  To enhance robotic grasping capabilities, we are developing new contactless
fingertip sensors to measure distance in close proximity and simultaneously
detect the type of material and the interior structure. These sensors are
referred to as pre-touch dual-modal and dual-mechanism (PDM$^2$) sensors, and
they operate using both pulse-echo ultrasound (US) and optoacoustic (OA)
modalities. We present the design of a PDM$^2$ sensor that utilizes a pulsed
laser beam and a customized ultrasound transceiver with a wide acoustic
bandwidth for ranging and sensing. Both US and OA signals are collected
simultaneously, triggered by the same laser pulse. To validate our design, we
have fabricated a prototype of the PDM$^2$ sensor and integrated it into an
object scanning system. We have also developed algorithms to enable the sensor,
including time-of-flight (ToF) auto estimation, ranging rectification, sensor
and system calibration, distance ranging, material/structure detection, and
object contour detection and reconstruction. The experimental results
demonstrate that the new PDM$^2$ sensor and its algorithms effectively enable
the object scanning system to achieve satisfactory ranging and contour
reconstruction performances, along with satisfying material/structure detection
capabilities. In conclusion, the PDM$^2$ sensor offers a practical and powerful
solution to improve grasping of unknown objects with the robotic gripper by
providing advanced perception capabilities.
