Stretch sensors are a type of sensor designed to measure soft structures, including soft robots and the human body. StretchSense, a company from New Zealand, has developed commercially available stretch sensors that can measure either stretch, bend, shear, or pressure. These stretch sensors are essentially flexible capacitors. When the sensor deforms (due to stretching or squeezing) its capacitance value changes. The sensor sends data about its capacitance to a Bluetooth enabled device, and the StretchSense App translates the data to give a user highly precise information about what the sensor is detecting. StretchSense sensors can be made in various sizes, elasticities, and sensitivities depending on desired application.
The inside of the stretch sensor consists of a stretchable signal electrode sandwiched between two ground electrodes which are separated by silicone dielectric insulators. Since the sensor is essentially a flexible parallel plate capacitor, the two dimensions that change when the sensor is stretched are thickness and area of the conductive layers. Stretching the sensor increases area and decreases thickness which both contribute to an increase in capacitance. Since capacitors resist a change in voltage, the capacitance can be indirectly measured through an applied voltage. By applying a voltage to the sensor and comparing the actual voltage output to what would be expected without a capacitor, the capacitance of the sensor can be determined.
StretchSense offers two versions of stretch sensor: A silicone based sensor and a fabric based sensor. The silicone variety offers a set of mounting holes to attach to your system, while the fabric variety has a thin sew-able region on each side of the sensor which is perfect for integrating with wearable applications.
Silicone stretch sensor
Fabric stretch sensor
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One sensor
Multiple sensors
The Apps are available to download for free on the Google Play Store.
The video below shows the stretch sensors sending data to a phone via bluetooth. It also shows an example of how multiple sensors can be easily incorporated into one system by showing a glove with stretch sensors over each finger.
Credit: A. Veale, Auckland Biomimetics Insitute
A group at the Auckland Biomimetics Institute is using StretchSense sensors to control an active soft elbow brace. The above video shows their elbow brace which is the first to feature slimline and flexible fluidic muscles (arranged antagonistically on either side of the brace in similar fashion to biological muscles).
Stretch sensors are sewn along the length of the muscles and are pre-tensioned so they move in concert with the muscles as they contract.
As the brace is actuated, the strain sensors provide feedback on the current orientation of the joint. This control can be seen at the beginning of the video when the lego model of the elbow joint moves in unison with the actuation of the elbow brace.