Device and process for measuring capacitance of capacitor

Abstract

A measuring system is configured with a reference capacitor, a capacitance measuring circuit which is coupled to the reference capacitor and a switch. The switch alternates between an open position in which the capacitance measuring circuit evaluates a capacitance of the reference capacitor, and a closed position in which the measuring circuit evaluates a combined capacitance of the variable capacitor unit and reference capacitor. The measuring circuit measures the capacitance of the variable capacitor unit upon processing the evaluated capacitances In respective open and closed positions of the switch.

Description

[0001] DEVICE AND PROCESS FOR MEASURING CAPACITANCE ()F CAPA<II1X)R

[0002] :BACKG:B:QIJND

[0003] Field of the invention.

[0004] The invention relates to a non-c-ontact technique for measuring displacement, distance

[0005] 5 and position, In particular, the invention relates to a capacitive measuring circuit tor measuring the physical quantity substantially independently from the influence qf temperature drifts of at 1 east one element of the "circuit.

[0006] Known Art

[0007] Measurement devices including capacitive censors are widely used in a variety of

[0008] 10 electronic systems. Various industrial applications taking advantage of the capacitive sensor may include precision positioning. scanning and measurement applications among which the measurement of small distances in a mm - sub-mm range is perhaps one of the primary beneficiaries. In the I alter -application, the value of ths measured capacitance Is small and typically does not exceed a few picofarads (pF? and its variation even smaller To provide the desired

[0009] 15 measuring accuracy, electronic components of the sensor should, not introduce substantial errors into the measurement resells -■ the errors resulting, among others, from the environmental temperature changes.

[0010] The traditional capacitive sensing methods, realized in the majority of specifically configured, chips, include the switched-cap architecture and charge-based. architecture, The

[0011] 20 switched-cap circuit exploits the charge transfer in and. out of a capacitor. The charge-based method can be used to measure current as a function of power supply and signal frequency. Both architectures are based on a charge / discharge algorithm implemented by means of electronic semiconductor switches and current sources. These methods are disclosed in respective USP 4,187,460 and USP 4.794,320. Briefly, these patents teach a method of minimizing the unwanted

[0012] 25 influence of the temperature-dependent components of a. measuring circuit on the measurement of the capacitance of the component of interest.

[0013] The temperature of practically all circuit components, on one hand, depends on. the environmental temperature variations affecting parameters of one or more components of the measuring circuit, On. the other hand, the temperature of the component depends on the Internal

[0014] 1 heating generation of the component itself. In the context of this disclosure, the temperatme dependent parameters of circuit s components are of utmost importance since they may increase errors of the measurement of capacitance of the component of interest

[0015] One of 'ordinary skill in the electronic arts is well aware that even the best chips, such as Capacitance-to-Digital Converter (CDC) with temperature sensor (AD7745 / AD7746}. remain temperature dependent and their usefulness in many applications is limited. The datasheetindicates a. large temperature drifts which may make this CDC impractical for determining small capacitances.

[0016] While the above temperature-related problem still exists^ the attempts to at least minimize its detrimental eileet have been made with some of the known results being encouraging. In particular, a resonant-sensing circuitry (RSC) has been employed in a variety of capacitance sensors. Such a circuit measures a variable capacitance value by evaluating the resonancefrequency of the circuit which is configured with a coil, having the known inductance, and capacitance to be determined. Due to a good Q-quality factor of both the inductor and capacitor of the LC frequency generator, the influence of the surrounding electronics on the capacitance measurement is drastically minimized. In other words, the measurement result mostly depends on the value of the inductance and capacitor, and not on parasitic parameters of semiconductors. Hence the measurements results and their accuracy have been markedly improved.

[0017] Yet, despite the improvement of the measurement results by the resonant technique, a noticeable temperature dependence of the inductance, resulting in. a temperature dependent resonance frequency, remains problematic. The reason for such a dependence is the temperature- dopendcnt magnetic permeability of the mdnctnr’s core and its geometrical parameters.

[0018] A few well known practical approaches based on the resonant technique have been developed for dealing with the thermal stability of LC frequency generators. According to one approach, the LC generator includes a temperature-dependent capacitance which compensates for the change of the coifs inductance. One of ordinary skill Is well aware of disadvantages associated with this approach. One of them is that the compensation is not fully satisfactory due to a complicated interdependence between these two properties. Still another disadvantage stems from the necessity of providing additional adjustments due to significant variations in the characteristics of these values during production. in accordance with another approach, the temperature of the LC genemtor ■components is measured, and the capacitance measurement result is subsequently corrected baaed on the prex-iously obtained temperature dependence of the result. The disadvantages of this method includes obtaining this dependence which requires & thermal calibration, and the need for

[0019] 5 additional electronic components, such as a temperature gauge arid signal transmission lines,

[0020] S till another approach minimizing the influence of the temperature drift on the capacitance in the LC resonator is disclosed in USP 8836349 fully incorporated herein by reference. The latter teaches alternately coupling variable and fixed capacitors to the inductor utilizing two switched As a result of the disclosed coupling scheme, the patent teaches two

[0021] 10 sequentially operating measurement LC circuits. One of the LC circuits is configured with, among others, only the fixed capacitor, whereas the other LC circuit includes only the variable capacitor coupled to other components, Both circuits operate to determine respective resonant frequencies of two LC resonators. Thereafter a ratio of determined frequencies is used io measure the capacitance of the variable capacitor.

[0022] 15 The use of two switches introduces additional. structural complexity and associated with it reduced cost effectiveness. Furthermore, the switches have to operate at the resonant, frequency associated with high voltage while featuring a low capacitance in their closed state. Witii two switches, the stray capacitance is doubled undermining the cardinal inventive concept of reducing the influence of parasitic parameters on the error of the measurement;

[0023] 2.0 A need, therefore exists for a capacitance measuring method and device for directly determining the physical quantity which, is proportional to or inversely proportional to the capacitance of the capacitor and independent from influence of temperature-dependent errors by the elements of the device.

[0024] Another need exists for a capaciiau.ce measuring method and device for determining the

[0025] 2S physical quantity with the desired accuracy arid reliability.

[0026] Still another need exists for a capacitance measuring device in which the measurement of the capacitance or the physical quantity to be measured is not affected by temperature variations of the components of the device.

[0027] SUMMARY OF THE INVENTION

[0028] 3 These needs are met by the disclosed capacitive measuring "system implementing the inventive method. The configuration of the inventive system includes a variable capacitor, reference capacitor, switch periodically coupling the variable and reference capacitors and measuring device which preferably includes an oscillator which may be selected from Rth LC circuits and their numerous modifications. The variable capacitor has its capacitance varying in response to the change of an external action affecting this capacitor, and the capacitance is measured by the measuring device. The reference capacitor is a thermostable capacitor with the constant, known capacitance. The measuring device is a computer-based controller including one or multiple units which are operable to evaluate the capacitance of the reference capacitor coupled to the measuring device. Based on this evaluation, the measuring device provides the measurement of the variable capacitor.

[0029] Although the reference capacitor has the constant, preset capacitance, the measurement of this capacitance brings different results due to a measurement error which is caused by the influence ofone or more factors, primarily temperature drifts, on optional inductance and semiconductor junction capacitance which are present In the system. Thus, if the time between the capacitance measurement of the reference capacitor and combined capacitance measurement of the reference and variable Capacitors is such that this factors remain substantially the same, the computational error-cancelling algorithm can be written to eliminate this error - the error that is constant in both measurements. The error-cancelling algorithm depends on the relationship between the measured capacitance and its evaluated capacitance-dependent parameter, as will be explained below. Most frequently, this parameter is the frequency (F) of an oscillator which is coupled to the measured capacitor. For example, for an LC oscillator, this relationship is F ~ whereas, for an RC oscillator this relationship is I7-"™ .

[0030] The implementation of the above is realized by the switch operating at a frequency high enough to ''preserve’' the environmental conditions and, particularly, the temperature drifts of various parameters which remain substantially unchanged during consecutive ineasurements. 'The elimination of the error during the computation improves the accuracy of the measurement of the capacitance of the variable capacitor.

[0031] The above discussed approach is used in the inventive system. In operation, the control unit operates the single pole single threw (SPST) switch which alternates between closed and open positions at such a switching .frequency that the above-mentioned factors, including temperature drifts, are presumed constant. In the dosed position, a circuit includes the variable and reference capacitors, the oscillator and measuring device. The open position of 8iPSl' switch corresponds to another circuit having all the same elements as discussed immediately above, bat without the variable capacitor. Both circuits are sequentially driven in response to the signal from the oscillator which is also coupled to the measuring device. As a consequence, the controller sequentially receives two periodic signals from the oscillator and evaluates each of these signals. Daring the evaluation, a characteristic of each periodic signal is detenniued based on some .reference value stored in the measuring device, The charaeterisde may be selected from the frequency or period of the evaluated periodic signals. The evaluated characteristics are then compared to one another and the result of the comparison is indicative of the capacitance of the variable capacitor.

[0032] Based on the foregoing, the disclosure: has two aspects. Ih accordance with one aspect, a system for measuring a capacitance of a variable capacitor unit is configured with a reference capacitor and a capacitance control system invariably coupled to the reference capacitor. A switch alternates between ait open position in which the measuring system is operable to evaluate a capacitance of the reference capacitor, and a closed position in which the measuring system is configured to evaluate a combined capacitance of the variable capacitor unit and reference capacitor, The controller is configured to measure the capacitance of the variable capacitor unit upon comparing the combined capacitance and the evaluated capacitance of the reference capacitor.

[0033] Still another aspect of the disclosure relates a method for measuring the capacitance of a variable capacitor including alternating a switch between open and closed positions. The circuits, corresponding to respective positions of the switch, am configured to output respective periodic signals which are evaluated in the measuring device. The subsequent comparison of the evaluated results is used in the measurement of the capacitance of the variable capacitor.

[0034] Various features of the disclosure are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included, to provide an illustration and a further understanding of the various features and aspects, and are incorporated in. and constitute a part of this specification, but are not intended as a definition of the limits of any particular embodiment. In the figures, each identical or nearly identical component that is illustrated; in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures:

[0035] 5 .FIG; 1 is a general schematic, of the disclosed system;

[0036] FIG. 2 is a flow chart of the operation of the disclosed system;

[0037] FIG. 3 is a graph illustrating the evaluation algorithm of the invention;

[0038] FIG, 4.is an exemplary schematic diagram of the system of FIG. 1;

[0039] FIG. 5 is still another exemplary-- schematic diagram of the system of FIG. 1 ;

[0040] 10 FIG, 6 is an exemplary sensor configured with the system of Figs, 1-5; and

[0041] FIG. 7 Is an electrical scheme of the switch of FIGs. 1 , 4 and 5.

[0042] FIG. 1 illustrates a measuring system 10 configured to limit erroneous measurements of temperature-dependent capacitance of variable capacitor unit Cx 12 due to a temperature drill of

[0043] 15 physical parameters of at Gast one of the elements of the system. The system 10 includes a switch 18, thermostable reference capacitor Gref 14 and a measuring circuit 22. The measuring system 10 is particularly useful for measuring small capacitances.

[0044] The measured, capacitor unit Cx 12 has the capacitance which varies in response io a physical phenomenon acting thereupon and Is shown as a single capacitor, but as one of ordinary

[0045] 20 skill readily realizes, the industrial environment may necessitate a sequence of capacitance elements coupled in scries with shown capacitor Cx 12. The last element of this sequence is grounded to the same ground as the rest of the illustrated system. The thermostable capacitor Gref 14 is configured with a preselected, capacitance which remains substantially constant during the measurement of the temperature dependent capacitance of variable capacitor Cx 12. Hence Gref

[0046] 25 14 is referred to as reference capacitor Gref 14.

[0047] Considering FIG. 1 in combination with FIG.2, the inventive technique of measuring the variable capacitance of capacitor Cx 12 depends on the ■ operation of a switch 18 which alternates between open and closed positions at a frequency between 10 Hz and 10 K.Hz. This range has been proven useful in multiple experiments. Clearly, the sequence of the switch positions is inessential

[0048] 30 as long as the switch positions alternate. Furthermore, instead of two consecutive4W’ and4iofF*

[0049] 6 switch positions, several alternating switching opemtions can occur before ths measurements takeplace, The -measuring algorithm discussed below involves two evaluation algorithms and one error cancelling algorithm. The evaluation algorithms are designed for obtaining the above- mentioned evaluated parameter. The cancelling algorithm serves for cancelling the error tan the evaluated parameters obtained in. accordance with respective evaluation algorithms. As a result of the above-disclosed algorithms, the measured result of capacitance is free from, the error,

[0050] The measuring algorithm is based on the evaluation of the capacitance of reference capacitor 14 twice: onetime independently form variable capacitor 12, mid the other time upon coupling these capacitors together. In the majority of if not all known configumtirms of frequency generators, there are parameters which influence the measurement results of the capacitance. During thi s measurement, the resul t is affected not only by the capacitance itself, but also by other various component values which each are multiplied by the value of the capacitance, for example, L*C, R*C or(U / I)*C; Any capacitance measuring device is characterized by a curve representing the dependence between the measured capacitance and certain evaluated parameter. For example, FIG. 3 illustrates the dependence between the parameter P and the capacitance C, i.e-., P::;P (G). Namely, the crave R represents the period of au RC generator and the curve L - the period of the LC genera tor.

[0051] Based on. the foregoing, the operation of the inventive system is illustrated by FlGs. 1 and 2. In response- to a command from circuit 22 of FIG. 1, switch 18 is brought in an open position of step 24. As a result, the system processes not a parameter Pj“P(Crcf), hut Pi«P(Cref*6), where S Is the error caused by components of the LC generator other than capacitor 14. The Cref*5 is further referred io as evaluation Cev; of Step 26 of FIG. 2 and presented as foeffo. (1 ) In the closed position of switch 18 corresponding to step 28 of FIG. 2, variable capacitor

[0052] 12 is coupled to reference capacitor 14 mid to measuring Circuit 22. Analogously to step 26, the evaluation step 10 of FIG, 2 provides the parameter p3::::P((Cx4- Crefffo) referred to as C^g. The result is as follows: Critically, the error b is the same for bo th evaluations of respective steps 26 and 30 because the temperature remains substantially the same since between two consecutive measurements. Based on the evaluation results of respective steps 26 and 30, as these results are processed together, the cancellation of the error o is realized in step 32,

[0053] The error cancelling algorithm of step 32 is as follows. .Solving equation (I ) for 6 produces the following:

[0054] Then solving the evaluation of the combined capacitance of step 30 using equation (3) in equation (2) results in: Finally, the measurement of variable capacitor 12 of FIG. 1 free from error o as follows: measuring s t> i

[0055] FIG. 4 illustrates one of examples of system 10 operating with the above-disclosed algorithm. Similarly to FIG. I, the shown schematic has variable capacitor 12 to be measured. by measuring system 10. The latter includes an oscillator 40 which may have a variety of configurations within the context of this disclosure. For example, as mentioned above, RC and LC may be integrated on-chip with control unit 44 or separately therefrom with switch 18. Other oscillators may include electromechanical system oscillators, crystal oscillators, ring oscillators and phased lock loops t PLLs) that are locked to external clock sources. The schematic operates in accordance with the algorithm disclosed in reference to FIGs. I and 2. As is known in the art, an electrical signal in oscillator 40 depends on a change of the capacitance. During the operation of system 10, the environmental factors, such as a temperature drift, cause the relative difference m xesonani trequencies or their respective derivatives, such as below-discussed periods, corresponding to respective positions of switch 18. The control unit 44, operating according to the disclosed algorithm of FIG, 2, is selected a microcontroller, microprocessor, application specific integrated circuit (ASIC), field programmable gale array (FPGA) or any other appropriate digital logic device. The control unit 44 can calculate the ratio between two frequencies or their derivatives periods of respective periodic signals generated in response to the operation of switch 18. The latter provides a relative measured valueof capacitance Cx of variable capacitor 12. Additionally, control unit 44 may be configured to calculate an absolute value of this capacitance by multiplying the obtained ratio by the fixed capacitance Cref of reference capacitor 14, In general, control unit 44 may have an internal memory or be operatively connected to an:external memory device or another controller measuring a physical value which is bused on the measured variable capacitance, Sines the shown schematics can he incorporated In a variety of sensors, such as a position sensor 70 of FIG. 6, the physical value may be the distance, pressure, thickness, height, density, flow and dielectric pemreability as well ax their combinations. I'hc control unit 44 executes theprogrammed instructions, 1.e., the invent! ve al gorithm disclosed above, which arc stored in its memory to identify the capacitance of variable capacitor 12. FIG. 5 illustrates an exemplary schematic of FIG. 4 In which the oscillator includes an LC generator 42 which, as known to one of ordinary skill, generates a reference oscillation signal with varying frequencies. The system 10 further includes, among others, sit inductor 46 advantageously integrated with freauenev generator 42.

[0056] When switch 18 connects variable and reference capacitors 12, 14, respectively., with. one another In its closed position, variable capacitor 12 and inductor 46 form an IX circuit that oscillates at a first resonant frequency. The amplifier integrated in oscillator 42 drives the oscillating current in this LC circuit which is coupled into control unit 44. In open position of switch 18, another LC circuit "including only reference capacitor 14 and inductance 46 is formed. Again, control unit is operable to evaluate the second resonant frequency. Thereafter, in accordance with inventive algorithm of FIG. 2, control unit calculates a relative value of Capacitance Of variable capacitor 12 based on the ratio of first and second frequencies or their respective derivatives.

[0057] Referring in FIG, 7, single switch 18 Is one of the salient features of the inventive eanfitmrations. One of the considerations for configuring switch 18 is that its switching freouenev should be high enough to establish a very short time of each of the switching states during which the inductance of the .inductor dues not undergo substantial changes. Depending on foe task at hand, the operation of switch 18 in a 10 Hz •••■ 10 KHz range renders the contribution of theinductance change oil for example, .inductor 46 of FIG. S, to the measurement results is insignificant and can be ignored. Another Consideration for selecting switch 18 is both a small capacitance of measured capacitor 12 not exceeding tenths of a single pF and its even smaller tsn-perature-drift capacitance dependence not exceeding hundredths of pF, The transistor-based switch cannot meet these requirements - its capacitance in the closed state is at least a few p.Fs, The electromechanical switches are too slow. The switching frequency of s witch 18 should be no lower than a few hundred times per second and as high as a few thousand times, for example 4000 which Is unattainable for the electromechanical switch.

[0058] The other key factor for selecting switch 18 is the specifics of the pm-diOde operation including one of Its parameters - charge carrier life time which should be considerably greater than the period of AC signal flowing though the switch. The greater the charge carrier life, the greater the capacity of the pin diode. Accordingly this consideration contradicts the one of the previousparagrapher and necessitates a carefoi selection of the pin diode.

[0059] Considering FIG, 7 in combination with FKfe, 1 and 4-5, a signal generated by control unit 44 and controlling the operation of switch 18 is a DC signal. However, the critical measurement operation of system 10 is. of course, based on an alternating current (AC), Accordingly, key 18 operates on the AC- This is accomplished by a circuit including optionally at least one or more pairs of inductors 52, Alternatively, the resistors may perform the same function. To prevent the AC signal from their iuilueuce, inductors 52 should have the impedance which is substantially greater (han that of the signal. In this case, the current flowing through inductor 52 is small compared to that of (he AC signal. Typically, pin-diodes operate in circuits with a resistance of 50 phms (D). To minimize the influence of the AC signal, inductors 52 are selected to have the impedance ranging between tens and hundreds of kilo ohms (kQ). .fa this mugs, the impedance is rather characterized by a resonant character forming a resonant contour with its own capacitance.

[0060] The aspects disclosed herein in accordance with tits present invention, are not limited in their application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. These aspects are capable of assuming other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, element and features discussed in connection with any of the above-disclosed schematics are not intended to be excluded from a similar role in any other similar schematics.

[0061] Having thus described several aspects of at least one example, it Is to be appreciated that various alterations', modifications, and improvements will readily occur to those skilled in the art. For instance, examples disclosed herein may also be used in other contexts. Such alterations, modifications, and im .p«. rovements are intended to be oart of th >is disclosure. and are intended to be within the scope of the examples discussed herein. Accordingly, the foregoing description and drawings are by way of example only.

Claims

CL AIMSWhat is claimed is:

1. A measuring system for determining a capacitance of a variable capacitor unit, comprising: a reference capacitor; a capacitance measufing cirnuit invariably coupled to the reference capacitor; and a switch alternating between an open position in which the capacitance measuring circuit is operable to evaluate a capacitance of the reference capacitor, and a closed position in which the measuring circuit is configured to evaluate a combined capacitance of the variable capacitor unit and reference capacitor; wherein the measuring circuit is configured to measure the capaeitance of the variable Capacitor unit upon processing the evaluated capacitances in respective open and closed positions of the switch.

2. The system of claim 1, wherein the variable capacitor unit includes one or more capacitors coupled in series with a last capacitor having a coiaftiGa ground with the reference capacitor and measuring system.

3. The system of claim 1, wherein the reference capacitor is at least partially or fully integrated in (he measuring circuit.

4. Ths system of claim 1 , wherein the measuring circuit includes an oscillator continuously coupled in parallel with the reference capacitance and configured to output: a first periodic signal in the open position of the switch, and a second periodic signal in the closed position of the switch.

5. The system of claim 4, whereinthe measuring circuit further includes a control Unit operable to; evaluate the first periodic signal, evaluate the second periodic signal, and output, based on the evaluations, a system signal.

6. The system of claim. 5, wherein the control unit evaluates the first and second periodic signal by determining a ratio between the frequencies or periods of respective first and second periodic signals.

7. 7 he system of claim 5, wherein the system signal represents a relative value of the measured capacitance of the variable capacitor unit, an absolute value of the measured capacitance of ihe variable capacitor unit er a physical value based on the measured capacitance of the variable capacitt rr unit, 8, The system of claim 7, wherein the physical value is elected from the group consisting of a distance, pressure, thickness, height, density, flow and dielectric permeability and a combination of these.

9. The system of claim 1 , wherein the measuring circuit comprises an inductor coupled to the reference capacitor. 1(1. The system of claim. U wherein the switch comprises a diode, pin diode or transistor operating at a switching frequency ranging between. 10 Hz. and 10 KHz.

11. A method for measuring a capacitance of a variable capacitor comprising the steps of alternating a switch between open and closed positions, thereby evaluating: a capacitance of the reference capacitor in the open position of the switch, and a combined capacitance of reference and variable capacitors in the closed, position of the switch; and processing the evaluated capacitances in a measuring circuit, which is invariably coupled to the reference capacitor, thereby measuring a capacitance of the variable capacitor.

12. The method of claim 11, wherein: the evaluation of the reference capacitance includes generating a first periodic signal by an oscillator of the measuring system in the open position of the switch, thereby evaluating a characteristic of the first periodic signal, and ihe evaluation of the combined capacitance includes generating a second periodic signal by the oscillator of the measuring system in the closed position of the switch, thereby evaluating a characteristic of the second periodic signal.

13. The method of claim 12, wherein the characteristics of the first and second periodic signals are selected from the group consisting of frequency , period and a combination of the frequency and perioa.14, The method of claim 12 further comprising outputting a system signal based on the evaluated characteristics of respective first and second periodic signals, 15. The method of claim 13, w herein measuring the capacitance of the variable capacitor includes determining the ratio between the characteristics of respective first and second periodic signals.

16. The method of claim I L wherein the measured capacitance of the variable capacitor represents Hs absolute value.

17. The method of claim 11 further comprising determining a pity steal value based on the measured capacitance of the variable capacitor.

18. The method Of claim 17, wherein the determination of the physical value includes determining a distance, thickness, height, density, flow and dielectric penneability or a combination of these.

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