A retrieving device for use in training dogs, the device being adapted to be cast, retrieved, and released from a casting line. The pressure variation in a second fluid, separated from the first by a pumping membrane, is measured in response to energy applied in a time-varying manner to the second fluid. The system further has a distributed network of nanoparticle ink based piezoelectric sensor assemblies deposited onto the structure. During the first period, voltages of first and second input terminals are input through first and fourth switches in the on-state to differential inputs of the second differential pair. The plane of the plate is arranged to be substantially perpendicular to the axis of the coaxial output.