DE102004035014A1 - Sensor array for accurate and reliable process temperature measurement, includes sensors with impedance-temperature coefficients differing in magnitude and sign - Google Patents

Abstract

All sensors (more correctly, sensor elements) have temperature-dependent electrical impedances, but differ in their temperature coefficients (A/B). They are thermally- coupled with each other and with the medium being measured. They are integrated into a measurement head. The electrical impedance-temperature coefficient of at least one sensor has negative sign, that of at least one other has a positive sign. They are connected in series. The potential drop across each impedance can be measured. Alternatively a parallel connection is made, with appropriate individual switching. In addition to primary impedance measurements, secondary variables are measured. These include e.g. thermoelectric potentials between various electrical connections of the sensor head. Currents and voltages are measured between sensors and the casing. Impedance variation due to self-heating by sensor currents of differing magnitudes is measured. Currents flowing into and out from the arrangement are compared and balanced. The lead resistance between sensor head and the sensor evaluation system is measured. Behavior of sensor temperature under transient conditions (temperature gradients) is measured at intervals. Error measurements are made to detect e.g. drift, degradation or complete failure of sensors. These are made by comparison of separately-measured temperatures by individual sensors. In addition, the secondary magnitudes are compared, and/or measured data are compared using previously-determined reference data sets. The sensors are embedded in a material (K) of relatively-low thermal resistance. This is at least partly enclosed by a material (R) of greater thermal resistance, forming the thermal connection to the medium under measurement. The first material has relatively high thermal capacity, forming with he second, a form of "thermal low-pass filter". The sensors are separated from the medium under measurement by a protective tube.

Description

The The invention relates to an arrangement of sensor elements for reliable measurement a temperature.

In Today's process and process engineering become temperatures in more diverse Form measured and used for process control and regulation. It is for reasons the product and process quality as well as the operational reliability a reliable and long-term stable temperature measurement with known measurement error limits more important.

to Minimization of maintenance and servicing, as well as plant downtime Beyond that self-monitoring, self-correcting and redundant systems for process measurement and Control sought (M.Frei: "Overview of the current state of self-diagnosis of sensor systems for process engineering ", VDI reports no. 1608, 2001; Trilling, Stieler, Schneider: "Self-monitoring and diagnosis of Field devices, magazine atp 11, 2001).

The common currently used in industry contact temperature measuring principles have age-related drifts and degradation of the sensor element on that raised to an Measuring errors or failures in the course of operation. Their occurrence is more random Nature and therefore unpredictable (B. Prause, A. Richter: "Aspects of Reliability and stability of thermocouples and resistance thermometers in industrial Application ", VDI reports, 1998; Xumo, Chen, Zhao: "An investigation into stability of industrial platinum resistance thermometer; Hart Scientific Inc., 2002; H. Kollmeier: "High-precision temperature measurement with Multislope A / D converter, "Shaker Verlag, 1st edition, 1996, chapter 4.1.3; "How does aging affect thermistor stability?", FAQ, YSI Inc. USA; "How do thermistors fail; what are typical failure modes for thermistors? ", FAQ, YSI Inc. USA).

Therefore is usually a regular, often too preventive calibration and adjustment of the temperature sensor performed to a tolerance band for to ensure the measurement error achieved. The time interval between two calibrations depends on permissible tolerance band and the operating conditions of the sensor. He is chosen on the basis of empirical values so that the tolerance band with big ones Probability between two successive calibrations will not leave. In most cases, this regular calibration will not leaving the permitted tolerance band because of the randomness however, the drift and degradation effects are not a better determination of the calibration interval possible. Every calibration involves considerable effort and costs because on the one hand the sensor must be removed (plant shutdown, dismantling of plant components) and second, a high accuracy reference temperature or temperature measuring device available for calibration have to be. Often exceed the accumulated calibration costs the acquisition cost of the Measuring system many times (O. Kanoun: "Novel models for calibration-free Temperature measurement with PN transitions ", progress reports VDI No. 905 chapter 2.2, VDI-Verlag, 2001). Another disadvantage this approach is that because of the randomness of drift and aging effects, such as also other errors within the measuring chain, despite a high calibration effort It can not be ruled out that it is between two calibrations leads to an unacceptable deterioration of the measurement accuracy.

Various temperature measuring methods and sensor arrangements are now known which permit a statement about the current measuring error and the condition of the sensor element. A whole range of these arrangements is based on sensor elements according to the resistance thermometer principle. In this case, other so-called secondary parameters such as insulation resistance to the housing, voltage between resistor and housing, self-heating index, etc. are measured on a single sensor element in addition to its temperature-dependent resistance. From these values, the actual temperature measured value, as well as an estimate of the sensor state and of the current measurement error are derived (WO 0011524, WO 0023776, US Pat. US 5828567 ).

To determine these secondary parameters, however, a not inconsiderable additional effort in the sensor evaluation is necessary. Above all, it must be ensured for reliable drift or degradation monitoring that the detection of the secondary parameters is stable over the long term, reliable and robust compared to the usual, troublesome industrial environment, and their own temperature dependence is compensated. As a result, the effort to determine the secondary parameters easily exceeds that of the actual temperature measurement. A considerable difficulty is also to infer from the present secondary parameters on the current measurement error and the state of the sensor element. For this purpose, an individual adjustment or learning process on the currently connected resistance measuring element is required. By using only one sensor element temperature measurement is also possible with immediate effect after the detection of an increased measurement error or after a complete failure of the sensor element with immediate effect. These reasons cause that until the At the time of the patent application, no serial meter based on these measuring methods was available.

A second group of self-monitoring Temperature measuring method is based on the combination of several sensor elements and their parallel evaluation.

In the methods according to WO 9808067, WO 0103099, US 6473708 and EP 0775897 Combinations of thermocouples with other temperature measuring principles are proposed in order to improve the limited long-term stability of a thermocouple measurement or to detect their drift effects can. For example, in many applications thermocouples have poorer long-term behavior than platinum resistance measuring elements, so that a reliable and stable measurement of the temperature is only possible over relatively short periods of time. Compared to a pure, individual thermocouple measurement, however, an estimate of the temperature from a plurality of sensor values over a relatively long time results in smaller measurement errors. In addition, an exceeding of the measurement error can be detected.

One Disadvantage of this method is that at least an initial, complex mutual calibration of the sensor combination is necessary, in which all temperature-dependent, in the subsequent measurement with included sizes above the Temperature recorded to be a reference database for later comparison with the current readings available (D.A. Barberree: "Dynamically Self-Validating Contact Temperature Sensors ", AccuTru International Corporation, 1996).

One Another disadvantage of this measurement principle is the need for Measurement and evaluation of several completely different measuring principles (e.g. Thermoelectric voltage and resistance value). This increases the complexity and susceptibility to interference the sensor evaluation, since two completely different measuring channels on the entire measuring range has a high accuracy and robustness against interference have to.

A redundant evaluation of two identical sensor elements (2x thermocouple, 2x platinum resistance probe) with comparison of the two measured temperatures is also known and available as an industrial sensor as a standard device. The realized here Homologous redundancy has the disadvantage of having the same measuring elements in the case of environmental effects (thermal cycles, aging) or fault conditions (e.g. Insulation error due to water in the sensor) behave similarly in the same direction and so by comparing the redundantly measured temperatures not sure a measured value drift can be detected.

Just in resistance temperature sensing elements, where in practice again and again isolation problems, for example, by penetrated Measuring medium in the sensor, as well as water in the sensor lines is due to the parallel to the resistance of the sensor element lying insulation resistance probable a faulty temperature measurement. Because such an insulation fault in two spatially close elements (in the same sensor protection tube) a similar one until same parallel resistance causes (common mode error), too the evaluated temperature difference of the two sensor elements stay small, although the absolute temperature measured already clear deviates from its nominal value. This leads even with arrangements several platinum resistance measuring elements of different nominal resistance (but same characteristic with positive temperature coefficient) to a bad, partly impossible detectability of a absolute temperature measurement error by comparing the measured individual temperatures.

Another possibility for reducing the calibration effort is the use of a so-called calibration-free temperature measurement based on semiconductor PN junctions ( DE 19710829 ; US 5982221 ; US 5195827 ; O. Kanoun: "Calibration-free temperature measurement based on bipolar transistors, new perspectives for metrology", Zeitschrift tm 4/2002, Oldenburg Verlag, 2002).

Here Temperature determination is based on various characteristic models the current / voltage characteristic a PN transition and the measurement of different current / voltage points this characteristic. Be based on this bevy of characteristic points all parameters of the characteristic model are determined. One parameter is doing the temperature of the barrier layer, which is then used as a temperature reading available is. By the universal validity This characteristic model can be almost any semiconductor PN junction without prior calibration with high accuracy to its temperature be evaluated.

there Although the methods are robust to parameter changes and scattering of the respective measured PN junction. They lead however then too unreliable Measured values, if the PN junction to be measured degrades in such a way, that his behavior (partially) no longer the underlying Characteristic model corresponds. Electrical or electromechanical Degradation within the measurement chain (e.g., electrical noise, EMC or line corrosion) also falsify the flock of measurement points so that the based on it Characteristic determination becomes inaccurate.

Of the Effort to record the characteristic points, as well as their further processing to a characteristic model is often considerably higher than the measurement e.g. a resistance temperature measuring element.

Of the Invention is based on the object, a long-term stable temperature sensor To develop, the leaving of defined measuring tolerance limits or the partial or complete Defect of the temperature measurement can detect automatically.

From Of particular importance here is the reliable detection of common mode errors.

to Temperature measurement, as well as for self-monitoring, should as possible only one type of measurement signal has to be detected. This can be the sensor belonging Evaluation electronics are kept simple, reliable and cheap.

The Asked object is achieved by the sensor arrangement solved according to claim 1. advantageous versions are the subject of further claims.

The inventive arrangement includes at least two sensor elements with temperature-dependent impedance, which are integrated thermally coupled within a sensor head.

there It should be emphasized that the temperature-dependent impedances as possible different temperature / resistance characteristics (characteristic "A" and "B") to have such a diversified Redundancy of the sensor elements to ensure.

The Raw sensor data from the sensor head enter the measured value processing. These are further processed here and the corresponding sensor output information (Such as a current proportional to the measuring temperature or a separate diagnostic output) generated therefrom, which then to further processing to a parent Control instance forwarded.

to Detection possible Errors or drift effects become the measured temperatures of all sensor elements independently determined from each other and then compared. exceeds the result of this temperature comparison defines a first Threshold, may be from a beginning degradation of the measurement be assumed. In this case, the temperature measuring system gives a warning.

exceeds the comparison result a second, higher threshold, so is from to assume a faulty temperature measurement. The temperature measuring system gives an alarm.

The particular advantages of the invention are that the temperature to be measured simultaneously detected by means of several diverse Temperatunnesskanäle which can be a high discrimination of common mode errors enable, but for all measuring channels only the measurement of electrical resistance required is. As a result, the evaluation connected to the sensor head can be relative realized simply and favorably become. The synchronization of the individual measuring channels within the evaluation via, for example Temperature and operating time can be reached with minimal effort.

By the use of several temperature measuring channels, the invention also has the advantage on, that in case of failure of temperature measuring channels a continuing temperature measurement with the remaining intact measuring channels is possible (backup function).

One Another advantage of this invention is that the sensor elements with a high long-term stability available and so are the resulting evaluated measurement temperature a high long-term stability having.

If after a long time Time but a degradation of the sensor elements used, this is independently from the sensor recognized. This is a condition-dependent calibration, maintenance and repair of the temperature sensor possible, which gives the operator a significant advantage in the total operating costs of its plant bestowed.

• • Temperaturmessungen, bei denen der Messwert für Sicherheitsaufgaben herangezogen wird und deshalb hochzuverlässig verfügbar sein muss.
• • Temperaturmessungen innerhalb von Anlagen bei denen die Messtemperatur einen wesentlichen Einfluss auf die Qualität oder die Ausbeute des hergestellten Produkts hat.
• • Temperaturmessstellen, die nach dem Einbau in einer Anlage nur sehr schwer oder mit großem Aufwand erreichbar sind und deshalb deren Wartungs- und Instandhaltungsaufwand minimal sein muss.
• • Temperaturmessungen, bei denen eine falsche oder ungenaue Messung finanzielle, ökologische oder gesundheitliche Folgen haben kann.
• • Temperaturmessungen, bei denen bei einer beginnenden Degradation der Messung sichergestellt sein muss, dass für einen beschränkten Zeitraum ein mindestens eingeschränkter Messbetrieb verfügbar bleibt (z.B. um die Anlage in einen sicheren Zustand zu bringen).

The invention is particularly suitable for the following fields of application:

• • Temperature measurements in which the measured value is used for safety tasks and therefore must be available with high reliability.
• • Temperature measurements within systems where the measurement temperature has a significant impact on the quality or yield of the product being manufactured.
• • Temperature measuring points, which are very difficult or difficult to reach after installation in a system and therefore their maintenance and repair costs must be minimal.
• • Temperature measurements where a false or inaccurate measurement can have financial, environmental or health consequences.
• • Temperature measurements which, if the measurement begins to degrade, must ensure that at least a limited measuring operation remains available for a limited period of time (eg around the system in one) to bring safe condition).

The Invention will be described below by way of exemplary embodiments and with reference to drawings explained in more detail. Show it:

1 a block diagram of the previously described general sensor arrangement

2 a detail of a sensor arrangement according to the invention in series connection

3 a detail of a sensor arrangement according to the invention in parallel

4 an exemplary mechanical construction of the sensor head

1 shows the basic sensor arrangement according to the invention. Within a sensor head ( 1 ) are resistance measuring elements ( 2 and 3 ) integrated with their different resistance / temperature characteristics A and B thermally coupled. Your sensor raw data ( 4 ) are used in the measured value processing ( 5 ) and from this the sensor output information ( 6 ), which is an overriding tax authority ( 7 ) to provide.

A first advantageous embodiment of the invention shows 2 , Here is a resistance temperature sensor element with a negative temperature coefficient ( 9 ) with a resistance temperature sensor element having a positive temperature coefficient ( 8th ) connected in series connection. Both temperature sensors are thermally coupled together in a sensor head ( 1 ) integrated. The connection to the measured value processing ( 5 ) via a 4- or 5-pole connection.

The sensor source ( 15 ) drives a measuring current ( 13 ) by the temperature-dependent resistances of the sensor elements ( 8th / 9 ). The voltages dropping at the sensor elements ( 14 ) can be connected to the measured value processing via electrical connections ( 5 ) are tapped. When using the conventional 3- or 4-wire resistance measurement technology, line resistances RI1..RI4 ( 10 ... 13 ) of these connecting lines not falsifying with the measurement result.

An extension of the series connection of two sensor elements according to the invention by further series-connected resistive sensor elements will be readily apparent to those skilled in the art. Likewise, it is readily apparent to one skilled in the art that the order of the two sensor elements ( 8th / 9 ) within the measuring current Im ( 13 ) can be arbitrary series connection.

It is advantageous in this sensor arrangement that a very good discrimination of common mode measurement errors can be achieved by the different signs of the temperature coefficient of the resistance measuring elements. Due to the series connection, the number of connecting lines between the sensor head ( 1 ) and measured value processing ( 5 ) are reduced.

A second solution variant for a sensor head according to the invention shows 3 , Here, unlike 2 the temperature resistance measuring elements ( 8th / 9 ) in a parallel connection. Here, too, both measuring elements have sign-different temperature coefficients and are thermally coupled.

The currents Im1 and Im2 through the sensor elements ( 13 ) are generated by the two sensor sources ( 15 ). The voltage drops across the sensor elements ( 14 ) are again determined by the measured value processing ( 5 ).

Next the advantages of the first embodiment leads here a faulty interruption one of the sensor elements not to complete failure of the sensor head, because by the parallel connection both sensor elements different Have rungs. Thus, the resistance measurement with the respective still intact measuring element will be continued.

A Extension of the described parallel connection by further parallel or series-connected resistive sensor elements is for those skilled in the art easily visible and possible without much effort.

Regardless of the solution chosen, the sensor sources ( 15 ) also via switching elements ( 16 ) operated switched. This has the advantage that the self-heating of the sensor elements ( 8th / 9 ) is reduced by pulse operation and partial defects in the sensor head by appropriate switching of the sensor sources a partial further operation is made possible (redundancy switching).

• – Thermospannungen im Sensorkopf und auf den Verbindungsleitungen zwischen Sensorkopf und Sensorauswertung zur Fehlerkompensation.
• – Vergleich des in den Sensorkopf (1) einfließenden Messstroms mit dem aus dem Sensorkopf ausfließenden Messstrom.
• – Leckströme zwischen den Anschlüssen des Sensorkopfs und dessen Gehäuse.
• – Messung des Selbsterwärmungseffektes durch Einprägen verschieden großer Ströme in die Temperaturfühler und Bestimmung der Änderung ihrer Sensordaten als Integritätskontrolle und zur Kompensation des Selbst erwärmungsfehlers der Fühlerelemente.
• – Zeitlicher Gradient der erfassten Sensordaten

Regardless of the chosen solution variant, by extending the sensor data acquisition ( 5 ), for example, the following secondary measurement data of the sensor head ( 1 ) to improve the measuring accuracy, reduce the response time of the sensor or to extend the drift and error monitoring are recorded:

• - Thermoelectric voltages in the sensor head and on the connecting cables between sensor head and sensor evaluation for error compensation.
• - Comparison of the in the sensor head ( 1 ) flowing measuring current with the from the sensor head outflowing measuring current.
• - Leakage currents between the terminals of the sensor head and its housing.
• Measurement of the self-heating effect by impressing currents of different magnitude into the temperature sensors and determining the change of their sensor data as an integrity check and to compensate for the self-heating error of the sensor elements.
• - Time gradient of the collected sensor data

Regardless of the chosen embodiment of the invention, it must be ensured that both measuring elements have the best possible thermal synchronization, so that even with temperature gradients at the sensor head, the two measured element temperatures do not diverge and thus the measured value evaluation ( 5 ) would erroneously detect a degradation of the sensor elements. This can be achieved by correspondingly 4 the two measuring elements are spatially closely embedded in a material "K" with relatively low thermal resistance and / or high thermal capacity, thereby ensuring a good thermal coupling between the two sensor elements The spatial arrangement of the sensor elements can be arranged side by side (as in FIG 4 ) as well as one behind the other in the sensor head and is dependent on the particular application (eg flow velocity of the measuring medium) or geometric design of the sensor head.

These Arrangement is at least partially enclosed by a second material "R", the one higher thermal resistance owns and forms the interface to the medium to be measured. This second material may also be formed as a protective tube, which is for a hermetic Separation between sensor elements and measuring medium ensures.

Learns the Sensor head now from the medium ago a temperature gradient, this is transmitted relatively poorly by the material "R" while passing through Material "K" ensured is that the then still arriving at the measuring elements temperature gradient preferably uniformly meets both measuring elements.

The thermal resistances The materials "R" and "K" thus form together with the thermal capacities the sensor elements as well as the materials "R" and "K" one Type thermal low-pass, the thermal synchronization of both Sensor elements improved.

Quoted Non-Patent Literature:

• 1. M.Frei: "Overview of the current state of self-diagnosis of sensor systems for process engineering ", VDI reports no. 1608, 2001
• 2. O. Kanoun: "Novel Models for Calibration-Free Temperature Measurement with PN Transitions ", VDI Progress Reports Series 8 No. 905, VDI-Verlag, 2001, Chapter 2.2
• 3. O. Kanoun: "Calibration free Temperature measurement based on bipolar transistors, new perspectives for the Metrology ", tm 4/2002, Oldenburg Verlag, 2002
• 4. D.A. Barberree: "Dynamically Self-Validating Contact Temperature Sensors ", AccuTru International Corporation
• 5. B. Prause, A. Richter: "Aspects the reliability and stability of thermocouples and resistance thermometers in industrial Application ", VDI reports 1998
• 6. Xumo, Chen, Zhao: "An investigation into stability of industrial platinum resistance thermometer, Hart Scientific Inc.
• 7. H. Kollmeier: "Highly accurate Temperature measurement with Multislope A / D converter ", Shaker Verlag, 1996, chapter 4.1.3
• 8. "How does aging affect thermistor stability? ", FAQ, YSI Inc. USA
• 9. "How do thermistors fail; What are typical failure modes for thermistors? ", FAQ, YSI Inc. USA
• 10. Trilling, Stieler, Schneider: "Self-monitoring and diagnosis of Field Devices ", atp 11, 2001

Claims (10)

Arrangement of a plurality of sensor elements for reliably measuring a temperature, characterized in that all sensor elements - have temperature-dependent electrical impedances which differ in the temperature coefficient (A / B) and - are thermally integrated with each other and coupled to the medium to be measured in a sensor head. Arrangement according to claim 1, characterized that - of the Temperature coefficient of the electrical impedance of at least one Sensor element has a negative sign, - while at least another sensor element has a positive temperature coefficient has the electrical impedance. Arrangement according to claims 1 or 2, characterized in that - the temperature-dependent impedances are arranged in an electrical series circuit ( 2 ) and - the voltage drops as needed across each individual impedance via an electrical connection for evaluation are electrically tapped. Arrangement according to claims 1 or 2 there characterized in that - the temperature-dependent impedances are arranged in an electrical parallel circuit ( 3 ) and - the voltage drops as needed across each individual impedance via an electrical connection for evaluation are electrically tapped. Arrangement according to claim 4, characterized that - the separate parallel circuit paths separately by appropriate Switching elements can be switched on and off. Arrangement according to one of claims 1-5, characterized that - Next the temperature-dependent Impedances more, secondary Characteristics of the Sensor elements are detected such as thermal voltages between the different electrical connections of the sensor head, - Currents and voltages between the sensor elements and the sensor housing, - Impedance change caused by self-heating by different sizes Sensor currents - Comparison and balance of the incoming and outgoing electrical currents in the arrangement, - Determination the line resistance between sensor head and sensor evaluation or - temporal Behavior of the individual measuring element temperatures with temperature gradients. Sensor evaluation to one of the arrangements of the claims 1-6 characterized in that a faulty measurement such as Drift, degradation or a complete defect of the sensor elements can be recognized by: - Comparison of separately recorded Temperatures of the individual sensor elements and / or - Comparison the secondary Characteristics accordingly Claim 6 and / or - Comparison the measured data of the sensor elements with an already the evaluation known reference data set. Arrangement according to one of claims 1-4, characterized that - the Sensor elements are embedded in a first material (K), the has a relatively low thermal resistance and this arrangement - from a second material (R) is at least partially enclosed, the one in comparison to the first material higher thermal resistance has and makes the thermal connection to the measuring medium. Arrangement according to claim 8, characterized that - the first material (K) additionally has a relatively high thermal capacity and so with the second Material (R) a "thermal Lowpass "forms. Arrangement according to one of claims 1-4, characterized that - the Sensor elements through a protective layer, such as a Protective tube, are separated from the measuring medium.