Abstract:
Systems, methods and apparatus are provided through which in some implementations determine the amplitude of an amplitude modulated signal, modulated by the position of an object being sensed. In some aspects, the apparatus accepts an excitation signal and the amplitude modulated signal and divides the amplitude modulated by the excitation signal to produce an output signal that is proportional to the position of the object being sensed. In other aspects, the division is performed only when the excitation signal is non-zero, such as close to the peaks in the excitation signal. In other aspects, the excitation signal and amplitude modulated signal are degraded due to an air gap and the degraded signals are used to correct for amplitude fluctuations due to the air gap, and produce an output signal, tolerant of the air gaps, that is proportional to the position of the object being sensed.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, as amended, Public Law 85-568 (72 Stat. 435, 42 U.S.C. §2457), and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to position sensors and more particularly to single coil absolute position sensors and inductive gap sensors. 
     BACKGROUND OF THE INVENTION 
     Single coil absolute position sensors that are used to sense the position of an object require an excitation signal that is of the form of a constant frequency sinusoidal wave. The output of the sensor is an amplitude modulated signal, modulated by the position of the object. The single coil absolute position sensor usually includes an excitation coil, and a sensor component. The sensor component is typically affixed to the object that is being sensed, whereas the excitation coil is not affixed to the object that is being sensed. Additionally, the excitation coil and the object being sensed are free to move relative to one another. In some aspects, the position of the excitation coil is fixed while the object being sensed and the affixed sensor component are free to move in a linear motion or in an angular motion. In other aspects, the position of the object being sensed and the affixed sensor component is fixed whereas the excitation coil is free to move in a linear motion or angular motion. 
     The function of the excitation coil is to transmit the excitation signal to the sensor component. The sensor component receives the excitation signal and uses this received excitation signal to output an amplitude modulated signal, modulated by the position of the object being sensed. 
     The amplitude modulated signal that is output by the single coil absolute position sensor may be demodulated to recover the position of the object being sensed. Conventional demodulator circuits may perform this demodulation of the amplitude modulated signal, but their performance is typically very sensitive to any variations in the amplitude of the excitation signal. Such variations in the amplitude of the excitation signal are quite common, and may result from degradation in the signal due to the air gap between the source of the excitation signal and the demodulator circuit. 
     Conventional demodulator circuits are usually composed of many individual components which each act on the excitation signal and the amplitude modulated signal. These extensive stages of electronics induce more noise and error in the demodulated signal. Additionally, conventional demodulator circuits yield a demodulated signal that contains ripples. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a demodulator apparatus that is insensitive to variations in the amplitude of the excitation signal, does not induce significant noise in the demodulated signal due to extensive stages of processing of the signals, and do not include ripples in the demodulated output signal. There is also a need for improved methods of accurately sensing the position of an object. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification. 
     Systems, methods and apparatus are provided through which an amplitude modulated signal, modulated by the position of an object being sensed, is demodulated such that the demodulation system, method and apparatus is insensitive to variations in the amplitude of an excitation signal, does not induce significant noise in the demodulated signal due to extensive stages of electronics to process the signals, and does not induce ripples in the demodulated output signal. 
     In one aspect, an apparatus to sense the position of an object includes a first receiver that is operable to receive an excitation signal, a second receiver that is operable to receive an amplitude modulated signal, modulated by the position of an object being sensed, and a demodulator that is operable to demodulate the amplitude modulated signal from the received excitation signal and is further operable to output a signal that is proportional to the position of the object being sensed. 
     In another aspect, an apparatus to sense the position of an object being sensed includes a receiver operable to receive an excitation signal, a position sensing device which is operable to output an amplitude modulated signal, modulated by the position of the object being sensed, a demodulator that is operable to demodulate the amplitude modulated signal from the received excitation signal and is further operable to output a signal that is proportional to the position of the object being sensed, and an excitation coil which is free to move relative to the rest of the apparatus. 
     In yet another aspect, a method to determine the position of an object being sensed includes receiving an excitation signal, receiving an amplitude modulated signal, modulated by the position of the object being sensed, and demodulating the amplitude modulated signal by dividing the amplitude modulated signal by the excitation signal to produce an output signal that is proportional to the position of the object being sensed. 
     Apparatus, systems, and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a system-level overview of an implementation to sense the position of an object. 
         FIG. 2  is a diagram of apparatus, according to an implementation to demodulate an amplitude modulated signal, modulated by the position of an object, from an excitation signal according to an implementation. 
         FIG. 3  is a diagram of apparatus, according to an implementation to receive, sample and demodulate an amplitude modulated signal, modulated by the position of an object, from an excitation signal according to an implementation. 
         FIG. 4  is a cross section block diagram of an apparatus to sense the position of an object, such that the output signal is insensitive to variations due to an air gap between a source of an excitation signal and the object being sensed. 
         FIG. 5  is a cross section block diagram of an apparatus to sense the position of an object by demodulating an amplitude modulated signal, modulated by the position of the object being sensed. 
         FIG. 6  is a flowchart of a method to demodulate an amplitude modulated signal according to an implementation. 
         FIG. 7  is a flowchart of a method to demodulate a degraded amplitude modulated signal according to an implementation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific implementations which may be practiced. These implementations are described in sufficient detail to enable those skilled in the art to practice the implementations, and it is to be understood that other implementations may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the implementations. The following detailed description is, therefore, not to be taken in a limiting sense. 
     The detailed description is divided into four sections. In the first section, a system level overview is described. In the second section, apparatus of implementations are described. In the third section, implementations of methods are described. Finally, in the fourth section, a conclusion of the detailed description is provided. 
     System Level Overview 
       FIG. 1  is a cross section block diagram of an overview of a system to sense the position of an object. System  100  solves the need in the art to demodulate an amplitude modulated signal, such that significant noise is not induced in the demodulated output signal, and ripples are not induced in the demodulated output signal. 
     System  100  includes a first receiver  102  that is operable to receive an excitation signal, a second receiver  104  that is operable to receive an amplitude modulated signal, modulated by the position of the object being sensed, an analog to digital converter  106  that is operable to sample the excitation signal and the amplitude modulated signal at exactly the same times when the excitation signal is non-zero, and a micro-controlled  108  that is operable to divide the amplitude modulated signal by the excitation signal, and further operable to output a signal that is proportional to the position of the object being sensed. 
     The system level overview of the operation of an implementation is described in this section of the detailed description. In some aspects an apparatus to sense the position of an object includes a first receiver that is operable to receive an excitation signal, a second receiver that is operable to receive an amplitude modulated signal, modulated by the position of the object being sensed, and a demodulator that is operable to demodulate the amplitude modulated signal from the received excitation signal, and further operable to output a signal that is proportional to the position of the object being sensed. 
     In other aspects, the demodulator includes an analog to digital converter that is operable to sample the excitation signal and the amplitude modulated signal at exactly the same time when the excitation signal is non-zero, and a micro-controller that is operable to divide the amplitude modulated signal by the excitation signal. In other aspects, the analog to digital converter includes an analog to digital converter with a dual simultaneously sampled sample/hold circuit. This provides a method to sample and hold the excitation signal and the amplitude modulated signal at the same moment in time. 
     In yet other aspects, the analog to digital converter is operable to sample the excitation signal and amplitude modulated signal at times close to the time corresponding to the peak amplitude of the excitation signal. As a result the signal to noise ratio in the samples is reduced since the rate of change of an arbitrary sinusoidal signal is less at or near the peak of the arbitrary sinusoidal signal. 
     While the system  100  is not limited to any particular receivers, demodulator circuits, analog to digital converters, or micro-controllers, for sake of clarity a simplified demodulator circuit which includes an analog to digital converter and micro-controller is described. 
     Apparatus Embodiments 
     In the previous section, a system level overview of the operation of an implementation was described. In this section, the particular apparatus of such an implementation are described by reference to a series of diagrams. 
       FIG. 2  is a cross section block diagram of apparatus  200  to demodulate an amplitude modulated signal, modulated by the position of an object, from an excitation signal according to an implementation. Apparatus  200  solves the need in the art to demodulate an amplitude modulated signal, such that significant noise is not induced in the demodulated output signal, and ripples are not induced in the demodulated output signal. 
     Apparatus  200  includes a first receiver  202  that is operable to receive an excitation signal, a second receiver  204  that is operable to receive an amplitude modulated signal, modulated by the position of an object, a demodulator  206  that is operable to demodulated the amplitude signal from the received excitation signal, and is further operable to output a signal that is proportional to the position of the object, and an output buffer  208  that is operable to receive the demodulated output signal that is proportional to the position of the object being sensed. 
     In some aspects, the excitation signal is a constant frequency periodic signal, and in other aspects, the excitation signal is a constant frequency sinusoidal signal of the form K*Sin(wt), where K is a constant value, w is the frequency of the sinusoidal signal, t is a variable representing time, and Sin(•) represents the sinusoidal function applied to the arguments within the parentheses. 
     In some aspects, the amplitude modulated signal is a constant frequency periodic signal modulated whose amplitude is modulated by the position of an object being sensed. In other aspects, the amplitude modulated signal is a constant frequency sinusoidal signal whose amplitude is modulated by the position of an object being sensed, where the amplitude modulated signal is of the form K1(t)*K*Sin(wt), where K1(t) is the position of the object being sensed, K is a constant value, w is the frequency of the sinusoidal signal, t is a variable representing time, and Sin(•) represents the sinusoidal function applied to the arguments within the parentheses. 
     In some aspects, determining the position of the object being sensed includes determining the value of K1(t) by using a demodulator circuit to divide the amplitude modulated signal by the excitation signal resulting in a normalized, ripple free signal that is proportional to the position of the object being sensed. 
       FIG. 3  is a cross section block diagram of apparatus  300  to receive, sample and demodulate an amplitude modulated signal, modulated by the position of an object, from an excitation signal according to an implementation. Apparatus  300  solves the need in the art to demodulate an amplitude modulated signal, such that significant noise is not induced in the demodulated output signal, and ripples are not induced in the demodulated output signal. 
     Apparatus  300  includes a first receiver  302  that is operable to receive an excitation signal, a second receiver  304  that is operable to receive an amplitude modulated signal, modulated by the position of an object, an analog to digital converter  306  that is operable to sample the excitation signal and the amplitude modulated signal at exactly the same times when the excitation signal is non-zero, a micro-controlled  308  that is operable to divide the amplitude modulated signal by the excitation signal, and further operable to output a signal that is proportional to the position of the object being sensed, a sync squared block  310  that is operable to generate a square wave that is exactly in phase with the excitation signal, a reference signal block  312  which normalizes the excitation signal and amplitude modulated signal to lie within a particular range, and an output buffer block  314  which stores the demodulated output signal. 
     In some aspects the sync squared block includes a circuit that generates a square wave synchronized with the excitation signal. The square wave is an input to the micro-controller. 
     In some aspects the micro-controller is operable to use the square wave signal from the sync squared block to determine the appropriate sampling times corresponding to the peaks of the excitation signal. The micro-controller reads the square wave signal from the sync squared block and determines the frequency of the excitation signal, and uses the frequency to delay the sampling of the excitation signal and amplitude modulated signal in order to sample the signal at the time corresponding to the peak in the excitation signal. Sampling at the times close to the peak in the excitation signal prevents the sampled excitation value from being zero which would make the demodulated output signal unstable. 
     In other aspects, the sampling of the excitation signal is performed several times and the average value of the samples is used as the value of the sampled excitation signal, and the sampling of the amplitude modulated signal is performed several times and the average value of the samples is used as the value of the sampled amplitude modulated signal. 
       FIG. 4  is a cross section block diagram of an apparatus  400  to sense the position of an object, such that the output signal is insensitive to variations due to an air gap between a source of an excitation signal and the object being sensed. Apparatus  400  solves the need in the art to sense the position of an object demodulating an amplitude modulated signal, modulated by the position of the object being sensed, such that the demodulated output signal is insensitive to variations in the amplitude of an excitation signal due to an air gap between a source of the excitation signal and the object being sensed. 
     Apparatus  400  includes a receiver  402  that is operable to receive a received excitation signal, a position sensing device  404 , an analog to digital converter  406  that is operable to sample the received excitation signal and an amplitude modulated signal, modulated by the position of an object being sensed, at exactly the same times when the received excitation signal is non-zero, a micro-controlled  408  that is operable to divide the amplitude modulated signal by the received excitation signal, and further operable to output a signal that is proportional to the position of the object being sensed, and an excitation coil  410  that is free to move relative to the rest of the apparatus. 
     In some aspects, the excitation coil is operable to transmit a transmitted excitation signal to the receiver and to the position sensing device, and the receiver and the position sensing device are each operable to receive the received excitation signal. 
     In other aspects, the position sensing device is further operable to use the received excitation signal to output an amplitude modulated signal, modulated by the position of the object being sensed, and transmit the amplitude modulated signal to a demodulator, and the demodulator is operable to receive the amplitude modulated signal. 
     In other aspects, the demodulator includes an analog to digital converter that is operable to sample the received excitation signal and the amplitude modulated signal at exactly the same time when the received excitation signal is non-zero, and a micro-controller that is operable to divide the amplitude modulated signal by the received excitation signal. In other aspects, the analog to digital converter includes an analog to digital converter with a dual simultaneously sampled sample/hold circuit. This provides a method to sample and hold the received excitation signal and the amplitude modulated signal at the same moment in time. 
     In other aspects, the analog to digital converter is operable to sample the received excitation signal and amplitude modulated signal at times close to the time corresponding to the peak amplitude of the received excitation signal. As a result the signal to noise ratio in the samples is reduced since the rate of change of an arbitrary sinusoidal signal is less at or near the peak of the arbitrary sinusoidal signal. 
     In other aspects, the demodulator includes a sync squared block that is operable to generate a square wave that is exactly in phase with the received excitation signal. In other aspects, the micro-controller is operable to use the square wave generated by the sync squared block to determine the appropriate sampling times corresponding to the peaks of the received excitation signal. In other aspects, the sampling of the excitation signal is performed several times and the average value of the samples is used as the value of the sampled excitation signal, and the sampling of the amplitude modulated signal is performed several times and the average value of the samples is used as the value of the sampled amplitude modulated signal. 
     In some aspects, the position sensing device includes a single coil absolute position sensor. In yet other aspects, the position sensing device includes an inductive gap sensor. 
       FIG. 5  is a cross section block diagram of an apparatus  500  to sense the position of an object by demodulating an amplitude modulated signal, modulated by the position of the object being sensed. Apparatus  500  solves the need in the art to sense the position of an object by demodulating an amplitude modulated signal, modulated by the position of the object being sensed, such that the demodulated output signal is insensitive to variations in the amplitude of an excitation signal due to an air gap between a source of the excitation signal and the object being sensed, and such that significant noise is not induced in the demodulated output signal, and ripples are not induced in the demodulated output signal. 
     Apparatus  500  includes a receiver  502  that is operable to receive a received excitation signal, a position sensing device  504  that is operable to receive the received excitation signal and output an amplitude modulated signal, modulated by the position of an object, a demodulator  506  that is operable to demodulated the amplitude signal from the received excitation signal, and is further operable to output a signal that is proportional to the position of the object, an output buffer  508  that is operable to receive the demodulated output signal that is proportional to the position of the object being sensed, and an excitation coil  510  that is free to move relative to the rest of the apparatus. 
     In some aspects, the transmitted excitation signal is a constant frequency periodic signal, and in other aspects, the transmitted excitation signal is a constant frequency sinusoidal signal of the form K*Sin(wt), where K is a constant value, w is the frequency of the sinusoidal signal, t is a variable representing time, and Sin(•) represents the sinusoidal function applied to the arguments within the parentheses. 
     In some aspects, the amplitude modulated signal is a constant frequency periodic signal modulated whose amplitude is modulated by the position of an object being sensed. In other aspects, the amplitude modulated signal is a constant frequency sinusoidal signal whose amplitude is modulated by the position of an object being sensed, where the amplitude modulated signal is of the form K1(t)*K*Sin(wt), where K1(t) is the position of the object being sensed, K is a constant value, w is the frequency of the sinusoidal signal, t is a variable representing time, and Sin(•) represents the sinusoidal function applied to the arguments within the parentheses. 
     In other aspects, the received excitation signal is a constant frequency sinusoidal signal that has been degraded by noise due to the air gap between the excitation coil and the receiver, and due to the excitation coil and the position sensing device, where the received excitation signal is of the form K(t)*Sin(wt), where K(t) is the noise due to the air gap, w is the frequency of the sinusoidal signal, t is a variable representing time, and Sin(•) represents the sinusoidal function applied to the arguments within the parentheses. 
     In other aspects, the amplitude modulated signal is a constant frequency sinusoidal signal that has been degraded due to noise, whose amplitude is modulated by the position of an object being sensed, where the amplitude modulated signal is of the form K1(t)*K(t)*Sin(wt), where K1(t) is the position of the object being sensed, K(t) is the noise due to the air gap, w is the frequency of the sinusoidal signal, t is a variable representing time, and Sin(•) represents the sinusoidal function applied to the arguments within the parentheses. 
     In some aspects, determining the position of the object being sensed includes determining the value of K1(t) by using a demodulator circuit to divide the amplitude modulated signal by the received excitation signal resulting in a normalized, ripple free signal that is proportional to the position of the object being sensed. 
     Method Embodiments 
     In the previous section, apparatus of the operation of an implementation was described. In this section, the particular methods performed by the various constituents of such an implementation are described by reference to a series of flowcharts. 
       FIG. 6  is a flowchart of a method  600  to demodulate an amplitude modulated signal according to an implementation. Method  600  solves the need in the art to provide a method to sense the position of an object by demodulating an amplitude modulated signal, such that significant noise is not induced in the demodulated output signal, and ripples are not induced in the demodulated output signal. 
     Method  600  includes receiving an excitation signal  602 , receiving an amplitude modulated signal  604 , modulated by the position of the object being sensed, dividing the amplitude modulated signal by the excitation signal  606 , and producing a demodulated signal that is proportional to the position of the object being sensed. 
       FIG. 7  is a flowchart of a method  700  to demodulate a degraded amplitude modulated signal according to an implementation. Method  700  solves the need in the art to provide a method to sense the position of an object by demodulating a degraded amplitude modulated signal, modulated by the position of the object being sensed, such that the demodulated output signal is insensitive to the degradation of the amplitude modulated signal. 
     Method  700  includes receiving a degraded excitation signal  702 , receiving an amplitude modulated degraded excitation signal  704 , modulated by the position of the object being sensed, dividing the degraded amplitude modulated signal by the degraded excitation signal  706 , and producing a demodulated output signal that is proportional to the position of the object being sensed and insensitive to the degradation of the excitation signal and amplitude modulated signal. 
     In some implementations, methods  600 - 700  are implemented as a computer data signal embodied in a carrier wave, that represents a sequence of instructions which, when executed by a processor, cause the processor to perform the respective method. In other implementations, methods  600 - 700  are implemented as a computer-accessible medium having executable instructions capable of directing a processor, to perform the respective method. In varying implementations, the medium is a magnetic medium, an electronic medium, or an optical medium. 
     CONCLUSION 
     An apparatus through which an amplitude modulated signal, modulated by the position of an object being sensed, is demodulated such that the apparatus is insensitive to variations in the amplitude of an excitation signal, does not induce significant noise in the demodulated signal due to extensive stages of electronics to process the signals, and does not induce ripples in the demodulated output signal is described. A technical effect of the demodulation system is to detect the absolute position of an object by producing an output signal which is proportional to the position of the object being sensed. Although specific implementations are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific implementations shown. This application is intended to cover any adaptations or variations. For example, although described in terms of an electronic circuit, one of ordinary skill in the art will appreciate that implementations can be made in firmware, software or other electronic circuits that provides the required function. 
     In particular, one of skill in the art will readily appreciate that the names of the methods and apparatus are not intended to limit implementations. Furthermore, additional methods and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in implementations can be introduced without departing from the scope of implementations. One of skill in the art will readily recognize that implementations are applicable to future position sensing devices, and different methods of sensing the position of object based on demodulating an amplitude modulated signal. 
     The terminology used in this application meant to include all demodulator circuits, analog to digital converters, micro-controllers, receiver and transmitter environments and alternate technologies which provide the same functionality as described herein.