US 20070179738 A1 Abstract An apparatus and method for determining a linear position from a linear variable differential transformer (LVDT) including a primary coil driven by an excitation signal, and two secondary coils coupled to two correlated signals. The method includes converting the correlated signals to a digital estimate, for each of the correlated signals, and evaluating an amplitude 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 substantially near the excitation frequency.
Claims(32) 1. An apparatus for determining a linear position of a movable component of a linear variable differential transformer (LVDT), which includes a primary coil driven by an excitation signal at an excitation frequency, a first positional output, and a second positional output, the apparatus comprising:
a first signal analyzer including an analog input operably coupled to the first positional output and a second signal analyzer including an analog input operably coupled to the second positional output, each of the first signal analyzer and the second signal analyzer comprising:
a feedback loop configured to generate a digital estimate that substantially tracks an amplitude of the analog input, the feedback loop comprising:
a comparator configured for comparing the analog input to an analog feedback signal and generating a comparison result;
a digital estimator operating at an estimation frequency and configured to sample the comparison result and modify the digital estimate by an incremental adjustment amount responsive to the comparison result to reduce a difference between the amplitude of the analog input signal and an amplitude of the analog feedback signal; and
a digital-to-analog converter configured to convert the digital estimate to the analog feedback signal; and
an amplitude analyzer configured to collect a digital estimate history by sampling the digital estimate at a sample frequency that is a binary multiple of the excitation frequency and analyze the digital estimate history to determine an amplitude of the digital estimate substantially near the excitation frequency; and
a result calculator configured to evaluate the amplitude of the digital estimate from the first signal analyzer and the amplitude of the digital estimate from the second signal analyzer to generate a numerical representation of the linear position of the movable component. 2. The apparatus of 3. The apparatus of 4. The apparatus of an amplitude generator configured for generating a digital representation of the excitation signal at each time period of a generation frequency; a pulse-width modulator configured for converting the digital representation to a digital pulse with a duty cycle correlated to the digital representation to generate a pulse-width modulated signal; and a low pass filter configured for filtering the pulse-width modulated signal to generate the excitation signal. 5. The apparatus of 6. The apparatus of 7. The apparatus of a first adder configured for adding the amplitude of the first signal analyzer to the amplitude of the second signal analyzer to generate a first term; a second adder configured for subtracting the amplitude of the second signal analyzer from the amplitude of the first signal analyzer to generate a second term; and a divider configured for dividing the second term by the first term to generate the linear position. 8. The apparatus of a pulse-width modulator configured for converting the digital estimate to a digital pulse with a duty cycle correlated to the digital estimate to generate a pulse-width modulated estimate; and a low pass filter configured for filtering the pulse-width modulated estimate to generate the analog feedback signal. 9. The apparatus of 10. The apparatus of ^{N }times the sample frequency wherein N is in a range from 2 to 10. 11. The apparatus of 12. A method for determining a linear position, comprising:
generating at least two correlated signals from a Linear Variable Differential Transformer (LVDT) by stimulating an excitation input with an excitation signal, wherein the at least two correlated signals are responsive to a linear position of a movable component of the LVDT and the excitation signal; converting the at least two correlated signals to a digital estimate for each of the at least two correlated signals, the converting comprising:
comparing the correlated signal to an analog feedback signal to generate a comparison result;
incrementally adjusting the digital estimate at an estimation frequency in response to sampling the comparison result such that the incremental adjusting reduces a difference between the correlated signal and the analog feedback signal;
converting the digital estimate to the analog feedback signal;
collecting a digital estimate history by sampling the digital estimate at a sample frequency that is a binary multiple of an excitation frequency of the excitation signal; and
analyzing the digital estimate history to determine an amplitude of the digital estimate substantially near the excitation frequency; and
evaluating the amplitude of the digital estimate substantially near the excitation frequency of the at least two correlated signals to determine a numerical representation of the linear position of the movable component. 13. The method of 14. The method of 15. The method of generating a digital representation of the excitation signal at each time period of a generation frequency; converting the digital representation to a digital pulse with a duty cycle correlated to the digital representation to generate a pulse-width modulated signal; and filtering the pulse-width modulated signal to generate the excitation signal. 16. The method of 17. The method of 18. The method of converting the digital estimate to a digital pulse with a duty cycle correlated to the digital estimate to generate a pulse-width modulated estimate; and filtering the pulse-width modulated estimate to generate the analog feedback signal. 19. The method of 20. The method of 2 ^{N }times the sample frequency wherein N is in a range from 2 to 10. 21. The method of 22. A method for determining a linear position of a movable component of a linear variable differential transformer (LVDT), which includes a primary coil driven by an excitation signal at an excitation frequency, a first signal from a first secondary coil, and a second signal from a second secondary coil, comprising:
converting the first signal to a first digital estimate, the convening comprising:
comparing the first signal to a first analog feedback signal to generate a first comparison result;
incrementally adjusting the first digital estimate at an estimation frequency in response to sampling the first comparison result to reduce a difference between an amplitude of the first signal and an amplitude of the first analog feedback signal;
converting the first digital estimate to the first analog feedback signal;
sampling the first digital estimate at the sample frequency to generate a first digital estimate history; and
analyzing the first digital estimate history to determine a first amplitude of the first signal substantially near the excitation frequency;
converting the second signal to a second digital estimate, the converting comprising:
comparing the second signal to a second analog feedback signal to generate a second comparison result;
incrementally adjusting the second digital estimate at an estimation frequency in response to sampling the second comparison result to reduce a difference between an amplitude of the second signal and an amplitude of the second analog feedback signal;
converting the second digital estimate to the second analog feedback signal;
sampling the second digital estimate at a sample frequency to generate a second digital estimate history; and
analyzing the second digital estimate history to determine a second amplitude of the second signal substantially near the excitation frequency; and
evaluating the first amplitude and the second amplitude to determine a numerical representation of the linear position of the movable component. 23. The method of 24. The method of 25. The method of generating a digital representation of the excitation signal at each time period of a generation frequency; converting the digital representation to a digital pulse with a duty cycle correlated to the digital representation to generate a pulse-width modulated signal; and filtering the pulse-width modulated signal to generate the excitation signal. 26. The method of 27. The method of adding the first amplitude to the second amplitude to generate a first term; subtracting the second amplitude from the first amplitude to generate a second term; dividing the second term by the first term to generate the linear position. 28. The method of converting the digital estimate to a digital pulse with a duty cycle correlated to the digital estimate to generate a pulse-width modulated estimate; and filtering the pulse-width modulated estimate to generate the analog feedback signal. 29. The method of 30. The method of analyzing the first digital estimate comprises:
collecting a first digital estimate history at a sample frequency that is a binary multiple of the excitation frequency of the excitation signal; and
analyzing the first digital estimate history to determine the first amplitude; and analyzing the second digital estimate comprises:
collecting a second digital estimate history at the sample frequency that is the binary multiple of the excitation frequency of the excitation signal; and
analyzing the second digital estimate history to determine the second amplitude.
31. The method of 2 ^{N }times the sample frequency wherein N is in a range from 2 to 10. 32. The method of 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: 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. By way of example, A result calculator The result calculator also may comprises dedicated circuitry for calculating the linear position as illustrated in In the signal analyzer The amplitude analyzer A digital converter Details of a representative embodiment of the digital converter A digital estimator The digital converter 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 Returning to The function of the amplitude generator Returning to The digital estimate history The number of bits N in the digital estimate history In operation, the implementation of Those of ordinary skill in the art will recognize that embodiments of the amplitude analyzer 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. Referenced by
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