|Publication number||USRE39803 E1|
|Application number||US 09/846,835|
|Publication date||Sep 4, 2007|
|Filing date||May 1, 2001|
|Priority date||Nov 14, 1996|
|Also published as||US5974869|
|Publication number||09846835, 846835, US RE39803 E1, US RE39803E1, US-E1-RE39803, USRE39803 E1, USRE39803E1|
|Inventors||Steven Danyluk, Anatoly Zharin, Elmer Zanoria, Lennox Reid, Kenneth Hamall|
|Original Assignee||Georgia Tech Research Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (14), Non-Patent Citations (1), Referenced by (8), Classifications (12), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is based and claims priority on United States of America provisional patent application Ser. No. 60/030,814, filed on Nov. 14, 1996.
Part of the work for this invention was funded by the United States of America Office of Naval Research under contracts numbers N00014-95-1-0903 and N00014-94-1-1074. The government of the United States of America has certain rights to this invention.
1. Field of the Invention
The present invention generally relates to non-contact sensors for monitoring surface variations of a component part, and more specifically relates to a non-vibrating capacitance probe which uses a variable capacitor to measure the contact potential difference between two surfaces, generally on the same component part, and thereby recognizes surface variations such as wear of an object subjected to, for example, a sliding contact.
2. Technical Field
Mechanical systems such as heat combustion engines have components that are dynamically in contact with another body. These components are subjected to cyclic motions that can involve impact loading, shear straining, plastic deformation, frictional heating, and fatigue of sub-surface regions. A combination of these mechanisms often leads to surface damage that impairs the performance of the component. In addition, the chemical interaction between the component surface and surrounding fluids also can accelerate surface degradation. Such problems, if unattended, can result in catastrophic malfunction of the machine and even compromise operational safety. In this regard, it is desirable to monitor the surface condition of a critical tribocomponent. The design of sensors to monitor the surface condition of the tribocomponents and the operation of machinery depends largely on the nature of tribological application.
A surface-monitoring method that exploits the spatial variation in the work function of a material is presented herein. The work function refers to an energy barrier to prevent the escape of electrons from the surface of the material. The work function is governed by the physio-chemical nature of the surface and also depends on the environmental conditions. From a tribological standpoint, the work function is a useful parameter for evaluating mechanical deformation features such as dislocation pile-ups and residual stresses. For example, it has been demonstrated that a metal subjected to different degrees of compressive stress exhibits a variation in the work function. Craig, P. P. and Radeka, V., “Stress Dependence of Contact Potential: The ac Kelvin Method,” Rev. Sci. Instrum., Vol. 41, pp. 258-264, 1969. The present invention is a non-vibrating capacitance probe as modified from that of the Kelvin-Zisman method, Zisman, W. A., “A New Method of Measuring Contact Potential Differences in Metals,” Sci. Instrum., Vol. 3, pp. 367-370, 1932, that uses a variable capacitor to measure the contact potential difference (CPD) between two surfaces.
Briefly described, in a preferred form, the present invention monitors the surface variations, such as surface wear, of a component. The surface wear is measured by the spatial variation in the work function of the component. The work function refers to an energy barrier to prevent the escape of electrons from the surface of the component. The invention detects the surface charge of the surface of the component through temporal variation in the work function of the component.
The present invention generally comprises the novel combination of a means for supporting the component and a non-vibrating capacitance probe, and the use of the non-vibrating capacitance probe in this combination to carry out the wear monitoring function of this invention. The component and non-vibrating probe are located in close proximity to each other. The relative motion between the component and the non-vibrating probe, the distance between them, and the contact potential difference between them, all are monitored. The work function of the component is found by monitoring the current induced by contact potential difference in the non-vibrating probe and relating it to the known work function of the electrode in the probe.
The present invention is directed to a non-vibrating capacitance probe which may be used as a non-contact sensor for tribological wear. Specifically, the present invention is a device which detects surface charge through temporal variation in the work function of a material. An artificial spatial variation in the work function is imposed on a shaft surface by coating a segment along the shaft circumference with a metal paint wherein the paint is compositionally different than the shaft surface. As the shaft rotates, the reference electrode senses changing contact potential difference with the shaft surface, owing to compositional variation. Temporal variation in the contact potential difference induces a current through an electrical connection. This current is amplified and converted to a voltage signal by an electronic circuit with an operational amplifier. The magnitude of the signal decreases asymptotically with the electrode-shaft distance and increases linearly with the rotational frequency.
In one embodiment of the apparatus, the component to be monitored for surface variations either is a cylindrical shaft composed of the material to be monitored, or wear-tested, or is a cylindrical shaft coated with the material to be monitored, or wear-tested. The component is supported by roller bearings on both ends of the shaft, allowing rotation of the shaft along its axis. The shaft is rotated by a motor and the rotational speed of the shaft is monitored. A non-vibrating capacitance probe is mounted on an xyz-positioning system, and a monitor detects the spacing between the shaft surface and probe. A monitoring device interprets the current induced in the non-vibrating capacitance probe as a difference in work function between the component and the known work fimction of the reference electrode in the probe. The process of measuring the work function of the component comprises the creation of relative rotational motion between the component and the non-vibrating capacitance probe. The relative motion of the component and probe, and the distance between the component and probe also are monitored.
One application of the non-vibrating capacitance probe is for detecting surface wear of an object subjected to sliding contact. One technique is to apply a thin coating of a material on the sliding body that is compositionally different from the substrate. Partial removal of this coating due to sliding contact creates sites where the substrate material is exposed. Formulation of these sites create lateral compositional variation, thus, heterogeneity in the work function of the wear surface. This yields an induced-current pattern that is unique from that of the unworn surface coating.
Accordingly, it is a primary object of the present invention to provide an apparatus comprising a non-vibrating capacitance probe which can be used as a non-contact sensor for tribological wear.
It is another object of the present invention to provide an apparatus comprising a non-vibrating capacitance probe which can be miniaturized and installed in systems that have moving parts.
These and other objects, advantages, and features of the present invention will become apparent to those skilled in the art upon reading the following specification in conjunction with the accompanying drawing figures, in which like reference numerals designate like parts throughout the several views.
FIGS. 3(a) and 3(b) show the theoretical variation of dV/dt with time for different values of x.
FIGS. 7(a) and 7(b) show experimental samples of probe output signal for different values of x.
The theoretical background detailed below provides a description of the Kelvin-Zisman method to monitor a surface probe, and demonstrates the operation of the probe on a rotating shaft.
where V, the CPD voltage, is defined by
and C, the capacitance, is expressed as
where e is the charge of an electron εr is the relative dielectric constant, ε0 is the permitivity in free space, A is the area of the reference electrode, and d is the spacing between the surfaces.
A typical experimental condition involves a reference electrode that does not detect a varying φdesired, thus, the term dV/dt in equation 1 is assumed to be zero. In most CPD-measurement studies, such a condition is implemented by having the vibrating reference electrode fixed in position on a particular site of the sample surface. The induced current is contributed solely by the change in the capacitance owing to the sinusoidal variation in d expressed as
where d0 is the mean spacing, d1 is the amplitude, ω is the angular frequency, and t is the time. Substituting equation 4 into equation 3 and 1 yields
i=−Vεrε0Ad1 cos ωt/(d0+d1 sin ωt)2 (5)
The Kelvin-Zisman technique to measure V is to provide a compensating voltage, VC, to the capacitor 10, shown in
In preferred form, the present invention comprises a non-vibrating capacitance probe for surface wear monitoring. The probe of the present invention forms a capacitor 10 between a reference electrode 12 and a surface 14 of interest, as described by the Kelvin-Zisman technique above. However, the spacing between the two surfaces, the electrode 12 being the first surface and the surface 14 being the second surface, in the present invention is fixed. Instead of the variable capacitance, the current is induced by the temporal change in CPD. Therefore, in reference to equation 1, the formulation for the induced current is simplified to
Varying the CPD with time can be achieved by imposing a lateral displacement between the reference electrode 12 and the sample surface 14 with a heterogeneous work function. A combination of equation 6 with equation 3, which yields
suggests that the magnitude of the induced current decreases asymptotically with the capacitor spacing, and increases with the area of the reference electrode and the rate of CPD change.
One embodiment of the present invention is the scanning of a cylinder 30 having a cylindrical surface 20 rotating along its longitudinal axis 22, as shown in FIG. 2. Using the geometry depicted in
As the cylinder 30 rotates at a constant speed, the reference electrode 40 senses a contact potential difference with material A, CPDEA, and another potential with material B, CPDEB. Also assume the CPDEB is zero. The variation in the CPD with time can be described by a rectangular wave function V(t) with an amplitude CPDEA, as shown in FIG. 2. The Fourier series of the function is
wherein V′=CPDEA−CPDEB, in volts, f is the fundamental frequency which is equivalent to the rotational frequency, x is the ratio of the arc length of A to the circumference of the cylinder, and n=1, 2, 3, . . . ∞. The derivative of this function is defined by
For CPDEB≠0, the derivative of V(t) is still identical to equation 9 where the dc component is eliminated.
Equation 9 indicates that the magnitude of the peak depends on the fundamental frequency. This is illustrated in
It should be noted that waves with smaller amplitude separate the major peaks as shown in FIG. 3. There should be a straight line (dV/dt=0) instead because of the absence of CPD variation between material boundaries. The appearance of minor waves between the large peaks is attributed to the limited number of harmonics included in the calculation. With the higher number of harmonics, the amplitude of these waves approaches zero.
Another embodiment of the present invention, as shown in
A non-vibrating capacitance probe 150 is mounted on an xyz positioning system 160 which is mechanically isolated from the above set-up. Stepper motors, not shown, control the lateral motion of the probe 150 along the longitudinal axis of the shaft 100 and the vertical position. The probe 150 is positioned such that a reference electrode 152 in the probe 150 is perpendicular to the shaft 100 surface. A separate positioning stage with a translational resolution of 0.01 mm is used to manually adjust the spacing between the shaft 100 and the reference electrode 152. Spacings ranging from 01. to 1.25 were used in experimentation.
Artificial variation in the work function was imposed on the sample shaft 100 surface by coating a segment along the shaft 100 circumference with a colloidal silver paint. Most of the tests were conducted for a silver strip 170 with an arc length x that was 1.3/100, or 0.013, of the circumferencial length of the shaft 100. One test was performed for a separate coating with a length x fraction of 0.3. The coating strips were approximately 14-μm thick and 5-mm wide for this experimentation.
The reference electrode 152 of the probe 150 was made of lead wire with a cross-sectional area of approximately 0.446 mm2. Electrical connection between the sample shaft 100 and the common ground of the probe's 150 electronic circuit was maintained through a brush in contact with the shaft 100. The current induced by the time-varying CPD between the electrode 152 and rotating shaft 100 surface was converted to a voltage output, as shown in
An interval of minor waves separates the large ones as shown in FIG. 7a. This interval could be the electrical signature of uncoated aluminum The fluctuation could reflect microstructural variation in the aluminum surface that also gives rise to heterogeneity in the work function. The microstructural variation could be linked to the machining history of the shaft 100.
The amplitudes of both the maximum and minimum peaks of the major wave is influenced strongly by the rotational frequency of the shaft 100 and the capacitance spacing. As an example, a quantitative analysis of the maximum peak measured for a silver strip with a length fraction of 0.013 is presented.
A mathematical equation for each curve in
1n(Vmax)=[s × 1n(d)]+y (10)
yields an asymptotic expression for Vmax
where c=ey. Equation 11 takes into account the negative slope indicated by the linearized plots in FIG. 9. Table 1 shows the values of c and s for each rotational frequency.
The empirical equation for Vmax conforms with the predicted model for the induced current (equation 7). Both equations are asymptotic; however, the experimental value of s in equation 9 range from 0.6 to 0.9. Except for f=10 Hz, these values are slightly below 1, which is the predicted value. It should be noted that the probe signal is acquired through a current-to-voltage conversion circuit with a gain factor of 3.9×109 V/amp. Taking this and equation 7 into account, it is proposed that the numerator, c, in the empirical equation, represents a product of the induced current, conversion factor, dielectric constants and dV/dt. Among these parameters, dV/dt which increases linearly with the rotational frequency, shown in
Therefore, the applicability of the non-vibrating capacitance probe for detecting surface variation in the work function has been presented. This variation is reflected by the nature of the current induced by the changing contact potential difference between the reference electrode and the surface in question. The magnitude of the induced current which indicates the sensitivity of the probe, decreases asymptotically with distance between the probe and sample, and increases linearly with the rate of CPD change. These results are consistent with the theoretical model.
While the invention has been disclosed in its preferred forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims.
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|U.S. Classification||73/105, 73/104, 324/686, 324/458, 324/663|
|International Classification||G01R27/26, G01N27/22, G01B21/30, G01R1/07, G01B7/34|