MEMS and Sensors

Measuring mechanics with flexible force sensors

18 September 2020
Force sensing resistor. Source: oomlout/CC BY-SA 2.0

Among the different forms of available flexible sensors, force sensors are crucial as they translate mechanical forces, including stress, tension, pressure, vibration, torque and strain, into electrical signals. In recent years, the standard manufacturing procedures for flexible force sensors are no longer meeting the demand of the latest complex and personalized structures.

Therefore, 3D printing technology is now being used for manufacturing them. This advanced additive processing technology can separately develop each layer of a physical system using computer models, delivering the incomparable simplicity and flexibility in complex 3D systems.

3D printing manufacturing methods

The main 3D printing techniques used for developing flexible force sensors include selective laser sintering (SLS), stereolithography (SLA), digital light processing (DLP), fused deposition modeling (FDM) and direct ink writing (DIW). The SLS printing technique has good mechanical properties and can be used for a broad range of materials. For applications requiring rapid prototyping and high resolution, SLA and DLP printing technologies are preferred. FDM is popular for manufacturing flexible force sensors at a lower cost and is easy to operate. Finally, DIW printing technology is also a low-cost solution, can print multi-materials and offers easy construction. However, it has certain drawbacks such as the resolution and printing speed is relatively low when compared with other 3D printing technologies. Further, it is also difficult to control the viscosity level of the ink. The other 3D printing methods also come with disadvantages such as SLS, which can cause shrinkage after cooling, DLP and SLA can lead to material waste, and FDM needs high temperatures for working and has low printing speed and resolution, similar to DIW technology.

Fabrication materials for flexible force sensors

Currently, the main materials being used for manufacturing flexible force sensors include polymer materials, carbon-based materials and metallic elements. Popular polymer materials that are used for flexible force sensors are polyvinylidene fluoride (PVDF), polyethylene naphthalate (PEN), polyurethane (PU), polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), polyimide (PI) and parylene. Thanks to their strong thermal conductivity, the flexibility of combination with conductive materials, high tense strength and chemical stability, these flexible matrix polymer materials are commonly used. For manufacturing active electrodes and resistive flexible sensors (which are discussed in the next section), the technique of filling elastic polymers with conductive active materials is popularly used.

Carbon-based materials are broadly used for the production of flexible force sensors because of their flexible nanostructures, exceptional electrical conductivity and efficient biocompatibility. The most commonly used carbon-based materials are carbon nanotubes (CNTs), carbon black, graphene oxide (GO) and graphene, which are mostly combined with polymers to develop conductive composites. Metal materials are perhaps the most widely used conducting materials for flexible force sensors. For flexible force sensors manufacturing, silver, copper, gold, zinc, aluminum, molybdenum, magnesium, nickel, titanium, chromium and others are commonly used. Moreover, the metallic materials are abundantly available as liquid metals, metal nanomaterials, metal films and metal oxides.

Sensing methods of flexible force sensors

For sensing the mechanical forces and translating them into the electrical parameter, flexible force sensors employ four main sensing mechanisms: piezoelectric, resistive, triboelectric and capacitive technologies. The piezoelectric flexible force sensors translate the pressure signal into an electrical voltage by using the piezoelectric behavior of the piezoelectric materials. An electric dipole moment is responsible for controling this piezoelectric behavior. When external deformation, or an external force of any direction is applied to the piezoelectric material, it results in electric polarization, which in turn, leads to the production of charges of opposite polarity on the two surfaces. After that, a potential difference is developed as soon as the external force is removed.

Resistive flexible force sensors work by translating the change in pressure into a change in resistance of the sensor. This sensor has a conductive dielectric whose area of contact changes with the applied stress force. This deformation of the sensor further causes a change in length of the conductive channel, and hence a change in resistance. Moreover, the resistive flexible force sensors can be further categorized into piezoresistive and strain type when pressure and tension signals are applied, respectively. However, many piezoresistive sensors have stability and biocompatibility problems. Therefore, they are commonly used in wearable devices.

Triboelectric flexible force sensors produce a charge when under pressure contact or after application of friction force. When the sensors are relieved, they produce a potential difference while separating from each other, and hence translating the mechanical stimuli into an electrical parameter. It is noteworthy that just like piezoelectric flexible force sensors, triboelectric flexible force sensors only generate electrical signals when they are subjected to a force and then get separated. Furthermore, one of the major components of triboelectric flexible force sensors is a triboelectric nanogenerator. Its operating principle is the integration of electrostatic and triboelectric induction. These nanogenerators are now being thoroughly researched and implemented in self-powered force sensors and low-power power supplies. Coming to the capacitive flexible force sensors, these sensors translate the change in pressure into a change in capacitance. When an external force is applied, the capacitance of a parallel plate capacitor is changed as the gap between the two plates is changed.

Applications

Flexible force sensors can be broadly used in consumer products, medical devices, automobiles, industrial motion control applications, aerospace, robotics and other similar areas. Many triboelectric and piezoelectric flexible force sensors are ideally suited for dynamic sensing. Furthermore, piezoelectric sensors have faced problems for the monitoring of static pressure signals. Capacitive flexible force sensors are perfect for sensing small deflection changes as they offer good frequency response, high spatial sensitivity and resolution, large dynamic range, and lower power usage. Even so, it is vital to address challenges, including electromagnetic interference and junction capacitance.

To contact the author of this article, email engineering360editors@globalspec.com


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