GB2142206A - Monitoring system - Google Patents

Abstract

A system for monitoring data signals representing a number of parameters of a plant or apparatus, periodically superimposes test signals on to some of the data signals so as to produce combined signals representing unacceptable states of the parameters. The selection of the data signals on which the test signals are superimposed is changed at each periodic examination of the data signals in a predetermined sequence, so that a pattern of acceptable and unacceptable signals is produced as an output to be compared with a reference pattern.

Description

SPECIFICATION Monitoring system This invention relates to monitoring systems for, and a method of monitoring, industrial process plants or apparatus, and one application ofthe invention is in nuclear reactors for the detection offault conditions.

A monitoring system is described in UK Patent No. 2 063021 B (U.S. Patent No. 4422140, French Patent Appln. No. 80 19664) in which data signals, originating from thermocouples, are supplied sequentiallyto a data processor by a multiplexer. To guard against failure ofthe multiplexer, some ofthe inputs to the multiplexer are wired to sources of test signals, rather than data signals; the test signals are equivalent to unacceptable data signals. The inputs to which the test signals sources are connected are chosen to produce a pattern of acceptable and unacceptable signals, which pattern can be recognised. The pattern should be the same for each scan ofthe entire set of input signals, and will change only if a data signal reaches an unacceptable value.Since the test signals sources are permanentlywired to multiplexer inputs, those inputs cannot be used for data signals. Furthermore malfunctioning between the thermocouple and the multiplexer input may not be detected.

According to a first aspect ofthe present invention there is provided a system for monitoring a plurality of data signals from a plant or apparatus, each data signal representing a parameterofthe plant or apparatus, the system comprising, means for periodically examining the data signals and for providing output signals representing whether each data signal represents an acceptable or an unacceptable state of the respective parameter, and means for supplying testsignalsto be superimposed on selected said data signals before each periodic examination thereof such as to produce corresponding combined signals equivalent to data signals representing the unacceptablestate,theselection of the data signals on which the test signals are superimposed being changed for each periodic examination of the data signalswithin a predetermined sequence so that the examining means produces a recognisable pattern of said output signals.

The test signal supply means may include signal generating means, andthe signal generating means may be linked to the examining means so as to control the test signal means in relation to a memory of past data signals.

Atiming means may control the examining means andlorthe test signal supply means, and the pattern of output signals may be compared in a pattern recognition means with a reference pattern.

In one application ofthe first aspect of the invention, a number of the monitoring sytems are arranged in parallel for monitoring the same number of data signals representing each parameter, such that each said data signal is monitored by a respective monitoring system, each monitoring system being arranged to provide the pattern of output signals thereof to voting means, the voting means being arranged to supply an output to a said pattern recognition means.

Preferably, a plurality of said voting means are provided, each monitoring system being arranged to provide the pattern of output signals thereofto each said voting means.

In another aspect, the invention provides a method of monitoring a plurality of data signalsfrom a plant or apparatus, each data signal representing a parameter ofthe plant or apparatus, the method comprising periodically examining each data signal and providing an output signal representing whether the data signal represents an acceptable or an unacceptable state of the respective parameter, and superimposing test signals on selected said data signals before each periodic examination thereof such as to produce corresponding combined signals equivalent to data signals representing the unacceptable state, the selection ofthe data signals on which the test signals are superimposed being changed within a predetermined sequence so that a recognisable pattern ofthe output signals is produced.

One advantage ofthe monitoring system of the invention is that it continually monitors its own operation and can provide a control signal indicating a fault if one ofthe parameters is in reality in an unacceptable state, or if the monitoring system is itself faulty, since in eithercasethe correct pattern will not be present.

The invention will now be further described by way ofexample only and with reference to the accompanying drawings, in which: Figure 1 shows a blockdiagram of a monitoring system; Figure 2 represents electrical signals attwo points in the monitoring system of Figure 1; and Figure 3 shows a block diagram of an alternative monitoring system to that of Figure 1.

Referring to Figure 1, a monitoring system 10 for monitoring temperatures atthirty-two locations in a core of nuclear reactor (not shown) has thirty-two inputterminals 12 (onlyfour are shown) to which leads carrying data signals from thirty-two tempera- tures sensors (not shown) are connected. Each input terminal 12 is connected to one input 14 of a respective two-input, signal adding-and-amplifying unit 16 (only four are shown), the other input 18 of which is connected to atestsignal generating system 22. The output of each unit 16 is connected to a respective input of a thirty-two input multiplexer 20, whose output is connected through an analogue-to-digital converter 24to a signal analyser 30 arranged to provide an input to a pattern recognition unit 40.

In operation ofthe monitoring system 10, the multiplexer 20 scans sequentially the signals it receives, which, in the absence of signals from the test signal generating system 22, will correspond to the data signals representing the temperatures of the temperature sensors connected to the inputterminals 12. Atthe end of each scan the polarity of the output from the multiplexer 20 is reversed, and the signal analyser 30 identifies this polarity change as signifying the beginning of a new scan. The signal received bythe signal analyser 30 hence consists of a sequence of signals in digital form, each representing the temperature of one of the temperature sensors.The signal analyser 30 is a computer which, from the signal from each sensor: (a) calculates the corresponding temperature; (b) determines whether the temperature lies within predetermined acceptable limits; (c) calculates the rate of change oftemperature, from the calculatedtemperature and a memory of the previously calculated temperature; (d) determineswhetherthe rate of change of temperature of that sensor lies within predetermined acceptable limits; (e) updates the memory ofthe temperature of that sensor if both the temperature and the rate of change oftemperature lie within the respective limits; and (f) gives an output signal "1" if both the temperature and the rate of change of temperature are acceptable, or an output signal "0" ifeitherthe tem peratu re orthe rate of change of temperature are unacceptable.

Hence, in the absence of signals from the test signal generating system 22, if the nuclear reactor is operating normallythe expected output signal produced bythesignal analyser30 isthesequence 1,1,1, 1, 1... etc., and this pattern of digits would be supplied as the inputtothe pattern recognition unit 40. Itwill be appreciated thatthe acceptable limitsfortemperature and for rate ofchange oftemperature maywell be different for sensors at different locations within the nuclear reactor. Hence the signal analyser 30 must be programmed with the acceptable limitsforeach sensor.

The test signal generating system 22 consists of a test signal generator 32 for producing a sequence of signals in digital form, and connected by a data link42 to the signal analyser 30 for receiving the memory of the currentvalues of the temperatures of the sensors.

The test signal generator 32 is connected through a digital-to-analogue coverter34to the inputofa thirty-two output demultiplexer36, the outputs of which are connected through respective sample-and hold amplifiers 38 (onlyfour of which are shown) to the inputs 18 ofthe units 16, each amplifier 38 being adapted to generate a steady signal corresponding to the most recent signal received from the demultiplexer 36 until the next signal from the demultiplexer 36 is received.

Thus when the test signal generating system 22 is in operation, the signals produced bythetestsignal generator 32 are superimposed in analogue form on the data signals from the sensors bythe signal adding-and-amplifying units 16. Some of the signals produced by the test signal generator 32 and referred to astestsignals, are such asto cause the multiplexer 20 and hence the signal analyser3Oto receive a signal representing a temperature outsidethe acceptable limits, or representing a temperature corresponding to an unacceptable rate of change of temperature, while other signals from the test signal generator 32 have no such effect on the respective data signals.The test signal generator32 is programmed to calculate suitable values of test signals, utilising where necessarythe temperature values received from the signal analyser 30 via the data link 42, so as to result in a desired test pattern of acceptable and unacceptable signals, i.e. to produce a desired sequence of "0" and "1" outputsignalsfrom the signal analyser 30. The presence of the desired pattern is then detected by the pattern recognition unit 40.

It is clearly importantforthetest signal generator 32 and the demultiplexer36 to operate in phase,sothat the testsignalsare supplied to the appropriate sample-and-hold amplifiers 38, and this is ensured by the use oftiming signals provided by a timer 44.

Timing signals from the timer 44 also control the multiplexer 20 so that the multiplexer 20 performs one scan of the inputs it receives for every scan of the demultiplexer36.

An example ofthe variation of voltage with time at the output ofthe multiplexer20 is shown in the graph of Figure 2, in which the change of polarity occurring attime A signifies the start of a scan by the multiplexer 20 ofthe signals received at its thirty-two inputs.

Below the graph are digits representing the corresponding output signals from the signal analyser 30.

The graph of Figure 2 and the corresponding digits are produced when the data signalsfrom the sensors are in reality all at acceptable values, each "0" signifying the presence of a test signal superimposed on the respective data signal (an unacceptable value), and each "1" signifying a data signal unaffected by a signal from the test signal generating system 22 (an acceptable value).

The overall pattern oftestsignals is produced in four scans ofthe multiplexer 20, the test signals being applied to the inputs 18 of a different selection of adding and amplifying units 16 on successive scans of the multiplexer 20.

The sequence of output signalsfrom the signal analyser 30 is shown in Table 1.

Table 1 1st Scan 2nd Scan 3rd Scan 4th Scan 1 1 1 1 1 0 1 0 0 1 0 1 1 0 1 0 1 1 1 1 0 1 0 1 1 0 1 0 0 1 0 1 0 1 1 0 1 1 0 1 1 0 1 0 o 1 1 1 } 0 0 1 1 1 1 0 O I O 1 1 0 1 1 1 0 0 1 0 1 1 0 1 1 1 1 1 0 0 1 0 1 1 0 1 0 0 1 1 1 1 1 0 1 1 0 0 1 1 0 1 0 1 1 0 1 0 1 1 1 1 0 1 0 0 1 0 1 1 1 1 0 I O 1 1 0 I It will be observed that with this test pattern, twenty inputs are tested twice, eight inputs are tested once, and four inputs are nottested at all.The structure of the test pattern ofTable 1 is shown more clearly in table 2, in which successive groups of eightsuccessive digits are arranged in Horizontal rows, com mencing atthe first eight digits ofthefirst scan. Table 2 1 1 0 1 1 0 1 0 o 1 1 0 1 1 0 1 I 0 1 1 0 1 1 0 O I O 1 1 0 1 1 1 0 1 0 1 1 0 1 1 1 0 1 0 1 1 0 0 1 1 0 1 0 1 1 1 0 1 1 0 1 0 1 1 1 0 I 1 0 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 1 1 1 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 0 1 1 0 1 The pattern of Table 2 may be generated bystarting with the eightdigits in the first row of Table 2, successive groups of eight digits being generated by sequentially shifting one place at a time eight places to the right and then eight places to the left, at each shift the digit at one end being moved to the other end of the group. It will also be observed that the pattern within any group of eight successive digits is asymmetrical, and also that the complete pattern cannot be reproduced by repetition of a portion ofthe complete pattern. These properties ensure that the pattern can only be reproduced if the multiplexer 20 scans all its inputs in the correct sequence. Consequently anyfault in the operation ofthe multiplexer 20 will produce a different pattern.

It will be appreciated thatthis is but one of many possible test patterns which the test signal generator 32 may be programmed to generate and which the pattern recognition unit 40 can reconise. Where it is advantageous to test every input, an alternative test pattern may be generated in the manner described above by starting with the eight digits 10110100.

The pattern recognition unit 40, which receives the sequence of digits shown in Table 1, comprises a shift register and a comparator, the shift register storing a reference pattern consisting initially of the sequence of digits shown in the first row of Table 2. The sequence of digits received from the signal analyser 30 is compared, eight digits at a time, with the reference pattern in the shift register. The next eight digits are then compared with the same reference pattern (which should give a mismatch), and then the reference pattern is shifted by one place and the comparison repeated. The output ofthe comparator is "0"for a mismatch and "1" for a match, and should therefore consist ofan alternating sequence 1 0 1 0 1 0 ... etc. as long asthe pattern is as shown in Table 1.If a match is not found afterthe reference pattern is shifted, then the shift-registerwill not shift the reference pattern again, and the next group of eight digitswill not be admitted to the comparator, and so the output signal will no longer alternate. The pattern recognition unit 40 may be reset by manually depressing a reset button (not shown), subsequent groups of eight digits then being admitted to the comparator until a match is found, and the alternating output sequence then resuming.

Itwill be appreciated thatfaulty operation of the pattern recognition unit 40 can also be expected to produce a non-alternating output.

The output signal from the pattern recognition unit 40 may be used as a control signal forthe nuclear reactor, the alternating sequence 1,0,1,0, etc.

signifying the detection of no faults, and anon- alternating sequence signifying:- (a) an unacceptablevalueofthetemperatureorof the rate of change oftemperature ofthe sensors, or (b) a hardwarefaultanywhere in the monitoring system 70, or (c) a software fault in the test signal generator 32 or the signal analyser30.

The monitoring system 10 thus continually checks values of temperature and rate of change oftempera- ture in the reactor, and continually checks its own hardware and software, and it can be expected to fail-safe if anyfaults do occur. If it is not required to check values of rate of change oftemperature but onlyvalues of temperature, then the signal analyser 30 need not maintain a memory of the values ofthe temperaturesofthe sensors, andthe data link 42 can be dispensed with.

If it is important to minimise the chance ofsoftware or hardware faults causing control signals which indicate an unacceptable value oftemperature and consequently trigger an unnecessary alarm, then a monitoring system similartothatofFigure 1 maybe provided with replication (and hence redundancy) of its components.

In Figure 3,to which reference is now made, is shown a monitoring system 50 for a nuclear reactor (notshown),similarin manyrespectstothemonitor- ing system 10 of Figure 1, identical components being referred to byte same reference numerals. The monitoring system 50 includes four identical independentsub-systems 52, each sub-system 52 differing from the monitoring system 10 only in not including a pattern recognition unit 40. Thus each sub-system 52 includes a test signal generating system 22, a multiplexer20,andasignalanalyser30, and hasthirty-two input terminals 12 (only two are shown for each sub-system 52) to which leads carrying data signals from sensors (not shown) measuring a variety of physical parameters are connected.Each sub-system 52 includes a timer 44 (not shown), the timers 44 being independent of each other and not synchronised.

The output from each signal analyser 30 is connected to four inputs of four identical four-input vote units 54, so that each vote unit 54 receives one input from each signal analyser 30. Each vote unit 54 is a computer programmed to storethe sequence of digits received at each of its inputs, and initially to lookforthe first sixteen digit sequence of an expected pattern so as to synchronise the sequences of digits from the sub-systems 52. Once the sequences of digits are synchronised, then each subsequent digit is voted on, the vote unit 54 giving an output signal the same as the majority of the input signals received.

The outputs ofthe vote units 54 are connected to respective pattern recognition units 40, whose outputs are connected through respective pulse-to-d.c.

converters 56to a common vote unit 58.

Each physical parameter relating to operation of the nuclear reactor is measured by four identical sensors each connected to an equivalent input terminal 12 of a respective one of the four subsystems 52.

Each sub-system 52 operates in the same manner as the monitoring system 10Of Figure 1,thetest signal generator32 generating a pattern oftest signals which are superimposed on the data signals from the sensors by the adding-and-amplifying units 16, the resulting signals being supplied by the multiplexer 20 to the signal analyser 30. Thus each signal analyser 30 receives an independentlygenerated sequence of test signals and data signals relating to the same set of physical parameters.

Each signal analyser 30 stores a memory of the currentvalues of each ofthe physical parameters, and this information is supplied serially bythe data link 42 to the respective test signal generator 32. On receiving each signal from the respective multiplexer 20, each signal analyser 30 is programmed to carry outthefollowing operations: 1. calculate the value of the corresponding parameter; 2. determinewhetherthe value lies within predetermined acceptable limits; 3. updatethe memory of the value of the parameter if the value is acceptable; and 4. give an output signal "1" ifthevalue is acceptable and an output signal "0" ifthevalue is unacceptable.

Itwill be appreciated that for some parameters, such as pressure,the signal analyser may need to ultilise its memory ofthe values of some ofthe other measured parameters, such as temperature, in deter miningwhetherthevalueofthe parameter is acceptable.

Ifthere are no faults in hardware or software ofthe sub-systems 52 and if the sensors function correctly, the sequence of signals received by each signal analyser30, and hence the pattern of output signals from all foursignal analysers 30 will be identical. A change in a value of one ofthe physical parameters from an acceptable to an unacceptable value will produce identical changes in the pattern of output signals from the four signal analysers 30, while a fault in a sensor or in one ofthe sub-systems 52 will cause the pattern of output signals from the corresponding signal analyser 30to differfrom those from the other three signal analyser 30.

Thevote units 54 are programmed to give an output signal the same as any signal received atthree or more of their inputs, and so the sequence of signals supplied to each pattern recognition unit40will be unaffected by a fault in one ofthe sensors or in one of the sub-systems 52. The pattern recognition units 40 operate in a similar mannerto that described above in relation to Figure 1, producing an alternating output as long as the correcttest pattern is received, and a non-alternating output if any change in the pattern occurs. Achange in the pattern is unlikely to be due to faults in sensors or in the sub-systems 52, asfaults are unlikelyto affecttwo identical sensors or two sub-systems 52 simultaneously, and so any change in the pattern can be interpreted as signifying the deviation ofthe value of one ofthe parameters from its accepted range.

The pulse-to-d.c. converters 56 convert the alternating output of the respective pattern recognition units 40 to a direct current, and the vote unit 58 gives an output alarm signal ifanytwo of its inputs receive no current Thus an alarm signal is given if any two of the pattern recognition units 40 agreethatthe pattern has changed, signifying the value of one of the parameters being unacceptable.

Itwill be appreciated thatthe replication ofthe vote units 54 and pattern recognition units 40 avoids the chance of a false alarm due to a fault in one ofthe pattern recognition units 40 or one ofthe vote units 54, asthree such faults must occur at onceto produce a false alarm. It will also be appreciated that the degree of redundancy (four-fold in this case) of the vote units 54 and the associated pattern recognition units 40 need not be the same as the degree of redundancy ofthe sub-systems 52 (which is also four-fold in this case).

It will be understood that monitoring systems of the invention may be used to monitor data signals from different numbers of sensors to those aforedescribed. For example, one alternative monitoring system, for use in a chemical plant, differs from that of figure 1 in having a twenty-foour output demulti plexer36 and a twenty-four input multiplexer 20, data signals being provided to twenty-four adding-andamplifying units 16 bytwenty-foursensors. The system monitors values of eight parameters relating to operation ofthe plant, the value of each monitored parameter being calculated from signals received at three adjacent inputs of the multiplexer 20, i.e. from groups of three successive signals received by the signal analyser 30.One monitored parameter, for example, might be the volume of a rectangular container calculated from signals representing the lengths of its sides, and another monitored parameter might be the quantity of gas in a container, calculated from signals representing the gas pressure,temperature and volume.

In operation ofthe alternative monitoring system, for each scan of the multiplexer 20 the signal analyser 30 receives twenty-fou r sig nals from the multiplexer 20, and produces eight output signals signifying whetherthe monitored parameters are within acceptable limits (1) or not (0). The complete test pattern is generated in sixteen scans of the multiplexer 20, and is shown in Table 3, in which each column of digits represents the states in consecutive scans of one of the monitored parameters A, B, C, etc., as determined from the corresponding three signals a, bc; d, e, f; g, h, j;, etc. received from the adding-and-amplifying units 16.

Table 3

Scan No. Monitored parameters H G F E D C B A 1 0 0 0 1 1 1 0 1 2 0 0 1 1 1 0 1 0 3 0 1 1 1 0 1 0 0 4 1 1 1 0 1 0 0 0 5 1 I 0 1 0 0 0 1 6 1 0 I 0 0 0 1 1 7 0 1 0 0 0 1 1 1 8 1 0 0 0 1 1 1 0 9 0 0 0 1 1 1 0 1 10 1 0 0 0 1 1 1 0 11 O 1 0 0 0 1 1 1 12 1 0 1 0 0 0 1 1 13 1 1 0 1 0 0 0 1 14 1 1 1 0 1 0 0 0 15 0 1 1 1 0 1 0 0 16 0 0 1 1 1 0 1 0 The test signal generator 32 is programmed to supply test signals to be superimposed onto a different selection of data signals each scan so as to generate the test pattern of Table 3.Some ofthe test signals, designated plus test signals, are such asto makethe monitored parameter (calculated from the corresponding combined signal and two othersignalsfrom the multiplexer 20) exceed its maximum acceptable limit, while other test signals, designated minus test signals, are such as to make the monitored parameter (calculated from the corresponding com bined signal andtwo othersignalsfromthe multiplexer 20) be less than its minimum acceptable limit, and such that if a plus test signal is superimposed on one data signal and a minus test signal issimul- taneously superimposed on another data signal within the same group ofthree signals, such ask, I, the third signal being untested, then the monitored parameter should be within its acceptable limits.

The complete sequence of test signals to generate the test pattern of Table 3 is shown in Table 4, the letters a, b,c, etc. representing the signals received by the signal analyser 30, the symbol "+" signifying the superposition of a plus test signal on the corresponding data signal, and the symbol "-" a minus test signal.

Table 4

Monitored Scan Numbers Parameters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a a+ a a a+ a a+ a+ a a- a- a a- a a a A b b b+ b b- b+ b b+ b b- b b- b+ b b- b c c c c+ c c- c- c+ c c- c+ c+ c c- c c d+ d d+ d d d+ d d+ d- d- d d- d d d- d B e+ e e e+ e e- e+ e e- e e- e+ e e- e e f+ f f f f+ f f- f- f- f+ f+ f f- f f f g+ g+ g g+ g g g+ g g- g g- g g- g g c h h+ h h h+ h h- h+ h h- h+ h h- h h h j- j+ j j 3 j+ j j- j+ j+ j j- j j j j k k+ k+ k k+ k k k+ k k- k k k- k k- k D 1+ 1 1+ 1 1 1+ 1 l- l- 1+ 1 l- 1 1 l- 1 m- m- m+ m m m m+ m m+ m m- m m m m- m+ n+ n n+ n+ n n+ n n n- n n n- n n- n- n E p- p+ p p+ p p p+ p p+ p p- p p p- p p- q q- q- q+ q q q q+ q q- q q q q- q+ q+ r r+ r r+ r+ r r+ r r r r- r r- r- r r F s B- s+ a s+ s s s+ s s- s s s- s s- s+ t+ t t- t- t+ t t t t- t t t t- t+ t+ t u u u+ u u+ u+ u u+ u u- u u- u- u u- u G v+ v v- v+ v v+ v v v- v v v- v v- v+ v w w+ w w- w- w+ w w w w w w- w+ w+ w w x+ x x x+ x x+ x+ x x- x x- x- x x- x x H y y+ y y- y+ y y+ y y y y- y y- y+ y y z z z+ z z- z- z+ z z z z- z+ z+ z z- z It will be observed that within each group ofthree signals every possible combination of single plus and minustestsignals is applied once, combinations involving two simultaneous plus test signals ortwo simultaneous minustestsignals are not applied, three simultaneous plus signals or minus signals are each applied once, and on two occasions no test signals are applied to any of the signals in the group.

The signal analyser30 in this case is programmed to determine the average value overthe previous sixteen scans of each signal a, b,c, d, etc., and these average values are the values supplied to the test signal generator 32 bythe data link42. In other respects this monitoring system operates in a similar manner to that described with reference to Figure 1.

The reference pattern againstwhich the sequence of digits from the signal analyser30 are compared is generated by starting with the eight digits in the first row ofTable 3, and moving sequentially, one place at a time, eight places to the left and then eight places to the right. Itwill be understood thatthis monitoring system may be provided with redundancy in the same way as described above with reference to Figure 3.

Thetestpattern maytake a variety offormsas long as it is recognisable bythe pattern recognition unit 40 independently ofthe test signal generating system 22, and an alternative test pattern could be in the form of a pseudo-random binary sequence.

Although the invention has been described in relation to the monitoring of parameters of a reactor ora chemical plant, it may be used to monitor other plant or apparatus.

The test patterns described above and shown in Tables 2 and 3 have used a sequence of eight binary digits, but it will be appreciated that the use of eight digits is not essential, being chosen for convenience in programming the computers.

Claims (14)

1. Asystemformonitoring a plurality of data signals from a plant or apparatus, each data signal representing a parameter of the plant or apparatus, the system comprising, means for periodically examining the data signals and for providing output signals representing whether each data signal represents an acceptable or an unacceptable state ofthe respective parameter, and means for supplying test signals to be superimposed on selected said data signals before each periodic examination thereof such as to produce corresponding combined signals equivalent to data signals representing the unacceptable state, the selection ofthe data signals on which the test signals are superimposed being changedfor each periodic examination ofthe data signals within a predetermined sequence so that the examining means produces a recognisable pattern of said output signals.

2. A monitoring system as claimed in Claim 1 wherein each output signal representswhethera monitored variable determined from the values of a number ofthe data signals lies within acceptable limits or not,the number being atleasttwo.

3. A monitoring system as claimed in Claim 2 wherein the selection of data signals is such that, for at least one periodic examination, test signals are superimposed on two of the said number ofthe data signals and are such thatthe monitored variable should liewithintheacceptable limits.

4. A monitoring system as claimed in Claim 1, Claim 2 or Claim 3furthercomprising a memory of the values ofthe data signals, and wherein the test signal supply means includes a signal generating means arranged to generate the test signals in relation to the memory ofthe values of the data signals.

5. A monitoring system as claimed in any one of the preceding Claims wherein, after an integral number ofthe periodic examinations ofthe data signals,the pattern is repeated.

6. A monitoring system as claimed in Claim 5 arranged such that at least one test signal is superimposed on each data signal during the said numberofthe periodic examinations ofthe data signals.

7. A monitoring system as claimed in Claim 5 or Claim 6whereinthetestsignals superimposed on anyone ofthe data signals, during the said numberof the periodic examinations, are such that the mean value of the corresponding combined signals is equivalentto the value ofthe said one data signal.

8. A monitoring system as claimed in any one of the preceding Claims wherein the selection of data signals on which the test signals are superimposed is such as to cause a pattern of output signals, represented as a series ofdigits, which is of a form which can be generated from a sequence of n digits, successive groups of n digits being generated by sequentially shifting one place at a time n placesto the right and then n places to the left and at each shift moving the digit at the end moved awayfrom to the other end ofthe group.

9. A monitoring system as claimed in any one of the preceding Claims,furthercomprising a pattern recognition means arranged to receive the said output signals and to comparethe pattern with a reference pattern.

10. Apparatusformonitoring a plurality of parameters of a plant or apparatus comprising, a number of monitoring systems as claimed in any one of Claims 1 to 8, each parameter being represented by as many data signals as there are monitoring systems and each data signal being monitored buy a respective monitoring system, a voting means arranged to receive the output signals from each monitoring system and to produce a signal representing the majority value of the output signals received, and a pattern recognition means arranged to receive the signalsfromthevoting means and to comparethe pattern ofthe received signals with a reference pattern, and to provide a signal indicating whether the patternsarethesame.

11. An apparatus as claimed in Claim 10 comprising a plurality of voting means, each voting means being arranged to receive the output signals from each monitoring system; a plurality of pattern recognition means, each arranged to receive the signals from a respective voting means; and afinal voting means arranged to receive the signals from all the pattern recognition means.

12. Asystem for monitoring a pluralityofdata signals, substantially as hereinbefore described with referenceto, and as shown in, Figure 1 ofthe accompanying drawings, or modified as hereinbefore described with reference to, and as shown in, Figure 3 ofthe accompanying drawings.

13. A method of monitoring a plurality of data signalsfrom a plant or apparatus, each data signal representing a parameterofthe plantorapparatus, the method comprising periodically examining each data signal and providing an output signal representing whetherthe data signal represents an acceptable or an unacceptable state ofthe respective parameter, and superimposing test signals on selected said data signals before each periodic examination thereof such as to produce corresponding combined signals equivalent to data signals representing the unacceptable state, the selection of the data signals on which fhetest signals are superimposed being changed within a predetermined sequence so that a recognisable pattern of the output signals is produce.

14. A method of monitoring a plurality of data signals, substantially as hereinbefore described with referenceto Figures 1 and 2, or Figure 3, ofthe accompanNing drawings.