Real-time Force Measurement Research Articles

3D printed smart elastomeric foam with force sensing and its integration with robotic gripper

Robotic grippers are extensively investigated as terminal actuators for gripping workpieces in robotic applications. Lately, implementing closed-loop force control for grippers to grip fragile workpieces securely is attractive. This paper aims to provide a dual-security solution combining gripping force control and stress-buffering capability. An integrated force-sensitive smart polydimethylsiloxane (PDMS) foam containing an embedded force-sensitive resistor (FSR) was designed and directly fabricated by 3D printing technology. The real-time force measurement was fed back into the control loop with proportional-integral (PI) rectification to precisely regulate the driving air pressure. In addition, the inherent stress plateau effect of the PDMS foam structure provides a constant stress buffer to absorb potential collision energy. The results show that the dual-security gripper with smart PDMS foam is able to precisely control the applied force and reliably grip fragile objects even in the presence of disturbances, enhancing the reliability of gripping applications. • Smart elastic foams with embedded force sensing capability are designed and 3D printed. • The printed smart elastic foams provide tunable mechanical and electrical properties. • The smart foam integrated with robotic gripper exhibits excellent stress buffering and closed-loop force feedback performance.

Signal processing of capacitive force sensors installed in the foot of an anthropomorphic robot

Introduction: The force-moment sensing of the functional surfaces in robots based on compact capacitive force sensors can significantly improve interaction with the environment and humans. Capacitive force sensors provide high measurement accuracy and speed, but the electromagnetic interference can affect significantly the signal. When processing signals the influence of external noise must be taken into account, which increases the computation time. Purpose: To apply the developed interface circuit for processing signals from capacitive primary force transducers in a real-world robot. Results: Experimental verification of the developed solutions implied simulation of a step of the pedipulator of an anthropomorphic robot with the calculation of the coordinates of the center of pressure exerted on the foot with the four installed capacitive sensors. Software filtering and measuring capabilities of the microcontroller made it possible to achieve a signal-to-noise ratio of approximately 62.24 dB, which allows a closed-loop control system to function correctly. The average time for calculating the coordinates of the center of pressure on the foot with software filtering of the signal on the on-board computer of the robot was from 3.1 to 6.1 ms and meets the requirements for the sensory system of a walking robot. Practical relevance: The interface circuit allows to scale the number of connected primary force transducers, while software processing allows to normalize transducer signals by applying the calculated correction factors. Proposed solutions can be used in different robotic systems for real-time force measurement.

Simulation and experimental research on magnetorheological finishing under dynamic pressure with a gap-varying

Magnetorheological finishing (MRF) is a novel machining method used to obtain ultra-smooth and even surfaces. To improve the material removal rate (MRR) while attaining smooth and undamaged surfaces, an MRF method with a varying machining gap between the polishing disks and workpieces is proposed to increase the polishing efficiency and improve the surface quality. According to the computational fluid dynamics (CFD) method, the behaviors of MRF fluids under the effect of magnetic fields are measured using a rheometer. Furthermore, a simulation model for fluids based on the gap-varying MRF is established to evaluate the influence of the varying machining gap on the behaviors of the flow field of MRF fluids. The results show that the flow field formed during gap-varying MRF presents a periodic dynamic change and formation of turbulence. Both the fluid pressure and velocity on the workpiece surface show a periodic change and the maximum fluid velocity on the workpiece surface in the x-direction is much greater than that in the z-direction; the transverse shear velocity contributes to the material removal and abrasive update. Under the same polishing time and rotational speed, the surface roughness is reduced from Ra 6.36 nm to Ra 0.155 nm after MRF without varying gaps for 5 h; in contrast, the surface roughness decreases from Ra 6.44 nm to Ra 0.104 nm after MRF with a varying gap, attaining an atomic step-structure. Properly increasing the frequency and amplitude of gap variations is conducive to achieving a better polishing effect. By conducting real-time force measurement and establishing a material removal function, it is shown that the simulation results match the test results. MRF with a varying gap can increase the polishing efficiency and surface quality by changing the behaviors of the flow field.

Feasibility of multilayer solid-state deposition via lateral friction surfacing for metal additive manufacturing

Lateral friction surfacing is a novel solid-state deposition process in which the radial surface of the rotating consumable tool is forced into the substrate surface, facilitating material transfer. This technique is an excellent alternative to create thin and ultra-smooth metallic deposit layers for repairing damaged surfaces or improving corrosion and wear resistance. The lateral friction surfacing approach results in a deposition process with lower generated process temperatures than conventional friction surfacing, which leads to reducing thermal effects on the microstructures and mechanical properties of the deposits. In this study, the extent of material transfer to the substrate was explored via multiple passes of the tool in an effort to create multiple layers of deposited material. Two types of substrate plates with different surface roughness as well as two different strategies for employing the consumable tools were experimented. A comprehensive assessment through conducting real-time force measurement, surface roughness measurement, hardness testing, optical microscopy, infrared thermography, scanning electron microscopy, and EDS analysis was performed to characterize the process and the fabricated deposits. The thickness of the coating was found to vary through work material transfer to the substrate and reverse material transfer from the coating to the radial surface of the rod, resulting in an approximately steady-state deposit thickness. The reverse material transferring process from the coating to the radial surface of the rod through rubbing off the previously fabricated coatings limits plasticizing more consumable material and built-up material.

Multi-layered polymer cantilever integrated with full-bridge strain sensor to enhance force sensitivity in cardiac contractility measurement.

In this study, we developed a multi-layered functional cantilever for real-time force measurement of cardiomyocytes in cell culture media. The functional cantilever with a full-bridge circuit configuration was composed of one polydimethylsiloxane (PDMS) and two polyimide (PI) layers, forming two resistive sensors on each upper side of the two PI layers. The PI layers were chemically bonded using an oxygen plasma treatment, with a thin composite layer consisting of Cr/SiO2/PDMS. These greatly improved the force sensitivity and the long-term reliability of the integrated strain sensor operating in liquids. The nanogrooved PDMS top layer bonded on the upper PI layer was employed to further improve the growth of cardiomyocytes on the functional cantilever. The difference in resistance changes and response characteristics was confirmed by evaluating the characteristics of the multi-layered polymer cantilevers with half-bridge and full-bridge circuit configurations. We also employed the cantilever devices to measure the contraction force of cardiomyocytes for 16 days and side effects in real time in human-induced pluripotent stem cells treated with the cardiovascular drug verapamil. The sensor-integrated cantilever devices are expected to be utilized as a novel biomedical sensor for evaluating the mechanobiology of cardiomyocytes, as well as in drug screening tests.

An experimental and numerical study of forces and residual stresses in AISI 316L stainless steel joints due to conventional and pulse gas tungsten arc welding

This paper aims to present the results of a numerical and experimental study of the temperature field, internal forces and the residual stresses in 2 mm thick autogenous welds of AISI 316L stainless steel produced by continuous and pulse current gas tungsten arc welding. A special experimental device was used to measure the temperature and the internal forces due to the welding. The welds were qualified for internal and external weld imperfections according to ISO 15614-1. FEM software ANSYS® Multyphysics™ was applied in order to solve the thermal and mechanical problems. Normal residual stresses were measured by the hole-drilling strain gauge method in the continuous current weld. The peak value of the longitudinal stress was 80 % of the base metal yield stress. The magnitude of the numerically obtained residual stress values was found to be 16 % to 19 % above the measured one in the longitudinal and transverse direction, respectively. The experimental device used in this study allowed for a real time measurement of forces far from the weld seam. On the basis of the correspondence between the calculated and measured forces the numerical results were verified. Therefore, this device might open up new possibilities for determining thermo-mechanical material data.

Does dynamic stability govern propulsive force generation in human walking?

Before succumbing to slower speeds, older adults may walk with a diminished push-off to prioritize stability over mobility. However, direct evidence for trade-offs between push-off intensity and balance control in human walking, independent of changes in speed, has remained elusive. As a critical first step, we conducted two experiments to investigate: (i) the independent effects of walking speed and propulsive force (FP) generation on dynamic stability in young adults, and (ii) the extent to which young adults prioritize dynamic stability in selecting their preferred combination of walking speed and FP generation. Subjects walked on a force-measuring treadmill across a range of speeds as well as at constant speeds while modulating their FP according to a visual biofeedback paradigm based on real-time force measurements. In contrast to improvements when walking slower, walking with a diminished push-off worsened dynamic stability by up to 32%. Rather, we find that young adults adopt an FP at their preferred walking speed that maximizes dynamic stability. One implication of these findings is that the onset of a diminished push-off in old age may independently contribute to poorer balance control and precipitate slower walking speeds.

A pilot study to precisely quantify forces applied by sonographers while scanning: A step toward reducing ergonomic injury.

There is a significantly high rate of work-related musculsokeletal injuries in sonography professionals. To date, assessment of risk factors for work- related injuries in sonographers has been based primarily on surveys, subjective reports, and observational methods. There is a need to develop quantitative techniques to better understand risk factors and develop preventive interventions. We pilot tested a high-resolution force-measuring probe capable of precisely measuring forces applied through the transducer by sonographers and used this novel direct measurement technique to evaluate forces during abdominal imaging. Twelve sonographers with varied experience, ranging from 1-33 years, performed routine abdominal scans on 10 healthy volunteers who had varied body mass indices (BMI). Imaging was conducted using the force-measuring probe, which provided real-time measurement of forces, and angles. Data were compared by sonographer years of experience and subject BMI. In total, 47 abdominal examinations were performed as part of this study, and all images met standards for clinical diagnostic quality. The mean contact force applied across all exams was 8.2±4.3 Newtons (N) (range: 1.2-36.5 N). For subjects in the high BMI group (BMI>25, n = 4) the mean force was 10.5 N (range: 8.9-13.2 N) compared to 7.9 N (range: 5.9-10.9 N) for subjects with normal BMI (BMI = 18.5-25, n = 6). Similarly, the mean maximum force applied for subjects with high BMI (25.3 N) was significantly higher than force applied for subjects with normal BMI (17.4 N). No significant difference was noted in the amount of force applied by sonographers with more than 5 years of experience (n = 6) at 8.2 N (Range: 5.1-10.0 N) compared to less experienced sonographers (n = 6), whose forces averaged 8.1 N (Range: 5.8-10.0 N). It is feasible to directly measure forces applied by sonographers using a high-resolution force measurement system. Forces applied during abdominal imaging vary widely, are significantly higher when scanning subjects with high BMI, and are not related to sonographer years of experience. This force measurement system has the potential to provide an additional quantitative data point to explore the impact of applied forces on sonographer related musculoskeletal injury, particularly in conjunction with various body positions, exam types and force durations.

Real-time force measurement in double wavelength optical tweezers

In optical tweezers, the trap stiffness varies across the sample area. To avoid this problem, the force measurement is often performed in a fixed place where the trap stiffness is well determined. However, for some experiments, bringing the sample to the fixed position is problematic. In this paper, we describe a precise and fast procedure for mapping the trap stiffness over the whole sample area. Such a map allows development of a real-time procedure for force measurement at any point of the sample area. The presented method is particularly suitable for measuring forces, for example, in living cells samples.

Real-time force measurement in double wavelength optical tweezers

Real-time force measurement in double wavelength optical tweezers

Real-time transverse force sensing using fiber Bragg grating through direct Stokes parameters measurement.

Characteristics of a fiber Bragg grating (FBG) transverse force sensor based on Stokes parameters are presented. Real-time force measurement is achieved through direct measurement of the Stokes parameters at single wavelength. A proportional relationship and linear fit are found between Stokes parameters and applied force. The sensitivity and dynamic range dependence on the state of polarization (SOP) of the incident light is investigated theoretically and experimentally. A maximum sensitivity of 0.037/N is experimentally achieved and it can be improved further by adjusting the incident SOP. This design significantly reduces system complexity and improves data processing speed, which has great practical value in real-time FBG sensing applications.

Regulation of Intracellular Structural Tension by Talin in the Axon Growth and Regeneration.

Intracellular tension is the most important characteristic of neuron polarization as well as the growth and regeneration of axons, which can be generated by motor proteins and conducted along the cytoskeleton. To better understand this process, we created Förster resonance energy transfer (FRET)-based tension probes that can be incorporated into microfilaments to provide a real-time measurement of forces in neuron cytoskeletons. We found that our probe could be used to assess the structural tension of neuron polarity. Nerve growth factor (NGF) upregulated structural forces, whereas the glial-scar inhibitors chondroitin sulfate proteoglycan (CSPG) and aggrecan weakened such forces. Notably, the tension across axons was distributed uniformly and remarkably stronger than that in the cell body in NGF-stimulated neurons. The mechanosensors talin/vinculin could antagonize the effect of glial-scar inhibitors via structural forces. However, E-cadherin was closely associated with glial-scar inhibitor-induced downregulation of structural forces. Talin/vinculin was involved in the negative regulation of E-cadherin transcription through the nuclear factor-kappa B pathway. Collectively, this study clarified the mechanism underlying intracellular tension in the growth and regeneration of axons which, conversely, can be regulated by talin and E-cadherin.

Development of a Force Sensor Prototype Integrated on a Coordinate Measuring Arm

This paper describes the design and manufacturing tasks of a force sensor for its integration on the standard probe of a Coordinate Measuring Arm (AACMM or CMA). The sensor design is based on the use of strain gauges located on the surface of the CMA hard probe. The strain gauges as well as their cables and connectors have been protected with a custom case, made by Additive Manufacturing techniques (Polyjet 3D). The same method has been selected to manufacture an ergonomic handle that includes a trigger and the electronic components (customized PCB) required for synchronizing the trigger signal when probing occurs. The force sensor is finally connected and commanded by a signal monitoring software given a real-time force measurement.

Near real-time measurement of forces applied by an optical trap to a rigid cylindrical object

An automated data acquisition and processing system is established to measure the force applied by an optical trap to an object of unknown composition in real time. Optical traps have been in use for the past 40 years to manipulate microscopic particles, but the magnitude of applied force is often unknown and requires extensive instrument characterization. Measuring or calculating the force applied by an optical trap to nonspherical particles presents additional difficulties which are also overcome with our system. Extensive experiments and measurements using well-characterized objects were performed to verify the system performance.

Development of a flexure-based, force-sensing microgripper for micro-object manipulation

This paper presents the design and development of a flexure-based microgripper, accompanied with a real-time, vision-based force sensing system to handle objects of various sizes ranging from 100 µm to 1 mm. A simulation-based design methodology is adopted to develop an initial microgripper design, which is then optimized using theoretical modeling. The final prototype developed generated a large stroke length of over 500 µm with high-deflection magnification (ratio of the end deflection (output) to the input deflection) of 3.52. A spring system has been incorporated into the microgripper for easy measurement and control of the gripping forces. The web-camera-based visual system enables real-time force measurement with a resolution of 2.37 mN and can be operated in both manual and automatic modes to control the applied forces. The system has successfully demonstrated gripping of a variety of micro-objects including a 100 µm human hair and a 1 mm steel rod with forces as small as 43 mN and 159 mN, respectively.

Validating a Plasma Momentum Flux Sensor to an Inverted Pendulum Thrust Stand

The accuracy of a plasma impact force sensor was compared with that of the more commonly used inverted pendulum thrust stand using a 5 kW Xe Hall effect thruster. An improved plasma momentum flux sensor was designed and constructed based on a previous design. Real-time force measurements were made with both the plasma momentum flux sensor and the inverted pendulum thrust stand. The plasma momentum flux sensor measured the force exerted onto it by the Hall effect thruster exhaust plume with a resolution of 0.1 mN and an average discrepancy of 2 % compared with thrust stand measurements. Experiments were completed using a 9 m by 6 m cylindrical vacuum chamber. The total force from the Hall effect thruster was modulated from 34 to 356 mN by varying both the anode voltage, from 150 to 500 V, and the neutral Xe gas flow rate, from 5 to 15 mg/s.

Global chassis control by sensing forces/moments at the wheels

The paper presents the expected improvement on the active safety of road vehicles deriving from the real time measurement of forces and moments at the wheels. A measuring wheel developed at the Politecnico di Milano is described. The active safety of road vehicles is mainly related to longitudinal motions (braking, etc.) and lateral motions (yaw moment control). Up-to date control strategies are described in the paper. New controls are presented, aiming to use all of the information available and provided by the measuring wheels. It is envisaged, a significant improvement of the vehicle active safety.

Improving the active safety of road vehicles by sensing forces and moments at the wheels

The paper reviews and presents which is the envisaged improvement of the active safety of road vehicles arising from the real time measurement of forces and moments at the wheels. There are many researches undertaken in the world attempting to find devices able to measure accurately the forces and the moments at the wheels of road vehicles. In particular, a six-component measuring wheel developed and patented at the Politecnico di Milano is presented. The active safety of road vehicles is related to longitudinal motions (braking, traction control), lateral motions (yaw moment control) and vertical motions (road holding). In each of these three fields the paper briefly reviews the up-to-date controls. Based on such a state-of-the art survey, the weak points of today's controls are highlighted. A proposal of new controls is presented aiming to use all of the information available and provided by six-axis measuring hubs at the wheels. A considerable improvement of the vehicle active safety can be achieved by sensing the forces at the wheels.

Control of Cutting Force for Creep-Feed Grinding Processes Using a Multi-Level Fuzzy Controller

In this paper, a multi-level fuzzy control (MLFC) technique is developed and implemented for a creep-feed grinding process. The grinding force is maintained at the maximum allowable level under varying depth of cut, so that the highest metal removal rate is achieved with a good workpiece surface quality. The control rules are generated heuristically without any analytical model of the grinding process. Based on the real-time force measurement, the control parameters are adapted automatically within a stable range. A National Instrument real-time control computer is implemented in an open architecture control system for the grinding machine. Experimental results show that the cycle time has been reduced by up to 25% over those without force control and by 10–20% compared with the conventional fuzzy logic controller, which indicates its effectiveness in improving the productivity of actual manufacturing processes. The effect of grinding wheel wear is also considered in the creep-feed grinding process, where the grinding force/power can be maintained around the specified value by the proposed MLFC controller as the wheel dulls gradually.

Atomic force microscopy measurement of leukocyte-endothelial interaction.

Leukocyte adhesion to vascular endothelium is a key initiating step in the pathogenesis of many inflammatory diseases. In this study, we present real-time force measurements of the interaction between monocytic human promyelocytic leukemia cells (HL-60) cells and a monolayer of human umbilical vein endothelial cells (HUVECs) by using atomic force microscopy (AFM). The detachment of HL-60-HUVEC conjugates involved a series of rupture events with force transitions of 40-100 pN. The integrated force of these rupture events provided a quantitative measure of the adhesion strength on a whole cell level. The AFM measurements revealed that HL-60 adhesion is heightened in the borders formed by adjacent HUVECs. The average force and mechanical work required to detach a single HL-60 from the borders of a tumor necrosis factor-alpha-activated HUVEC layer were twice as high as those of the HUVEC bodies. HL-60 adhesion to the monolayer was significantly reduced by a monoclonal antibody against beta1-integrins and partially inhibited by antibodies against selectins ICAM-1 and VCAM-1 but was not affected by anti-alphaVbeta3. Interestingly, adhesion was also inhibited in a dose-dependent manner (IC50 approximately 100 nM) by a cyclic arginine-glycine-aspartic acid (cRGD) peptide. This effect was mediated via interfering with the VLA-4-VCAM-1 binding. In parallel measurements, transmigration of HL-60 cells across a confluent HUVEC monolayer was inhibited by the cRGD peptide and by both anti-beta1 and anti-alphaVbeta3 antibodies. In conclusion, these data demonstrate the role played by beta1-integrins in leukocyte-endothelial adhesion and transmigration and the role played by alphaVbeta3 in transmigration, thus underscoring the high efficacy of cRGD peptide in blocking both the adhesion and transmigration of monocytes.