DE1303861C2 - ELECTRICAL CIRCUIT FOR THE INDEPENDENT CALIBRATION OF AN IMPEDANCE BRIDGE - Google Patents

Description

The invention relates to an electrical circuit for the automatic balancing of an impedance measuring bridge, in whose bridge branch a number of switches for changing the current flowing through this bridge branch are arranged, with a phase-controlled rectifier emitting an error signal corresponding to the phase of the equalizing current and a reversible counter which can be reversed by this and which can be incremented by means of a clock generator for controlling the switching state of the switches and with a display and / or output device controlled by the reversible counter for the component of the impedance to be measured.

The most varied types of impedance measuring bridges are known. To make an automatic adjustment the impedance, current or voltage ratios in the comparison branch of the Bridge can usually be switched digitally. In particular, a number of comparison impedances can be optional or switched together to get the

xo to change the current flowing through the comparison branch. Alternatively or in addition to this, at the transformer measuring bridge, which is particularly suitable for automatic balancing, has various taps the secondary winding of the transformer

X5 can be switched to the To change the voltage and thereby the current flowing through this bridge branch.

A circuit for the automatic adjustment of a measuring bridge of the type mentioned is, for example, from British Patent 744,364 known. In the known circuit, the unknown impedance z. B. consist of a temperature sensor, which its value within relatively lower Borders changes. Since changes in impedance are possible in both directions, the counter must be reversible being. To this Zv. corner, an alternating voltage is fed in at two bridge corner points and the equalizing current flowing between the other two corner points via a transformer Secondary center tap for phase control fed two diodes, each with one Winding of a changeover relay are connected to the neutral rest position. By means of the relay the reversible counter controlled. The counter changes the comparison impedance by opening and closing switches within given limits. The step-by-step change in the comparison impedance takes place continuously during the adjustment. Such a one However, circuit is poorly suited for measuring impedances in a range of several decadal orders of magnitude, because on the one hand there is a far too large number of comparison impedances would have to be provided and, moreover, the adjustment would take far too long.

Another known circuit (US Pat. No. 3,082,373) attempts this deficiency for the to avoid automatic adjustment of a measuring bridge in that comparison impedances of different Orders of magnitude can be switched in such a way that first a coarse adjustment and then riding on an ever finer adjustment is carried out. Such circuits are z. B. from decades of resistance well known. With this well-known bridge, an »error detection

folgs discriminator «is used, the following Compares the adjustment step with the previous one and decides whether the next adjustment step has led to an improvement in the bridge balance. If this is not the case, then will the last switching step is reversed and the next finer series of comparison impedance cη advanced. A particular disadvantage of this known circuit is the requirement for a memory unit, which must store the result of the previous switching step, and the difficulty wedge the adjustment of active and reactive components exhibiting impedances because of the dei during adjustment individual components with respect to the other cones

component of the adjustment point that often changes significantly. binding with a capacitance transformer measurement. This has the consequence that the balancing of the individual bridge, _c · ι Vmn

Components must be repeated alternately. Fig. 2 is a circuit diagram according to Fig. 1 under verai-

The invention is based on the object of providing an illustration of further details of the circuit of the type mentioned at the outset, 5 Fig. 3 a transistor switch which can be used in a scnaiwelche a quick adjustment when measuring device according to FIGS. 1 and 2 , impedances in one Range of several deka- Fig. 4 and 5 possible arrangements of the more precise

dischen orders of magnitude with low circuitry for the reference impedance and

technical effort and in particular

which also has these advantages when effective measurement of capacitance and conductance. _Door _

and minor components are to be matched. In Figs. 1 and 2, the tekaiinten ^ irans-

This object is achieved according to the invention formator-measuring bridge assumed the "lmarwK through between the clock and the individual lung P of the transformer T is of ernenn, TSOW decade inputs of the reversible counter overall rator fed 6 with alternating current, so that a span switched from a second counter controllable voltage is induced in the secondary winding 5 with a g ^ee r ™> AND element and a phase-controlled center tap G. The rectifiers connected downstream, each in phase with the secondary winding S, form bridge branches.11 reversal of the equalizing current a counting signal is applied to the other two bridge branches, a voltage comparator which emits an unsecond counter, a known capacitance C _x and a known vergieicn * - whereby the outputs of the second counter are assigned to the AND capacitance C ₅ . that from the second the bridge is balanced when the currents in

Counter when it is incremented, the AND gate assigned to the bridge branches with the comparative capacitance l next decade of the reversible counter and the unknown capacitance C gieicn ^{SI ™ 'is activated.} By means of the fact that there is no equalizing current in the equalization of this circuit, a phase-controlled Oleicnncnter λι ikbiι around the balance point 25. convergent oscillating adjustment achieved. The connection between the bridge corner point 1 and the control of the individual counter decades by means of the ground connection G 'on the grounded "back-AND" element and the second counter is corner point G. The phase-controlled rectifier * is very simple. determines whether the bridge branch is marked with C

Expediently, the current flowing in the voltage curve is greater or smaller than that rather than a current flowing from the phase-controlled rectifier through the bridge arm, hi-generated direct voltage with a fixed reference voltage, which indicates an error signal whether to; Bridge identifier compared and the comparison signal is fed to the second counter _{via a comparison in the bridge branch C 5 via a larger OR element.} On lower tension is required. I11il. If this is the way the second counter will run as long as 35 magnitude of the voltage ^mi » ^el ^ ^u /? is supplied ^erschiedll f ^h J _m ^A "'current Once delivered as the phase-controlled rectifier taps on the secondary winding and a reter an error signal. ER- gelbaren contact T changed the balance, wherein (he selection ranges, while the second counter is de-energized the tapping points are shut down via electronic digital switches and the reversible counter is thereby shut down.

In a particularly advantageous manner, the logic circuit described in more detail can be used for the circuit according to the invention in the known transformer and a reversible counter IV_ zugeformator bridge circuits with a number! leads, which controls _{the voltage at C s} via lines 10 by means of the electrically switchable taps of the secondary winding use circuit. To that the. A measuring device constructed in this way is able to reduce the error signal. In the Reconciled condition of 100 _M F and a balance within less than bridge the value of the unknown ^Ka P ^Aztat ^ half a second with an accuracy of at an "in-line" -Digitalanzeige 21 shown, α> 0 jo "for example in the capacity units indicates. This

'If active and reactive information are also to be compared for use with component at the same time, are two identical 50 printing devices or other? ^atenvera ^ 'built, independent of each other, a knitting and processing system at 22 in the block-locked deci-one blind element in the comparison branch of the bridge painting system. ,

assigned units, consisting of phase In the embodiment according to F1 g. 1 adjust

controlled rectifier, AND gates, second len the electronic digital switch 41, the tap counter and reversible counter, are provided. The 55 fung Γ on the secondary winding S, with the demphakonvergent pendulous simultaneous adjustment of the bit-controlled rectifier 3 em S rom ^to S ^etuhr J the components prevents the occurrence of the analog value of the Dgta state of the different adjustment points, so that an alternating counter 19 corresponds. The adjustment of the two components is automatically omitted. The tapping point selected which enables a circuit in accordance with the bridge compensation therefore leads to a 60 equal. , · _F · "

high synchronization speed. A preferred arrangement of the switches 4 is shown in FIG

The advantage here is that the two compo- F i g. 2 shown. In this case, successive circuits used have the same structure. Tests that produce different voltage values _are existing taps T via corresponding switches

Embodiments according to the invention advertising 6 ₅ T _1, T _n ... T ₃ in same normal capacity C _s angeden below with reference to the drawings in greater detail closed ER. The switches are shown schematically, refined. It preferably shows transistor switches as shown in Fig.

1 shows a block diagram of the circuit in FIG.

5 6

If the bridge is unmatched, a signal arises at the output and generates a current in the normal

dcs phase-controlled rectifier has a DC capacitance C _s , which corresponds to the analog value of the count in

voltage that corresponds to a voltage to the reversible counter via line 5.

Compare 9 is supplied. The voltage Vcr- The switches 'T ₁ , 7'. ,, T ₃ etc. can beispicls-

same 9 belongs to the logic circuit 20, the 5 wise two-stage transistor switches with parallelgc-

other circuit elements below, more closely connected base outputs and common Emil-

bcschricbcn are. The DC voltage will be in the span (Fig. 3). The changeable tension off

Voltage comparator with one at 7 supplied to the various taps in FIG. 1 and 2 too

tensile stress that use up earth potential, for example, is only one possible way. Another

knows, compared. In the case of an incomplete bridge, the ₁₀ way is shown in Fig. 4, where instead of several, one

Output signal of the phase-controlled rectifier at the same size comparison capacitances according to FIG

higher or lower than the reference voltage so that F i g. 2, each with a different voltage

A signal can be fed at the output of an OR element 11, several capacitances C ₅₁ ... C ' _s3 etc.

will hold that over a line 15 are a counter with graduated values and a common

13 controls. The counter 13 has in FIG. 2 four output connections of the switches T ₁ - T ₃ etc. to the upper one

inputs D ₁ , D., D., and D ₄ , which with the corresponding connection of the secondary winding S exists. This

the AND gates G ₁ . G.,. G., and G ₁ in connec-

stand that the synchronous clock pulses from a strict (for example, each doubling

(via 8 capacitance values connected to the power source) in the upper branch of the bridge,

the) clock 17 received to corresponding Dc- _ao Furthermore, as shown in Fig. 5, a single

cade inputs 8 'of the reversible counter are used to store Vcrglcichskapacity C _s , the

sen. To indicate the direction in which the current flowing through them according to the state

reversible counter 19 is to count, the switch T ₁ , 7 \ "T., etc. and the associated

a directional control DC voltage to Neten resistors R ₁ , R. " /?., etc., which depends

the reversible counter 19 is applied. Has the ver-as from the count of the reversible counter 19 gc-

mood control signal a value that the counter will be switched. The tension at B is created by

is higher than what is required for a comparison.

Liche, causes the counter 19 reverse directions to a common amplifier A, the

counts and vice versa. to generate a high level of linearity and stability

If the bridge is unbalanced, the counter 13 indicates - with considerable negative feedback at R arbeilet. With-

a signal at the output D ₁ , so that by means of this digital-to-analog conversion, the

the gate circuit G ₁ pulses from the clock 17 ratio of the alternating voltage bet B compared to the

passes and the highest decade of the reversing AC voltage changed at the input, so that for

ble counter 19 supplies. The highest decade counts. Effecting the adjustment of the impedance value of the standard

further until it counts beyond the balancing point of the voltage capacitor C _s in the bridge circuit effectively

has, whereby the polarity of the error signal is changed.

returns. This reversal causes a change in the For illustrative purposes, the bridge is

State of the output of the voltage comparison circuit according to FIGS. 1 and 2 on the basis of sclbst-

chers 9, what has been described about the OR gate 11 and the capacitance measurements made,

Line 15 is passed to the counter 13 and 40 although if desired also inductance and

leads to the fact that the counter at output D., conductance measurements can be made,

emits a signal and thus the gate circuit G _2. In the circuit according to FIG. 6, the capacitance

öfinet. As a result, the next decade as shown in FIG. 1 and 2, for example, by a

input 8 'clock pulse c are supplied. Additional conductance measurement added. The unknown conductance

Changes in the state of the comparison 9 cause 45 G _x is connected in parallel to the unknown capacity

ken a corresponding control of the counter 13, city C _x determined. A digital-to-analog conversion

so that this continues to the outputs / λ or D ₄ sctzcr 4 'with an additional phase connection

is switched and the gate circuit G., or G ₄ uses rectifier 3 ', which has a 90-phase

is opened. The counter 19 has the normal shifted output to a separate set

Transfer paths from each decade to the next 5 "logic circuits 20 'and one reversible

higher decade. Counter 19 'creates, with the individual connections

Which determines the counting direction and thus the direction of the reference signs corresponding to FIG.

necessary correction of the tapping point at the sc- 7.cichnct.

Secondary winding 5 of the transformer T indicating that in practice there can be bridge circuits. The output 10 of the reversible counter 19 can have the 55 prescribed construction with F.success mi, in FIG. 2 schematically illustrated switch 4 that an automatic adjustment in the tax above. The tapping point T can therefore be reached in the manner described. For example, successive switches T ₁ . T ", _Τ Λ etc. capacity Serving self at a 400 Hz power source 6 continuously sought in order Spannungsvcrhält- the area (parallel connection) of 0.01 pF to 100 (iF nissc to change in the bridge arms. Count- 60 mark, an I. ), namely C \ actuated in a suitable manner and for all Karichtigcn Ί ranslormaloranzapf location with an accuracy of 0.1 ° "T 'associated capacity and conductance and / o fliettt of 1 ° for the Ab-Schaltcr. This current in the bridge branch (65 icscgcnauigkcit tics loss factor. Without Bcrcichsbei balance the current j-liich in the bridge changes, an automatic adjustment lättl χ branch f. Dk switch 4 so take the digital in 12:35 reach lake.

1 sheet of drawings

Claims (4)

Patent claims: 1. Electrical circuit for the automatic adjustment of an impedance measuring bridge, in whose bridge branch a number of switches for changing the current flowing through this bridge branch is arranged, with a phase-controlled rectifier emitting an error signal corresponding to the phase of the compensating current and a phase-controlled rectifier that can be reversed by this and reversible counter that can be incremented by means of a clock generator to control the switching state of the switches and with a display and / or output device controlled by the reversible counter for the impedance component to be measured, characterized by between the clock generator (17) and the individual Delcade inputs (8 ') of the reversible counter (19), AND elements (G ₁ ... G ₄ ) which can be controlled by a second counter (13) and a phase-controlled rectifier (3) connected downstream of the phase-controlled rectifier (3), a counting signal to the second counter ( 1 3) emitting voltage comparator (9), the outputs of the second counter (13) being assigned to the AND gates (G ₁ ... G ₄ ) in such a way that the second counter (13), when it is incremented, is that of the next lower decade of the reversible counter (19) associated AND gate is controlled. 2. Circuit according to claim 1, characterized in that the voltage comparator (9) When the DC voltage supplied by the phase-controlled rectifier (3) is compared with a fixed reference voltage, the comparison signal is compared to the second via an OR gate (11) Counter (13) is supplied. 3. Circuit according to claim 1 or 2, characterized by the use for a transformer bridge circuit with a number of switchable taps on the secondary winding (S). 4. A circuit according to claim 1, 2 or 3, characterized in that for the adjustment of the Active and reactive components of the bridge detuning two identically constructed, from each other independent, one active and one dummy element in the comparison branch of the bridge assigned Units consisting of a phase-controlled rectifier, AND gates, and a second counter and reversible counter are provided.