MEASURING DEVICE FOR THE ELECTRICAL MEASUREMENT OF A RESISTANCE AS WELL AS ASSOCIATED MEASURING PROCEDURE

The invention refers to a Meß device gemäß Generic term the requirement 1 as well as a Meß amplifier gemäß Generic term of the requirement 8.

The electrical resistance of a medium or a solid body serves very frequently for the regulation a physical Größ e, for example the temperature. The measurement of the resistance takes place during a resistance telemetering, as a constant current is led by the resistance which can be measured, whereby the tension is proportional to the resistance.

In addition the so-called two-leader procedure is well-known, with from the Meß turns out to the resistance of two leaders which can be measured are led, over whom the constant current fließ t and the tension are measured at the same time. Thus the Ohm's resistance of both leaders of the Me&szlig adds itself for the actual resistance of the test specimen; turns out to the test specimen. If these two lines over putting or clamping link are attached, additionally the transition resistance the junction point adds itself to the resistance actually which can be measured. This becomes altogether apparent as substantial errors. The Meß turns out muß therefore to the attached line to be adjusted. The resistance of the line by change of temperature and/or the transition resistance of the putting and/or compression joint, for example by oxidation, changes muß the Meß turns out to be calibrated new. This is a substantial disadvantage of the Zweileiter Meß technology.

Over when measurements, where the evaluation equipment cannot be placed directly at the test specimen, actually only the voltage drop at the test specimen to measure and not additionally still the voltage drop over the inlets for the constant current, the so-called four-leader procedure is used. Two further lines are led to the test specimen apart from the current feeds. To it directly the tension over the test specimen can erfaß t become.

Is unfavorable with the Vierleiter Meß technology among other things, daß a increased wiring expenditure is necessary. In practical applications the four lines must be led frequently across contacts - plug contacts, sliding contacts or such a thing -, so daß on the one hand also here a increased expenditure develops and on the other hand one größ ere susceptibility to interference and the danger of Meß errors are present.

From the WO-A-85/03348, from which the generic term of the requirement 1 was derived, is a Meß arrangement for a tank level announcement admits, with to a Me&szlig designed as potentiometers; a condenser resisted is parallel switched. With one in the Meß circle fließ ends constant current at the Meß resisted sloping tension becomes over a Meß instrument indicated and is a Maß for the respective level. The condenser serves here for the oscillation damping of the Meß instrument and also with varying level. Auß erdem the condenser causes an absorbed resetting of the Meß worth after switching the Meß range.

Schließ lich it is also well-known, direct with the sensor resistance an electronics circuit to the Meß to place worth admission and processing. This is frequently used in particular with temperature measurements. At the exit of the electronics circuit then a Me&szlig stands; signal in form of an unit signal or in digital form on, over größ ere distances to be transferred knows. Unfavorably here one is substantially vergröß erte design of the sensor, since in the sensor also still active electronics is. This electronics muß very sturdily its and over one groß EN temperature range to work can, in order to be able to be used in rough industrial environment.

By inserted electronics the sensor needs additionally also still another power supply.

Task of the available invention is it, a Meß device and a Meß to create proceeded, with that in each case the advantages of the Zweileiter Meß technology and the Vierleiter Meß technology receive, whose disadvantages are however avoided. In particular is a high Meß accuracy practically independently of the length of the inlets and thus the resistance in the Meß circle available its, whereby only two inlets are intended. Within the Meß circle at contact points arising thermovoltages are with the Meß proceeded for the increase of the Meß accuracy to be along-considered know.

Further is with the Meß proceeded with changing conditions a self alignment available its.

The solution of this task is in the marking parts of the requirement for device 1 and regarding the Meß procedure in requirement 8 shown.

By to the Meß parallel switched condenser can after switching the constant current off the charging voltage of the condenser, those sensor-resisted the voltage drop over the Meß sensor-resisted corresponds, to be measured. Line resistances do not affect themselves here any longer unfavorably, there in the Meß circle over the two Meß lines at the time of the measurement practically no river fließ t.

Itself between the time of switching the constant current off and measuring the condenser charging voltage by unloading over the parallel switched Meß sensor-resisted adjusting tension sinking can on the one hand by a short time interval between switching the constant current and Me&szlig off; time small to be held and on the other hand is computationally with high accuracy determining the Meß tension over the condenser at the time of switching the feeding constant current off well possible. Thus here one of the conduit length is and concomitantly from the line resistance between the Meß sensor-resisted also parallel switched condenser and the Meß within wide limits independent measurement turns out possible, during the one two-leader connection to the Meß sensor is sufficient.

A further training of the Meß device plans, daß the power source pole reversalable is da&szlig and; preferably two alternatively adjustable power sources with different Stromfluß direction are present.

Before the actual measurement of the Meß sensor resistance can thereby the Meß circle successively with different Stromfluß directions to be bestromt. The voltage drops measured in each case imgesamten Meß circle can be used then for the determination of the arising thermovoltage, as the difference of these tensions is formed. The thermovoltage stands thus as Größ e for the order and can be considered with the measurement, which likewise for the increase of the Meß accuracy contributes.

Zweckmäß igerweise points the Meß amplifier of two entrances up, whereby to the power source anschließ cash reference resistance is present, in place of the lines with the entrances of the Me&szlig, attached to parallel to it the reservoir capacitor switched; amplifier to be connected can.

The constant current can be determined thereby practically directly before the measurement of the sensor resistance exactly, so daß itself a Langzeitdrift of the regulated current source not unfavorably on the Meß result affects.

A further training of the Meß procedure plans, daß after switching the constant current the tension off at the reservoir capacitor in time intervals one bases several times and one calculates the tension on the sensor at the switching time in particular by linear interpolation.

The measurement of the condenser tension at several times supplies well evaluable Meß rate, by which the condenser tension can be calculated at the time of switching the constant current off accurately.

The measurement takes place thereby zweckmäß igerweise only with short time interval to the point of current switching off and also within the almost linear range of the condenser final load curve. One considers here also, daß the measurements auß erhalb by transients on engagement of the tension which can be measured, which arise due to line inductances, to be made.

Additional arrangements of the invention are specified in the further Unteransprüchen.

Below the invention with its substantial details is still more near described on the basis the designs.

It shows:

1. Fig. 1 a circuit diagram of a resistance Meß arrangement
2. Fig. 2 a block diagram of a Meß mechanism,
3. Fig. 3 a flow chart of a resistance Meß procedure and
4. Fig. 4 a diagram of the potential gradient in the Meß circle.

One in Fig. 1 Me&szlig represented in the basic principle; device 1 serves for the electrical measurement a Meß sensor 2 of forming resistance 3. the Meß sensor 2 is attached to two lines 4, which lead to evaluation equipment 5, which in Fig. 1 and 2 is paint-lined bordered. The conduit length knows several meters, e.g. 25 meters amount to so daß a use of the Meß device 1 is possible, where a Meß signal processing and transformation into digital form in the spatial proximity of the sensor are not possible. For example thereby a temperature measurement is on far from the Meß distant places turn out, e.g. in earth boreholes or with Prozeß temperature measurements in the chemical industry and such a thing possible.

The evaluation equipment 5 contains among other things power sources 6a, 6b, a Meß amplifier 7 as well as a reference resistance 8th Auß erdem still another change over switch 9 as well as intended with the power sources 6a, 6b switch 10a, 10b is.

By the lines of 4 formed resistances are marked with 12. Parallel to the Meß 3 is switched a reservoir capacitor 11 sensor-resisted. This reservoir capacitor 11 offers together with that erfindungsgemäß EN Meß proceed the possibility, those at the resistance 3 with constant Meß circle stream sloping tension without Einfluß the line resistances 12 einschließ to determine lich from transition resistances at if necessary existing plug contacts and such a thing.

In principle this takes place via it, daß the reservoir capacitor 11 with the constant Bestromen of the Meß circle and sloping Meß tension at the resistance 3 is loaded and daß anschließ end the constant current one interrupts and the tension on the condenser one bases. There in the Meß phase practically no river in the Meß lines 4 fließ t, have also the line resistances 12 accordingly not an unfavorable effect.

To consider participates daß the Meß , that circle-resisted 12 and the Me&szlig from a row number of the line resistances; resisted 3 is formed, is very many smaller than the input impedance of the attached Meß vertärkers 7. usually becomes as Meß a Pt-100 with a resistance value sensor-resisted by 100 ohms used and the line resistances 12 moves within the range of approximately 1 to 10 ohms. An input impedance of the Me&szlig stands for that; equipment of more than 1 megaohm against.

By that erfindungsgemäß e Meß principle comes one despite high Meß accuracy with only two lines 4 as feeder lines between the Meß sensor 2 and the evaluation equipment 5 out. Around the Meß accuracy still further to improve, points the Meß device 1 mechanisms up, one considering in the Meß circle arising thermovoltage make possible and also errors by Langzeitdrift with the power sources avoid.

A Meß cycle becomes following on the basis the Fig. 1 in connection with Fig. 3 described.

The power sources 6a and 6b are power sources for different Stromfluß directions, whereby the power source 6a for a positive Meß river and the power source 6b for a negative Meß river by the Meß serve circle. Such power sources exhibit very constant values over short time intervals, whereby errors can occur only by a Langzeitdrift. In order to elimenieren this, becomes before each Meß procedure first the respective constant current of the power sources 6a and 6b examines. In addition serves for the entrances of the Meß amplifier 7 parallel switched reference resistance 8. is operated now the change over switch 9 and connected with it with the reference resistance 8 and connected at the same time the power source 6a by the switch 10a, fließ t over the reference resistance 8 by the power source 6a supplied the constant current and at the Meß amplifier 7 lines up a tension proportional dropping at the reference resistance 8 to the constant current. This Meß worth one stores. In Fig. this sequence of functions is shown 3 by the blocks 13 and 14. In the same way the constant current of the power source 6b with then becomes reverse Stromfluß direction examines and likewise as Meß worth stored (see blocks 15 and 16). The two Meß results place the Größ EN of the two Meß , those flows directly into the Meß accuracy are received. There the collection of the constant currents directly before the measurement of the Meß sensor resistance 3 effected, against the long-term stability of the power sources 6a, 6b no special requirements are placed.

After determining the constant currents the change over switch becomes 9 again in Fig. 1 starting position shown brought and by Schließ EN of the switch 10a takes place a Bestromung of the Meß circle first with positive Meß river. The Meß circle tension U₁ (see also Fig. 4) sits down thereby from the tensions over the resistance 3 which can be measured, to the tension over the line resistances 12 as well as the arising thermovoltage in the Meß circle together.

This Meß cut off is in Fig. 3 by the blocks 17 to 19 shown. In the same way takes place anschließ end a Bestromung with negative Meß river according to the functional modules 20 to 22.

The Meß results according to the functional modules 19 and 22 are related to the initially measured constant currents in each case. This is on the one hand and on the other hand explanatory to the functional module 16 and block 21 by the respective connection between the functional module 14 and the functional module 18.

From the two Meß results of the positive and the negative Meß Circle Betromung takes place anschließ end gemäß Block 23 a difference formation, which as result the thermovoltage in the Meß circle results in (see block 24).

It becomes now a Bestromung of the Meß circle with positive constant current made, whereby the switch 10a is closed and itself the change over switch 9 in Fig. 1 position shown finds. The Bestromung takes place to the reservoir capacitor 11 reliably on the tension U₁ over the resistance 3 is loaded. Then the switch 10a is opened and the Meß circle tension measured and stored. The blocks 25 to 27 give this part of the Meß procedure again. At the time of switching off the existing tension is direct the tension over the reservoir capacitor 11 without Einfluß the line resistances 12, there during the Meß phase no river fließ t.

In practice directly there among other things the change-over process cannot be measured time at the switching time, e.g. some microseconds needs. The tension at the reservoir capacitor 11 becomes from there within the final load curve running after an exponential function (Fig. 4) in time intervals several times measured, whereby the time interval between switching the constant current off at the time t₀ (Fig. 4) and the first measurement at the time t₁ than the time constant of the discharge current circle is substantially smaller. For example this time interval amounts to some microseconds (3 to 10 microseconds) and a further measurement at the time t₂ becomes then in the distance to the first measurement again after some microseconds, e.g. 8 microseconds made. The distance of the first measurement of the point of current down time is also in such a way selected, daß by line inductances caused, brief voltage drops (Fig. 4) or rises no disturbing Einfluß have, because these at the time of the first measurement (t₁ ,) faded away.

From the measured condenser tensions, that from block 24 the available thermovoltage of the Meß circle as well as from block 14 Konstantstromme&szlig the available; worth in block 28 the charging voltage of the condenser is then computed when switching the constant current off. Can for the monitoring of the Meß circle the line resistance 12 (Fig. 1) also still with to be considered, there on the one hand either from block 18 or block 25 the Meß circle tension with positive Bestromung and on the other hand the Meß tension at the reservoir capacitor 11 after switching the Bestromung off is available. The difference of these Meß tensions is related to the positive Meß river proportionally to the line resistances. This line Widerstandsmeß worth can also to examination of the Meß lines 4 to be used.

With the computation of the condenser charging voltage at the time of switching the constant current off gemäß Functional module 28 is considered the tension sinking arising between switching the Bestromung off and measuring the tension at the condenser by unloading of the reservoir capacitor 11 taken place up to then over the resistance 3 with. The unloading curve becomes thereby as straight line (in Fig. 4 paint-lined suggested) treats, so daß by means of linear interpolation on the tension for the point of fall time of the constant current to be reckoned back can.

In the functional module 29 a computation of the current resistance value of the sensor resistance 3 takes place, by the Spannungsmeß worth from block 28 as standardized Größ e with the resistance value of the reference resistance, with that the Meß flows were determined, are mulipliziert. As result one obtains the value of the resistance 3 in ohms direct.

In Fig. 4 is the process of the Meß circle tension during a Meß cycle represented.

First is a Meß circle tension U₁ during the Bestromungsphase available, itself from and concomitantly at the reservoir capacitor the 11 lining up sensor tension U&sub2 dropping at the sensor resistance 3; and 12 of the lines 4 for the voltage drop up over the line resistances builds.

Becomes now the time t₀ the constant current switched off, sinks at the Meß amplifier 7 lining up tension on U₂ , there now without Konstantstromfluß also (practically) no more voltage drop over the Meß lines is present.

Like already above-mentioned, the tension becomes U₂ also not directly measured, since in this off or switching range tension oscillations can occur, which would impair an accurate measuring. The tension U₂ , those at the time t₀ at the condenser lines up, can from the Spannungsmeß rate Ut1 and Ut2 at the times t₁ and t₂ accurately to be computed.

Also the value of the line resistances 12 can be computed and considered for monitoring purposes directly. The difference becomes from the measured tension of the Meß circle with Stromfluß and the calculated tension over the sensor 2 without Stromfluß formed. Divides one this difference by the positive Meß , then one receives a Grö&szlig to river; e according to the error by line resistances. Mulipliziert one this Größ , then one obtains the resistance value of the used line between the Me&szlig direct to e with the value of the reference resistance 8; - and/or evaluation equipment 5 and the sensor 2. the reference to the positive constant current is intended, because with it the determination of the line resistance independently of the Größ e of the river becomes.

Becomes too gro&szlig by influences of noise the line resistance; , an alarm can be released. Contrary to the past two-leader technology thereby also the Me&szlig can; line quality to be supervised. It is thus also a Selbstdiagnose of the Meß lines possible, whereby the Meß security to be substantially increased can.

If the Meß 3 as fixed against sand is trained, can the line resistance resisted as Meß größ e to be determined. That erfindungsgemäß e Meß proceeded can be used thus in this modified form also for the accurate examination by lines.

Against the reservoir capacitor 11 certain demands are to be made, so that a good quality and reproductibility of the measurement are present. The capacity of the condenser plays a subordinated role and can within a range from for example 1 microfarad to 20 microfarads vary with a sensor resistance of approx. 100 ohms. The lower limit is certain by the time constant of the Entladungskreises, which is not to fall below a certain value because of the intended, linear interpolation. The upper border of the Kapazitätswertes is by the Meß cycle determines, in which the condenser is to be completely loaded in each case.

An important parameter of the reservoir capacitor 11 is its insulation resistance. It should so groß like possible its, thus by it no important falling Meß arise to errors. Because of the high demanded insulation resistance foil condensers or condensers with metalized foil are applicable as reservoir capacitors. This wise substantially higher insulation resistance than at least demanded - größ it as 500 Kilo ohms - up, whereby usual values are more than 1,000 megaohms. The reservoir capacitor 11 should be also inductance-poor.

Into the Fig. 1 and 2 paint-lined circumscribed evaluation equipment 5 contains auß it in Fig. 1 recognizable components still further, which are necessary for signal processing. Signal processing essentially happens in four steps. As the first the signal becomes around the factor 10 in the Meß amplifier 7 strengthens. At the exit of the Meß amplifier 7 a precision electric rectifier 30 is attached, that in a second step the amount from the output signal of the Meß amplifier forms 7. By the output voltage of the precision electric rectifier 30 becomes a constant tension with a following reversal amplifier 31 (U₀) of the Meß signal subtracts. This constant tension is derived directly from a reference supply terminal of the analogue-digital converter 32. By this tension reduction one gets along with the analogue-digital converter - transducer 32 with a dissolution of 12 bits instead of 14 bits.

Behind the reversal amplifier 31 SAM-polarizes & Hold is an amplifier 33 attached. This serves for the intermediate storage of the Meß rate for the period of the analog-digital transformation. In that itself anschließ ends analogue-digital converter - transducers 32 become the similar Meß rate into digital form converted.

With the analogue-digital converter 32 still another alphanumeric announcement 34 as well as a micro controller system 35 are connected. The announcement 34 serves on the one hand for the expenditure of the current resistance and/or temperature level and can be used also for the announcement by error messages in the plain language. Additional display elements are no longer necessary thereby.

The micro controller system 35 takes over the flow control of the measurement gemäß Fig. 3.

The Bestromung of the Meß circle with constant current one makes intermittently. By this intermittent Meß proceeded the constant Me&szlig can; river by the sensor so groß are selected, daß a signal which can be processed well develops, without daß but the sensor thereby is warmed up. An additional source of error is avoided thereby.

In attempts showed themselves, daß by means of a Pt 100 of resistance a temperature within the range of 0°C to 100°C with an accuracy of 0,1 °C to be measured could. In particular thereby also attempts with lines of different lengths became 4 between the sensor 2 and the Meß 5 turns out accomplished. The conduit length became thereby of first 30 centimeters on anschließ end 15 meters changed. Here a coaxial cable was used. In usual ZweidrahtMeß technology became thereby a change of the Meß worth-indicate from 1,8°C devoted.

With that erfindungsgemäß EN Meß device took place automatically an error correction and the indicated Meß worth were exactly alike with 30 centimeters conduit length and 15 meters of conduit length. Also as additional to the Meß line a carbon film resistor of up to 10 ohms was switched into the line 4, showed the Meß turns out accurately for the same Meß worth like before. To notice is here, daß 10 ohms of change of resistance with the Pt 100-Meß resisted a change of temperature of 20°C would correspond.

From these test results is also recognizably, daß with changing resistance of the lines 4 by change of temperature, change of the transition resistances of putting and/or clamping links e.g. by oxidation and such a thing, with that erfindungsgemäß EN Meß device 1 no new calibration of auß EN is necessary, an automatic error correction is made there here and the accurate Meß result despite these Störgröß EN remains.

Also during continuous operation of 96 hours the indicator value remained within the given margins of error. The announcement remained also stable, as the Meß value registration hardware as well as the reservoir capacitor 11 parallel to the Pt 100-Meß 3 with a hair hair dryer resisted several minute with warm air was blown on.

With a further attempt a thermovoltage simulator in the Me&szlig became; circle switched. A thermovoltage of 100 millivolts did not have Einfluß on the Meß result, whereby is to be considered, daß the thermovoltage lead/copper only - 0.31 millivolts/100 °C and/or with copper/nickel 1.22 millivolts/100 °C amounts to.

With the aforementioned attempts could be shown clearly, daß with the help of that erfindungsgemäß EN Meß device and/or the associated Meß proceed a resistance telemetering with the accuracy of the Vierleiter Meß technology to be reached could, although only two lines 4 are needed.