In this paper, we present a new version of tactile sensor GelSlim 2.0 with the capability to estimate the contact force distribution in real time. The sensor is vision-based and uses an array of markers to track deformations on a gel pad due to contact. A new hardware design makes the sensor more rugged, parametrically adjustable and improves illumination. Leveraging the sensor's increased functionality, we propose to use inverse Finite Element Method (iFEM), a numerical method to reconstruct the contact force distribution based on marker displacements. The sensor is able to provide force distribution of contact with high spatial density. Experiments and comparison with ground truth show that the reconstructed force distribution is physically reasonable with good accuracy.
There are many force sensor arrays with various working principles developed to measure force distributions @cite_0 @cite_1 @cite_19 . However, most of them can only sense the distribution of normal force @cite_13 @cite_7 @cite_7 with spatial resolution of about @math . An 8 @math 8 flexible capacitive tactile sensor array built by Lee . could measure both normal and shear force distribution @cite_3 . Four capacitors in the sensor formed a cell to decompose the contact force into normal and shear components. The sensor also worked with 2 mm spatial resolution. Compared to the potential of optical-based sensors, the sensors discussed above have lower spatial resolution and much smaller amount of sensing elements, but often feature higher bandwidth.
@cite_7 @cite_1 @cite_3 @cite_0 @cite_19 @cite_13
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Although major research efforts were devoted in the past to the development of tactile sensing systems for robots, a truly usable, robust, reliable and cheap system is not available yet. This paper presents the design of a tactile sensor system which could fulfil the above requirements. The sensor incorporates multiple sensing subsystems for detecting distributed contact forces and surface characteristics. The fabrication and experimental evaluation of the tactile system and of its electric interface are described. Results indicate that the system provides reasonable performances for practical applications requiring manipulation with tactile feedback. This paper presents the construction and theory of operation of a large area, flexible, tactile sensor and its applications. The sensing mechanism is based on the novel contact piezoresistive effect. Furthermore, the sensor's resolution, size and shape can be easily tailored to the applications' requirements. This versatility facilitates the use of the sensor in smart applications where tactile information is used to create system intelligence. Future improvements in the tactile sensing arena are discussed along with the potential benefits of using polymer electronics. In this paper, we report a new flexible capacitive tactile sensor array with the capability of measuring both normal and shear force distribution using polydimethylsiloxane (PDMS) as a base material. A tactile cell consists of two thick PDMS layers with embedded electrodes, an air gap, and a pillar structure. The pillar structure is formed at the center of each tactile cell between the air gap under a large bump. There are four capacitors in a cell to decompose the contact force into normal and shear components. Four capacitors are arranged in a square form. If a normal force is applied on the bump, the PDMS layer on the pillar structure is compressed, and the air gap between the top and bottom electrodes decreases, resulting in the increase in all four capacitances. If a shear force is applied, a torque is induced around the pillar. Therefore, the capacitance of the two capacitors increases, whereas that of the other two decreases. The bump and the pillar structure play a critical role to generate a torque for shear force measurement. The sensor has been realized in an 8 x 8 array of unit sensors, and each unit sensor responds to normal and shear stresses in all three axes, respectively. Measurement of a single sensor shows that the full-scale range of detectable force is about 10 mN, which corresponds to 131 kPa in three directions. The sensitivities of a cell measured with a current setup are 2.5 mN, 3.0 mN, and 2.9 mN for the x-, y-, and z-directions, respectively. Normal and shear force images are also captured from a 4 x 4 array of the fabricated sensor. Distinctive characteristic patterns appear when a shear force is applied to the sensor. We present the development of a polyimide-based two-dimensional tactile sensing array realized using a novel inverted fabrication technique. Thermal silicon oxide or Pyrex® substrates are treated such that their surfaces are OH group terminated, allowing good adhesion between such substrates and a spun-on polyimide film during processing through what are suspected to be hydrogen bonds that can be selectively broken when release is desired. The release of the continuous polyimide film is rapidly accomplished by breaking these bonds. This process results in robust, low-cost and continuous polymer-film devices. The developed sensor skin contains an array of membrane-based tactile sensors (taxels). Micromachined thin-film met al strain gauges are positioned on the edges of polyimide membranes. The change in resistance from each strain gauge resulting from normal forces applied to tactile bumps on the top of the membranes is used to image force distribution. Response of an individual taxel is characterized. The effective gauge factor of the taxels is found to be approximately 1.3. Sensor array output is experimentally obtained. The demonstrated devices are robust enough for direct contact with humans, everyday objects and contaminants without undue care. This paper suggests a force sensor array measuring contact force based on intensity change of light transmitted throughout optical waveguide. For transparency and flexibility of the sensor, two soft prepolymers with different refractive index have been developed. The optical waveguide consists of two cladding layers and a core layer. The top cladding layer is designed to allow light scattering at the specific area in response to finger contact. The force sensor shows a distinct tendency that output intensity decreases with input force and measurement range is from 0 to −13.2 dB. Human symbiotic robots, such as for nursing, which share humans' environments and interact with them, should be covered with soft areal tactile sensors for safety and dexterous manipulation. Unfortunately, we do not have definitive and standard tactile sensors for this purpose. In order to solve this problem, we are now developing tactile sensors by embedding semiconductor pressure sensors, as pressure-sensing elements, into an elastic body. We have established a method of embedding an array of semiconductor pressure sensors in an elastic body using a flexible substrate. We have also proposed a structure for the elastic body consisting of hard and soft elastic materials, for improving sensitivity. We first describe the structure of our soft areal tactile sensors, for detecting normal stress. Then we report results of their simulations and experiments. Finally we describe the extension of our sensor for detecting shearing stress.