Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2001-10-02
2004-10-12
Le, N. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207120, C324S207260, C324S239000
Reexamination Certificate
active
06803757
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates generally to eddy current proximity probe systems for determining displacement motion and position of an observed conductive target object and in particular, to a multi-coil eddy current proximity probe system utilized for monitoring, for example, rotating and reciprocating machinery.
BACKGROUND OF THE INVENTION
Proximity probe systems that analyze and monitor, for example, rotating and reciprocating machinery are known in the art. These systems typically include one or more proximity probes: noncontacting eddy current displacement devices operating on the eddy current principle for measuring displacement motion and position of an observed conductive target object relative to one or more of the displacement devices. Typically, each proximity probe is located proximate a target object such as a rotating shaft of a machine or an outer race of a rolling element bearing being monitored and is connected to signal conditioning circuitry which in turn is coupled to analyzing apparatus for data reduction and display. By known techniques, these systems analyze and monitor rotating and reciprocating machinery for providing, inter alia, indications of incipient problems. A variety of proximity probes and systems are at the present time being sold by the assignee of this application, Bently Nevada Corporation of Minden, Nev.
Generally, a proximity probe system includes a proximity probe comprised of a multi-conductor probe cable coupled to an inductor or coil that is situated at a forward most end of the probe. The coil is coupled to the signal conditioning circuitry of the system via the probe cable and is driven by a radio frequency signal from the signal conditioning circuitry and in turn creates an alternating magnetic field in any proximate conductive target object. This magnetic field produces eddy currents in the object that induce a counter electromotive force (emf) in the coil that alters the impedance of the probe and thus the output of the probe as a function of distance between the probe and observed target object. The signal conditioning circuitry demodulates the probe output and provides output signals proportional to a distance or gap interposed between the proximity probe and the observed conductive target object. Thus, the signal conditioning circuitry is also sometimes referred to as an oscillator-demodulator device.
The above-delineated system is burdened by temperature errors due to temperature variations in the multi-conductor cables, the coil and the electronics associated with the signal conditioning circuitry. Additionally, temperature variations in the targets themselves cause temperature stability problems within the system. Furthermore, component and manufacturing variations also generally burden the system.
Moreover, and more particularly, sense coil resistance of the probe is one principle source of temperature drift error. Thus, the process of measuring gap as a function of the impedance of the coil is susceptible to this error thereby resulting in inaccurate proximity probe measurements as a consequence of the drift error causing a false appearance of a gap change between the target and probe. 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.
U.S. Pat. No. 5,854,553 to Barclay, et al. teaches the use of a digitally linearizing eddy current probe wherein the output of an eddy current probe is demodulated and subsequently linearized using an analog to digital converter, a digital signal processor, and a memory. The linearized digital output signal is converted back to an analog signal, the voltage of which is used as being directly proportional to the position of the conductive target in relation to the probe. Hence, the U.S. Pat. No. 5,854,553 to Barclay, et al. measures the gap, as a function of the impedance of the coil being driven to engender or set up eddy currents in the target object. Thus, U.S. Pat. No. 5,854,553 to Barclay, et al. also suffers from, inter alia, the same temperature instability problems delineated hereinabove.
Moreover, known multiple coil proximity switch devices, distance measurement devices, and metal detecting circuits are also plagued by, inter alia, temperature drift error and component and manufacturing variations.
For the foregoing reasons, it would be highly desirable to provide an eddy current proximity probe system that would be accurate, reliable, and substantially unaffected by temperature, component and manufacturing variations. More specifically, it would be highly desirable to provide a proximity probe system that compensates for different probe cable lengths, resistance changes in the cable and the probe coil, and temperature changes in the probe cable, coil, and signal conditioning devices.
SUMMARY OF THE INVENTION
The instant invention is distinguished over the known prior art in a multiplicity of ways. For one thing, the instant invention provides a multi-coil eddy current proximity probe system that provides accurate and reliable measurements over a wide range of circuit and environmental conditions. Particularly, the instant invention provides an eddy current proximity probe system that includes a unique multi-coil proximity probe and processing and control circuitry that generally eliminates temperature errors and manufacturing and component variations. For example, the unique multi-coil probe, and processing and control circuitry provides a stable output with different probe cable lengths, with resistance changes in the cables and probe coils, and with temperature changes in the probe cables, coils, and circuitry thereby providing accurate and reliable measurements of machine operating characteristics. Furthermore, the eddy current proximity probe system generally solves the problem of compensating for changes in the conductivity, permeability, and temperature profiles of different target materials. Moreover, and in stark contrast to the known prior art, the instant invention detects a current in a sense coil separate from a coil driven to engender or set up eddy currents in a target object for determining gap values.
In one form of the instant invention, the eddy current proximity system includes a multi-coil proximity probe including a sense coil, a drive coil, and a reference coil. The coils are located adjacent one another with their interiors or hollow cores coaxially arranged along a common longitudinal axis. The drive coil is interposed between the sense coil and the reference coil such that the sense coil is positioned at a forwardmost end of the probe. The system is further comprised of a signal conditioning and control system having a feed back loop comprised of a first phase detector or first multiplier circuit and a signal generator having an automatic gain controller. The first multiplier includes an input coupled to the reference coil via a multi- conductor cable and an output electrically connected to the automatic gain controller. An output of the automatic gain controller is coupled to the signal generator that is coupled to the drive coil via a multi-conductor cable. The signal generator drives the drive coil with an alternating drive signal that generates a first magnetic field that radiates from the drive coil and induces an alternating reference signal in the reference coil. Any phase discrepancy in the reference signal results in a control signal being sent from the multiplier to automatic gain controller which provides a corrective control signal which automatically adjusts the gain of the signal generator for controlling the amplitude of the drive signal. Thus, the input signal driving the drive coil is a function of its own magnetic field output. Additionally, any attenuation by the cable coupling the reference coil to the first multiplier is corrected for by the closed feedback loop between the drive coil and the reference coil. Furthermore, temperature variations, manufacturing variat
Bentley Nevada, LLC
DeBoo Dennis A.
Kinder Darrell
Le N.
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