FBG-based three-dimensional micro-force sensor with axial force sensitivity-enhancing and temperature compensation for micro-forceps.

A research pump intended for both flow visualization studies and direct measurement of hydrodynamic radial and axial forces has been developed. The impeller and the volute casing are constructed from Plexiglas which facilitates optical access for laser velocimetry measurements of the flow field both inside the impeller and in the volute casing. The pump housing is designed for flexibility allowing for each interchange of impellers and volute configurations. The pump rotor is supported by three radial magnetic bearings and one double acting magnetic thrust bearing. The magnetic bearings have been calibrated to characterize the force versus coil current and air gap relationship for each bearing type. Linear calibration functions valid for rotor eccentricities of up to 2/3 of the nominal bearing clearances and force level of ±58 N (13 lbf) and ±267 N (60 lbf) for the radial and axial bearings, respectively, were found. A detailed uncertainty analysis of the force calibration functions was conducted such that meaningful uncertainty bounds can be applied to in situ force measurements. Hysteresis and eddy current effects were quantified for each bearing such that their effect on the in situ force measurements could be assessed. By directly measuring the bearing reaction forces it is possible to determine the radial and axial hydraulic loads acting on the pump impeller. To demonstrate the capability of the magnetic bearings as active load cells representative hydraulic force measurements for a centered 4 vane 16 degree log spiral radial flow impeller operating in a single tongue spiral volute casing were made. At shut-off a nondimensional radial thrust of 0.084 was measured. A minimum nondimensional radial thrust of about 0.007 was observed at the nominal design flow. The nondimensional radial thrust increased to about 0.019 at 120 percent of design flow. The nondimensional axial thrust had a maximum at shut-off of 0.265 and decreased steadily to approximately 0.185 at 120 percent of design flow. Two regions of increasing axial thrust, in the flow range 75 to 100 percent of design flow, were observed. The measurements are compared to radial and axial force predictions using classical force models. The direct radial force measurements are compared to a representative set of radial force measurements from the literature. In addition, the directly measured radial force at design flow is compared to a single representative radial force measurement (obtained from the literature) calculated from the combination of static pressure and net momentum flux distribution at the impeller exit.

An experimental method of determining the axial forces in power transformers is applied to a transformer constructed so that various tapping arrangements can be set up, and curves are given of the forces which occur with typical methods of tapping concentric windings.It is considered that the simple “residual ampere-turn” method of calculating axial forces is sufficiently accurate for design purposes, and a number of formulae are given which should cover most of the requirements of the transformer designer.The distribution of axial force around the periphery of the coils is investigated experimentally.

A fiber Bragg grating (FBG) accelerometer using transverse forces is more sensitive than one using axial forces with the same mass of the inertial object, because a barely stretched FBG fixed at its two ends is much more sensitive to transverse forces than axial ones. The spring-mass theory, with the assumption that the axial force changes little during the vibration, cannot accurately predict its sensitivity and resonant frequency in the gravitational direction because the assumption does not hold due to the fact that the FBG is barely prestretched. It was modified but still required experimental verification due to the limitations in the original experiments, such as the (1) friction between the inertial object and shell; (2) errors involved in estimating the time-domain records; (3) limited data; and (4) large interval ~5 Hz between the tested frequencies in the frequency-response experiments. The experiments presented here have verified the modified theory by overcoming those limitations. On the frequency responses, it is observed that the optimal condition for simultaneously achieving high sensitivity and resonant frequency is at the infinitesimal prestretch. On the sensitivity at the same frequency, the experimental sensitivities of the FBG accelerometer with a 5.71 gram inertial object at 6 Hz (1.29, 1.19, 0.88, 0.64, and 0.31 nm/g at the 0.03, 0.69, 1.41, 1.93, and 3.16 nm prestretches, respectively) agree with the static sensitivities predicted (1.25, 1.14, 0.83, 0.61, and 0.29 nm/g, correspondingly). On the resonant frequency, (1) its assumption that the resonant frequencies in the forced and free vibrations are similar is experimentally verified; (2) its dependence on the distance between the FBG's fixed ends is examined, showing it to be independent; (3) the predictions of the spring-mass theory and modified theory are compared with the experimental results, showing that the modified theory predicts more accurately. The modified theory can be used more confidently in guiding its design by predicting its static sensitivity and resonant frequency, and may have applications in other fields for the scenario where the spring-mass theory fails.

This paper presents a new methodology for noncontact measurement of the axial force of bolts in their tightening processes using laser-generated ultrasound waves. This method employs ultrasound waves scattered in a bolt shaft to detect axial force changes, while most conventional ultrasonic methods use ultrasound waves propagating linearly along the bolt axis. The ultrasound waves in this study are generated by laser irradiation on the top surface of a bolt. Subsequently, they propagate deeply into the shaft and return towards the top of the bolt through complicated paths due to the multiple scattering in the bolt shaft. Finally, they are detected at the top surface using another laser and a speckle knife edge detector. With an examination based on the finite element analysis and verification experiments, it has been shown that the waveform of the scattered ultrasound shifts in time linearly with increasing the axial force. The time shifts were estimated using the cross-correlation analysis between the measured waveforms and the reference waveform with no axial force. This result demonstrates the feasibility of estimating the change in axial force during tightening processes once the relationship between the time shifts and axial force is obtained for the specific type of bolt to be used in products. Furthermore, the proposed technique does not require machining to flatten a bolt's head and the end, while conventional ultrasonic methods need the flattening procedures, enabling fast, cost-effective axial force measurement in mass production manufacturing processes.

The simultaneous inflation and extension of a tube is investigated as a method for conducting elongational flow experiments. This generally unsteady inhomogeneous deformation is restricted so that the material particles at the outer radius of the tube (r 0 ) experience steady elongational flow. The solution to the stress equations of motion, including inertia and interfacial tension, is given for a tube made of an incompressible, isotropic simple material. This solution is used to relate the contributions of interfacial tension, inertia and the normal stress differences to two measurable surface tractions: the pressure difference across the tube wall and the axial force on the ends of the tube. Explicit predictions for dimensionless force and pressure functions (related to the axial force and pressure difference) are obtained using a simplified form ofLodge's rubberlike liquid constitutive equation to evaluate the integrals containing the normal stress differences. For typical elongational flow experiments with high-viscosity materials, the contribution of inertia and interfacial tension to the predicted force and pressure functions is negligible, but the effect of the cylindrical geometry is significant, except for uniaxial elongation which is a homogeneous flow. For certain elongational flows the cylindrical geometry causes an “overshoot” in the predicted pressure function which is not observed in the related normal stress difference for the corresponding steady elongational flow atr 0 . As the time since the inception of any inhomogeneous elongational flow increases, the ratio of the tube wall thickness to the outer radius of the tube decreases and the force and pressure functions approach values predicted for the corresponding steady elongational flow atr 0 , provided that a critical value of one strain rate is not exceeded. Under these conditions the normal stress differences for the steady elongational flow atr 0 can be determined approximately from experimental measurements of the pressure difference and axial force. The restriction on one strain rate noted above indicates that it is not always valid to assume that the effect of the cylindrical geometry on the measured surface tractions decreases as the ratio of the tube wall thickness to the outer radius decreases.

PurposeThe use of a force sensor to estimate the external force of manipulator not only needs to deal with the signal noise of the sensor itself but also needs to solve the coupling interference of the sensor itself, especially the axial force. The purpose of this paper is to develop a three-dimensional fiber Bragg grating (FBG) wrist force sensor, which has a simple structure and reduces the coupling influence between several axes.Design/methodology/approachA particular separation elastic structure with four FBGs is devised for the three-axial force sensor. One FBG is suspended on the profile of central cylinder and the other three FBGs are pasted on the elastic beam surface of the over and under measuring bodies, respectively. Finite element analysis (FEA) simulation has been implemented to the strain distribution characteristics, the output characteristics of each direction and the coupling effects of the structure. Furthermore, theoretical derivation and experimental results are used to compare, which have a good consistency.FindingsThe experiment results show that the maximum repeatability error of the sensor is 6.75%, the maximum nonlinear error is 5.36%, the maximum coupling interference is 4.73% and the minimum sensitivity is 1.58 pm/N.Originality/valueA three-dimensional force sensor based on FBG adopts a particular separation elastic structure. The sensor can reduce the coupling influence between several axes, especially the coupling interference in the z-direction is 0.

The force acting on centrifugal compressors is an important parameter to be considered throughout the operating life of these turbomachines. When the compressor is operating in surge conditions, these forces can become highly dangerous for the mechanical and aerodynamic structures. This instability is usually avoided in industrial applications, but the antisurge system may not react in time when emergency shutdowns or power failures take place. During these rapid transients, surge can develop, generating unsteady forces which can harm the close clearance components of the compressor. Therefore, the capability to predict the characteristics and the dynamics of these surge forces would allow the estimation of the off-design fatigue cycles produced on these components by surge. Currently, no validated method exists to predict the frequency and amplitude of the surge forces and determine the potential damage of these components. In this paper, a lumped parameter model, developed by using the bond graph approach to predict the dynamic surge fluid-dynamic oscillations, is presented. The model requires the geometry and the steady-state performance maps of the compressor as inputs, together with the piping system configuration characteristics. The simulator is provided with a supplementary tool to estimate the axial force frequency and amplitude, taking into consideration all the contributions to the axial fluid-dynamic thrust, the stiffness-damping of the thrust bearing, and the mass of the rotor. The model was tuned and validated using the test case axial force data from the Southwest Research Institute (SWRI) facility. The model has shown good agreement with the experimental results which implies that it can offer significant information about the severity of a surge event and the quantification of the machine performance losses together with possible damage to the close clearance components. This study is a first important step that can lead to schedule optimization for maintenance and repair activities.

This paper presents the results obtained from fibre Bragg grating (FBG) sensors integrated into a railway current-collecting pantograph for accurate measurement of contact force and contact location when it is infjected to various temperature conditions. The temperature change of the pantograph is simulated, at the industrial laboratory of Brecknell Willis in the UK, by changing the DC current applied to pantograph from 0 to 1500 A. This test is primarily designed to verify the effectiveness of the temperature compensation mechanism built in the FBG sensor design. For this verification, 3 thermocouples co-located with the FBG sensor packages are used to measure the temperature change seen from 25 °C to 55 °C. The tests were repeated several times and the sensor system has shown its temperatureindependence, confirming that the intrinsic cross-sensitivity of FBGs to temperature variation for strain measurement has been fully compensated through the use of this innovative sensor design and data processing.

A fiber optic sensor developed for the measurement of tendon forces was designed, numerically modeled, fabricated, and experimentally evaluated. The sensor incorporated fiber Bragg gratings and micro-fabricated stainless steel housings. A fiber Bragg grating is an optical device that is spectrally sensitive to axial strain. Stainless steel housings were designed to convert radial forces applied to the housing into axial forces that could be sensed by the fiber Bragg grating. The metal housings were fabricated by several methods including laser micromachining, swaging, and hydroforming. Designs are presented that allow for simultaneous temperature and force measurements as well as for simultaneous resolution of multi-axis forces.The sensor was experimentally evaluated by hydrostatic loading and in vitro testing. A commercial hydraulic burst tester was used to provide uniform pressures on the sensor in order to establish the linearity, repeatability, and accuracy characteristics of the sensor. The in vitro experiments were performed in excised tendon and in a dynamic gait simulator to simulate biological conditions. In both experimental conditions, the sensor was found to be a sensitive and reliable method for acquiring minimally invasive measurements of soft tissue forces. Our results suggest that this sensor will prove useful in a variety of biomechanical measurements.

The transverse force experienced by the radial head during axial loading of the forearm: A cadaveric study

1. Studies of the axial hydrodynamic forces permitted determining the loads which are transmitted by the heel and the counterheel in a bulb unit. 2. The results indicate that the value of the equivalent axial hydrodynamic force depends basically upon the opening of the turbine control and upon the equivalent discharge. The type of runner and the speed exert an indirect effect, since they affect the variation of Q'I. In this connection, the obtained data can be used for preliminary analyses of the operating regimes of bulb units with similar geometrical designs of the internal passages, but with different types of runners. 3. In the absence of the possibility of direct measurement of the axial hydrodynamic force under steady regimes, this force can be calculated with a departure of 10–15% from the true value, by using the results of the pressure measurements along the radius on the turbine cover or directly inside the flow in the runner zone.

In this study, radial and axial thrust forces working on the whole rotor in a three-stages centrifugal pump are measured in a wide range of flow rate. The forces are measured at two floating journal bearings and one floating ball bearing, which are supported by the individual load cells. The effects of the offset of rotor position in the axial direction on the thrust forces are investigated. It is found that the effect of the axial offset is significant for the axial thrust force in the low flow rate range, whereas it has little influence on the head and efficiency performances in the whole flow rate range.

This paper presents the results of a series of wind-tunnel tests conducted to measure the force in ribbon parachute reefing lines. Three-foot diameter ribbon parachute models with porosities of 10-25% were tested in the Vought Systems Low-Speed Wind Tunnel. Simultaneous measurement of parachute axial force and reefing line force allowed the latter to be expressed either as a percent of axial force or as a dimensionless force coefficient. Maximum reefing line forces for 15, 20, and 25% porosity chutes were 4-5% of the axial force. Forces for the 10% porosity chute were considerably higher. The 10% porosity chute also showed a significant dynamic effect, with maximum values considerably higher than those in steady state.

In this study, the impact of tool pin profile on the dissimilar joining characteristics of aluminum alloy AA2017 and polycarbonate in friction stir welding (FSW) was examined. The tool pins made of SKD11 tool steel were used in the experiments of the FSW to join AA2017 and polycarbonate plates in a butt configuration at a tool rotation speed of 1320 and 1760 rpm and tool transverse speed of 60 mm/min. The tool shoulder diameter and tool pin height were set to 10 mm and 2 mm. The shapes of tool pin were cylindrical with and without groove, tapered cylindrical with and without groove, bobbin shaped with and without groove, square, triangular, pentagonal, hexagonal, oval, and ellipsoidal. The measurement of axial force was performed using a dynamometer. The changes in workpiece temperature at six locations during the process were measured using K-type thermocouples embedded in the plates. Microstructures of welded joints were examined by an optical microscope. Vickers Microhardness mapping was performed in and around the weld nugget zone, which is formed by the material flow between the two different base materials. From the result, the grooved tool pins generated less heat; while cylindrical, square, and oval type pins are also result in the superior properties of the welds during joining processes. Each tool pin had unique material flow behavior and weld formation, and the weld defects such as tunnel/voids occurred at the joint interface. The oval tool pin gives rise to the highest joint efficiency at a higher tool rotation speed, and the ellipsoidal tool pin generates the highest joint efficiency at a lower tool rotation speed was noticed.

Hull monitoring system with Fiber Bragg Grating (FBG) sensors increasingly receives people’s attentions. However, for the ship hull monitoring, the deformation of hull girder changes a lot as is subjected to a huge temperature variation. Therefore, the compensation method with only FBG temperature self-correction is not suitable for the hull monitoring sensors because no material thermal expansion effects are reasonably included. In this paper, the new compensation method of hull monitoring FBG sensor based on the sensor theory with both FBG temperature self-correction and steel thermal expansion effects correction is studied. The coupled compensation method suitable for hull monitoring sensor is obtained by theoretical derivation. As the comparison, the coupled compensation experiment was carried out. The results show that the relative error under the temperature compensation method is large in the case of drastic strain and temperature changes, and the correction results of the tested method will be closer to the true level.