A Planar Coil-Based Inductive Bend Sensor for Soft Robotic Applications

Bend angle measurement of a soft structure is valuable for effective grasping of different types of objects by a soft robotic gripper. A novel, easy to integrate planar coil-based bend sensor is presented in this work to provide a measure of the bend angle of a soft robotic finger (or any soft and bendable structure). An efficient method to convert the bend angle to a displacement, within the constraint of limited space available for the sensor and at the same time not changing the soft nature of the finger is presented. The developed sensor comprises of three tightly packed flexible printed circuit boards (PCBs) engraved with a set of planar spiral coils. An effective sensing technique is proposed where a set of adjacently placed primary coils are excited and the induced voltages of two secondary coils are recorded. The two output signals exhibit push-pull pattern and therefore a ratiometric output can be derived. This ratiometric output contributes to a linear characteristic and provides a measure of the cumulative bending of the structure. A prototype sensor was developed and tested which delivered repeatable characteristic.