This paper proposes and describes a charge amplifier-based interface electronics design methodology approach for piezoelectric polymer tactile sensors to be used for humanoid robot applications. The tactile sensor–consisting of a PVDF single sensor element–is illustrated, the electromechanical thickness mode behavior is modeled and a fabricated prototype is presented. The model allows associating the PVDF charge response to the effective load applied on the outer surface of the sensor. The model is also used as a key in the proposed design methodology to estimate the charge to be detected by the charge amplifier. A case study based on the design methodology approach is reported. The system (PVDF single taxel + interface electronics) is analyzed firstly, considering it as ideal and then when the nonidealities come into play. A charge amplifier-based interface electronics prototype is also presented. The implementation is used to validate the followed approach and then defining the design specs towards the dedicated IC design. Some experimental results are also reported.

Charge amplifier design methodology for pvdf-based tactile sensors

One of the most efficient ways to connect a resistive sensor array is using columns and rows in a 2-D structure. The main problem in this implementation is crosstalk appearing due to non-idealities in the readout circuits. Crosstalk causes measurement errors which restrict the usable range in array size or resistive sensors values. This paper analyses the way in which these factors affect accuracy, setting out a new approach to eliminate or reduce their influence based on just an additional operational amplifier and some calibration resistors without a significant increase in readout time. The first method involves introducing an additional column in order to eliminate the combined effect of the output resistance buffers for row selection, offset voltages and bias current of the operational amplifier. In the second method, a row of resistors is added in order to reduce the effect of finite gains. Theoretical analysis has been performed and validated by simulation. Results show that an improvement in accuracy compared with other proposals in the literature is possible.

New Approaches for Increasing Accuracy in Readout of Resistive Sensor Arrays

Resistive sensor arrays are formed by a large number of individual sensors which are distributed in different ways. This paper proposes a direct connection between an FPGA and a resistive array distributed in M rows and N columns, without the need of analog-to-digital converters to obtain resistance values in the sensor and where the conditioning circuit is reduced to the use of a capacitor in each of the columns of the matrix. The circuit allows parallel measurements of the N resistors which form each of the rows of the array, eliminating the resistive crosstalk which is typical of these circuits. This is achieved by an addressing technique which does not require external elements to the FPGA. Although the typical resistive crosstalk between resistors which are measured simultaneously is eliminated, other elements that have an impact on the measurement of discharge times appear in the proposed architecture and, therefore, affect the uncertainty in resistance value measurements; these elements need to be studied. Finally, the performance of different calibration techniques is assessed experimentally on a discrete resistor array, obtaining for a new model of calibration, a maximum relative error of 0.066% in a range of resistor values which correspond to a tactile sensor.

/pdf/accuracy-and-resolution-analysis-of-a-direct-resistive-2jks4v9z2k.pdf

Accuracy and Resolution Analysis of a Direct Resistive Sensor Array to FPGA Interface

Tactile sensing arrays for robotic applications become more and more popular these days. This allows us to equip robots with sensing abilities similar to those of our human skin. This paper presents an approach to tactile-based recognition of objects and evaluates the utility of various feature extractors for tactile processing. Extracting these features from a tactile database, we describe a system that combines a discretization step with the well-known C4.5 algorithm in an object classification task. We analyse the usefulness of the features in terms of entropy-based considerations taking into account the generated decision trees and report our results that give important hints for feature selection.

Using entropy for dimension reduction of tactile data

For humans, the sense of touch is essential for interactions with the environment. With robots slowly starting to emerge as a human-centric technology, tactile information becomes increasingly important. Tactile sensors enable robots to gain information about contacts with the environment, which is required for safe interaction with humans or tactile exploration. Many sensor designs for the application on robots have been presented in literature so far. However, most of them are complex in their design and require high-tech tools for their manufacturing. In this paper, we present a novel design for a tactile sensor that can be built with low-cost, widely available materials, and low effort. The sensor is flexible, may be cut to arbitrary shapes and may have a customized spatial resolution. Both pressure distribution and absolute pressure on the sensor are detected. An experimental evaluation of our design shows low detection thresholds as well as high sensor accuracy. We seek to accelerate research on tactile feedback methods with this easy to replicate design. We consider our design a starting point for the integration of multiple sensor units to a large-scale tactile skin for robots.

/pdf/a-flexible-and-low-cost-tactile-sensor-for-robotic-4ealdetw9d.pdf

A flexible and low-cost tactile sensor for robotic applications

Tactile sensors can be used in many applications from object recognition in industrial grippers and medical devices to human-robot-interaction in service robotics. Among the different working principles of tactile sensors, the resistive is of particular interest. Those sensors are constructed in a simple way, are insensitive to shock and overdrive and can be designed with both, a high spatial resolution and a large sensing area. Conventional resistive tactile sensors are based on two electrode layers, located on opposed sides of a conductive polymer. Since this design has disadvantages affecting lifetime, we use a onesided contacting by the use of a regular electrode matrix. However, this increases the signal processing circuit's complexity. This paper introduces a novel kind of resistive tactile sensor matrix with an inter-electrode sampling technique for high speed measurements at an reduced number of electrodes and therefore a reduced complexity. The design of such sensor matrices is described and a method for comparing their performance based on the matrix efficiency is introduced. In addition, a working prototype was build and successfully tested.

Resistive tactile sensor matrices using inter-electrode sampling

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Resistive tactile sensor matrices using inter-electrode sampling