Capacitive Force Sensor Research Articles

Design and Analysis of a Decoupled XY MEMS Microgripper with Integrated Dual-Axis Actuation and Force Sensing

This paper presents the design, analysis, and simulation study of a novel micro-electromechanical system (MEMS) microgipper with two decoupled electrostatic actuators and two capacitive force sensors dedicated to the testing in compressive and shear directions of soft samples. The gripper mechanism is designed based on the two-prismatic-prismatic (2-PP) parallel structure and folded leaf flexure guiding mechanism. Owing to the design of decoupled actuation structure, the translations of the two gripping arms are parallel, which ensures the reaction force between two tips and grasped object in sole gripping direction. At the same time, this structure also offers a good decoupling property. The analytical model is verified by carrying out finite element analysis (FEA) simulation study, which confirms the feasibility and effectiveness of the proposed design. The fabrication procedure by SOIMUMPs process is also given, and the microgripper will be fabricated for future experimental study.

静電容量式力センサを有するロボットハンドによる把持

Lunch boxes usually contain soft food such as fried chickens and boiled eggs. It is difficult to manipulate soft food objects due to their deformation and variety in shape and size. In this research, we focus on the design of a sensor for grasping such objects using a robot hand. During food object manipulation, a robot hand often applies excess grasping force to the object, causing much deformation of the food, or destroying it. A capacitive force sensor was designed to deal with this problem. Capacitance changes according to the distance between electrodes so, by using this property, a robot hand was equipped with a couple of electrodes are able to measure the grasping force. Some experiments performed showed that the proposed capacitive force sensor successfully detects the change in the force during grasping.

Technology of reducing the parasitic capacitance of capacitive force sensor by stacking layers having different dielectric constant.

Technology of reducing the parasitic capacitance of capacitive force sensor by stacking layers having different dielectric constant.

Fabrication and Analysis of a Composite 3D Printed Capacitive Force Sensor

Fiber encapsulation additive manufacturing (FEAM) and thermoplastic elastomer additive manufacturing (TEAM) are combined to create a composite capacitive force sensor. This work presents analysis on a capacitive force sensor developed by combining the advantages of FEAM and TEAM. The capacitive sensor consists of a 3D printed rigid frame with embedded wires in a tight spiral pattern that emulates a flat plate capacitor and a thermoplastic elastomer dielectric spacer that compresses under applied force. An approximately linear response for the corresponding capacitance change versus applied load is obtained to evaluate the printed capacitor effectiveness as a force sensor.

Enhanced performance in capacitive force sensors using carbon nanotube/polydimethylsiloxane nanocomposites with high dielectric properties.

Force sensors have attracted tremendous attention owing to their applications in various fields such as touch screens, robots, smart scales, and wearable devices. The force sensors reported so far have been mainly focused on high sensitivity based on delicate microstructured materials, resulting in low reproducibility and high fabrication cost that are limitations for wide applications. As an alternative, we demonstrate a novel capacitive-type force sensor with enhanced performance owing to the increased dielectric properties of elastomers and simple sensor structure. We rationally design dielectric elastomers based on alkylamine modified-multi-walled carbon nanotube (MWCNT)/polydimethylsiloxane (PDMS) composites, which have a higher dielectric constant than pure PDMS. The alkylamine-MWCNTs show excellent dispersion in a PDMS matrix, thus leading to enhanced and reliable dielectric properties of the composites. A force sensor array fabricated with alkylamine-MWCNT/PDMS composites presents an enhanced response due to the higher dielectric constant of the composites than that of pure PDMS. This study is the first to report enhanced performance of capacitive force sensors by modulating the dielectric properties of elastomers. We believe that the disclosed strategy to improve the sensor performance by increasing the dielectric properties of elastomers has great potential in the development of capacitive force sensor arrays that respond to various input forces.

Integration of a Thin Film PDMS-Based Capacitive Sensor for Tactile Sensing in an Electronic Skin

We present a capacitive force sensor based on a polydimethylsiloxane (PDMS) film integrated into a printed circuit board (PCB) on a flexible substrate whose layout is defined by inkjet printing. The influence of the dielectric thickness on the sensor behavior is presented. The thinner PDMS film of about 45 μm shows a sensitivity of up to 3 pF/N but poorer dynamic response. The dielectrics with thicknesses above 200 μm show a significantly reduced sensitivity. The best compromise between sensitivity and dynamic response is found for PDMS film of about 100 μm, showing about 1.1 pF/N and less than 15 s of recovery time. This film is integrated into a flexible PCBS including a microcontroller capable of evaluating the sensor. Interconnects of the circuit are defined by silver nanoparticles deposited by inkjet printing. The working principle of the circuit is demonstrated, proving that this simple approach can be used for artificial skin applications.

Flexible Distributed Pressure Sensing Strip for a Urethral Catheter.

A multi-sensor flexible strip is developed for a urethral catheter to measure distributed pressure in a human urethra. The developed sensor strip has important clinical applications in urodynamic testing for analyzing the causes of urinary incontinence in patients. There are two major challenges in the development of the sensor. First, a highly sensitive sensor strip that is flexible enough for urethral insertion into a human body is required and second, the sensor has to work reliably in a liquid in-vivo environment in the human body. Capacitive force sensors are designed and micro-fabricated using polyimide/PDMS substrates and copper electrodes. To remove the parasitic influence of urethral tissues which create fringe capacitance that can lead to significant errors, a reference fringe capacitance measurement sensor is incorporated on the strip. The sensing strip is embedded on a catheter and experimental in-vitro evaluation is presented using a bench-top pressure chamber. The sensors on the strip are able to provide the required sensitivity and range. Preliminary experimental results also show promise that by using measurements from the reference parasitic sensor on the strip, the influence of parasitics from human tissue on the pressure measurements can be removed.

A novel tri-axial capacitive-type skin sensor

This paper introduces a novel tri-axial capacitive force sensor. The sensor can measure the force vector, is embedded in soft 7 mm-thick silicone skin, enables temperature sensitivity compensation and has digital output. To measure the force vector, tilted capacitive sensor elements are used which are facing in different directions to differentiate the tangential forces. The sensor is intended for distributed contact sensing in a robotic skin, but could be also used for other applications such as novel haptic user interfaces in wearable devices. A series of experiments was performed and showed good sensor characteristics. The concept of the tilted force transducers has been proven to have the capability of detecting the force vector acting on the local sensor surface.

Design, Fabrication, and Testing of an MEMS Microgripper With Dual-Axis Force Sensor

This paper presents the design and fabrication of a new microelectromechanical systems microgripper with integrated electrostatic actuator and capacitive force sensor. One uniqueness of the proposed microgripper is that it possesses a single force sensor which can measure the gripping force and environmental interaction force in two axes in an alternate manner. The gripper structure is devised based on compliant rotary bearing and linear guiding flexure mechanisms, which enable the generation of a compact size. Analytical models are established to facilitate the parametric design of the gripper, which are verified by performing finite-element analysis simulation study. The microgripper is fabricated by the silicon-on-insulator-based process. Experimental calibrations are conducted to demonstrate the gripping and sensing performances. The feasibility and effectiveness of the developed gripper device are validated through the experimental investigations on gripping a human hair.

Differential capacitive sensing circuit for a multi-electrode capacitive force sensor

A multi-electrode differential capacitive sensing circuit is designed and realized for the read-out of a multi-axis capacitive force–torque sensor. The sensing circuit is based on a differential relaxation oscillator, to which multiple capacitances can be connected. For selecting the capacitances, reprogrammable asynchronous logic can be used, such that any desired combination of differential or single-ended capacitance can be determined. The noise performance of the oscillator in the system is analysed and measured, revealing the influence of individual component values on the noise performance of the system. Capacitance measurements show that a deviation of 0.9fF is obtained at an acquisition rate of 225Hz including auto-calibration, which is mainly limited by the quantization noise due to the frequency counter. The lowest obtained deviation is 0.12fF at an acquisition rate of 3.5Hz. The system is successfully interfaced to the multi-axis capacitive force–torque for the read-out of six capacitor configurations at an acquisition rate of 38Hz.

(Keynote) Heterogeneous Integration of MEMS by Adhesive Bonding

Technology called microsystems or MEMS (micro electro mechanical systems) has been developed and applied as value added devices for systems. It is based on semiconductor microfabrication integrating not only circuits but also sensors, actuators and microstructures. MEMS as switches and filters fabricated on CMOS LSI are needed for future multi-band wireless systems, in which good mechanical properties or piezoelectric materials are required for the MEMS and state of the art for the LSI. Such heterogeneous integration can be performed by transferring MEMS devices fabricated on a carrier wafer to a LSI wafer by adhesive bonding (Figure). Wafer level selective transfer from one carrier wafer to multiple target wafers are needed for cost-effective integration of different size chips. The selective transfer technology was applied to multiple SAW (surface acoustic wave) resonators on LSI and tunable SAW filter having variable ferroelectric capacitor. Distributed tactile sensors are needed on the skin of robots to ensure their safety in nursing care robots. A tactile sensor network acquires sensing data from each tactile sensor element by autonomous data transmission (event driven). Capacitive tactile force sensors were formed on a communication LSI by adhesive bonding. An electron emitter array integrated with a 100×100 active-matrix driving LSI has been developed using the adhesive bonding. Massive parallel EB (electron beam) exposure systems for maskless lithography (digital fabrication of LSI) are under development. A nc (nanocrystalline)-Si emitter consists of cascaded tunnel junctions can be controlled at low voltage (10V). Resist patterning by the emitted electron was successfully confirmed. Amperometric biosensor in which 20×20 boron doped diamond electrode array are formed on LSI using the adhesive bonding. Distribution of chemicals like neurotransmitter can be detected owing to a wide potential window of the diamond electrode. Cost effective development is required for micro systems because they are versatile and not standardized. A hands-on access fabrication facility in our University is reuse of a production line for power transistor and donated equipments. Companies can easily access and utilize for their prototyping or small-volume production. It is equipped with 4 and 6 inch facilities. The users are charged depending on their amount of usage. They can access a lot of technology and know-how accumulated and can be assisted by skilled engineers. More than 150 companies are using this facility and it is allowed to sell produced small volume devices. Figure 1

Design of electrostatic actuators for suppressing vertical disturbances of CMOS-MEMS capacitive force sensors in bio applications

The objective of this work is to design electrostatic actuators for a CMOS-MEMS nano-newton capacitive force sensor to suppress vertical vibrations disturbances. Electrostatic actuators are selected because the movable part of this force sensor is anchored to the fixed parts. In the first step, we propose a framework for simulation of the force sensor based on finite element method. The proposed model is modified utilizing comparison between the simulation and experimental models to improve the performance of the model. Then, 14 pairs of electrostatic actuators are designed for applying the control algorithm and their pull-in voltage is calculated. In next step, Modal Analysis is applied to find dominant natural frequencies and mode shape vectors. In addition, an observer is proposed to estimate the velocity of the modal coordinate. Finally, an optimal controller is designed employing state-space approach to suppress vertical vibration due to undesired out-of-plane excitations generated by environment during manipulation. Simulation results illustrate that employing optimum LQR control approach, the maximum out-of-plane disturbance input is suppressed less than 0.4 s with acceptable range of voltage less than pull-in voltage.

2A2-W01 A Design for Distributed Capacitive-Type Skin Sensor : A new tactile sensor design aimed for robotic skin

Humanoid robots require a skin sensor that allows them to identify objects and to perform in-hand manipulation accurately. Current robots, however, are still unable to achieve the desired accuracy. To achieve such a task, this paper introduces a new design of a capacitive force sensor for the skin. Equipped with temperature sensitivity compensation and digital output. Although the sensor is intended for distributed contact sensing on the skin, it has a wide range of applications. The experiment conducted gave satisfying results, allowing the concept to be tested further to confirm its functionality.

A Dual-Axis Electrostatically Driven MEMS Microgripper

This paper presents the design of a new monolithic two-axis electrostatically actuated MEMS microgripper with integrated capacitive position and force sensors working at the micro-scale level. Each of the two jaws of the microgripper possesses two degrees-of-freedom (DOF) and is capable of positioning in both x-and y-axes. Unlike existing works, where one gripper arm is actuated and other one is sensed, both arms of the proposed microgripper are actuated and sensed independently. A sensing scheme is constructed to provide the position and force signals in the noncontact and contact phases, respectively. By applying a 120V driving voltage, the jaw can provide 70 μm x-axis and 18 μm y-axis displacements with the force of 190 μN. By this design, the real-time position and grasping force information can be obtained in the dual sensing mode. Both analytical calculation and finite-element analysis (FEA) were performed to verify the performance of the proposed design. A scaled-up prototype is designed, fabricated and tested through the experiment to verify the structure design of the microgripper.

Optimal PID control of a nano-Newton CMOS-MEMS capacitive force sensor for biomedical applications

This paper presents closed loop simulation of a CMOS-MEMS force sensor for biomedical applications employing an optimal proportional-integral-derivative controller. Since the dynamic behavior of the sensor under investigation is nonlinear the iterative feedback tuning approach was proposed for optimal gains tuning of the proposed controller. Simulation results presented in this research illustrate that the proposed controller suppresses the undesired in-plane vibration induced by environment or gripper 40 times faster than the nonlinear controller proposed in the literature. To suppress the maximum input disturbance the maximum voltage was approximately 18 V which was less than the pull-in voltage of 30 V. The proposed controller is served to actuate two stator fingers adjacent to a rotor finger in order to provide both the attractive and repellent forces during manipulation. Employing the proposed mechanism not only resolves the drawbacks corresponding to the nonlinear controller presented in the literature but also improves its performance of the closed loop system by using the complete nonlinear dynamics of the force sensor. Also, applying complete non-linear dynamic of model improves the performance of controller and is one of superior features of proposed PID controller in comparison with the classical controller presented in literature.

Note: Resistance spot welding using a microgripper

Interest in thin-film nanostructures as building blocks for nanoelectronics and nanoelectromechanical systems (NEMS) is increasing. Resistance spot welding (RSW) on a nano or micro scale can play a significant role; similar to that of its macro counterpart for forming connections in device assembly processes. This Note presents a novel micron scale RSW technique using a microgripper as mobile spot welding electrodes to assemble ultra-thin film nanostructures. As an example, assembly of three-dimensional helical nanobelt (HNB) based device was successfully demonstrated using the proposed system. The spot-welding process was fully monitored by the built-in capacitive micro force sensor of the microgripper. Experiments show that RSW, using the microgripper, provides a stable electrical contact with sufficient mechanical strength for the construction of devices such as HNB based devices demonstrated here.

Accuracy of Displacement Estimation and Selection of Capacitors for a Four Degrees of Freedom Capacitive Force Sensor

Accuracy of Displacement Estimation and Selection of Capacitors for a Four Degrees of Freedom Capacitive Force Sensor

In situ SEM electromechanical characterization of nanowire using an electrostatic tensile device

This paper presents a method for simultaneously performing mechanical tensile test and electrical test on nanowire in SEM. For this purpose, an electrostatic tensile device is designed, fabricated and tested. In order to simplified the testing system, microforce sensor beam is used to measure the tensile force applied to the nanowire instead of traditional capacitive force sensor. Special care is taken on the design and fabrication process to improve the resolution of mechanical measurement by SEM imaging. Mechanical properties such as Young's modulus, elastic limit and fracture strength, and electrical resistance of nanowire can be obtained in the process of tensile testing using this device. Cu nanowire and SiC nanowire are tested as examples in this paper. Smaller Young's modulus is found in both nanowires due to the imperfection in crystallization. On the other hand, electrical properties under different tensile stress are characterized for both nanowires. Nonlinear and identical I–V curves are revealed for Cu nanowire, and linear I–V curves and the piezoresistive effect are revealed for SiC nanowire. The gauge factor of SiC nanowire is calculated and turns out to be compatible with the values of bulk SiC published in the literature.

MEMS Microgripper Actuators and Sensors: The State-of-the-Art Survey

In recent years, microelectromechanical systems (MEMS) have been widely applied in diverse science and engineering domains. MEMS-based microgrippers provide advantages in terms of compact size and low cost, and hence play an important role in microassembly and micromanipulation fields for manipulating micromechanical elements, biological cells, etc. During the past two decades, microactuators based on different actuation principles such as shapememory alloys, electrostatic, electrothermal, piezoelectric, pneumatic and electromagnetic approaches have been devised to drive MEMS microgrippers. Moreover, the integrated position and force sensors can deliver real-time feedback signals to protect both the microgripper and grasped object from damaging. In addition, a number of patents have been devoted to this area. This paper presents a state-of-the-art overview of recent development in the actuators (electrostatic, electrothermal, micro-pneumatic, electromagnetic, shape memory alloy and other actuators) and sensors (optical method, piezoresistive force sensor, capacitive force sensor and other developments). By providing detailed comparisons among them, some guidelines of selection have been underlined for different application scenarios such as biomedical and biological applications, micro-manufacturing and so on.

Battery-Less Wireless Instrumented Knee Implant

Over 400,000 total knee replacement procedures (TKR) are performed annually in the United States. This paper focuses on the development of a battery-less wireless instrumented tibial tray for performance feedback in TKR implants. The proposed instrumented tibial tray is powered internally by an integrated piezoelectric energy harvesting system. Energy is harvested during the walking of the patient when forces are exerted on the tibial component. The sensors and wireless electronics are entirely powered from the harvested energy. This tibial tray is also instrumented with capacitive force sensors and an ultra low-power method to measure the capacitive force sensors. A bench top test rig is developed for testing the battery-less wireless knee replacement implant. For a person with a body weight of 55 kg, the energy harvesting system can fully charge the storage capacitors in 11 steps and can harvest an average of 1051 μJ per step. To power the force measurement system for ten seconds and to transmit the data, the piezoelectric energy harvesting system must be charged before the force measurement process is initiated by a minimum of 11 steps and a minimum of two steps must be taken during the force measurement process. During the force measurement process, each force sensor is sampled at a frequency of 10 Hz for ten seconds; thereafter, all of the data is transmitted to the RF base station. The resulting capacitive force sensors adequately represented cyclic loads; however, the sensors demonstrated some issues with repeatability.