Patent Description:
Flowmeters sense the flow of liquids or gasses in conduits and produce a signal indicative of the flow. The presence of an obstacle known alternatively as a shedding bar, bluff body, or vortex generator, in a flow conduit causes periodic vortices in the flow. The frequency of these vortices is directly proportional to the flow velocity in the flowmeter. The shedding vortices produce an alternating differential pressure across the bluff body. This differential pressure is converted to an electrical signal by piezoelectric crystals or other differential pressure devices. The magnitude of the differential pressure or electric signal is proportional to ρV<NUM>, where ρ is the fluid density and V is the fluid velocity. The vortex flowmeter produces pulses having a frequency proportional to the flow rate.

The vortex flowmeter is a measurement transmitter that is typically mounted in the field of a process control industry installation where power consumption is a concern. The vortex flowmeter can provide a current output representative of the flow rate, where the magnitude of current varies between <NUM>-<NUM> mA on a current loop. It is also desirable for the vortex flowmeter to be powered completely from the current loop so that additional power sources need not be used. <CIT> relates to a vortex flow meter and a method for measuring the quality of process and installation conditions of such a vortex flow meter. <CIT> relates to a measurement abnormality determination method. <CIT> relates to diagnostics of processes using a process variable sensor signal.

It is known to incorporate a microprocessor into a vortex flowmeter. The microprocessor receives digital representations of the output signal from the vortex sensor and computes desired output quantities based on parameters of the digital representation.

Certain conditions in the flow of the process fluid through the flowmeter can cause errors in flow rate measurements by the flowmeter. It would be desirable to detect such conditions and/or correct for errors caused by such conditions.

The invention provides a vortex flowmeter as recited in the independent claim. A vortex flowmeter for measuring a flow rate of a process fluid includes a vortex generator arranged to generate vortices in a flow of the process fluid. A vortex sensor is arranged to sense the vortices in the flow of the process fluid and responsively provide a sensor output related to the flow rate of the process fluid. Measurement circuitry is configured to receive the sensor output and provides a digital output. A memory is configured to store measurements based upon the digital output. Diagnostics circuitry coupled to the memory detects instability in the flow of the process fluid based upon the measurements stored in the memory; wherein the diagnostics circuitry is configured to correct for errors in a measurement flow rate based upon detected instability in the flow of the process fluid.

This invention relates to a vortex meter diagnostic that detects a flow of process fluid that is unstable. In particular, flows that are sufficiently unstable result in an erroneous measurement that can be detected. Most flow technologies require stable or slowly varying flowrates. By determining the variation in period for each shedding cycle, the device can alert an operator when the flow rate periodically varies too quickly to obtain an accurate flow measurement.

<FIG> illustrates an embodiment of a vortex flowmeter <NUM> of the present invention. Generally, the vortex flowmeter <NUM> includes a vortex sensor <NUM> that senses vortices <NUM> in a fluid <NUM> that flows through a conduit <NUM>. The vortex sensor <NUM> is operably coupled to an electronic circuit that produces a <NUM>-<NUM> mA current on a current loop <NUM> indicative of flow as well as an optional square wave output Fout (not shown) having a frequency proportional to fluid flow.

The vortex flowmeter <NUM> includes a vortex meter housing <NUM> having a bluff body <NUM> located therein. When the fluid <NUM> flows past the bluff body <NUM>, shedding vortices <NUM> having a frequency indicative of the flow rate are produced. A vortex sensor <NUM> preferably located at the bluff body <NUM>, senses a pressure difference associated with the shedding vortices <NUM>. The vortex sensor <NUM> can include, for example, a piezoelectric sensor. The sensor <NUM> has characteristics approximated by a potential source Es and a series capacitor Cs. The magnitude of the output signal from the piezoelectric sensor <NUM> is proportional to the differential pressure, which is proportional to the ρV<NUM>, where ρ is the fluid density and V is the velocity of the fluid <NUM>, and also proportional to ρD<NUM>F<NUM>, where D is the inside diameter of the meter housing <NUM> and F is the shedding frequency of the vortices <NUM>.

The output of the piezoelectric sensor <NUM> is coupled to an amplifier <NUM> which includes capacitor CF and a resistor RF. The amplifier <NUM> provides an analog output signal on line <NUM>. The signal on line <NUM> is provided to input circuitry <NUM> including an anti-aliasing filter <NUM> and an analog-digital (sigma-delta) converter indicated at <NUM>. The anti-aliasing filter <NUM> filters the signal from line <NUM> to remove unwanted high-frequency noise and performs anti-aliasing filtering.

The analog-digital converter <NUM> samples the signal from filter <NUM> and outputs a single bit datastream which is indicative of the amplitude and frequency of the vortices <NUM>. The relative number of ones and zeros, sometimes called the bit density, is representative of the amplitude of the vortices <NUM>. The digital datastream is transmitted across an electrical isolation barrier <NUM> required for sensors which are grounded or have leakage current to ground.

Digital filter <NUM> is an optional component and can be used for digitally preprocessing the digital data stream from the analog to digital converter <NUM>. A microprocessor <NUM> can be used to calculate an output signal related to fluid flow using the equations discussed in the Background section. Microprocessor <NUM> operates in accordance with instructions stored in memory <NUM>. The microprocessor <NUM> provides a desired output value to a digital to analog converter <NUM> for converting the digital value into a <NUM>-<NUM> mA current representation of flow of the process fluid. This current level is applied to the two-wire process control loop <NUM>. A digital communication circuit <NUM> can also be employed for sending information on the process control loop <NUM> related to flow using known formats. Communication circuitry <NUM> can be used for both sending and receiving data. A display <NUM> provides a user interface for the vortex flowmeter <NUM>. Power supply <NUM> is connected to loop <NUM> and can be used for providing power to the flowmeter <NUM>.

As discussed in the Background section, certain conditions and the flow of process fluid through the process piping <NUM> can cause errors in flow measurements. One such condition is instability in the process flow. For example, during the start up operations, some vortex flowmeters may produce erroneous readings due to unstable flow of the process fluid. This type of instability may be readily apparent when observing the output from the flowmeter. However, in certain situations, during steady state operation, the flowmeter may erroneously produce a stable output even though the flow itself is experiencing instability. For example, if during steady state operation the flow is widely varying, the flowmeter may not detect such instability if the flow variations are in a certain frequency range. For example, a <NUM> instability may go undetected. Such an instability can cause the flowmeter to report a flow measurement which is significantly less than the actual flow rate. This type of error is introduced independent of the technology used to measure the vortex shedding such as a piezoelectric sensor, differential pressure sensor, optical or acoustic based sensor, etc..

<FIG> is a graph of the output of a vortex sensor and is a graph of signal amplitude versus time. <FIG> illustrates a significant frequency in amplitude modulation that occurs in a cyclic manner with a time period of about <NUM> seconds.

<FIG> is an illustration of the signal of <FIG> converted into the time domain and is a graph of amplitude versus frequency and illustrates numerous peaks having roughly the same amplitude. Such a frequency domain signature in which numerous peaks are detected can be used by microprocessor <NUM> to provide an output indicating the occurrence of flow instability. (As illustrated in <FIG>, the output signal in the frequency domain should have a single peak.

In contrast to <FIG>, <FIG> is a graph of amplitude versus time for a stable, steady state flow of process fluid as sensed by a vortex sensor. <FIG> shows the frequency spectrum of the signal from <FIG> and is a graph of amplitude versus frequency. As illustrated in <FIG>, the flow signal is readily apparent at about <NUM>.

<FIG> shows flow rate calculated per pulse using the data sample illustrated in <FIG>. As illustrated in <FIG>, the calculated flow rate varies greatly between samples. Using this data, a histogram was created as illustrated in <FIG> of the shedding cycles collected over <NUM> seconds. <FIG> clearly illustrates a bi-modal distribution in the frequency of each shedding cycle. Thus, it is apparent that the flowrate <NUM> is constantly in a transient state. However, the output as determined by microprocessor <NUM> is stable because the average is roughly consistent over the <NUM> to <NUM> second measurement span. This introduces an error in the measured flow rate.

As such variations may introduce erroneous measurements, it is desirable to alert an operator that the vortex sensor may be providing such erroneous readings. In one configuration, in order to provide an alert regarding the periodic variations in flowrate, the shedding cycle period is measured and a number of such measurements are stored in memory <NUM>, as shown in <FIG>. These stored periods may then be examined using any number of statistical and/or signal processing techniques and a flag can be set to indicate flow instability. For example, if the threshold value is reached, the flowmeter <NUM> can use the digital communication circuitry <NUM> to provide an alert. Such alert may be provided over, for example, HART®, Fieldbus, Modbus, or other communication techniques. Similarly, in an SIF (Safety Instrumented Function) application, an alarm can be provided to indicate that the flowmeter <NUM> is operating outside of a safety accuracy range.

In one specific configuration, microprocessor <NUM> stores vortex shedding period information in memory <NUM>. Using this stored information, the microprocessor <NUM> calculates a standard deviation of the period using the collective samples. During typical steady state operation, the standard deviation for a vortex shedding period should vary between about four and seven percent depending upon the particular meter body design and flow regime. The standard deviation in other flowmeter configurations may range between eight and ten percent.

The microprocessors <NUM> can compare the calculated standard deviation to an acceptable threshold. If the standard deviation exceeds such a threshold, a warning can be provided. In another example configuration, if the standard deviation of the samples is greater than a percentage of the mean of the samples, for example <NUM> percent, a diagnostic flag can be set indicating flow instability.

In another example configuration, the collected period data is tested to determine if it is unimodal. Various tests are known in the art for detecting if a dataset is unimodal and include the use of the histogram illustrated in <FIG>. However, any appropriate test for unimodality may be employed. If unimodality is not detected through the test, a warning can be provided indicating that the process flow is in stable.

In another example configuration, the collected data is analyzed in the frequency domain, for example, using a fast Fourier transform performed by microprocessor <NUM>. In such a configuration, digital samples from the vortex sensor are stored in the memory <NUM> and used to perform the fast Fourier transform. Using the frequency domain data, changes in the shedding frequency over time with respect to frequencies which exceed a threshold level may be observed. For example, a threshold of <NUM>% of the highest peak may be employed within a measurable frequency range. This configuration eliminates structural resonances from causing a possible false alarm. Changes which are more rapid than a threshold value, for example, three times a standard deviation of the signal, can be used to trigger a flow instability diagnostic warning.

<FIG> is a simplified block diagram <NUM> showing steps implemented by microprocessor <NUM> in accordance with instructions stored in, for example, memory <NUM> for detecting flow instability using the above mentioned techniques. The block diagrams <NUM> starts at block <NUM> and block <NUM> vortex shedding period data is obtained. At block <NUM>, the obtained period data is stored in memory <NUM>. Blocks <NUM>-<NUM> may independently operate as illustrated by the dashed arrow whereby period data is continuously collected. At block <NUM>, the period data stored in memory <NUM> is analyzed using statistical or other analyzing techniques including both techniques implemented using time and/or frequency domains. The analyzed data is then compared to a baseline at block <NUM>. The baseline comparison can be through a simple threshold, a dynamic threshold, or a more complex baseline including a particular signature, data peak configuration, graphical analysis, etc. At block <NUM>, based on the comparison an output warning is provided at block <NUM> or controlled is passed to block <NUM> for further analysis. The configuration allows the data to be collected in the background and a rolling analysis performed on the stored data. The storing and analyzing of data can operate continuously or can be triggered based upon some event, for example, periodically, in response to the observance of a unusual peak or other signature in the data from the vortex sensor, receipt of a command from the process control loop, or some other mechanism to initiate the procedure.

Although in one configuration the diagnostics are performed by microprocessor <NUM> and optionally digital filter <NUM> within the flowmeter <NUM>, in another example configuration the computations are performed at a remote location, for example at a control room. In such a configuration the flowmeter may be configured to output raw data. This may allow more advanced diagnostics to be performed as the remote location will not be subject to the power restrictions found in a field device such as flowmeter <NUM>. In another example configuration, the diagnostics are only performed periodically or as desired. This allows the field device <NUM> to enter a high power mode in order to perform such diagnostics. In such a configuration, the power supply <NUM> may include some type of a power storage unit such as a capacitor or battery which is used to provide additional power during diagnostic computation.

If a sufficient amount of vortex shedding information is collected, it may be compared with known signatures. This comparison can be used to identify possible causes for the flow instability. For example, an oversized regulator may cause a particular type of flow instability. In such a configuration, the diagnostic output provided by the meter <NUM> can also include information related to the possible cause of the flow instability thereby allowing an operator to change the process configuration. For example, a randomly distributed signal may be an indication of a valve in a "noisy" control scheme.

Claim 1:
A vortex flowmeter (<NUM>) for measuring a flow rate of a process fluid (<NUM>), comprising:
a vortex generator (<NUM>) arranged to generate vortices (<NUM>) in a flow of the process fluid (<NUM>);
a vortex sensor (<NUM>, <NUM>) arranged to sense the vortices (<NUM>) in the flow of the process fluid (<NUM>) and responsively provide a sensor output related to the flow rate of the process fluid (<NUM>);
measurement circuitry (<NUM>) configured to receive the sensor output and provide a digital output indicative of the amplitude and frequency of the vortices (<NUM>);
a memory (<NUM>) configured to store measurements based upon the digital output; and
diagnostics circuitry (<NUM>, <NUM>) coupled to the memory (<NUM>) arranged to detect instability in the flow of the process fluid (<NUM>) based upon the measurements stored in the memory (<NUM>);
wherein the diagnostics circuitry (<NUM>, <NUM>) is configured to correct for errors in a measurement flow rate based upon detected instability in the flow of the process fluid (<NUM>).