US3876997A - Analog data acquisition system - Google Patents

Analog data acquisition system Download PDF

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Publication number
US3876997A
US3876997A US411496A US41149673A US3876997A US 3876997 A US3876997 A US 3876997A US 411496 A US411496 A US 411496A US 41149673 A US41149673 A US 41149673A US 3876997 A US3876997 A US 3876997A
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United States
Prior art keywords
measurement
operative
lines
pair
transmission lines
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Expired - Lifetime
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US411496A
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English (en)
Inventor
Earl T Farley
Andras I Szabo
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Westinghouse Electric Corp
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Westinghouse Electric Corp
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Priority to US411496A priority Critical patent/US3876997A/en
Priority to JP12198874A priority patent/JPS5619676B2/ja
Priority to FR7436240A priority patent/FR2250167B1/fr
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Publication of US3876997A publication Critical patent/US3876997A/en
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    • G—PHYSICS
    • G08—SIGNALLING
    • G08C—TRANSMISSION SYSTEMS FOR MEASURED VALUES, CONTROL OR SIMILAR SIGNALS
    • G08C15/00—Arrangements characterised by the use of multiplexing for the transmission of a plurality of signals over a common path
    • G08C15/06—Arrangements characterised by the use of multiplexing for the transmission of a plurality of signals over a common path successively, i.e. using time division
    • G08C15/08—Arrangements characterised by the use of multiplexing for the transmission of a plurality of signals over a common path successively, i.e. using time division the signals being represented by amplitude of current or voltage in transmission link

Definitions

  • the invention relates to analog multiplexing systems used in an industrial environment to derive small signals from remote locations. Since the signals received 2% 'i 340/183 45 2; might mask a failure of the sensing devices a bias f source is used to generate an anomalous signal when 1 1 0 can sensing is defective.
  • the bias source is common to all 56 R f d points of measurement, It has a small time constant so 1 1 e erences that it can be switched at the multiplexing frequency,
  • FIGIO ANALOG DATA ACQUISITION SYSTEM BACKGROUND or THE INVENTION Digital computer process control or monitoring requires many input data in analog form which are derived by sensing devices located at various points of the controlled industrial process. These input data are converted into digital form for fast and complex treatment within a. computer system before output signals can be supplied forcontrol or monitoring action.
  • a computer system includes digital circuitry which controls the opening and closing of relays associated with the analog multiplexing system. As a result of such relay closing and opening actions, input data acquisition is obtained from the multiplexed system through cables providing communication between the interface of the computer system and the various and remote locations where the transducers sense process variable instantaneous conditions.
  • the multiplexed cable system leads to terminals which form the input to an analog digital converter providing the necessary transformation from analog to digital of the analog signals sequentially received each during a time interval of measurement generally of l/60 of a second duration. Since the sensing devices are remotely located, long cables are necessary for the transmission of the measurement signals. These signals, however, have a small magnitude, so that the cables have to be shielded from parasitic interference to minimize noise. and these shields are connected electrically to form a potential floating system from the point of measurement to the input of the analog digital converter.
  • lt is an object of the present invention to provide an improved fault detector for a multiplexed analog data acquisition system which is less costly, more reliable and which leaves measurement signals unaffected.
  • the computer processor is associated with well known input systems including conventional contact closure input system which scans contacts or other signals representing the status of various process conditions, a conventional analog input system which scans and converts process analog signals.
  • the invention is more particularly concerned with the analog data acquisition system scanned by the analog input system.
  • the analog input data are provided by a plurality of transmission cables which extend to various sensing devices. such as thermocouples, which are remotely connected and located within the industrial environment of the controlled process.
  • the invention resides in an analog data acquisition system of the multiplex type connecting various points of measurement to a common multiplex terminal, in which a fault detecting circuit is provided selectively connected at the time of data acquisition to the multiplex terminal. Normally the fault detecting circuit discharges itself in the connected transmission line including the selected sensing device. If there is a fault in the sensing device and the transmission line, the fault detecting circuit maintains a potential on the multiplex terminal which is indicative of a fault.
  • the invention also provides for discharging the transmission lines of the multiplex system before fault detection.
  • FIG. 1 represents an analog data acquisition system provided with a fault detector according to the prior art
  • FIG. 2 shows an analog data acquisition system provided with a fault detector according to the present invention
  • FIG. 3 provides four curves A, B, C, D which illustrate respectively A) the timing of the selection of a point of measurement, B) the duration of response availability by the associated computer system to a measurement signal, C) the timing of the purging action from system from accumulated charges according to the invention, and D) the timing of the fault detecting curcuit according to the invention;
  • FIG. 4 shows a combination of curves typical of a fault condition and of a correct condition at the point of measurement during operation of the fault detecting circuit according to the invention
  • FIG. 5 shows three successive electrical states of a transmission line 1000 feet long having an open circuit 1) immediately before switching at the point of measurement, 2) immediately after such switching, and 3) long after such switching, when none of the features according to the invention is used;
  • FIG. 6 is similar to FIG. 5 but for a transmission line 100 feet long;
  • a multiplexed data acquisition system comprising n transducers such as T,, T, connected by cables K,, K,, respectively to bus lines 1, 2, 3 via corresponding contacts A,,, A B,,, 8, C,, and C, of a relay R, and A,,,, A B,,,, B C,,, and C of a relay R,,.
  • Each cable is shielded cable having two signal wires such as L,, and L, for cable K,, L,,, and L,, for cable K and shieldconnections such as S, for cable K,, S,, for cable K,,.
  • Wires L,, and L,,, are connected to bus line 1; wires L, and L,, are connected to bus line 2; the shield connections 8,, S,, are connected to the third bus line 3.
  • Each transducer T,, T, represents a point of measurement in an industrial process 99 under monitoring and/or control by a process control system 100.
  • the terminals of cables K,, K are mounted on a multiplexer card MC, which supports the contacts of relays R,,, R,, and the bus lines 1, 2, 3.
  • a card relay CR connecting the bus lines 1, 2, 3 of card MC, to the input terminals of a shielded bus cable BK,, having signal wires LB,, LB, and a shield connection L5,.
  • cable BK is connected to three analog bus lines 4, 5, 6 forming the multiplex terminal.
  • LB is connected to line 4
  • L8 is connected to line 5
  • LS is connected to line 6.
  • the system comprises M multiplexer cards such as MC,.
  • M multiplexer cards such as MC
  • FIG. 1 On FIG. 1 is shown the multiplexer card of the m"' order MC,,,, which is itself connected with the analog bus lines 4, 5, 6 through a bus cable BK,,,.
  • Relays such as R,, R, and CR,, CR, are controlled by the process control system 100, in order to connect each point of measurement, one at a time as selected by the computer, with the analog bus lines 4, 5, 6. Therefore, on those bus lines appear measurement signals from the transducers, representing the sensed values at each point of measurement.
  • the points of measurement are at various remote locations where information is required relative to the operative process. For instance if temperature is measured the transducer is a thermocouple. Since the transmission cables have a non-negligible length, and since the measurement signals derived from the transducers may be very small, all cables are shielded, and, the shields are allowed to float together in order to minimize noise and outride interference on the signal wires such as L,,. L,,, LB,, L8,.
  • the three bus lines 4, 5, 6 provide analog data which must be inputted for treatment by a digital computer 7.
  • a digital analog converter 18 receives the signals from bus lines 4, 5 and convert them into corresponding digital signals which are interfaced with the process computer via transformers or optical couplers 19, as generally know.
  • the A/D converter is floating, and receives for this reason the third bus line 6.
  • transducers T,, T, all provide a true signal, e.g. a signal which is representative of the magnitude of the variable sensed, for instance temperature.
  • Asensing device can fail, by a short or an open circuit due to poor connections, damaged cable Thus, a zero signal would appear which would not indicate the actual temperature and such indicating could be erroneously used for monitoring or control.
  • FIG. 1 shows a prior art solution to this problem.
  • Each cable is provided with a battery 10 connected in parallel with the two signal wires (L,,, L, or L,,,, L,,) of the particular cable, through resistors R,, R0
  • the battery operates as a bias source with the associated transducer (T, or T,,).
  • the transducer T, (or T,,) may be represented as a series combination of a resistor R and a voltage V V is much smaller than the potential of the battery 10. If the values of R,, R are large compared to R the battery 10 will develop a small voltage only in the circuit R V and the measurement of a signal along L,, and L,, will be appreciably impaired by the presence of the battery 10. Should, however, there be an open circuit in the parallel circuit R V of transducer T,, or in the adjoining wires L,,, L,,, then, the potential of battery 10 will appear in full between L,, and L,,. When this happens, the bus lines 4, 5 indicate a potential much higher than the signal normally developed when responding to V In such event, the reading of the A/D converter 18 will fall outside the operating range for which the converter is set when responding to a normal operation of transducer T,.
  • FIG. 2 the preferred embodiment of the present invention will be hereinafter described.
  • the same multiplexed data acquisition system is here illustrated as in FIG. I.
  • the same elements have received in FIG. 2 the same reference numerals as in FIG. 1.
  • a circuit card 20 is provided mounted for connection with the bus lines 4, 5, 6 which are the common output terminal of the cables and transducers and also are the input terminals to the A/D converter 18.
  • the circuit card 20 carries a fault detecting circuit 61 comprising an isolated power supply 21 having two supply lines 22, 23 leading via resistors R to a buffer circuit 24, 25 including a large capacitor 24 and a resistor 25 in parallel on lines 22, 23.
  • the two supply lines are connected to two contacts 26, 27 which belong to one position of a two-pole changeover relay FR.
  • the two other contact positions of relay FR are 28, 29 which are connected to bus lines 4 and 5.
  • a series capacitor-resistor circuit 30, 31 is connected between the two moving cotacts of relay FR so that when the relay FR is in the 26, 27 position it is charged by the buffer circuit 24 of power supply 21.
  • the series circuit 30, 31 is connected in parallel with bus lines 4, 5.
  • the circuit card 20 also carries a discharging circuit 60 consisting of a two-pole discharging relayDR having two contacts 50, 52 each associated with two others 51, 53. These opposite contacts when closed provide a short circuit between respectively lines 4 and 5, and the shield connection to line 6 of thefloating system.
  • the discharging circuit- 60 when closed discharges cables BK,, BK,, removing any charge left from a previous connection established with one of the cables (K,, K,,).
  • FIG. 3 there are shown four curves A, B, C, D which are timing diagrams.
  • the scanning system determines opening and closing of relays such as PR,, PR, and of relays such as CR, in order to selectively connect the output circuit V R of each transducer T,, T,,, one at a time, with the multiplex terminal 4, 5.
  • This is the process of selecting each point of measurement.
  • Curve A shows time intervals such as 1 1;, during which one particular point of measurement is effectively selected e.g. connected to lines 4, 5.
  • Curve B shows the time interval m during which the A/D converter 18 is in factresponding to the measuring signal so received. This time interval t,! is typically l/60 of a second.
  • Curve C represents the time interval ta during which the discharging circuit 60 is in fact closed.
  • Curve D illustrates the time interval during which the line testing circuit 61 has its relay FR connected in the discharging position.
  • the latter time interval coincides with the time interval 1,,[ which is the measurement time interval.
  • the operation of the line testing circuit 61 will be explained with more particularity.
  • the scanning system for instance energizes relays PR, and CR, connecting transducer T, and transmission lines L,,, L of cable K, to the common output terminal 4, 5.
  • relay FR moves to position 28, 29.
  • capacitor 30 which was initially charged to a potential V,. (point A on FIG. 4) is being discharged in the circuit comprising transmission lines L,,, L and the output circuit V R of the transducer T,.
  • the discharge curve is AF.
  • the time constant (C X R) of circuit 30, 31 is such that in 0.1 ms the potential V,. will have decreased to more than of its value (at F on the discharge curve).
  • capacitor 30 0.l MF and resistor 31 10 ohms.
  • the 10 ohms of resistor 3l are negligible compared to the I000 ohms of the source.
  • the time constant is therefore: l0 X 10 II) sec., or 0.1 ms. Accordingly, at time 1, (FIG. 4) when the A/D converter becomes responsive to V from transducer T, capacitor 30 is no longer of any moment on the transmission lines L,,, L,-,.
  • relays FR and DR are controlled as shown on FIG. 3 by the digital circuitry of the computer system. Relays such as PR,, PR CR,, CR,,,, FR or DR instead of being electromechanical, could as well be solid state switches, like FET.
  • storing means for providing a test potential having a magnitude substantially different from said predetermined operative range of said measurement signals; switch means for selectively connecting said storing means between a selected one pair of transmission lines and said output terminals for providing a discharge path for said storing means to reduce said test potential to a magnitude within said operative range when the sensing device associated with said selected one pair of transmission lines has electrical continuity;
  • sensing devices are divided into groups, each of, said pairs of transmission lines of one group being selectively connected to a corresponding pair of group lines, said output terminals being connected to said pairs of group lines, said switch means being operative to connect said storing means with said pairs of group lines and with said output terminals; means being provided for discharging said pairs of group lines after a supply of a measurement signal from a selected pair of transmission lines and before selection of another pair of transmission lines.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Arrangements For Transmission Of Measured Signals (AREA)
  • Selective Calling Equipment (AREA)
US411496A 1973-10-31 1973-10-31 Analog data acquisition system Expired - Lifetime US3876997A (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
US411496A US3876997A (en) 1973-10-31 1973-10-31 Analog data acquisition system
JP12198874A JPS5619676B2 (enrdf_load_stackoverflow) 1973-10-31 1974-10-24
FR7436240A FR2250167B1 (enrdf_load_stackoverflow) 1973-10-31 1974-10-30

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US411496A US3876997A (en) 1973-10-31 1973-10-31 Analog data acquisition system

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JP (1) JPS5619676B2 (enrdf_load_stackoverflow)
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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4075607A (en) * 1975-11-25 1978-02-21 Ricoh Co., Ltd. System for collecting data from meters placed at remote places
US4371953A (en) * 1970-12-28 1983-02-01 Hyatt Gilbert P Analog read only memory
US4495497A (en) * 1982-01-07 1985-01-22 National Semiconductor Corporation Circuit arrangement for controlled interconnection of signal sources and signal destinations
EP0153437A1 (en) * 1983-05-24 1985-09-04 Radiometer A/S Electrochemical measuring electrode device and method for transmission of signals therefrom
US4796025A (en) * 1985-06-04 1989-01-03 Simplex Time Recorder Co. Monitor/control communication net with intelligent peripherals
US5512890A (en) * 1992-02-19 1996-04-30 Namco Controls Corporation Sensor connection system

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS63155599U (enrdf_load_stackoverflow) * 1987-03-27 1988-10-12

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2833862A (en) * 1953-09-11 1958-05-06 William A Tolson Multiplexing commutators
US2937369A (en) * 1955-12-29 1960-05-17 Honeywell Regulator Co Electrical signal measuring apparatus
US3059220A (en) * 1959-07-02 1962-10-16 Dimeff John Apparatus for coupling a plurality of ungrounded circuits to a grounded circuit
US3271651A (en) * 1962-09-24 1966-09-06 Internat Systems Control Ltd Signal transfer apparatus incorporating means to suppress spurious interference signals
US3281808A (en) * 1962-04-27 1966-10-25 Cons Controls Corp Data measuring and transmission system
US3513460A (en) * 1965-04-12 1970-05-19 Geophysical Res Corp Method and apparatus for the remote indication of data
US3688292A (en) * 1970-02-16 1972-08-29 Robertshaw Controls Co Analog telemetry system having frequency signal transmission

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2833862A (en) * 1953-09-11 1958-05-06 William A Tolson Multiplexing commutators
US2937369A (en) * 1955-12-29 1960-05-17 Honeywell Regulator Co Electrical signal measuring apparatus
US3059220A (en) * 1959-07-02 1962-10-16 Dimeff John Apparatus for coupling a plurality of ungrounded circuits to a grounded circuit
US3281808A (en) * 1962-04-27 1966-10-25 Cons Controls Corp Data measuring and transmission system
US3271651A (en) * 1962-09-24 1966-09-06 Internat Systems Control Ltd Signal transfer apparatus incorporating means to suppress spurious interference signals
US3513460A (en) * 1965-04-12 1970-05-19 Geophysical Res Corp Method and apparatus for the remote indication of data
US3688292A (en) * 1970-02-16 1972-08-29 Robertshaw Controls Co Analog telemetry system having frequency signal transmission

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4371953A (en) * 1970-12-28 1983-02-01 Hyatt Gilbert P Analog read only memory
US4075607A (en) * 1975-11-25 1978-02-21 Ricoh Co., Ltd. System for collecting data from meters placed at remote places
US4495497A (en) * 1982-01-07 1985-01-22 National Semiconductor Corporation Circuit arrangement for controlled interconnection of signal sources and signal destinations
EP0153437A1 (en) * 1983-05-24 1985-09-04 Radiometer A/S Electrochemical measuring electrode device and method for transmission of signals therefrom
US4796025A (en) * 1985-06-04 1989-01-03 Simplex Time Recorder Co. Monitor/control communication net with intelligent peripherals
US5512890A (en) * 1992-02-19 1996-04-30 Namco Controls Corporation Sensor connection system

Also Published As

Publication number Publication date
JPS5619676B2 (enrdf_load_stackoverflow) 1981-05-08
JPS50113246A (enrdf_load_stackoverflow) 1975-09-05
FR2250167A1 (enrdf_load_stackoverflow) 1975-05-30
FR2250167B1 (enrdf_load_stackoverflow) 1981-06-12

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