WO2009033845A1

WO2009033845A1 - Apparatus for determining and/or monitoring a process variable

Info

Publication number: WO2009033845A1
Application number: PCT/EP2008/058970
Authority: WO
Inventors: Thomas Schönstein
Original assignee: Innovative Sensor Technology Ist Ag
Priority date: 2007-09-07
Filing date: 2008-07-10
Publication date: 2009-03-19
Also published as: DE102007042500A1; DE102007042500B4

Abstract

The invention relates to an apparatus for determining and/or monitoring at least one process variable, having at least one first measurement path (1) which has at least one electronic reference element (3) and/or an electronic measurement element (11), having at least one second measurement path (2) which has at least one electronic reference element (4) and/or an electronic measuring element (12), and wherein at least one electronic element (3, 11) in the first measurement path (1) and at least one electronic element (4, 12) in the second measurement path (2) can be interconnected such that a reference unit (6) with at least one known characteristic is obtained. The invention also relates to a method for determining and/or monitoring a frequency of a clock unit.

Description

description

Device for determining and / or monitoring a process variable

The invention relates to a device for determining and / or monitoring at least one process variable. The process variable is, for example, moisture, temperature or flow of a medium, which is for example a liquid, a bulk material or a gas. Furthermore, the invention relates to a method for determining and / or monitoring a frequency of a clock unit.

In the prior art, a number of measuring devices or sensors is known, which serve the determination and / or monitoring of process variables. In most cases, measured quantities are generated that depend on the process variable or a change in the process variable. Some of these measuring devices have at least one reference element for the evaluation of the measured variable, which has known characteristics. Furthermore, it is sometimes necessary for the measurement that a clock unit specifies a frequency. However, problems usually exist in that, if necessary, the reference element or the clock unit drifts over time or as a result of process parameters.

A variant is to perform the corresponding elements redundant and make occasional comparisons between the elements. However, this is costly and not easy to implement in terms of space requirements in the meter or sensor.

The invention has for its object to provide a cost-effective solution to the problem of unsafe components or elements.

The invention solves the problem on the one hand by a device for determining and / or monitoring a process variable and on the other by a method for determining and / or monitoring a frequency of a clock unit.

The invention solves the problem by a device for determining and / or monitoring at least one process variable, with at least a first measuring branch, which at least one electronic Reference element and / or an electronic measuring element having at least one second measuring branch, which has at least one electronic reference element and / or an electronic measuring element, and wherein at least one electronic element of the first measuring branch and at least one electronic element of the second measuring branch are interconnected such that there is a reference unit with at least one known parameter. The device thus has at least two measuring branches which measure and / or monitor different or the same measuring or process variables. The measuring branches have electronic components or elements which possibly serve for referencing within the respective measuring branch or the actual measurement. According to the invention, at least two electronic elements of the two measuring branches are interconnected, so that there is a new reference unit with a new characteristic. In this case, for example, the reference elements are interconnected or, for example, the measuring elements, the latter combination then also having to take account of the respective measured quantities. For the interconnection of the elements corresponding switches and connections are provided. The characteristic of the measuring branches cross-reference unit then also refers, for example, the corresponding electronic properties. An embodiment provides that the reference element of the first measuring branch and the reference element of the second measuring branch are interconnected such that a reference unit results with at least one known parameter. The device is thus provided with at least one first measuring branch, which has at least one electronic reference element and with at least one second measuring branch, which has at least one electronic reference element, and wherein the reference element of the first measuring branch and the reference element of the second measuring branch are interconnected such that provides a reference unit with at least one known characteristic. The device has Thus, at least two measuring branches, which each have at least one reference element. With the two measuring branches, for example, the determination and / or monitoring of two process variables is possible or, for example, a measured variable can be measured redundantly. The parameter of the switched reference unit preferably results from the characteristics of the individual reference elements. These are, for example, the parameters of components or units which describe their specific electrical properties. In particular, the reference unit is designed in such a way that its parameter makes it possible to measure at least a part of the measuring device or a unit or a component of the measuring device. If, for example, a frequency or a clock rate is to be measured, then the parameter is, for example, a time constant.

The two measuring branches serve to determine and / or

Monitoring at least two different process variables (for example temperature, humidity, flow, level, pH, pressure, gas, etc.). In another embodiment, at least one process variable is redundantly and / or additionally diversely measured by the two measuring branches, i. the measurements are identical or done in different ways. The device according to the invention refers to any number, i. at least two measuring branches, so that any number of process or measured variables measured, or even redundant and / or diverse can be measured.

An embodiment of the device according to the invention includes that at least one clock unit is provided, which generates at least one frequency, and that the reference unit has at least one time constant. The characteristic of the reference unit in this embodiment was therefore in particular a time constant.

For the control of the clock unit now these are at least two

Reference elements connected in such a way that a reference unit results. This reference unit is characterized by a time constant. The time constant is, for example, at least partially dependent on the dimensioning of the two reference elements. Thus, starting from the known properties of the two reference elements, the time constant of the newly designed reference unit is known. By means of this reference unit, the clock unit can then be checked by, for example, comparing the time constant determined by the reference unit via the clock unit with the suitably deposited value. The time constant relates, for example, to an electronic behavior such as the charging or discharging behavior of the reference unit.

Time measurements are often realized via pulse counts. With the aid of a known counting frequency f ₀ , the events within the time t or the time window t ₂ -t- ₁ are incremented to N (t) via timer arrangements and provided in digital form for further processing, for example: N (t) = f _t) * t

The frequency f ₀ provides the clock unit, so for example an oscillator. The accuracy of the time measurement depends essentially on the errors of the oscillator frequency f ₀ . In particular, her Temperaturgang is considered below. Assuming, for example, that the linear temperature response is given by: f {τ) = f _o * [l ± a * (T -T _l) )], where (T - To) represents a deterministic temperature range, we have: Af (τ) = f _o * a * (T-T _tl ) = f (τ) -f _o

The maximum frequency deviation with respect to the temperature range is given as follows: A / _ (r) = /, * «* (/ _ - T _u ) = f (T _mj Jf ₍ ,

For the temperature coefficient α of the oscillator frequency results from this:

The measurement error due to the temperature response of the oscillator is

Thus: - AA (J) = a * [/ T - T ₀ \) * / * _± 1 _\ 0 _n 0 _n = Percent _* ual i ERF _T -eh _I / ier

[0016] with AN (T) = f _n * t * a * (TT _n) and N (/,) <JV (0 <^ (/ _2). [0017] ΔΛ ^ _X = N {t ₂ ) - NH ₁ ) or AN _^ = /, Hh ~ O

Not infrequently are time windows to be evaluated in the range: ~ - _< t _{x <}

^"~ t ₂ ^~

Practical values such as T -T ₀ = 6OK and α = 5 * 10 ⁴ A "give relative errors in the value of 5% to 15% of the measuring range at the measuring range end value.

Starting from the accuracy to be achieved then in turn specify the components to be interconnected for the reference unit.

An embodiment of the device according to the invention provides that it is the clock unit to a microprocessor or a microcontroller or a resonant circuit.

An embodiment of the device according to the invention includes that the first measuring branch is used to measure a capacitive measuring transducer. If the capacitive measuring sensor is realized, for example, as a measuring capacitor whose capacitance depends on the process variable or on a change in the process variable, it is possible to deduce the process variable from the capacitance. For example, the measurements of the humidity or the level of a medium in a container are known. In both cases, the dielectric changes - in one case by the penetration of moisture, in the other case by the increase of the medium as a dielectric between two "capacitor plates" - and thus the capacity.

An embodiment of the device according to the invention provides that the first measuring branch comprises at least one RC element. By measuring the charging time of the RC element, the capacitance can be determined, for example, with a known value of the electrical resistance, since the charging constant is equal to the product of the electrical resistance and the capacitance.

An embodiment of the device according to the invention includes that the second measuring branch is used to measure a measuring resistor. For example, resistance thermometers are known in which the electrical resistance is dependent on the temperature, so that the Measurement of the resistance value in a known dependence allows the determination or monitoring of the temperature.

An embodiment of the device according to the invention provides that the second measuring branch comprises at least one RC element. The resistance of the temperature-dependent resistor can be determined via the charging curve of an RC element, insofar as, for example, a capacitor is specified as a known reference value.

An embodiment of the device according to the invention includes that the reference element of the first measuring branch is an electrical resistance, and that the reference element of the second measuring branch is an electrical capacitance. In this embodiment, the above embodiments for the measuring branches are thus summarized.

In the following embodiment, it is provided in particular that the measuring device according to the invention is a measuring device for determining and / or monitoring two variables or, in particular, temperature and humidity. In alternative embodiments, both measuring branches monitor at least one identical process or measured variable.

An embodiment of the device according to the invention provides that the first measuring branch serves to measure the moisture, and that the second measuring branch is used to measure the temperature. This refinement thus belongs to those in which the reference elements of the two measuring branches serve for the measurement of different process variables and in which the reference unit allows the measurement of a parameter which in turn is different therefrom.

An embodiment of the device according to the invention includes that the reference unit comprises at least one RC element. In this embodiment, it is thus specified concretely that the two reference elements allow the circuit of a complete RC element.

An embodiment of the device according to the invention provides that the clock unit at least the evaluation of the charging process and / or the discharging process of the first measuring branch and / or the second measuring branch is used.

In the specific case of the evaluation of two RC elements - consisting of Cref and Rx for the measurement of the temperature-dependent resistance or from Cx and Rref for the monitoring of the moisture-dependent capacitance value - over, for example, a processor-internal RC oscillator as a clock unit is thus according to the invention a compensation of the temperature response of the clock unit by forming a reference time constant Rref ^* Cref performed as a reference unit.

A great advantage is that the temperature T of the oscillator than

Clock unit, and the characteristic of its temperature response must not be known.

Cx defines a size-dependent capacity, here for example for the measurement of moisture, and Rx a size-dependent, e.g. temperature-dependent resistance.

For measured value compensation of the temperature error of the clock unit, which AN (T) = f _o * t * a * (TT _n ) or AN (T) = f (τ) * t ₀ - / J ₀ corresponds, is from each current counter reading for the corresponding measurement of Rx or Cx the error is subtracted in each case. The value f (T) corresponds to the counting frequency at the unknown temperature T. With f _o t _o as a reference count for N ₀ at the time t ₀ , which is known and largely constant.

The accuracy of the drift compensation depends on the

Measurement uncertainty with which the reference RC element, so the reference unit can be evaluated by the measuring algorithm, as well as its temperature response. Assuming that a more precise evaluation of the reference RC element by the measuring algorithm in comparison to the evaluation of the measuring RC elements is not necessary, only the temperature coefficient of the reference variable t _{0 is} considered. This results to:

[ _Ar :, _{0 + k)] + k} , _tti , _{Tm _ _τ)

In this case, the relationship between the two a values of the reference capacitor and the reference resistance is the following: a _r = k * a _R For example, a values of 50 ppm / K are acceptable for both Cref and Rref. Thus, the above influence of the temperature coefficient of the clock unit can be reduced from a = 5 * 10 ^"4 AT ¹ to about a = 1 * 10 ^ AT ^1. Even in the worst case, if both a values have the same sign.

Furthermore, the invention solves the problem with a method for

Determining and / or monitoring a frequency of a clock unit, wherein the clock unit is connected to at least one first measuring branch and a second measuring branch, each measuring branch having at least one reference element and / or at least one measuring element, wherein at least one element from the first measuring branch and at least one Element of the second measurement branch are interconnected such that there is a reference unit, which is characterized by at least one known time constant, wherein using the clock unit, a time constant of the reference unit is determined, and wherein the known time constant and the determined time constant are compared. The method thus consists in that at least two electronic elements or components of two different measuring branches are connected to one another in such a way that a new reference unit results which has a size, via which the clock unit is then measured. Thus, for example, if a measuring unit consists of at least two measuring branches, a new reference unit encompassing the measuring branches is generated by means of a combination of electronic components of these at least two measuring branches.

In particular, the method is part of a method for determining and / or monitoring at least one process variable, wherein the process variable is, for example, temperature, humidity, flow and / or the determination of a gas. The frequency or clock rate of the clock unit is thereby used to increase or guarantee the accuracy of the measured values or to monitor the clock unit itself.

An embodiment of the method provides that each measuring branch has at least one reference element, that at least the two reference elements of the two measuring branches are interconnected such that there is a reference unit, which is characterized by at least one known time constant. By interconnecting the reference elements thus results in a reference unit, which has a time constant, which is predetermined and known by the properties of the reference elements. If this time constant is measured with the clock unit and a deviation from the previously known value results, then a deviation of the frequency of the clock unit from the frequency should be seen, over which the clock unit should actually have and with what value the evaluations and calculations are made ,

The invention will be explained in more detail with reference to the following drawings. It shows:

Fig. 1: a schematic representation of a measuring device, and

2 shows a circuit of a sensor according to the invention.

Fig. 1 shows the basic structure of a sensor which serves both the measurement of moisture, as well as the measurement of the temperature. For this purpose, two separate measuring branches 1, 2 are provided. However, both measuring branches 1, 2 also use, for example, the same clock unit 5, so that both measuring branches 1, 2 can share the same elements otherwise. In each case, for example, both measuring branches are an RC element whose time constant is evaluated for the charging process or the discharging process.

The two measuring branches 1, 2 differ in that in the first measuring branch 1 the measuring capacitance 11 Cx is dependent on the humidity and that in the second measuring branch 2, the resistance value of the measuring resistor 12 Rx is dependent on the temperature. In both measuring branches 1, 2, however, a correspondingly known reference element-in the first measuring branch 1 the resistor 3 Rref and in the second measuring branch 2 the capacitor 4 Cref-is provided whose characteristic values are correspondingly stored in the measuring device / sensor or in a suitable evaluation unit. For a process variable can it may also be, for example, the detection or measurement of a gas.

The two measuring branches 1, 2 are controlled here by a microprocessor as a clock unit 5, which charges the two RC elements and, for example, evaluates the charging time. From the measured time, the time constant of the respective RC element, and thus the knowledge of the respective reference element, of the unknown variable of the temperature-dependent resistance or of the moisture-dependent capacity results.

It is thus necessary to know which parameters the two reference elements 3, 4 have. Furthermore, it must be known which clock frequency the microprocessor 5 uses. The latter, to be able to determine the time exactly.

The clock frequency of the microprocessor 5 may change due to aging or, for example, by the action of temperature or other process properties, whereby the measurement of the process variables or the evaluation of the measurement signals is uncertain or inaccurate. As shown above, the timing refers to the timing of the clock unit.

FIG. 2 shows how the two reference elements 3, 4 are connected to one another in such a way that a new reference unit 6 results.

For this purpose, the three switches 7 are provided. This results in a new RC element as a reference unit 6, the time constant resulting from the resistance of the reference resistor 3 and the capacitance of the reference capacitor 4.

The reference unit 6 thus results advantageously from the already existing components, so that there is a further measurement and thus a higher degree of accuracy without additional costs (except for the switch 7). For checking the clock frequency of the microprocessor 5, the reference unit 6 is appropriately controlled here. The reference elements 3, 4 are preferably precision components. Another advantage is that for the Measurement of the two process variables temperature and humidity and for the monitoring of the clock unit 5 always the same measuring principle is applied.

If the value measured for the time constant of the reference unit 6 deviates from the stored value, which results from the dimensioning of the participating reference elements 3, 4, and it can be assumed that the reference elements 3, 4 have not changed, it can thus be concluded that the clock rate or the frequency of the clock unit 5 no longer coincides with the corresponding stored value. This deviation or the newly determined value can then in turn be used for the more accurate evaluation of the time constants of the two measuring branches 1, 2.

The invention thus allows, for example, that clock units in general or especially microprocessors can be replaced or replaced more easily, since the clocking of the invention can be measured very accurately. In one embodiment, at least two clock units are provided. In the event that the frequency or clock rate of one of the two clock units falls below a certain value, it is switched to the other clock unit, so that it is active for the further measurements. In a further embodiment, in the event that a deviation of the determined clock rate is outside a predetermined tolerance range, an alarm or error signal is generated and transferred, for example, to a higher-level unit.

LIST OF REFERENCE NUMBERS

Table 1

measuring capacitor

measuring resistor

Claims

claims

1. Device for determining and / or monitoring at least one process variable, with at least one first measuring branch (1), which at least one electronic reference element (3) and / or an electronic measuring element

(11), with at least one second measuring branch (2), which at least one electronic reference element (4) and / or an electronic measuring element

(12), and wherein at least one electronic element (3, 11) of the first measuring branch (1) and at least one electronic element (4, 12) of the second measuring branch (2) are interconnectable such that a reference unit (6) gives at least one known characteristic.

2. Device according to claim 1, characterized in that the reference element (3) of the first measuring branch (1) and the reference element (4) of the second measuring branch (2) are interconnected such that a reference unit (6) having at least one known characteristic results.

3. Device according to claim 1 or 2, characterized in that at least one clock unit (5) is provided, which at least one

Frequency generated, and that the reference unit (6) has at least one time constant.

4. Apparatus according to claim 2 or 3, characterized in that it is the clock unit (5) is a microprocessor or a microcontroller or a resonant circuit.

5. Device according to one of claims 1 to 4, characterized in that the first measuring branch (1) of the measurement of a capacitive Transmitter (11) is used.

6. Device according to one of claims 1 to 5, characterized in that the first measuring branch (1) comprises at least one RC element.

7. Device according to one of claims 1 to 6, characterized in that the second measuring branch (2) of the measurement of a measuring resistor (12) is used.

8. Device according to one of claims 1 to 7, characterized in that the second measuring branch (2) comprises at least one RC element.

9. Device according to one of claims 1 to 8, characterized in that it is the reference element (3) of the first measuring branch (1) is an electrical resistance, and that it is in the reference element (4) of the second measuring branch (2 ) is an electrical capacity.

10. Device according to one of claims 1 to 9, characterized in that the first measuring branch (1) is used to measure the moisture, and that the second measuring branch (2) is used to measure the temperature.

11. Device according to one of claims 1 to 10, characterized in that the reference unit (6) comprises at least one RC element.

12. Device according to one of claims 1 to 11, characterized in that the clock unit (5) at least the evaluation of the charging process and / or the discharging process of the first measuring branch (1) and / or the second measuring branch (2) is used.

13. Method for determining and / or monitoring a frequency of a clock unit (5), wherein the clock unit (5) is connected to at least one first measuring branch (1) and one second measuring branch (2), each measuring branch (1, 2) having at least one reference element (3, 4) and / or at least one measuring element (11, 12 ), wherein at least one element (3, 11) of the first measuring branch (1) and at least one element (4, 12) of the second measuring branch (2) are interconnected in such a way that a reference unit (5) results, which at least one known time constant is characterized, wherein using the clock unit (5), a time constant of the reference unit (5) is determined, and wherein the known time constant and the determined time constant are compared with each other.

14. The method of claim 13, wherein each measuring branch (1, 2) at least one reference element (3, 4), wherein at least the two reference elements (3, 4) of the two measuring branches (1, 2) are interconnected such that a Reference unit (5) results, which is characterized by at least one known time constant.