Novel piezoelectric sensors monitor robotic movement

Flexible piezoelectric sensors are essential to monitor the motions of humans and humanoid robots; however, existing designs are either expensive or have limited mobility. Now, a research team from Shinshu University in Japan has developed a novel piezoelectric composite material made from electrospun polyvinylidene fluoride fibres combined with dopamine. The sensors made from this material showed performance and stability improvements at a low cost, which could help advancements in medicine, health care and robotics.

The world is moving towards the intelligent era — a stage marked by automation and interconnectivity, achieved by leveraging technologies such as artificial intelligence (AI) and robotics. Sensors are a sometimes-overlooked requirement for this transformation. However, now that robots are becoming more agile, flexible sensors (which provide better comfort and higher versatility) are becoming an active area of study. Piezoelectric sensors are particularly important in this regard, as they can convert mechanical stress and stretching into an electrical signal.

Now, the researchers from Shinshu University have improved the design of flexible piezoelectric sensors by using an established manufacturing technique known as electrospinning. The study, which was led by Distinguished Professor Ick Soo Kim, was published in the journal Nature Communications.

The proposed flexible sensor design involves the stepwise electrospinning of a composite 2D nanofibre membrane. First, polyvinylidene fluoride (PVDF) nanofibres with diameters in the order of 200 nm are spun, forming a strong uniform network that acts as the base for the piezoelectric sensor. Then, ultrafine PVDF nanofibres with diameters smaller than 35 nm are spun onto the pre-existing base. These fibres become automatically interweaved between the gaps of the base network, creating a particular 2D topology.

After characterisation via experiments, simulations and theoretical analyses, the researchers found that the composite PVDF network had enhanced beta crystal orientation. By enhancing this polar phase, which is responsible for the piezoelectric effect observed in PVDF materials, the piezoelectric performance of the sensors was improved. To further increase the stability of the material, the researchers introduced dopamine during the electrospinning process, which created a protective core-shell structure.

“The sensor fabricated from using PVDF/DA composite membranes exhibited superb performance, including a wide response range of 1.5–40 N, high sensitivity of 7.29 V/N to weak forces in the range of 0–4 N and excellent operational durability,” Kim said.

These qualities were demonstrated using wearable sensors to measure a variety of movements and actions. The proposed sensors, when worn by a human, could also produce a distinguishable voltage response to natural motions and physiological signals. This included finger tapping, knee and elbow bending, foot stamping, speaking and wrist pulses. Because these piezoelectric sensors use environmentally friendly organic materials instead of harmful inorganics, this study could have technological implications for health monitoring, diagnostics and robotics.

“Despite the current challenges, humanoid robots are poised to play an increasingly integral role in the very near future. For instance, the well-known Tesla robot ‘Optimus’ can already mimic human motions and walk like a human. Considering high-tech sensors are currently being used to monitor robot motions, our proposed nanofibre-based superior piezoelectric sensors hold much potential not only for monitoring human movements but also in the field of humanoid robotics,” Kim said.

The researchers will now focus on improving the material’s electrical output properties so that flexible electronic components can be driven without requiring an external power source.

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