Low-Cost Flexible ShArc Sensors Use Capacitance, Geometry for Accurate Multi-Bend and Shape Sensing

Built using flexible printed circuit board technology, the ShArc sensors are claimed to be largely self-correcting.

Gareth Halfacree
a year ago β€’ Sensors / Wearables
The ShArc sensors are built using existing flexible PCB technology. (πŸ“·: Shamiri et al)

Researchers from Tactual Labs, working with Georgia Tech, have demonstrated a new technique for multi-bend and shape sensing using flexible strips: ShArc.

"We present ShArc, a precision, geometric measurement technique for building multi-bend/shape sensors," researchers Fereshteh Shamiri and Paul H. Dietz write in the abstract to their paper. "ShArc sensors are made from flexible strips that can be dynamically formed into complex curves in a plane. They measure local curvature by noting the relative shift between the inner and outer layers of the sensor at many points and model shape as a series of connected arcs."

"Unlike jointed systems where angular errors sum with each joint measured, ShArc sensors do not accumulate angular error as more measurement points are added. This allows for inexpensive, robust sensors that can accurately model curves with multiple bends. To demonstrate the efficacy of this technique, we developed a capacitive ShArc sensor and evaluated its performance."

The ShArc sensor is constructed from two strips: A reference strip and a sliding strip, separated by a constant distance. For the prototypes, the researchers took inspiration from commercial digital calipers based on capacitive sensing: Transmit and receive strips are constructed using a flexible printed circuit board process, with eight electrodes on the transmit strip and eight differential electrode pairs on the receiving strip. When the sensor is assembled, the differential capacitance is zero; as the strip is bent and the two strips move out of alignment, the differential capacitance is changed in a predictable manner.

The pair tested the prototype sensor, and found it capable of accurate measurements with a self-correcting nature: "When a measured curve starts to drift off of the ideal," the researchers claim, later segments help pull it back on. Again, this is in sharp contrast to systems that independently encode a series of joints." To prove ShArc's efficacy in real-world scenarios, the researchers looked at several potential implementations: Gesture sensing by tracking joints in the wrist and index finger, posture monitoring for healthcare, and as an angular ruler for measuring curvature of real-world objects.

"ShArc devices are able to sense complex curves which are modelled as a series of connected, circular arcs," the researchers conclude. "We outlined the operating principle of measuring relative shift between inner and outer layers of the sensor at many points and showed the theoretical tolerance to measurement errors. A practical capacitive implementation was described, and its performance characterized. Compared to traditional bend sensors, ShArc sensors are inexpensive, precise and do not suffer from drift of strain characteristics. This makes them ideal for a number of applications."

The pair's work has been published as part of the Proceedings of the 2020 CHI Conference on Human Factors in Computing Systems (CHI '20) on the ACM Digital Library under open-access terms.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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