This application is related to U.S. Patent

Application Serial No. 200,788, entitled Capacitive

Pressure Sensor and filed on October 27, 1980. That application is incorporated by reference herein.

BACKGROUND OF THE DISCLOSURE

This invention relates to measurement systems, and more particularly, to measurement systems having a capacitive sensor.

Capacitive sensors have often been used to provide the input signal for many forms of prior art transducer instrumentation, for example, scales or balances, accelerometers, and pressure transducers, or proximity gauges. In such systems, the precfsion at which measurements can be made is very much a function of the stability of the circuit interfacing the sensor portion and the read-out portion of the system. In general, the prior art systems have permitted relatively low precision measurements due to sensitivity of the interface circuit to combinations of various factors, such as drive signal wave shape and drive signal ampli¬ tude.

In practice, these shortcomings of the prior art systems have been in part overcome by systems, such as those disclosed in U.S. Patent No. 4,093,915, where a feedback network is used for linearizing the nominal system output. While such systems are generally effec¬ tive, in many applications, the feedback networks add substantially to the circuit complexity, and are correspondingly expensive.

Accordingly, it is an object of the present invention to provide a system for generating a signal

linearly related to the capacitor reactance of a sensor capacitor.

Another object is to provide a system for determining the capacitance associated with a sensor capacitor with the system being substantially indepen¬ dent of the wave shape of the driving signal.

SUMMARY OF THE INVENTION

Briefly, the invention includes a capacitive sensor with two fixed plate elements having opposed pla- nar, electrically conductive surfaces serving as electrodes. The opposed surfaces are substantially parallel and spaced apart by a predetermined distance. The sensor further includes an electrically conductive diaphragm having a substantially planar portion posi- tioned between and parallel to the electrodes . The por¬ tion of the diaphragm between the electrodes is movable in the direction perpendicular to the plane of the electrodes.

At least one diode detector network is coupled across the electrodes. The diode detector networks each include a pair of oppositely directed series connected diodes with the junction between the diodes of each net¬ work being coupled to the diaphragm. In embodiments having only one diode detector network, the junction between the two diodes may be directly or capacitively coupled to the diaphragm. In embodiments having two such diode detector networks, the diodes of the second network are oppositely directed with respect to the corresponding diodes in the first network, and the junc- tion between the diodes in the second network are capacitively coupled to the diaphragm while the junction between the diodes in the first diode network may be either directly or capacitively coupled to the diaphragm.

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The system further includes a coupler for coupling a constant amplitude a.c. signal across the electrodes of the sensor. An output network may provide an output signal representative of the net d.c. voltage across the diodes of the diode detector networks.

With this configuration, an a.c. signal may be applied across the fixed electrodes of the sensor. Since the diaphragm is substantially parallel to the fixed electrodes, that diaphragm lies on an equipoten- tial plane within the sensor, and therefore, is charac¬ terized by the voltage at that equipotential plane. Since the equipotential planes increase in voltage from zero volts a.c. at one electrode up to a peak amplitude a.c. voltage at the other electrode, the amplitude of the signal at the diaphragm is a linear function of the position of the diaphragm with respect to the sensor electrodes.

The detector ^" networks convert the a.c. signals to a linear, ground referenced d.c. output signal. Particularly, in an embodiment with one diode detector network, the a.c. across the portion of the detector between the diaphragm and one of the electrodes is clamped by the detector diode connected across those elements, such that the average voltage at the junction of the two diodes is offset by a d.c. voltage corresponding to the peak amplitude of the a.c. voltage at that one of the electrodes.

Similarly, the second diode of the detector provides an oppositely directed d.c. offset to the voltage at the other electrode. This second offset corresponds to the average value of the peak signal between the diaphragm and the other electrode. When the diaphragm is exactly centrally located between the two

electrodes, a net voltage d.c. voltage of zero is across the two diodes of the detector network. When the diaphragm is offset from center, since the two diodes are oppositely directed, the d.c. voltage offsets are in the opposite sense so that the resultant net d.c. voltage across both detector diodes corresponds to the unbalance between the sensor halves (established by the movement of the diaphragm with respect to the electrodes) .

This net d.c. voltage across the two diodes of the detector is a linear function of the position of the diaphragm between the electrodes. This form of the invention is particularly effective for symmetric a.c. drive signals and provides a low component count (and correspondingly inexpensive) system.

In embodiments having two diode detector net¬ works, a first diode detector operates in the manner similar to the diode detector described above. The second detector network is directly coupled to one end of the first network and capacitively coupled to the other end of that network and provides a similar type operation but where the diodes are oppositely directed. Consequently, this two diode detector system also provides an output representative of the net d.c. voltage across the diodes of both detectors. However, in this case, the opposite sensing of the second detector exactly offsets errors due to drive waveform asymmetry.

BRIEF DESCRIPTION OF THE DRAWING The invention disclosed herein will be better understood with reference to the drawings, in which:

Fig. 1 shows in generalized schematic form, an embodiment of the present invention;

Figs. 2-7 and 14 show in schematic form, exemplary embodiments having a single diode detector network; and

Figs. 8-13 and 15 show in schematic form, exemplary embodiments having the diode detector networks.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 shows in a generalized schematic form, an embodiment of the present invention, where the dotted lines represent electrical couplings. The illustrated system 10 includes a capacitive sensor 12 having a pair of fixed plate elements, or electrodes, 14 and 16. Electrodes 14 and 16 have opposed planar, electrically conductive surfaces that are substantially parallel and spaced apart by a predetermined distance. The sensor 12 also includes a substantially planar conductive diaphragm 18 which is positioned between electrodes 14 and 16. The diaphragm 18 is movable in the direction perpendicular to the plane of the opposed surfaces of electrodes 14 and 16. By way of example, diaphragm 18 may be an edge supported diaphragm of the form disclosed in the incorporated reference, having its edges sup¬ ported with respect to the two fixed electrodes 14 and 16. With this configuration, a central portion of diaphragm 18 between electrodes 14 and 16 may be substantially planar and parallel to those electrodes (although minor deviations from this are possible due to pressure differentials across the diaphragm 18, or alternatively, acceleration induced motion of the diaphragm 18).

In Fig. 1, the electrode 14 is coupled to ter¬ minal Ti and electrode 16 is coupled to terminal T2. In various embodiments, this coupling may have the form of

either direct or capacitive coupling, as noted more par¬ ticularly below.

The system of Fig. 1 also includes two detec¬ tor networks outlined by dashed lines and identified by reference designations N^ and 2, respectively. Both of networks N- _j _ and N2 include two series connected, opposi¬ tely directed diodes and associated parallel resistors (D and D2 and Ri and R2» respectively, for network , and D3 and D4 and R and R2 respectively, for network 2). The diodes in network N2 are oppositely directed with respect to their counterparts in network N^. Both networks are coupled to terminals ± and 2. In both networks, the junction point between the two diodes is coupled to the diaphragm 18, although for N2 that coupling is by way of the junction of N- _j _. The coupling to the diaphragm 18 may be direct or capacitive, as described more fully below. A signal generator 20 is provided which couples (directly or capacitively, in various embodiments) a constant amplitude a.c. signal across terminals T-^ and T2-

It should be noted at this point that the directions of all of the diodes in networks N^ and N2 may be reversed without affecting the functioning of the system.

With this configuration, a constant amplitude a.c. signal is applied across the terminals T^ and T2- The net d.c. voltage across the diodes "of networks

- ^~ 1 and N2 is representative of the position of the diaphragm 18 with respect to the electrodes 14 and 16.

Figs. 2-11 schematically illustrate ten par¬ ticular configurations of the general system shown in Fig-. 1. These systems will now be explained in detail.

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Generally, Figs. 2-7 show single diode detec¬ tor network embodiments, while Figs. 8-11 show two diode detector network embodiments. In these figures, ele¬ ments having corresponding elements in the embodiment of Fig. 1 are identified by the same reference designa¬ tions.

Fig. 2 shows an embodiment adapted for coupling to a signal generator 20 which provides a ground referenced, constant amplitude a.c. signal to terminal T _j . A diode detector network including diodes T)± and D2 and resistors R and R2 is directly connected between terminals T and T2» The terminal T2 is coupled to ground by way of a by-pass capacitor C. In opera¬ tion, the diaphragm 18 may be responsive to a pressure differential, or acceleration, to move between the electrodes 14 and 16 of the sensor 12. Thus, the diaphragm 18 lies on an equi-potential plane and there¬ fore carries the a.c. potential at that level. The a.c. signal across electrode 14 and diaphragm 18 has its base line shifted by diode Di so that there is an average d.c. potential equal to the peak value of the a.c. potential produced across diode D » The a.c. across diaphragm 18 and electrode 16 is similarly rectified. Imbalances in a.c. on the sensor 12 (that is, due to offsetting of the diaphragm 18 from the center) provides a corresponding d.c. unbalance at the output terminal 22. Thus the net d.c. voltage at terminal 22 is a linear function of the position of diaphragm 18.

Fig. 3 shows a similar embodiment, but where the a.c. signal from generator 20 is capacitively coupled to T-]_ by a coupling capacitor C. In this con¬ figuration, the terminal T ₂ is maintained at the ground reference potential. The diode D]_ is directly coupled to terminal T]_ and the diode D2 is directly coupled to

terminal T2. In operation, the a.c. signal across diaphragm 18 and terminal T2 is clamped by diode D2 so that the average voltage at the cathode of D2 must be positive and vary proportionally with the amplitude of the a.c. signal. A similar clamping occurs at the diode D^, but since D is reversed with respect to D2 _* the resultant d.c. voltage is reversed. Therefore, the net d.c. voltage at D (at terminal 22) across the diodes D^ and D2 represents the unbalance of the sensor 12.

The embodiment of Fig. 4 is substantially similar to that of Fig. 3, except that the diode D2 is capacitively coupled to the ground reference potential by way of capacitor C and the output terminal 22 is connected to diode D2. This embodiment provides input- output isolation, where the output signal is the ent d.c. across terminals 22 and 22a.

The embodiments of Figs. 5-7 are substantially similar to the respective ones of Figs. 2-4, except that the plate elements 14 and 16 of sensor capacitor 12 are capacitively coupled from terminals Tl and T2 by capaci¬ tors denoted by Z and Z', respectively, in those figures.

Figs. 8-10 show two diode detector network embodiments which are similar to the embodiments of

Fig. 3, but which include a second diode detector having diode D3 and resistor R3 directly coupled to terminal Tl and diode D4 and resistor R4 capacitively coupled to terminal T ₂ by capacitor C . Terminal T2 is directly coupled to the ground reference potential. In Fig. 8 the junction between diodes D3 and D4 is directly coupled to the diaphragm 18, while in Figs. 9-11 that junction is capacitively coupled to diaphragm 18. In

all of Figs.8-11, the junction between the diodes of the second network is capacitively coupled to diaphragm 18. The diode D of the second detector network is capaciti¬ vely coupled to the ground reference potential of ter- minal T2 by way of by-pass capacitor C.

In operation of the circuits of Fig. 8, a signal generator 20 (not shown in that Figure) provides an a.c. voltage at terminal Tl. As in Fig. 3, the d.c. portion of the voltage at terminal Tl is representative of the d.c. drops across diodes Dl and D2. The diodes D3 and D4 of the second network operate in substantially similar to the diodes Di and D2 of the first network, and provide oppositely directed drops across those diodes so that the net d.c. voltage across diodes Dl, D2, D3 and D4 at terminal 22 is representative of the position of diaphragm 18 between electrodes- 14 and 16.

The circuits of Figs. 9-10 operate in a simi¬ lar manner to the embodiment of Fig. 8, where the a.c. signal is capacitively coupled to terminal Tl. Again, diodes D3 and D4 act similarly, but in an opposite direction from those in detector Dl and D2 and provide a net diode drop at terminal 22 which is representative of the position of diaphragm 18.

Fig. 11 shows a two diode detector network which is similar to the embodiment of Fig. 2, but which includes a second diode detector network having diode D3 and resistor R3 capacitively coupled to terminal T_ and diode D4 and resistor directly coupled to terminal T2. The junction of diodes D3 and D4 is capacitively coupled to diaphragm 18. The output terminal 22 is coupled directly to diode D3, and provides an output signal corresponding to the net d.c. voltage across diodes D^, D ₂ ,-D ₃ and D .

Figs. 12 and 13 show two diode detector embodiments which are similar to the embodiments of Figs. 8 and 11, respectively, but which include a capa¬ citor C" between diode D ₂ and terminal T2 (in Fig. 12) and between diode D-j_ and terminal T (in Fig. 13). These embodiments provide input-output isolation, with the output being the d.c. portion of the potential dif¬ ference between terminals 22 and 22a.

For a given diaphragm position, the systems of Figs. 8-13 provide substantially twice the output net d.c. voltage compared to similar systems of Figs. 2 and 3. In addition, the systems of Figs. 8-13 are substan¬ tially immune to error due to driving waveform asym¬ metry. Any of the systems of Figs. 8-13 may also include capacitors Z and Z' in a manner similar to the embodiments of Figs. 5—7.

By way of example. Figs. 14 and 15 show sche¬ matic diagrams corresponding to the embodiments depicted in Figs. 2 and 9, respectively. In those figures, the oscillators corresponding to signal generator 20 provide peak-to-peak signals which have twice the peak-to-peak value of the supply voltage with a minimum number of component parts. The series inductors in the output of the oscillator de-couple the collector of the transistor driver from the resonant circuitry, purifying the wave¬ form at the sensor and doubling the a.c. voltage there. In the oscillator of Fig. 12, the collector-to-base diode at the oscillator transistor stabilizes the level of oscillation.

The invention may be embodied in other speci¬ fic forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustra-

tive 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.