EP2193339A1 - Appareil de terrain à sortie analogique - Google Patents

Appareil de terrain à sortie analogique

Info

Publication number
EP2193339A1
EP2193339A1 EP08834820A EP08834820A EP2193339A1 EP 2193339 A1 EP2193339 A1 EP 2193339A1 EP 08834820 A EP08834820 A EP 08834820A EP 08834820 A EP08834820 A EP 08834820A EP 2193339 A1 EP2193339 A1 EP 2193339A1
Authority
EP
European Patent Office
Prior art keywords
digital
analog
signal
pulse width
output signal
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP08834820A
Other languages
German (de)
English (en)
Inventor
Eric Chemisky
Michael Geppert
Ulrich Hahn
Simon Rohrbach
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Siemens AG
Original Assignee
Siemens AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Siemens AG filed Critical Siemens AG
Publication of EP2193339A1 publication Critical patent/EP2193339A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D3/00—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups
    • G01D3/02—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups with provision for altering or correcting the law of variation
    • G01D3/024—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups with provision for altering or correcting the law of variation for range change; Arrangements for substituting one sensing member by another
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters
    • H03M1/68—Digital/analogue converters with conversions of different sensitivity, i.e. one conversion relating to the more significant digital bits and another conversion to the less significant bits
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters
    • H03M1/82—Digital/analogue converters with intermediate conversion to time interval
    • H03M1/822—Digital/analogue converters with intermediate conversion to time interval using pulse width modulation

Definitions

  • 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.
  • transducers with a 4-20 mA interface are used in a variety of ways to measure physical or chemical parameters, such as pressure, temperature or pH. These usually have a sensor, the sensor signal 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, in this case an output current in the range of 4-20 mA, and via a two-wire line to an evaluation device, for example a programmable logic controller in an automation network. transfer.
  • an evaluation device for example a programmable logic controller in an automation network. transfer.
  • 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.
  • digital / analog converters with different modes of operation are known.
  • digital / analog converters with an R2R network implemented as integrated components are available.
  • a disadvantage of these components are the associated costs and also their high power consumption.
  • this can represent a significant disadvantage, since the available energy is very limited.
  • Another possibility for digital / analog conversion can be seen to use a timer output of the microcontroller to control a pulse width modulator on which a highly accurate reference voltage out and the smoothing of the output signal is followed by a low-pass filter.
  • a compromise must be made between the achievable dynamics and the setting accuracy of the analog signal.
  • 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.
  • 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.
  • the invention has for its object to provide a field device with an analog output, in particular a transmitter for process instrumentation with a 4-20 mA interface as analog output, which is characterized by low power consumption and with which an analog output signal in high resolution and can be generated with great dynamics.
  • the invention is based on the recognition that the conflict between dynamics and accuracy of the analog output signal can be resolved by a stepwise digital / analog conversion.
  • a stepwise digital / analog conversion For this purpose, in a first stage, two first Analog signals generated with lower resolution, which are above and below the desired analog output signal. These are used in a downstream, second stage as a voltage level for generating a pulse width modulated signal whose pulse pause ratio only has to be set with an accuracy which corresponds to the resolution to be achieved compared to the coarse resolution.
  • the microcontroller can now be clocked at a lower frequency, so that the
  • Energy consumption of the microcontroller decreases and as a result more energy is available for the actual measuring task of a transmitter. This can be used to improve the measurement accuracy of the transmitter.
  • a microcontroller Since a microcontroller is present in most field devices anyway, a particularly simple implementation of the digital / analog converter can be achieved if, due to suitable programming, it divides the digital value into a digital coarse fraction and a digital fine fraction and controls it the pulse width modulators required time signals generated.
  • Realized digital / analog converter with passive components and are dimensioned such that their input resistance compared to the Schwarzwidertand the pulse width modulators is much higher.
  • the dynamics of the stages should be as equal as possible.
  • FIG. 1 shows a basic structure of a transmitter
  • FIG. 2 is a block diagram of a digital / analog converter
  • FIG. 3 shows a timing diagram for explaining its functional principle
  • FIG. 4 shows a circuit of a digital / analog converter.
  • 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.
  • the measurement signal 3 is amplified and digitized.
  • the thus preprocessed measurement signal is supplied to a microcontroller 5 which, for example, carries out a compensation of non-linearities and temperature influences and calculates the measured value to be output.
  • a digital / analog converter 6 the digital measured value ascertained in the microcontroller 5 is converted into an analog
  • a microcontroller ⁇ C For digital / analog conversion, a microcontroller ⁇ C according to FIG. 2 generates three time signals PWM 1 , PWM 2 and PWM 3 .
  • the time signals PWMi and PWM 2 are determined according to a coarse fraction of the digital value, the time signal PWM 3 corresponding to a digital fine fraction.
  • a buffer BUFi which has the function of a pulse width modulator, generates a pulse width modulated signal corresponding to the time signal PWMi, the upper
  • Function of a pulse width modulator has. On the switch SWi the first analog signal Vi and the second analog signal V 2 are performed. By a low pass TP 3 , which is the switch SWi downstream, the pulse width modulated signal is again smoothed, so that finally the analog output signal V O u ⁇ is present.
  • the following is an example of 17-bit resolution digital-to-analog conversion.
  • the coarse fraction has a resolution of 9 bits, the fines also.
  • a resolution of one bit, which remains at a summation of the resolutions of coarse and fine fraction compared to the resolution of the digital value is, as explained later, needed to realize a hysteresis of the coarse fraction.
  • the first analog signal Vi can be calculated according to the formula N.
  • V i V ref
  • Ni essentially corresponds to the most significant bits of the digital value and has a value range between 0 and 2 9 -1.
  • the level of the second analog signal V 2 can be calculated according to the formula:
  • N 2 also largely corresponds to the most significant bits of the digital value and has the same value range as Ni.
  • a hysteresis is used to avoid noise in the analog output signal V O u ⁇ . For this it is determined
  • the value m corresponds to the fine fraction determined with the aid of the microcontroller .mu.C, which is used to set the timer for generating the time signal PWM 3 . . It also has a range of values between 0 and 2 9 -l Substituting in the last formula Ni by 2 N + 2 and N 2 through N, is obtained for the level of the analog output signal V O u ⁇ :
  • the digital values for the coarse fraction and the fine fraction which are determined in the microcontroller .mu.C, need not exactly correspond to the most significant bits or the least significant bits of the digital value at all times, but are set differently from them under certain conditions.
  • the analog output voltage is set as a function of the first analog signal Vi and of the second analog signal V2. If the value of the nine least significant bits is close to its limits, that is close to the value 0 or close to the value 2 9 -l, it might happen that the digital coarse fraction and thus the two analog signals when these limits are exceeded in the absence of hysteresis Vi and V 2 constantly switch back and forth. This would result in unnecessary noise of the analog output signal V O u ⁇ .
  • FIG. 3 shows a time diagram in which a profile 31 of an analogue output signal V O u ⁇ , a profile 32 of a first analogue signal Vi, a profile 33 of a second analogue signal V 2 and a profile 34 corresponding to the respective digital fine component corresponds, are plotted over time.
  • the digital coarse fraction is set to the value N.
  • the value N 2 for setting the time signal PWM 2 is equal to N
  • the value Ni for setting the time signal PWMi is N + 2.
  • This setting of the digital coarse fraction remains constant as long as the digital fines within its limits between 12.5% and 87 , 5% of the value range is located.
  • the digi- Valley value such that the digital fine fraction falls below the 12.5% limit and correspondingly the analog output signal 31 approaches the second analog signal 33.
  • the values Ni and N 2 are decremented by one.
  • the digital fine fraction is increased by about 50% of its value range, so that no switching jump can be detected in the course of the analogue output signal 31.
  • the new value m NEW of the digital fine fraction results according to the formula:
  • m NEW is about 62.5% and is below the 87.5% limit at which the values Ni and N 2 would again be increased by 1. It is thus realized a hysteresis, by which a noise is prevented at switching points.
  • the digital fine fraction corresponding to the curve 34 exceeds the 87.5% limit of its value range. This is followed immediately by an increment of the values Ni and N 2 by 1 and a reduction of the digital fine fraction by 2 8 . In the direct connection of the switchover, the value of the digital fine fraction is approximately 37.5% of its value range.
  • the division of the digital value, which corresponds to the analog output signal V 0U T, into the digital coarse fraction and the digital fines content with hysteresis of the coarse fraction is carried out in the microcontroller ⁇ C on the basis of its programming. An additional circuit complexity is advantageously not connected with it.
  • FIG. 4 shows a circuit 41 which is suitable for implementing the digital / analog converter.
  • the microcontroller used is an integrated module 42 of the type MSP430 which has three timer outputs 43, 44 and 45 which are used to realize pulse width modulated time signals PWMi, PWM 2 and PWM 3 are used.
  • the timing signals PWMi and PWM2 are at two Buffer 46 or 47 of the type 74LVC04 supplied, which are supplied by a diode 48 with a high-precision reference voltage.
  • the buffers 46 and 47 are each followed by a low pass, which consists of a resistor Ri and a capacitor Ci or of a resistor R2 and a capacitor C 2 .
  • the resistors each have 51 k ⁇ , the capacitors 33 nF each.
  • the analog signals smoothed in this manner are fed to two inputs of a Texas Instruments 3157 type switch 49.
  • the time signal PWM 3 is used to actuate the switch 49.
  • the switch 49 is again followed by a low-pass filter with a resistor R3 of 150 k ⁇ and a capacitor C 3 of 100 nF.
  • This low pass finally provides an analog output signal 50 corresponding to the predetermined digital value.
  • the low-pass filters are deliberately passive RC filters used and no circuits with active components, since they provide a very good accuracy. It is important that the output resistance of the buffers 46 and 47 and the output resistance of the switch 49 in comparison to the input impedance of the respective downstream low-pass filter is small.

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  • Engineering & Computer Science (AREA)
  • Technology Law (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Analogue/Digital Conversion (AREA)

Abstract

L'invention concerne un appareil de terrain à sortie analogique, en particulier un transducteur (1) de mesure pour l'instrumentation de processus, comprenant une interface 4-20 mA en tant que sortie analogique (7). La conversion numérique/analogique est effectuée par division d'une valeur numérique en une composante grossière et une composante précise. En fonction de la composante numérique grossière, un premier signal analogique (V1) se situant au-dessus du signal de départ analogique (VOUT) est créé au moyen d'un modulateur d'impulsions en durée (BUF1) dont la sortie est connectée à un filtre passe-bas (TP1), et un second signal analogique (V2) se situant en dessous du signal analogique de départ (VOUT) est créé au moyen d'un modulateur d'impulsions en durée (BUF2) dont la sortie est connectée à un filtre passe-bas (TP2). Les deux signaux analogiques (V1, V2) sont envoyés dans un troisième modulateur d'impulsions en durée (SW1), régulé en fonction de la composante numérique précise et dont la sortie est connectée à un filtre passe-bas (TP3). On obtient ainsi un signal de sortie analogique (VOUT) présentant une résolution élevée et une bonne dynamique. Cet appareil de terrain est en outre caractérisé en ce qu'il comprend un convertisseur numérique/analogique dont le coût de production est particulièrement bas.
EP08834820A 2007-09-28 2008-09-26 Appareil de terrain à sortie analogique Withdrawn EP2193339A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE102007046560A DE102007046560A1 (de) 2007-09-28 2007-09-28 Feldgerät mit einem Analogausgang
PCT/EP2008/062946 WO2009043821A1 (fr) 2007-09-28 2008-09-26 Appareil de terrain à sortie analogique

Publications (1)

Publication Number Publication Date
EP2193339A1 true EP2193339A1 (fr) 2010-06-09

Family

ID=40344611

Family Applications (1)

Application Number Title Priority Date Filing Date
EP08834820A Withdrawn EP2193339A1 (fr) 2007-09-28 2008-09-26 Appareil de terrain à sortie analogique

Country Status (5)

Country Link
US (1) US20100302085A1 (fr)
EP (1) EP2193339A1 (fr)
CN (1) CN101809413A (fr)
DE (1) DE102007046560A1 (fr)
WO (1) WO2009043821A1 (fr)

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DE102009034419B4 (de) * 2009-07-23 2016-09-29 Siemens Aktiengesellschaft Feldgerät zur Prozessinstrumentierung
DE102009050645A1 (de) * 2009-10-26 2011-04-28 Siemens Aktiengesellschaft Feldgerät zur Prozessinstrumentierung
DE102010006634B4 (de) 2010-02-03 2021-07-15 Conti Temic Microelectronic Gmbh Verfahren und Vorrichtung zum Umwandeln eines digitalen Eingangssignals in ein analoges Ausgangssignal
EP2413300B1 (fr) * 2010-07-29 2016-04-13 Rockwell Automation Limited Procédé et appareil d'envoi et de réception de formes d'ondes de transmission pour communications HART
DE102011005128B4 (de) * 2011-03-04 2021-11-25 Endress + Hauser Wetzer Gmbh + Co. Kg Messeinrichtung mit Kompensation eines verzörgerten Ansprechverhaltens
RS55775B2 (sr) * 2011-06-23 2022-10-31 Ablynx Nv Tehnike za predviđanje, otkrivanje i smanjenje nespecifične proteinske interferencije u testovima koji uključuju pojedinačne varijabilne domene imunoglobulina
US8471746B2 (en) * 2011-07-08 2013-06-25 Tektronix, Inc. Digital-to-analog conversion with combined pulse modulators
DE102011085271A1 (de) 2011-10-27 2013-05-16 Siemens Aktiengesellschaft Feldgerät mit einem Analogausgang
DE102011087157B4 (de) 2011-11-25 2015-10-15 Siemens Aktiengesellschaft Prozessautomatisierungsgerät
US20140009137A1 (en) * 2012-07-03 2014-01-09 Nvidia Corporation System, method, and computer program product for single wire voltage control of a voltage regulator
DE102012223706A1 (de) 2012-12-19 2014-06-26 Siemens Aktiengesellschaft Feldgerät mit einem Analogausgang
US10461749B1 (en) * 2018-10-25 2019-10-29 Shenzhen GOODIX Technology Co., Ltd. Ground intermediation for inter-domain buffer stages

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Also Published As

Publication number Publication date
DE102007046560A1 (de) 2009-04-02
US20100302085A1 (en) 2010-12-02
WO2009043821A1 (fr) 2009-04-09
CN101809413A (zh) 2010-08-18

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