IMPROVEMENTS IN THE MEASUREMENT OF IMPEDANCE RATIOS

Abstract: Apparatus is provided for measuring the ratio of first and second impedances such as capacitors (C₁, C₂) of a capacita­ tive transducer. A carrier driver (S₁) supplies an amplitude modulated voltage (V₁) at a constant frequency to the first im­ pedance (C₁) and a second driver (S₂) supplies a voltage (V₂) to the second impedance (C₂) that is of opposite polarity to the voltage (V₁), is amplitude modulated at the same frequency and is of variable amplitude. A comparator constituted by a summing amplifier (A₁, C₃), a demodulator (S₃) and an integra­ tor (A₂, R₁, C₄) responds to the output of said first and second impedances (Ct, C₁) and supplies an imbalance signal that is a function of (V₁ C₁ - V₂ C₂). That signal modifies the output of variable waveform generator (G₂) and compara­ tor A₃, the output being fed through chopper (S₄) and low pass filter (F₁) to driver (S₂) to provide a variable amplitude signal that is the voltage (V₂). The mark/space ratio of the compara­ tor A₃ represents the ratio of (C₁) to (C₂).

Description: IMPROVEMENTS IN THE MEASUREMENT OF IMPEDANCE RATIOS This invention relates to apparatus for measuring the ratio of two electrical impedances that provides an output that is readily digitised. It is particularly, but not exclusively, intended for providing a digitised output representing the setting of a capacitative transducer that may be used for a variety of purposes eg. measuring strain, displacement, pressure load or level. Our UK Patent No.2036982 describes and claims electrical apparatus for measuring the ratio of first and second impedance means C1, C2 each connected at one side to an input to a first direct current amplifier that outputs an error signal, means for supplying a repetitive waveform synchronously to the other side of the first and second impedance means, a detector driven synchronously with the waveform supplied to said impedance means and fed with the error signal of the first direct current amplifier to generate a direct current output in response thereto, a second direct current amplifier arranged as an integrator and fed with the output of the detector to output a voltage V2 that determines the amplitude of the waveform fed to one of said first and second impedance means, the amplitude of the waveform fed to the other of said impedance means being determined by a reference voltage V11 the arrangement being such that the circuit reaches a balance so that V2/V1 = C2/C1. In that apparatus the voltage V2 fed to one of the impedances is derived by means of a feedback loop that includes the demodulator and integrator and is arranged so that the voltage at the summing junction of the first direct current amplifier is always zero by reason of the high loop gain of the feedback circuit. It is an object of this invention to provide a ratiometric circuit for measuring a variable impedance that provides an output as the ratio of two time periods, thereby facilitating digital representation. Broadly stated the invention provides apparatus for measuring the ratio of first and second impedance means and producing a time ratio output proportional to the ratio of said impedances comprising: means for supplying a voltage V1 to the first impedance amplitude modulated at a constant frequency; means for supplying a voltage V2 to the second impedance that is of opposite polarity to the voltage V1 is amplitude modulated at the same frequency as the voltage V1 and is of variable amplitude; comparator means responsive to the outputs of said first and second impedance means to supply an imbalance signal that is a function of (V1 C1 - V2 C2); and a variable squarewave waveform generator whose mark/space ratio is modified by the imbalance signal from the comparator means and whose output waveform is fed through a low-pass filter to produce a variable amplitude signal that is said voltage V2. The ratio of the mark/ space that2 therefore represents the ratio c1/c2. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure l is a circuit diagram of an apparatus for measuring the ratio of two capacitances; and Figure 2 is a waveform diagram. In the drawings the impedances to be measured are capacitances C1, C2 one of which is fixed and the other is variable and which form elements of a capacitative transducer. Such transducers operate ratiometrically when one of the elements is fixed and differentially when both are variable, as described, for example, in Patent Specification No.1274856. A voltage V1 from a dc reference source is applied to capacitance C1 through a chopper S1 that comprises a pair of field effect transistor switches driven by a fixed frequency square wave generator G. The second switch is driven from the first switch through an inverter (not shown) and is connected to earth whereby the input to capacitor C1 is grounded when V1 is not applied to C1, the two FET switches being driven in antiphase. The same voltage V1 is applied through switches Sqr low-pass filter F1 and chopper S2 to one side of capacitor C2. The capacitors C1 and C2 are both connected to the summing junction of operational amplifier A1 having a feedback capacitor C3. Like the chopper S1, the chopper S2 comprises a pair of field effect transistors driven from the square wave generator G in antiphase, one of the switches being earthed so that capacitor C2 is grounded when reference voltage V1 is removed by the carrier generator G. The output from amplifier A1 is fed to switch S3 of a demodulator also driven synchronously from the carrier generator G and the dc output of the demodulator is applied to an integrator constituted by an operational amplifier A2 and feedback elements R1 and C4 that cause the integral of the signal applied via S3 to accumulate at the output of amplifier A2. The integrator is arranged to provide most of the gain of the loop. The amplified dc output from A2 is proportional to the integral of the input signal at A1 and is applied to one input of comparator A3. The other input of the comparator amplifier A3 is driven from a generator G2 of triangular or sawtooth waves which may typically be an integrator circuit driven from generator G via a divider unit D1. Alternatively the generator G2 may be a free-running repetitive ramp waveform generator that may optionally be synchronised to carrier generator G, D1 conveniently divides the frequency output of the carrier generator G by a convenient factor eg. 16 where the carrier generator has a frequency of 1600 Hz. The output from comparator A3 is now a fixed frequency square wave whose mark to space ratio varies between logic 0 and logic 1 depending on the level of the dc voltage V2 at one input compared with the equivalent level of the triangle waveform 10 (Figure 2) at the other input. The output of the comparatorA3 is taken to provide a digitisable output and is also used to control the switches S4 which are field effect transistors, again driven in antiphase with one of them earthed. The voltage V1 applied to capacitor C2 is chopped by switches S4 with a variable mark to space ratio so that the average dc voltage level fed to capacitor C2 represents the voltage level needed to drive C2 and produce a zero error voltage at the input of amplifier A1. In order to reduce the dynamic range of requirements of the amplifier system the output of chopper S4 is smoothed. Therefore the variable mark/space ratio waveform is applied to the low-pass filter F1 which filters out all the ac components of the waveform and leaves only a dc component whose amplitude will be a direct function of the mark/space ratio. This resultant variable dc level is applied to the switch S2 which is driven from the same generator G1 as Sl and therefore applies the same frequency of carrier to C2 but of an amplitude determined by the mark/space ratio. If the voltages are in the correct phase the current through C2 will tend to cancel that due to C1 at the summing junction of the amplifier A1. The system therefore forms a feedback network in which the amplitude driving the capacitor C2 increases until there is no further drive signal at the input of integrator A2 and the balance process has arrived at a steady state. It will be apparent that in a steady state condition the ratio of the voltage at the input to S2 and the output of filter F1 to the reference voltage V1 represents the ratio of the impedance of the capacitor C2 to the impedance of the capacitor C1. Also the mark to space ratio at the output of comparator A3 also represents the ratio of C1 to C2. In Figure 2 the waveforms are shown to demonstrate the principles of operation. Waveform 10 is the triangular wave with a period T1 which is derived from a divider which is in turn driven from a carrier generator G in Figure 1. Also in Figure 2 line 11 is the dc voltage V2 which occurs at the output of integrator A2 in Figure 1. The amplitude of V2 intersects waveform 10 at an interval of time T2 after the commencement of the triangular waveform and again at T3 on the reverse slope of waveform 10 and the process is repeated at intervals of T1 assuming a steady state condition. It will be apparent that as the amplitude of V2 varies the period T3 - T2 varies also. If T3 - T2 is represented as T4 then the ratio T4/T1 = /" C1/C2. If T4 and T1 are used to gate a high frequency clock then these time intervals are digitised and may be used to represent the ratio of C1 to C2 in digital form. Various modifications may be made to the embodiment described above without departing from the invention, whose scope is defined in the appended claims. For example, it is also possible to use a reversible counter to establish the mark/space ratio which represents the capacitance ratio. This reversible counter would take the place of integrator A2 and would be used to generate the waveform applied to filter F1. It will be appreciated that there are a range of possibilities for generating this mark/space ratio but that they all involve representing the capacitance ratio as a time ratio for ease of transmission from a sensor to the outside world. It will also be appreciated that the same technique can be used for other electrical impedances such as resistors or inductors or a combination thereof. The invention can be put into integrated circuit form and will provide a compact and accurate interface with an external control system.

Claims: CLAIMS: 1. Apparatus for measuring the ratio of first and second impedance means and producing a time ratio output proportional to the ratio of said impedances comprising: means (S1) for supplying a voltage (V1) amplitude modulated at a constant frequency to the first impedance (cl); means (S2) for supplying a voltage (V2) to the second impedance (C2) that is of opposite polarity to the voltage (V1) is amplitude modulated at the same frequency as the voltage (V1) and is of variable amplitude; comparator means (A1, C3, 83, A2, R1, C4) responsive to the outputs of said first and second impedance means (C1, C2) to supply an imbalance signal that is a function of (V1 C1 - V2 C2); and a variable squarewave waveform generator (G2, A3) whose mark/space ratio is modified by the imbalance signal from the comparator means and whose output waveform is fed through a low-pass filter (F1) to produce a variable amplitude signal that is said voltage (V2). 2. Apparatus according to claim 1, wherein the voltages (V1) and (V2) are derived from a common source (G). 3. Apparatus for measuring the ratio of first and second impedance means (C1, C2) each connected at one side to an input to a first amplifier (A1) that outputs a signal, means (G, S1, S2) for supplying a repetitive waveform synchronously to the other side of the first and second impedance means (C1, C2), a detector (S3) driven synchronously with the waveform supplied to said impedance means (C1, C2) and fed with the signal of the first amplifier (A1) to generate a direct current output in response ther'eto, a second amplifier arranged as an integrator (A2, C4) and fed with the output of the detector S3 to output a voltage V2 that determines the amplitude of the waveform fed to one of said first and second impedance means (C2), the amplitude of the waveform fed to the other of said first and second impedance means being determined by a reference voltage (V1) characterised in that the amplitude of the waveform fed to said one impedance means (C2) is determined by the average level of voltage (V) supplied from a voltage source via first chopper means (S4) controlled by a comparator (A3) supplied at one input with the output (V2, 11) from the integrator and at the other input with triangle or sawtooth waves (G2, 10) whereby said circuit reaches a balance such that tl/t4 = C2/C1 wherein tl represents the period of the triangular waveform (10) fed to the comparator, t4 represents the period for which the output of the comparator is at logic 1 and (C1, C2) represent the values of the impedances being measured. 4. Apparatus according to claim 3, wherein the first chopper means (S) controlled by the comparator comprises a pair of switches driven in antiphase therefrom, one switch connecting the said one impedance means (C2) to the reference voltage source and the other to earth. 5. Apparatus according to claim 4, wherein the first chopper means (S4) controlled by the comparator is connected via a low pass filter (F1) to the said one impedance means (C2). 6. Apparatus according to claim 4 or 5, wherein a dc reference voltage source is connected via second chopper means to said other impedance means (C1) and via said first chopper means (84) and third chopper means (S3) to said one impedance means (C2), said first and third chopper means being driven synchronously. 7. Apparatus according to claim 6, wherein said first and said third chopper means (S1, S2) each comprise a pair of switches driven in antiphase, one switch being connected to the voltage source and the other being connected to earth. 8. Apparatus according to claim 7, wherein said switches are field effect transistors. 9. Apparatus according to any of claims 6 to 8, wherein the source of triangle or sawtooth waves (G2) is driven via a divider (D1) from a carrier generator (G1) that also controls the first and third chopper means (S1, S3). 10. Apparatus according to any preceding claim, wherein said first and second impedance means (C1, C2) are elements (C1, C2) of a capacitative transducer.

Inventors: DAYKIN, CHRISTOPHER ISRAEL, WOLFENDALE, PETER CALEB FREDERICK, YEWEN, JOHN DAVID
Applicants: AUTOMATIC SYSTEMS LABORATORIES LIMITED
Application year: 1985
Main classification: G01R19
IPC codes: G01R27, G01R19