A tactile sensing array is a tactile sensor composed of multiple tactile sensing units. The tactile sensing unit can measure the contact force of a single point, while the tactile sensing array has the ability to detect distributed contact force due to the integration of multiple sensing units. At present, there are various methods for implementing tactile sensing, which can be classified according to their principles as piezoresistive, piezoelectric, capacitive, fluidic, optical, transistor, etc.
1. Piezoresistive tactile sensing
When subjected to external pressure, the resistance value of piezoresistive materials will change. By detecting the change in resistance value, the applied pressure can be measured. Therefore, using piezoresistive materials as pressure-sensitive materials for tactile sensors can achieve the detection of tactile force.
The preparation of common piezoresistive materials can be achieved by mixing conductive substances in insulating polymer materials to form composite materials with pressure-sensitive properties. Conductive particles, conductive nanowires, and other conductive materials are distributed in an insulating polymer matrix. When compressed, the distance between conductive materials decreases, the number of conductive pathways increases, and the overall resistance decreases.
In 2012, Tee et al. from Stanford University in the United States mixed nickel particles with nano microstructures and supramolecular polymers to prepare highly sensitive piezoresistive materials, which were used in the design of flexible electronic skin. The developed flexible electronic skin exhibited excellent contact force detection performance.
In 2014, Takei et al. from the University of California, California, added conductive CNTs (carbon nanotubes) and AgNPs (silver nanoparticles) into a polymer insulation matrix as sensitive materials for tactile sensing arrays, as shown in Figure 1. When the sensing array is subjected to tension, compression, and bending, its resistance values undergo significant changes.
In addition to conductive polymers, metal thin films with piezoresistive effects are also used in the design and manufacturing of tactile sensing arrays.
In 2008, Kim et al. from Goryeo University in South Korea designed a tactile sensing array based on PI (polyimide) substrate. The PI substrate is made of nickel chromium alloy, and its resistance changes when subjected to force deformation, thus achieving the measurement of tactile force.
In 2016, Park et al. from Yonsei University in South Korea developed a tactile sensing array using MoS2 and graphene electrodes, which exhibited high detection sensitivity and excellent repeatability in testing. Using metal thin films with piezoresistive effects as pressure-sensitive materials for tactile sensing arrays, the sensing array has the characteristics of good dynamic performance and fast response speed.
2. Piezoelectric tactile sensing
Piezoelectric materials generate charges when subjected to external pressure, and by measuring the amount of charge generated, the applied external force can be detected. PVDF (polyvinylidene fluoride) has piezoelectric properties after polarization treatment and can be used as a pressure-sensitive material for tactile sensors.
In 2006, Hosoda et al. from Osaka University in Japan randomly distributed PVDF membranes in simulated human fingers. The contact between fingers and objects generated charges on the PVDF membranes, which were then converted into voltage to measure the indirect contact force between fingers and objects.
In 2016, Li et al. from Sun Yat sen University designed a planar piezoelectric thin film as the force sensitive layer of the tactile sensing unit, as shown in Figure 2. When subjected to force, charges are generated on the upper and lower surfaces of the piezoelectric thin film, and the generated charges and voltages are amplified by transistors to achieve the measurement of tactile force.
In 2016, Yu Pingping from Zhejiang University integrated four PVDF films into each sensing unit of the tactile sensing array. When a three-dimensional force is applied to the sensing unit, the four PVDF films generate different amounts of charge. The difference in charge can be measured to decouple and calculate the three-dimensional force received by the tactile sensing unit.
For piezoelectric tactile sensors, the amount of charge generated by piezoelectric materials will decrease over time, so the static detection performance of piezoelectric tactile sensors is poor, but they are suitable for detecting dynamic forces.
3. Capacitive tactile sensing
Capacitors typically consist of two capacitor plates and an intermediate dielectric layer. Under external force, the dielectric layer is compressed, causing a change in the distance between the upper and lower capacitor plates, resulting in a change in capacitance value.
The capacitive tactile sensing array using flat electrodes as capacitive plates has the characteristic of simple structure and easy measurement of normal tactile force.
In 2008, Pritchard et al. from the University of Tennessee designed a thin layer of poly (p-xylene) as a dielectric layer between two flat electrodes to create a 10x10 tactile sensing array. Under pressure, poly (p-xylene) was compressed, increasing the capacitance value to detect the magnitude of tactile force.
Zhang et al. from City University of Hong Kong designed a capacitive tactile sensing unit based on PDMS dielectric layer and fabricated a 3x3 sensing array, as shown in Figure 1.14. The various tactile sensing unit structures of capacitive plates have achieved good three-dimensional force detection performance.