When Stretchable Sensing Meet Flexible Robotics
Authors: Ren Hongliang 
Published: February 2020

Advantages of soft, flexible materials with developments in stretchable electronics can be intertwined for a promising sensorized flexible robotics platform to work on a resilient, adaptable manipulator aimed to meet ever-increasing demands in safe regulated medical environments. Taking advantages of these soft magnetic polymers, we propose a novel, soft-squishy and flexible bend sensor by determining the relationship between inductance changes with bending angle. This bend sensor employs flexible wire embedded in a silicone elastomer with the different permeable core. The principle notion is to have a comprehensive analysis of the change in morphology of the sensor with bending angle which can be translated to inductance generated therein. The performance of the sensor is evaluated with various experimental trials while analytical modeling elucidates that the bend angle is linearly proportional to the sensor signal citing R-square value up to 0.9204. The proposed sensor produces the desired output in the electromagnetic frequency range of 8-10 MHz with a tunable sensitivity of 0.418 mV/rad. The sensor is robust enough to stretch up to twice of its original length. The main advantage of this bend sensor is its simple fabrication technique, flexibility, robustness, and economical. Conclusively, this induction based tactile bending sensor is proved to produce robust output and can be extrapolated to sense bending angle using induction principle for the rehabilitative device, wearable robots, and related biomedical applications requiring low cost, soft and flexible operations.

To equip robotic manipulators with intelligence, tactile sensors are extensively used for detecting various external stimuli like slippage, grasping force and imitate the tactile perceptions. Since past few decades, there have been various researches in realising sensing element on a soft surface to obtain stretchable, foldable and flexible sensors. Apart from these external stimuli measurements, various bend sensors were developed to determine the curvature of bending and utilized in different applications like measurements of flexion and extension of body joints. These bend sensors were realised on different hardware and software technology namely optical fibre bend sensor, resistive and capacitive-based sensor. For small size, increased MEMS sensors were developed. Resistive based sensors consist of pressure sensitive element which changes its resistance based on force or induced deformation. The sensitive element is usually comprised of conductive ink, rubber or elastomer. These sensors are affordable and commercially available in the market with a higher spatial resolution that employs fewer electronics which makes signal processing easier to work in mesh configurations.

Apart from these above-mentioned technologies, inductive based tactile sensors are based on the principle of magnetic coupling. Modulating the inductance of the coil by varying the length and the permeable core, in turn, modulates the sensing voltage. The sensors based on this technology are still under development and not widely explored like others. Although they do not fix all the disadvantages of the above-mentioned technologies, they have a high dynamic range, linear output and can be employed at a higher frequency. This makes the construction bulky and incurs higher losses. Therefore, we aimed to develop a sensor with only one coil that acts as both primary and secondary/sensing coil.

Specifically, we propose a novel soft inductive Solenoidal Bend Sensor (SBS) completely different from the existing technology. This sensor comprises of a flexible coil with a protective covering made of elastic material. This silicone elastomer provides mechanical compliance and robustness to the sensor. Due to SBS flexible yet resilient properties, this has promising applications in the field of wearable electronics, soft robotics, drug delivery, medical implants, human-machine interaction. In this paper, the geometry of the sensor is considered like a single cantilever beam hung through certain height and bent with tangential and normal force. Thus, the characterization and mathematical modelling of the sensor is simpler due to the simpler geometrical structure. This developed sensor uses a passive L–R circuitry that consists of a fixed resistor and a variable inductor. The variable inductor is coupled with elastomer element that makes dimensional changes in response to the target parameter to modify the inductance.

The material and the design of the sensor is economical, readily available and does not require complex fabrication procedure. SBS can be modified by varying the wire gauge length. This novel SBS is sensitive to bidirectional force and can be calibrated in near terms for the measurement of the angle or the force in the finger joints for rehabilitation.