Patent Abstract:
A vibration detector suitable for field use and associated systems and methods are disclosed. A representative apparatus includes a vibration sensor in contact with the vibrating structure. The vibration sensor can be in contact with the vibration isolators to eliminate the frequencies of the operator&#39;s hand. In some embodiments, a contact force between the vibration sensor and the vibrating structure can be measured using, for example, contact resistors. Since the sensitivity of the vibration sensor can be a function of the contact force, the vibration amplitude measurements can be adjusted for a known contact force to improve the precision of the vibration amplitude measurement.

Full Description:
TECHNICAL FIELD 
     The present technology is directed generally to vibration meters, and associated systems and methods. 
     BACKGROUND 
     Vibration can be an important consideration when designing, testing, and maintaining machinery. For example, significant levels of vibration can indicate poor design or an impeding failure of the machinery. The presence of unexpected frequency peaks in a vibrating structure may indicate nonlinear interactions among the natural frequencies of the subassemblies, which can cause premature failure of the machinery. In some applications, detecting an increase in vibration amplitude is a trigger for initiating equipment maintenance and/or service. 
     Vibration detection is often performed in the field by attaching one or more accelerometers or other vibration sensors to the rotating machinery or other vibrating structure. Vibration sensors produce output signals that can be used to determine the amplitude and frequency of vibration. It is known that contact between the vibration sensor and the vibrating structure can be improved by rigidly attaching vibration sensors (i.e., accelerometers) to the rotating machinery. In general, rigidly attaching the vibration sensor improves the transmission of vibrations from the vibrating structure to the sensor. However, such rigid attachment may not be possible or at least not practical with hand held vibration meters, which are preferred for the field use. Instead, a hand held vibration detector is typically kept by hand in contact with rotating machinery in the field to measure the vibrations. 
       FIG. 1  is a plan view of a conventional vibration meter  100 . In operation, a wand  3  of the vibration meter  100  contacts a vibrating structure  5 . The vibrations are transmitted through the wand  3  to an accelerometer  2  inside a housing  4 . When subjected to vibrations, the accelerometer  2  produces an output signal to be routed to electronics within the unit  100  via wires  6 . The unit  100  then determines vibration amplitude/frequency based on the signal coming from the accelerometer. The output (i.e., the amplitude and frequency of the vibration) can be displayed using display  7 . However, such a conventional device is sensitive to the quality of contact between the device and the vibrating structure. Therefore, the accuracy of reading for the vibration sensor in a hand-held device remains a problem. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Instead, emphasis is placed on clearly illustrating the principles of the present disclosure. Furthermore, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a plan view of a hand held vibration meter in accordance with the prior art. 
         FIG. 2  is a plan view of a hand held vibration meter in accordance with an embodiment of the presently disclosed technology. 
         FIG. 3A  is an exploded view of a hand held vibration meter in accordance with an embodiment of the presently disclosed technology. 
         FIG. 3B  is an isometric view of a force sensor assembly in accordance with an embodiment of the presently disclosed technology. 
         FIG. 4  is a graph of a sensitivity of the vibration sensor as a function of frequency. 
         FIG. 5  is a schematic diagram of the vibration sensor output linearization in accordance with an embodiment of the presently disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     Specific details of several embodiments of representative vibration meters and associated methods for vibration measurements are described below. In some embodiments of the present technology, the vibration meter is a hand held device that contacts a vibrating structure (e.g., rotating machinery) and measures vibrations of the vibrating structure. The vibration meter can also include a force sensor to measure force between the vibration sensor and the vibrating equipment. The measurement of force can be used to improve accuracy of the vibration reading (e.g., amplitude and frequency) because the output of the vibration sensor generally changes with the intensity of the contact force between the vibration meter and the vibrating structure. Therefore, in some embodiments, the output of the vibration sensor can be combined with the force reading to produce an adjusted output that automatically takes into account the contact force without further input from the user. Furthermore, one or more vibration isolators can contact the vibration sensor to filter the noise created by the unsteadiness or shaking of the hand of the operator, which, if unfiltered, would appear as low frequency vibrations at the output of the vibration sensor. A person skilled in the relevant art will also understand that the technology may have additional embodiments, and that the technology may be practiced without several of the details of the embodiments described below with reference to  FIGS. 2-5 . 
       FIG. 2  illustrates a vibration meter  200  in accordance with embodiments of the presently disclosed technology. The vibration meter  200  has a housing  70  that can be made from molded plastic or other suitable materials. In operation, the vibration meter  200  can be hand held against a vibrating structure  5  such that a vibrating sensor  10  contacts the vibrating structure. In other embodiments of the technology, the vibrating sensor may contact the vibrating structure  5  through an intermediary element (not shown) that transfers the vibrations from the vibrating structure to the vibrating sensor. The vibrating sensor  10  can be protected from mechanical or environmental damage by a jacket  25 , which can be made of rubberized molded plastic, metal, textile, or other suitable materials. In some embodiments, the vibrating sensor  10  can have a tip  11  that is harder than the rest of the vibrating sensor  10  to improve the contact between the vibrating sensor  10  and the vibrating structure  5 . A person skilled in the art would know of many examples of vibrating sensors including, for example, accelerometers. The vibrating sensor  10  is connected to signal processing electronics (not shown in  FIG. 2 ) inside the housing  70 . The signal processing electronics can determine the amplitudes and frequencies of the vibration based on the output from the vibration sensor  10 . For example, the amplitude of vibration can be determined by twice integrating the signal from the vibration sensor  10  (e.g., an accelerometer). A person skilled in the art would know of many methods for numerically or electronically integrating a vibrating sensor signal to determine the corresponding displacement of the vibrating structure under the measurement. Command buttons  80  and a display  90  can be used to select and display the frequencies and amplitudes of vibrations corresponding to the vibrating structure  5 . As discussed in detail with reference to  FIGS. 3A-5  below, the low frequency noise can be filtered from the vibration sensor  10 . Furthermore, the signal from the low frequency noise can be processed in conjunction with the signal from the force sensor to yield a more accurate vibration reading. 
       FIG. 3A  shows an exploded view of the vibration meter  200  configured in accordance with embodiments of the presently disclosed technology. Starting from the upper right corner of the figure, the vibration sensor jacket  25  can at least partially house the vibration sensor  10  for protection from environmental or mechanical damage. Additionally, in at least some embodiments, the vibration sensor  10  can be at least partially contacted by vibration isolators  16 ,  20 . With conventional hand held vibration meters, the vibration of the operator&#39;s hand can be transferred to the vibration sensor  10  and may be erroneously interpreted as being generated by the vibrating structure itself. The vibration of the operator&#39;s hand is typically in the low frequency range (e.g., less than about 50 Hz). In at least some embodiments of the inventive technology, the vibration isolators  16 ,  20  can filter out these low frequencies before they reach the vibration sensor  10 . The vibration isolators  16 ,  20  can be made of rubber-like material or other material that attenuates vibrations of the vibration sensor  10  for the frequencies of interest. For example, the rubber-like material can be selected based on its known frequency attenuating properties. The vibration isolators  16 ,  20  can have lips  17 ,  21 , respectively, for a more secure engagement with the vibration sensor  10 . An output signal from the vibration sensor can be transferred through a cable  11  to an interface board  61  and further to signal processing electronics (not shown). 
     The vibration meter  200  also can include a force sensor  30 . When the vibration meter  200  presses against the vibrating structure (not shown), the contact force is transferred from the vibration sensor  10  to the force sensor  30 , as explained in more detail with reference to  FIG. 3B  below. The force sensor can be supported by a structure, for example, a combination of load boss  65 , a load beam  55  and a load beam brace  50 , to keep the force sensor in place. Screws  70  can engage with receiving threaded holes  26  to hold the parts of the vibration meter  200  in contact. 
       FIG. 3B  shows an isometric view of the force sensor  30  positioned between a pad  45  and a plunger  40 . A person skilled in the art would know of many force sensors available on the market including, for example, load cells and film resistor force sensors. A plunger  40  having a generally flat first surface  41  can transfer contact force from the vibration sensor to the force sensor  30 , which can be sandwiched between the plunger  40  and the pad  45 . In some embodiments, the force sensor  30  can be preloaded to precondition its output within the range of sensitivity. The preloading can be achieved by, for example, pressing the plunger  40  against the force sensor  30  that is supported by the elastic pad  45  on the opposite side. When loaded, the force sensor  30  changes its electrical resistance. This change in the resistance, corresponding to the change in force, can be measured through a connector  35 . As explained below with reference to  FIG. 4 , the measurements of vibration amplitude can be improved based on the value of contact force between the vibrating structure and the vibrating sensor. 
       FIG. 4  is a graph of frequency response of the vibration sensor  10 . The horizontal axis of the graph shows a range of frequencies on a logarithmic scale. The vertical axis on the left shows a sensitivity of the vibration sensor in dB. The sensitivity of a vibration sensor can be interpreted as, for example, a ratio of the amplitude indicated at output of the vibration sensor  10  and the vibration amplitude of the vibrating structure itself. The sensitivity that is close to zero on the logarithmic scale of graph  300  corresponds to a sensitivity value of about one on the linear scale. Conversely, a positive value on the vertical axis indicates a higher sensitivity and a negative value on the vertical axis indicates a lower sensitivity. In general, the sensitivity of a vibration sensor is a function of the vibration frequency. Furthermore, if the sensitivity of vibration sensor is known, an appropriate coefficient or other adjustment can be used to determine the relevant vibration amplitude of the vibration structure at particular frequency of vibration. 
     The vertical axis on the right shows the vibration amplitude. Normally, the sensitivity of the vibration sensor as a function of frequency can be obtained from the vibration sensor manufacturer or it can be determined experimentally. Therefore, the amplitude of vibration can be back-calculated for a particular frequency of vibration. However, if the sensitivity of the vibration sensor is also a function of the contact force between the sensor and the vibrating structure, a measurement of the vibration amplitude that does not take the contact force into account may reduce the accuracy of the measurement. For example, curves F 1 , F 2 , F 3  in  FIG. 4  may correspond to the vibration amplitude measurements over a range of frequencies, but using different contact force. A person having ordinary skill in the art would know that for a given frequency of vibration an amplitude of vibration can be calculated by integrating the acceleration signal twice and by adjusting the result based on the known sensitivity of the vibration sensor. 
     In the illustrated example, for the frequency of vibration of about 1.4 kHz, the vibration sensor would indicate vibration amplitudes A 1 , A 2 , or A 3  for the respective sensitivity curves F 1 , F 2 , F 3 , depending on the magnitude of the contact force between the vibration sensor and the vibrating structure. To obtain more precise vibration amplitude measurements the contact force can be measured and used to select appropriate sensitivity curve, e.g., F 1 , F 2  or F 3 . The amplitude of the vibrating structure can then be determined from the appropriate sensitivity curve. For example, the force sensor  30  (described with reference to  FIGS. 3A-3B ) can measure contact force, which is in turn used to select the correct vibration sensitivity curve among the sensitivity curves F 1 , F 2  and F 3 . In at least some embodiments, the sensitivity curves can be available in a tabulated form for easier calculations per relevant frequencies of vibration. The tabulated sensitivity curves can be accessed using suitable electronics based on the force sensor reading, and then further processed to calculate the vibration amplitude using, for example, signal integrating algorithms. In some embodiments, the sensitivity curves can be linearized using linearization circuits. For example, the sensitivity curves F 1 , F 2 , F 3  can be linearized to yield linearized sensitivity curves L 1 , L 2 , L 3 , respectively. In at least some embodiments, the linearized sensitivity curves make the vibration amplitude calculation easier and/or faster. 
       FIG. 5  is a schematic diagram of a linearization circuit  500  in accordance with embodiments of the presently disclosed technology. A non-linear input  110  (for example the sensitivity curves F 1 , F 2 , F 3  shown in  FIG. 4 ) can be fed to a function generator  120  which outputs a function which can be attenuated by an attenuator  130 . Next, the non-linear input  110  and the output of the attenuator  130  can be summed up in a summing amplifier  140  to produce a linearized output  150  (for example the linearized sensitivity curves L 1 , L 2 , L 3  shown in  FIG. 4 ). The linearized output  150  can be used for easier determination of the vibration amplitudes. Many function generators and linearization circuits are commercially available on the market, for example function generators AD640, AD639, AD538 and linearization circuits AD7569 by Analog Devices, Norwood, Mass. 
     From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. For example, in some embodiments, a function analyzer can be used in conjunction with the disclosed technology to help in determining dominant frequencies. In other embodiments, the output of the vibration sensor can be acquired by an analog-to-digital conversion circuit for a subsequent data processing which may be done outside of the vibration detector. Furthermore, the vibration detector may include analog or digital frequency filters for eliminating the unwanted harmonics or subharmonics of the main frequencies of the vibration structure. Moreover, while various advantages and features associated with certain embodiments have been described above in the context of those embodiments, other embodiments may also exhibit such advantages and/or features, and not all embodiments need necessarily exhibit such advantages and/or features to fall within the scope of the technology. Accordingly, the disclosure can encompass other embodiments not expressly shown or described herein. The following examples provide further embodiments of the present technology.

Technology Classification (CPC): 6