Flexible haptics patch senses pressure too

Technology News |
By eeNews Europe

In a paper titled “A Flexible Piezoelectret Actuator/Sensor Patch for Mechanical Human-Machine Interfaces” published in ACS Nano, the authors describe 150um thin sandwich-structured piezoelectret-based device able to assume both the sensor and actuator functions. The flexible device consists of top and bottom fluorinated ethylene propylene (FEP) electret films interspaced by an Ecoflex spacer (with 2mm diameter holes patterned throughout), finalized with gold (Au) and aluminium (Al) electrodes at the top and bottom surfaces, respectively.

The actuator/sensor structures as a bracelet-shaped flexible
device for mechanical human-machine interfaces providing
real-time haptics and sensing through electrostatic actuation
and charge generation under mechanical deformation,

To turn the device into an electret, a corona charging process is used to generate megascopic electrical dipoles inside the air cavities formed between the FEP films sandwiching the holed-out Ecoflex spacer. Then, the mere application of an alternating voltage generates alternating electrostatic forces across the device, inducing small vibrations that can be felt by touch for haptics applications.

A 3×3 matrix of piezoelectret pixels intermixing 5
sensor (S) pixels and 4 actuator (A) pixels on the
same piezoelectret film.

Reversibly, under a mechanical deformation such as body motions (if the sensor patch is worn on the skin across a joint) or a tactile input, the flexible piezoelectret outputs small charges which can be correlated to the applied pressure or motion. By connecting different areas or piezoelectret pixels with driving electrodes (for the haptic effects) or with sensing electrodes (to sense local pressure), one simple piezoelectret film can be designed into an integrated actuator/sensor device with multiple pixels individually driven for real-time sensing and actuation feedback across a wearable skin patch. To prove their concept (for which a patent has been filed), the authors fabricated a 3×3 array intermixing 5 sensor pixels with 4 actuator pixels on the same piezoelectret film, each about one square centimetre.

The equivalent piezoelectric coefficient of such a piezoelectret device (d33 value) was characterized to be 4050 pC/N, exceeding those of many traditional piezoelectric or piezoelectret materials or devices, meaning the soft actuator was able to produce a strong vibration force close to the vibration mode of a cell phone at 20mN. Meanwhile, the flexible pressure sensor part, operating without power, exhibited a minimum pressure detection limit of 1.84 Pa (the weight of dandelion seeds) with less than 1% readout variations for more than 6000 operations.

The sandwich-structured piezoelectret including two FEP layers on the top and bottom and a soft Ecoflex layer at the centre with circular-shape holes.

Power consumption for the prototype actuator was only 8.25mW under a driving input of 3.33V/μm, making the device suitable for low power wearables, whereas tapping the sensor with the fingertip generated enough charges to illuminate selected liquid crystal display pixels (showing the number of “1” in an experiment).

The researchers anticipate they could further optimize the geometry design of their sandwich-structured piezoelectret and its performances with other materials, possibly leading to large area actuator/sensor patches for practical mechanical human-machine interface applications.

Related article:

Thin flexible tactile actuator for wearable haptics


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