US20070179738A1 - Digital method and apparatus for sensing position with a linear variable differential transformer - Google Patents
Digital method and apparatus for sensing position with a linear variable differential transformer Download PDFInfo
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- US20070179738A1 US20070179738A1 US11/341,738 US34173806A US2007179738A1 US 20070179738 A1 US20070179738 A1 US 20070179738A1 US 34173806 A US34173806 A US 34173806A US 2007179738 A1 US2007179738 A1 US 2007179738A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
- G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
- G01D5/20—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying inductance, e.g. by a movable armature
- G01D5/22—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying inductance, e.g. by a movable armature differentially influencing two coils
- G01D5/2291—Linear or rotary variable differential transformers (LVDTs/RVDTs) having a single primary coil and two secondary coils
Definitions
- This invention relates generally to determining linear position and, more particularly, to using digital means for determining linear position.
- This invention relates to determining linear position by using the electrical outputs of a a Linear Variable Differential Transformer (LVDT). It may be desireable to determine a linear position for many applications. For example, actuation systems on aircraft or rocket motors. One method of doing this is to use a LVDT to sense the position.
- the LVDT may use a sinusoidal excitation and measurement of two inductively coupled output signals.
- LVDTs have one primary winding and two secondary windings. The two secondary windings are mechanically arranged so an excitation signal on the primary winding will proportionally couple onto the secondary windings based on the position of a ferrous core. By measuring the voltages on the secondary windings it is possible to determine the linear position of the LVDT.
- the present invention reduces the complexity and number of analog devices needed to resolve linear position.
- An embodiment of the present invention comprises an apparatus for determining a linear position of a LVDT.
- the apparatus includes a first signal analyzer, a second signal analyzer, and a result calculator.
- the LVDT includes an excitation input coupled to an excitation signal with an excitation frequency, a first positional output coupled to an analog input of the first signal analyzer, and a second positional output coupled to an analog input of the second signal analyzer.
- Each of the first signal analyzer and the second signal analyzer includes a comparator, a digital estimator, a digital-to-analog converter, and an amplitude analyzer.
- the comparator is configured for comparing the analog input to an analog feedback signal and generating a comparison result.
- the digital estimator uses the comparison result to modify a digital estimate by an incremental adjustment amount at an estimation frequency in response to the comparison result.
- the digital estimate is converted to the analog feedback signal by the digital-to-analog converter.
- the amplitude analyzer collects a digital estimate history at a sample frequency that is a binary multiple of the excitation frequency and determines an amplitude of the digital estimate substantially near the excitation frequency. With the amplitudes determined, the result calculator evaluates the amplitude for each of the first signal analyzer and second signal analyzer to generate the linear position.
- Another embodiment of the present invention comprises a method for determining a linear position.
- the method comprises resolving an excitation signal at an excitation frequency into at least two correlated signals, converting the at least two correlated signals to a digital estimate for each of the correlated signals, and evaluating an amplitude of the digital estimate of the correlated signals to determine the linear position.
- the process of converting the correlated signals comprises comparing the correlated signal to an analog feedback signal to generate a comparison result and incrementally adjusting the digital estimate in response to sampling the comparison result at an estimation frequency.
- the converting process also includes converting the digital estimate to the analog feedback signal, collecting a digital estimate history at a sample frequency that is a binary multiple of the excitation frequency, and analyzing the digital estimate history to determine the amplitude of the digital estimate substantially near the excitation frequency.
- Yet another embodiment, in accordance with the present invention comprises a method of determining a linear position.
- This method includes resolving an excitation signal at an excitation frequency into a first signal and a second signal, converting the first signal to a first digital estimate, converting the second signal to a second digital estimate, evaluating a first amplitude and a second amplitude to determine the linear position.
- Converting the first signal includes comparing the first signal to a first analog feedback signal to generate a first comparison result.
- Converting the first signal also includes incrementally adjusting the first digital estimate in response to sampling the first comparison result at an estimation frequency, converting the first digital estimate to the first analog feedback signal, and analyzing the first digital estimate to determine the first amplitude of the first signal substantially near the excitation frequency.
- converting the second signal includes comparing the second signal to a second analog feedback signal to generate a second comparison result. Converting the second signal also includes incrementally adjusting the second digital estimate in response to sampling the second comparison result at the estimation frequency, converting the second digital estimate to the second analog feedback signal, and analyzing the second digital estimate to determine the second amplitude of the second signal substantially near the excitation frequency.
- FIG. 1 is a schematic depiction of a linear variable differential transformer (LVDT);
- LVDT linear variable differential transformer
- FIG. 2A illustrates a representative excitation waveform for input to a LVDT
- FIG. 2B illustrates a representative first positional output waveform from a LVDT when the excitation input is the excitation waveform of FIG. 2A and the core of the LVDT is moving at a constant rate;
- FIG. 2C illustrates a representative second positional output waveform from a LVDT when the excitation input is the excitation waveform of FIG. 2A and the core of the LVDT is moving at a constant rate;
- FIG. 3 is a schematic depiction of a representative embodiment of the present invention including a LVDT
- FIG. 4 is a schematic depiction of a representative embodiment of a signal analyzer according to the present invention.
- FIG. 5 is a schematic depiction of a representative embodiment of a digital converter according to the present invention.
- FIG. 6 is a schematic depiction of a representative embodiment of a digital-to-analog converter according to the present invention.
- FIG. 7A illustrates a representative pulse width modulation signal for generating a sine wave
- FIG. 7B illustrates a sine wave that may be produced by low pass filtering the pulse width modulation signal of FIG. 7A ;
- FIG. 8 is a schematic depiction of a representative embodiment of an excitation signal generator according to the present invention.
- FIG. 9 is a schematic depiction of a representative embodiment of an amplitude analyzer according to the present invention.
- the present invention reduces the complexity and number of analog devices needed to resolve linear position obtained from a linear variable differential transformer (LVDT).
- LVDT linear variable differential transformer
- circuits and functions may be shown in block diagram form in order not to obscure the present invention in unnecessary detail. Conversely, specific circuit implementations shown and described are exemplary only and should not be construed as the only way to implement the present invention unless specified otherwise herein. Additionally, block definitions and partitioning of logic between various blocks is exemplary of a specific implementation. It will be readily apparent to one of ordinary skill in the art that the present invention may be practiced by numerous other partitioning solutions. For the most part, details concerning timing considerations and the like have been omitted where such details are not necessary to obtain a complete understanding of the present invention and are within the abilities of persons of ordinary skill in the relevant art.
- signals may represent a bus of signals, wherein the bus may have a variety of bit widths and the present invention may be implemented on any number of data signals including a single data signal.
- signals may be referred to as asserted and negated. Those of ordinary skill in the art will recognize that in most instances, the selection of asserted or negated may be arbitrary and the invention could be implemented with the opposite states for such signals.
- FIG. 1 illustrates a LVDT 100 .
- the LVDT 100 includes an excitation input 110 attached to a primary coil 102 and at least two secondary coils 104 attached to modulation outputs ( 120 and 130 ).
- the LVDT also includes a ferrous core 115 , which moves linearly relative to the primary coil 102 and secondary coils 104 .
- the core may be attached to a linkage or other means for mechanical linking (not shown) to external parts (not shown).
- the primary coil 102 and secondary coils 104 include windings positioned such that when an electrical signal is induced in the primary coil 102 inductive coupling produces electrical signals in the secondary coils 104 windings.
- the secondary coils 104 may be positioned such that, the amount of inductive coupling to each secondary coils 104 may be different and dependent on the position of the core 115 .
- the modulation outputs ( 120 and 130 ) may also be referred to as correlated signals ( 120 and 130 ) or as a first positional output 120 and a second positional output 130 .
- FIG. 2A illustrates an excitation signal 112 that may be applied to the excitation input 110 coupled to the primary coil.
- the excitation signal 112 is a sinusoidal signal driven through the primary coil, which inductively induces modulated signals in the correlated outputs ( 120 and 130 ).
- FIGS. 2B and 2C illustrate the modulation outputs that may be present when the primary coil 102 is moving at a substantially constant rate from one extreme to the other.
- a first signal 122 on the first positional output 120 includes a modulated amplitude that is modulated along an increasing envelope 106 having a substantially linear rate corresponding to the movement of the core 115 .
- a second signal 132 on the second positional output includes a modulated amplitude that is modulated along a decreasing envelope 108 having a substantially linear rate corresponding to the movement of the core 115 .
- FIGS. 2A, 2B , and 2 C are simple examples illustrating general operation with a simple constant linear movement.
- the envelopes ( 106 and 108 ) representing movement of the core 115 may be complex, and perhaps vibratory waveforms.
- the excitation frequency should be substantially higher than an expected rate of change of the core 115 displacement.
- the excitation signal 112 may have an excitation frequency generally in the range of about 1 to 10 kHz, but the scope of the invention is not limited to this range.
- FIG. 3 illustrates a representative embodiment of the present invention.
- the excitation input 110 couples to the primary coil of the LVDT 100 .
- the secondary coils of the LVDT 100 are coupled to a first positional output 120 and a second positional output 130 .
- Each of the first positional output 120 and the second positional output 130 are coupled to a signal analyzer ( 200 A and 200 B).
- Each signal analyzer ( 200 A and 200 B) generates an amplitude ( 290 A and 290 B) for its respective input signal.
- a result calculator 300 receives the outputs from the signal analyzers ( 200 A and ( 200 B) to calculate the linear position 340 .
- the position of the core 115 may be determined by the difference between the two secondary windings divided by their sum.
- the result calculator 300 may comprise an arithmetic unit configured for calculating the equation (A ⁇ B)/(A+B) wherein A represents the amplitude 290 A from the first signal analyzer 200 A, B represents the amplitude 290 B from the second signal analyzer 200 B, and the calculation result represents the linear position 340 .
- the result calculator also may comprises dedicated circuitry for calculating the linear position as illustrated in FIG. 3 .
- a subtractor subtracts the second amplitude 290 B from the first amplitude 290 A.
- an adder add the first amplitude 290 A and second amplitude 290 B.
- a divider 320 divides the result from the subtractor by the result from the adder to determine the linear position 340 .
- the final result is a digital representation of the linear position 340 .
- An optional filter 336 may be used to filter the linear position 340 with a conventional digital filtering algorithm to further reduce noise and generate a filtered linear position 350 .
- FIG. 4 illustrates a representative embodiment of the signal analyzer 200 .
- the signal analyzer 200 is the same for both the first signal analyzer 200 A and the second signal analyzer 200 B.
- the first positional output 120 and the second positional output 130 couple to the input signal 205 of their respective signal analyzers ( 200 A and 200 B).
- the amplitude 290 output from the signal analyzer 200 couples to the corresponding amplitude output of the first signal analyzer 200 A and the second signal analyzer 200 B.
- the input signal 205 couples to a digital converter 210 , which converts the analog input signal to a digital estimate 240 .
- the digital estimate 240 is used by an amplitude analyzer 400 to generate the amplitude of the input signal 205 .
- the amplitude analyzer 400 repeatedly samples the digital estimate 240 using the sample clock 405 to create a digital estimate history, which may be used to convert the time varying digital estimate 240 from the time domain to the frequency domain.
- the outputs of the amplitude analyzer 400 is a digital signal indicating the amplitude 290 of the modulated signal substantially near the excitation frequency.
- the output of the first amplitude analyzer is a digital value indicating the amplitude 290 A of the first positional output 120 substantially near the excitation frequency
- the output of the second amplitude analyzer is a digital value indicating the amplitude 290 B of the second positional output 130 substantially near the excitation frequency.
- the amplitude analyzer 400 is explained more fully below.
- a digital converter 210 is used to provide a continuously available estimate of the input signal 205 accurate to within one bit.
- a continuously available estimate may be advantageous in that it does not have the sample and hold characteristics of many conventional analog-to-digital converters and may not need to be synchronized to other clocks within the system.
- the input signal 205 couples to a simple fast analog comparator 220 .
- the other input of the analog comparator 220 is coupled to an analog feedback signal 260 .
- the comparison result 225 is a digital signal that may be asserted if the input signal 205 is larger than the analog feedback signal 260 and negated if the input signal 205 is smaller than the analog feedback signal 260 .
- a digital estimator 230 which is controlled by an estimation clock 215 , analyzes the comparison result 225 to update the digital estimate 240 .
- the update rate at which the estimation clock 215 runs is selected such that the estimate will always be able to track the input signal 205 .
- the estimation frequency may be a substantially higher frequency than the excitation frequency.
- the estimation clock 215 may run at or above one Mhz for an excitation frequency of about 10 Khz.
- the digital converter 210 is a feedback loop that begins by selecting a starting digital estimate 240 of the signal amplitude, which is stored in an estimate register 238 .
- the comparison result 225 is used by adjustment logic 232 to determine whether the digital estimate 240 should be improved by modifying the digital estimate 240 by an incremental adjustment amount.
- the adjustment logic 232 may generate an adjustment signal 236 for incrementing, decrementing, or maintaining the digital estimate 240 .
- the resulting new digital estimate couples to a digital-to-analog converter 250 , which generates the analog feedback signal 260 for comparison in the comparator 220 .
- the feedback loop continues until the digital estimate 240 is an accurate representation of the input signal 205 . Then, as the input signal 205 changes, the digital converter 210 can easily track the changes through the adjustment logic 232 and feedback loop.
- this method may be unacceptably slow.
- the present invention takes advantage of the A Priori knowledge that the input signal will be substantially a sinusoidal wave of known frequency and limited, but varying, amplitude.
- the update rate i.e., the estimation frequency
- this method is faster than other conversion methods, provides an estimate accurate to within one bit, and provides an estimate that is continuously available to other circuitry in the system.
- the digital-to-analog converter 250 may be implemented in a variety of ways known to those of ordinary skill in the art.
- One representative embodiment of the digital-to-analog converter 250 that may be simple to implement is illustrated in FIG. 6 .
- the digital estimate 240 is used by a pulse-width modulator 252 (PWM), which converts the digital estimate 240 into a pulse-width modulated estimate 254 , which is a series of pulses with varying duty cycles corresponding to the digital estimate values.
- An analog filter 256 filters the pulse-width modulated estimate 254 to generate the analog feedback signal 260 to represent variations in the digital estimate 240 .
- FIG. 7A illustrates an example pulse-width modulated signal 254 .
- the width of the pulses i.e., the duty cycle
- the pulse-width modulated estimate 254 includes a varying amount of energy, which corresponds to the high portion of the pulses. Therefore, the pulse-width modulated estimate 254 may be filtered by a simple low pass analog filter 256 to generate the analog feedback signal 260 (shown as signal plot 285 in FIG. 7B ).
- the excitation input 110 may be configured as a sine wave with an excitation frequency.
- a simple excitation generator 500 may be used for generating the excitation input 110 , as illustrated in FIG. 8 .
- An amplitude generator 510 creates a digital excitation signal 520 with values that vary at a generation frequency.
- the digital excitation signal 520 is used by a PWM 252 ′, which converts the digital excitation signal 520 into a pulse-width modulated signal 530 .
- An analog filter 256 ′ filters the pulse-width modulated signal 530 to generate the excitation input 110 .
- the function of the amplitude generator 510 including the pulse-width modulator 252 ′ and analog filter 256 ′ is similar to what was described for the digital-to-analog converter 250 of FIGS. 6, 7A and 7 B.
- the amplitude generator 510 creates the desired signal.
- the amplitude generator 510 creates a digital excitation signal 520 , which is a time varying digital representation for emulating a sine wave.
- the amplitude generator 510 may be some type of arithmetic unit for calculating sine waves, or it may be a simple look-up table with the proper amplitudes for generating a sine wave.
- the amplitude analyzer 400 converts the time varying digital estimate 240 from the time domain to the frequency domain. Generally, converting a time domain signal to the frequency domain generates a function with amplitudes at a variety of frequencies.
- the output of the amplitude analyzer 400 is a digital signal indicating the amplitude 290 of the modulated signal substantially near the excitation frequency.
- a number of implementation for finding the amplitude 290 of the modulated signal substantially near the excitation frequency may be used, such as, for example, implementing a conventional Fast Fourier Transform (FFT). However, a simpler implementation may be used for the present invention because only the amplitude 290 at the excitation frequency is needed.
- FFT Fast Fourier Transform
- FIG. 9 illustrates an implementation of an amplitude analyzer 400 .
- a sample clock 405 running at a sample frequency feeds a history shift register 420 configured to sample and shift values of the digital estimate 240 .
- the history shift register 420 generates a digital estimate history ( 425 ) N bits long.
- the digital estimate history 425 is coupled to a set of summing units 430 in a butterfly pattern recognizable to those of ordinary skill in the art in performing a Discrete Fourier Transform (DFT), except that only the calculations necessary to determine the amplitude at the excitation frequency are performed.
- a first set of difference units 435 perform subtractions on the results from the first set of summing units.
- a set of multipliers 440 multiply the subtraction results from the first set of difference units by the appropriate constants for a DFT.
- a second set of difference units 445 perform subtractions on the results from the multipliers 440 .
- a third set of difference units 450 perform subtractions on the results from the second set of difference units 445 .
- a set of squaring units 455 square the absolute value of the results from the third set of difference units 450 .
- a summing unit 460 adds the results from the set of squaring units 455 , and a square root unit 465 calculates the square root of the result from the summing unit 460 to arrive at the final amplitude 290 .
- the number of bits N in the digital estimate history 425 may be chosen to be a binary multiple. In the example of FIG. 9 , the number of bits is chosen as 16 to generate the digital estimate history 425 of signals td 0 -td 15 .
- the sample frequency of the sample clock 405 is chosen to correspond to the number of bits such that the sample frequency is a binary multiple of the excitation frequency. Thus, in the example of FIG. 9 , the sample frequency is set at 16 times the excitation frequency such that digital estimate history 425 comprises samples of one full cycle of the excitation frequency.
- the implementation of FIG. 9 performs a limited DFT in that it only calculates the amplitude at the excitation frequency. For each sample point, the limited DFT is taken to provide the instantaneous amplitude of the input signal at the base excitation frequency. As each sample is taken, the new value is operated on along with the previous 15 samples using the limited DFT.
- the amplitude analyzer 400 may encompass other bit widths and sample rates for the limited DFT.
- the limited DFT may use a sample frequency that is of 2 N times the sample frequency wherein N may be in a range from 2 to 10.
- the amplitude analyzer 400 also encompasses implementations that perform a full DFT or FFT to determine the amplitude of the digital estimate at the excitation frequency.
Abstract
Description
- The present application is related to concurrently filed U. S. patent application Ser. No. (2507-7532US) (22154-US) and entitled DIGITAL METHOD AND APPARATUS FOR RESOLVING SHAFT POSITION.
- 1. Field of the Invention
- This invention relates generally to determining linear position and, more particularly, to using digital means for determining linear position.
- 2. Description of Related Art
- This invention relates to determining linear position by using the electrical outputs of a a Linear Variable Differential Transformer (LVDT). It may be desireable to determine a linear position for many applications. For example, actuation systems on aircraft or rocket motors. One method of doing this is to use a LVDT to sense the position. The LVDT may use a sinusoidal excitation and measurement of two inductively coupled output signals. LVDTs have one primary winding and two secondary windings. The two secondary windings are mechanically arranged so an excitation signal on the primary winding will proportionally couple onto the secondary windings based on the position of a ferrous core. By measuring the voltages on the secondary windings it is possible to determine the linear position of the LVDT.
- These output signals from the two secondary windings are generally analog signals, which may require a significant amount of analog electronics to evaluate the signal amplitudes and derive the shaft position. As a result, many proposals use analog-to-digital converters to convert the analog signals to digital signals, which may then be manipulated digitally to determine the respective amplitudes and calculate arithmetic functions to determine the shaft position. However, even these solutions may require complex analog-to-digital converters, and complex arithmetic engines for determining the signal amplitudes.
- There is a need for a method and apparatus that reduces the complexity and number of analog components used in determining linear position by using simple analog components coupled to flexible digital logic and digital signal processing.
- The present invention reduces the complexity and number of analog devices needed to resolve linear position.
- An embodiment of the present invention comprises an apparatus for determining a linear position of a LVDT. The apparatus includes a first signal analyzer, a second signal analyzer, and a result calculator. The LVDT includes an excitation input coupled to an excitation signal with an excitation frequency, a first positional output coupled to an analog input of the first signal analyzer, and a second positional output coupled to an analog input of the second signal analyzer. Each of the first signal analyzer and the second signal analyzer includes a comparator, a digital estimator, a digital-to-analog converter, and an amplitude analyzer. The comparator is configured for comparing the analog input to an analog feedback signal and generating a comparison result. The digital estimator uses the comparison result to modify a digital estimate by an incremental adjustment amount at an estimation frequency in response to the comparison result. The digital estimate is converted to the analog feedback signal by the digital-to-analog converter. The amplitude analyzer collects a digital estimate history at a sample frequency that is a binary multiple of the excitation frequency and determines an amplitude of the digital estimate substantially near the excitation frequency. With the amplitudes determined, the result calculator evaluates the amplitude for each of the first signal analyzer and second signal analyzer to generate the linear position.
- Another embodiment of the present invention comprises a method for determining a linear position. The method comprises resolving an excitation signal at an excitation frequency into at least two correlated signals, converting the at least two correlated signals to a digital estimate for each of the correlated signals, and evaluating an amplitude of the digital estimate of the correlated signals to determine the linear position. The process of converting the correlated signals comprises comparing the correlated signal to an analog feedback signal to generate a comparison result and incrementally adjusting the digital estimate in response to sampling the comparison result at an estimation frequency. The converting process also includes converting the digital estimate to the analog feedback signal, collecting a digital estimate history at a sample frequency that is a binary multiple of the excitation frequency, and analyzing the digital estimate history to determine the amplitude of the digital estimate substantially near the excitation frequency.
- Yet another embodiment, in accordance with the present invention comprises a method of determining a linear position. This method includes resolving an excitation signal at an excitation frequency into a first signal and a second signal, converting the first signal to a first digital estimate, converting the second signal to a second digital estimate, evaluating a first amplitude and a second amplitude to determine the linear position. Converting the first signal includes comparing the first signal to a first analog feedback signal to generate a first comparison result. Converting the first signal also includes incrementally adjusting the first digital estimate in response to sampling the first comparison result at an estimation frequency, converting the first digital estimate to the first analog feedback signal, and analyzing the first digital estimate to determine the first amplitude of the first signal substantially near the excitation frequency. Similarly, converting the second signal includes comparing the second signal to a second analog feedback signal to generate a second comparison result. Converting the second signal also includes incrementally adjusting the second digital estimate in response to sampling the second comparison result at the estimation frequency, converting the second digital estimate to the second analog feedback signal, and analyzing the second digital estimate to determine the second amplitude of the second signal substantially near the excitation frequency.
- In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:
-
FIG. 1 is a schematic depiction of a linear variable differential transformer (LVDT); -
FIG. 2A illustrates a representative excitation waveform for input to a LVDT; -
FIG. 2B illustrates a representative first positional output waveform from a LVDT when the excitation input is the excitation waveform ofFIG. 2A and the core of the LVDT is moving at a constant rate; -
FIG. 2C illustrates a representative second positional output waveform from a LVDT when the excitation input is the excitation waveform ofFIG. 2A and the core of the LVDT is moving at a constant rate; -
FIG. 3 is a schematic depiction of a representative embodiment of the present invention including a LVDT; -
FIG. 4 is a schematic depiction of a representative embodiment of a signal analyzer according to the present invention; -
FIG. 5 is a schematic depiction of a representative embodiment of a digital converter according to the present invention; -
FIG. 6 is a schematic depiction of a representative embodiment of a digital-to-analog converter according to the present invention; -
FIG. 7A illustrates a representative pulse width modulation signal for generating a sine wave; -
FIG. 7B illustrates a sine wave that may be produced by low pass filtering the pulse width modulation signal ofFIG. 7A ; -
FIG. 8 is a schematic depiction of a representative embodiment of an excitation signal generator according to the present invention; and -
FIG. 9 is a schematic depiction of a representative embodiment of an amplitude analyzer according to the present invention. - The present invention reduces the complexity and number of analog devices needed to resolve linear position obtained from a linear variable differential transformer (LVDT).
- In the following description, circuits and functions may be shown in block diagram form in order not to obscure the present invention in unnecessary detail. Conversely, specific circuit implementations shown and described are exemplary only and should not be construed as the only way to implement the present invention unless specified otherwise herein. Additionally, block definitions and partitioning of logic between various blocks is exemplary of a specific implementation. It will be readily apparent to one of ordinary skill in the art that the present invention may be practiced by numerous other partitioning solutions. For the most part, details concerning timing considerations and the like have been omitted where such details are not necessary to obtain a complete understanding of the present invention and are within the abilities of persons of ordinary skill in the relevant art.
- In this description, some drawings may illustrate signals as a single signal for clarity of presentation and description. It will be understood by a person of ordinary skill in the art that the signal may represent a bus of signals, wherein the bus may have a variety of bit widths and the present invention may be implemented on any number of data signals including a single data signal. Furthermore, signals may be referred to as asserted and negated. Those of ordinary skill in the art will recognize that in most instances, the selection of asserted or negated may be arbitrary and the invention could be implemented with the opposite states for such signals.
-
FIG. 1 illustrates aLVDT 100. TheLVDT 100 includes anexcitation input 110 attached to aprimary coil 102 and at least twosecondary coils 104 attached to modulation outputs (120 and 130). The LVDT also includes aferrous core 115, which moves linearly relative to theprimary coil 102 andsecondary coils 104. Generally, the core may be attached to a linkage or other means for mechanical linking (not shown) to external parts (not shown). Theprimary coil 102 andsecondary coils 104 include windings positioned such that when an electrical signal is induced in theprimary coil 102 inductive coupling produces electrical signals in thesecondary coils 104 windings. Thesecondary coils 104 may be positioned such that, the amount of inductive coupling to eachsecondary coils 104 may be different and dependent on the position of thecore 115. Thus, by measuring the voltages on thesecondary coils 104 it is possible to determine the linear position of thecore 115 and, as a result, the linear position of a linkage attached thereto. Throughout this description, the modulation outputs (120 and 130) may also be referred to as correlated signals (120 and 130) or as a firstpositional output 120 and a secondpositional output 130. - By way of example,
FIG. 2A illustrates anexcitation signal 112 that may be applied to theexcitation input 110 coupled to the primary coil. In this example, theexcitation signal 112 is a sinusoidal signal driven through the primary coil, which inductively induces modulated signals in the correlated outputs (120 and 130).FIGS. 2B and 2C illustrate the modulation outputs that may be present when theprimary coil 102 is moving at a substantially constant rate from one extreme to the other. Afirst signal 122 on the firstpositional output 120 includes a modulated amplitude that is modulated along an increasingenvelope 106 having a substantially linear rate corresponding to the movement of thecore 115. Similarly, asecond signal 132 on the second positional output includes a modulated amplitude that is modulated along a decreasingenvelope 108 having a substantially linear rate corresponding to the movement of thecore 115.FIGS. 2A, 2B , and 2C are simple examples illustrating general operation with a simple constant linear movement. Those of ordinary skill in the art will recognize that the envelopes (106 and 108) representing movement of thecore 115 may be complex, and perhaps vibratory waveforms. As a result, the excitation frequency should be substantially higher than an expected rate of change of thecore 115 displacement. For example, theexcitation signal 112 may have an excitation frequency generally in the range of about 1 to 10 kHz, but the scope of the invention is not limited to this range. -
FIG. 3 illustrates a representative embodiment of the present invention. Theexcitation input 110 couples to the primary coil of theLVDT 100. The secondary coils of theLVDT 100 are coupled to a firstpositional output 120 and a secondpositional output 130. Each of the firstpositional output 120 and the secondpositional output 130 are coupled to a signal analyzer (200A and 200B). Each signal analyzer (200A and 200B) generates an amplitude (290A and 290B) for its respective input signal. - A
result calculator 300 receives the outputs from the signal analyzers (200A and (200B) to calculate thelinear position 340. The position of thecore 115 may be determined by the difference between the two secondary windings divided by their sum. Thus, theresult calculator 300 may comprise an arithmetic unit configured for calculating the equation (A−B)/(A+B) wherein A represents theamplitude 290A from the first signal analyzer 200A, B represents theamplitude 290B from thesecond signal analyzer 200B, and the calculation result represents thelinear position 340. - The result calculator also may comprises dedicated circuitry for calculating the linear position as illustrated in
FIG. 3 . InFIG. 3 , a subtractor subtracts thesecond amplitude 290B from thefirst amplitude 290A. Similarly, an adder add thefirst amplitude 290A andsecond amplitude 290B. Adivider 320 divides the result from the subtractor by the result from the adder to determine thelinear position 340. The final result is a digital representation of thelinear position 340. Anoptional filter 336 may be used to filter thelinear position 340 with a conventional digital filtering algorithm to further reduce noise and generate a filteredlinear position 350. -
FIG. 4 illustrates a representative embodiment of thesignal analyzer 200. Thesignal analyzer 200 is the same for both the first signal analyzer 200A and thesecond signal analyzer 200B. Thus, the firstpositional output 120 and the secondpositional output 130 couple to theinput signal 205 of their respective signal analyzers (200A and 200B). Similarly, theamplitude 290 output from thesignal analyzer 200 couples to the corresponding amplitude output of the first signal analyzer 200A and thesecond signal analyzer 200B. - In the
signal analyzer 200, the input signal 205 couples to adigital converter 210, which converts the analog input signal to adigital estimate 240. Thedigital estimate 240 is used by anamplitude analyzer 400 to generate the amplitude of theinput signal 205. - The
amplitude analyzer 400 repeatedly samples thedigital estimate 240 using thesample clock 405 to create a digital estimate history, which may be used to convert the time varyingdigital estimate 240 from the time domain to the frequency domain. The outputs of theamplitude analyzer 400 is a digital signal indicating theamplitude 290 of the modulated signal substantially near the excitation frequency. In other words, the output of the first amplitude analyzer is a digital value indicating theamplitude 290A of the firstpositional output 120 substantially near the excitation frequency and the output of the second amplitude analyzer is a digital value indicating theamplitude 290B of the secondpositional output 130 substantially near the excitation frequency. Theamplitude analyzer 400 is explained more fully below. - A
digital converter 210 is used to provide a continuously available estimate of theinput signal 205 accurate to within one bit. A continuously available estimate may be advantageous in that it does not have the sample and hold characteristics of many conventional analog-to-digital converters and may not need to be synchronized to other clocks within the system. - Details of a representative embodiment of the
digital converter 210 are illustrated inFIG. 5 . In thedigital converter 210, the input signal 205 couples to a simplefast analog comparator 220. The other input of theanalog comparator 220 is coupled to ananalog feedback signal 260. Thecomparison result 225 is a digital signal that may be asserted if theinput signal 205 is larger than theanalog feedback signal 260 and negated if theinput signal 205 is smaller than theanalog feedback signal 260. - A
digital estimator 230, which is controlled by anestimation clock 215, analyzes thecomparison result 225 to update thedigital estimate 240. The update rate at which theestimation clock 215 runs is selected such that the estimate will always be able to track theinput signal 205. Thus, the estimation frequency may be a substantially higher frequency than the excitation frequency. For example, and not limitation, theestimation clock 215 may run at or above one Mhz for an excitation frequency of about 10 Khz. - The
digital converter 210 is a feedback loop that begins by selecting a startingdigital estimate 240 of the signal amplitude, which is stored in anestimate register 238. Thecomparison result 225 is used byadjustment logic 232 to determine whether thedigital estimate 240 should be improved by modifying thedigital estimate 240 by an incremental adjustment amount. Thus, based on thecomparison result 225, theadjustment logic 232 may generate anadjustment signal 236 for incrementing, decrementing, or maintaining thedigital estimate 240. The resulting new digital estimate couples to a digital-to-analog converter 250, which generates theanalog feedback signal 260 for comparison in thecomparator 220. The feedback loop continues until thedigital estimate 240 is an accurate representation of theinput signal 205. Then, as the input signal 205 changes, thedigital converter 210 can easily track the changes through theadjustment logic 232 and feedback loop. - For many applications, this method may be unacceptably slow. However, the present invention takes advantage of the A Priori knowledge that the input signal will be substantially a sinusoidal wave of known frequency and limited, but varying, amplitude. The update rate (i.e., the estimation frequency) is selected such that the estimate will always be able to accurately track the input signal. As a result, this method is faster than other conversion methods, provides an estimate accurate to within one bit, and provides an estimate that is continuously available to other circuitry in the system.
- The digital-to-
analog converter 250 may be implemented in a variety of ways known to those of ordinary skill in the art. One representative embodiment of the digital-to-analog converter 250 that may be simple to implement is illustrated inFIG. 6 . Thedigital estimate 240 is used by a pulse-width modulator 252 (PWM), which converts thedigital estimate 240 into a pulse-width modulatedestimate 254, which is a series of pulses with varying duty cycles corresponding to the digital estimate values. Ananalog filter 256 filters the pulse-width modulatedestimate 254 to generate theanalog feedback signal 260 to represent variations in thedigital estimate 240. -
FIG. 7A illustrates an example pulse-width modulatedsignal 254. The width of the pulses (i.e., the duty cycle) is varied in proportion to the magnitude of the digital estimate to generate the pulse train with varying pulse widths. As a result, the pulse-width modulatedestimate 254 includes a varying amount of energy, which corresponds to the high portion of the pulses. Therefore, the pulse-width modulatedestimate 254 may be filtered by a simple lowpass analog filter 256 to generate the analog feedback signal 260 (shown assignal plot 285 inFIG. 7B ). - Returning to
FIG. 3 , theexcitation input 110 may be configured as a sine wave with an excitation frequency. Asimple excitation generator 500 may be used for generating theexcitation input 110, as illustrated inFIG. 8 . Anamplitude generator 510 creates adigital excitation signal 520 with values that vary at a generation frequency. Thedigital excitation signal 520 is used by aPWM 252′, which converts thedigital excitation signal 520 into a pulse-width modulatedsignal 530. Ananalog filter 256′ filters the pulse-width modulatedsignal 530 to generate theexcitation input 110. - The function of the
amplitude generator 510 including the pulse-width modulator 252′ andanalog filter 256′ is similar to what was described for the digital-to-analog converter 250 ofFIGS. 6, 7A and 7B. However, for theexcitation generator 500 theamplitude generator 510 creates the desired signal. Thus, inFIG. 8 theamplitude generator 510 creates adigital excitation signal 520, which is a time varying digital representation for emulating a sine wave. Theamplitude generator 510 may be some type of arithmetic unit for calculating sine waves, or it may be a simple look-up table with the proper amplitudes for generating a sine wave. - Returning to
FIG. 4 , theamplitude analyzer 400 converts the time varyingdigital estimate 240 from the time domain to the frequency domain. Generally, converting a time domain signal to the frequency domain generates a function with amplitudes at a variety of frequencies. The output of theamplitude analyzer 400 is a digital signal indicating theamplitude 290 of the modulated signal substantially near the excitation frequency. A number of implementation for finding theamplitude 290 of the modulated signal substantially near the excitation frequency may be used, such as, for example, implementing a conventional Fast Fourier Transform (FFT). However, a simpler implementation may be used for the present invention because only theamplitude 290 at the excitation frequency is needed. -
FIG. 9 illustrates an implementation of anamplitude analyzer 400. Asample clock 405 running at a sample frequency feeds ahistory shift register 420 configured to sample and shift values of thedigital estimate 240. Thus, thehistory shift register 420 generates a digital estimate history (425) N bits long. - The
digital estimate history 425 is coupled to a set of summingunits 430 in a butterfly pattern recognizable to those of ordinary skill in the art in performing a Discrete Fourier Transform (DFT), except that only the calculations necessary to determine the amplitude at the excitation frequency are performed. A first set ofdifference units 435 perform subtractions on the results from the first set of summing units. A set ofmultipliers 440 multiply the subtraction results from the first set of difference units by the appropriate constants for a DFT. A second set ofdifference units 445 perform subtractions on the results from themultipliers 440. A third set ofdifference units 450 perform subtractions on the results from the second set ofdifference units 445. A set of squaringunits 455 square the absolute value of the results from the third set ofdifference units 450. A summingunit 460 adds the results from the set of squaringunits 455, and asquare root unit 465 calculates the square root of the result from the summingunit 460 to arrive at thefinal amplitude 290. - The number of bits N in the
digital estimate history 425 may be chosen to be a binary multiple. In the example ofFIG. 9 , the number of bits is chosen as 16 to generate thedigital estimate history 425 of signals td0-td15. In addition, the sample frequency of thesample clock 405 is chosen to correspond to the number of bits such that the sample frequency is a binary multiple of the excitation frequency. Thus, in the example ofFIG. 9 , the sample frequency is set at 16 times the excitation frequency such thatdigital estimate history 425 comprises samples of one full cycle of the excitation frequency. - In operation, the implementation of
FIG. 9 performs a limited DFT in that it only calculates the amplitude at the excitation frequency. For each sample point, the limited DFT is taken to provide the instantaneous amplitude of the input signal at the base excitation frequency. As each sample is taken, the new value is operated on along with the previous 15 samples using the limited DFT. - Those of ordinary skill in the art will recognize that embodiments of the
amplitude analyzer 400 may encompass other bit widths and sample rates for the limited DFT. For example, and not limitation, the limited DFT may use a sample frequency that is of 2 N times the sample frequency wherein N may be in a range from 2 to 10. In addition, as stated earlier, theamplitude analyzer 400 also encompasses implementations that perform a full DFT or FFT to determine the amplitude of the digital estimate at the excitation frequency. - Although this invention has been described with reference to particular embodiments, the invention is not limited to these described embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods that operate according to the principles of the invention as described.
Claims (32)
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