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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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. This paper presents a fiber optic based tactile array sensor that can be employed in magnetic resonance environments. In contrast to conventional sensing approaches, such as resistive or capacitive-based sensing methods, which strongly rely on the generation and transmission of electronics signals, here electromagnetically isolated optical fibers were utilized to develop the tactile array sensor. The individual sensing elements of the proposed sensor detect normal forces; fusing the information from the individual elements allows the perception of the shape of probed objects. Applied forces deform a micro-flexure inside each sensor tactel, displacing a miniature mirror which, in turn, modulates the light intensity introduced by a transmitting fiber connected to a light source at its proximal end. For each tactel, the light intensity is read by a receiving fiber connected directly to a 2-D vision sensor. Computer software, such as MATLAB, is used to process the images received by the vision sensor. The calibration process was conducted by relating the applied forces to the number of activated pixels for each image received from a receiving fiber. The proposed approach allows the concurrent acquisition of data from multiple tactile sensor elements using a vision sensor such as a standard video camera. Test results of force responses and shape detection have proven the viability of this sensing concept. 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. This letter presents a strategy to represent data from a tactile array sensor and match it to an object’s geometric features. Using that representation, a method is presented to localise a grasped object within a robot hand. The method consists of computing the covariance matrix in the tactile sensors’ pressure data and computing the eigenbasis from its principal axes. A global search is carried out to find a pose in which the object’s local geometry in the vicinity of the contact is coherent with that basis, i.e., is aligned with the principal axes and has similar variances. This approach, which can be used as a measurement model for tactile sensors, is compared and outperforms methods using the distance between the tactile sensor elements and object surface.