Methods and systems for controlling the temperature stability of an inductor

Methods and systems a method of assembling a proximity probe are provided. The method includes determining a coil wire dimension and coil geometry for the probe such that a resistance versus temperature profile of the coil is approximately constant when the coil is excited with an excitation frequency of approximately 150 kilohertz to approximately 350 kilohertz, and adjusting the coil geometry such that a resistance versus temperature profile of the coil is substantially constant.

BACKGROUND OF THE INVENTION

This invention relates generally to controlling the temperature stability of an inductor, and more particularly to methods and apparatus for controlling the temperature stability of a sensing coil of a proximity probe.

At least some known rotating and reciprocating machinery use an eddy current or proximity probe to facilitate monitoring machine vibration or rotor position characteristics. The environment that the proximity probe operates may be relatively harsh.

Typically, the proximity probe, outputs a signal correlative to a spacing between a target object such as, a rotating shaft of a machine or an outer ring of a rolling element bearing and a sensing coil of the proximity probe. The gap or spacing between the target and the sensing coil of the proximity probe needs to remain within the linear range of the proximity probe for providing accurate and reliable measurements of machine vibration characteristics. Accordingly, to provide accurate and reliable measurement, a proximity probe should remain in the linear range of operation under all operating environmental conditions.

The electronics associated with the proximity probe typically incorporates an oscillation circuit whose amplitude of oscillations is dependent on the conductance of the sensing coil. When the circuit is oscillating, the sensing coil has an alternating current flowing therein which causes the sensing coil to radiate energy in the form of an alternating magnetic field. The target object absorbs some of the radiated energy from the sensing coil when it is placed within the alternating field emanating from the sensing coil. This absorption of energy is a result of the alternating field generating eddy currents in the object which circulate so as to oppose the alternating field which created them. The amount of energy absorbed by the target object is correlative to the spacing between the target object and the sensing coil. The closer the target is to the sensing coil, the more energy the target will absorb from the sensing coil as a result of the eddy current principle. Therefore, the amplitude of oscillations of the oscillation circuit will vary as a function of spacing between the sensing coil and the target.

The ability to provide accurate and reliable measurements over a wide range of circuit and environmental conditions is dependent, at least partially on, the characteristics of the sensing coil including the material and diameter of the wire used to wind the coil, the operating frequency of the system and the electronics of the proximity probe system. The sensing coil and the remaining electronics usually have a wide range of tolerances, such as gain, bias voltage, bias current and temperature coefficients. Accordingly, each production unit has to be initially calibrated to incorporate those tolerances. Moreover, the sensing coil of the proximity probe usually contains sources of temperature drift error which are attempted to be compensated for in the final product.

A source of the temperature drift error in the sensing coil is due to a temperature dependent resistance of the coil and an inductance of the coil. This temperature dependent resistance and inductance of the sensing coil effects a source of temperature drift error resulting in inaccurate proximity probe measurements as a consequence of the false appearance of a gap change between the target and sensing coil. Such inconsistencies in temperature stability of the proximity probe result in unpredictable and unreliable measurements even when the proximity probe is functioning in its linear range of operation. Compensation for such inconsistencies is usually only partially effective to facilitate temperature dependent errors.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a method of assembling a proximity probe includes determining a coil wire dimension and coil geometry for the probe such that a resistance versus temperature profile of the coil is approximately constant when the coil is excited with an excitation frequency of approximately 150 kilohertz to approximately 350 kilohertz, and adjusting the coil geometry such that a resistance versus temperature profile of the coil is substantially constant.

In another embodiment, a proximity probe configured to monitor a rotatable machine by measuring a gap between the probe and a target associated with the rotatable machine includes a coil configured to be located magnetically proximate the target on a first side of the coil, said coil comprising a coil wire dimension and coil geometry for the probe such that a resistance versus temperature profile of the coil is approximately constant when the coil is excited with an excitation frequency of approximately 150 kilohertz to approximately 350 kilohertz and a tuning disk positioned proximate a second opposite side of the coil wherein a distance between the first coil and tuning disk is changeable to generate a predetermined scale factor for the coil.

In yet another embodiment, a system for determining a gap defined between an eddy current proximity probe and a target includes a proximity probe including a coil configured to be located magnetically proximate the target on a first side of the coil such that an output signal correlative to a distance between the proximity probe and the target, said coil comprising a coil wire dimension and a coil geometry for the probe such that a resistance versus temperature profile of the coil is approximately constant when the coil is excited with an excitation frequency of approximately 150 kilohertz to approximately 350 kilohertz, and a tuning disk positioned proximate a second opposite side of the coil wherein a distance between the first coil and tuning disk is changeable to generate a predetermined scale factor for the coil. The system also includes an electronic circuit coupled to the proximity probe configured to transmit an excitation frequency to the proximity probe and to receive the output signal from the proximity probe.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a schematic view of a proximity probe system100. Proximity probe system100is configured to monitor a target object, for example, a rotating shaft102of a machine (not shown). Proximity probe system100includes a proximity probe104including a temperature stable inductor106, an electronic circuit108and a monitoring system110. Typically, proximity probe104is operatively coupled to electronic circuit108and is positioned adjacent rotating shaft102for monitoring the vibrational or rotor position characteristics thereof. Proximity probe104uses eddy currents to generate a signal indicative of a gap112between rotating shaft102and inductor106. Proximity probe104includes a tuning disk114magnetically coupled to inductor106for setting a scale range for the output of proximity probe104.

Proximity probe104defines a common enclosure116housing both inductor106and tuning disk114. In the exemplary embodiment, proximity probe104is formed from a cured monolith of molded material118defining an encapsulation having a front end120and a back end122. Inductor106is generally symmetrically positioned within proximity probe104. An integrally formed protective probe tip124of a substantially uniform thickness126is fabricated along a forwardmost portion of proximity probe104. Molded material118substantially surrounds inductor106proximate front end120and ensconces a portion of an electrical conduit128that emanates from back end122. Tuning disk114is adjustable such that a distance138between inductor106and tuning disk114may be selected and maintained to facilitate controlling the output characteristics of proximity probe104.

In the exemplary embodiment, electrical conduit128comprises a multi-axis cable including a center conductor140electrically connected to a first lead142of inductor106and a coaxial conductor144electrically connected to a second lead146of inductor106wherein the center conductor72and coaxial conductor74are separated by an insulator or dielectric148. In the exemplary embodiment, a braided sheath150circumscribes coaxial conductor144and is separated therefrom by an insulator152. Braided sheath150provides additional shielding and mechanical integrity to electrical conduit128. Electrical conduit128extends out of the back end116of the encapsulation118and couples inductor106to electronic circuit108to provide signal output and power input therebetween.

Electronic circuit108is operatively coupled to inductor106for radiating energy in the form of an alternating magnetic field158from inductor106to rotating shaft102and for receiving signals from inductor106. Rotating shaft102absorbs some of this radiated energy and the received signals by electronic circuit108, from inductor106, are a function of the spacing between rotating shaft102and inductor106. Thus, the closer rotating shaft102is to inductor106, the more energy rotating shaft102will absorb as a result of the eddy currents induced in rotating shaft102.

Specifically, the absorption of the radiated energy by shaft102is a result of the alternating magnetic field158generating eddy currents in shaft102which circulate so as to oppose magnetic field158, which created them. This action causes the resistance of the coil to change. In addition, the mutual effect of the currents in the closely adjacent conductors, turns of inductor106, produce a temperature dependent change in resistance in response to the alternating current. Thus, the mutual effect of the currents in the closely adjacent turns of inductor106and the effect of the currents in shaft102produce temperature drift and gain variations in response to the alternating current. In the exemplary embodiment, dimensions of inductor106and the wire comprising inductor106are preselected based on system performance requirements and the environment where proximity probe104is to operate. For example, a distance112may determine a diameter of proximity probe104and inductor106. A system operating frequency is determined based on the dimensions of inductor106and the dimensions and electrical characteristics of the wire used to form inductor106. The system operates at a specified frequency in order to provide an acceptable level of temperature stability. This frequency is theoretically determined, for example, using a computer programmed to determine the frequency based on the wire composition, diameter, and the geometrical configuration of the coil windings. This provides a baseline frequency at which the system will operate. The system is tested iteratively over its operating temperature range to determine the optimal operating frequency. In the exemplary embodiment, the eddy current system is temperature stable when operating at a frequency between 150 kilohertz and 350 kilohertz. Specifically, for a 50 millimeter diameter probe configured as described herein the system is temperature stable when operating at a frequency of approximately 267 kilohertz.

Using the determined frequency, a linearly of a resistance versus temperature curve is determined empirically. To improve the linearity of the resistance versus temperature curve over the desired range, the inductance of inductor106is adjusted, for example, by adding or removing winding turns to inductor106such that temperature drift and gain variations generated by both inductor106and target object (i.e., shaft102) for effecting the temperature stable inductor10are substantially canceled out which results in a temperature stable proximity probe104. Thus, the temperature compensated proximity probe maintains an accurate indication of the gap112between rotating shaft102and inductor106under the temperature variations found in the environment of machine monitoring.

FIG. 2is an exploded perspective view of an exemplary proximity probe104in accordance with an embodiment of the present invention. Proximity probe104includes common enclosure116housing inductor106and tuning disk114. In the exemplary embodiment, proximity probe104is formed from molded material118defining an encapsulation having a front end120and a back end122. Inductor106is generally symmetrically positioned within proximity probe104. An integrally formed protective probe tip124of a substantially uniform thickness126is fabricated along a forwardmost portion of proximity probe104. Molded material118substantially surrounds inductor106proximate front end120. A feed through boss202extends from back end122to guide and protect electrical conduit128(not shown inFIG. 2) from inductor106. Boss202includes a bore204therethough and a threaded portion206configured to threadably engage a threaded portion208of tuning disk tuning disk114. Tuning disk tuning disk114is adjustable such that a distance between inductor106and tuning disk114may be selected and maintained to facilitate controlling the output characteristics of proximity probe104. A weld ring210facilitates sealing a case212to enclosure116.

FIG. 3is a flow chart of an exemplary method300of assembling proximity probe104(shown inFIG. 2). The method includes determining302a coil wire dimension and coil geometry for the probe such that a resistance versus temperature profile of the coil is approximately constant when the coil is excited with an excitation frequency of approximately 150 kilohertz to approximately 350 kilohertz, and adjusting304the coil geometry such that a resistance versus temperature profile of the coil is substantially constant.

The coil geometry is selected based on an application for the proximity probe. In the exemplary embodiment, a fifty millimeter diameter probe is selected. Selecting the coil geometry also includes selecting a wire diameter and material. An excitation to be applied to the probe during operation is determined using for example, a computer-based algorithm that determines a frequency of operation that facilitates reducing a temperature dependence of the output of the probe. In the exemplary embodiment, the algorithm uses the selected coil geometry and the coil wire dimensions and electrical characteristics to determine an operating frequency between approximately 150 kilohertz to approximately 350 kilohertz. The coil is subjected to output tests at various gap distances between the coil and a target. In one such test, the ambient temperature of the probe is varied between temperature extremes expected to be experienced in the particular application the probe will be used in. The coil geometry is iteratively adjusted to attain a resistance versus temperature profile of the coil is substantially constant. For example, coil turns are added or removed to control the coil inductance such that the excitation frequency changes correlatively. In the exemplary embodiment, an operating frequency of approximately 267 kilohertz is the final operating frequency. Additionally, a tuning disk comprising, in the exemplary embodiment, a stainless steel disk is positioned on an opposite side of the coil from the target. The tuning disk is magnetically coupled to the coil when an excitation signal is applied to the coil. The tuning disk is threadably coupled to a boss extending from the side of the coil enclosure. The scale factor of the coil is controlled by threading the tuning disk closer to the coil or further away from the coil. When the output scale factor is determined to be at the predetermined value, the tuning disk is locked in place and the proximity probe is further assembled.

The above-described embodiments of temperature stable proximity probe system provides a cost-effective and reliable means for monitoring machinery. More specifically, the coil of the proximity probe is fabricated such that an operating frequency is selected that substantially mitigates a temperature dependence of the output of the coil. As a result, an proximity based monitoring system is provided that facilitates improving the accuracy and repeatability of the proximity probe.

Exemplary embodiments of monitoring systems are described above in detail. The monitoring system components illustrated are not limited to the specific embodiments described herein, but rather, components of each monitoring system may be utilized independently and separately from other components described herein. For example, the monitoring system components described above may also be used in combination with other monitoring system.