Abstract:
This invention relates to transducers for indicating position. In particular, this invention relates to circuitry for accurately measuring either rotary or linear displacement of an inductive coupling member within a voltage displacement transformer (LVDT or RVDT).

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an improved linear variable displacement transducer (LVDT) or rotational variable displacement transducer (RVDT) and improved circuitry for the operation thereof. 
     LVDTs and RVDTs are used to generate an electrical output proportional to the mechanical displacement of a moveable core generally of ferromagnetic material. The electrical output provides an electrical measurement of distances and displacements which in turn may represent, for example: force, torque, pressure, velocity, or acceleration. As such, LVDTs and RVDTs are in common use in a number of well known configurations. The basic elements include a moveable core, a primary coil and a pair of secondary coils which are so arranged as to have the moveable core magnetically link the primary coil with the secondary coils. 
     The advantages associated with the use of differential transformers over other displacement transducers, such as the resistance potentiometer, are the absence of contacts, infinite resolution, low and near constant output impedance, and input-output isolation. The disadvantage is that operation has heretofore been limited to excitation by conventional alternating voltages, with the accompanying drawbacks of residuals, transients, harmonic generation, and shielding problems. 
     Two recent patents representative of the state of the art for conventional AC excitation of the primary coil within an LVDT are U.S. Pat. No. 4,514,689 issued to William A. Gerard and U.S. Pat. No. 4,450,443 issued to Charles R. Dolland. The 4,514,689 patent detects the peak value and polarity of each secondary coil output voltage and provides an output voltage representative thereof. The 4,450,443 patent converts the output of the secondary coil from a sinusoidal (AC) waveform to a square-wave signal prior to sampling a value proportional to the peak value of the output waveforms. Both feature high resolution and accuracy, however, both also retain some of the inherent limitations associated with the use of an AC excitation signal. 
     SUMMARY OF THE INVENTION 
     The transducer of the present invention utilizes a signal generator to initiate a voltage pulse which is transmitted through one or a plurality of variable displacement transformers, each having a primary coil and a pair of secondary coils magnetically linked by a moveable core, to a plurality of sample and hold elements associated with the secondary coils. Each transformer is arranged such that the degree of flux linkage between the primary and secondary coils is dependent upon the physical position of the core. In a neutral position, the flux linkage will produce an equivalent voltage in the two secondary coils. Movement of the core from the neutral position will cause an uneven linkage in the two secondary coils resulting in different voltages being induced within the secondary coils. These voltages are measured by the pair of sample and hold elements at a predetermined time after the signal generator initiates the voltage pulse. This time delay enables the sample and hold elements to examine a desired segment of the induced pulses generated within the secondary coils, which segment is substantially linear, thereby avoiding sampling of distortions which occur near the beginning and end of the induced pulses. This enables a more accurate measurement with less possibility of error. Signal processing circuitry analyzes the sampled and held voltages to produce a digital output signal indicative of the position of the moveable core. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a detailed schematic of the circuitry of the present invention including a plurality of linear variable displacement transformers; 
     FIG. 2 illustrates a rotational variable displacement transformer arrangement useable in the circuit of FIG. 1; and 
     FIG. 3 is a graphic representation of the operation of the apparatus of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematically illustrates apparatus in accordance with the present invention having a plurality of linear variable displacement transducers (LVDTs) 12 and associated signal processing circuitry 14. The operation of the circuit will first be described with reference to one LVDT 12. Each LVDT 12 includes a moveable member 16, connected to an external mechanical link 18, member 16 being translatable along the y-axis in response to the external mechanical link 18. The LVDTs 12 also include a magnetic core 20 coupled to the moveable member 16, a primary coil 22, and a pair of secondary coils 24, 26. The primary coil 22 and secondary coils 24, 26 are coaxially positioned and arranged such that the amount of magnetic coupling between the primary coil 22 and each of the two secondary coils 24, 26 is dependent upon the position of the magnetic core 20. A signal generator such as an oscillator 28 and line driver 30, is electrically connected by positive leads 32 to the primary coil 22 of LVDT 12. The oscillator 28, which may include an internal reference clock, excites the primary coil 22 with a voltage pulse, having a preferred repetition rate of between 500 Hz and 4000 Hz. This preferred repetition rate may be increased if required for higher accuracy. The primary coil 22 also has a ground lead 34 and may include clamping diode 36 placed across the primary coil 22 to allow the primary coil 22 to discharge. 
     The secondary coils 24, 26 are adapted to provide voltages, inductively generated in response to the excitation of the primary coil 22, to signal processing circuitry 14. Accordingly, the secondary coil 24 is electrically connected at its output terminal to a sample and hold element 38 via conductor 40. Similarly, secondary coil 26 is electrically connected at its output terminal to a sample and hold element 42 via conductor 44. The other terminal of secondary coils 24 and 26 is referenced to ground through ground wire 46. The secondary coils 24, 26 also may include capacitors 50 and resistors 52 placed across their terminals to allow discharge thereof (diodes such as those shown across primary coil 22 may also be used). A delayed pulse generator 54 is electrically connected to the oscillator 28 and each of the sample and hold elements 38 and 42 to provide a timing signal to the sample and hold elements 38, 42. The sample and hold elements 38, 42 are also electrically connected to a multiplexer 58 through signal leads 60, 62. The multiplexer 58 is connected to an analog to digital converter 38 via a multiplexer output bus 66. The analog to digital converter 64 is in turn connected via a data bus 68 to a central processing unit (CPU) 70, which is either a computer or simply a programmable microprocessor. The CPU 70 is also electrically connected to the multiplexer 58. The signal processing circuitry 14 includes the sample and hold elements 38, 42, the delayed pulse generator 54, the multiplexer 58, the analog to digital converter 64, the CPU 70, as well as the electrical conductors linking each of these elements. 
     FIG. 2 represents a rotational variable displacement transducer (RVDT) 72 which may be used to replace the LVDT 12 within the circuitry of FIG. 1. The RVDT 72 includes an E-shaped core 74 having a primary coil 76 encircling the center arm of the E-shaped core 74, and secondary coils 78, 80 encircling opposite arms of the E-shaped coil 74. A rotatable pivot 82 may be mechanically connected to an external mechanical link (not shown). A displacement input member 84, which operates to magnetically link the primary coils 76 and secondary coils 78, 80, is translatable in response to rotation of the rotatable pivot 82, thereby providing variable magnetic linking of the RVDT 72 in response to the external mechanical link. The RVDT 72 may also include clamping diodes 86, 88 and 90 across the primary coil 76 and the secondary coils 78 and 80 respectively. 
     Generally, when the primary coil 22 of the LVDT 12 in FIG. 1 is energized with a positive square-wave pulse, positive voltages are induced within the secondary coils 24, 26. The magnetic core 20 provides a path for the magnetic flux linking the primary coil 22 and secondary coils 24, 26. When the magnetic core 20 is moved from the center position, the induced positive voltage in the secondary coil toward which the magnetic core 20 is moved increases, while the induced positive voltage in the other secondary coil decreases. Thus, the secondary coils 24, 26 produce a positive voltage of varying magnitude in response to the energizing of the primary coil 22. The sample and hold elements 38, 42 receive the positive voltage ouputs from the secondary coils 24, 26. The sample and hold elements 38, 42 wait for a signal from the delayed pulse generator 54, timed to allow the initial transients of the received voltage outputs to dissipate, then the sample and hold elements 38, 42 sample the voltage output and hold the sampled values. The sampled voltage values are communicated from the sample and hold elements 38, 42 to the multiplexer 58 which subsequently transfers the sampled voltage values to the analog to digital converter 64. Within the analog to digital converter 64, the analog sampled voltage values are converted to a pair of digital numbers A and B, A representing a digital value for the output of each secondary coil 24 and B representing a digital value for the output of secondary coil 26. These digital values, A and B, are then sent to the CPU 70. 
     The CPU 70 uses the values A and B to perform a calculation which determines the position of the moveable member 16 within the LVDT 12, and thus also indicate the position of the external mechanical link 18. The position value (P) to be calculated by the CPU is determined by the equation: 
     
         P=(A-B)/(A+B) 
    
     It is readily observable that when the magnetic core 20 is in its center position intermediate to secondary coils 24, 26, the values of A and B will be equivalent, therefor P=0. When the value of A is high with respect to B, the quantity calculated for P would approach a positive 1. If the value of B is high with respect to A, the quantity calculated for P would approach a negative 1. Thus, the relative position of the member 16 can be determined by a value for P within the range -1 to +1 representing the limits on the displacement of the magnetic core 20. Since the values of A and B used to calculate P are digital numbers, the value of P will be easily determined and highly accurate. By the use of this circuitry, the incorporation of the position of the member 16 into external analysis and control programming means will be facilitated. 
     The operation of the apparatus of FIGS. 1 and 2 is more clearly described with reference to FIG. 3 which illustrates various quantities as a function of time. FIG. 3 shows the voltage waveforms across elements indicated by the corresponding reference numerals in FIG. 1. The various waveforms and output signals are arranged so that vertically aligned values occur simultaneously. The output of the signal generator i.e., oscillator 28 and driver 30 is illustrated as a unipolar square-wave pulse 92. The output of secondary coils 24 and 26 are represented by output voltage waveforms 94 and 96 respectively. The well known overshoot and transients 98 which are associated with a transformer responding to an energizing signal exist at the beginning and end of each output voltage waveform 94 and 96. The center of the output voltage waveforms 94, 96, however, are substantially linear. The sample and hold elements 38 and 42 are caused to sample the output voltage waveforms 94, 96 of secondary coils 24, 26 within a window 100 centered about halfway into the waveform, the samples taken therein are free of the transient distortions 98. The width of the window 100 within which acceptable samples can be taken may be adjusted depending upon the voltage and current characteristics of the signal generated and the distortion characteristics of the particular LVDT 12 or RVDT 72 being used. The sampled voltages held within sample and hold elements 38, 42 are transmitted through the multiplexer 58 to the digital converter 64 where the sampled voltages 48, 49 are converted to the digital numbers represented by A and B respectively. These digital representations of the sampled voltages A and B are then passed to the CPU 70. Within the CPU 70, the calculation for the position P is made, wherein P=(A-B)/(A+B). The values calculated for P are shown in the bottom line of FIG. 3 for each of the input unipolar square-wave pulses 92. 
     Returning again to the circuitry of FIG. 1, there is depicted a plurality of LVDTs 12(1-N), all having their primary coils 22 connected in parallel, while all of the secondary coils 24 and 26 have associated sample and hold elements 38 and 42 connected to a common multiplexer 58. There are several unique advantages to this arrangement. First, only one electrical wire is required to traverse from the driver 30 to all of the primary coils 22 within each LVDT 12 (1-N). This alone accounts for a significant weight and volume savings for a given application such as an aircraft wing which may include from 10-30 LVDTs spaced throughout its length. Second, the CPU 70 can control the multiplexer 58 and direct the multiplexer 58 as to which sampled voltages (A, B) from any one or any number of LVDTs 12 should be processed. This arrangement allows the sampling rates for various of the individual LVDTs 12 to be tailored to the particular application, or to isolate one LVDT for troubleshooting or testing. Additionally, the circuit of FIG. 1 may include a switch 102 and a one shot signal generator 104 as well as electrical conductors 106, 108, to allow the CPU 70 to be switched into electrical contact with line driver 30 (switching oscillator 28 out of contact) thus allowing the CPU 70 to control the frequency and shape of the voltage pulse delivered to the primary coils 22. For many applications involving a large number of LVDTs 12 or RVDTs 72 or any combination of the two, the circuitry of the present invention enhances the flexibility of the system while reducing the associated system complexity, weight, and bulk. 
     It should be evident from the foregoing description that the present invention provides many advantages over conventional AC excited position transducers. Although preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teaching to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.