  Soft robots could bring robotic systems to new horizons, by enabling safe
human-machine interaction. For precise control, these soft structures require
high level position feedback that is not easily achieved through conventional
one-degree-of-freedom (DOF) sensing apparatus. In this paper, a soft two-DOF
dielectric elastomer (DE) sensor is specifically designed to provide accurate
position feedback for a soft polymer robotic manipulator. The technology is
exemplified on a soft robot intended for MRI-guided prostate interventions. DEs
are chosen for their major advantages of softness, high strains, low cost and
embedded multiple-DOF sensing capability, providing excellent system
integration. A geometrical model of the proposed DE sensor is developed and
compared to experimental results in order to understand sensor mechanics. Using
a differential measurement approach, a handmade prototype provided linear
sensory behavior and 0.2 mm accuracy on two-DOF. This correlates to a 0.7\%
error over the sensor's 30 mm x 30 mm planar range, demonstrating the
outstanding potential of DE technology for accurate multi-DOF position sensing.
