WO2014095474A1

WO2014095474A1 - Field device with an analogue output

Info

Publication number: WO2014095474A1
Application number: PCT/EP2013/076019
Authority: WO
Inventors: Patrick Balle; Eric Chemisky
Original assignee: Siemens Aktiengesellschaft
Priority date: 2012-12-19
Filing date: 2013-12-10
Publication date: 2014-06-26
Also published as: DE102012223706A1

Abstract

The invention relates to a field device with an analogue output, particularly a measuring transducer (1) used for process instrumentation with a 4-20 mA interface as an analogue output (7). In the steady-state, a highly precise digital/analogue converter (21, 22) sets only a signal level smoothed by a first RC lowpass filter (22) at the analogue output on the basis of a digital value (D). If an abrupt change in the digital value (D) occurs, a pulse is additionally applied to the output of the first RC lowpass filter (22). The application (25, 26) of the pulse is again deactivated after the level is provided. Thus a digital/analogue converter that provides an analogue output signal with high resolution and improved dynamics is made available with relatively low production costs.

Description

description

The invention relates to a field device with an analog output, in particular a transmitter for process instrumentation with a 4-20 mA interface as an analog output, according to the preamble of claim 1. In the process instrumentation transmitter with 4-20 mA interface are versatile to Measurement of physical or chemical quantities, for example, a pressure, a temperature or a pH used. These usually have a sensor whose sensor signal is amplified, digitized and then evaluated in a microcontroller and corrected for linearity and temperature behavior. The thus processed sensor signal is converted in an output circuit with a digital / analog converter into an analog output signal, here an output current in the range of 4-20 mA, and transmitted via a two-wire line to an evaluation device, such as a programmable logic controller in an automation network.

On the other hand, a programmable logic controller as a field device may have an analog output, for example for the transfer of a manipulated variable to a control valve as an actuator with a corresponding analog input.

To generate the analog output signal, digital / analog converters with different modes of operation are known. For example, implemented as integrated components digital / analog converter with R2R network available. A disadvantage of these components, however, are the associated costs and also their high power consumption. This can be a significant disadvantage, especially for field devices that are supplied with their power required for operation via a 4-20 mA interface, since the available energy is very limited. From DE 10 2007 046 560 AI a field device with an analog output is known in which a microcontroller is used to generate a pulse width modulated signal, which is guided on a low-pass filter for signal smoothing. The pulse-width-modulated signal is obtained by means of a timer integrated in the microcontroller and a buffer, on which a highly accurate reference voltage is guided for accurate level adjustment. However, the problem arises that a compromise between the achievable dynamics and the setting accuracy of the analog signal must be concluded. The frequency of the pulse width modulated signal, which has a direct effect on the dynamics, results from the bit resolution of the digital / analog conversion and the clock frequency of the microcontroller and is proportional to the product of these two variables. The timing of the microcontroller has a direct effect on its power consumption and can not be increased arbitrarily. On the other hand, the frequency of the pulse width modulated signal can not be arbitrarily reduced in order to achieve a higher bit resolution, since this is decisive for the dynamics of the generated analog output signal. In order nevertheless to produce an analog output signal with fine resolution and high dynamics, a stepwise digital / analog conversion is carried out with a first stage in which first two analog signals with a lower resolution are generated, which are above and below the desired analog output signal lie, and with a downstream, second stage, in which the two analog signals generated in the first stage are used as the level of another pulse width modulated signal whose pulse-pause ratio must be set only with an accuracy that compared to the coarse resolution corresponds to the first stage still to be achieved further resolution. Since each stage takes over a part of the resolution, when using a microcontroller to generate the time signals for the pulse width modulation at the same timing much higher dynamics can be achieved. On the other hand, to achieve a given dynamic, the microcontroller can now be clocked at a lower frequency, so that the power consumption of the microcontroller As a result, more energy is available for the actual measuring task of the transmitter and, for example, for diagnostic functions. This can increase the measurement accuracy and reliability of the transmitter.

The invention has for its object to further improve the dynamics of an analog output for a field device with relatively little overhead. To solve this problem, the new field device of the type mentioned in the characterizing part of claim 1 features. In the dependent claims advantageous developments of the invention are described. The invention is based on the finding that the dynamics of a high-resolution digital-to-analog converter can be increased in a particularly simple manner if, at times of otherwise insufficient dynamics, namely the occurrence of sudden changes in the analog signal to be converted Digital value, by applying a pulse to the output of a low-pass filter used for smoothing in the digital / analog converter, the change tracking is accelerated. In normal operation, that is, when no jumps in the measured value and thus the digital value occur in the stationary case, however, the PulsaufSchaltung is disabled so that it does not distort the output of the high-precision converter. The invention thus has the advantage that the accuracy of the analog output signal can be maintained, but at the same time the reaction time is significantly increased at sudden changes. In addition, advantageously, highly accurate digital / analog converters of any type can be combined with the pulse-on circuit to increase the dynamics of the high-precision digital / analog converter. In a particularly advantageous embodiment of the invention, a microcontroller for detecting a sudden change in the digital value is provided. The same microcontroller generates a pulse-width-modulated signal as a function of the high-resolution digital value, which in a subsequent Switched RC second-order low-pass filter is smoothed. Furthermore, the microcontroller delivers the pulse for rapid change tracking on a tristate output. This

Tristate output is activated for the predetermined period of the pulse immediately after the detection of sudden changes in the digital value and switched back into the high-impedance state after their expiration. The charge state of a capacitor in an RC low-pass connected downstream of the tristate output is therefore changed only after the occurrence of discontinuous changes of the digital value by the pulse signal. Since both the first RC low-pass and the second at their output each have a capacitor and the two outputs of the low-pass filters are connected, there is a parallel connection of the two capacitors and it can be used to further reduce the manufacturing cost of the digital / analog converter the two capacitors are omitted.

In a further advantageous embodiment of the invention, a significant improvement in the dynamics is achieved even with different, strongly divergent jump heights of the detected changes in the digital value, if the respective duration of the pulse is predetermined in dependence of the respectively determined jump height. With knowledge of the dynamics of the digital / analog converter, the time duration is advantageously predetermined in such a way that after renewed deactivation of the tristate output, the signal level at the analog output corresponds at least approximately to the digital value. This can be done, for example, by calculating an analytical function which describes the relationship between detected jump height and suitable time duration, or by means of a characteristic curve or a characteristic field.

Since a microcontroller is present in most field devices anyway, a particularly simple realization of the high-precision digital / analog converter with improved dynamics can be achieved if the microcontroller detects a sudden change in the digital value due to suitable programming, depending on the respective step. height determines a suitably predetermined period of time for activating the tristate output of the pulse timer, using two integrated timer generates the pulse width modulated signal and the pulse for rapid change tracking and the

Tristate output of the second timer, which provides the pulse, activated for the determined period of time after the detection of a step change.

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.

1 shows a basic structure of a transmitter,

Figure 2 is a functional block diagram for explaining the

Digital / analog conversion and Figure 3 timing diagrams to explain their principle of operation.

According to FIG. 1, a measuring transducer 1 for detecting a physical or chemical quantity X of a process has a pick-up 2, which converts this quantity into a measuring signal 3. In a pre-processing 4, the measurement signal 3 is amplified and digitized. In digital form, the thus preprocessed measurement signal is fed to a microcontroller 5, which makes, for example, a compensation of non-linearities and temperature influences and calculates the measured value to be output. In a digital / analog converter 6, the digital measured value ascertained by the microcontroller 5 is converted into an analog output signal which is transmitted via a 4-20 mA interface 7 for further use in a process plant in which the measuring transducer 1 is used. is issued.

The block diagram according to FIG. 2 shows the essential functional components which are connected to the digital / analogue Implementation are involved. A microcontroller 20, in which the digital value D to be converted has been calculated with a resolution of, for example, 16 bits, additionally determines from the digital value D a coarse fraction G corresponding, for example, to the more significant 8 bits of the digital value D. The digital value D is fed to a digital / analog converter whose essential components form a first timer 21 and a first RC low-pass filter 22. By means of the timer 21, a pulse width modulated signal Ul is generated in a known manner, the frequency of which is 16 hertz, for example, due to the high resolution and the clock frequency of the microcontroller 20. In the downstream first RC low-pass filter 22, the pulse-width-modulated signal U1 is smoothed and, in the stationary case, an analog output signal U0 is generated at the output, which is characterized by high accuracy due to the high resolution of the digital / analogue converter. In order to obtain a good smoothing effect, the first RC low-pass filter 22 consists of two first-order delay elements 23 and 24.

Since the dynamics of the digital / analog converter is still relatively low without further measures, that is, since it reacts comparatively slowly to changes in the digital value D, means for connecting a pulse to the output of the first RC low-pass filter 22 are additionally present essential components are a second timer 25 and a downstream, second RC low pass 26. The second RC low-pass filter 26 consists in the embodiment shown only of a first-order delay element. The first RC low-pass filter 22, like the second RC low-pass filter 26, has a capacitor at its output. Both capacitors are in a real circuit to reduce the

Component overhead to a smoothing capacitor for the analog output signal U0 summarized. The analog output signal U0 is used to implement a 4-20 mA

Interface as an analog output of a field device, for example, as an analog voltage with a voltage level corresponding to the digital value on the basis of a field effect transistor. tor, which serves in a known manner for adjusting the loop current on the 4-20 mA interface.

To implement the high-resolution digital / analog converter with the first RC low-pass filter 22 at its output, in principle any converter principles can be used. Preferably, however, the already known from the aforementioned DE 10 2007 046 560 AI known converter is used, which is characterized by high accuracy with already comparatively good dynamics. The coarse fraction determined there can then be used simultaneously as a digital coarse part G for the second timer 25. The second timer 25 has a tristate output 27, which in the stationary case, that is to say when no jumps of the digital value D occur which significantly exceed the resolution of the digital coarse part G, is switched to high-resistance state by programming the microcontroller 20. This prevents the supplemented means for pulse switching in the stationary case from impairing the accuracy of the high-precision digital / analog converter. The static accuracy of the digital / analog converter thus remains the same. If, unlike the illustrated embodiment, the second timer does not have a tristate output, the tristate output can be implemented in a simple manner by means of a tristate buffer connected downstream of the second timer.

If a sudden change of the digital value D occurs, this is detected by suitable programming of the microcontroller 20 and the tristate output 27 of the second timer 25 is switched to the active state for a predetermined period of time immediately after the jump has been detected.

In the timing diagram according to FIG. 3, in which the pulse signal U2, the signal U0 at the analogue output and a fictitious analogue output signal U0 'are shown over the time t, this happens for a period of time Atl after the detection of a sudden change in the digital value D. at a time t1 and for a period of time At2 after the detection of a negative jump of the digital value D at a time t2. The Height of the two jumps from an output voltage ul to a final voltage u2 or vice versa is identical.

Since the sequence control by the microcontroller 20 is largely the same after both jumps, it suffices to explain in the following only the sequence for the first jump at the time t 1 on the basis of FIGS. 2 and 3:

Until the time t1, the stationary case is present, in which the tristate output 27 of the second timer 25 is switched to high-impedance state. At time t1, the stepwise change of the digital value D from the initial voltage ul to the final voltage u2 is detected and, in order to achieve a rapid response to the change, the pulse lock is activated by activating the tristate output 27 and one of the End voltage u2 corresponding pulse signal U2 is generated with a high level on, for example, a reference voltage. In this case, a possibly existing tristate output of the first timer 21 still remains in the active state, so that the pulse width modulated signal U1 is also generated corresponding to the new digital value for the final voltage u2. Deviating from this, however, it would also be possible to deactivate the digital / analog converter 21, 22 for the predetermined period of time Atl of activation of the pulse input, if the design of the downstream low-pass filter 22 does not cause an inadmissible delay in the setting of the signal level at the analog output , The adjustment of the level of the signal U0 at the analogue output to the final voltage u2, which now additionally takes place with the pulse-up circuit, reacts very quickly to the change of the digital value D at the time t1 because of its high dynamics. This can be seen on the steep flank in the course of the signal U0 in Figure 3 after the time tl. For a qualitative comparison with a conventional embodiment of a digital / analog converter, in FIG. 3, broken lines additionally show an output of an analogue output signal U0 ', which would be set if the signal level at the analogue output not only in the stationary case but also in the event of jumps in the digital value solely as a function of the digital tal / analog converter 21, 22 would be set, which works with high accuracy but with relatively low dynamics.

The time period Atl is predetermined in such a way that, after its expiry, the signal level at the analog output corresponds approximately to the final voltage u2 and thus to the new digital value. For this purpose, the microcontroller 20 calculates the time period Atl as a function of the respective jump height and in dependence on the position of the initial voltage ul and / or the final voltage u2 in such a way that the level accuracy reached at the time of the deactivation of the tristate output 27 is already approximately equal to the resolution of the digital coarse fraction G. of the digital value D corresponds. After expiration of the period of time Atl, the steady state case again sets in, in which the signal level at the analogue output is determined solely by the digital / analogue converter 21, 22.

The sequence controlled by the microcontroller 20 upon detection of a sudden change in the negative direction at the time t2 largely corresponds to that of the positive jump at the time t1. Only the time duration At2 and the level of the pulse signal U2, which is now switched to low level, for example GND, differ, since they correspond here to the sudden change from the voltage u2 to the voltage ul to be readjusted.

The invention was exemplified with a digital / analog converter, which based on

Pulse width modulation works. However, it is equally applicable to other types of converters. Depending on the requirements, in addition to the described embodiment, in which the digital coarse fraction G has half the bit width of the digital value D, any other bit width ratios can be selected.

Claims

claims

1. Field device with an analog output, in particular transmitter (1) for process instrumentation with a 4-20 mA interface as analog output,

with a digital / analog converter (21, 22), to which a digital value (D) is guided and in whose dependency, in the stationary case, a signal level smoothed by a first RC low-pass filter (22) is adjustable at the analog output, characterized .

in that means (20) are present for detecting a sudden change in the digital value (D) and means (25, 26) for applying a pulse to the output of the first RC low-pass filter (22) for rapid change tracking immediately after a detection a jump-like change.

2. Field device according to claim 1, characterized in that a microcontroller (20) is provided as the means for detecting a step change and

in that a timer (25) of the microcontroller (20) with tristate output (27) is provided as means for switching on the pulse, which is controlled in such a way that it is switched to high-resistance state after the predetermined time period (Atl, At2) has elapsed and

that the output (27) of the timer, an RC low pass (26) is connected downstream.

3. Field device according to claim 2, characterized in that the time duration (Atl, At2) of the pulse depending on the detected jump height is predetermined in such a way that after

Switching the tristate output (27) in the high-impedance state of the signal level at the analog output at least approximately corresponds to the digital value.