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Towards Tactile Sensing System on Chip for Robotic Applications Ravinder S. Dahiya, Member, IEEE, Davide Cattin, Andrea Adami, Cristian Collini, Leonardo Barboni, Maurizio Valle, Member, IEEE, Leandro Lorenzelli, Member, IEEE, Roberto Oboe, Senior Member, IEEE, Giorgio Metta, Member, IEEE, Francesca Brunetti
Abstract—This paper presents the research on tactile sensing system on chip. The tactile sensing chips comprise of 5×5 array of POSFET (Piezoelectric Oxide Semiconductor Field Effect Transistor) devices and temperature sensors. The POSFET devices are obtained by spin coating piezoelectric polymer, P(VDF–TrFE) (poly(vinylidene fluoride-trifluoroethylene)), films directly on to the gate area of MOS (Metal Oxide Semiconductor) transistors. The tactile sensing chips are able to measure dynamic contact forces and temperatures. The readout and the data acquisition system to acquire the tactile signals are also presented. The chips have been extensively tested over wide range of dynamic contact forces and temperatures and the experimental results are presented. The paper also reports the research on tactile sensing chips with POSFET array and the integrated electronics. Index Terms—Tactile Sensing, POSFET, Touch Sensing System on Chip, Robotic Skin, Humanoid Robots.
I. I NTRODUCTION
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UTURE robots are expected to work closely and interact safely with humans as well as real-world objects. Among various sensing modalities needed for this purpose, the sense of touch is particularly important. Unlike other senses (e.g. vision, audio), it involves complex physical interaction, and plays a fundamental role in estimating properties such as shape, texture, hardness, material type and many more. Such properties can be better estimated by touching or physically interacting with the objects – as humans do. The sense of touch also provides action related information, such as slip, and helps in carrying out actions, such as rolling an object between fingers without dropping it. All these, highlight the importance of sense of touch and call for equipping various parts of robot’s body with intrinsic force sensors and extrinsic touch or tactile sensors. The work presented here pertains to the extrinsic touch or tactile sensing. It is desirable to have tactile sensors over whole body of a robot. However, the robotic hands, especially the fingertips, are accorded higher priority due to their involvement in majority Ravinder S. Dahiya*, Andrea Adami, Cristian Collini and Leandro Lorenzelli are with Bio-MEMS group, Center for Materials and Microsystems, Fondazione Bruno Kessler, Trento, Italy. e-mail: (dahiya, andadami, ccollini, lorenzel)@fbk.eu. Maurizio Valle and Leonardo Barboni are with DIBE, University of Genoa, 16145, Italy. e-mail: (maurizio.valle, l.barboni)@unige.it. Davide Cattin and Roberto Oboe are with University of Padova, Italy. email: (
[email protected];
[email protected]). Giorgio Metta is with Robotics, Brain and Cognitive Sciences, Istituto Italiano di Tecnologia, Genova, Italy. e-mail: (
[email protected]). Francesca Brunetti is with University of Rome, Tor Vergata, Italy. e-mail: (
[email protected]). Manuscript received ****, ****.
of the daily tasks (such as exploration, manipulation and interaction). Over last more than two decades, a large number of tactile sensors, based on a wide variety of transduction techniques (e.g. resistive [1], capacitive [2], piezoelectric [3], optical [4], quantum tunneling [5] etc.), materials (conductive rubber [6], [7], Ferroelectric [8], Nanocomposites [5], Fibers and Yarns [9], Carbon Nanotubes [10], elastomers [11] etc.), and innovative designs (e.g. MEMS (Micro-Electro-MechanicalSystem) [12], OFETs (Organic Field Effect Transistors) [7], [11], MOS transistor [3] and Flexible Printed Circuit Boards etc.) have been reported in literature. Many such solutions have been discussed in some of the recent survey/review papers on tactile sensing [13]–[15]. However, owing to their large size, many sensing components are unsuitable for body parts such as fingertips - where human like spatial resolution (∼ 1 mm) is desired. The desire to obtain human like touch sensing capability therefore calls for employing a large number (high density) of miniaturized and fast responding tactile sensors, especially for such body parts as fingertips of a robot. This work focuses on the tactile sensing for body sites like fingertips and therefore only the techniques adopted to obtain small sized tactile sensors are discussed here. The detailed discussion on various other tactile sensing techniques is reported elsewhere [13]. The miniaturized touch sensors are generally based on MEMS approach [12]. These sensors are in general quite sensitive and realized with diaphragms and cantilevers [16], [17]. However, MEMS based touch sensor can detect contact forces (∼ 0.25 N ) that are at best equal to the lowest range of forces experienced by humans in normal manipulative tasks (∼ 0.15 − 0.9 N ) [13]. Thus, their usage is limited to those applications where low magnitude contact forces are involved. The mechanically flexible OFETs too have been employed as miniaturized touch sensors. Electronic skin using OFETs have also been reported [7], [11]. OFET based tactile sensing solutions have the advantage of being mechanically flexible and low fabrication cost. However, organic devices typically have short life and their operational stability is influenced by various factors, including dependence on stress voltage and duty cycle, gate dielectric, environmental conditions, light exposure, and contact resistance [18]. Further, their low mobility makes them much slower in comparison to the standard silicon based devices [19] and thus limits their usage to measurement of slow varying contact forces only. For sensitive body parts such as fingertips of a robots, the sensors should have a fast response over wide range of dynamic contact events and sense multiple contact parameters. The work presented here is an
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in FeRAM (usually, 100 − 200 nm) devices is an order of magnitude lower than that used in POSFETs [22]. Using thicker polymer films in POSFETs, however, brings up new challenges that are discussed in the following section. The remanent polarization (Pr ) of the polarized polymer and the principle of charge neutrality lead to the appearance of fixed charges ±Q, as shown in Fig.1(a). For piezoelectric polymers in thickness mode, as in this work, the mechanical stress T3 , electric field E3 and electric displacement D3 are related as [23]: Fig. 1: (a) The structure and working of a POSFET touch sensing device; (b) SEM image of a POSFET device. The cross section of piezoelectric polymer film is shown in inset.
effort in this direction. This work extends our research on POSFET touch sensing devices [3], [20] and arrays [21] towards tactile sensing system on the chip. The tactile sensing chip consists of 5×5 POSFET devices array and the integrated temperature sensors. The tactile sensing chips are extensively tested over wide range of dynamic contact forces and temperatures. The tactile sensing chips are able to detect complex dynamic contact events such as rolling of an object, where stimuli varies, both, in time and space. The readout and the data acquisition systems, needed to acquire the tactile signals, are presented. The tactile arrays, with 1 mm×1 mm sized POSFET devices and 1.5 mm center–center distance between two adjacent POSFETs, have human fingertip like spatial resolution (∼ 1 mm) and spatial acuity [13]. The research towards POSFETs with integrated electronics on the chip is also presented. This paper is organized as follows: The concept and working of POSFET touch sensing devices, their comparison with state of the art and their relative merits are presented in Section II. The design of tactile sensing arrays is also presented in Section II. The experimental evaluation of the tactile sensing chips is given in Section III. The development of the data acquisition for acquiring outputs of POSFETs and the further research on the POSFETs, with integrated electronics on the chip, are discussed in Section IV. Finally, section V summarizes the results with a note on the future work. II. POSFET TACTILE S ENSING C HIP A. POSFET Touch Sensing Device – Concept and Working The structure of a POSFET touch sensing devices is shown in Fig.1(a). It can be noticed that the piezoelectric polymer film is present over the gate area of the MOS device. Thus, transducer material is an integral part of a POSFET device. The structure of POSFET device is similar to that of metalferroelectric-metal-insulator-semiconductor type FeRAM (Ferroelectric Random Access Memory) devices [22], which are used for memory applications. However, working of POSFET devices is fundamentally different from that of FeRAM - as former responds to changes in mechanical stimulus and the output in latter results from electric field switching. Further, the typical thickness of ferroelectric material (Lead Zirconate Titanate (PZT), poly(vinylidene fluoride) polymer etc.) layer
D3 = d33 × T3 + ε33 × E3
(1)
Where, d33 and ε33 are the piezoelectric and dielectric constants of piezoelectric polymer respectively. Following (1), the electric displacement can be controlled either by varying the electric field E3 and/or by the mechanical stress T3 (or the contact force). While former is used in FeRAM to switch the polarization state, latter is used in the POSFETs to modulate the charge in the induced channel of underlying MOS device. It may be noted that unlike FeRAM, the polarization state of P(VDF–TrFE) in POSFETs is not reversed during operation and only changes in polarization arising from applied forces are reflected in the induced channel. This also means that POSFET is relatively free of issues like fatigue observed in FeRAM because of polarization reversals [24]. Thus, the (contact) force variation is directly reflected as variation in the channel current of POSFET devices - which can be further processed by an electronic circuitry that may also be integrated on the same chip. It should be noted that silicon based devices are also known to exhibit piezoresistive effect, i.e. change in resistivity when stressed. This raises an important question about the source of the change in channel current – piezoelectric action of P(VDF–TrFE) polymer film present on the gate area or the piezoresistive behavior of silicon? In practice, both piezoelectric and piezoresistive effects contribute. However, as demonstrated elsewhere [3], the contribution of piezoresistive effect is about 1% of total output. Therefore, the change in the channel current of a POSFET, and hence its output, is primarily because of the piezoelectric action of the P(VDF– TrFE) polymer film. The POSFETs can thus be termed as “sensotronic” units – comprising of transducer and the transistor – that are capable of ‘sensing and (partially) processing the tactile signal at same site’. In this context, POSFETs can be loosely compared with the mechanoreceptor in human skin that not only sense the contact parameters, but also partially process the tactile data at same site [25]. The marriage of sensing material and the electronics, as in POSFETs, is advantageous in a number of ways. Among others, some of the immediate advantages are: better signal to noise ratio, faster response, wider bandwidth, better force sensitivity, and no wire is needed to connect transducer and electronic devices. Similar approaches, but using extended gates, have been reported in the past for ultrasonic [26], force and tactile sensing [8], [27]. In extended gate approach, the gate terminal of a MOS device is connected to a bigger electrode or the extended gate that is located elsewhere on the chip. Like POSFETs, the extended gate approach too brings the sensor and
IEEE SENSORS JOURNAL, VOL. *, NO. *, **** 2011 (ARTICLE IN PRESS)
Fig. 2: (left) A part of the 5 × 5 POSFET tactile sensing chip before polymer deposition; (right) SEM image of the POSFET tactile sensing chip after polymer film deposition.
conditioning electronics closer and hence the overall response is better than the conventional approach - where the sensor and conditioning electronics are placed apart. However, extended gates introduce a large substrate capacitance (value depends on the substrate), which in turn, significantly attenuates the voltage available at the gate terminals of MOS transistors. Thus, benefits of closely located sensor and electronics are not fully exploited with extended gate approach. With piezoelectric polymer on the gate of the MOS itself, the POSFET touch sensing devices are relatively free from such issues. Further, unlike extended gate approach, the POSFETs occupy lesser area on the chip. The silicon real estate thus saved can be used to accommodate on–chip electronics and signal processing circuitry. The local processing of the tactile signal will also help reduce the amount of tactile data transferred to higher perceptual levels of a robot. B. POSFET Tactile Sensing Chip – Design and Fabrication The tactile sensing chips presented in this work consist of an array of 5×5 POSFET touch sensing devices. The design constraints of a tactile sensing structure depends on the body site i.e. the part of robot’s body where sensing structure is to be placed. Considering human body, as an example, the sensitivity and pressure thresholds vary across the body. A similar, if not the same, scenario can be envisaged for tactile sensing in robotics. For body sites like fingertips, it is desirable to have the tactile structures with high spatio-temporal response. Further, such structures should be multifunctional, i.e. they should be able to measure multiple contact parameters such as contact force, temperature etc. The 5×5 POSFET devices based tactile sensing chips, partly shown in Fig. 2, have been designed to have spatial resolution and acuity similar to that of human fingertips. The overall dimension of the tactile sensing chip is 1.5 cm × 1.5 cm. Each POSFET taxel on the array is designed to be 1 mm × 1 mm in size, thus ensuring human like spatial acuity. The center–center distance of 1.5 mm between two adjacent taxels ensures human like spatial resolution. The MOS part of the POSFET device is obtained by using the n-MOS transistor of a 4 µm aluminum gate CMOS (Complementary Metal Oxide Semiconductor)
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Fig. 3: The threshold voltages (VT H ) and transconductance (gm ) (at VDS = 0.5 V and VGS = 2.5 V ) of all POSFET elements on the chip.
technology. The MOS gates are designed with interdigitated structure, for high aspect ratio (Channel width, W = 7500 µm; Channel length, L = 12 µm) and hence large transconductance. In addition to the array of POSFETs, two diodes for temperature sensing have also been placed on the tactile sensing chips. The temperature diodes have been designed to work at 100 µA forward bias current and for linear response over a wide range of temperature. These diodes can be used to measure the ambient as well as contact temperatures. It should be noted that, in addition to being piezoelectric, the P(VDF– TrFE) polymer films are also pyroelectric in nature, i.e. they respond to the variations in the ambient temperature. Thus, temperature variation could be a potential source of error in a POSFET’s response, especially when only contact force is needed to be measured. Such an error can be compensated by observing response of P(VDF–TrFE) at various temperatures and presence of temperature diodes on the chip is helpful in this regard. The fabrication steps adopted to realize the tactile sensing chips are same as those used to develop single POSFET devices. Detailed description of the fabrication steps is reported elsewhere [20]. The fabrication of tactile sensing chip, however, involves additional challenges, such as: (a) depositing polymer film with uniform thickness over all touch sensing elements, (b) fabricating the chip with minimum spread in the characteristics of MOS devices, and (c) simultaneous polarization of the polymer film (a step needed to orient the dipoles in the thickness direction) over all the touch sensing elements so as to as ensure uniform polarization. A number of experiments were performed on dummy silicon wafers (without any MOS device) to investigate the steps for obtaining uniform and controlled thickness of polymer films over large areas. The concentration of solution, spinner’s speed and spinning time were used as variables in these experiments. A 10% P(VDF–TrFE) solution spin coated with 3000 rpm for 30 seconds resulted in a uniform (
