Accurate Force Measurements Research Articles

Developing Miniature Force-Sensing Technologies for Measuring the Forces Applied to a Computer Mouse

Force-sensing technologies, 1.4 − 2.5 mm thick, were developed to measure the forces applied to the button and sides of a computer mouse. A force-sensing computer mouse was developed, built and tested for force measurement accuracy. The force-sensing technology could accurately measure the forces applied to the sides of a mouse and identify the location of force application. The force-sensing technology also accurately measured the forces applied to the mouse button with slight measurement errors at low forces. The force-sensing technologies developed in this study were made from commercially available materials, were relatively low cost and will be readily exportable to different mouse designs. These force-sensing technologies should assist manufacturers, designers and researchers in better understanding how various hand held devices and tools are gripped and operated.

An apparatus for quantitative fretting testing

The design and construction of an apparatus for performing quantitative fretting fatigue experiments is described. The device allows accurate measurement and control of normal contact force, tangential contact force, relative displacement between contacting surfaces and bulk fretting loads, as well as measurement of average friction coefficients. Its design is simple, and includes interchangeable fretting contact pads, allowing the use of various pad geometries without major adjustment. The device incorporates many points of adjustment for alignment and compliance, making it a robust frame for a wide variety of fretting fatigue conditions involving different materials. The capabilities of this device are also verified by results of fretting fatigue experiments conducted on a 7075‐T6 aluminium alloy.

Computer Modeling of Atomic Force Microscopy Force Measurements: Comparisons of Poisson, Histogram, and Continuum Methods

A computer model of the AFM pull-off process was used to test the Poisson method for calculating bond forces and to compare the Poisson and histogram methods of finding single-molecule bond-rupture forces. The Poisson method appeared to work well, producing accurate individual bond-rupture forces even in the presence of moderate to large amounts of different types of variations or noise, while the histogram method only produced good results at low noise levels, even with large sample sizes. Three cases were found where use of the Poisson model could lead to inaccurate measurement of individual bond forces, the most important being related to the bond-formation probability, which must be low (ideally <5%) for accurate results. The Johnson−Kendall−Roberts (JKR) continuum method was also examined. Poisson and JKR methods are not expected to produce similar results, but are complementary, and if used together may produce accurate measurements of both individual bond-rupture forces and bond-formation probabilities, even if neither method alone is successful.

An historical review of surface force measurement techniques

The measurement of surface forces has developed from a pursuit to test the theories of van der Waals and colloidal interactions to a field which encompasses issues as varied as cell interactions, lubrication, rheology, phase transitions, DNA binding and materials science. The desire to continually improve the accuracy and the applicability of surface force measurements has driven consistent improvements in the instrumentation employed. Until recently, macroscopic surfaces were required to enable accurate measurement of surface forces to be performed. These measurements are found to describe accurately the behaviour of a wide variety of colloidal systems. With the advent of the techniques of atomic force microscopy, total internal reflection microscopy and colloidal particle scattering and their application to force measurement, the sensitivity of force measurement has increased sufficiently to enable microscopic surfaces to be studied. It now seems possible that the next generation of instrumentation may be used to investigate interactions on a molecular scale. Here the techniques of measuring surface forces, both direct and indirect, are illustrated, from early measurements to the present. Further, recent developments and techniques which may be applied to the measurement of surface forces in the future are discussed.

The accurate measurement of a micromechanical force using force-sensitive capacitances

A sensor for the precise and accurate measurement of micromechanical forces is presented. The sensor is based on the capacitive detection of force-induced deflection of a microstructure and integrated charge readout. The mechanical performance of the sensor is evaluated and verified with experiments. The structure has been designed to enable the separation of the force-to-deflection and deflection measurements in order to result in a sensor structure in which the response is linear with force and little affected by fringe fields. The sensor is 0.25 mm/sup 2/ in size and has a force range of 0.2 mN, a zero-force capacitance of 0.5 pF, a sensitivity of 1-5 fF//spl mu/N, and a resolution that corresponds to a capacitance variation of 2 fF. >

Fully inverse dynamics of very flexible beam using a finite element approach and lagrange formulation

The problem of fully inverse dynamics of a very flexible planar Bernouli-Euler beam, where all six boundary conditions are imposed on one end, was analyzed and discussed. The problem was solved by using a finite element approach and Lagrange formulation. Theoretically, the solution of a beam divided into n finite elements requires 2 n time derivatives of the boundary condition functions. In practice, a much smaller number of time derivatives was required for an accurate solution. The fully inverse solution was verified by comparing the elastic curves of the beam for both direct and inverse dynamics. The developed model can be applied for remote sensing of forces and motion, where accurate measurements of both forces and motion at one end of a beam enable the reconstruction of the elastic curve of the beam and extract the location and the loads applied on the other end. Another application of the proposed model may be in the field of robotics, by predicting the input needed at one end of a flexible arm in order to perform a specific task at the other end of the arm.

On Some Issues in Triaxial Extension Tests

Abstract Results of triaxial extension tests on dry sand are presented. Special attention is paid to the factors influencing the results in triaxial extension tests. It is found that factors such as accuracy of the axial force measurement, axial force carried by the rubber membrane surrounding the specimen, gravity, and inhomogeneous deformation have a much larger influence on the results in triaxial extension tests than in triaxial compression tests. Accurate measurement of the axial force is achieved by placing the load cell inside the pressure chamber. The influence of the confining pressure on the axial force measurement is avoided through a proper construction. The force carried by the rubber membrane is corrected with reference to the result of a tension test on a strip of rubber membrane. The effect of gravity is accounted for by adding the axial stress due to gravity to the applied axial stress. The inhomogeneous deformation is traced by three lateral strain collars placed along the specimen height. Results of triaxial extension tests cannot be appreciated if these influential factors are not taken into account. The failure mode of the specimen is found to be influenced largely by the initial density. Inhomogeneous deformation in the form of necking develops in the upper part of dense specimens and in the middle of loose specimens. For dense specimens, the necked region becomes wider with advanced deformation and develops into one single or two intersecting shear bands. For loose specimens, however, no shear bands can be observed.

The ‘Minituft’ surface flow visualisation method; experience of use in the RAE 5m pressurised low-speed wind tunnel

SummaryThe development and use of a ‘minituft’ surface flow visualisation technique in the RAE 5m Pressurised Low-Speed Wind Tunnel is described.The method, due to Crowder, uses very fine nylon monofilaments to show surface flow direction. These tufts are made visible by coating them in a fluorescent dye and then illuminating them with a powerful ultra-violet light source.Because of their small size, a large number of tufts may be used without interfering with the flow to a great extent. However, it is shown that if accurate force measurements are required, then it is advisable to test the model in the absence of tufts.

Hydrodynamic Forces on Multitube Production Risers Exposed to Currents and Waves

This paper presents the experimental results of a joint industry riser test program. Over a five-year period, an extensive experimental investigation of hydrodynamic loads acting on various production risers was carried out. The program included the laboratory testing of many multitube riser configurations as well as a single cylinder. Several major oil companies sponsored the project. The tests were made with rather large-scale models which were: 1) held stationary in steady current, 2) moved at constant speed or constant acceleration through a quiescent fluid or through a moving fluid, and 3) held stationary in oscillating flow or in steady current superimposed on oscillating flow. The primary objective of this investigation was to establish an accurate measurement of the hydrodynamic forces on bundle-type risers. The findings from this study will be presented in two papers. In the first, we shall present the hydrodynamic force coefficients for some marine risers consisting of one or more cylinders interacting either only with currents or oscillatory flow separately. In a companion paper, the pertinent data for the joint presence of the oscillatory flow and currents will be discussed. Among the results outlined here are the drag, inertia, and lift coefficients for various composite risers as a function of the Reynolds and Keulegan-Carpenter numbers. An analysis and synthesis of the test data are also presented.

Circular myotomy and esophageal length and safe esophageal anastomosis: An experimental study

The indication for circular myotomy is often unclear at the time of esophageal atresia repair because the maximal permissible tension at the anastomosis and the precise effect of myotomy on esophageal length and anastomotic tension are not known. This study was undertaken to answer these questions. In piglets, 0 to 6.5-cm segments of thoracic esophagus were excised and approximation force was measured by dynamometers attached to traction sutures in each cut end of the esophagus prior to anastomosis. When approximation force was greater than 75 g. esophageal leaks occurred and increased fibrosis was noted at the anastomosis. Circular myotomy of the upper segment reduced the force necessary to approximate the ends of the esophagus after a 5- to 6-cm resection by about 50%, and double circular myotomy by as much as 75%. When the upper third of the esophagus of piglets and neonatal cadavers was stretched by the application of traction forces of 0 to 75 g. esophageal length increased by about 40%. Thereafter, length increase ws minimal. For any given traction force, each circular myotomy increased esophageal length by about 5 mm. These studies suggest that accurate measurement of approximation force would be useful at the time of esophageal atresia repair. On the basis of these data, circular myotomy(s) is indicated to elongate the esophagus and reduce anastomotic tension when a force greater than 75 g is needed to approximate the esophageal ends.

Air Model Tests of Labyrinth Seal Forces on a Whirling Rotor

Steam-excited whirl of high pressure turbine rotors is caused by shroud and shaft labyrinth seal forces and by flow forces on the blades. Accurate test results are needed to guide development of a valid method for calculating labyrinth seal whirl forces and to verify the method. This paper describes apparatus for accurate measurement of labyrinth seal forces on a whirling model rotor. The effects of some system parameters on the whirl excitation constant (whirl force/whirl amplitude.) and radial stiffness of a model seal are shown. The seal whirl force is destabilizing for some conditions and stabilizing for others. A method is given for predicting the net seal and blade-row excitation constant that will cause self-excited whirl of rotors having specified shaft and bearing parameters.

Evaluation of errors in finding the strength characteristics of refractory metals

1. Increased accuracy in temperature measurement is one of the main ways of improving the metrological characteristics of high-temperature test machines. 2. Stabilization of temperature with an accuracy of ΔTstab≤ΔTmeas/(2–3) should be regarded as entirely satisfactory, and it is hardly necessary to improve the existing accuracy of force measurement (±1%) for the purposes under consideration.

Some remarks on lifting surface theory

This paper gives a brief review of the theory of unsteady lifting surfaces as applied to ships' propellers. The possibilities and limitations of this theory are discussed, together with some results recently obtained by different authors. Mathematical methods of solution and their implications will be avoided wherever possible. The accurate measurement of unsteady forces acting on a propeller is rather difficult. Some investigations have been performed and the results of these are compared with results obtained with the lifting surface theory. Further systematic investigations are necessary and some suggestions for these are given.

The Measurement of Friction and Friction-Induced Vibration

The design, development, and application of a pin-on-disk tribometer for the measurement of friction and friction-induced vibration is described. Careful isolation techniques eliminated many of the errors in measurement which are associated with vibration arising from the apparatus or the surroundings. The use of acceleration, velocity, and displacement transducers in association with a one-cycle sequence triggering circuit and other electronic devices permitted the accurate measurement of kinetic friction forces in the presence of friction-induced vibration.

Hypersonic viscous effects on free-flight slender cones

An experimental study was performed in order to determine the effects of various parameters upon cone aerodynamics in the hypersonic regime at Reynolds numbers as low as 7800/in. The use of free-flight models eliminated the need for evaluating sting interference and made practical accurate measurement of drag forces down to 8 X 10 ~4 Ib and pitching moments as low as 2 X 10 ~4 in-lb. The effects of model to total temperature ratios of 0.45 (gun launch) and 0.87 (wire release) for 6° through 15° half-angle cones on total drag and static stability were investigated at Mach numbers from 6 to 10 for values of the viscous Knudsen number up to M 00/(KI)00)1/W = 0.28. Limited nose-blunting effects were also investigated. Conical shock viscous drag theory agrees quite favorably with the experimental data. Use is made of trends predicted by theory in order to compare the data of this paper writh available hypersonic viscous cone drag data.