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Soft multi-axis force sensors are essential in automation and robotic manipulations for dexterous applications. Carbon-based composites are quality candidates for sensing elements of soft multi-axis force sensors due to its piezoresistivity originated from electron tunneling effect. However, low cost and fast-prototyped soft multi-axis force sensors based on tunneling effect of carbon-based composites are still in their infant. This work investigated a soft multi-axis forces sensor utilizing Velostat film strips as the core sensing elements. A unique serpentine structure of Velostat strips has been designed and implemented as piezoresistors for the soft force sensor. The sensitivity of the proposed multi-axis force sensor has been characterized in x, y and z directions. Experimental results show that the sensor can achieve 2.1%/N and 2.2%/N in x and y axis, and 4.2%/N in z axis, respectively. The force sensor has been implemented in dexterous manipulations with a robotic gripper as the proof-of-concept, which demonstrates great potential for multi-axis force sensing applications.

In this article, a methodology for wireless and passive pressure measurement systems has been presented based on the principle of inductive-capacitive (LC) sensing. In the proposed method, two flexible carbon composite materials: Velostat and carbon conductive foam have been chosen as the pressure sensing element. Both the materials have the property of piezoresistivity and show the change in resistivity with applied pressure. In the designed sensor, the LC circuit has been modified into an LCR tank circuit with a variable resistor as a sensing element. During the measurement, pressure has been monitored and measured using a reader coil, connected with the impedance analyzer. The change in resistance in Velostat or conductive foam with applied pressure, changes the input impedance, quality factor, and resonance frequency. The proposed measurement system has shown an innovative technique based on an LC pressure measurement system with a resistive sensing element. Both the sensing materials have been taken separately and individual experiments have been performed. The method has been tested and evaluated experimentally, which also analyzes the two carbon-composite-based materials for this type of wireless pressure sensor. [ABSTRACT FROM AUTHOR]

Printed and flexible sensors are in the focus of recent efforts to establish the advantages of low-cost manufacturing techniques such as screen printing or inkjet printing for printed electronical applications. Devices based on conductive carbon nanotube (CNT) networks within polymeric matrices such as polydimethylsiloxane (PDMS) are already exceeding mere technological demonstrations. Therefore, we investigate the application-oriented behaviour of fully inkjet-printed CNT/PDMS strain sensors under different conditions such as short- and long-term performance. The sensors exhibit a quasi-linear piezoresistive behaviour with vanishing hysteresis to tensile strain. Significant differences in the resistive response between compressive and tensile strain suggest complex re-orientation mechanisms of CNTs inside the matrix. No clear indication for this phenomenon could be observed in the evolution of the CNT network resistance during fatigue measurements within an uncured or cured PDMS matrix, where both scenarios exhibit no visual degradation. However, these measurements over thousands of cycles show different permanent changes in the overall device resistance exhibiting damages but also recovery in the network. Considering these findings facilitates the development of printed sensor devices.

A new 3-D stress sensor that features strain engineering is developed with the capability of extracting the six stress components in a full temperature compensation manner. This article presents the detailed testing of the out-of-plane shear and normal stress for the developed chip. The shear state is produced via applying horizontal force on a microfabricated two-point shear bridge structure, which is mounted on a chip on a printed circuit board. On the other side, a direct vertical force is utilized to induce normal stress directly at the sensing rosette. Finite element analysis is conducted for both loading cases to evaluate the level and uniformity of these stress components. The tested chip captures the out-of-plane shear and normal stress with 16% and 11% full scale, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a new hybrid temperature compensation system for doped silicon-based piezoresistive 3D stress sensors. The developed compensation system integrates a temperature sensor, placed in close proximity to the stress sensing rosettes, with the artificial neural networks (ANNs). In this work, the n-type circular piezoresistor featured over (111) silicon plane was employed to capture the local temperature variations, within the sensing chip. The extracted temperature changes, along with the resistance changes, are fed, as inputs, to the ANNs to compensate the temperature effect on the acquired signals for more accurate stress measurement. The proposed compensation system was experimentally evaluated while extracting stress applied up to 60 MPa at different temperatures within a range from 0 °C to 50 °C. The developed system was successfully able reduce the maximum full scale error, obtained from using only a temperature sensor for compensation, by ~55%. The new system has merit since it has the capability to compensate for both resistance and sensitivity, for 3D stress sensor, with no need for additional circuitry. Moreover the employed temperature sensor shares the same thermal environment with the stress sensing rosette. [ABSTRACT FROM AUTHOR]

In this paper, a developed n-type piezoresistive three dimensional (3D) stress sensor with full temperature compensation is presented. The proposed sensing rosette benefits from the stress insensitivity of the full-circular n-type piezoresistor, oriented over (111) silicon plane, to detect only temperature changes for compensation. Moreover, the unique behavior of shear piezoresistive coefficient ($\pi _{{{44}}}$) of n-Si is utilized to construct a piezorestive stress sensing rosette over (111) silicon plane that is capable of extracting 3D stress components. A prototype stress sensing chip was microfabricated to test the capability of the developed sensing rosette to accurately extract stress applied on structures at different thermal environments. The fabricated sensing chip was subjected to different mechanical loads using a loading rig with four-point bending fixture. The testing was carried out over a temperature range of 0–50 °C. The results showed that the proposed sensing chip has the capability of capturing the stress applied at different temperatures. Also, the developed sensing rosette showed less sensitivity to the uncertainty in piezoresistive coefficients’ values compared to the other developed 3D piezoresistive stress sensors. [ABSTRACT FROM AUTHOR]

A stretchable pressure sensitive piezoresistive film was fabricated through the polymerization of a mixture containing an ionic liquid (IL, 1-ethyl-3-methyl-imidazolium tetrafluoroborate) and a commercially available 3D printable photopolymer. This piezoresistive layer was placed between two carbon-nanotube (CNT)-based stretchable electrodes and was encapsulated with stretchable top and bottom insulating layers to form a multilayer piezoresistive pressure sensor. The intersecting electrodes create a sensing unit, referred to as a taxel. The sensor was connected to a Wheatstone half-bridge circuit, and a normal force was applied on the taxel to evaluate the sensor in terms of change in electrical resistance as the sensor underwent deformation. Sensor performance was investigated for various concentrations of IL and the thicknesses of the intermediate piezoresistive layer. The experimental results showed that the IL concentration and the thickness of the intermediate layer affect the sensitivity of the sensor. Next, various forces were applied to a sensor that was fabricated using reasonable values for IL concentration and layer thickness to determine the reliability and responsiveness. The proposed IL-based sensor displayed superior performance with high sensitivity and reliability. While the sensors for these experiments were manufactured using screen printing and molding processes, the materials used for sensor fabrication are 3D printable, as 3D printing is the target manufacturing technique for stretchable pressure sensors in the future. [ABSTRACT FROM AUTHOR]

In this paper, a new temperature transducer using full-circular n-type piezoresistor over (111) silicon plane is proposed as an accurate solution for local temperature compensation for piezoresistive three-dimensional (3-D) stress sensors. Analytical study, in this paper, showed that the proposed n-type full-circular piezoresisor over (111) has near-zero response under 3-D state of stress. As an experimental verification, a test device with n-type piezoresistive stress sensing elements oriented over (111) plane at 0° and 90° to crystallographic direction $[ {\bar{1}10} ]$ was microfabricated to test the capability of the proposed temperature compensator. The test device was subjected to a uniaxial load up to 50 MPa using a loading rig with four-point bending fixture. The testing was performed over a temperature range of 0–50 °С. The results showed that the proposed temperature transducer has a linear response to temperature excitation. The fabricated transducer is capable of accurately capturing the local temperature changes with maximum absolute error of ±0.8 °С. This compensation system is advantageous for piezoresistive stress/strain sensors, since the temperature transducer has the same thermal environment with the stress/strain-sensing rosette. Moreover, this technique does not require additional circuitry on the sensing device itself. [ABSTRACT FROM AUTHOR]

Strain has been integrated into many silicon devices, as it has an essential impact on carrier mobility and crystal symmetry. Those parameters respond differently under both biaxial and uniaxial, so their effect needs to be quantified to successfully employ the strain engineering in different silicon applications. As an extended step toward utilizing strain in enhancing the sensitivity and the temperature independency of a 3-D stress sensor, the effect of uniaxial transverse strain onto the piezoresistive (PR) coefficients of heavily doped n-type silicon will be experimentally characterized. A new design was developed to apply the transverse tensile and compressive uniaxial stresses onto the silicon substrate using a highly compressive nitride layer. This stressing technique was integrated with six PR elements rosette to fully calibrate the PR coefficients, where unstrained, tensile, and compressive strained PRs are fabricated within the same chip to accurately quantify the strain impact. Four-point bending, stress-free temperature, and hydrostatic test were used to typically measure the PR coefficients. Strain values of 0.065% and 0.083% $\varepsilon $ were achieved locally using both the tensile and compressive stressors, respectively. Under this level of strain, the typical result shows opposite impact for both the tensile and compressive transverse strains on the longitudinal and transverse PR coefficients. Moreover, an increase up to 80% can be achieved for the pressure coefficient of heavily doped n-type silicon due to the compressive transverse strain. [ABSTRACT FROM AUTHOR]

To develop the differential pressure sensor made of carbon nanotube-filled silicone rubber composites with a simplified structure, an electrode-shared differential configuration is designed. The configuration includes the top and bottom sandwich layers. The top sandwich layer is composed of the top film, the intermediate film, and the composite film with low carbon nanotube content between them. The bottom sandwich layer is composed of the intermediate film, the bottom film, and the composite film with high carbon nanotube content between them. The electrode in the intermediate film is shared by the top and bottom sandwich layers, which can make the sensor probe more concise. The electrical resistance of the composite film with high/low carbon nanotube content increases/decreases with the increase of the pressure, and increases with the increase of the temperature. The top and bottom sandwich layers are connected to the neighboring arms of an electrical bridge to construct a differential system. The experiments verify the feasibility of using the electrode-shared configuration to construct the differential piezoresistive sensor made of the composite to decrease the temperature-induced drift and increase the sensitivity. [ABSTRACT FROM AUTHOR]

The existing piezoresistive sensor array adopts the classical configuration called “unconnected structure”, in which the piezoresistive materials in the subelements are separated with each other. Consequently, the sensor array cannot detect the pressure applied on the region among the sub elements (i.e., blind area). To eliminate the blind area, the traditional method is to use more subelements, leading to the increase of the number of the electrodes which complicates the sensor probe. To overcome the limitation, a connected structure, in which a whole piezoresistive sheet is used in all subelements, is designed. The pressure applied on any region of the connected structure can cause the changes in the resistances of the subelements. Therefore, the region bearing pressure can be located and the pressure value can be obtained without adding electrodes. The piezoresistive sensor array made of conductive polymer composite is used as an example. The experiments show that the pressure position can be evaluated, and the pressure measurement error is less than 30 kPa within the range from 0 to 0.4 MPa. The results indicate that the usage of the connected structure can eliminate the blind area by using less electrodes than the traditional method. [ABSTRACT FROM AUTHOR]

This paper reports a successful design, fabrication, and characterization of a new microstructure, that utilizes piezoresistivity, for locally applying a pre-strain state onto silicon substrates and measuring it. This technique allows for applying tensile and compressive transverse local strain for the first time, using the same stressing layer rather than using nitride capping for tensile or silicon germanium for compressive strain. It is well-known that the only strain that enhances both electron and hole mobility simultaneously is the transverse uniaxial strain. Moreover, the utilized sensing structure evaluates directly the actual stress transmitted to the substrate. Therefore, it is a valuable tool for strain engineering applications, where modulating and determining the pre-strain state onto the silicon at infinitesimal areas are required. The proposed structure composed of highly compressive plasma enhanced chemical deposition nitride layer that was patterned in a way allowing for inducing both local tensile and compressive uniaxial pre-strain onto the silicon. Using this method, the stress transferred to silicon is around 40 to 150 MPa, however the developed sensing structure was capable of detecting it for both tensile and compressive stresses. [ABSTRACT FROM PUBLISHER]

Numerical and experimental sensitivity analyses in this paper indicate that the accuracy of a silicon multi-element piezoresistive (PR) stress sensor can be dramatically influenced by the microfabrication non-uniformity and the uncertainties in the values of the PR coefficients and the thermal coefficient of resistance (TCR). The results showed that errors as large as 70% FS or more, in the extracted stress values, may be obtained due to uncertainty of about 2.5% in the values of PR coefficients. This paper aims to evaluate the capabilities of the artificial neural network (ANN) to eliminate the error in stress measurement, due to the fabrication non-uniformity within wafer, wafer-to-wafer, and batch-to-batch, for multi-element PR sensing rosettes. In this paper, sensing chips from two different batches were integrated in building the ANN and testing its performance. The proposed calibration technique employs the neural network fitting Toolbox in MATLAB to generate a two-layer feed-forward network, with sigmoid hidden neurons and linear output neurons. Three different configurations of calibration were designed to test the generalization abilities of the ANN in capturing the in-plane stress components exerted on the silicon chip. The results showed that ANN is capable of accurately predicting the stresses applied to the sensing chip with maximum stress error of 1.5% FS with no need for individual, expensive, and time-consuming calibration process for each sensor. [ABSTRACT FROM PUBLISHER]

The performance of titanium nitride (Ti-N) thin films deposited by reactive magnetron sputtering for use as a strain gauge material was investigated. Films of different composition ranging from pure titanium to over-stoichiometric Ti-N were prepared by varying the nitrogen flow to total flow ratio ( $\alpha )$ . The atomic composition was measured using Rutherford backscattering spectrometry and energy-dispersive X-ray spectroscopy. The structure of the films was studied using X-ray diffraction and scanning electron microscopy, revealing the formation of a columnar film morphology consisting of cubic-Ti-N grains with preferential crystal orientations depending on the flow ratio $\alpha $ . The stability of the film resistivity was studied at 200 °C for 360 h. Films deposited at low ( $\alpha \leq 0.08$ ) and high ( $\alpha = 0.60$ ) nitrogen flows exhibited a stable behavior relative to films deposited with intermediate nitrogen flows. The longitudinal piezoresistive gauge factor was found to increase with increasing nitrogen flow before settling at values of 6–6.35. Finally, the temperature coefficient of resistivity was found to be less than 300 ppm/°C for the films with the highest gauge factors. The results demonstrate that the Ti-N films have a good potential as a strain gauge material, especially due to the relatively high gauge factor. [2016-0040] [ABSTRACT FROM PUBLISHER]

An electrical bridge system based on differential structure is designed to improve the sensitivity of compressive pressure sensor based on the piezoresistivity of conductive polymer composite. As the sensor works in the condition where all of the subsensing elements have to bear the same pressure, the classical “excitation-induced differential structure” (i.e., the properties of the subsensing elements are the same, and they are arranged on the special positions to bear the reversed excitations) cannot be applied. To solve this problem, a “property-induced differential structure” (i.e., the subsensing elements are endowed with reversed properties by adjusting the mass ratio of conductive filler to polymer in the composite, and the responses of them are opposite when the pressure exerted on them are the same) is designed. If the mass ratio is lower/higher than the critical mass ratio, the destruction/formation effect of the conductive network of the composite is dominant under compression, inducing that the changing tendency of the resistance of the element is consistent/opposite to that of the pressure. By using the subsensing elements with reversed properties as the arms of a bridge, the pressure is converted to output voltage. The experimental results verify the feasibility of using the bridge system based on the “property-induced differential structure” to improve the sensitivity. [ABSTRACT FROM PUBLISHER]

To improve the sensitivity and reduce the thickness of the pressure sensor based on conductive polymer composite, a piezoresistive sensor probe with a novel structure called transverse-electrodes-probe (TEP), which is different from the traditional sandwich-structure-probe (SSP), is designed. As the composite and the two electrodes are located in the same layer/three layers of TEP/SSP, TEP is thinner than SSP. The resistivity of TEP increases with the increase in the pressure, contributing to the increasing tendency of the resistance, and the increasing extent of the resistivity of TEP is larger than that of SSP during compression. The uniaxial direction of the electrodes in TEP/SSP is perpendicular/parallel to that of the pressure. Consequently, the thickness of the composite in TEP/SSP is invariant/decreased and the cross-sectional area of the composite in TEP/SSP is decreased/invariant with the increase in the pressure, contributing to the increasing/decreasing trend of the resistance of the composite during compression. The effect of the change in the dimensions on the resistance for TEP/SSP is consistent/opposite with/to the effect of the change in the resistivity. The experimental results verify that the sensitivity of TEP is higher than that of SSP. [ABSTRACT FROM AUTHOR]

To lower the temperature interference on the piezoresistive sensor made of conductive polymer composite, a differential structure composed of “positive piezoresistive and positive thermoresistive unit (PPPTU)” and “negative piezoresistive and positive thermoresistive unit (NPPTU)” is designed. The mass ratio of carbon black to silicone rubber in the composite in PPPTU is 0.06, which results in the increase of the electrical resistance of the composite with the increase of the compressive pressure/temperature. The mass ratio in NPPTU is 0.12, which results in the decrease/increase of the resistance of the composite with the increase of the compressive pressure/temperature. PPPTU and NPPTU are placed on the neighboring arms of an electrical bridge to construct a differential structure. The experimental results verify the feasibility of using the differential structure based on PPPTU and NPPTU to reduce the temperature interference. [ABSTRACT FROM PUBLISHER]

This paper presents the development process for a new kind of compliant piezoresistive transducer based on carbon-nanotube-filled silicone rubber composite. The methods for the composite fabrication and the sensor probe encapsulation are specified. The piezoresistive mechanism of the composite is researched based on the tunneling effect theory. The calibration and transforming method is designed based on the piezoresistivity of the composite. The signal processing system is developed to realize the data conversion, acquisition, processing, and pressure displaying. The transducer system is tested within the measurement range from 0 to 2 MPa at room temperature. The measuring error is 4% full scale. The transducer system can be used to measure the interlaminar pressure of the industrial equipment. Its key technologies can also be used to fabricate an electronic skin and a robotic haptic sensing system. [ABSTRACT FROM PUBLISHER]

For CMOS technology, the increase of interconnects metal density is responsible for heterogeneous mechanical stress fields in active regions of silicon. Coupled mechanical–electrical measurements are performed to evaluate the impact of stress at circuit and device levels. This mismatch originated by interconnects metal lines stress is measured through the use of piezoresistive test structures. Local mechanical stress can thus be monitored in a simple process control compatible approach. [ABSTRACT FROM AUTHOR]

Feedback control of microactuators holds potential to significantly improve their performance and reliability. A critical step to realize the feedback control of microactuators is feedback sensing. In this paper, we report the feasibility of using a Z-shaped thermal actuator (ZTA) as a simultaneous force or displacement sensor. An in situ scanning electron microscope nanomanipulation process is used to characterize the piezoresistive response of ZTAs, which shows that ZTAs can be used as piezoresistive sensors. The experimental results agree very well with multiphysics (electric–thermal-structural-piezoresistive) simulations. A new feedback scheme is further explored, where the ZTA is treated as a two-input (applied current and external force) and two-output (displacement and electric resistance) system. Based on the calibrated relationships between the inputs and the outputs, a feedback system is developed, which can simultaneously sense the external force and generate updated current to actuate the ZTA to the desired position. We demonstrate preliminary results of this feedback control by holding the ZTA at a constant position under various external forces. The device and method presented in this paper are valuable for a range of microelectromechanical systems applications, including on-chip nanoscale mechanical testing and nanopositioning.\hfill [2011-0282] [ABSTRACT FROM PUBLISHER]

This paper presents the experimental evaluation of a new piezoresistive microelectromechanical systems strain sensor. The sensing chip is highly capable of measuring biaxial state of strain/stress. The sensing elements are p-type piezoresistors on (100) single crystal silicon aligned along [110] and its in-plane transverse. The concept of introducing geometric features to enhance the sensor sensitivity is investigated. The results of experimental evaluation and finite-element analysis (FEA) proved the viability of this concept to improve the sensor sensitivity. The microfabrication process utilizes five doping concentrations to explore the effect of doping level on the sensor performance. The sensor is developed considering applications under varying temperature conditions. Therefore, high doping concentration (more than 1\times 10^19\ atoms/cm^3) is favorable to reduce the sensor thermal drift. As a result, the sensor sensitivity is significantly reduced. Hence, geometric features are introduced in the sensor silicon carrier to compensate for the signal loss through stress concentration effect, which magnified the strain field in the proximity of the sensing elements. In addition, the use of full-bridge configuration reduced the overall temperature coefficient of resistance (TCR). At doping concentration of \sim 5\times 10^19\ atoms/cm^3, the measured strain sensitivity is 0.035 mV/\mu\varepsilon for input voltage of 5 volts, which corresponds to an effective gauge factor of \sim7 and piezoresistive gauge factor of \sim44. The effective gauge factor includes all the signal losses and the effect of bonding adhesive. Design and analysis, prototyping, and experimental evaluation are presented. Finally, guidelines to select the bonding adhesive and packaging scheme are provided. [ABSTRACT FROM PUBLISHER]

This paper reports on the temperature-dependent electrical resistivity and piezoresistive characteristics of boron-doped ultrananocyrstalline diamond (UNCD) and the fabrication of piezoresistive microcantilevers using boron-doped and undoped UNCD. The devices consist of 1-\mu\m-thick doped UNCD on either 1- or 2-\mu\m-thick undoped UNCD. The electrical resistivity of doped UNCD is 0.1 \Omega\cdot \cm at room temperature, which is five orders of magnitude smaller than the electrical resistivity of undoped UNCD. Over the temperature range of 25 ^\circ\C–200 ^\circ\C, the doped UNCD has a temperature coefficient of electrical resistance of (-1.4 \times 10^-3) per ^\circ\C. The doped UNCD exhibits a significant piezoresistive effect with a gauge factor of 7.53 \pm 0.32 and a piezoresistive coefficient of 8.12 \times 10^-12\ \Pa^-1 at room temperature. The piezoresistive properties of UNCD are constant over the temperature range of 25 ^\circ\C–200 ^\circ\C. Microcantilevers having a length of 300 \mu\m have a deflection sensitivity of 0.186 \m\Omega/\Omega per micrometer of cantilever end deflection. These measurements of electrical and piezoresistive properties of doped UNCD could aid the design of future diamond microsystems.\hfill[2010-0020] [ABSTRACT FROM PUBLISHER]

Resistance dependence on temperature and mechanical deformation (compression and tension) issues have to be solved, in order to utilize carbon nanotubes (CNT) polymer composite for practical heat-related applications. We report a resistance compensated laser printer fuser element using CNT composite that shows rapid heating properties. During electric heating (200 °C), highly conductive CNT/polydimethylsiloxane (PDMS) composites show a negative temperature coefficient (NTC) of resistivity resulting in the decrease of resistance (−14%), due to interconnection resistance between the CNTs. We redeem the NTC property through the use of the positive pressure coefficient effect of CNT/PDMS. In a paper-feeding test using a laser printer fuser system, CNT/PDMS film that was properly compressed by pressing roller (120 N/cm2) and heating roller showed a constant normalized resistance until 200 °C. The proposed laser printer fuser element could be used as a basis for heating unit applications. [ABSTRACT FROM PUBLISHER]

We report a novel piezoresistive microelectromechanical system (MEMS) differential displacement sensing technique with a minimal footprint realized through a standard MEMS fabrication process, whereby no additional doping is required to build the piezoresistors. The design is based on configuring a pair of suspension beams attached to a movable stage so that they experience opposite axial forces when the stage moves. The resulting difference between the beam resistances is transduced into a sensor output voltage using a halfbridge readout circuit and differential amplifier. Compared with a single piezoresistive flexure sensor, the design approximately achieves 2, 22, and 200 times improvement in sensitivity, linearity, and resolution, respectively, with 1.5-nm resolution over a large travel range exceeding 12 μm. [ABSTRACT FROM AUTHOR]

This letter investigates junctionless nanowire field-effect transistor (NWFET) (JL-NWFET) parameters such as piezoresistance and low-frequency noise (LFN) with respect to channel doping and gate bias. The JL-NWFET is piezoresistive, and its gauge factor (GF ) is increased from 24 to 47 by reducing the channel doping ten times from \6.7 \times \10^19 to \6.7 \times \10^18\ \cm^-3. Significant variations of GF and LFN are observed when the JL-NWFET is operated from subthreshold to on-state regime, and resolution (minimum detectable strain) is improved four times compared to inversion-mode NWFET. The simple fabrication and superior resolution formulate JL-NWFET as a promising sensing element for miniaturized nanoelectromechanical sensors. [ABSTRACT FROM AUTHOR]