WO2013053905A2

WO2013053905A2 - Pressure sensor and method for checking the function of a pressure sensor

Info

Publication number: WO2013053905A2
Application number: PCT/EP2012/070295
Authority: WO
Inventors: Markus Messmer; Rolf Mueller; Heinz Walter
Original assignee: Ifm Electronic Gmbh
Priority date: 2011-10-14
Filing date: 2012-10-12
Publication date: 2013-04-18
Also published as: DE112012002911B4; DE102011084514A1; WO2013053905A3; DE112012002911A5

Abstract

The invention illustrates and describes a method for checking the function of a pressure sensor for sensing the pressure prevailing in a medium, wherein the pressure sensor has a pressure measurement cell with at least one measuring transducer and a temperature measuring sensor which is in the form of a diode, in particular a zener diode, and is intended to sense the temperature prevailing in the pressure measurement cell, and such a pressure sensor. According to the invention, the diode is alternately operated in the forward direction and in the reverse direction and a temperature-dependent forward voltage or a temperature-dependent zener voltage is measured in this case. The two characteristic curves of the forward voltage and of the zener voltage against the temperature were previously stored in a look-up table in an adjustment procedure. The associated temperature from the look-up table is respectively associated with the two measured values of the forward and zener voltages and the difference between the two temperatures is then determined. If the value of zero is significantly exceeded or undershot, a defective function of the temperature measuring sensor can therefore be detected.

Description

Pressure sensor and method for checking the function of a pressure sensor

The invention relates to a pressure sensor for detecting the pressure prevailing in a medium, wherein the pressure sensor comprises a pressure measuring cell with at least one transducer and a temperature sensor designed as a diode, in particular as a Zener diode for detecting the temperature prevailing at the pressure measuring cell, and a method for Checking the function of a pressure sensor.

Pressure sensors are used in many industries for pressure measurement, with the capacitive pressure sensor being a typical design. capacitive

Pressure sensors often have a ceramic pressure measuring cell, as a transducer for the process pressure, and an evaluation for signal processing.

In pressure measurement, the pressure within a medium adjacent to a measuring cell is often to be detected. The measuring cell is part of the

Measuring device and typically consists of a ceramic body and a membrane, wherein between the base body and the membrane, a glass solder ring is arranged. The resulting cavity between the body and membrane allows the longitudinal mobility of the membrane due to a pressure influence. On the underside of the membrane and on the opposite upper side of the main body electrodes are provided, which together form a measuring capacitor. By pressure it comes to a

Deformation of the membrane, resulting in a capacitance change of the measuring capacitor. With the aid of an evaluation unit, the capacitance change is detected and converted into a pressure measurement value. Typically, these pressure sensors are used to monitor or control processes. They are therefore often connected to higher-level control units (PLC).

Measuring cells and measuring devices of this type have been known for a long time and are used, for example, in many areas of process measuring technology for metrological process monitoring. Such measuring devices are manufactured by the applicant, for example, under the device names PNxx and Plxx and placed on the market. The measuring ranges are currently usually up to 600 bar.

Also in the pressure measurement of the type in question, the temperature infiuence on the measurement result plays a major role. The evaluated in the pressure measurement Capacity change is also dependent on the permittivity of the medium, which is located within the ceramic body and thus between the electrodes.

Usually this medium is air. By a temperature change, this permittivity may change, whereby a pressure change is detected, the actual amount may differ from the measured value. For example. For example, the actual pressure change may be zero while the meter is reading

Determines pressure change.

Because of this, the transducer must be temperature compensated, i. the proportion of the temperature prevailing in the immediate vicinity of the converter must be taken out of the measured capacitance value or voltage or current value, so that all measured values without temperature influence are comparable with each other.

For this purpose, a temperature sensor can be provided on the measuring cell. As a temperature sensor is usually an NTC or PTC temperature probe in question, for example, as a Pt100 resistance thermometer

executed thermistor device. In an evaluation unit, e.g. one

Microcontroller, then can mathematically the measured pressure value to the

Error influence of the temperature are cleaned up.

However, the problem now is that the temperature sensor does not always measure reliably, for example, aging can occur over a longer service life. Overall, it can be seen that temperature sensors drift and

Degradation effects are subject, which can lead to a false measurement of the temperature and thus to an incorrect compensation of the measured pressure value. In order to perform a reliable compensation of the pressure values, a reliable temperature measurement is necessary. Especially at

safety-oriented applications, it comes down to a safe

Temperature detection or errors in the temperature detection must be detected safely.

A common method for permanently checking the temperature sensor is a redundant design. The measured values of at least two temperature sensors are compared in terms of magnitude and a malfunction is detected with a correspondingly large deviation. A disadvantage of this simple redundant design is In addition to the temperature gradient between the two components that systematic errors are difficult or impossible to detect. In addition, drift and aging effects are subject to a certain degree of randomness, so that it can not be ruled out that both temperature sensors have the same or similar error behavior over time. The magnitude comparison of both temperature readings would not reveal any irregularities in this case.

For this reason, DE 10 2004 035 014 A1 proposes a construction with diverse redundancy. This arrangement includes at least two sensor elements with temperature-dependent impedance, which are integrated thermally coupled within a sensor head. In this case, the temperature-dependent impedances have different temperature / resistance characteristics, so the diversified

To ensure redundancy of the sensor elements. Due to the diverse structure in combination with redundancy, systematic errors as well as drift and aging effects can now be reliably detected. However, the disadvantage is - as with any redundant structure - the higher manufacturing costs resulting from the additional components and electrical connections.

The object of the invention is to further improve the reliable temperature measurement while taking up space and in particular the manufacturing costs

reduce.

The indicated object is achieved by a method with the features of claim 1, a method having the features of claim 2 and a pressure sensor with the features of claim 7. Advantageous

Embodiments of the invention are specified in the subclaims.

According to the invention, the pressure sensor has a pressure measuring cell with at least one measuring transducer and a temperature measuring transducer designed as a diode, in particular a Zener diode, for detecting the temperature prevailing at the pressure measuring cell. The diode is now operated alternately in the forward direction and in the reverse direction and thereby becomes a temperature-dependent

Continuity voltage or a temperature-dependent Zener voltage measured. In a previous calibration procedure, the temperature characteristics of

Continuity and Zener voltage stored in a lookup table, so that in current operation can be assigned to the two measured values of continuity and Zener voltage in each case the corresponding temperature from the lookup table. Now, if the difference between the two temperature amounts from the lookup table to each other is determined, can significantly overrun or undershoot the value zero, an erroneous function of

Temperature sensor and thus the pressure sensor can be detected.

The adjustment procedure essentially involves the storage of the

Temperature characteristic of both the passing and the Zener voltage, i. the behavior of the voltage over a temperature range which is typically between -40 ° C and + 125 ° C. With these stored characteristics, it is then possible to determine the associated temperature for each measured continuity and Zener voltage.

The essential thing is that continuity and Zener voltage each

different temperature dependencies and thus different

Have temperature characteristics. The zener diode thus works or acts as two redundant, diverse temperature sensors, since two components of this one component

Temperature information can be obtained. The advantage of this is that with a Zener diode, these two properties are combined in a semiconductor crystal and thus a temperature gradient is negligible.

Are caused by environmental conditions, e.g. Heat, radiation, EMC or aging, which changes the properties of the component, will thereby also their

Change temperature property differently, i. the temperature characteristics of continuity and zener voltage do not change to the same extent but differently. If only the characteristic changes with one polarity, the measured values of through and zener voltage are each assigned different temperature values, which is noticeable in the last method step, the comparison of the two temperature values. As a result, an uncertain temperature determination can be detected.

Instead of a Zener diode, it is also possible to use another electronic component, with the prerequisites that its thermal properties are as different as possible in both polar directions. For example, instead of a Zener diode also a circuit network - eg transistor with resistors - used which is then used as a bipolar. The alternative may be necessary for a more accurate temperature measurement or for a larger temperature range.

As an alternative to the previously proposed embodiment, to compare the two temperature amounts with one another, it is also provided to form a pressure value p1, which is compensated with the temperature determined by the forward voltage, from the pressure value measured by the pressure measuring cell, and from the pressure value measured by the pressure measuring cell to form a pressure value p2, which is compensated with the temperature determined by the Zener voltage. Now the difference between the two temperature-compensated pressure values p1 and p2 is determined. In the event of a significant overshoot or undershoot of the value zero, a faulty function of the temperature measuring transducer and thus of the pressure sensor can be detected. The difference is therefore that in the

Alternative solution not the temperature amounts are compared with each other, but the measured pressure value in each case already determined with the two

Temperature values is compensated and then the compensated

Pressure values are compared. This has the advantage that there is already a further processable signal, if the comparison does not give any irregularities, and any irregularities during the temperature compensation can be monitored with.

In an advantageous embodiment of the invention, it is provided that the lookup table is stored in a microcontroller, for example in an EEPROM. In a first alternative, the microcontroller has two ports, one acting as an input and the other as an output. In a second alternative, the microcontroller has three ports, two acting as an output and one as an input. One of the outputs works advantageously as an inverter. In this way, it is possible to take advantage of the available voltage range of typically 0V to 5V to a greater extent. Thus Zener diodes with a higher zener voltage than the temperature measuring element are also possible.

The object is further achieved by a pressure sensor in which

According to the invention provided for the temperature detection diode is connected to a microcontroller, the diode via its I / O port with a

AC signal applied and this AC signal is received by the microcontroller again. It will be both the sent and the the received alternating voltage signal in each case used to compensate for the influence of temperature on the pressure value determined by the pressure measuring cell. By means of the method described above, a faulty function of the temperature measuring transducer, ie the diode can now be detected.

The invention will be described in connection with figures with reference to FIG

Embodiments explained in more detail.

Show it:

1 shows a first embodiment of a pressure measuring cell with a Zener diode as

temperature sensor,

Figure 2 shows a second embodiment of a pressure measuring cell with a Zener diode as

temperature sensor,

FIG. 3 shows a measuring circuit for reliable temperature detection;

Figure 4 shows the voltage waveforms at I / O pin and A / D input pin of

microcontroller,

Figure 5 shows a measuring circuit with two voltage sources and

6 shows a measuring circuit with a voltage source.

In the following figures, unless otherwise stated, like reference numerals designate like parts with the same meaning.

FIG. 1 shows a first embodiment of a pressure measuring cell 1 with a Zener diode D1 as a temperature sensor. The pressure measuring cell 1 consists of a ceramic base body 4 with a membrane against which the pressure to be measured is applied. Such measuring cells are well known and require no further description at this point. On the upper side of the ceramic base body 4 is spaced apart a component carrier 3 for receiving the evaluation of the capacity change necessary electronic circuit 2. The component carrier 3 is designed as a board or also made of ceramic. The spacing is effected by the Kontaktierungspins, through which the evaluation circuit 2 with the

Capacitance change measuring electrodes is connected.

The pressure measuring cell 1 is influenced by the surrounding temperature. Even if the temperature in Fig. 1 is indicated only by the arrow from below, so nevertheless completely surrounds the measuring cell 1. But since a change in temperature predominantly emanates from the medium to be measured, the arrow was drawn on the underside. Ultimately, a temperature change affects that

Measurement result, because thereby u.a. the mechanical expansion or

Clamping of the measuring cell changes. For this reason, the must

Temperature sensor as close as possible to the measuring cell. In Fig. 1, the temperature sensor is designed as a Zener diode D1 and is located on the underside of the component carrier. 3

FIG. 2 shows a second embodiment of a pressure measuring cell 1 with a Zener diode D1 as a temperature sensor. Instead, the entire evaluation circuit 2 together with the Zener diode D1 is located on the upper side of the ceramic base body 4. Otherwise, the measuring cell 1 is comparable to the measuring cell known from FIG.

FIG. 3 shows a schematic diagram of how a measuring circuit for

Implementation of the method according to the invention could look like. At the center is the Zener diode D1 and the microcontroller 10. At the I / O port of the

Microcontroller 10 is a square wave signal. The resistor R1 serves as

Series resistor for limiting the current flowing through the Zener diode D1 current. The resistors R2 and R3 form a voltage divider for influencing the

Forward voltage, wherein R3 can also be designed as a voltage reference in the form of a diode.

The second, denoted by ADC port of the microcontroller 10 is an input at which received by the Zener diode D1 square wave signal is received, digitized and forwarded for further processing. The appearance of the signals on the two ports is illustrated in the following FIG. 4. The circuit shown, as mentioned, only a schematic diagram to illustrate the

Method is because in this embodiment, high losses occur at the two resistors R2 and R3. An improvement is shown in FIG

Alternative with two voltage sources.

The signal curve marked "A" in FIG. 4 depicts the rectangular signal generated by the microcontroller 10 and applied to the I / O port, while the lower diagram, labeled "B", represents the modified signal at the ADC input of the MicroController 10 represents. For example, VSS is at 0V and VCC at 5V. The generated square wave alternates from OV to 5V. By the Zener diode D1, the signal is changed so that it only between the zener voltage U _z and the

Forward voltage U _F alternates, where U _{F represents} the lower and U _{z represents} the upper limit. When the polarity of the Zener diode D1 is reversed, "high" and "low" are interchanged, ie UF then represents the upper limit and Uz the lower limit. Depending on the temperature "measured" by the Zener diode D1, the respective shoulder heights now change the signal.

FIG. 5 shows a first exemplary embodiment for implementing the method according to the invention. In contrast to the measuring circuit shown in Fig. 3, the two resistors R2 and R3 were replaced by two voltage sources of 2.5V each to avoid the losses occurring at the resistors. The microcontroller 10 is operated at 5V, while the Zener diode D1 is at a potential of 2.5V. Accordingly, the Zener diode D1 can be dimensioned to about 2.4V. The resistor R1 is as explained above a series resistor for limiting the current flowing through the Zener diode D1 current. At the I / O port is a rectangular signal of 0-5V, which is alternately received at the ADC port of the microcontroller 10 according to the diagram in Fig. 4 from UF to Uz. The signal is digitized in an A / D converter. Depending on the clock of the originally generated square wave signal, a switch for "high" or logic "1" and for "low" or logical "0" is alternately actuated. This should be indicated by the dashed line.

In the polarity of the Zener diode D1 shown here, Uz is equal to "1" and UF is "0". If the path for Uz is switched through, the amount for Uz is determined and compared with the stored temperature characteristic curve 12b. This characteristic curve 12b was stored in a memory 11, for example an EEPROM, at the factory and represents the voltage curve of Uz over the temperature. UF is processed in the same way. If the path for UF is switched through, the amount for UF is determined and compared with the stored for U _F temperature characteristic 12a, which is also stored in the memory 1 1.

As already explained, Uz and UF are temperature-dependent, so that after the adjustment with the characteristic curves 12a, 12b the respectively measured temperature can be determined. In a memory 1 1 downstream comparator unit 13, the two determined temperature amounts are compared. If - in the frame an agreed tolerance - it is determined that the amounts are the same, this is an indication that the Z-diode D1 works correctly and that the determined amount can be further processed for temperature compensation. Are the

But unequal amounts, that is a sure indication that the

Temperature measurement is not done correctly, which can be pointed out accordingly by a signal output, not shown, an optical and / or audible warning.

The actual temperature compensation then takes place either by taking a correction value dependent on the measured temperature from a further measured value table, with which the measured pressure is calculated, so that the actual pressure adjusted by the temperature error can be output. Alternatively, both characteristic curves 12a, 12b shown in FIG. 5 can each already output the pressure values corrected by the measured temperature, which are then compared in the comparator unit 13 in order to obtain information about the reliability of the temperature measurement. The advantage here is that in the event that the difference between the two compared pressure values is zero, this pressure value can be passed on directly for further processing.

FIG. 6 shows a second exemplary embodiment for implementing the

inventive method. Unlike the previous one

Embodiment, the Zener diode D1 is applied in Fig. 6 with 5V. This increases the choice of Zener diodes since the Zener voltage can be in the range of 2V to 4.7V. Depending on how accurate the temperature measurement should be, Z-diodes with 1 mV / K or 2-3mV / K can be used, just to name two examples. The advantage of Zener diodes with greater Zener voltage is that the evaluable range increases and thus the characteristic curve is more accurate. The subsequent A / D conversion may then be at a lower resolution

be performed.

The larger voltage range is achieved by the microcontroller 10 having a further output which outputs the rectangular signal applied exactly inverted at the first output. In this way, the edge height of the rectangular signal doubles. It can be dispensed with the second voltage source, so that only a voltage source with 5V is necessary. Receiving and Further processing of the signal is otherwise in the same way as in

Embodiment in Fig. 5, which is hereby referred to avoid repetition.

Claims

claims

1 . Method for checking the proper functioning of a pressure sensor for detecting the pressure prevailing in a medium, the pressure sensor comprising a pressure measuring cell (1) with at least one measuring transducer and a diode (D1), in particular Zener diode

Temperature sensor for detecting the prevailing at the pressure measuring cell (1) temperature,

characterized in that

- The diode (D1) is operated alternately in the forward direction and in the reverse direction and thereby a temperature-dependent forward voltage or a temperature-dependent Zener voltage is measured,

in a calibration procedure, the characteristic curve (12a, 12b) of transmission and zener voltage have been stored above the temperature in a look-up table (11),

- the respective measured temperature from the lookup table (1 1) is assigned to the two measured values of through and zener voltage, and

- Determines the difference between the two temperature amounts to each other and when a significant overrun or undershoot of the value zero, a faulty function of the temperature sensor is detected.

2. A method for checking the proper functioning of a pressure sensor for detecting the pressure prevailing in a medium, wherein the pressure sensor comprises a pressure measuring cell (1) with at least one transducer and a diode (D1), in particular as a Zener diode

characterized in that

the characteristic curve (12a, 12b) of transmission and zener voltage over the temperature has been stored in a look-up table (11) in a calibration procedure, the respective measured temperature from the lookup table (11) is assigned to the two measured values of through and zener voltage,

- From the pressure of the measuring cell (1) measured pressure value, a pressure value p1 is formed, which is determined with the on the forward voltage

Temperature is compensated,

- From the pressure of the measuring cell (1) measured pressure value, a pressure value p2 is formed, which is compensated with the temperature determined by the Zener voltage, and

- Determines the difference between the two temperature-compensated pressure values p1, p2 to each other and in case of significant overrun or undershoot of the value zero, a faulty function of the temperature sensor is detected.

3. The method according to claim 1 or 2,

characterized in that the lookup table (1 1) in one

Microcontroller (10) is stored.

4. The method according to any one of claims 1 to 3,

characterized in that the microcontroller (10) has two terminals, one acting as an input and the other as an output.

5. The method according to any one of claims 1 to 3,

characterized in that the microcontroller (1 0) has three terminals, with two acting as an output and one as an input.

6. The method according to claim 5,

characterized in that an output of the microcontroller (10) operates as an inverter.

7. Pressure sensor, consisting of a pressure measuring cell (1) with at least one transducer and a diode designed as a diode (D1), in particular as a Zener diode temperature sensor for detecting the at

Pressure measuring cell (1) prevailing temperature,

characterized in that

the diode (D1) is connected to a microcontroller (10) which applies an alternating voltage signal to the diode (D1) via an I / O port and this AC signal from the microcontroller (10) is received again, wherein both the transmitted and the received AC voltage signal is used in each case for compensating the influence of temperature on the pressure value determined by the pressure measuring cell (1).

8. Pressure sensor according to claim 7,

characterized in that by means of a method according to claim 1 or 2, a faulty function of the temperature measuring transducer is detected.