WO2008024910A2

WO2008024910A2 - Lidar-based level measurement

Info

Publication number: WO2008024910A2
Application number: PCT/US2007/076638
Authority: WO
Inventors: Sigifredo Nino; Andre Beaulieu; Jean-Louis Dumont; Bart Di Ciero; Ronald Fisher; Nathalie Marcotte; Sylvain Nadeau; Trevor Nowlan; Andre Raymond; Richard Steele
Original assignee: Invensys Systems, Inc.
Priority date: 2006-08-25
Filing date: 2007-08-23
Publication date: 2008-02-28
Also published as: WO2008024910A3

Abstract

A level meter for measuring a liquid surface level in a tank includes a housing that attaches to the tank, an optical source and an optical sensor in the housing that transmit a laser pulse toward the liquid surface and receive the reflected pulse. A signal processor calculates a raw distance to the liquid surface based on the time of flight of the laser pulse, performs a filter process on the raw data and outputs a measured distance between the optical sensor and the liquid surface. A method of measuring the liquid surface level includes transmitting a laser pulse toward the liquid surface, receiving the reflected pulse, determining the time of flight of the pulse, calculating a raw distance measurement based on the time of flight, performing a filter process on the raw data and outputting a measured distance to the liquid surface level.

Description

LIDAR-Based Level Measurement

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 60/823,626, filed August 25, 2006, which is incorporated herein by reference.

TECHNICALFIELD

This disclosure relates to apparatus, systems and methods for measuπng a liquid surface level, and more particularly to using LIDAR (Light Detection and Ranging) to measure the level of liquefied natural gas in vessels, such as onboard transport ships.

BACKGROUND Liquefied natural gas (LNG) is often transported on large ships having several spherical tanks for holding the LNG. It is important to know the amount of LNG in a tank for custody transfer A known method and device for measuπng the level of LNG in a vessel is to measure the capacitance of a vertical metal pipe extending into the LNG. Since LNG is a dielectric, the capacitance varies with the liquid level in the pipe, and the measured capacitance corresponds to the LNG level. However, it is desirable to obtain greater measurement accuracy than is currently possible using this method and device

SUMMARY

According to one general implementation, a level meter for measuring a liquid surface level of a clear fluid in a tank includes a housing with a flange for attachment to an interface of the tank, a rotatable platform, and a mounting bracket attached to the platform to adjust a pitch angle. A tube extends from the housing into the tank. The level meter also includes an optical source disposed within the housing and affixed to the mounting bracket. The optical source includes a semiconductor laser diode that transmits a pulse of coherent electromagnetic radiation into the tube. The level meter further includes an optical sensor that receives the pulse reflected by the liquid surface, as well as a signal processor that determines a time of flight of the pulse, calculates a raw distance measurement based on the time of flight, performs a Kalman filter process and a second- order Butterworth filter process on the raw distance measurement, and outputs a measured distance between the optical source and the liquid surface based on performing the processes. i According to another general implementation, a level meter for measuring a liquid surface level in a tank includes a housing that attaches to the tank, an optical source in the housing that transmits a pulse of coherent electromagnetic radiation with a wavelength of less than about two micrometers toward a liquid surface, and an optical sensor that receives the pulse reflected from the liquid surface level A signal processor calculates a raw distance measurement between the optical source and the liquid surface based on a time of flight of the pulse

Implementations may include one or more of the following features For example, the optical source may transmit a sequence of pulses at a predetermined pulse rate. The liquid surface level may be a liquid surface level of a clear liquid (such as a liquefied gas or cryogenic liquid) m a tank The signal processor further may perform a filter process on the raw distance measurement and outputs a measured distance between the optical sensor and the liquid surface based on performing the filter process. The signal processor may include a first processor and a second processor. The filter process may use a recursive optimal state estimator that produces an estimated distance based on the raw distance measurement, as well as on at least one previous distance measurement that corresponds to at least one previous pulse The filter process may use a Kalman filter process The filter process may use a second-order Butterworth filter process The housing may include a positioning mechanism to orient the optical source perpendicular to the liquid surface.

The housing also may include a rotatable platform and a pivoted mounting bracket attached to the platform, where the optical source is affixed to the mounting bracket, the mounting bracket is configured to adjust a pitch angle of the optical source and the rotatable platform is configured to adjust a yaw angle of the optical source. The housing may further include a calibrated pitch angle adjustment device and a calibrated yaw angle indicator. The mounting bracket may include a shim adjustment to adjust a roll angle of the optical source. The mounting bracket may include a resilient element to bias the pitch angle The level meter may include a straight tube extending from the housing into the tank The level meter may include a reflective target floating on the liquid surface within the tube, where the pulse is reflected by the target. The optical source may include a semiconductor laser diode. The tank may be a storage or transport vehicle tank containing liquefied natural gas. The transport vehicle may be a ship.

According to another general implementation, a method of measuring a liquid surface level in a tank includes transmitting a pulse of coherent electromagnetic radiation with a wavelength of less than about two micrometers toward a liquid surface, receiving the pulse reflected from the liquid surface level, determining a time of flight of the pulse and calculating a raw distance measurement based on the time of flight. The method also includes performing a filter process on the raw distance measurement, and outputting a measured distance to the liquid surface level based upon performing the filter process.

Implementations may include one or more of the following features. For example, performing the filter process may include recursively performing an optimal state estimation. Performing the filter process may include performing a Kalman filter process. Performing the filter process may include performing a second-order Butterworth filter process The method may include transmitting a sequence of pulses at a predetermined pulse rate. Transmitting the pulse may include transmitting the pulse into a straight tube extending into the tank. The method may include reflecting the pulse from a reflective target floating on the liquid surface. The method may include orienting a laser perpendicular to the liquid surface, wherein the laser transmits the pulse. The method may include adjusting a pitch angle of a laser using a pivoted mounting bracket, and adjusting a yaw angle of the laser using a rotatable platform, where the laser transmits the pulse.

According to another general implementation, a computer program product is tangibly embodied in a machine-readable storage medium, and the computer program product includes instructions that, when read by a machine, operate to cause data processing apparatus to receive a raw distance measurement, perform a Kalman filter process on the raw distance measurement and at least one previous distance measurement, perform a second-order Butterworth filter process on a result of the Kalman filter process and at least one previous result, and output a measured distance based upon performing the processes.

Implementations may include one or more of the following features. For example, the instructions may operate to cause the data processing apparatus to determine a time of flight of a transmitted pulse of coherent electromagnetic radiation reflected from a liquid surface level and calculate the raw distance measurement based on the time of flight, where the previous distance measurement and the previous result correspond to at least one previous pulse. The instructions may operate to cause the data processing apparatus to trigger an optical source to transmit a sequence of pulses of coherent electromagnetic radiation at a predetermined pulse rate. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OFDRAWINGS

FIG. 1 is a schematic vertical cross-sectional view of a spherical tank with an example LIDAR level measuring system.

FIG 2 is a front perspective view of a LIDAR device in a mounting bracket. FIG. 3 is a rear view of the LIDAR device in the mounting bracket. FIG. 4 is a cross-sectional side view of the mounting bracket.

FIG. 5 is a plan view of the mounting bracket.

FIG. 6 is a partial cross-sectional view of a still pipe at the liquid surface level inside the tank.

FIG. 7 is a block diagram of an exemplary computer system that can implement level measurement.

FIG. 8 is a block diagram of a signal processing process.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, a level measurement system 10 is shown attached to a spherical tank 12 containing a liquid 14, for example, an onboard vessel on a ship containing liquefied natural gas (LNG) under pressure. Since the tank 12 is only partially filled with the liquid 14, a liquid surface 16 interfaces with a pressurized vapor 18 at a liquid surface level 20. The vapor 18 is substantially saturated vapor from the liquid 14. The tank 12 has a port 22, such as one of several ports provided on LNG transport tanks to provide access to the interior of the transport tanks, to which a housing 24 is attached, for example, by way of a coupler 26. The liquid may also be a cryogenic liquid, such as liquid oxygen, nitrogen, argon, helium or hydrogen, or other liquid with a boiling point below -15O⁰C (-238⁰F).

The housing 24 has a base 28 and a body 30 that enclose a laser ranging device, or LIDAR (Light Detection and Ranging) device, that transmits a pulsed laser beam through a shroud that provides a vapor shield, such as a substantially straight tube, or still pipe 32, to the liquid surface level 20, where the laser beam is reflected back to the LIDAR device. Since, in one example implementation, the tank may be 40 meters tall, the still pipe 32 may also be approximately 40 meters long to extend up to the entire height of the tank. In another example implementation, the still pipe 32 is omitted.

In general, the still pipe 32 should be sufficiently straight that its walls do not interfere with or block the laser beam. The LIDAR device is coupled to an external signal processor 34 that is programmed to generate a measured distance between the LIDAR device and the liquid surface level 20 based on the time the laser beam takes to travel from the LIDAR device to the liquid surface level 20 and back. The external signal processor 34 is coupled to an output device 36, such as a display, a logging or recording device, or a transmitter, to communicate the measured distance to a user or to another device.

The housing base 28 is shown in FIG. 2 supporting a mounting bracket 38 that holds a LIDAR unit 40. The LIDAR unit 40 can be a semiconductor laser diode device that generates and transmits pulsed coherent electromagnetic radiation in the visible to the near infrared spectral range. The LIDAR unit 40 includes a transmitter lens that collimates the electromagnetic radiation to form a laser beam with low divergence and a receiver lens that focuses the reflected light pulses on a light sensor, such as a photodiode. For example, in one implementation, the level measurement system 10 uses a Reigl Model LD-3 LIDAR distance, level and speed meter, which emits laser pulses of about 910 nanometer (nm) wavelength at a rate of about 2,000 per second (2 kHz) collimated into a three milliradian (mrad) by one-half mrad diverging beam. In addition, the LIDAR unit 40 includes an internal clock to measure the time of flight of the laser pulse, that is, the time from the transmission of a pulse to the time of reception of the corresponding reflection. While a 910 nm wavelength is used in the exemplary implementation, other wavelengths less than 2000 nm may also be used, and preferably between 500 to 1000 nm. In another example implementation, the optical source emits a laser pulse having a wavelength about 650 nm.

The mounting bracket 38 supports the LIDAR unit 40 in the housing 24 and provides adjustments to facilitate aiming the transmitted laser beam toward the liquid surface 16. The LfDAR unit 40 is mounted between two vertical support arms 42 that are rigidly attached to a platform 44. The platform 44 is rotatably coupled to the housing base 28 to allow a swivel or yaw adjustment of the mounting bracket 38 and the LIDAR unit 40 about a vertical axis X. The platform 44 includes a locking mechanism, such as the locking bolts 46 in the elongated slots 48 (see also FIG. 5), that can releasably lock the platform 44 in a desired swivel position. For example, the curved slots 48 allow the platform 44 to rotate through a swivel range of approximately 45 degrees when the locking bolts 46 are loosened, and the platform 44 can be locked m any position throughout the range by tightening the locking bolts 46. Furthermore, if the locking bolts 46 are removed, the platform 44 can rotate 360 degrees to be locked m any position by reinserting and tightening the locking bolts 46.

Referring to FIG. 3, the LDDAR unit 40 is attached to the vertical support arms 42 by way of pivoted swing arms 50, each of which is pivotally attached to one of the support arms 42 by a pivot pm 52, or a bolt, such that the swing arms 50 can freely pivot about the respective pins 52 The LIDAR unit 40 is rigidly attached to the swing arms 50, for example, by the countersunk bolts 56 Thus, the pivotable swing arms 50 allow a tilt, or pitch, adjustment of the LIDAR unit 40 about a transverse axis Y In addition, each of the swing arms 50 includes a locking mechanism, such as the locking bolt 54 in the elongated slot 58 (see FIG. 2), that can lock the swing arms 50 in a desired tilt position. For example, the curved slots 58 may allow the swing arms to be locked in any position through a tilt range of approximately five degrees (± 2.5 degrees), through a tilt range of approximately ten degrees (± 5 degrees from vertical), or through a tilt range of approximately fifteen degrees (± 7 5 degrees) depending on the particular implementation. In order to facilitate precision adjustment of the tilt angle, the mounting bracket 38 includes a micrometer head 60 attached to an upper transverse swing member 62 that spans the two swing arms 50 As shown in FIGS. 4 and 5, a telescoping spmdle 64 of the micrometer head 60 contacts an anvil surface, such as a spheπcal ball 66 embedded in an upper transverse support member 68 that spans the two support arms 42 In addition, one or more resilient elements, such as the two coil springs 70, apply a biasing force to the swing arms 50 to bias the swing arms 50 toward the upper support member 68 Thus, the micrometer head 60 is adjusted to push the swing arms 50 away from the upper transverse support member 68 or allow the swing arms 50 to be pulled toward the upper transverse support member 68 by the springs 70 In one implementation, as shown in FIG 4, the swing arms 50 are not as long as the support arms 42 and are rounded at the lower edge to provide clearance between the swing arms 50 and the platform 44 to permit the pivoting motion of the swing arms 50. Thus, the swivel and tilt adjustments are used to position the LEDAR unit 40 m an installation orientation such that the laser pulses are aligned substantially perpendicular to the liquid surface 16. For example, the swivel and tilt adjustments can be adjusted in the field to position the LIDAR unit 40 perpendicular to the liquid surface 16 at the time that the level measurement system 10 is installed on the tank 12 Since the liquid surface 16 may not he level or smooth (e g., if the tank 12 is mounted on a transport ship, which may be a seagomg or oceangoing vessel), the liquid surface 16 may indicate a representative surface level, such as a mean surface level or a surface level at the center of the tank 12.

Referring again to FIG. 3, the mounting bracket 38 further provides a shim adjustment 72 underneath each of the support arms 42 to allow an inclination, or roll, adjustment about an axis Z. For example, the shim adjustment 72 can be set during a calibration procedure at the time that the mounting bracket 38 is assembled at the factory

When installed in the mounting bracket 38 and housing 24, the LIDAR unit 40 rests against a lens divider 74 As seen in FIGS 2 and 3, the lens divider 74 separates the transmitted laser beam from the reflected laser beam to prevent reflection, or bad echoes, between the two beams A transparent protective plate 76, shown m FIG. 3, such as a borosilicate glass plate, protects the lens divider 74 and the LIDAR unit 40 from the pressurized vapor 18 and environmental elements The protective plate 76 is separated from the lens divider 74 by a gasket 78 made of felt, silicone, or any other material having suitable resilience and sealing properties. A groove 80 in the housing base 28 (see FIG. 2) receives the housing body 30, which is made from a high-strength, corrosion- resistant material (such as stainless steel) to protect the LIDAR unit 40 from environmental elements. The housing base 28, the housing body 30 and the protective plate 76 are assembled with appropriate gaskets, fittings and sealants to create a hermetic seal and to withstand an LNG explosion The housing base 28 includes an interface flange 82 to facilitate attachment of the housing 24 to the tank 12. Referring again to FIG. 1, the still pipe 32 extends vertically (approximately perpendicular to the liquid surface 16) directly below the housing 24 and the LfJDAR unit 40 from approximately the level of the upper inner surface of the tank 12 to a level near the lower inner surface of the tank 12. In one implementation, the still pipe 32 includes a nominal six-mch stainless steel tube; however, other materials, such as aluminum, can be used Based on the coefficient of thermal expansion of the still pipe material, a gap 33 exists between the lower tip of the still pipe 32 and the inner surface of the tank 12 to allow for contraction and expansion of the still pipe 32 between an ambient temperature and a storage temperature (typically about -160 ± 0 5 C for LNG) The still pipe 32 is held in place independently of the housing 24 by brackets (not shown) inside the tank 12 that have sleeves to allow for contraction and expansion of the still pipe 32. At installation, or at a later time, the swivel and tilt of the LIDAR unit 40 is adjusted so that the laser beam does not reflect off the inner surface of the still pipe 32.

Referring to FIG. 6, the level measurement system 10 also includes a reflective target 84 that floats on the liquid surface 16 inside the still pipe 32. The target 84 has a reflective upper surface, for example, a reflective layer of silver, a silver alloy or gold, with fine grooves or serrations stamped into the surface, that reflects the transmitted laser beam back toward the LIDAR unit 40. The reflective target 84 ensures that the reflected signal is of sufficient intensity to be sensed by the light sensor. For example, if the liquid 14 is relatively transparent, the liquid surface 16 alone may not sufficiently reflect the laser beam. As another example, if the vapor 18 above the liquid surface 16 is relatively dense, the vapor 18 may partially block the laser beam, and the liquid surface 16 may not sufficiently reflect the laser beam to penetrate the vapor blanket

In order to determine the distance between the LIDAR unit 40 and the hquid surface level 20, the LIDAR unit 40 includes an internal clock that measures the time of flight of the laser pulses, that is the time from the transmission of each pulse to the time of reception of the corresponding reflection. The time of flight is then converted to distance traveled by Equation (1)

2 (1)

In Equation (1), D is the distance between the LIDAR unit 40 and the liquid surface level 20, c is the velocity of light in the medium (e.g., the vapor 18), and t is the time of flight of the laser pulse. The individual distance measurements include variations due to a number of factors, or process variables, including the ability of an individual laser pulse to penetrate the vapor 18 in order to reach the liquid surface level 20, and absorption of the reflected beam by the vapor 18.

The raw data, for example, the distance measurements, are communicated to the external signal processor 34, which includes software to perform signal processing. The signal processor 34 receives the raw data at a predetermined incoming rate (e.g., 2 kHz) and produces a smooth output curve with a high degree of accuracy, for example, less than one-tenth of one percent of the span (i e., the valid measurement range) of the LIDAR unit 40. This is accomplished by performing a zero/span data validation, followed by two levels of filtering and a correction for sensor non-lrneaπty, to yield highly accurate liquid-level measurement results. In one implementation, the resulting output provides a measured distance with an accuracy of about ± 7.5 mm.

FIG. 7 is a diagram of an exemplary computer system 100 that implements the level measurement. According to one general implementation, the system 100 is used for the operations described in association with the process 100.

The system 100 includes a processor 110, memory 120, a storage device 130, and input/output devices 140. Each of the components 110, 120, 130, and 140 are interconnected using a system bus 150. The processor 110 is capable of processing instructions for execution within the system 100. In one implementation, the processor 110 is a single-threaded processor. In another implementation, the processor 110 is a multi-threaded processor. The processor 110 is capable of processing instructions stored in the memory 120 or on the storage device 130 to display graphical information for a user interface on an appropriate mput/output device 140. In a further implementation, a first processor is used to calculate the raw distance measurement between the optical source and the liquid surface, and a second processor is used to perform the filter process on the raw distance measurement and output the measured distance between the optical sensor and the liquid surface based on performing the filter process.

The memory 120 stores information within the system 100. In one implementation, the memory 120 is a computer-readable medium. In one implementation, the memory 120 is a volatile memory unit. In another implementation, the memory 120 is a non-volatile memory unit.

The storage device 130 is capable of providing mass storage for the system 100. In one implementation, the storage device 130 is a computer-readable medium. In various different implementations, the storage device 130 may be a floppy disk device, a hard disk device, an optical disk device, a tape device, or a flash memory device.

The input/output device 140 provides mput/output operations for the system 100. hi one implementation, the input/output devices 140 includes a keyboard and/or pointing device, hi another implementation, the input/output device 140 includes a display unit for displaying graphical user interfaces.

In alternative implementations, the LIDAR unit can generate a laser beam having a shorter wavelength or a longer wavelength, such as in the visible or ultraviolet spectral ranges. In various implementations, the signal processor can be implemented by a single processor, as described above, or by multiple processors. For example, the LIDAR unit may include an embedded microprocessor that performs part of the signal processing and sends a result to an external processor that performs the remainder of the signal processing. As a specific example, in one implementation the distance between the LDDAR unit 40 and the liquid surface level 20 is calculated by an embedded microprocessor m the LIDAR unit 40 and additional filtering and smoothing are performed m the external signal processor 34.

In applications where the reflection of the laser beam from the liquid surface has sufficient intensity to be reliably detected by the photodiode, the level measurement system 10 can omit the reflective target 84 In applications where the vapor over the liquid surface does not block the laser beam, the level measurement system 10 can omit the shroud, or still pipe 32 In addition, the level measurement system 10 can be modified for use with tanks of various sizes and shapes, including vertical and horizontal cylindrical tanks, elliptical tanks, square and rectangular tanks, and the like

FIG 8 is a flowchart illustrating a process 200 for refining the raw data received from the LEDAR unit 40 The raw data is received (S202) and zero/span validated (S204) to exclude data values that are clearly erroneous. For example, the signal processor 34 may exclude values that do not lie within the span of the instrument, or values outside of a range between predetermined minimum and maximum values, such as zero and the depth of the tank 12. The data validation ensures that reflections from objects or surfaces other than the liquid surface level 20, including the vapor blanket, windows, or any other erroneous source, generally are ignored Ideally, only reflections from the liquid surface level 20 are taken into consideration.

Estimation and vicinity validation are performed (S206) on the data using a modified Kalman filter The Kalman filter is a recursive optimal state estimation process that performs conditional probability density propagation for cases in which the system can be described by a linear Markovian model m which both process and measurement noises are white and Gaussian. The Kalman filter was originally descπbed m Kalman, R E., A New Approach to Linear Filtering and Prediction Problems, Transactions of the ASME -Journal of Basic Engineering, Vol. 82, Series D, (Mar 1960), pp. 35-45, which is hereby incorporated by reference

In this context, the term optimal does not necessarily mean that a more accurate estimate could never be possible, but rather that the filter attempts to find the best solution. Thus, the Kalman filter is optimal in the sense that the estimates are determined by statistically minimizing the error. The Kalman filter generates estimates using knowledge of the system (process) dynamics and the measurement device (sensor) dynamics, including a state-space model of the process; statistical parameters that describe noise related to the process, noise introduced by the sensor, measurement errors, and uncertainty in the dynamic model; and any other available information regarding the initial conditions of the variables of interest.

Because the process is recursive, the Kahnan filter does not require that all previous data, or large amounts of previous data, be stored in memory for subsequent processing (e.g., upon receipt of each raw distance measurement). Instead, the Kalman filter requires storage of, for example, two readings. In one implementation, the Kalman filter is modified to exclude in real time any data points that are determined to be statistical outliers. After the Kahnan filter is applied, the estimated value is smoothed (S208) by applying a second-order Butterworth filter. In addition, a nonlinear correlation is performed (S210) on the estimated value to correct for sensor nonlinearity. Thus, the process used to perform the signal processing proceeds as follows for each successive raw distance measurement:

The stochastic state-space model of the process, which is dπven by white, Gaussian noise, is expressed below in Equations (2) and (3):

X_k+ι = ΛX_k + w_k (2)

X_k is the pπor estimate. (On the first pass, an initial estimate X₀ is used.)

P_k is the error covariance of X_k . (On the first pass, an initial covariance P₀ is used.)

X_k is the true, or actual, state vector at sample time k . X _p is the best estimate of the state vector at time k . P is the covariance of the best state estimate at time k .

A is the transition matrix w_k is the process noise vector (additive, white, zero-mean and Gaussian). Q is the covariance matrix of the state model uncertainties (w) . Y_k is the system observable output vector at sample time k . C is the raw distance measurement matrix. v_k is the measurement noise vector (additive, white, zero-mean, Gaussian, and uncorrelated with process noise).

R is the covariance matrix of the observation noise (v) . X_k is the estimation of the system state vector at time k . P is the state estimation covariance matrix for X_k .

K is the Kalman filter gain matrix. Inn is the Innovation vector / is the Identity matπx.

The state is projected ahead and the associated error covariance is determined using Equations (4) and (5): The Kalman filter gain is calculated according to Equation (6):

^K^=^p _Pk+p^T^₊f^{T +R}r rø

If the current measurement is not determined to be an outlier, then the innovation vector results from Equation (7):

Inn_k+1 = K_k+l(Y_k+l - CX_pkJ (7)

Otherwise, if the current measurement is determined to be an outlier, then the innovation vector is set to zero per Equation (8):

M₊₁ = O (8)

The previous state estimate and its error covariance are corrected using the current measurement according to Equations (9) and (10):

P_k+l = (I-K_k+1C)P_nJI-K_k+lQ^T (10)

The current best estimate for the system observable output is calculated according to Equation (11):

Y_M = CX_k+1 (H)

The estimated value is smoothed using to Equation (12):

JU₊₁ = /(Y₁₊I. %, Time Constant) (12)

The estimated value is corrected for sensor nonlinearity using a nonlinear correlation, thereby outputting LIDAR measurements that exclude data values that are clearly erroneous, such as those values that do not lie within the span of the instrument, or values outside of a range between predetermined minimum and maximum values. Additionally, the output LIDAR measurements exclude reflections from objects or surfaces other than the liquid surface level 20, generally ignoring the vapor blanket, windows, or other erroneous sources. If an additional distance measurement is available, the process 200 is repeated for the next raw data point, beginning at Equation (4) above, using X_k+X as the prior estimate and P_k+l as its error covariance.

The features described can be implemented in digital electronic circuitry, or in computer hardware, firmware, application, or in combinations of them. The apparatus can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described implementations by operating on input data and generating output. The described features can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment.

Suitable processors for the execution of a program of instructions include, by way of example, both general and special purpose microprocessors, and the sole processor or one of multiple processors of any kind of computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memories for storing instructions and data. Generally, a computer will also include, or be operatively coupled to communicate with, one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of nonvolatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits).

To provide for interaction with a user, the features can be implemented on a computer having a display device such as a CRT (cathode ray tube) or LCD (liquid crystal display) monitor for displaying information to the user and a keyboard and a pointing device such as a mouse or a trackball by which the user can provide input to the computer.

The features can be implemented in a computer system that includes a back-end component, such as a data server, or that includes a middleware component, such as an application server or an Internet server, or that includes a front-end component, such as a client computer having a graphical user interface or an Internet browser, or any combination of them. The components of the system can be connected by any form or medium of digital data communication such as a communication network. Examples of communication networks include, e.g., a LAN, a WAN, and the computers and networks forming the Internet.

The computer system can include clients and servers. A client and server are generally remote from each other and typically interact through a network, such as the described one. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Additionally, elements of different described implementations may be combined, supplemented, modified, or removed to produce other implementations. Accordingly, other implementations are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A level meter for measuring a liquid surface level in a tank, comprising: a housing configured for attachment to the tank; an optical source disposed within the housing and configured to transmit a pulse of 5 coherent electromagnetic radiation with a wavelength of less than about two micrometers toward a liquid surface; an optical sensor configured to receive the pulse reflected from the liquid surface level; and a signal processor configured to calculate a raw distance measurement between the o optical source and the liquid surface based on a time of flight of the pulse.

2. The level meter of claim 1, wherein the liquid surface level is a liquid surface level of a clear liquid in the tank. 5

3. The level meter of claim 2, wherein the clear liquid is a liquefied gas or a cryogenic liquid.

4. The level meter of claim 1, wherein the optical source is further configured to transmit a sequence of pulses at a predetermined pulse rate. 0

5. The level meter of claim 1 , wherein the signal processor is further configured to perform a filter process on the raw distance measurement and output a measured distance between the optical sensor and the liquid surface based on performing the filter process. 5

6. The level meter of claim 5, wherein the signal processor further comprises: a first processor configured to calculate the raw distance measurement between the optical source and the liquid surface; and a second processor configured to perform the filter process on the raw distance measurement and output the measured distance between the optical sensor and the liquid0 surface based on performing the filter process.

7. The level meter of claim 5, wherein the filter process comprises a recursive optimal state estimator that produces an estimated distance based on the raw distance measurement and on at least one previous distance measurement corresponding to at least one previous pulse.

8. The level meter of claim 5, wherein the filter process comprises a Kalman filter process or a second-order Butterworth filter process.

9. The level meter of claim 1, wherein the housing comprises a positioning mechanism to orient the optical source perpendicular to the liquid surface.

10. The level meter of claim 1, wherein the housing comprises: a rotatable platform; and a pivoted mounting bracket attached to the platform, wherein the optical source is affixed to the mounting bracket, the mounting bracket is configured to adjust a pitch angle of the optical source and the rotatable platform is configured to adjust a yaw angle of the optical source.

11. The level meter of claim 10, wherein the housing further comprises a calibrated pitch angle adjustment device and a calibrated yaw angle indicator.

12. The level meter of claim 10, wherein the mounting bracket further comprises: a shim adjustment to adjust a roll angle of the optical source; and a resilient element to bias the pitch angle.

13. The level meter of claim 1, further comprising a reflective target configured to float on the liquid surface within the tube, wherein the pulse is reflected by the target.

14. The level meter of claim 1 , wherein the tank is a storage or transport vehicle tank.

15. A method of measuring a liquid surface level in a tank, comprising: transmitting a pulse of coherent electromagnetic radiation with a wavelength of less than about two micrometers toward a liquid surface; receiving the pulse reflected from the liquid surface level; determining a time of flight of the pulse; calculating a raw distance measurement based on the time of flight; performing a filter process on the raw distance measurement; and outputting a measured distance to the liquid surface level based upon performing the filter process.

16. The method of claim 15, wherein performing the filter process comprises recursively performing an optimal state estimation.

17. The method of claim 15, further comprising transmitting a sequence of pulses at a predetermined pulse rate.

18. The method of claim 15, wherein transmitting the pulse further comprises transmitting the pulse into a straight tube extending into the tank.

19. The method of claim 15, further comprising: adjusting a pitch angle of a laser using a pivoted mounting bracket; and adjusting a yaw angle of the laser using a rotatable platform, wherein the laser transmits the pulse.

20. A computer program product tangibly embodied in a machine-readable storage medium, wherein the computer program product comprises instructions that, when read by a machine, operate to cause data processing apparatus to: receive a raw distance measurement; perform a Kalman filter process on the raw distance measurement and a previous distance measurement; perform a second-order Butterworth filter process on a result of the Kalman filter process and a previous result; and output a measured distance based upon performing the processes.