Abstract:
A monitoring device adapted to monitor a condition includes a sensor that produces an output signal representative of the condition, a filter configured to at least partially operate on the output signal, a sampling arrangement adapted to sample the output signal at a predetermined frequency to thereby collect a plurality of samples, and an analysis arrangement adapted to at least partially analyze the plurality of samples to thereby produce data. The filter has a controllable knee the knee is related to the sample frequency.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a non-provisional of, and claims the benefit of, co-pending, commonly-assigned, Provisional U.S. Patent Application No. 60/621,510 (Attorney Docket No. 040050-002400US) entitled “DIGITALLY SYNTHESIZED ACQUISITION,” filed on Oct. 21, 2004, the entirety of which application is incorporated herein for all purposes.  
         [0002]     This application is related to co-pending, commonly-assigned, Provisional U.S. Patent Application No. 60/624,637 (Attorney Docket No. 040050-002300US) entitled “SENSOR ANALYSIS MONITOR,” filed on Nov. 2, 2004, the entirety of which application is incorporated herein for all purposes. 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0003]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00024-02-C-4124 awarded by the Navy. 
     
    
     BACKGROUND OF THE INVENTION  
       [0004]     Embodiments of the present invention relate generally to monitoring systems. More specifically, embodiments of the invention relate to sensors and associated processing systems.  
         [0005]     Industrial complexes often require a variety of parameters to be measured and analyzed. This promotes safety, security, efficiency, and a number of other desirable features. The more efficiently the parameters are measured, the more efficient the complex operates, generally.  
         [0006]     Many complexes are distributed over vast distances. Many locations that require monitoring have limited access to power and communications infrastructure. Hence, for these and other reasons, it is desirable to have adaptable sensors capable of operation in such environments.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Embodiments of the invention thus provide a monitoring device adapted to monitor a condition. The monitoring device includes a sensor that produces an output signal representative of the condition, a filter configured to at least partially operate on the output signal, a sampling arrangement adapted to sample the output signal at a predetermined frequency to thereby collect a plurality of samples, and an analysis arrangement adapted to at least partially analyze the plurality of samples to thereby produce data. The filter has a controllable knee and the knee is related to the sample frequency.  
         [0008]     In some embodiments the monitoring device includes a Direct Digital Synthesizer configured to generate a clock signal determinative of the sample frequency and the knee. The filter may be a low-pass filter. The filter may be an anti-aliasing filter. The device may include a storage arrangement configured to at least temporarily store the data and a communications arrangement configured to transmit at least a portion of the data. The communications arrangement may be a wireless communication arrangement. The condition may be vibration and the sensor may be an accelerometer. The data may relate to a frequency of the vibration. The data may relate to an amplitude of the vibration. The data may relate to a phase of the vibration. The analysis arrangement may be configured to perform a Discrete Fourier Transform on the plurality of samples. The device may include a battery for powering the monitoring device. The device may include a digital to analog converter for converting the output signal into a digital data stream. The data may have at most three states indicative of the condition. A first state may be a normal state and a second state may be an upset state.  
         [0009]     In other embodiments, a data processing circuit includes a data sampling arrangement configured to sample a signal at a sample frequency to thereby produce a plurality of samples, a processing arrangement configured to at least partially process the plurality of samples into data, and a filter configured to operate on the signal. A corner frequency of the filter is at least partially adjustable by the processing arrangement. The corner frequency and the sample frequency are related.  
         [0010]     The data processing circuit may include a Direct Digital Synthesizer configured to generate a clock signal determinative of the sample frequency and the corner frequency. The processing arrangement may be further configured to perform a Discrete Fourier Transform on the plurality of samples. The circuit may include a communications arrangement configured to wirelessly communicate at least a portion of the data to a remote receiver. The data may include at most three states indicative of a condition monitored by the monitoring arrangement. The sample frequency may be a multiple of the corner frequency.  
         [0011]     In still other embodiments, a monitoring device includes an accelerometer configured to monitor vibration and produce a signal representative of the vibration, a programmable low-pass filter configured to receive the signal and produce a conditioned signal, an analog-to-digital converter configured to produce a digital data stream relating to the conditioned signal, and a processor configured to at least partially process a sampling of the digital data stream by performing a Discrete Fourier Transform of at least a portion of the digital data stream to thereby produce data, control the low-pass filter, via a direct digital synthesizer in response to the data, and at least temporarily store the data. The monitoring device may include a wireless communications arrangement configured to transmit at least a portion of the data to a remote receiver. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     A further understanding of the nature and advantages of the present invention may be realized by reference to the figures which are described in remaining portions of the specification. In the figures, like reference numerals are used throughout several figures to refer to similar components. In some instances, a sub-label consisting of a lower case letter is associated with a reference numeral to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sub-label, it is intended to refer to all such multiple similar components.  
         [0013]      FIG. 1  provides a distributed monitoring system according to embodiments of the invention.  
         [0014]      FIG. 2  illustrates a vibration sensor according to embodiments of the invention, which vibration sensor may be employed in the system of  FIG. 1 .  
         [0015]      FIG. 3  illustrates an exploded view of the vibration sensor of  FIG. 2 .  
         [0016]      FIG. 4  illustrates a pressure sensor according to embodiments of the invention, which vibration sensor may be employed in the system of  FIG. 1 .  
         [0017]      FIG. 5  illustrates an exploded view of the pressure sensor of  FIG. 4 .  
         [0018]      FIG. 6  illustrates a temperature sensor according to embodiments of the invention, which vibration sensor may be employed in the system of  FIG. 1 .  
         [0019]      FIG. 7  illustrates an exploded view of the temperature sensor of  FIG. 6 .  
         [0020]      FIG. 8  illustrates a first exemplary acquisition circuit according to embodiments of the invention, which acquisition circuit may be used with the vibration sensor of  FIG. 2 .  
         [0021]      FIG. 9  illustrates a second exemplary acquisition circuit according to embodiments of the invention, which acquisition circuit may be used with the vibration sensor of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the invention. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment of the invention. It is to be understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention as set forth in the appended claims.  
         [0023]      FIG. 1  illustrates a distributed monitoring system  100  according to embodiments of the invention. Those skilled in the art will appreciate that the system  100  is merely exemplary of a number of possible systems according to embodiments of the invention. The system  100  may be distributed across a vast geographical area or may be locate within a single facility. The system includes a data processing server  102  and a network  104  through which data are collected. The system  100  also includes a database for housing data. The data processing server  102  may be any of a variety of well-know computing devices, including, for example, a server, a workstation, a personal computer, a mainframe, and/or the like. The network  104  may be a Wide Area Network, a Local Area Network, the Internet, and/or any of a number of other types and varieties of networks, as is apparent to those skilled in the art. The database  106  may be nay of a variety of storage systems, including, for example, magnetic, optical, solid state, and/or the like.  
         [0024]     The system  100  includes two monitored devices  108 , although other exemplary systems include may additional monitored devices. The monitored devices  108  may be, for example, tanks, piping systems, processing systems, fluid and gas systems, electrical systems, and/or the like. Sensors  110  are placed at various points on the monitored devices  108  to collect, and in some cases process, data. As will be explained in more detail hereinafter, the sensors may be, for example, temperature sensors, pressure sensors, vibration sensors, and/or the like.  
         [0025]     The sensors  110  transmit data to the processing server  102  through any of a variety of paths. For example, the sensors  110 - 1  transmit information via a generally terrestrial path. Signals from the sensors  110 - 1  are transmitted via a land-based receiving tower  112  that may be hard wired to the network  104 , although retransmitting wireless signals is also a possibility. The sensors  110 - 2  transmit signals to a satellite  114  that retransmits the signals to a ground-based receiver  116 . Those skilled in the art will appreciate many such examples in light of the disclosure herein.  
         [0026]     The signaling of the system  100  may follow any of a variety of protocols. For example, the sensors  110  may be polled periodically by the data processing server  102 . When a sensor  110  is interrogated, it responds with either real-time or stored data. In some embodiments, the data is a compilation of processed data, while in other embodiments, the sensor  110  transmits raw data. In some embodiments, sensors are configured to transmit upon the detection of an upset condition or upon the occurrence of any of a number of predetermined events. In still other embodiments, the sensors  110  transmit data according to a predetermined schedule. Although impractical, some sensors may be configured to transmit continuously. In any of the foregoing, handshaking may be employed to ensure data are received. Many other examples exist, including combinations of the foregoing.  
         [0027]     As previously stated, some sensors  110  may do some level of pre-processing. Such sensors have the advantage of low power utilization, since data transmission is typically the highest power consumption function the sensors perform. This will be described in greater detail hereinafter.  
         [0028]      FIGS. 2 and 3  illustrate a vibration sensor  200  according to embodiments of the invention. Any of the sensors  110  in the system  100  may be a vibration sensor. The vibration sensor  200  is a self-contained unit that can be attached to anything that might move or vibrate. Movement is sensed by an accelerometer. The sensed movement is processed by circuitry within the sensor and transmitted to other networked devices or a central processor, i.e., the data processing server  102 . Acceleration may be measured in one, two or three axis, depending upon the embodiment.  
         [0029]     The vibration sensor includes an acquisition module  202  and a sensor module  204 . The acquisition module  202  includes a body  206 , an acquisition module cover  208 , an antenna cover  210 , and fastening hardware  212 . A gasket  214  may be placed between the cover  208  and the body  206  to form a weather tight seal. The body  206  forms two compartments: a battery compartment  216  and an electronics compartment  218 . The battery compartment  216  houses a battery  220 . The electronics compartment  218  houses one or more printed circuit boards (PCB)  222 . An antenna  224  is attached to one of the PCB  222 .  
         [0030]     The sensor module  204  includes a sensor cover  224  and fastening hardware  226  that attaches the cover  224  to the body  206 . A gasket  228  may be included. The sensor module  204  houses an accelerometer  230  that is held fast with a hold down ring  232 . The PCB is designed with flexible material such that a direct connection can be made with the accelerometer.  
         [0031]     In a specific embodiment, the vibration sensor includes two PCBs and an 800 mAH battery in a single Li-ION cell. The first circuit card includes a radio and processor. The second circuit card includes circuitry to condition and bias the accelerometer. In other embodiments, the various circuit elements could be divided in any way between the two PCBs. A connector couples the first and second circuit cards together, and a flex circuit couples the PCBs to the battery.  
         [0032]     As will become apparent from the ensuing description, the acquisition module  202  generally may be considered “universal” in that core components of the acquisition module  202  are adaptable for use with a variety of sensor modules. For example, the body  206 , acquisition module cover  208 , antenna cover  210 , and fastening hardware  212 , are the same components in each of the sensors herein described. Further, the body  206  has a form factor in which standard components having a compatible form factor may be housed. For example, while different batteries, antennas, and PCBs may be used with different sensors, the specific battery, PCB, and sensor used may be chosen or designed to fit within the standard acquisition module  202 . The acquisition module  202  may be configured for integration with any of a variety of sensors, including, for example, a pressure sensor or a temperature sensor, as will be described. Other parameters that may be monitored include speed, distance, illumination, acidity, time, location, depth, fill level, motion, and/or the like.  
         [0033]     The body  206  may be made of a durable material suitable for the environment in which the sensor  200  is to be deployed. For example, the body may be made of stainless steel, titanium, carbon fiber, any of a variety of plastic materials, other metals, and the like. In a specific embodiment, the body displaces no more than 1.62 cubic inches. Various other embodiments could displace 1 or more, 2 or more, 3 or more, or 4 or more cubic inches.  
         [0034]     In a specific embodiment, the battery  220  is not rechargeable, but provides months or years of power depending on the frequency of sensor measurements and/or radio communications. In other embodiments, the battery  220  could be rechargeable. Some embodiments could include a photovoltaic cell to recharge the battery.  
         [0035]     Since frequent radio transmissions generally decrease battery life, radio transmissions may be made infrequent by performing some amount of analysis within the sensor. The sensor can be configured to receive programming, configuration, and/or firmware updates via wireless transmission. In a specific embodiment, transmissions are via unlicensed 900 Mhz frequency band, but other embodiments could use any licensed or unlicensed frequency.  
         [0036]      FIGS. 4 and 5  illustrate a pressure sensor  400  according to embodiments of the invention. The pressure sensor  400  is self-contained and can be mounted anywhere a pressure reading is desired. The pressure can be measure in dry or wet environments. The measured data can be radio-transmitted to a network of other devices. The pressure sensor  400  includes an acquisition module  402  and a sensor module  404 . The battery  420 , PCB(s)  422 , and antenna  424  are specifically designed to work with the sensor module  404 , but may share many common features with the analogous components of the vibration sensor  200 . The pressure sensing module  404  of the pressure sensor assembly  400  is attached to body using an adapter bushing  446  and screws  450 . The actual pressure sensing mechanism makes use of a standard resistive bridge and diaphragm configured in a custom form factor,  440 . Gaskets  448  and  446  may be included.  
         [0037]      FIGS. 6 and 7  illustrate a temperature sensor  600  according to embodiments of the invention. The temperature sensor  600  may be self-contained or may be configured to connect to external temperature sensing devices via a hardwired connection. A connector is provided  640  to accommodate an external temperature sensing probe. The temperature sensor can measure temperature in a dry or wet environment. The temperature sensor  600  includes an acquisition module  602  and a sensor module  604 . The battery  620 , PCB(s)  622 , and antenna  624  are specifically designed to work with the sensor module  604 , but may share many common features with the analogous components of the vibration sensor  200  and the pressure sensor  400 . The temperature sensor module/interface module  604  of the temperature sensor  600  includes a probe connector  640 . The temperature sensor module  605  is attached to the body  206  using fastening hardware  644 . A gasket  642  may be included.  
         [0038]     Having described several sensors according to embodiments of the invention, attention is directed to  FIG. 8 , which illustrates a block diagram of a vibration acquisition and analysis circuit  800  according to embodiments of the invention. The circuit  800  may be employed in the vibration sensor  200 , as will be appreciated by those skilled in the art. The circuit  800  is configured to process measurements to thereby decrease the amount of information to be transmitted, which conserves power.  
         [0039]     In some embodiments of the vibration acquisition and analysis circuit  800 , it is desirable to reduce the data set to a minimum without compromising the analysis. This reduces the computation energy and/or reduces the amount of data to be transmitted if raw samples or resulting discrete Fourier Transform (DFT) set is to be sent by radio, both of which increase battery life. Longer battery life leads to lower maintenance. For example, halving the battery energy used with each computation or transmission may increase the maintenance period by up to a factor of two, e.g. typically from 6 months to one year.  
         [0040]     In such embodiments, a DFT is generated from a set of equally spaced samples of instantaneous signal taken from an accelerometer, which samples are transformed to yield the amplitude of a frequency of period equal to the full sample set duration, and all harmonic frequencies thereof up to a frequency of period equal to just two sample periods. These values at each of the discrete set of frequencies are also often known as bins. A typical sample set comprises 256 samples, and after the DFT transform provides amplitudes of a frequency of period spanned by the 256 samples and 127 harmonics thereof. If for example the sample rate were 256 k samples/sec, then the DFT would yield amplitude of signals at 1 kHz, 2kHz, etc., up to 128 kHz.  
         [0041]     In analyzing vibrations from a rotating machine, it is assumed that the vibrations are primarily at the rotation frequency and/or harmonics thereof. The DFT must be computed so as to provide sufficient resolution and information about the rotation frequency and its harmonics. Since the DFT only analyzes a discrete set of frequencies, an arbitrary sample set must be large and finely spaced to ensure the resulting DFT is sufficiently detailed to resolve the frequencies of interest.  
         [0042]     It is useful, then, to make the sample set duration exactly equal to one rotational period for the machine. Then the resulting DFT evaluates exactly the harmonics of the rotational frequency, which is exactly what is required, without any superfluous data. Conversely, if one uses an arbitrary predefined and fixed sample rate one must ensure that the sample set (a) spans the slowest expected rotational period and (b) is of sample rate faster then twice the maximum required harmonic at the maximum rotational speed. If the rotational speed is not taken into account, this might require a huge, detailed sample set to be collected and analyzed.  
         [0043]     In embodiments of the present invention, the sampling clock is precisely variable so that it may be set at a precise multiple of the rotational speed. Aligning the fundamental rotational frequency within a given bin is an iterative process that can be done with little user interaction, assuming you have sufficient ADC clock granularity. Essentially the aligning algorithm is written to maximize the amplitude of the fundamental frequency in the desired bin by taking several sample sets and slightly varying the sample rate until the amplitude peaks. A typical sample set of 256 samples is then adequate for deriving the amplitude of harmonics up to the 127 th . Conversely, if the machine speed range is known only within a factor of, say, eight, the sample set would need to be 2048 samples in order to achieve the same information if the sampling frequency were not adjusted as described herein. DFT computational time and energy typically increases as the square of the number of samples. Thus it causes a considerable penalty in the use of valuable battery energy, and in the above examples the non-frequency locked design could use 256 times as much computational energy as the frequency locked design.  
         [0044]     In the exemplary embodiments of  FIGS. 8 and 9 , the sampling frequency is generated from a crystal stabilized clock reference by a direct digital synthesizer (DDS) circuit, which is generally available as a power efficient integrated circuit device ( 808  and  908 ). DDS IC allows a simple processor to provide sub-hertz sample rate resolution. Using a DDS the sampling frequency may be digitally programmed into the DDS, for example from a simple microprocessor ( 804  and  902 ), with great accuracy and resolution. For precision in the sampling process it is necessary to low-pass-filter the incoming waveform to attenuate frequencies above the maximum DFT frequency (i.e. half the sample rate) to avoid aliasing distortion. In the current implementation the sampling rate is variable and so the low-pass-filter frequency must also be varied to track the sampling rate. This is achieved using a digital filter whose characteristic frequency is controlled by an input clock frequency. In this case the input clock frequency is also taken from the output of the same DDS as generates the sampling rate clock so that it tracks the sampling rate exactly.  
         [0045]     There is a further reason for aligning the rotational frequency within the DFT frequency bins. In order to make an assessment of overall machine health, a good baseline sample set is typically needed. This reference set is more precise if the fundamental rotation frequency accurately aligns and peaks within the bins. With an accurate baseline and the ability to perform bin alignment, accurate comparative measures can be taken at any time in the future, or from other similar systems, and even to some extent from systems operating at different rotational rates. The system can then assess how the various harmonics deviate with respect to the baseline. Thresholds can be set to trigger alarms or warnings based on these changes. This ability allows a simple wireless device to transmit minimal data in the form of alarms or warnings, thus minimizing data transmission and maximizing battery life. Proper sample rate resolution and bin alignment allows the use of historical data in the analysis process. Using a DDS as the clock source in a wireless sensor analysis monitor permits the required frequency setting precision in a design that is compatible with the miniature, very low power requirements of a battery operated wireless sensor analysis monitor.  
         [0046]     In a specific embodiment, the circuit  800  accomplishes this by performing a DFT on samples taken by an accelerometer to determine the frequencies of vibration sensed by the accelerometer. A direct digital synthesis circuit controls both a low pass filter, or anti-aliasing filter, and an analog-to-digital converter (ADC).  
         [0047]     The circuit  800  includes an accelerometer  824  and an adjustable low-pass filter  820 . The knee of the filter is adjusted by a direct digital synthesis circuit  808  under the direction of a processor  804 . The measurements passed by the filter  820  are converted to digital signals by an analog-to-digital converter  812 . The signals are thereafter passed to a processor  804 .  
         [0048]     The processor  804  receives the digital stream and performs a DFT. The processed signals may then be stored in memory  836  and/or transmitted by a radio  832 . The circuit is powered by a battery  828 .  
         [0049]     Having described a general embodiment, attention is directed to  FIG. 9 , which illustrates a specific example of a vibration acquisition and analysis circuit  900  according to embodiments of the invention. In this embodiment, the output of an accelerometer  924  is conditioned in an amplifier  940 , the gain of which is adjustable by a controller  902 . The amplified output is passed to an anti-aliasing filter  920 , which performs a low pass filter. In this specific embodiment, the filter  920  is a switched capacitor filter. The knee of a switched capacitor filter is proportional to the rate of a source clock. In this embodiment, a direct digital synthesis (DDS) circuit  908 , also adjustable by the controller  902 , serves as the source clock. The output from the filter  920  is conditioned in another amplifier  944  before being received by the controller  902  for further processing.  
         [0050]     In this embodiment, the controller  902  includes a one megabyte memory  936 , a programmable divider circuit  916 , an ADC  912 , and a microprocessor  904 . The output of the DDS  908  is fed back into the divider circuit  916  which reduces the DDS  908  output clock rate so that a single clock source drives both the ADC  912  and the filter  920 . The filter  920  uses a faster clock rate than the ADC in this embodiment. The divider circuit  916  is selected such that the ADC clock rate is a multiple (e.g., 2×, 3×, 4×, etc) of the corner frequency of the filter. In this way, the DDS  908  controls both the filter  920  and the ADC  912  via the same signal.  
         [0051]     The ADC  912  converts the signal originating from the accelerometer  924  into a stream of digital samples. The microprocessor  904 , which is a 16 bit core with about 8 MIPS of processing power, receives the digital stream and performs a DFT to determine the frequency domain of the accelerometer signal. The precise control of the direct digital synthesizer  908  affords extreme accuracy in gathering frequency information. For example, a 0.1 Hz resolution for the sample rate can be performed in this embodiment.  
         [0052]     In some embodiments, the DFT alone does not necessarily reduce the transmitted data because the DFT generates as many values as the time domain data set. For example, a 1024 point DFT (1024 time domain samples) produces 512 real and 512 imaginary values from the DFT calculation. This results in zero savings in transmitted data if all the values are transmitted. If, however, the user wants only the magnitude or phase (mag=sqrt (realˆ2+imagˆ2), phase=arctan (imag/real)), the data set can be reduced in half. Additionally, knowing the frequency domain data allows, in some embodiments, a data set reduction by windowing (e.g., only a subset of frequencies are of interest), thresholding (e.g., only current values above or below×dB are of interest), and/or comparing (e.g., current frequency domain sample sets are of interest only if they exceed historical values by X). In some embodiments, no time or frequency domain data is sent. Instead, the sensor only reports good, bad, or marginal performance. In summary, performing the DFT onboard ‘enables’ the device to locally assess the condition of the machine being monitored by the acquisition circuit  900 .  
         [0053]     A radio  932  operates in a bi-directional manner. A remote radio can accept the processed frequency information from the radio  932  and can pass handshaking, configuration information, and the like to sensor via the radio  932 . An unlicensed spectrum in the 900 MHz range is used in this embodiment, but other embodiments could use licensed or unlicensed frequency ranges (e.g, 2.4 GHz, 5.8 GHz, etc.).  
         [0054]     A battery  928  powers the circuit  900 . In this embodiment, the battery is a 3 volt, lithium/manganese dioxide battery that has an 800 mAH capacity. The acquisition circuit  900  spends most of its time in a low power mode that draws about 25 micro-Amps. Periodically, the acquisition circuit  900  powers itself up, takes a reading from the accelerometer, processes that reading and forwards the processed frequency information to another device using the radio  932 . In powered mode, about 10-20 mili-Amps is consumed for a 1-3 second period before returning to low power mode. Using a battery of this type, hourly readings would allow the battery  928  to last about 2 months. Daily samples would allow the battery  928  to last about a year. Some embodiments can last up to five years on the same battery.  
         [0055]     Although this embodiment processes information from an acceleration sensor, other embodiments could process information from any type of sensor. For example, the sensor could measure pressure, temperature, flow, or other parameters.  
         [0056]     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.