Robust and high-performance soft inductive tactile sensors based on the Eddy-current effect

Highlights

• The first Soft Inductive Tactile Sensor (SITS) is proposed.

• Working principle and design methodology of SITS are discussed.

• A SITS prototype achieves a resolution of 0.82 mN in a range of over 15 N.

• The presented SITS can operate in water or other harsh environments.

• The SITS systems are low cost, durable, low hysteresis, and high performance.

Abstract

Tactile sensors are essential for robotic systems to interact safely and effectively with the external world, they also play a vital role in some smart healthcare systems. Despite advances in areas including materials/composites, electronics and fabrication techniques, it remains challenging to develop low cost, high performance, durable, robust, soft tactile sensors for real-world applications. This paper presents the first Soft Inductive Tactile Sensor (SITS) which exploits an inductance-transducer mechanism based on the eddy-current effect. SITSs measure the inductance variation caused by changes in AC magnetic field coupling between coils and conductive films. Design methodologies for SITSs are discussed by drawing on the underlying physics and computational models, which are used to develop a range of SITS prototypes. An exemplar prototype achieves a state-of-the-art resolution of 0.82 mN with a measurement range over 15 N. Further tests demonstrate that SITSs have low hysteresis, good repeatability, wide bandwidth, and an ability to operate in harsh environments. Moreover, they can be readily fabricated in a durable form and their design is inherently extensible as highlighted by a 4 × 4 SITS array prototype. These outcomes show the potential of SITS systems to further advance tactile sensing solutions for integration into demanding real-world applications.

Introduction

Tactile sensors are essential components that enable robotic systems to interact safely and effectively with humans and the environment [1,2]. They also offer significant potential for use in modern healthcare systems [3], including prosthetics [4], wearable health monitoring devices [5], and smart surgical instruments [6]. Compared to the visual and auditory senses, the tactile sensory capabilities provided by human skin [7,8] are complex, combining large arrays of high performance, multi-modal sensory elements (receptors) within a mechanically compliant substrate (skin tissues) to extract information through deformation during interaction with objects [1,9]. These attributes have provided a natural benchmark for those seeking to develop tactile sensors and achieve a comparable performance to biological systems such as human hand [10], notably in resolution, accuracy, bandwidth and mechanical compliance. To be effectively applied in real-world environments, it is advantageous that they are durable and robust to the repeated mechanical interaction inherent in tactile sensing. Research into tactile sensors which attempts to meet these challenging objectives has been catalysed by recent advances in enabling technologies, particularly printed organic electronics [11] and advanced materials [12]. Remarkable progress has been made in developing compliant sensory systems, with notable examples including an ultra-lightweight, tactile sensing array with integrated organic electronics [11], a printed flexible tactile sensing skin [13], self-powered sensors employing triboelectric effects [14] and systems using organic electronics that simulate the signalling outputs of the mechanoreceptors found in human skin [15]. Nevertheless, challenges remain in the ease with which tactile sensors can be fabricated, interfaced and integrated into robotic and healthcare systems. Researchers seeking innovations in tactile sensing have explored and exploited new materials, novel composites/structures, fabrication techniques and transducer mechanisms [16]. In this work we focus on the opportunity to use an alternative transducer mechanism to develop high-performance, robust and durable tactile sensors.

Tactile sensors are typically derived from modes of transducer sensitive to strain, stress, displacement or vibration. A wide variety of different transducer mechanisms have been exploited to date [17], with common modalities including piezoresistivity/resistance, piezoelectricity, triboelectricity, capacitance, optics/laser, and magnetic field [16]. Piezoresistive/resistive tactile sensors [18] are prevalent, and operate by measuring the resistance variations caused by changes in contact area between conductive materials, changes in conductive path in conductive elastic composites, or changes in the geometry of conductive liquids [19]. They are low-cost, and require simple readout electronics, but they also encounter low sensitivity, slow response, small dynamic range and large hysteresis. Recently, an ultra-sensitive resistive pressure sensor (1 Pa resolution) [12] has been developed by using an elastic hollow-sphere microstructured conductive polymer, however large hysteresis was observed. Piezoelectric sensors [20] generate electrical charges when force is applied. They are typically highly sensitive, but rigid, and only detect dynamic forces. Recently, novel piezoelectric composites and structures (e.g. PVDF [21] and piezoelectric nanowires [22]) have been developed for flexible tactile sensors. Capacitive tactile sensors [23] obtain force information by measuring the capacitance variations caused by the movement of one electrode toward another when force is applied to the elastic body. Flexible single-axis and three-axis forms of capacitive tactile sensors have been developed, for instance using conductive textile as electrodes [24]. Despite their high sensitivity and rapid response, capacitive tactile sensors require complex fabrication processes [25], and they are sensitive to environmental contaminants [26] (such as oil, dust, liquid and vaporous water etc.). Other research has exploited optical transducers for tactile sensing, typically using camera/photodetectors to monitor the deformation of soft skins [27]. This approach yields highly deformable systems, insensitive to electromagnetic interference and environmental contaminants. A notable example is a thin (<150 μm), transparent, flexible tactile sensor array based on a polymer-waveguide system [28]. However, it has relatively low sensitivity, poor repeatability, and large hysteresis in comparison to other sensing modalities. Magnetic field-based tactile sensors [29] have seen recent enhancements from the availability of integrated, compact, Hall-Effect sensing chips, providing sensors which are deformable, durable and low-cost [30]. This type of sensor has been integrated into fingertip of robotic hand to provide tactile sensing feedback [31]. However, they are directly affected by external magnetic sources or ferro-magnetic objects which change the local magnetic field distribution. These factors make such sensors prohibitive for applications involving objects made of ferro-magnetic materials (e.g. Iron, nickel, cobalt and their alloys).

One notable physical phenomenon which remains undeveloped in soft tactile sensing is the eddy-current effect, despite its ubiquity in industry [32] in the form of eddy current sensors (ECSs), which enable non-contact displacement sensing with high sensitivity, wide bandwidth and robustness to environmental contaminants [33,34]. ECSs operate by monitoring the distance between an AC current excited coil and an electrically isolated conductor (sensing target) [34], a configuration which minimises the need to expose potentially vulnerable electronic elements near the external environment. Based on this principle, Texas Instruments developed a demonstration system to detect touch/force on metal buttons [35] by monitoring the inductance change of a coil situated underneath. We sought to exploit the advantages brought by ECSs and translate this technology into a form-factor that can be readily designed, fabricated and optimised toward soft tactile sensing for robotic and medical applications. Thus, in this work, we present the first Soft Inductive Tactile Sensor (SITS) which exploits an inductance-transducer mechanism based on the eddy-current effect. We explain the operating principle and discuss design methodology, and develop physical exemplars to evaluate their performance and illustrate their potential for real-world applications.