  Detecting and preventing slip is a major challenge in robotic hand operation,
underpinning the robot's ability to perform safe and reliable grasps. Using the
robotic hand design from the authors' earlier work, a sensing and control
strategy is proposed here to prevent object slippage. The robotic hand is
cable-driven, single-actuated, has five fingers, and is capable of replicating
most human hand motions. The slip sensing approach utilizes a piezoelectric
vibration sensor, namely, polyvinylidene fluoride (PVDF), which is a flexible,
thin, cheap, and highly sensitive material. The power of the filtered PVDF
signal is shown to exhibit identifiable signatures during slip, thus providing
a suitable slip detection mechanism. Using the PVDF feedback, an integral
controller is implemented to prevent the grasped object from falling and ensure
a safe, powerful, and reliable grasp. The extension movement of the robotic
hand is controlled using a bend sensor, through a proportional-integral (PI)
controller. The robotic hand weights 338 gr. The functionality and robustness
of the proposed slip-detection sensory system and control logic implementation
are evaluated through experiments.
