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BACKGROUND OF THE INVENTION
The present invention relates to an automation system for an industrial
plant, especially a chemical plant the operating parameters of which are
subject to supervision and correction by digital electronic calculators
with memorized program, as a function of signals proportional to the
values of the said operating parameters.
For continuous correct operation of a industrial plant it would be
necessary for the operating parameters to remain constant in time at a
predetermined value, whereas in the case of non-continuous (batch
processing) plants it would be necessary for the operating parameters to
be varied in time in accordance with a strict predetermined logic
sequence.
It is in fact obvious that any excessive variation in the value of one of
the parameters would upset the smooth operation of the system, whereas
such variations are unavoidable in practice. Consequently it is important
to continually, if not continuously, maintain surveillance on the
operating conditions of an industrial plant so as to be able to intervene
promptly if the conditions in the plant tend to deviate undesirably from
predetermined values, which values may be determined by calculations on
the basis of memorized criteria.
In this connection there is at present a tendency to assign the task of
supervision and control of industrial plants to checking apparatus
including at least one electronic digital computer, this partially or
wholly replacing the usual instruments.
The adoption of electronic computers for these purposes has been
slowed-down however, because of problems of reliability of such systems.
When using conventional instrument supervision and control means, all
signals representing the operating parameters of the plant are transmitted
to a central control room and processed in parallel, which allows
supervision with a very high overall reliability. When digital computers
are employed for such purposes the parameters cannot be processed in
parallel because, as is known, digital computers are essentially
sequential in operation and can only detect in series the signals coming
from the plant, and can only intervene serially upon the control members
of the said plant.
The use of computers therefore involves the loss of the "parallelism" which
is a feature of conventional instrumentation and control techniques with
consequent loss of reliability in the system.
OBJECTS OF THE INVENTION
One object of the invention is to ensure continuity of operation of a
control system in the event of malfunctioning, or failure of one or more
control computers.
Another object of the present invention is to improve reliability of
computer controlled systems to a degree such that large scale use of
computers in the supervision and control of industrial plants can be
undertaken.
A further object of the present invention is to provide a highly reliable
system for the automation of an industrial plant, more particularly a
chemical plant.
SUMMARY OF THE INVENTION
The present invention is particularly distinguished in that it provides a
system for the automation of an industrial plant, such as a chemical plant
the operating parameters of which are subject, at least partly, to
monitoring and adjustment by digital electronic computers in dependence on
signals representing the values of the aforesaid parameters, wherein said
system comprises:
a plurality of sensors for detecting the parameters of said plant to be
monitored,
four digital electronic computers each with a central memory, in which
there are stored identical programs for processing the parameters of said
plant which are to be monitored, said computers being paid out according
to a matrix circuit;
connection means operatively and selectively interconnecting said computers
to each other and to said sensors on the plant to be checked, one of said
computers including:
means for reading in sequence all the incoming signals representing
detected values of said parameters arriving from said sensors on said
plant based on said memorized program,
means for transmitting to the other computers signals representing said
readings,
means for comparing said signals read with corresponding stored or
calculated nominal or limit values based on said memorized program,
signalling and checking means connected to said other computers,
means for activating said signalling and checking means in the event of a
discrepancy between said detected values and said corresponding stored or
calculated nominal or limit values, based on said memorized program; and
the three said other computers including,
means for receiving from said one computer signals representing which of
said of said readings are being taken,
means for receiving from said one computer signals representing said
readings, and
means for comparing said signals from said one computer with corresponding
memorized or calculated nominal, or limit, values based on their memorized
programs;
four operational watch-dog detectors, each having one input and three
outputs,
means connecting the input of each watch-dog detector to a respective
computer,
means connecting the three outputs of each watch-dog detector to the three
computers other than the one to which its input is connected,
means on each computer for feeding its associated watch-dog detector with
check signals at predetermined time intervals; said watch-dog detectors
operating to generate a failure signal which is fed to the other three
computers is said check signal fails to arrive at the predetermined time,
means on said computers for selecting, from said other three computers,
upon receipt of a failure signal from the watch-dog detector of said one
computer, which of said computers is to take over from said one computer.
According to the present invention, the signals arriving from individual
sensors which detect the value of the parameters of the plant being
monitored are processed individually according to their type, and
converted into serialized digital signals.
The normal routine of the said one digital computer consists of
interrogating in sequence all the sensors in a continuous cyclic series,
based upon the program memorized in it and upon informing the remaining
three computers as to the interrogation carried out. In this way the
remaining three computers are able correctly to link their programs with
the signals arriving in parallel to the four computers.
The said one computer moreover provides for energizing signalling and
checking means in the event of a discrepancy between the values read and
those memorized or calculated, by means of its outputs, which are
connected to the plant; these outputs are also connected to one or more
control desks having video monitors, key boards, optical and/or acoustic
warning devices etc.
According to a preferred embodiment of this invention the four computers
each have a central memory storing identical programs for processing
signals representing the primary parameters of the plant, two of said
computers also being provided with a peripheral memory for storing
programs for processing secondary parameters of said plant and also for
preparing programs,
the matrix arrangement of said computers being such that the computers
having peripheral memories are one of:
both in the same row, and
both in the same column, said one computer being, in normal operating
conditions, one having a peripheral memory as well as a central memory.
With this arrangement the said one computer is preferably, in normal
operating conditions, one of those having a peripheral memory as well as a
central memory.
Further features and advantages of the invention will become apparent
during the course of the following description which is provided purely by
way of non-restrictive example, and in which reference is made to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating the lay out of the four computers in
accordance with the principles of the present invention; and
FIG. 2 is a diagram illustrating an example of the manner in which the
signals arriving from the plant are processed and transmitted to the
appropriate digital computer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1 there are shown four digital electronic computers A, B, C, D each
housing a central memory, with magnetic or semi-conductor cores, in which
there are memorized the programs for processing the basic parameters of a
chemical plant to be supervised and controlled by the supervision and
control system of which the computers A, B, C, D form part.
The computers C and D are additionally provided with an identical
peripheral memory DC and DD respectively, which may be a magnetic disc or
drum memory, intended for memorising the programs for processing the
secondary parameters of the plant, and possibly for program planning.
Each of the four computers has an associated teletype service machine (TA,
TB, TC, TD), and the computers C and D are also provided with a reader
unit and tape puncher (LC and LD).
A plurality of substations OS-1, OS-2, . . . OS-n are positioned close to
the sensors (not shown) which detect the operating parameters of the plant
to be checked, the substations receive signals arriving from the plant,
and pass them on to the computers having first converted them into digital
form (the sensors themselves would usually generate analogue signals), in
a manner which will be described in greater detail below. The overall
number of parameters will be different for each plant, but each substation
is capable of handling signals representing fifty parameters. Thus, the
substation OS-1 handles signals which for convenience may be labelled
1-50, whilst substation OS-2 handles signals 51 to 100 and so on. Since
the number of parameters to be checked will be different for different
plants, the number of substations will also be different: for example, for
a chemical plant the number of substations may be between twenty and
fifty.
The digital signals from the substations are also serialized and coded, and
they pass first to a unit for shunting and switching, and from this unit
pass in parallel to the four computers after passing through a master
station to be decoded. The unit for shunting (for the input signals) and
for switching (for the output signals from the digital computer) is shown
as UC-1 in FIG. 1 and will hereinafter be called the switching unit UC-1
for brevity.
Each substation of the plant is connected to four master stations, each
connected to a computer. Thus, as indicated in FIG. 1, the four master
stations to which the substations OS-1 are connected comprise the master
station MS-1A connected to the computer A, MS-1B connected to the computer
B, MS-1C connected to the computer C and MS-1D connected to the computer
D.
Similarly, the substation OS-2 is connected to four master stations MS-2A,
MS-2B, MS-2C and MS-2D; more generally, a substation OS-n is connected to
four master stations MS-nA, NS-nB, MS-nC and MS-nD, which latter are
connected to the computers A, B, C, D respectively. Thus, a signal
originating from the substation OS-1 arrives along a line 10 at the
switching unit UC-1, and from this is passed to the computer A via a line
10A and the master station MS-1A, to the computer B via line 10B and the
master station MS-1B, to the computer C, via a line 10C and the master
station MS-1C, and to the computer D via a line 10D and the master station
MS-1D.
Likewise, signals coming from the substations OS-2 and OS-n arrive at the
switching unit UC-1 via lines 11, 12, and from there are fed in parallel
to the four computers via lines 11A, 11B, 11 C and 11D and, 12A, 12B, 12C
and 12D respectively. Signals from stations between OS-2 and OS-n are
treated similarly, each of the lines being a telephone loop circuit.
The computers themselves are interconnected by synchronization lines which
are connected to the respective computers by synchronization master
stations which are shown in FIG. 1 as MS-XA (for computer A), MS-XB (for
computer B), MS-XC (for computer C) and MS-XD (for computer D).
The synchronizing lines are indicated 13, 14, 15, 16, 17 and 18 and connect
the computers A and B, A and C, A and D, B and C, B and D and C and D
respectively. Over these lines each digital computer supplies the other
computers with a check reading.
In addition, for surveillance purposes, there are also provided four
"watch-dog" detectors, one to each computer, indicated WD-A, WD-B, WD-C
and WD-D in an obvious notation, each "watch-dog" detector operating to
detect any irregularity in the operation of the computer to which it is
connected. Thus, the "watch-dog" detector WD-A is connected to the
computer A to detect its operation, and is connected via the circuit lines
19, 20 and 21, to the computers B, C and D respectively. Likewise, the
"watch-dog" detector WD-B is operatively connected to the computer B, and
is connected via the lines 22, 23 and 24 to the other computers A, C and D
respectively; in the same way "watch-dog" detector WD-C is operatively
connected to the computer C, and linked via lines 25, 26 and 27 to the
computers A, B and D respectively, and "watch-dog" detector WD-D is
operatively connected to the computer D and linked via the lines 28, 29
and 30 to the other three computers A, B and C respectively.
The outputs of the four computers A, B, C, D are connected to a switching
unit UC-2 which is connected to two control desks 37 and 38 by lines 35
and 36; three lines 39, 40 and 41 connect the switching unit UC-2 to the
service teletypes, and one or more video display devices for displaying
the values of the parameters are also connected thereto by lines 42, 43
and serve as repeaters for the warning signals when such are generated.
These video display devices for the most part replace conventional
recorders and alarms, such as regulators, indicators, synoptics,
interblocks, sequence logic, controls, and others.
Conveniently, the control desks include one or more video display devices
by means of which it is possible to see visually the lay-out of the plant,
with the current positions of the control members and the instantaneous
values of the measurements; one or more push-button panels in which the
buttons correspond to particular areas of the plant; a warning signal in
one area causes intermittent illumination (flashing) of the corresponding
button, and when this button is depressed, the values of the parameters
corresponding to this area are displayed on the video display device; in
addition there are one or more alphanumerical keyboards which can be
operated for activating further controls or request and for expansion or
modifying of the display on the video, and an indicator panel showing the
state of the computers and of their peripherals: finally there are printer
units for providing printouts of information for written documentation of
events, bulletins or the like, relating to the running of the plant.
In the FIG. 2 there is shown a case in which analogue signals, partly at
low and partly at high level, arrive at the substation OS-1 and from this
digital signals are fed to the digital computer C, as well as to the other
computers, via the switching unit UC-1. The sensors associated with
substation OS-1 are shown as S-1, S-2, . . . S-50, and each of these
supplies an analogue (current) signal, proportional to the value of the
operating parameter to which that sensor is sensitive.
The substation OS-1 comprises a multiplexer 45 which, in the embodiment
illustrated, has fifty inputs, thirty of them being connected to the
sensors S-1 . . . S-30, via respective amplifiers 44 which raise the level
of the signal from the sensors from a value in the millivolt region to a
value of the order of Volts. The remaining twenty inputs to the
multiplexer 45 are connected to further sensors, similar to the others,
but which feed to the multiplexer analogue current signals, which indicate
other parameters, which are already of the order of volts and which
therefore do not need to be amplified.
The substation OS-1 also includes an analogue-to-digital converter 46 which
feeds output digital signals to an encoder 48. From the encoder 48 signals
are fed on lines 49 to a receiver/transmitter unit 50 which has a feedback
connection 51 to the multiplexer 45.
The receiver-transmitter unit 50 receives orders through the loop circuit
10 from the associated digital computer C. The normal routine of the
computer C consists, inter alia, of interrogating, according to the
program, the 50 inputs of the multiplexer 45 in order to receive their
respective signals (and, of course, of doing the same for the other inputs
of each of the other substations shown in FIG. 1). Thanks to the memorized
program, the computer C "knows" which sensor it is about to interrogate at
any given time and to which part of the plant it belongs. For example,
when the sensor 30 is to be interrogated, then the computer C sends the
interrogation signal, through the master station MS-1C to the address of
that input of the multiplexer 45 of the substation OS-1 to which the
sensor S-30 is connected.
In compliance with the inquiry of the computer C, the multiplexer 45 sends
the analogue signal of the sensor S-30 to the analogue-to-digital
convertor 46 where it is converted into a corresponding digital signal
composed, for example, of eight bits. The output of the A/D convertor 46
consists of the 8 lines 47 (one per bit) which lead to the encoder 48
which completes the "message", adding to the eight "information" bits a
"beginning-of-message" bit, two "end-of-message" bits and a parity bit.
The output of the encoder 48 therefore consists of twelve circuit lines 49
which lead into the receiver/transmitter unit 50 for serialization.
The receiver/transmitter station 50 serializes the individual bits, sending
them one after the other to the switching unit UC-1 via the loop circuit
10. From the switching unit, however, they are fed in parallel to the
master stations MS-1A, MS-1B, MS-1C and MS-1D connected to the respective
computers. In the embodiment shown in FIG. 2 the master station MS-1C
confirms the authenticity of the message (in the possible presence of
noise), and then eliminates the four bits added by the encoder 48, and
passes the eight information bits to the computer C, in parallel on the
eight output lines 52.
Wholly similar operations take place in the corresponding master stations
connected to the other computers. If the value of the check reading
arriving at the computer C from the other computers, on the
synchronization lines 13, 14, 15, 16, 17, and 18 does not correspond to
the value (or interval of values) memorized in the said computer (or
calculated based upon its program) then an error signal is generated,
based upon which the master station sends a correcting signal via the
corresponding substation. This correcting signal could be sent directly
from the computer. At the same time the computer C sends a warning signal
over the line 33.
FIG. 2 illustrates an embodiment in which analogue signals are sent from
the sensors, but in which other signals can arrive at the substations,
such as, for example, digital signals, impulse trains, and so on.
Obviously such signals will be processed according to their nature, and
will, if necessary, be converted into digital form, coded and serialized.
In the embodiment illustrated the configuration of the four computers is a
matrix with duality according to the lines and the possibility of
degradation according to the columns. In this case, by considering the
initial operation being undertaken by a computer having a peripheral
memory, there is a total reserve and two partial or degraded reserves.
As can be seen from the drawings, the inputs to the system from the
substations and from the control desk arrive in parallel at all the
computers which will therefore at any moment be brought up to date with
the situation of the plant and the operators' requests.
Moreover the operating computer informs the other calculators via the
synchronization lines, about the interrogations carried out, so that the
latter can accurately link the data they receive. The output of the system
to the plant and to the control desks, on the other hand, is effected
solely by the operating computer. This is achieved by means of the
switching units which connect the peripherals to the outputs of the
operating computer.
One of the operations of the operating computer consists of sending a
renewal signal to the relative watch-dog detector at predetermined
intervals. In the event of this signal not reaching it, due to
malfunctioning of the operating computer, the watch-dog detector sends a
priority signal to the other computers. In this event a program for
selecting a new operating computer is put into effect on each of the
remaining computers. This program takes account of which computer was the
operating computer, of which computers remain available, and, based on
memorized criteria, induces switching of the output circuit lines on the
computer which involves the minimum operational degradation of the system.
This program also takes into account possible incompatabilities between
switching actions erroneously effected by the computers.
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