Level measurement using correlation between a pair of secondary reference signals

A method of measuring a level of a material in a tank. A first secondary reference (FSR) signal is generated when a reference pulse is transmitted into the tank and a second secondary reference signal (SSR) signal is generated when an echo signal responsive to the reference pulse is received. A plurality of FSR signals and a plurality of SSR signals are stored. The plurality of FSR signals and plurality of SSR signals are transferred to a correlator block which calculates a plurality of correlation function values (CFVs) that have magnitudes reflecting a degree of matching between pairs of the SSR signals with their associated FSR signals. Using a maximum CFV result a time interval (Tm) is calculated between the FSR signal and SSR signal associated with the maximum CFV result, and the level of material in the tank is determined from Tm.

FIELD

Disclosed embodiments relate to radar or ultrasound level measurement.

BACKGROUND

Conventional level measurement implements time domain reflectometry (TDR) by transmitting microwave or ultrasound pulses (reference pulses) from an antenna or waveguide to the surface (or interface) of a material in a container (or tank) and measuring signals reflected (echo signals) including signals from that surface or interface. For example, when a radar reference pulse reaches a material with a different dielectric constant, part of the microwave energy is reflected back which is received by a receiver as an echo pulse. The echo pulse has an associated echo amplitude. Generally, the echo pulse will have the same shape as the reference pulse that is sent down the waveguide, but its sign and magnitude depend on the change in impedance level. Known radar level measurement methods include non-contact radar typically being pulsed radar and contact radar typically being frequency modulated continuous wave (FMCW) radar.

For TDR-based level measurement devices or systems, the reference pulse is superimposed on the echo signal reflected by an object or interface in an at least partially filled tank, whose distance (or level) is to be measured. In known methods, in order to determine the time position of the echo pulse in the echo signal to determine travel time T to enable calculation of a level value, the time profile of the echo signal is compared with a stored reference echo signal that was generated by the level measurement system without any objects or product material in the measurement path or an otherwise empty tank. The echo signal generated for an at least partially filled tank is different from the reference echo signal generated for an empty tank by at least one additional pulse sometimes referred to as an interface pulse that results from the reflection of the reference pulse at the interface(s) in the tank (e.g., an interface between a product liquid and the gas above). The time position determination of the interface pulse requires comparing the reference echo signal with the echo signal, with the simplest case being the reference echo signal subtracted from the echo signal performed incrementally through subtraction of amplitude values that are located at corresponding time positions of the signal profiles of the reference echo signal and the echo signal.

Due to the change in impedance at the reflection surface (e.g., the interface), points on the echo signal curve including the interface pulse generally have an amplitude that deviates from the points on the reference echo signal curve. Making the comparison of the echo signal and the reference echo signal to determine the travel time T can thus become difficult since significant amplitude offsets between the echo signal and the reference echo signal are generally present which can lead to errors in the level measurement.

SUMMARY

Disclosed embodiments eliminate problems described above with measuring the time interval (travel time) between the reference echo signal and the echo pulse used for calculating the material level in tanks by eliminating the conventional need for measuring amplitude offsets between the echo pulse (or interface pulse) and the reference echo signal by using a correlation measure between a pair of new signals referred to herein as secondary reference signals. Unlike a conventional reference echo signal obtained when the tank is empty, disclosed secondary reference signals are used while there is product material in the tank, and the tank can be operating. A first secondary reference (FSR) signal is generated when a reference pulse is transmitted by a transmitter into the tank and a second secondary reference (SSR) signal is generated when an echo signal including an echo pulse is received responsive to the reference pulse.

During each measurement cycle, a correlator block (hardware or software) calculates a plurality of correlation function values (CFVs) which each reflect a match between the SSR signals and their associated FSR signals. “Correlation” as used herein compares each FSR signal generated when the reference pulse is transmitted and its associated SSR signal (which both have the same shape) generated when the echo signal responsive to the reference pulse is received, so that the SSR signals are time-delayed versions of the FSR signals which reflect the travel time by their delay. When the plurality of CFVs is determined to have a maximum CFV result that is above a predetermined minimum level, a time interval between the FSR signal and SSR signal associated with the maximum CFV (corresponding to the travel time shown inFIG. 3in the examples section below and referred to herein as Tm) is calculated, and the level of the material in the tank can be calculated from Tm.

DETAILED DESCRIPTION

Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate certain disclosed aspects. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the disclosed embodiments.

One having ordinary skill in the relevant art, however, will readily recognize that the subject matter disclosed herein can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring certain aspects. This Disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the embodiments disclosed herein.

Also, the terms “coupled to” or “couples with” (and the like) as used herein without further qualification are intended to describe either an indirect or direct electrical connection. Thus, if a first device “couples” to a second device, that connection can be through a direct electrical connection where there are only parasitic in the pathway, or through an indirect electrical connection via intervening items including other devices and connections. For indirect coupling, the intervening item generally does not modify the information of a signal but may adjust its current level, voltage level, and/or power level.

Disclosed embodiments provide methods and systems for radar and ultrasound-based level measurement which enables accurate level measurements through calculating a match between a plurality of FSR signals and a plurality of associated SSR signals generated each measurement cycle, and selecting the best matching FSR and SSR pair to calculate Tm. The FSR and SSR signal pulse shapes and amplitudes do not depend on properties of an object (or interface) for which distance to the object (or interface) is to be measured. As a result, disclosed embodiments solve conventional level measurement system problems with measuring the travel time between the reference echo signal and the echo pulse by eliminating the conventional need for measuring amplitude offsets between these signals.

FIG. 1Ais a block diagram depiction of an example FMCW radar range level measurement system (level measurement system)100using correlation for calculating matches between a plurality of FSR and SSR signal pairs, and for selecting the best matching FSR and SSR pair(s) to calculate Tm. System includes a waveguide110inside of a tank (or container)105. A FSR signal131is generated by a pulse generator circuit (PG)130at essentially the same time when a transmitted reference pulse (reference pulse) is sent by a transmitter (TX)126shown provided by a transceiver (TX/RX)125also including a receiver127through a feed-through120sealed to the top of the tank105with a flange121to the waveguide110into the tank105having a product material112therein.

PG130can comprise an electronic circuit or a piece of electronic test equipment. A SSR signal132is also generated by PG130at essentially the same time when the echo signal including an echo pulse responsive to the reference signal is received by the RX127. Typically, the level measurement system100components including the TX/RX125are within a single common enclosure or housing, such as a flame-proof and/or explosion-proof housing.

The generation of FSR signal131and SSR signal132are thus synchronized with the reference pulse and the echo pulse, respectively, where the SSR signals132are time-delayed versions of the FSR signals131which reflect Tm by their relative delay. There are a variety of possible methods available to achieve the synchronization of the FSR signals131with the reference pulse with and synchronization of the SSR signals132with the echo pulse. The synchronization method can select a “marker” used to determine which signal characteristic for both the reference pulse and the echo pulse is used for triggering the generation of FSR signals131and SSR signal132, respectively. Possible markers include an amplitude of the signal, slope of a rising edge of the signal, or a point of zero crossing of the signal, or a combination of all above.

A disclosed algorithm, which is stored in a memory block that is executed by the processor150can use a “marker”-based method for generating trigger control signals (shown as “trigger”156) typically being pulses, which are coupled to the PG130for synchronizing the triggering a FSR signal131with the transmitting of the reference pulse and triggering of a SSR signal132with the receipt of the echo pulse. Level measurement system100also has a system clock153shown provided by the processor clock152to provide clocking for the processor150and for controlling other timing therein in the level measurement system100.

Although the system clock153is shown provided by the processor clock152, there are numerous other ways the system clock153can be implemented, such as a separate (dedicated) clock circuit. The system clock153is typically a square wave that synchronizes and controls functions of PG130(other than FSR signal131and SSR signal132generation), and synchronizes and controls the first memory135, correlator block140, counter145, and the second memory145a. The PG130is shown coupled to the counter145for determining when for the counter145to start, and the correlator block140is shown determining when for the counter145to stop. Although described herein as having first memory135, second memory145a, and memory151associated with processor150, level measurement system100may utilize other memory arrangements such as a single memory, or a memory151associated with processor150and one other memory.

The frequency of the FSR signal131depends on frequency of the reference pulse transmitted by the TX126via waveguide110into the tank105. The frequency of FSR signal131can be the same as the frequency of the reference pulse in a simplest implementation. However, the use of any frequency for the FSR signal131that is an integer multiple of the frequency of the reference pulse can also be used.

The pulse associated with the FSR signal131is defined by its shape, its amplitude and its frequency. Regarding the pulse shape for the FSR signal131, generally any signal shape can be used as long as the FSR signal131has a single rising edge, a single peak (amplitude) and a single falling edge. For simplicity, the pulse width of the FSR signal131can be the same as the pulse width of the reference pulse. Using a FSR signal131with a smaller pulse width compared to the reference pulse width can also be used as long as the shape of FSR signal131is symmetrical with respect to its peak or the peak of the signal is placed at equal distances from both edges. Using signals for the FSR signal131with larger pulse width as compared to the reference pulse width can cause additional errors in the level measurement and can increase complexity of the system. A simple example shape for the FSR signal131is a triangle waveform shape, such as shown inFIG. 3described below in the Examples section. It is expected to have a single echo signal generated in response to a single reference signal, so that there will generally be the same number of FSR signals131and SSR signals132.

The SSR signal132and FSR signal131can both be generated by the same PG130. Separate PGs (two PGs) can also be used provided the respective PGs are synchronized which can add some complexity, which can be minimized by having matched first and second PGs on the same chip/die to ensure that FSR signal131and SSR signal132are essentially identical which is generally a desired arrangement as it simplifies the design and implementation. However, typically a most important system feature is to generally ensure that the pulses from both FSR signal131and SSR signal132have the same frequency (or an integer multiple thereof). A measurement system where the FSR signal131and SSR signal132have different amplitudes can generally be used as long as symmetry of the amplitude conditions described above are met. The same applies for the pulse shape, with the triangular shape being one example that is generally a simplest implementation option.

For the FSR and SSR signals, PG130allows control of the pulse repetition rate (frequency), pulse width, delay with respect to an internal or external trigger and the high- and low-voltage levels of the pulses. PG130also controls the rise time and fall time for the FSR and SSR signals.

The FSR signals131and SSR signals132generated by PG130are both shown stored in memory135. The correlation function implemented by correlator block140by its nature performs calculations on a series of samples that represent the FSR and SSR signals over an interval of time (measurement cycle). Memory135can comprise a static random access memory (SRAM) a dynamic random access memory (DRAM), or a flash memory, which provides the FSR signals131and SSR signals to the correlator block140at each system clock153signal. The memory storing the FSR signals131and SSR signals132can be located in other locations other than memory135as shown, such as on the same chip as the processor150shown as memory151(e.g., SRAM/DRAM/flash memory).

The correlator block140calculates in real-time values of a correlation function (correlation function values “CFVs”) determined by comparing the degree of correlation (match) between each SSR signal132and its associated FSR signal131. CFV values being stored enables detecting a maximum value from a wide range (large number) of calculated CFV values acquired during each measurement cycle. The calculating of CFVs can be repeated several times for each pair of FSR and SSR signals, which will produce a range of several maximum CFVs that can be averaged out to obtain a maximum CFV result that is essentially free of noise errors which represents the time Tm used for calculating the level L. The actual shape, pulse width or signal amplitudes for the FSR signal131and the SSR signal132is generally not important as long as certain requirements for both of these signals are met as described above. The correlator block140can be implemented as hardware solution such as a Field-Programmable Gate Array (FPGA) or application-specific integrated circuit (ASIC), a software algorithm run by a processor150, or a combination of both hardware and software.

The CFV values reflect quantitatively how well each SSR signal132when overlayed on its associated FSR signal131matches (correlates). A maximum CFV result is indicated when a given SSR signal132is essentially an exact copy (i.e., shape) of its associated FSR signal131where the higher the CFV, the better the FSR/SSR signal match is. Known algorithms for determining CFVs may be used, such as based on calculating maximum autocorrelation values. Disclosed embodiments thus recognize in any practical level measurement system implementation it will be almost impossible to ensure that FSR and SSR signals are identical due to a variety of generally random errors (noise), which is overcome by calculating a maximum AFV result from a large number of FSR and SSR signal pairs to determine Tm.

The level measurement system100generally includes one or more algorithms stored in memory151run by the processor150for performing calculations including real-time calculations of the correlation function to generate the CFVs. A result of these calculated CFVs is an array (a plurality of) of CFVs that is stored in a memory, such as shown as memory145aassociated with counter145. These CFVs will range from a minimum value (where the minimum value can be zero or close to zero depending on how much noise is present in the level measurement system100) to a maximum (highest) value.

The actual value of the maximum CFV result will generally depend on a plurality of different factors, e.g. an amplitude, or the scaling of the FSR signals131and SSR signals132. For any particular level measurement system a practical maximum CFV reference value can be measured during a calibration process, where an empty tank can be used for the calibration process. For the purpose of this Application a maximum CFV result is defined as the highest value from a plurality of calculated CFVs, acquired during a measurement cycle, such as stored in memory145a. Based on properties of the correlation function and requirements for the FSR signal131and SSR signal132there should be only one highest maximum CFV in the range (for a particular defined period of time representing one measurement cycle).

When the SSR signal132and FSR signal131are determined to sufficiently match one another (i.e., results in a maximum CFV) a CFV result is generated and the output of the correlator block140can then change its logical output value, from logic LOW to logic HIGH or logic HIGH to logic LOW, which when a logic state change is received by the processor150(via the counter145as shown) can cause the processor150to read the maximum CFV result (stored in memory145a) and its associated FSR signal and SSR signal. The counter145can be implemented as an algorithm (e.g., by processor150) or as hardware.

Using the Tm associated with the maximum CFV result, the processor150calculates the distance (D) to the object (e.g., interface) in the tank105and can display this distance (or level, L) value on display device160shown inFIG. 1A, generally in real-time. At the end of this measurement cycle the processor150can reset the counter145, and erase the CFV contents of memory145a, and FSR signals131and SSR signals132in memory135, and start a new level measurement cycle.

FIG. 1Bis a block diagram depiction of an example non-contact radar system having level measurement system100including a correlator block140for calculating a match between FSR and SSR signals to calculate Tm, according to an example embodiment. In this embodiment, a horn antenna172is located near the top of the tank105. Level finding algorithms generally stored in memory151will generally be customized for the non-contact radar system. As known in the art ultrasonic level sensors are another non-contacting level measurement method that works by the “time of flight” principle using the speed of sound. The ultrasound sensor emits a high-frequency pulse, generally in the 20 kHz to 200 kHz frequency range, and then waits for an echo. The pulse is transmitted in a cone, usually about 6° at the apex. The pulse is incident at the level surface and is reflected back to the ultrasound sensor now acting as a receiver.

FIG. 1Cis a block diagram depiction of an example ultrasound level measurement system having level measurement100system including a correlator block140for calculating a match between FSR and SSR signals to calculate Tm, according to an example embodiment. The ultrasound level measurement system includes a connector181having an ultrasound transducer182connected thereto. Level finding algorithms generally stored in memory151will generally be customized for the ultrasound system.

FIG. 2is a flow chart for an example method200of measuring a level of a material in a tank by calculating a match between FSR and SSR signals to calculate Tm, according to an example embodiment. Step201comprises generating a FSR signal when a reference pulse is transmitted into the tank and generating a SSR signal when an echo signal including an echo pulse responsive to the reference pulse is received. Step202comprises storing a plurality of FSR signals and a plurality of SSR signals. Step203comprises transferring the plurality of FSR signals and plurality of SSR signals to a correlator block. Step204comprises the correlator block calculating a plurality of CFVs which have magnitudes reflecting a degree of matching between pairs comprising ones of the plurality of SSR signals with their associated FSR signals including identifying a maximum CFV result. Step205comprises using the maximum CFV result calculating Tm associated with the maximum CFV result. Step206comprises determining the level of material in the tank from Tm.

Disclosed level measurement systems can be used for non-contact radar such as pulsed radar and frequency modulated continuous wave (FMCW) radar. Moreover, as noted above disclosed level measurement systems can be applied to ultrasound systems.

Examples

Disclosed embodiments are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way.

FIG. 3shows an example profile of a conventional echo signal310including a transmitted reference pulse (reference pulse)301and an echo pulse302resulting from an interface in the tank that is captured during operation by a disclosed level measurement system, described as being a GWR system for measuring a location of the interface (e.g., between gas and liquid in a tank), and its timing relationship with disclosed FSR and SSR signals shown as being triangular waveforms above the conventional echo signal310. It is noted thatFIG. 3represents only an example not a real profile of an echo signal. In a real situation the actual profile may be significantly different. The main purpose ofFIG. 3is to show time dependence between the disclosed secondary reference signals (FSR and SSR) and the relationship of the disclosed secondary reference signals FSR and SSR to the reference pulse301and echo pulse302of the echo signal. As noted above, for determining the time position (shown as Ts inFIG. 3) of the echo pulse302in the signal profile of the echo signal310conventional level measurement systems must obtain a “reference echo signal” (not shown) generated by the level measurement system without any objects or product material in the measurement path or an otherwise empty tank for comparing with the echo signal310.

The reference pulse301marks the time of reference signal transmission, and the echo pulse302represents the reflection of the reference pulse301in the waveguide at the interface at which there is change in impedance in the waveguide. The negative pulse303represents a negative going signal that results from the reflection of the reference pulse301at the transition between the feed-through (marked as120inFIGS. 1A, 1B and 1C) and the flange (marked as121inFIGS. 1A, 1B and 1C).

The presence of negative pulse303assumes the feed-through and flange have different impedances. The pulse304represents a signal that results from the reflection of the transmit pulse at the end of the waveguide (the bottom of the tank marked as105inFIGS. 1A, 1B and 1C). The time shown as Ts represents the travel time for the reference pulse301to travel up to the point where this pulse gets reflected from the interface. For simplicity, Ts is shown being measured between the reference pulse301and the echo pulse302.

Now turning to disclosed FSR signal131and SSR signal132waveforms above the conventional signal, the time interval Tm represents the time difference between peaks of the FSR signal131and SSR signal132that as described above are both generated by the same pulse generator (PG130inFIGS. 1A, 1B and 1C). For simplicity it is shown that these two pulses (FSR signal131and SSR signal132) reach their respective maximum values (amplitude) when the reference pulse301and the echo pulse302reach their peaks. However, other methods of synchronization can be used as long as the level measurement system can maintain correlation between these two sets of signals (FSR signal131, SSR signal132, and reference pulse301, echo pulse302) so that the condition Ts=Tm remains true.

Tm is determined by calculating the correlation between the FSR signal131and SSR signal132pairs to generate CFV values, and Tm is calculated when there is a maximum CFV result calculated that is a sufficiently high value (e.g., compared to a predetermined minimum CFV value from a calibration process). The maximum CFV result is essentially free of noise errors which represents Tm. The level of the liquid (or other material) in the tank can be determined from Tm using time domain reflectometry (TDR) as Tm is essentially=conventional Ts.

Since Tm is derived directly from the properties of the correlation function that samples an array of CFV values to identify a maximum CFV result during each measurement cycle that provides averaging and eliminates the need for measuring amplitude offsets, Tm provides a more accurate time measure for the time difference between the echo pulse302and the reference pulse301as compared to conventional TSdetermination. As described above conventional TSdetermination requires comparing the reference echo signal with the echo signal310performed incrementally through subtraction of amplitude values that are located at corresponding time positions of the signal profiles of the reference echo signal and the echo signal310which involves measuring amplitude offsets which is subject to amplitude offset-based time measurement errors.

For the purpose of explaining the principle of level measurement disclosed herein, negative pulse303and pulse304can be omitted from the analysis. However, as known in the art, these pulses can be used for level system calibration or diagnostics using correlation as well.

While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.