DE102013206646A1 - Transducer for process instrumentation and method for its diagnosis - Google Patents

Abstract

The invention relates to a measuring transducer (1) for process instrumentation with a sensor (S) for detecting a physical or chemical variable, the sensor (S) being connected to form a Wheatstone bridge measuring resistors (R1 ... R4) for generating an analog measuring signal ( To) has. To carry out a transmitter diagnosis, a center tap (X +) of at least one first voltage divider of the Wheatstone bridge is temporarily connected to reference potential through a series connection of at least a first resistor (Rb1) and a first switch (Q1). When the first switch (Q1) is open or closed, a control and evaluation device (2) determines the respective values (p1; p2) of a digital measurement signal (p), which is generated from the analog measurement signal (ADC) by means of an analog-digital converter (ADC). To) was won. If the deviation (Δ) of the two values (p1; p2) is outside a predetermined tolerance range, an error in the measuring transducer (1), in particular the analog-to-digital converter (ADC), is detected and displayed. This has the advantage that no second analog-digital converter is required to monitor the analog-digital converter (ADC) for malfunctions.

Description

The invention relates to a transmitter for process instrumentation with a sensor for detecting a physical or chemical quantity according to the preamble of claim 1 and a method for diagnosing the transmitter according to the preamble of claim 6.

In process engineering plants, a variety of field devices are used for process instrumentation to control processes. Transmitters, for example, are used to capture process variables, eg. As temperature, pressure, flow, level, density or gas concentration of a medium. Using a sensor that has commonly connected to a Wheatstone bridge measuring resistors, which are applied to a planar substrate, the physical or chemical quantity is converted into an electrical measurement signal. This substrate then serves as a mechanical attachment of the sensitive sensor on a support and also as protection against external influences, for example, to improve the electromagnetic compatibility. In particular, in the case of piezoresistive pressure sensors, one possibility for applying measuring resistances of the sensor to the substrate is to embed these resistors in the substrate and to dope the substrate in the vicinity of these resistors, wherein there is an electrical contact between the resistors and the substrate. A diode-type PN junction, when properly applied, provides electrical isolation of the resistors from the substrate. Electrical lines which conduct signals from the sensor to external contacts are connected to a control and evaluation device for evaluating the signals, which outputs, for example via a field bus, a measured value corresponding to the respective pressure to a control station or a programmable logic controller.

From the EP 2 269 009 B1 For example, a process instrumentation transmitter is known in which the state of a substrate carrying sensing resistors of a sensor for generating a measurement signal is monitored for changes due to chemical contamination. In addition, the measuring resistors and their connecting cables are monitored for breakage, ie for electrical interruption. The detection of a sensor rupture can take place, for example, by measuring the current consumption of the measuring resistors connected to a Wheatstone bridge. A detection of a malfunction of an analog-to-digital converter, which converts the analog measurement signal into a digital measurement signal in the form of a temporal sequence of digital values for further processing, is not described.

As in more and more applications of transmitters a very high reliability in the measurement of physical or chemical quantities is required, which by appropriate certifications, eg. B. after IEC 61508 , is to be confirmed, but it is less and less possible to dispense with monitoring of the analog-to-digital converter for malfunction. To detect a malfunction of the analog-to-digital converter, for example, a second converter could be connected in parallel to the first, so that a malfunction could be detected based on a simple comparison of the digital measurement signals generated by the two converters. However, adversely, the use of a second analog-to-digital converter would be associated with a significant increase in power consumption and device cost for the transmitter. In addition, the additional converter would increase the space requirement of the control and evaluation on an electronic board and it may be necessary additional measures for filtering analog and digital measurement signals.

The invention has for its object to provide a transmitter for process instrumentation, in which malfunction of the analog-to-digital converter with comparatively simple means and yet reliably detectable.

To solve this problem, the new transmitter of the type mentioned in the characterizing part of claim 1 features. In the dependent claims advantageous developments of the invention and in claim 6, a diagnostic method are described.

The invention has the advantage that in a particularly simple manner and with little effort, an analog-to-digital converter, which is provided in a transmitter for converting an analog measurement signal into a digital measurement signal, can be monitored for correct function. In this case, the above-mentioned disadvantages of a redundant design of the analog-to-digital conversion by using a second analog-to-digital converter, which is connected in parallel to the first, are avoided. In particular, in the novel diagnosis of the converter, the current consumption of the transmitter is increased at best negligible and there is no further, associated with high costs analog-to-digital converter required. Since only one or two additional resistors and switches are needed for the diagnosis of the analog-to-digital converter, the additional circuitry and space required for implementing the invention is comparatively low.

Furthermore, the comparatively simple processing of the digital measuring signals recorded in the various switch positions is advantageous, as this only consumes a small part of the computing power available in the transmitter. For the evaluation, only simple arithmetic and comparison operations are to be carried out.

If an error occurs in the analog-to-digital converter, which is detected, for example, if deviations between the digital measuring signals detected in different switching states leave a permissible tolerance range, the transmitter issues an error message and a process in which the transmitter is used if necessary be brought to a safe state. This increases the proportion of possible errors that lead to a safe state, the so-called "Safe Failure Fraction (SFF)".

For the diagnosis of the analog-to-digital converter, it is sufficient to provide only one voltage divider of the Wheatstone bridge a series connection of a resistor and a switch, through which the center tap of the voltage divider is temporarily connectable to a predetermined reference potential. By evaluating the thereby adjusting digital measurement signals in addition a possible sensor break in the region of the relevant voltage divider or a rupture of the bonding wires can be determined. To further increase the safe-failure fraction, it is also advantageous to arrange a corresponding series connection on the other voltage divider of the Wheatstone bridge. This allows the analog-to-digital converter and complete sensor condition to be monitored for faults, such as sensor breakage or cracking of the bonding wires at the bridge outputs.

In a particularly advantageous embodiment of the invention ground potential of the transmitter is predetermined as a reference potential, with which the center tap of the voltage divider of the Wheatstone bridge is connectable through the series connection of a resistor and a switch. The value of the resistor is advantageously selected to be approximately ten to one hundred times the value of a measuring resistor connected in the bridge. For example, if a sense resistor has a value of 6 kOhm, the value of the resistor used in the series connection should be between about 60 kOhm and 600 kOhm, preferably between 100 kOhm and 600 kOhm. This has the advantage that the relative deviations of the digital measurement signals respectively determined in the various switching states are in the range of a few percent and can thus be easily evaluated for diagnosis.

On the basis of the resistors used in a practical embodiment, it can be easily calculated which jump heights of the digital measured values are to be expected during switching operations in the error-free state. The diagnostic evaluation is particularly simple and yet leads to a reliable diagnosis statement if, for the detection of an error, the deviations determined during the diagnosis are monitored to see whether they are outside a tolerance range which includes the expected jump height. The upper limit and the lower limit of the respective tolerance range can be precalculated so that fluctuations that are due to component variations or temperature changes and are within a permissible range, do not lead to an erroneous fault diagnosis.

In a further, particularly advantageous embodiment of the invention, the digital measurement signal, which was determined when the first or second switch is closed for the purpose of diagnosis, is corrected by the deviation expected in the respective operating point from the measurement when the switch is open. Thus, the correct measured value of the physical or chemical quantity can be output even when the switch is closed for diagnostic purposes and there is no interruption of the measurements by the diagnostic procedure.

With reference to the drawings, in which an embodiment of the invention is shown, the invention and refinements and advantages are explained in more detail below.

Show it:

1 a circuit diagram of an embodiment and

2 a timing diagram of a digital measurement signal.

In 1 are merely the parts of a transmitter contributing to a better understanding of the invention 1 shown, which serves to detect a physical or chemical quantity, such as a pressure. On a substrate, which is designed as a pressure-sensitive membrane, four strain-sensitive resistors R1 ... R4 of a sensor S are connected to form a Wheatstone bridge. In this case, the two resistors R1 and R2 form a first voltage divider with a center tap X +, which is connected between a reference voltage Vref and ground potential. The two resistors R3 and R4 with a center tap X- are connected as a second voltage divider to the first voltage divider in the Wheatstone bridge in parallel. A current-limiting resistor R5 is connected between a supply voltage VCC and the reference voltage Vref, which is kept substantially constant during operation of the sensor S. Between the center taps X + and X- is tapped off a voltage whose level corresponds to the respective pressure and which in the illustrated embodiment, an analog measurement signal Um, which is generated by the sensor S represents. The measurement signal Um is applied to two inputs In1 and In2 of an analog-to-digital converter ADC, which converts the analog signal into a temporal sequence of digital values, which is referred to below as a digital measurement signal p. To improve the measurement accuracy, the reference voltage Vref is additionally routed to an input Ref of the analog-to-digital converter ADC. This allows a ratiometric measurement of the analog measurement signal Um.

In the exemplary embodiment shown, the analog-to-digital converter ADC is part of a drive and evaluation device 2 in which the analog measurement signal Um is further processed to a physical or chemical quantity reading, in this example of pressure, using the transmitter 1 in a process plant, for example to an automation device or a control station via a fieldbus 3 is issued.

The center tap X + of the first voltage divider with resistors R1, R2 is connectable via a series circuit of a resistor Rb1 and a switch Q1 with ground potential as a reference potential. A corresponding series connection of a resistor Rb2 and a switch Q2 is provided for the center tap X- of the second voltage divider, which is formed from the resistors R3 and R4. Using a digital output Out1 of the control and evaluation 2 For example, the first switch Q1 can be temporarily closed by means of a digital output Out2 of the second switch Q2. In normal operation, the two switches Q1 and Q2 are open. If one of the two switches Q1 or Q2 is closed temporarily, a change of the analog measuring signal Um occurs. If the analog-to-digital converter ADC is operating correctly, this leads to corresponding deviations of the digital measuring signal p, which are used to diagnose the transmitter 1 in one 2 later described in more detail by the control and evaluation 2 to be analyzed. Which deviations between values of the digital measuring signal p, which are generated, for example, when the first switch Q1 is open and the first switch Q1 closed, can be expected to be in the faultless state, can be determined by dimensioning the resistors R1... R5, the resistor Rb1 and the supply voltage VCC be calculated and stored in a simple manner. Alternatively, it is possible to expect values for the deviations during commissioning of the transmitter 1 To determine in good condition and in a memory of the control and evaluation 2 for later use during transmitter diagnostics. Expected values of the deviation Δ, values of the upper limit Δmax and the lower limit Δmin, which in 2 are exemplified, predetermined in the form of characteristics and in the control and evaluation 2 be deposited if they vary depending on operating point and / or temperature.

2 shows in a time chart an example of a course 20 of the digital measurement signal p. The history 20 in 2 is obtained with faultless transmitter. Different states Z1, Z2 and Z3 are marked on the axis of time t. In a state Z1, the two switches Q1 and Q2 are opened, and a digital measuring signal p corresponding to the respective pressure sets with a value p1. By closing the switch Q1 is transferred from the state Z1 in a new state Z2, in which the digital measurement signal p has the value p2. A deviation .DELTA. Between the values p.sub.2 and p.sub.1 corresponds to an expected value determined during commissioning or by calculation. It can thus be stated that in the analog-to-digital converter ADC and in the region of the first voltage divider with the resistors R1 and R2 (FIG. 1 ) there is no error. During the transition from state Z2 to state Z3, the first switch Q1 is opened again and the second switch Q2 is closed. In this case, a value p3 of the digital measuring signal p sets in which is higher by the deviation Δ than the value p1 in the state Z1. Since the deviation Δ also corresponds to the expected deviation in the state Z3, the resistors R3 and R4 (FIG. 1 ) no error.

So that correct measured values of the pressure are also determined in states Z2 and Z3 and output by the measuring transducer, the values p2 and p3 are corrected on the basis of the expected deviations Δ. A corrected value pk2 is calculated by adding the expected deviation Δ to the determined value p2 of the digital measurement signal p, as shown in FIG 2 through the formula pk2 = p2 + Δ is shown. A corrected value pk3 is obtained by subtracting the expected deviation Δ from the detected value p3 in accordance with the 2 attached formula pk3 = p3 - Δ receive. In the state Z1, no such correction is required, so that the determined value p1 can also be further processed as corrected value pk1.

State Z3 is again followed by state Z1, in which the two switches Q1 and Q2 are open for normal measuring operation. For the transmitter diagnosis additionally provided states Z2 and / or Z3 can, controlled by the control and evaluation 2 cyclically at periodic intervals, event-driven or on request, for example via the fieldbus 3 or a control device is given on site, be taken.

For the state Z2 are in 2 a predetermined upper limit .DELTA.max and a predetermined lower limit .DELTA.min of the deviation .DELTA. are defined, which define a tolerance range within which a good condition of the transmitter is determined during the diagnosis. If, on the other hand, a deviation greater than the upper limit Δmax is present in the diagnostic operating state Z2 compared to the normal operation in the state Z1, then an error is detected which is, for example, a rupture of the sensor S, a rupture of a bonding wire or can be a defect of the analog-to-digital converter. If the deviation is smaller than the lower limit Δmin, then an error is detected, the cause of which may be, for example, the analog-to-digital converter or a possibly upstream multiplexer, which may for example be provided in transmitters with different analog signals with the same analogue converter. Digital converter to digitize.

In state Z3, the deviation is monitored for compliance with a predetermined criterion in a manner analogous to the type of monitoring that has already been described in detail for state Z2.

In summary, a diagnosis is thus obtained which makes it possible to detect errors in an analog-to-digital converter or a possibly upstream multiplexer as well as a sensor break or rupture of the bonding wires at bridge outputs of the sensor. In this case, no additional analog-to-digital converter is required in an advantageous manner. The absence of an additional analog-to-digital converter has a favorable effect on the current consumption of the transmitter. This is a great advantage, especially for transducers with 4 to 20 mA interface, since only a very limited power supply is available for the operation of the transmitter here. In addition, the abandonment of a second analog-to-digital converter represents a significant reduction in manufacturing costs and the space required for the control and evaluation. Since now errors of the analog-to-digital converter in addition to sensor, resistance or lead break detectable is achieved a further increase of the safe failure fraction by the novel diagnosis.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2269009 B1 [0003]

Cited non-patent literature

• IEC 61508 [0004]

Claims (6)

Transducer for process instrumentation with a sensor (S) for detecting a physical or chemical quantity, wherein the sensor (S) to a Wheatstone bridge connected measuring resistors (R1 ... R4) for generating an analog measuring signal (Um) has, and with a Control and evaluation device ( 2 ) for determining and outputting a measured value of the physical or chemical quantity as a function of the analog measuring signal (Um), wherein the control and evaluation device ( 2 ) has an analog-to-digital converter (ADC) for converting the analog measurement signal (Um) into a digital measurement signal (p), characterized in that a center tap (X +) of at least one first voltage divider of the Wheatstone bridge by a series circuit of at least one first resistor (Rb1) and a first switch (Q1) for performing a transmitter diagnosis is temporarily connectable to a predetermined reference potential and that the control and evaluation device ( 2 ) is designed to determine respective values (p1, p2) of the digital measurement signal (p) when the first switch (Q1) is open or closed and by monitoring the deviations (Δ) of the values (p1, p2) to maintain a predetermined value Criteria a fault of the transmitter ( 1 ) to detect. Transmitter according to claim 1, characterized in that a center tap (X-) of the second voltage divider of the Wheatstone bridge by a series circuit of at least one second resistor (Rb2) and a second switch (Q2) temporarily connectable to a predetermined reference potential and that Control and evaluation device ( 2 ) is designed to determine respective values (p1, p3) of the digital measuring signal (p) when the second switch (Q2) is open or closed and by monitoring the deviations (Δ) of the values (p1, p3) to maintain a predetermined value Criteria a fault of the transmitter ( 1 ), wherein the first switch (Q1) is opened or closed. Transmitter according to claim 1 or 2, characterized in that the predetermined reference potential is ground potential and that the value of the first resistor (Rb1) is one to two orders of magnitude higher than the values of the measuring resistors (R1 ... R4). Transmitter according to one of the preceding claims, characterized in that as a predetermined criterion, the deviations (Δ) are monitored for whether they are outside a tolerance range with a predetermined upper limit (Δmax) and predetermined lower limit (Δmin). Transmitter according to one of the preceding claims, characterized in that the control and evaluation device ( 2 ) is designed to correct the value (p2) of the digital measuring signal (p) determined with the first switch (Q1) closed by an expected deviation (Δ) for determining and outputting the measured value of the physical or chemical quantity. Method for diagnosing a transmitter ( 1 ) for process instrumentation with a sensor (S) for detecting a physical or chemical quantity, wherein the sensor (S) to a Wheatstone bridge interconnected measuring resistors (R1 ... R4) for generating an analog measuring signal (Um) and with a drive and evaluation device ( 2 ) for determining and outputting a measured value of the physical or chemical quantity as a function of the analog measuring signal (Um), wherein the control and evaluation device ( 2 ) has an analog-to-digital converter (ADC) for converting the analog measurement signal (Um) into a digital measurement signal (p), characterized in that a center tap (X +) of at least one first voltage divider of the Wheatstone bridge by a series circuit of at least one first resistor (Rb1) and a first switch (Q1) for performing a transmitter diagnosis is temporarily connected to a predetermined reference potential and that the drive and evaluation means respective values (p1, p2) of the digital measurement signal (p) with the first switch open or closed (Q1) and by monitoring the deviations (Δ) of the values (p1; p2) for compliance with a predetermined criterion, an error of the transmitter ( 1 ) is detected and displayed.

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