Temperature compensation for magnetostrictive position detectors

A system, method, and apparatus is provided for magnetostrictive position detectors to compensate fluid level measurements for temperature conditions associated with the process without the use of a built-in or external stand-alone temperature sensor. Also disclosed is an algorithm to compensate for temperature conditions associated with the process by determining thermal error coefficients for temperature compensation that are proportional to the process temperature via digital processing of the signals of the position detector.

BACKGROUND

The present disclosure relates in general to magnetostrictive position detectors, and, more particularly, to compensation of position measurements taken by the magnetostrictive position detector due to temperature changes associated with the environment in which the magnetostrictive position detector is located.

Magnetostrictive position detectors can be used in process control and other applications to measure a position of a movable float, such as to determine a fluid level in a vessel such as a tank. Temperature changes during the process can result in inaccuracy in the position detection of the float, causing inaccuracy fluid level measurements. This is typically resolved by using a temperature sensor built into the detector, or by using a dedicated external temperature sensor to measure the temperature changes, and then determine a temperature compensation error for the position measurement that is based on the measured temperature.

The use of stand-alone temperature sensors to determine the position measurement error and compensation for the same suffers from several drawbacks. For example, costs for the instrumentation are higher due to having to provide separate instruments and/or sensors, in addition to the extra costs for installation, wiring, additional process connections, and additional inputs into the control system. Magnetostrictive position detectors with built-in sensors can alleviate some of these extra costs, but still require the addition of a temperature sensor component to the magnetostrictive position detector. Such detectors may also only provide limited temperature range capabilities. The present disclosure is directed to a system, method, and/or apparatus that provides temperature compensation for magnetostrictive position detector position measurements without requiring the use of temperature sensors.

SUMMARY

In accordance with the present disclosure, a system, method, and/or apparatus is provided for magnetostrictive position detectors to compensate position measurements for temperature conditions without the use of a built-in and/or an external stand-alone temperature sensor. As a result, savings can be realized on the cost of the magnetostrictive position detector and/or in the installation of the magnetostrictive position detector into, for example, a process control system. The present disclosure provides an algorithm to compensate for temperature conditions associated with the environment in which the magnetostrictive position detector is located by determining thermal compensation coefficients that are proportional to the environment temperature via digital processing of the signals of the magnetostrictive position detector. Also provided in accordance with the present disclosure is a computer system that is operable to perform the foregoing.

This summary is provided to introduce a selection of concepts that are further described below in the illustrative embodiments. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring toFIG.1there is shown a magnetostrictive position detector20for measuring a position12in an environment, such as a vessel10. In one embodiment, position12is a liquid level, vessel10is a tank or other object containing the liquid, and the environment is a process that involves the liquid in vessel10. However, the present disclosure has application in determining a position12in any application or environment in which the magnetostrictive position detector20is employed.

Magnetostrictive position detector20includes a sensor22connected to a sensor tube or probe24. Probe24contains a sensor wire26that is connected to an electronic circuit30in the detector20. A position marker such as a magnetic float28is mounted about probe24and is movable along the probe24in response to a change in level or position12of the liquid, as shown by the position of float28′ along probe24.

In operation, the electronic circuitry30in detector20generates a low energy current pulse32at fixed intervals which travel down wire26in probe24at the speed of light, which creates a magnetic field around wire26. The interaction of the magnetic field around wire26and the magnet associated with float28causes a torsional stress wave to be induced in the wire26, which propagates along the wire26at a known velocity from the float28toward the sensor22. Sensor22is configured to convert the received mechanical torsional stress wave into an electronic return pulse. An electronic device, such as computer system50is connected to sensor22, measures the elapsed time between the start and return pulses (time of flight) and converts the measurements into a position measurement of position12, which is proportional to the level of the float28and indicative of the level of the liquid in vessel10.

Computer system50receives input signals from sensor22and is configured to output a position determination for float28that is compensated for temperature effects. The environmental temperature changes and/or changes in temperature of the liquid creates an inaccuracy in the position measurement obtained by the time of flight determination described above. The computer system50may be incorporated into detector20, or may be a stand-alone device connected to magnetostrictive position detector20in any suitable manner, including wired and wireless connections. Computer system50may also connected in any suitable way to, or may be part of, a process controller (not shown) that may use the compensated position measurement for position12in process control. The computer system50and/or process controller may provide the temperature adjusted position measurement determination according to the present disclosure as an alarm, reading, printout, e-mail, text, process variable, signal, etc. without requiring the use of a temperature sensor to determine the temperature compensation. As a result, the functioning of the computer system50is improved and the cost of magnetostrictive position detector20and/or the associated process hardware is reduced.

The computer system50may include one or more computers for performing one or more tasks related to the production process and/or may be dedicated to magnetostrictive position detector20. At least one of the computers of the computer system50may include a user interface devices (UI)52that include one or more display devices, such as a monitor (with or without a touch screen) or a hand-held device such as a smart phone, tablet, laptop computer, or other device cell phone for displaying graphics, and one or more entry devices, such as a keyboard, a mouse, a track ball, a joystick, a hand-held device, and/or voice-activated device.

The computer system50may be configured to compensate for any one or more temperature error contributors. For example, temperature errors can be contributed to by the material of the probe24, such as metal, since the material may elongated or contract depending on the direction of the process or environmental temperature change. Temperature errors may also be introduced by changes in density of the process material (liquid) due to temperature fluctuations, which affect the buoyancy of the float28. The propagation speed of the torsional wave may also be affected by the temperature of the process and/or environment.

These sources of error each have a linear characteristic with respect to temperature change. The computer system50is configured to compensate the error introduced by the temperature changes by using a temperature factor without the need for any temperature sensors, resulting in cost reduction of the instruments and the corresponding installations. The computer system50is operable to determine the probe length PL as shown inFIG.2, which remains constant at any given temperature independently of the position of the magnet.

InFIG.2, the distance travelled by the transmitter signal reflected by the magnet of float28to the end of the probe24and back to the sensor22is R, and the distance travelled by the transmitter signal reflected from the magnet of the float28back to sensor22is L. The measured distance to the end of the probe, also known as the probe length PL, can be determined by averaging L and R as follows:
PL=(R+L)/2  Equation 1

The average times of flight measured for the first reflected signal generated by the magnet of float28, which travels from the magnet of float28to the end34of the probe24and back to the sensor22, and the second reflected signal, which travels from the magnet of float28to the sensor22, is equivalent to the time of flight for the length of the probe. Computer system50calculates the coefficient of thermal error (CTE) as the ratio between the measured PL and the reference PL (PLREF) previously measured at a calibration temperature (such as room temperature), during factory configuration.
CTE=PL/PLREFEquation 2

The coefficient of thermal error can then be applied as an inverse correction factor to the measured position12and also to the parameters of the float28to compensate for buoyancy errors as follows:
LCOMPENSATED=L/CTE−H*K/CTEEquation 3

LCOMPENSATEDis the compensated position measurement, L is position measurement, H is the immersion height of float28at room temperature, and K is a buoyancy coefficient determined by the float design. The parameters H and K can be retrieved by computer system50from, for example, one or more lookup tables stored in memory containing the design details of various floats28, or input directly into computer system50if float28is a custom designed float.

The calculation of probe length PL can be performed at any given temperature of the environment and at any position of float28along the probe24. In addition, the buoyancy correction factor H*K/CTE can be disabled for applications in which detector20is used to measure distance or position and not a liquid level.

Referring toFIG.3, one embodiment of a method for compensating for temperature effects on a position measurement by magnetostrictive position detector20is shown. Method300includes an operation302of transmitting a currently pulse generated by circuitry30down wire26to the magnetic position marker, such as float28in liquid applications.

Method300further includes an operation304to induce torsional stress waves in wire26by the interaction of the current pulse with the magnetic position marker. Method300continues at operation306to measure first and second distances based on the time of flight of the induced stress waves travelling along the wire26, which generate reflected signals detected by sensor22. The first measured distance is measured along a path traveled by a first reflected signal from the magnetic position marker to the end34of the probe24and from the end34of the probe24to the sensor22. The second measured distance is measured along a second path traveled by a second reflected signal from the magnetic position marker back to the sensor22.

Method300continues at operation308to average the first and second measured distances to determine a measured probe length for probe24. Method300continues at operation310to determine CTE in response to a ratio of the measured probe length to a reference probe length. The reference probe length can be determined in the factor at a predetermined calibration temperature and stored in computer system50. Method300continues at operation312to compensate a position measurement for the magnetic positon marker based on the determined CTE.

In environments in which the magnetic position marker is a float to determine a liquid level, the buoyancy error for the float may also be temperature compensated using the CTE. The buoyancy error is a product of an immersion height of the float at room temperature and a buoyancy coefficient of the float divided by the coefficient of thermal error. The immersion height and buoyancy coefficient can be retrieved from look-up tables based on the design of float28, or entered by the user if float28is custom.

The schematic diagrams and procedures described above are generally set forth herein. As such, the depicted order and labeled steps are indicative of representative embodiments. Other steps, orderings, combinations of steps, and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the methods illustrated in the schematic diagrams.

Additionally, the format and symbols employed are provided to explain the logical steps of the schematic diagrams and are understood not to limit the scope of the systems, apparatus, and methods illustrated by the diagrams. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown. It will also be noted that each block of the block diagrams and/or flowchart diagrams, and combinations of blocks in the block diagrams and/or flowchart diagrams, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and program code.

Many of the functional units described in this specification have been labeled in order to more particularly emphasize their implementation independence. For example, one or more aspects of detector20and/or computer system50may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. Detector20and/or computer system50may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like.

One or more aspects of detector20and/or computer system50may also be implemented in machine-readable medium for execution by various types of processors. In some instances, the machine-readable medium for execution by various types of processors may be implemented in the aforementioned hardware circuit. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions, which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified circuit need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the circuit and achieve the stated purpose for the detector20and/or computer system50.

For example, computer readable program code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data such as measurements may be identified and illustrated herein within a module, monitor, or circuit, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. Where a module, monitor, or circuit or portions thereof are implemented in machine-readable medium (or computer-readable medium), the computer readable program code may be stored and/or propagated on one or more computer readable medium(s).

The computer readable medium may also be a computer readable signal medium. A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electrical, electro-magnetic, magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport computer readable program code for use by or in connection with an instruction execution system, apparatus, or device. Computer readable program code embodied on a computer readable signal medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, Radio Frequency (RF), or the like, or any suitable combination of the foregoing.

The program code may also be stored in a computer readable medium that can direct a controller, computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified herein.

Various aspects of the present disclosure are contemplated as indicated in the claims appended hereto.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described. Those skilled in the art will appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.