Force Sensor Based on Optoelectronic Technology

Force Sensor Based on Optoelectronic Technology

Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 826 – 829 30th Eurosensors Conference, EUROSENSORS 2016 Imp...

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Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 168 (2016) 826 – 829

30th Eurosensors Conference, EUROSENSORS 2016

Improved version of the tactile/force sensor based on optoelectronic technology A. Cirilloa, P. Cirilloa, G. De Mariaa, C. Natalea, S. Pirozzia,* a

Dipartimento di Ingegneria Industriale e dell’Informazione, Seconda Università degli Studi di Napoli, via Roma 29, Aversa, 81031, Italy

Abstract The aim of this paper is to introduce an improved version of the tactile sensor already presented by the authors. The tactile sensor provides an accurate estimation of the contact wrench and a tactile map related to the contact geometries. With respect to the old sensor version, the improvements concern: the spatial resolution, increased by exploiting 25 couples of infrared LED and photodetector; the full-scale, become four times higher; the new digitalization technique, introduced in order to reduce the overall wire number and power consumption. © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Tactile/Force sensor; Optoelectronic sensor;

1. Introduction In industrial and advanced robotics, the use of force/tactile sensors is necessary to provide to robots capabilities for interacting with the environment. The knowledge of the contact wrench is fundamental for grasping and handling objects of different shapes, stiffness and materials, without producing damages and avoiding object slipping. Nowadays, a significant number of tactile sensors are able to provide a pressure map related to the external contacts or, sometimes, an estimate of the 3D contact force. Differently, it is not simple to design a sensor that can provide more complex information regarding the exerted contact wrench and the contact location and geometry [1]. Starting from a previous version [2], the authors propose in this paper a new prototype of the optoelectronic tactile sensor with

* Corresponding author. Tel.: +39 0815010433. E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.283

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improved performance. With respect to the old sensor version, the new one exploits a greater number of optoelectronic devices in order to increase the spatial resolution. This choice required the adoption of a new digitalization technique, introduced in order to reduce the overall wire number and power consumption. Moreover, the full-scale become four times higher. In Section 2, the sensor is described highlighting the differences with the previous version. The section provides also a description of the adopted acquisition technique that emphasizes the advantages and the limitations of the proposed approach. Section 3 focuses on the sensor calibration procedure and accuracy. Finally, in Section 4, the conclusion and future extensions are discussed. Nomenclature LED PD ADC GND MCU

light emitting diode photo detector analog to digital converter ground signal microcontroller

2. Sensor description 2.1. Sensor technology The tactile/force sensor is constituted by two layers (see Fig. 1), an optoelectronic layer and a deformable silicone layer. The optoelectronic layer of the sensor presented in [2] was constituted by 16 infrared LED/PD couples (taxels) organized in a 4x4 matrix. Each taxel was formed by two components installed on the PCB side by side: an SFH4080 infrared LED (880 nm) and an SFH3010 PD (860 nm), both by OSRAM, with a total surface encumbrance of 1.7x2.0 mm2 resulting in a spatial resolution of 16 taxels/cm2. Because of the limited dimensions, such components not allowed the use of an automatic positioning and soldering process, requiring mandatorily a soldering by hand. The optoelectronic layer of the new prototype consists of an electronic PCB where 25 LED/PD couples are organized in a 5x5 matrix. Differently from the previous version, the presented prototype uses the NJL5908AR, by JRC, as unique optoelectronic device. It is a compact SMT photo reflector, that integrates, on a unique component, both an infrared LED (920 nm) and a PD (880 nm), with a total surface encumbrance of 1.06x1.46 mm2 allowing to reach a spatial resolution of 20 taxels/cm2. The new component has been selected pursuing two targets: it makes easier the automation of the assembly process with a robotized pick-and-place procedure and it allows to reduce the uncertainties on the relative orientation between two separated devices due to the soldering by hand, which can negatively impact on the estimation accuracy. The electronic layer is covered by a deformable layer that transduces the mechanical deformation of the surface, due to external force/torque applied to the sensor, in electrical variation of the PD current. These variations are converted in voltage variations by using a single resistor, adequately selected to determine the sensitivity of the sensor. The mechanical properties of the deformable layer can be adequately chosen to select the full-scale of the sensor. The presented prototype uses a shore hardness of 26 A, four time higher with respect to the previous version, increasing the full-scale for the measured forces up to 14 N. 2.2. Sensor interrogation To keep unaltered the sensor dimensions, the use of 25 couples makes difficult the adoption of an on-board A/D converter. However, an external acquisition system requires at least a number of wires equal to the number of the taxels. For these reasons, an innovative acquisition procedure has been adopted [3],[4], with the aim to reduce the total number of wires and, then, the dimensions of the on-board connector. Fig. 1 shows a picture of the designed PCB, where some annotations are reported to support the explanation of the digitalization technique. The 25 taxels are grouped in 5 rows (marked in orange color), which are independently powered by using a digital I/O of an MCU.

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Fig. 1. Pictures of the developed sensor (left) and Designed PCB with the interrogation scheme (right).

At each PD emitter, an ADC channel of the MCU is connected. In particular, the PDs in the same column (marked in green color) share both the resistor on the emitter and the ADC channel. Powering alternatively the groups, five analogue channels are acquired, and in only 5 acquisition cycles the 25 taxel voltages are digitized. This approach is based on the fact that the turned off PDs behave as an open circuit that does not influence the A/D conversion of the voltage of the turned on PDs. In this way, the mentioned technique allows to reduce the total number of wires to 11: 5 wires for the digital I/O, 5 wires for the A/D channels and 1 wire for the GND signal. Such digitalization system provides a benefit also in terms of power consumption. In each time instant, just 5 taxels are contemporaneously turned on. By considering a power supply of 3.3 V and a current absorption of 1mA for each taxel, the sensor power consumption results to be 16.5 mW, five time lower than the power required for the whole sensor. The major drawback of the discussed technique concerns the maximum reachable sampling frequency. The choice of turning on alternatively the taxel groups decreases the maximum sampling rate of a factor equal to the group number (5 for the presented sensor). Actually, the most important factor affecting the sampling frequency is the Inrush Current generated when the LEDs are switched on [5],[6]. Typically, the inrush current is a short duration current, whose amplitude is much greater than the operating or steady-state current. Basically, the taxels are powered on through a square wave voltage generated with an MCU digital I/O (see Fig. 2). On each square wave rising edge, the current absorbed by the LED reaches a maximum value and, then, it exponentially decreases down to the working current. The LED behavior influences the PD response through the emitted light. So, the PD voltage signal has a similar trend that reaches a steady-state value with a time constant that depends on the resistor connected to the PD emitter, properly chosen to select the component sensitivity. The inrush current introduces an important delay in the digitalization process given that the signal conversion can be performed just when the steady-state condition is reached. The mitigation of the inrush current effects still remains an open challenge and, in general, it requires complex electronic systems that would increase the sensor dimensions. For the presented sensor, the delay value is 2 ms, which fixes the maximum sampling frequency to 100 Hz.

Fig. 2. Inrush Current issue.

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Fig. 3. Validation dataset. Forces on the left, torques on the right.

3. Sensor calibration and results A calibration procedure allows the estimate of a calibration matrix K that represents the linear mapping between the taxel voltage variations and the force/torque components measured in the sensor reference frame. Figure 3 reports a validation dataset. The sensor shows an estimation error of 0.24 N and 0.46 N for the shear and the normal component of the force, respectively, with a full scale of 4 N and 14 N. Concerning the torques estimation, the maximum estimation error is 5.56 Nmm, with a full scale of 200 Nmm. 4. Conclusion and future works In this paper, a new design of the optoelectronic tactile/sensor has been presented. With respect to the previous version, the sensor adopts 25 couples of optoelectronic devices and an innovative interrogation technique that allows the minimization of the connection wires and of the power consumption. The presence of an inrush current due to the described technique penalizes the sampling frequency, so new techniques and possibilities will be evaluated in order to workaround the problem. Acknowledgements This work was supported by the European Commission’s Seventh Framework Programme (FP7/2007-2013) partly under grant agreement no. 608849 (EUROC project), and partly, under grant agreement no. 601116 (ECHORD Plus Plus project - WIRES Experiment). References [1] [2] [3] [4] [5] [6]

Hanna Yousefa, Mehdi Boukallela and Kaspar Althoeferb, Tactile sensing for dexterous in-hand manipulation in robotics - A review, Sensors and Actuators A: Physical 167, 171-187, 2011. G. De Maria, C. Natale, and S. Pirozzi, Force/tactile sensor for robotic applications, Sensors and Actuators A: Physical 175, 60-72, 2012. Y. Ohmura, Y. Kuniyoshi and A. Nagakubo, Conformable and scalable tactile sensor skin for curved surfaces, IEEE International Conference on Robotics and Automation, Orlando, Florida, 1348-1353, 2006 A. Cirillo, P. Cirillo, G. De Maria, C. Natale, and S. Pirozzi, An artificial skin based on optoelectronic technology, Sensors and Actuators: A. Physical 212, 110-122, 2014. H. Tech, D. Leber and A. Schwarzmeier, A novel approach to efficient inrush current limitation for LED power applications, PCIM Europe 2014, Nuremberg, Germany, 1-8, 2014. Shiquan Fan, Yan Wang, Yen Wang and Li Geng, Minimized start-up transient and initial inrush current of boost converter for LED lighting, IEEE International Symposium on Industrial Electronics (ISIE), Taipei, Taiwan, 1-6, 2013.

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