinforcing fibers that provide a way of sensing the deformation and force output of fiber-reinforced actuators without any external transducers. Typically the length of the actuator would be deduced from a sensor attached to a rigid link (like a potentiometer or an optical encoder). Smart Braids provide a soft sensor that sense the actuator contraction without external mechanical parts. A Smart Braid changes in inductance and resistance in response to the movement and force output in fiber-reinforced actuators. This can be accomplished by using conductive fibers in a circuit to form the reinforcing structure of a Pneumatic Artificial Muscle, FREE, or other fiber-reinforced actuator. When the actuator contracts, the fibers become more aligned and the inductance increases.  The inductance is related to the strength of the magnetic field created by the wires. When the wires are aligned, the magnetic field created by each wire builds on the magnetic field created by its neighbors and the inductance is high. When the wires are not aligned, they cancel each others magnetic fields and the inductance is lower. If the wires are far apart, they have a smaller effect on each other's fields.  When the wires are connected in series, these small changes in magnetic field intensity can turn into a valuable signal.

As the actuator contracts, the fibers become more aligned and the inductance increases.

Additionally, external forces and internal pressure create a strain in the fibers that can be measured through changes in resistance (similar to a strain gauge). That is, the tension on the wires causes them to stretch slightly. As they stretch, the current in the wires is forced to travel through a narrower space and it encounters more resistance. We can measure this electrical resistance to estimate the amount of force the wires are being subjected to. We tested  the "Smart Braid" by building Pneumatic Artificial Muscles with a braided sleeve made out of off-the-shelf wire. We found we can characterize the actuator contraction quickly and accurately by measuring the inductance of the braid.

Adapted from: Felt, Wyatt and Remy, C David. “Smart Braid: Air Muscles that Measure Force and Displacement.” IROS 2014.

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