PVDF touch sensors and time-of-flight force sensors for robot gripper applications

PVDF film is a very versatile material which is well suited to robot gripper sensor applications. The authors have used the material on the fingers of a novel robot gripper which has a wide grip range. Initial long range finger movement is controlled by a low power electric motor and final full grip force is applied by a pneumatic cylinder. Information from the sensors is used to detect when the gripper fingers have contacted the object so that full grip pressure may be applied. The results from work on time of flight sensors using a continuous signal applied to the PVDF transmitting element and using an excitation frequency of 33 kHz show that the phase shift between the transmitted and received signals can change over a range of 15 mu sec as force is applied and removed from a 2 mm thick rubber film. This result is at variance with the expected result of approximately 1 usec which has been reported by other groups. Calculations which assume that the speed of sound in silicone rubber is 1000 metres/sec. predict a signal transmission time of 2 mu sec through the uncompressed film. Because of this effect the authors' sensor is significantly more sensitive than those previously described.<>