  Soft robots are intrinsically capable of adapting to different environments
by changing their shape in response to interaction forces with the environment.
However, sensing and feedback are still required for higher level decisions and
autonomy. Most sensing technologies developed for soft robots involve the
integration of separate sensing elements in soft actuators, which presents a
considerable challenge for both the fabrication and robustness of soft robots
due to the interface between hard and soft components and the complexity of the
assembly. To circumvent this, here we present a versatile sensing strategy that
can be retrofitted to existing soft fluidic devices without the need for design
changes. We achieve this by measuring the fluidic input that is required to
activate a soft actuator and relating this input to its deformed state during
interaction with the environment. We demonstrate the versatility of our sensing
strategy by tactile sensing of the size, shape, surface roughness and stiffness
of objects. Moreover, we demonstrate our approach by retrofitting it to a range
of existing pneumatic soft actuators and grippers powered by positive and
negative pressure. Finally, we show the robustness of our fluidic sensing
strategy in closed-loop control of a soft gripper for practical applications
such as sorting, fruit picking and ripeness detection. Based on these results,
we conclude that as long as the interaction of the actuator with the
environment results in a shape change of the interval volume, soft fluidic
actuators require no embedded sensors and design modifications to implement
useful sensing. We believe that the relative simplicity, versatility, broad
applicability and robustness of our sensing strategy will catalyze new
functionalities in soft interactive devices and systems, thereby accelerating
the use of soft robotics in real world applications.
