We developed a sensor that measures multi-axial shear stresses based on optical coupling between a red, green, and blue (RGB) light- emitting diode (LED) and a photodiode. This is a contactless design, meaning that all electronics and wiring are confined to one side of the sensor. A schematic of the sensing principle is shown on the left. The instrumented side of the sensor has a windowed pattern (Surface A), whereas the opposite side of the sensor (Surface B) displays a colored grid consisting of green, red, blue, and magenta (red + blue) squares. The contactless design offers versatility in how the sensor is implemented. For example, the pattern on Surface B could be printed on a second sensor component, or on an adjacent stationary surface or existing device (e.g., a shearing body on a robot). Controlling electronics cycle the LED color while measuring reflected light intensity as a resistance change at the photodiode during red (Rr), green (Rg), and blue (Rb) light illumination.

A schematic of the sensing principles is shown on the right. All sensing of shearing is based on the alignment of the pattern on Surface B with the window on Surface A. When there is no shear force applied to the sensor, Surfaces A and B are perfectly aligned, and thus only green appears in the window on Surface A. Under this condition, the photodiode only measures a resistance change when the LED is emitting green light, since there is no blue or red color surface to reflect the blue and red lights, respectively (R_g > 0, R_r = R_b = 0). A vertical shear force causes misalignment of Surfaces A and B, and the red squares on Surface B are exposed through the window of Surface A, leading to resistance changes in R_g and R_r. The values for R_g and R_r are inversely proportional and proportional to the magnitude of vertical shear force, respectively. Similarly, a horizontal shear force causes the blue on Surface B to be visible in the window on Surface A. When the sensor experiences both vertical and horizontal shear forces, the magenta surface appears through the opening of Surface A, which reflects both blue and red lights. As a result, vertical shear force can be calculated as the ratio R_r / R_g, and horizontal shear force can be determined as R_b / R_g.

Highlights of this sensor include its ability to accurately measure multi-axial shear stresses and strains, small and lightweight form, and potential to be a wireless standalone device. The design is also scalable based on the material properties of an intermediate elastomer layer, making it ideally suited to sense shear stress for a variety of applications. These properties support the use of this sensor for a variety of applications in robotics, sport performance, and orthopedics. For example, the sensor could be embedded in the sole of a shoe, providing a powerful tool to improve sport performance or aid in footwear design. The sensor could also be integrated within a prosthetic socket system to help manage socket fit and residual limb tissue health.