Patent Publication Number: US-4093915-A

Title: Capacitance measuring system

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 648,490, filed Jan. 12, 1976 and assigned to the assignee of this application. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to measurement systems, and more particularly, to capacitance measurement systems. 
     Capacitive sensors have often been used to provide the input signals for many forms of prior art transducer instrumentation, for example, scales or balances, accelerometers, and pressure transducers or proximity gauges. In such systems, the precision at which measurements can be made is very much a function of the stability of the circuit interfacing the sensor portion and read-out-portion of the system. In general, the prior art systems have permitted relatively low precision measurements due to the sensitivity of the interface circuit to combinations of various factors such as drive signal waveshape, drive signal amplitude, drive signal frequency, and temperature dependent component parameter variation. 
     More particularly, the class of system exemplified by U.S. Pat. No. 3,318,153 to Lode includes circuit interface which generates an output signal as derived from a rectification and summing of current signals whose amplitude is dependent on both the magnitude and frequency of an applied drive signal, thereby requiring a high voltage drive signal at relatively low frequencies and establishing a relatively large power requirement. Thus, that system has a relatively high sensitivity to both drive signal amplitude and frequency, and, as a result, means is provided by Lode to maintain the amplitude-frequency product for the drive signal to be constant. 
     Other forms of prior art systems, such as that illustrated by U.S. Pat. No. 3,572,319 to Bittner are directed to systems for differential capacitance measurement which are drive signal waveshape sensitive as well as amplitude and frequency sensitive. More particularly, that system utilizes a means for extracting the average of the rectified voltage applied across the sensor, that average being a function of the waveshape of the driving signal. 
     Accordingly, it is an object of the present invention to provide a system for determining the capacitance associated with a sensor capacitor with the system being relatively independent of the amplitude, frequency, and waveshape of the driving signal. 
     Another object is to provide a signal linearly related to the capacitive reactance of a sensor capacitor. 
     SUMMARY OF THE INVENTION 
     According to one form of the present invention, the system includes an oscillator for providing an oscillatory signal characterized by a peak amplitude proportional to a control signal, a capacitive network and associated peak detecting network, and a feedback network. The capacitive network includes a drive capacitor and a sensor capacitor coupled in series between the oscillator and a first reference potential. The peak detecting network provides a peak signal representative of the peak voltage at the junction of the drive and sensor capacitors. The feedback network is responsive to the peak signal and to a reference potential to generate a control signal and apply that signal to the oscillator whereby the peak amplitude of the voltage across the drive capacitor is constant. 
     In another form of the present invention, an oscillator is adapted to provide an oscillatory signal characterized by a predetermined peak-to-peak voltage. This signal is applied to a pair of capacitive networks and associated peak detectors in a capacitive bridge configuration. Each capacitive network includes a coupling, or drive, capacitor and a sensor capacitor connected in series from the oscillator to a common potential, with its associated peak detector having is input terminal connected to the junction point between the sensor and coupling capacitor. The sensor capacitors of the capacitive networks may both vary with external parameters (such as pressure) or, alternatively, one may be fixed to serve as a reference while the other varies with an external parameter. 
     The system further includes a combining network for providing an output signal proportional to the difference in magnitude of the peak signals generated by the peak detectors. In one form of the invention, the oscillator includes a limiter which restricts the voltage excursion of the oscillator output signal to be within predetermined uper and lower limits defining the predetermined peak-to-peak voltage. The limiter may be in the form of a diode limiter. The limited oscillator output signal is applied to the capacitive voltage dividers established by the coupling and sensor capacitors. The peak-to-peak voltage across each of the sensor capacitors is converted to d.c. by the associated peak detector to form peak signals. The peak signals are combined to generate a signal representative of the difference in peak-to-peak magnitude of the signals across the sensor capacitors. 
     Since the only peak voltage is used, the system may utilize a driving signal at a frequency which is relatively high, so that the capacitor reactances are low and the two peak voltage signals generated by the peak detectors may be combined in a resistive summing network. Furthermore, the use of a relatively high frequency drive signal permits operation with resonant energy exchange between the oscillator and capacitive networks, with consequent low power consumption and increased stability. As a result of using peak detection, the present invention is relatively insensitive to frequency of the driving signal, and, in addition, is relatively insensitive to waveshape of the driving signal (in contrast to prior art systems using an average of the rectified drive signal). 
     As a further advantage of the present circuit, wherein diode limiters are utilized to establish a predetermined peak-to-peak driving signal and wherein further the peak detectors utilize a well-known diode networks, temperature dependent variations in this circuitry due to temperature varying diode forward drops are offset so that the system is also relatively temperature insensitive. 
     In another form, the present invention provides a relatively large dynamic rang and may accommodate relatively large changes in sensor capacitance, such as produced by the reduction in separation of the sensor capacitor plate elements towards zero. In this form, the output signal of the system, which is representative of the difference in capacitance of the associated sensor capacitors, may be fed back to the oscillator and diode limiting network in order to produce a change in the excitation voltage, i.e., the peak-to-peak value associated with the oscillator output signal. At such times when the sensor capacitors are characterized by relatively large differences in capacitance, a relatively low excitation may be applied to the circuit, thereby reducing the sensitivity so that the operating point of the system does not fall outside its dynamic linear range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: 
     FIG. 1 shows, in block diagram form, a differential capacitance measuring system in accordance with the present invention; 
     FIG. 2 shows, in detailed schematic form, the system of FIG. 1; 
     FIG. 3 shows, in detailed schematic form, an alternative configuration embodying the present invention; 
     FIG. 4 shows, in block diagram form, a capacitance measuring system in accordance with the present invention; and 
     FIGS. 5-11 show, in detailed schematic form, additional configurations of the system of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention is shown in FIG. 1 having an oscillator 12 connected to a capacitive network 14 and an associated peak detector 16, and to a capacitive network 18 and an associated peak detector 20. Each of networks 14 and 18 include a coupling, or drive, capacitor C c  and a sensor capacitor C s  connected in series between the oscillator output terminal and a common reference (ground) potential. The peak detectors 16 and 20 each have their output signals applied to a combining network 22 which in turn provides an output signal on line 22a. It will be understood that detectors 16 and 20 and combining network 22 are arranged so that network 22 provides an output signal on line 22a which is representative of the difference between the magnitude of the peak-to-peak values of the signals applied to the respective inputs of detectors 16 and 20. 
     FIG. 2 shows an embodiment of the circuit of FIG. 1 in detailed schematic form. The oscillator 12 of the FIG. 2 circuit includes an oscillatory signal generator 26 and a diode limiter 28 which is a.c. coupled to the oscillator 26 by way of capacitor C5. The exemplary generator 26 is a common base oscillator comprising Q, C1 - C4, R1 - R3, D1 and L1. The oscillator 12 peak-to-peak output level is maintained relatively constant in this configuration. The outputs of the capacitive networks 14 and 18 are respectively applied to the composite network 30 in FIG. 2. Network 30 represents one embodiment of the combination of peak detectors 16 and 20 and combining network 22. In this network 30, the peak-to-peak voltage across the sensor capacitor of network 14 is converted to a positive d.c. voltage by D4, D5, and C6, and the peak voltage across the sensor capacitor of network 18 is converted to a negative d.c. voltage by D6, D7, and C9. Resistors R4 and R5 sum these d.c. voltages to form a resultant signal with respect to the common point of diodes D5 and D6 and capacitors C6 and C7 across the output terminal pair 22a. Capacitors C8 and C9 provide by-pass filtering to ground. 
     Since only the peak-to-peak voltage is utilized via the network 30, the output signal across terminals 22a is relatively insensitive to frequency and waveshape of the drive signal applied from oscillator 12. Furthermore, the system is relatively stable with respect to changes in temperature since temperature dependent changes in the diode drop across limiting diodes D2 and D3 are effectively offset by similar drops in the peak detector diodes. Furthermore, the use of the peak-to-peak voltage via network 30 permits relatively high frequency operation with resonant energy exchanged between L1 of oscillator 26 and the capacitive networks 14 and 18, thereby permitting relatively low power consumption and high stability. As a further result of the high frequency operation, the relatively low capacitive reactances render small resistive leakages across the sensor capacitors to be of little importance. 
     Accordingly, with the present invention, a relatively high frequency drive signal may be utilized and the detection network is relatively insensitive to the waveshape of the frequency of the drive signal. Furthermore, as indicated, the resultant network is relatively insensitive to temperature changes. 
     FIG. 3 illustrates a further embodiment of the present invention. In this embodiment, circuit elements having corresponding counterparts in the FIG. 2 embodiment are identified with identical reference symbols. In the embodiment of FIG. 3, the power supply for the oscillator 12 is provided by the feedback signal generated from the feedback network 32 by way of line 34. This feedback signal is equal to the d.c. signal produced by the external supply V cc  and voltage divider resistors R6 and R7, as reduced by a signal representative of the difference in capacitance of the sensor capacitors of networks 24 and 18. The signal across terminal pair 32a is representative of the difference in capacitance of the sensor capacitors of networks 14 and 18. 
     For the illustrated embodiment, exemplary values for the circuit elements are listed in the following table for nominal operation with an approximately 4 volt peak-to-peak, 5 MHz drive signal (with the sensor capacitors approximately equal), and utilizing the capacitive sensor elements of Type 90H, manufactured by MKS Instruments, Inc., Burlington, Mass. with nominal capacitance on the order of 30 picofarads. For operation with minimum sensitivity to temperature, the following diode pairs include matched diodes: D2 and D3, D4 and D7, D5 and D6, D8 and D9. 
     
                       TABLE                                                       
______________________________________                                    
R1                                                                        
       47     ohms       C1                                               
                       .01   uf                                           
                       R2    8,200 ohms C2                                
                             470 pf                                       
                       R3    510 ohms C3                                  
                             68 pf                                        
                       R4    100,000 ohms C4                              
                             1000 pf                                      
                       R5    100,000 ohms C5                              
                             2700 pf                                      
                       R6    10,000 ohms C6                               
                             270 pf                                       
                       R7    10,000 ohms C7                               
                             270 pf                                       
                       R8    47,000 ohms C8                               
                             .01 uf                                       
                       R10 - 20 ohms C9                                   
                             .01 uf                                       
                       R11 - 20 ohms C10 - 270 pf                         
                       R12 - 20,000 ohms C11 - 270 pf                     
                       R13 - 20 ohms C12 - .01 uf                         
                       R14 - 20,000 ohms C13 - 270 pf                     
                       L1    10 uh C.sub.c                                
                             33 pf                                        
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     R9 may be selected to match the particular sensors used in order to establish operation in a desired dynamic range. 
     In operation, wherein the sensor capacitors are characterized by relatively similar capacitance values, oscillator 12 provides the relatively constant peak-to-peak amplitude signal to the common point of capacitors C c  in both networks 14 and 18, with the resultant bridge providing at terminals 22a a signal proportional to the difference in capacitance of the sensor capacitors C s  of network 14 and 18. In the event the capacitance of the sensor capacitors becomes greatly different (for example, in a pressure transducer as the diaphragm plates of one sensor capacitor are pushed almost together with the resultant capacitance associated with that capacitor becoming very large while the other sensor capacitor has its diaphragm plates located at their nominal separation), the feedback network 32 lowers the level of the power supply signal on the feedback line 34 to the oscillator 12. As a result, the amplitude of the oscillatory signal produced by network 26, is correspondingly reduced, thereby decreasing sensitivity of the overall circuit to accommodate the external condition of the sensor capacitor. 
     In the configuration of FIG. 3, the capacitive reactance of the drive capacitors C 1  of networks 14 and 18, and the sensor capacitor C 3  of network 14 may be maintained constant and equal while the capacitive reactance of the sensor capacitor C s  of network 18 may vary with an external parameter, such as pressure. For the circuit values set forth in the Table, the peak amplitude of the a.c. voltage across the drive capacitor C c  of network 18 is constant and the feedback signal to network 12 is a linear function of the capacitive reactance of the sensor capacitor of network 18. 
     The configuration of FIG. 3 may also be set forth in the form illustrated in FIG. 4, for example, where the ratio R4/R5 is large. In FIG. 4, elements corresponding to those in FIGS. 1-3 are denoted with identical reference numerals. The capacitive network 18 includes a drive capacitor and sensor capacitor connected in series between a system input terminal 50 and a first reference potential V1. As noted more fully below, the relative position of the capacitors within this series connection may vary for different embodiments. The peak detector 20 couples the junction between the drive and sensor capacitors to the input of a feedback network 32. The feedback network 32 is responsive to the signal from the peak detector and to a second reference potential V2 to provide a feedback signal by way of feedback path 34 to a control terminal 56 for oscillator 12. Oscillator 12 is adapted so that the peak amplitude at terminal 50 is proportional to the signal at control terminal 56. 
     FIGS. 5-11 illustrate further embodiments wherein the feedback signal on path 34 is a linear function of the capacitive reactance of the sensor capacitor C s . In the embodiments of FIGS. 5-7, the first reference potential V1 is ground potential and the sensor capacitor C s  is coupled to that potential, with the drive capacitor C D  being coupled to the system input terminal 50. FIGS. 8 and 9 depict further embodiments wherein V1 is ground potential and the sensor capacitor C s  is coupled to the system input terminal 50 and the drive capacitor is coupled to potential V1. 
     In the configuration of FIG. 5, the ratio R A  /R B  equals unity to ensure a linear relationship between the feedback signal and the reactance of C s . In the FIG. 6 embodiment, the gain between terminals 56 and 50 may be set to be proportional to (R1 + R2)/R1. In this case, the output from the peak detector 20 is directly proportional to the capacitive reactance of the sensor capacitior of network 18. 
     In all the configurations of FIG. 5-FIG. 9, the constant of proportionality for the feedback signal of path 34 is related to the gain between terminals 56 and 50. Accordingly, for maximum stability of these capacitance measuring systems with respect to temperature, this gain may be temperature compensated, for example, by way of a limiting network such as network 28 in the FIG. 3 configuration. 
     FIGS. 10 and 11 illustrate additional embodiments of the present invention, and in those figures, elements corresponding to those in the earlier shown embodiments are denoted by identical reference numerals. Each of the configurations of FIGS. 10 and 11 includes a first capacitive network 14 and associated peak detector 16 and a second capacitive network 18 and associated peak detector 20. The peak detector output signals are combined in the feedback network 32 together with a reference potential V2 to provide a feedback signal on path 34. In each system, the output signal on terminal 60 may be proportional to the capacitive reactance of the sensor capacitor in network 18. In the circuit of FIG. 11, this latter characteristic occurs when R A  equals R B . By adjusting the ratio of R A  to R B , different relationships may be attained. 
     For the circuits of FIGS. 10 and 11, the output signals at terminal 60 are substantially independent of variations in the oscillator gain (i.e., between terminals 56 and 50) such as might be due to temperature. In addition, these circuits are substantially independent of characteristic changes in the diodes in the detector networks. Particularly with respect to the circuit of FIG. 11, the diodes in peak detectors 16 and 20 are at substantially the same operating point, thereby providing optimal stability with respect to temperature. 
     In the configuration of FIGS. 10 and 11, since the output signals are independent of the gain between terminals 56 and 50, a relatively straightforward oscillator circuit may readily be used without a temperature compensating network such as network 28 illustrated in the FIG. 3 configuration. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.