Patent Application
20110213571
The invention relates generally to a non-contact measurement of rotating shaft parameters and more particularly to a method and system for sectional magnetic encoding and sensor arrangement for measuring shaft parameters.
Generally, there are numerous applications of rotating shafts in industries for accomplishing some form of work or energy conversion. Rotating shafts are still used on current windmills and hydroelectric plants, however they incorporate advanced technology and processing. Rotating shafts are also used in electronic equipment such as computer disk drives, media recorders/players, and household appliances, and are generally of a smaller length and width such that the torque is relatively small. Larger rotating shafts experience larger torque and are deployed in applications including locomotives, airplanes, ships, and energy conversion to name just a few examples. The modern usage of equipment utilizing larger rotating shafts typically incorporates sensing and processing capabilities to achieve safe and efficient operation. This necessitates the requirement of measuring various shaft parameters such as angular velocity, acceleration, torque, and rotational anomalies to address the design and operation of equipment utilizing rotating shafts. Conventional technologies employ a number of different systems of sensing or measuring the shaft parameters such as strain gauge systems, encoder/tooth systems, acoustic wave systems, elastic systems, magnetostrictive systems, and magnetoelastic systems. Each of these systems has certain characteristics and applications.
Strain gauges provide for local strain measurements of the shaft and typically require some form of coupling to the rotating shaft that can be via a physical connection (e.g.: slip rings) or telemetry. The gauges generally suffer from low stability, have limitations in the bandwidth and tend to have calibration and environmental correction requirements. The limited operating temperature range of strain gauges limits their use in a harsh environment.
The encoder/tooth-wheel pickup style of shaft parameters sensing usually has at least some partial attachment to the rotating shaft such as by a magnetic tooth-wheel. The tooth-wheel design tends to be costly and impractical for many implementations. Such a design is not practical for higher speed applications and although stable, lacks high resolution and can cause reliability issues in harsh environment.
The acoustic wave systems utilize sensors such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices that use acoustic waves to detect strain-induced changes to the shaft via telemetry with transducers connected on the shaft. The application of acoustic wave technology to shaft parameter sensing is relatively new and the present systems are being used for smaller shafts that have high manufacturing tolerances.
The elastic torque systems measure the twisting of the shaft by using markers across a length of the shaft and measuring the angular displacement. This system has accuracy issues when applied to large diameter shafts, and there are practical implementation problems. Moreover, making direct use of the magnetostrictive effect for measuring shaft parameters in ferromagnetic material requires complex sensor arrangements, difficult calibration procedures and typically results in limited accuracy.
Accordingly, there exists a need for efficient non-contact measurement of high resolution rotating shaft parameters in a harsh environment to address the design and operation of equipment utilizing rotating shafts.
In accordance with an embodiment of the invention, a method for non-contact measurement of multiple shaft parameters is provided. The method includes magnetically encoding multiple sections of the shaft using pulsed currents. The method also includes sensing a magnetic field of the encoded sections of the shaft using multiple sensors arranged circumferentially about the shaft. Further, the method includes generating a spectrum of periodical variations based on sensed magnetic field of the encoded sections of the shaft during rotation and determining multiple parameters based on recurrences of patterns of the spectra.
In accordance with another embodiment of the invention, a system for measuring a plurality of operating parameters of a shaft is provided. The system includes multiple magnetic encoded regions circumferentially disposed about a shaft. The system also includes one or more sensors proximate to the shaft and at least some of the encoded regions, wherein the sensors enable measurement of magnetic field properties of the encoded regions. Further, the system includes a processor for processing the magnetic field properties and computing shaft parameters.
In accordance with yet another embodiment of the invention, a method for encoding of magnetic sections in a shaft is provided. The method includes disposing at least one conducting member in close proximity about a section of the shaft. The method further includes disposing electrodes onto the shaft proximate to a first end and a second end of the conducting member. The method includes providing a second end electrode coupled to the second end of the conducting member. The method also includes electrically coupling a first end electrode to a current source and further coupling the current source to the first end of the conducting member. Finally, the method includes applying unipolar current pulses to the conducting members thereby inducing sectional encoding regions.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, embodiments of the invention are directed towards a non-contact measurement of rotating shaft parameters. As used herein, the phrase ‘magnetic encoding’ refers to magnetization of a section of shaft by flowing current in an axial direction of the shaft. The present invention addresses a system and method of sectional magnetic encoding and sensor arrangement about the magnetic encoded shaft for measuring various shaft parameters during rotation of the shaft.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters are not exclusive of other parameters of the disclosed embodiments.
The encoding can be done during manufacture of the shaft 12 or post installation and is permanent when applied to the right type of material and created with high current densities. In one embodiment as illustrated, the encoding structure 14 is depicted as encircling the shaft 12 and may include additional frame elements (not shown) to maintain its orientation and position about the shaft 12. This may include frame supports (not shown) to ensure the conducting members 18, 20 are arranged properly and sufficient for the encoding operation. While the encoding structure 14 goes around the shaft 12, there is no requirement for the encoding structure 14 to encircle the shaft with conducting members. In a further embodiment, the encoding structure 14 is located proximate to a portion of the shaft 12 and may include multiple encoding structures arranged about the shaft such that each of the encoding structures 14 generates magnetic polarized regions.
The conducting members 18, 20 are disposed proximate the shaft 12 with a gap between the members 18, 20 and the exterior surface of the shaft 12. Non-limiting example of the conducting members 18, 20 include reinforced isolated copper bars. The use of any other suitable conductors for the conducting members 18, 20 are also within the scope of the encoding system 10. Further, non-limiting examples of the conducting members 18, 20 can be bars with a shape that can be round, oval, square or rectangular. The length of the conducting members 18, 20 can vary depending upon the design criteria. Longer conducting members 18, 20 can provide greater surface area for sensing. The diameter of the conducting members 18, 20 should have sufficient rigidity and provide for the required current pulses.
The conducting members 18, 20 include a first end 19 and a second end 21. Each of the first end 19 and the second end 21 is coupled to an encoding source 26 and the shaft 12 respectively. In order to electrically couple the conducting members 18, 20 to the encoding source 26, electrical connectors 22, 24, 28, 30 are provided at the first end 19 of the conducting members 18, 20. The encoding system 10 includes multiple first end electrodes 32 that are used to establish electrical connections from the shaft 12 to the encoding source 26. The system 10 further includes multiple second end electrodes 34 that are used to establish electrical connections from the second end 21 of the conducting members to the shaft 12. The first and second end electrodes 32, 34 refer to the electrical coupling to the shaft 12. In one embodiment, the first end electrodes 32 are conductive elements coupled about the non-conductive frame 16 that contact the shaft 12. In another embodiment, the second end electrodes 34 refer to a conductive element that extends from the second end 21 of the conducting members 18, 20 to the shaft 12. The electrodes may also be points of contact with jumpers or wires that connect to the shaft 12.
Furthermore, the first end 19 of the positive conducting members 18 is coupled to the positive terminal of the encoding source 26 along the positive electrical connector 22. The negative terminal of the encoding source 26 is coupled to the electrode 32 and the shaft 12 via an electrical connector 24. For the negative conducting members 20, the negative terminal of the encoding source 26 is coupled to the first end 19 of the negative conducting member 20 along electrical connector 30. The positive terminal of the encoding source 26 is connected to the electrode 32 and the shaft 12 via electrical connector 28 with regard to the conducting member 20.
In one embodiment, an electrical current 36 as shown, travels through the shaft 12 such that magnetized regions are generated on the shaft 12. One of the features of the encoding system 10 is the ability to magnetically encode channels or magnetic polarization regions in the shaft 12. In particular, ferromagnetic steel shafts have a high relative permeability and the electric currents that travel through the steel shaft create distinct encoded channels. As illustrated, the positive polarity current pulse at electrical connector 22 is coupled to the conducting member 18 and the current pulse travels along the conducting member to an electrode 34 that contacts the shaft 12 about the second end 21. The current discharged by the second end electrode 34 travels back along the shaft 12 to the first end electrode 32 and the negative connection of the encoding source 26 via the electrical connector 24. The electrical current 36 flowing along the shaft 12 creates a polarized magnetic channel on the shaft 12. Each of the adjacent conducting members in the encoding structure 14 have alternating polarities and the pulse encoding may be used to simultaneously encode the conducting members all at one time, grouped, or individually. For example, the first set of positive conducting members 18 can be encoded simultaneously followed by the negative set of conducting members 20.
In one embodiment, the conducting members 18, 20 include rigid or semi-rigid bars that define a path for the current flow in a longitudinal direction, circumferentially or diagonally along the shaft 12. According to one embodiment, a cage assembly is utilized to position such conducting members about the shaft in a secure manner for the encoding process. In another embodiment, the cage with the conducting members is affixed about the shaft 12 such that the shaft 12 and the cage are in a fixed relationship to each other until encoding is completed.
In one embodiment, the encoding system employs four conducting members uniformly distributed approximately ninety degrees apart. Such an encoding system including the shaft may have four segments. One encoding source can be used to apply the current pulses to each of the four current encoding sources with alternating polarities. In another embodiment, there are four separate encoding sources, thereby avoiding short-circuits between the different encoding currents during the encoding process. In another example, a switching scheme can be employed to apply the current pulse signals with alternating polarities.
While the conventional techniques rely upon total circumferential shaft magnetization, one embodiment of the system herein encodes magnetic channels in the shaft using the return currents. The sectional magnetic encoding takes advantage of the asymmetrical skin effect and the fact that a current always takes the path of least impedance. The impedance is dominated by inductance if the frequency of the current is high enough. In the case of a short current pulse the return current flowing in the shaft will be more localized than in the case of a longer pulse, enabling polarized and well defined/narrow magnetic patterns. This effect is used to magnetize sections of a shaft with more localized channels that lead to faster changes in the magnetic field during sensing. Therefore, the pulse length during encoding affects the signal frequencies observed during sensing application.
In one embodiment, the current pulses are generated by discharging a capacitor bank, wherein the size of the discharge resistor determines the discharge time constant and therefore the depth of the current penetration. In another embodiment, the sectional encoding method in one example uses five consecutive 500 A current pulses with a pulse length of about 5 ms each to generate permanent magnetic flux densities of about 5 Gauss used to encode an industrial steel shaft with a diameter of 60 mm. The shorter the DC current pulse length, the higher the current and flux density near the shaft surface. This is advantageous for measuring shaft parameters based on magnetic field measurements because the highest magnetic flux densities are created close to the shaft surface, in about one or more millimeters radial distance to the magnetic field sensors.
According to a simple encoding approach in one embodiment, a magnetized section is encoded one circuit at a time. For example, a positive polarity current pulse can be applied to encode a first encoded section followed by another section magnetized by applying a second circuit with a negative polarity. Subsequent sections are encoded using alternating polarity current pulses.
Such a sequential encoding process with alternating polarity current pulses creates multiple almost identical encoded sections. If only one current pulse is applied to each section to be magnetized, the sections are generally not identical because magnetizing the second section also affects the first magnetized section. This undesired interaction is higher in the middle of the encoding tool than at the beginning and end of it, where the electrodes contact the shaft. Almost identical encoded sections can be achieved in performing sequential current pulses, alternating the sections while magnetizing and by performing the magnetic field measurements close to the regions where the electrodes contact the shaft. Another example for sequentially creating magnetized zones in the shaft measures the field strength created in each segment or zone and adapts the amplitude of the current pulses for the subsequent encoding steps. To avoid that the influence of sequential magnetization of one section by the next magnetization, another encoding embodiment is to apply the same current amplitude to all the conducting members and encoding all the sections at once. In one embodiment, the conducting members would use separate or split encoding sources to accommodate the multiple conducting members. In one example, separate capacitor banks would be used for each conducting member.
As illustrated, the number of magnetically encoded polarized regions 106 depends upon the encoding and the design criteria, such as the diameter of the shaft 102. However, the number of sensors 106 disposed around the magnetic encoded shaft 102 may vary depending upon the need for acquiring sampling rates for processing the measured magnetic field. In one embodiment, as shown in
Furthermore, the processing of the measured magnetic field is typically carried out using a processor (not shown). The processor is further configured to compute various shaft parameters such as angular velocity, angular acceleration, rotational frequency spectrum and torque of the shaft. Various rotational anomalies such as vibration, shock, misalignment and imbalance in the rotating shaft can also be computer by the processor. Thus, the processor may compute the sensor outputs to measure shaft parameters with high long-term stability. It should be noted that embodiments of the invention are not limited to any particular processor for performing the processing tasks of the invention. The term “processor,” as that term is used herein, is intended to denote any machine capable of performing the calculations, or computations, necessary to perform the tasks of the invention. The term “processor” is intended to denote any machine that is capable of accepting a structured input and of processing the input in accordance with prescribed rules to produce an output. It should also be noted that the phrase “configured to” as used herein means that the processor is equipped with a combination of hardware and software for performing the tasks of the invention, as will be understood by those skilled in the art.
According to another embodiment, the measuring system 300 deploys sensors such as sensing coils and therefore obtains greater frequency of readings and greater reliability. The sensing coils are relatively inexpensive and multiple coils can be easily deployed in a sensor holder 310. In one example, there are multiple sensors 306 disposed within the sensor ring assembly 304. According to one embodiment the multiple sensors 306 are used to provide greater reliability by allowing more frequent measurements. The multiple sensors 306 can also be used to provide redundancy so that the sensing functions are operable even with some sensor failure. In another example, different types of sensors are deployed such that different types of data can be measured. The multiple sensor types can take advantage of the sensing properties of the particular sensor or otherwise allow for enhanced sensing functionality. In a further embodiment, the sectional encoding process includes different encoding sections having different encoding properties such that the sensors can obtain multiple forms of data.
Advantageously, the encoding method and system according to one embodiment enables highly accurate measurements of angular velocity, angular acceleration, rotational frequency spectrum, direct power, torque, and/or bending moment for rotating machinery. The present invention makes it possible to locate the sensor electronics at some distance from the sensors and up to several meters away from the sensor installation to enable measurement in harsh environments (oil, grease, dust, temperatures of about 300 degrees, etc.). The magnetic encoding within the shaft thus remains unaffected due to pollution. Also, the magnetic field sensors may have encapsulation or metallic sensor holder for screening against external magnetic field components. Further, one of the features of the system detailed herein is the non-contact measurement of speed, acceleration, shaft power, torque and/or rotational anomalies based on sensing AC field components with respect to the shaft. High sampling rates can simply be achieved in shafts rotating at high speed, for example, high-speed electrical machines (e.g. 20000 rpm) where no attachments to the rotor are desired. Moreover, the present invention is applicable to measuring high-resolution speeds.
Furthermore, the present invention also provides a non-contact measurement system as nothing is attached to the rotating shaft during operation. This non-contact system enables direct monitoring of the shaft power that is extremely relevant for detecting efficiency decreases in different sections of a shaft system such as large turbine trains.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
