DE2350450A1 - Accurate ratio measurement using phase-detector - gives method of measuring physical parameters accurately normalized with respect to standard - Google Patents

Abstract

A phase measurement bridge arrangement is used by which various physical parameters may be measured accurately in the form of a ratio. The example described is the measurement of the translucence of some medium compared with that of air. Basically, a light beam is modulated at some unspecified frequency and is directed via a set mirrors of which one is semi-reflecting, along two paths to two photoelectric cells. One path is through air, and the other through the medium concerned. The output sinusoidal signals from these photocells are processed by integrators, adders and inverters constructed of liner amplifiers, to produce a signal of which the phase is a measurement of the ratio of the amplitude of the two received signals.

Description

Prof. Dr. W. Kroebel 2308 Schellhorn near Preetz i, Holst.d, October 3, 1973

Wehrbergallee 43 Tel. Preetz 04342/257 «

Description u η g (Appendix 1)

Two-phase AC voltage bridge for measurement primarily for measuring the quotient of physical quantities

In the case of physical measurements, there is often an incentive to measure the quotient of physical quantities precisely. To solve this problem, for example, _t for the case of measuring an electrical resistance R in relation to a normal resistance R, that is to say in the case

the measurement of a quotient χ is usually carried out by bridge

TT ""
Circuits made use of, ο Both direct and alternating voltages can serve as operating voltage for such bridge circuits. For such measuring tasks, the applicant has considered a special AC voltage bridge circuit with a fixed frequency and registered it with the German Patent Office;

In applying this ^x ness as a two-phase AC bridge to characteristic measuring device on a measurement of the light transmittance of a z, B, the liquid medium by determining the quotient D, where I is the intensity percentage of takes place the liquid passed ο light and I that of the incident light means the Measurement of this quotient via a change in phase angle

A.if or, with a fixed frequency f, over the time interval At assigned to Af . However, this measuring arrangement described in the Of f en Ie supply writing presupposes that the alternating primary intensity of a light source modulated with the frequency f remains constant in amplitude. This condition is not easy to meet, especially when using light-emitting diodes as light sources.

The subject of the present application is now a further development of the two-phase _{AC voltage bridge of the applicant 8} in which the size of / \ * £ or, dt ' as a measure of the quotient of ¹ D is invariant to intensity fluctuations of the light source,

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To explain the idea of the invention, reference is made to fig is closed off by a reflection mirror H , so that the light beam passes through 3 in the path 5 and via 4 back through 3 and 2 to a light-sensitive cell or photodiode 6. The measurement signal obtained from 6 is amplified in the amplifier 7 and thereby obtained in the form of an alternating measurement voltage Ai ^. The alternating light intensity coming from 1 is also radiated at the semitransparent mirror 2 to a second light-sensitive cell or photodiode 8 and the alternating voltage obtained from 8 is amplified in 9 so that it is present at the output of 9 as a reference voltage _{+ Αλ ο} ,

/ U ^ is variable due to the light transmission properties of the medium in the measuring section 5, W lrd_ the mean value lxtj ^ ~ | around the

AJCfy can fluctuate most appropriately ^ . I = fiig / g *** acA 4 then a voltage ^ ^ will appear at the output of the differential amplifier 10, the value of which can fluctuate between Au ₄ _ / itn to ^^ - AaO , (see Fig. 2), from J & £ z, B, a voltage + 'fik'Uii and - yVCvf »^ is produced via an integrator 11 with ^ - /. or, </ £ i1 t In the addition or differential amplifier 12 is off

ΙΛΑ get a voltage ΑΓ ^ χ and from the differential amplifier 13 with ^ l'dvU ^ & U ^ f a voltage "^ Hy · ^The associated phasor diagram is shown in Fig. 2, J ^ r and

ν, * ζ

have a phase angle ψ with each other or a time shift

The phase angle ψ is assigned to the quotient D or to the

ι τ Ά "^ Τ *" * quotient ο "D _t Δψ or jyis obtained £ ο with known circuits in 14, ο Ai can then ζ, Β ^ be counted according to known methods in order to obtain the quotient ο ~ D digitally,

~ ΤΓ ~

A variation of the AC intensity of the modulated light source 1 influences the phase angle ψ not because - ^ - ^ "V) or, ^ '^' o also varies in the arrangement of Figure 1 as the

Difference in voltage At / ^ y, - *** β «And d, R ,, that with the circuit according to Fig. 1 the quotient ο" D in the selected example is

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corresponding phase angle ty * or phase angle changes df with changes in I _D and thus also Δΐ are invariant to fluctuations in I, because of the use of a difference

Stronger 10 is plotted in the vector diagram of Fig, 2 Λ * β in the negative coordinate direction and - ^ /, ^ in positive. The difference voltage S8fin between S ^ u ,. - aad and Λ * ^ - λ * Γ _ο - i.e. between the corresponding arrows in Fig, 2,

If you choose fskß / ⁼ i ™ Hyl », the differential voltage ^. / Til at the output of 10 fluctuates around the earth or mass point

of the circuit, the alternating voltages Ϊ3 '^^ fi derived from / CCn and shifted by 90 compared to ^ tJa or their partial voltages "t- ^ f' ^ 'y ^ / l at points Α _Λ and A ^ on the - / - The axis of the phasor representation in Fig. 2 are voltages relative to the earth or mass point of the bridge circuit, instead of measuring the phase angle - ^ Y ^ ,, / i ^ _t ) , the phase angle - ^ NJy Hk M can also be used for measurement 12 u, 13 umml as a differential as well as summation amplifiers can be switched,

For the realization of the bridge circuit shown in Fig. 1 only in principle and schematically, various designs are possible with the integrated electrical components available today. They are included in the concept of the invention for producing an invariance of ^ f A ^ or, Ai: by deriving the partial stresses + -i ^^ fi shifted by 90 ° from Atn or, A ^ and their use according to the principle diagram in Fig, I,

For the measuring arrangement according to Fig. 1 it is useful from time to time Tent to carry out a recalibration or to carry it out automatically. For the first case, a possibly, also electromagnetic Remote-controlled reflection mirror 15 inserted into the beam path, e.g. in front of window 3, around different calibration points to be able to check, calibrated gray filters or the like can also be used be inserted into the beam path. With such a gray wedge 16 in the beam path from 2 to 8 can also shift the range or adjust the beam intensities of the measuring arrangement be effected.

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The measuring arrangement described is frequency-independent up to one Relatively high cut-off frequency, it can therefore also be operated in primarily periodic time intervals of duration 4-T ". Here practically nothing changes in the measurement sensitivity if

^ T ^ - ^L ^ j> remains, If the period of the measurement time repetition is eg »> 2dtr» ^then the respective intermediate time in connection with an interconnection of the reflecting mirror 15 in Fig. I in the beam path can also be used for a calibration test. The intensity of the light source 1 can also be controlled to be higher in pulse mode if necessary. This also applies to an application of the inventive concept according to laid-open specification 2 205 989 when the two-phase bridge circuit according to the phasor diagram in FIG. 2 is applied to a resistor network.

A continuous automatic re-calibration is possible with the measuring arrangement according to Fig. 1 and its addition by Fig. 3, Compared to Fig. 1, a second light source I ^{1 is} added in Fig. 3. It is operated with an alternating voltage of a frequency f · - which differs sufficiently from the modulation frequency of 1 with f from this, so that the light intensities of 1 and 1 'after the photodiodes 8 and 6 via filter amplifiers 9 and 9 · or, 7 and 7 ^! can be separated. For the light source 1 'with the semitransparent mirror 2' and the reflection mirror 15 'there is then a relation of the measurement of the intensity quotient ο of the light sources 1 and 1' which is independent of the light attenuation in the medium according to Fig. 1 for the light source. with an addition to the measurement order of Fig. 3 according to Fig. 1 after the output of 9 ¹ and 7 ¹ ,

The same inventive idea can also be used with an optical Apply turbidity measuring arrangement, which has been presented in the patent application 2 205 989 of the applicant of 16.8.1973. at it is assumed that the light sources have less fluctuations than light-sensitive cells such as, for example, in form of photodiodes, The measuring arrangement is therefore designed in such a way that sensitivity fluctuations of the photodiodes affect the measurement result are ineffective,

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Such a measuring arrangement is shown schematically in Fig. 4. In it 1 means a light source modulated with the frequency f, its rays fall through a semitransparent mirror 2 and a window 3 into a medium 5 and onto a reflection mirror 4 2 thrown to the photosensitive cell 6, or over respectively, at 6, a subsequent amplifier 14 then produces a voltage U ^^ · This voltage is a second superimposed as Referenzspannung- sho, by modulation of the light source 12 in anti-phase modulation over that of the Light source 1 arises, so the same phasor diagram applies to the superposition of the two light change intensities / of 1 and 12 as in Fig. 2, i.e. that at the output of 14 a differential voltage corresponding to ^ Ky ^ ^ P ⁿ a ° ⁿ Fig. 2 can be seen is. For precision measurements, however, it is necessary that the light intensity of 12 is controlled or regulated according to that of 1 for equality of amplitude.

This happens because in the light-sensitive cell intensity components of 1 and 12 over the mirror 2 for the rays 1 and. over the reflective mirror 8 and the semitransparent 13 a superimposition in 7 is effected, after aligning the beams intensity in terms of their losses on the beam paths then equal intensity for the alternating light component of the modulation frequency f, when the difference signal at the output of the amplifier disappears. With an in-phase gain in 16 or 16, the control voltage required to correct equality deviations can therefore be obtained from 16. Instead of of the semitransparent mirror 13, a corresponding mirror 9 can also be used, with only certain beam paths swap.

When regulating the light change intensities of 1 and 12 for equality, this itself can still fluctuate, so that the differential voltage would also fluctuate after l4, in order to be one of these instead To be able to obtain fluctuations independent physical quantity as a measurement result, is about the semi-permeable or partial transparent mirror 10 an alternating light component of the light source 1 " branched off; in the light-sensitive cells / in an alternating voltage

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converted and from it, according to the description with reference to Fig. 1, an alternating voltage - ^ "^ - ä is generated, which therefore fluctuates with fluctuations of ^. or the difference saving A \. ~ A. 2 and a corresponding circuit arrangement as in Fig. 1, the quotient ο "D can therefore be determined from the phase angle ^^^» Aj ^ j or the time interval Λ ~ t assigned to pn , as well as independent of sensitivity fluctuations of 6 and 7,

With regard to a calibration of the measuring arrangement, corresponding measures can also be taken in the case of Fig. K , as has been explained above in connection with the measuring arrangement of Figs. 1 and 3.

The inventive concept described lends itself to using the two-phase voltage alternating current bridge when applied to, for example, a light-optical turbidity measurement at the same time for measuring, for example, the light intensity and, if necessary, several scattering angles. Fig. 5 demonstrates how this is to be carried out as an example of an additional component. In Fig. 5, the explanations given above with reference to Fig. 1 are based on the meaning of the functions for the numbered blocks became. Changed from Fig, I is initially in the beam arrangement of the reflecting mirrors 4 shown in Fig, 5 is preferably as Pliaeenspiegel central on a limited by the direct-mapped primary beam of the light source 1 cut an imaging lens ¹ P positioned ^ eic ^ Zi. The light scattered, for example, from the spatial region 5 'of the medium is collected by 4' on a photo cell 17, which corresponds to that of FIG. The scattered light signal amplified in 18 arrives at an adder or subtracter. 19, which corresponds to the function in IO from Fig, I. Since the scattered light will usually be much weaker than that which has passed through, the reference signal (s, a, phasor diagram Fig. 2) is weakened in 20 to the amount /? '* Ϊ́σ. The output signal is then processed as shown in FIG. 1 with the proviso that the corresponding components 21, 22 and 23 are added instead of 10, 12 and 13 for the measurement of the scattered light quotient. The measured value is then obtained at the output of 23,

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Further similar · additional arrangements are also, for possible to measure other physical quantities so, B _t for the measurement of scattered light intensities among several scattering angles, and also for the simultaneous measurement of the light transmittance of light of different wavelengths, for the latter purpose, the Fig. 5 is to be combined with the Light beam arrangement according to fig, h ,

The idea of the invention is analogous to a real or complex resistor network applicable, as described in the laid-open specification 2 205 989 has been made known,

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Claims (1)

Claim / 1 / σ * Two-phase AC voltage bridge for measuring physical quantities or for measuring the quotient of physical quantities, characterized in that the measurement takes place over the phase angle (V) between two V / echselspannungen ^^ _} - <λΉ _κ Λθγ, which are composed of a real and an imaginary component, of which the real for both AC voltages ^ n, - ^^ J is common and is formed from the sum or difference of a reference voltage and the one dependent on the value of the measured variable, while the negative imaginary component of one ^ t ^) and the positive imaginary C ** / u \ Λ belongs to the other AC voltage, whereby to achieve an invariance of the phase angle ( ψ) of the mutually shifted AC voltages A * ^ i ^ u ^ Viie AC voltage for the imaginary and real axis is derived from one source or, when using two sources the same sources or sources from one of these two sources that are fixed at the same alternating amplitude by regulation, Claim 2 Two-phase alternating voltage bridge for measurement · - according to claim characterized in that instead of the phase angle ( ψ ) or the phase angle (£ <f) which changes with the change in the value of the measured variable at a fixed frequency of the source voltage, the time interval which can be derived from (γ) and the frequency ^ £ = ^ X / serves, whereby this can then be counted in a known manner in order to obtain the measured value digitally, Claim 3 Two-phase AC voltage bridge for measurement according to Claims 1 and 2, characterized in that the measured variable is determined by the The intensity ratio f% AJ of a light intensity measured by a medium and that primarily incident on the medium is given, or by the quotient of the difference between the primary alternating light intensity minus that weakened by a medium to the primary (J. 509815/0 7 62 Claim 4 Two-phase AC voltage bridge for measurement -----—.- after Claims 1 to 3, characterized in that the measured variable according to claim 3 u, 1 u, 2 relates to the intensity of an acoustic beam, Claim 5 Two-phase AC voltage bridge for measurement ——— «—after Claims 1-4, characterized in that, in order to simplify the measuring circuit for measuring the physical quantity or its »quotients, the phase angle of only one of the two alternating voltages Qu ^ and / ti' / tfj.j is used with respect to the voltage of the real or the imaginary axis, " Claim 6 Two-phase AC voltage bridge for measurement ——— - according to Claim 1-5 characterized in that a light source with two alternating voltage ^en / erschiedener frequency (fu, f) is modulated or two in use "light sources, the one with a frequency (f), the other with the other (T ^1)" whereby, through the use of selective amplifiers, the alternating light intensities can each be used in terms of electronic circuitry according to claims 1-5, 5098 15/0762