  Soft robotic systems offer benefits over traditional rigid systems through
reduced contact trauma with soft tissues and by enabling access through
tortuous paths in minimally invasive surgery. However, the inherent
deformability of soft robots places both a greater onus on accurate modelling
of their shape, and greater challenges in realising intraoperative shape
sensing. Herein we present a proprioceptive (self-sensing) soft actuator, with
an electrically conductive working fluid. Electrical impedance measurements
from up to six electrodes enabled tomographic reconstructions using Electrical
Impedance Tomography (EIT). A new Frequency Division Multiplexed (FDM) EIT
system was developed capable of measurements of 66 dB SNR with 20 ms temporal
resolution. The concept was examined in two two-degree-of-freedom designs: a
hydraulic hinged actuator and a pneumatic finger actuator with hydraulic beams.
Both cases demonstrated that impedance measurements could be used to infer
shape changes, and EIT images reconstructed during actuation showed distinct
patterns with respect to each degree of freedom (DOF). Whilst there was some
mechanical hysteresis observed, the repeatability of the measurements and
resultant images was high. The results show the potential of FDM-EIT as a
low-cost, low profile shape sensor in soft robots.
