Analog Data Acquisition System

Abstract

The invention relates to analog multiplexing systems used in an industrial environment to derive small signals from remote locations. Since the signals received might mask a failure of the sensing devices a bias source is used to generate an anomalous signal when sensing is defective. The bias source is common to all points of measurement. It has a small time constant so that it can be switched at the multiplexing frequency, and it is operative only at the inception of the time interval for signal derivation so that measurement is not affected. Provision is made for purging successively the signal transmitting cables before signal derivation.

Description

BACKGROUND OF 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 for control 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 1/60 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.

Although such data acquisition system often operates satisfactorily, there remains the possibility that the sensing device, or even the transmission line, be defective so that the signal received on the common output terminal be not a true measurement signal. In such a case there is the risk that the computer system command control or monitoring actions which are not warranted or even which may be detrimental to the industrial operation. Therefore, it is desirable to be able to detect at any instant whether data received on the multiplex terminal is a true or false measurement signal.

This problem has already received a solution in the prior art by providing a bias source in parallel with the measuring device and arranged in such a way that interference be minimized during measurement. When the measuring device is defective the bias source becomes preponderant and a signal characteristic of such condition is applied by the bias source on the multiplex terminal. In such case either the computer system does not respond, because for instance such characteristic signal would be out of range for normal control, or more simply, source emergency signal is displayed by the characteristic signal to alert the operator.

This prior art solution is not fully satisfactory. First, in a complex multiplexing system of cables it requires a bias source for each cable, which makes the solution costly. Secondly, the presence of the bias source in parallel with the sensing device during measurement is a cause of error in the data derived. A large bias current makes an emergency action more reliable but introduces a large error in the transducer's response. Conversely, a small bias current keeps the accuracy but may cause the response to emergency to be marginal. Actually, the lower limit acceptable for the bias current is determined by the resistance of the insulation between the input terminals.

It 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.

For the sake of illustration, the invention will now be described in the context of an industrial process controlled by a digital process control system embodying a digital computer system such as a Prodac 2000 (P2000) sold by Westinghouse Electric Corporation. A descriptive book entitled "Prodac 2000 Computer Systems Reference Manual" has been published in 1970 by Westinghouse Electric Corporation and made available for the purpose of describing in greater detail this computer system and its operation.

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

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;

FIGS. 7 and 8 show an open circuited transmission line under the three conditions 1 to 3 of FIGS. 5 or 6, when the fault detection according to the invention is used. FIG. 7 is for a 1000 feet long cable; FIG. 8 for a 100 feet long cable; and

FIGS. 9 and 10 are, for respectively a 1000 feet and a 100 feet long open circuited cable, the three above mentioned electrical states when in addition to using the fault detecting circuit according to the invention, the cable is also purged in advance from any accumulated electrical charge .

DESCRIPTION OF THE PRIOR ART SYSTEM

Referring to FIG. 1, a multiplexed data acquisition system is shown comprising n transducers such as T.sub.1, T.sub.n connected by cables K.sub.1, K.sub.n respectively to bus lines 1, 2, 3 via corresponding contacts A.sub.11, A.sub.12, B.sub.11, B.sub.12, C.sub.11 and C.sub.12 of a relay R, and A.sub.n1, A.sub.n2, B.sub.n1, B.sub.n2, C.sub.n1 and C.sub.n2 of a relay R.sub.n. Each cable is shielded cable having two signal wires such as L.sub.11 and L.sub.12 for cable K.sub.1, L.sub.n1 and L.sub.n2 for cable K.sub.n, and shield connections such as S.sub.1 for cable K.sub.1, S.sub.n for cable K.sub.n. Wires L.sub.11 and L.sub.n1 are connected to bus line 1; wires L.sub.12 and L.sub.n2 are connected to bus line 2; the shield connections S.sub.1, S.sub.n are connected to the third bus line 3. Each transducer T.sub.1, T.sub.n 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.sub.1, K.sub.n are mounted on a multiplexer card MC.sub.1 which supports the contacts of relays R.sub.11, R.sub.n and the bus lines 1, 2, 3. Also, on the multiplexer card MC.sub.1 are provided contacts CR.sub.11, CR.sub.12, CR.sub.13, CR.sub.14 and CR.sub.15, CR.sub.16 of a card relay CR connecting the bus lines 1, 2, 3 of card MC.sub.1 to the input terminals of a shielded bus cable BK.sub.1, having signal wires LB.sub.1, LB.sub.2 and a shield connection LS.sub.1. At the output, cable BK.sub.1 is connected to three analog bus lines 4, 5, 6 forming the multiplex terminal. LB.sub.1 is connected to line 4, LB.sub.2 is connected to line 5 and LS.sub.1 is connected to line 6.

The system comprises M multiplexer cards such as MC.sub.1. On FIG. 1 is shown the multiplexer card of the m.sup.th order MC.sub.m, which is itself connected with the analog bus lines 4, 5, 6 through a bus cable BK.sub.m.

Relays such as R.sub.1, R.sub.n and CR.sub.1, CR.sub.m 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.sub.11, L.sub.12, LB.sub.1, LB.sub.2.

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.

The circuit just described of a multiplexed data acquistion system is satisfactory provided transducers T.sub.1, T.sub.n all provide a true signal, e.g. a signal which is representative of the magnitude of the variable sensed, for instance temperature. A sensing 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.sub.11, L.sub.12 or L.sub.n1, L.sub.n2) of the particular cable, through resistors R.sub.1, R.sub.2.

The battery operates as a bias source with the associated transducer (T.sub.1 or T.sub.n). The transducer T.sub.1 (or T.sub.n) may be represented as a series combination of a resistor R.sub.T and a voltage V.sub.T. V.sub.T is much smaller than the potential of the battery 10. If the values of R.sub.1, R.sub.2 are large compared to R.sub.T, the battery 10 will develop a small voltage only in the circuit R.sub.T V.sub.T and the measurement of a signal along L.sub.11 and L.sub.12 will be appreciably impaired by the presence of the battery 10. Should, however, there be an open circuit in the parallel circuit R.sub.T V.sub.T of transducer T.sub.1, or in the adjoining wires L.sub.11, L.sub.12, then, the potential of battery 10 will appear in full between L.sub.11 and L.sub.12. When this happens, the bus lines 4, 5 indicate a potential much higher than the signal normally developed when responding to V.sub.T. 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.sub.1.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring now to 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. 1. The same elements have received in FIG. 2 the same reference numerals as in FIG. 1. By comparison with FIG. 1, it is observed that there is no longer any bias source such as 10 associated with cables K.sub.1, K.sub.n. Instead, 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. When relay FR is in the opposite position, namely on contacts 28, 29, the series circuit 30, 31 is connected in parallel with bus lines 4, 5. It is clear that by a proper choice of capacitor 30 and resistor 31 energy is stored which will be discharged at 28, 29 over lines 40, 41 through the signal wires of the particular cable K.sub.1, K.sub.n which at a given moment has been connected in circuit by the associated point relay (PR.sub.1 or PR.sub.n) and the corresponding card relay (CR.sub.1, CR.sub.m). This discharge occurs early in the time interval that the point of measurement is switched ON, the circuit 30, 31 normally will not be affecting the measuring signal derived through the cable (K.sub.1 or K.sub.n) and inputted at 4, 5 to the A/D converter. If however the circuit of the transducer (T.sub.1 or T.sub.n) is open, the increased resistance of the circuit will prevent capacitor 30 from being discharged, and the voltage of capacitor 30 will appear on lines 4, 5. If the capacitor has been initially charged to a voltage which is large enough, the voltage on buses 4, 5 will be large enough to be outside the range of response of the A/D converter 18. In such case the faulty transducer or transmission line will have been detected.

The circuit card 20 also carries a discharging circuit 60 consisting of a two-pole discharging relay DR 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 the floating system. As will be explained hereinafter, with more detail, the discharging circuit 60 when closed discharges cables BK.sub.1, BK.sub.n removing any charge left from a previous connection established with one of the cables (K.sub.1, K.sub.n).

The operation of the line testing circuit 61 and the purging circuit 60 will now be explained by reference to FIGS. 3 and 4. Referring first to 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.sub.1, PR.sub.n and of relays such as CR.sub.1, in order to selectively connect the output circuit V.sub.T, R.sub.T of each transducer T.sub.1, T.sub.n, 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 t.sub.0 t.sub.3 during which one particular point of measurement is effectively selected e.g. connected to lines 4, 5. Curve B shows the time interval t.sub.1 t.sub.2 during which the A/D converter 18 is in fact responding to the measuring signal so received. This time interval t.sub.1 t.sub.2 is typically 1/60 of a second. Curve C represents the time interval t.sub.4 t.sub.5 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 t.sub.0 t.sub.3 which is the measurement time interval. These time interval relationships need not to be strictly observed. It is clear that they delineate zones of operations allowing a certain margin of safety on either side of the time scale. The frequency of measurement is typically 40 points of measurement in a second.

Referring to FIG. 4, the operation of the line testing circuit 61 will be explained with more particularity. At time t.sub.0 the scanning system for instance energizes relays PR.sub.1 and CR.sub.1 connecting transducer T.sub.1 and transmission lines L.sub.11, L.sub.12 of cable K.sub.1 to the common output terminal 4, 5. At the same time relay FR moves to position 28, 29. As a result capacitor 30 which was initially charged to a potential V.sub.c (point A on FIG. 4) is being discharged in the circuit comprising transmission lines L.sub.11, L.sub.12 and the output circuit V.sub.T R.sub.T of the transducer T.sub.1. The discharge curve is AF. The time constant (C .times. R) of circuit 30, 31 is such that in 0.1 ms the potential V.sub.c will have decreased to more than 60% of its value (at F on the discharge curve). For instance capacitor 30 = 0.1 MF and resistor 31 = 10 ohms. The 10 ohms of resistor 31 are negligible compared to the 1000 ohms of the source. The time constant is therefore: 10.sup.-.sup.7 .times. 10.sup.3 = 10.sup.-.sup.4 sec., or 0.1 ms. Accordingly, at time t.sub.1 (FIG. 4) when the A/D converter becomes responsive to V.sub.T from transducer T.sub.1, capacitor 30 is no longer of any moment on the transmission lines L.sub.11, L.sub.12.

At time t.sub.0 transducer T.sub.1 being in circuit with lines 4, 5 to the A/D converter, voltage V.sub.T appears as input thereto, which signal is effectively received after time t.sub.1 without being impaired by the line testing circuit 61. If there is, however, an open circuit in the V.sub.T R.sub.T circuit of transducer T.sub.1, then capacitor 30 will not discharge and the A/D converter will have a potential V.sub.C at the input. The faulty condition is therefore detected.

After the time t.sub.2 (1/60 of a second after time t.sub.1) the A/D converter is no longer responsive. At time t.sub.3 transducer T.sub.1 is no longer a point of measurement and (see FIG. 3 curve D) relay FR is moved back on positions 26, 27 to allow recharging of capacitor 30. At time t.sub.4 relay DR is moved to closed positions 50, 51 and 53, 52 thereby to purge cables BK.sub.1, BK.sub.m and at time t.sub.5 relay DR is open again (curve C, FIG. 3). The cycle repeats itself from one of the cables (K.sub.1 to K.sub.n) to another.

It is observed here that relays FR and DR are controlled as shown on FIG. 3 by the digital circuitry of the computer system. Relays such as PR.sub.1, PR.sub.n, CR.sub.1, CR.sub.m, FR or DR instead of being electromechanical, could as well be solid state switches, like FET.

The importance of using a line testing circuit such as 61 and a discharging circuit such as 60 (on FIG. 2) can be better ascertained from a comparison of FIGS. 5, 7 and 9 relative to a 1000 feet cable and FIGS. 6, 8 and 10 relative to a 100 feet cable, such as cable K.sub.1 when such cable has an open circuit, for resistance at OC on one of the signal wires (L.sub.12) from the transducer circuit V.sub.T R.sub.T.

Charge condition of the cable is shown for three successive states: A) immediately before switching by the scanning system of the relays, such as PR.sub.1 and CR.sub.1 (FIG. 2), B) immediately after each switching and C) a long time thereafter.

The results in the six above different cases correspond to FIGS. 5 to 10, respectively.

It should be observed that a cable exhibits between the two wires L.sub.11, L.sub.12 and the guard cable of the shield S.sub.1 a delta connection of three stray capacitors (C.sub.HS, C.sub.HL and C.sub.LS). The Figures show the results as follows:

On FIG. 5 (case 1) in its last state the cable exhibits a high voltage Y.sub.PN = 413 mv between lines L.sub.11, L.sub.12, which does not meet the conditions for detection which is Y.sub.PN <- 10 mv.

On FIG. 7, (case 3) it is assumed that the line testing circuit 61 is used without the discharging circuit 60. In this instance Y.sub.PN is 64.4 mv. This does not satisfy the condition for detection Y.sub.PN <- 10 mv.

On FIG. 9 (case 5) it is assumed that the line testing circuit 61 and the discharging circuit of FIG. 2 are both used, e.g. after an initial discharging (t.sub.4 t.sub.5) of the cable by the discharging circuit 60, the fault detecting circuit has been connected (t.sub.0) and the cable charge condition is at a time past t.sub.1. Case 5 reveals that Y.sub.PN = -16.21 mv which would satisfy the condition for detection Y.sub.PN <- 10 mv.

The same can be said of cases 2, 4 and 6 which are in similar situations but with a cable 100 feet long. The results are readily seen from corresponding FIGS. 6, 8 and 10.

It is observed here that the above considerations do not establish that the use of the discharging circuit 60 is required in all instances of line testing by circuit 61 according to the present invention. It has been shown that the line testing circuit according to the present invention exhibits definite desirable functions which are accomplished with any data acquisition system. The discharging circuit such as circuit 60 in FIG. 2, however, although a useful additive in many cases, is only necessary in particular instances of a cable system.

Claims

We claim:

1. In an analog data acquisition system operative with output terminals for deriving in a multiplex manner respective measurement signals having a predetermined operative range from a plurality of sensing devices with each of said sensing devices being associated with a different pair of transmission lines, the combination of:

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; and

means operative with said output terminals for detecting a defective sensing device.

2. The analog system of claim 1 with said storing means and the associated discharging circuit having a time constant which is small relative to a measurement time interval defined by said scanning means.

3. The analog system of claim 2 wherein said storing means includes means operative with said switch means for charging said storing means during a time period following said measurement time interval.

4. The analog system of claim 3 wherein said 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.

5. The analog system of claim 4 wherein said switch means is operative during a measurement time interval during which said measurement signal is supplied to said output terminals by said selected one pair of transmission lines, and wherein said means for discharging said pairs of group lines is operative during a time period between two consecutively established measurement time intervals.

6. The analog system of claim 5 wherein said time period is established in response to operation of said means for discharging.