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Structure and Mode of Operation of a Computerized Process Control System for a Chemical Installation
STRUCTURE AND MODE OF OPERATION OF A COMPUTERIZED PROCESS CO:iTROL SYSTE,,! FOR A CHEMICAL INSTALLATION
M. Beger W. Koch Division Chief Engineer Senior Associate Engineer Control Engineering Division Kalle AG Wiesbaden-Biebrich, W. Germany
Abstract A DDC process control system is described, which was installed for the control ~nd monitoring of a foil production machine in a chemical plant. The structure and operation of the used process control system is drawn and the possibilities with regard to control, alarm and maximum rating monitorin~, manipulation and communication are shown. The algorithms used for the various types of control loops are presented and the _chosen sampling cycles explained. Furthermore a special solution for the control of pneumatic valves and the used bang-bang algorithm is introduced.
Introduction Constantly new possibilities for the application of plastic films in many variations are found and therefore foils with new or special properties must be developped. The production of modern plastic foils requires complicated methods of manufacturin~ which introduce new demands for the control technique used. Effective systems with a flexible and adaptable control strate~y are needed which often cannot be achieved by conventional means. The collection and evaluation of a ~reat number of data are a necessary requirement for the ~owing quality demands to which a product is subjected, and for an economical method of production. A computer is particularly suited for the collection of all informations about a process at a central controlling station. Its capacity and adaptability ensures a good total view over the entire process and the software ability and flexibility make an optimization of the process possible (10).
I. System Description and Capabilities of the Process Control Installation at the KALLE Plant 1. Previous Scone of the System In the sum~er of 1969 a FERRANTI process control computer "Argus 500" was installed at the KALLE company at Wiesbaden-Biebrich for the control and monitoring of a foil production machine which produces plastic film for various applications. A block schematic of the machine is shown in Fig. 1. The total system consists of 430 loops of which 290 are exclusively used for the collection of data, the lo~ging and balancing, as well as the monitoring of the alarms and the maximum ratings. Of the 140 "closed control loops" 45 are equipped with valve actuators of various kinds. The heaters of the two step action temperature loops are induction and resistance heaters. Including these temperature loops the computer has to control a total of 95 loops in a bang-bang mode. Only this portion of the installation has a back-up control. This ensures that in case of a severe disturbance in the computer system a freezing of the melted plastic in the extruder is prevented. The subsequent portions of the machine do not have a backup control, and therefore the proper production cannot be maintained in case of break-down of the comnuter system. ¥.owever in such a case the operator is in the nosition to manually control each valve from a special control desk. 2. :4onitoring of Alarms and i-!aximum Ratings An essential aspect for the application of a process control computer is its capability to provide monitoring of the alarms and maximum ratings in an extensive, central, effective, flexible and documentative way. Frequently this by itself is enough reason for the application of a computer. Such an initial system often constitutes the first step in the direction towards a more comprehensive system, which can perform the task of process control and other more sophisticated objectives. Basicly 2 different groups of alarms can be distinguished. 1: Analogue measuring data and their derivatives.
399
2: Pure digital data. In the KALLE system all analogue loops are examined for the following limits.
determined and cO!lloared with a desired reference value in accordance with the computer output. In checkinp a deviation from a reference value the ener~izinl': time of the resoective actuator is taken into consideration ani a deviation is evaluated to produce an alarm only after a oreset time. This enables an immediate reco~ition of a fault in the operation of relays, po...er-plugs, heaters and solenoid valves, avoiding a delayed re sponse through the control loop feedback. Thereby a severe deterioration of the quality of the product can be nrevented in case of a fault, and often the maintenance time can be drastically reduced.
2.1.1 Deviations Each control loop is checked with regard to deviations presetable to any extent by the operator. The tolerance is symmetrical to the selected set poi nt, and is therefore automatically superimposed on any selected set point. 2.1.2 Upper and Lower Disturbance Limit Both limits are independently selectable for all loops and monitoring circuits and are preset on the basis of physical units. An overlappin? of t~e two limit s or selection of a set point outside these limits in the case of a control loo, is prevented.
All fault- and alarm signals are printed out on the alarm printer. The printout contain s time, loop identification, tYDe of fault, and is printed out in "red" UDon the transition into alarm condition. Transition.lpon return into the normal operating condition is printed in "black".
2 .1.3 Upper and Lower Alarm Limit A second pair of limits can be introduced. In chemical proces ses this is frequently required in order to provide a possibility to perform essential safety measures in case that one of the limit s is exceeded. Therefore these special limits should be selectable only outside the normal limits. Furthermore for a limited number of loo ps the gradient of the measured value can be detected.
3.
Loggin ~
Facilities
In the follo,Tin!,;, some of the more imoortant available data lo ggin~ facilities ...ill be briefly explained. For data logging purposes the system contains a special log~inp, printer. All logging data can be demanded by the operator through a simple keyboard operation. The colour of the printout sirnifies the condition of the loop.
2.1.4 Life Reference Point Whenever possible each input was provided with a life reference point. A thresho ld value defined by the software permits the examination of a measured signal for credibility. In case of failure or of an open si/1:nal line a quick dial':nosis is made possible and in control loops a wrong calculation of the output signal is prevented.
3.1 Aeasured Value Loe: The measured value log prints upon request all measured and calculated values important for the production . Frequently the operator needs only information fro!ll a portion of the system. Therefore the foil machine has been subdivided into 8 different p,roups, which can be logged separately upon demand.
All different l imi ts described here have the common property , that in case the limit is exceeded by the measured value auto~atically a reduction of the ran ge of these li~its will take place. This measure prevents, that an oscillation of the measured data around the limit then therefore causes a repetition of the alarm printout . Upon return of the measured value to the reduced limits these limits are returne1 to their ori'ginal levels.
3.2 Alarm Lol'; The alarm lo~ Drints information of all 100P3 beinp, in an alarm condition. This lo~ provides to the operator an indication of any difficulties in the Dresent production condition, ...hich may be an important factor when operators are changed at ...orkshift ti"le. 3.3 Salance Lo g The balance log is periodically called for automatically. It nrovides the management ... ith a record for the energie s used as well as the amounts of ra... material consQmed and foil produced. In case of a production break down for instance because of a foil breakage, balancing is suporessed until a defined length of foil has been produced .
2.2 DiE!ital Alarms
2 .2.1 Disturbance Contact Alarm3 These alarms comprise limit switches and proximity switches. An interruption in a 3i ,~al line is also reco gnized as an alarm situation . 2.2.2 Check Contact Alar~s An effective evaluation of this category of alarms would be very difficult without the use of a comput er. The KALLE s:r ste!ll contains a great number of bang-bang control loops with actuators consisting of solenoid valves, relays and power semiconductors. Through the use of limit switches, rotation monitors, discriminatin~ relays, etc., the operatin~ condition of an actuator (on-off, open-closed) is
4. Description and Capabilities of the Software Package "Consul B" The soft ...are Dackap:e used differs only in a fe ... minor details ' from the package mean ...hile developped by FERRANTI known as standard package "Consul B" , and allows all operations listed
400
Drie ARGUS 500
/
/
I
Casting
Wind-up
Orientation and Heat Set
Scope: Analogue Inputs 430 Digital Innuts 384 Fast Pulse Inputs 64 Incremental Outputs: 48 Digital Outputs 112
Figure 1.
Block Schematic of a Ferranti Process Control Computer "Argus 500" and the Scope of the System.
Static Lists
Dynamic Lists
_ __ -I
- - - 4 - - - - _ -...J
Key:
o Program Cl _ ---
Data Activation Pointer Data transfer
Figure 2. Relationship Between Data Lists (Re: Ferranti User Specification).
401
Preset Lists
below in the same way as "Consul B" does. On-line can be done: a.
alarm monitorine of analogue inputs a store savinp. version can be used. These so called "read only loops" cannot be used in connections with algorithms.
Establishin~, changing, or deleting of control loops of all kind.
All informations about the individual measurin g and control loons which are made available to the cOMputer through the "Consul B" panel, are stored in a se quence of li s ts wit h in the operating system. These lists contain for eac h loop the required and desired data, such as type of the loop ("control" loop or "read only" loop), loop identification, kind of operation (Auto/~anual), format of the di solay , the physical un i t, measurin ~ ran ~ e, set noint, process parameter, limits, sequence and t :rpe of the al~orithms used. These different lists (see Fi ~ . 2 ) can be divided into three groups.
b. Opening and closin~ of cascade control loon switches, nrovidin~ a bumpless transfer. c. Activation of all possible loop states. d. Display and access of process and control narameters. e. Interface with f. Alarm
Auto/t~anual
monitorin~
g. Data logein~ and values. h. Selective gra:ns.
back-up stations.
of all looos. recordin~
initializin~
of all measured
4.1 Predefined Lists In each case the length of all lists must be determined during the development of the so f tware, becaus e it is de p endin ~ on the ha rdware configuration. In addition the li s ts have to be establ i s hed for the lineariz ation and s calin g o f the analorue inputs and the valve out outs, as they cont a in th e c orres ponding constants for the different t ypes o f sen so rs, si gnal levels, actuator sensitivitie s , a nd energ izing time s .
of all users pro-
"Consul B" is an operating system with hi ph degree of flexibility and universal application. It was s pecially developped for application in the field of "data loggin g" and "direct digital control". tor a majority of a great variety of control loop calculations, a combination of standard al gorithms can be used. Each algorithm can be applied unchanged in various control loops in an unrestricted sequence, whereby corresponding data are applied to the aooronriate loons. This means that normally the d~ta ~re differ~nt for each loo p , while the progra'!l of the al gorithm remains the same.
4.2 Lists to be Established from the "Consul
:3"
Panel
~~g~!_~~~_~~~~~~~_~!~!_i~~l Each measuring and control 1000 has access to thi s list, in wh ich the loo n identific a tion (G c har a cters, i. e. ~ T 1234), t he s cal i n ~ f a ctors for the measurin g ran r e, the format , a nd th e nhy sic a l un it of t h e loop have b e en stored . In a dd ition t h i s li s t cont ai n s t h e s'Lmpling ner iod , a nd t he entry po int to th e next list.
As the storage requirement for "Consul B" is relatively small, the package may be used without any external background store. We feel that all necessary pro grams ~used for the control and the basic operation s of a production process (like for ins tance alarm monitoring ) should be core resident. The production of an installation workin ~ on a 24 hours basis is not be dependin ~ on electromechanical units. The production facilit y should be able to continue operation even if an externally connected store should fail. In such a case some restrictions in the data lo ~q ing capability could be considered acceptable.
~~g~g_b22~_ Q~fi~ili2~_1i§1_i12bl In the cas e of a control loo p t he pro q,ram will here find t he t yp e and t he s equenc e o f the al ~orithm s us ed a nd an ind icat i on wh er e t he appro priate d ata for t ho se a l gorith'!ls can be fou nd.
a. The collection of meas urin g data.
~~g~3_~be2ri!~2_2~!~_bi§! This lis t c ontain s th e data fo r the a l ~o rit hms us ed f or a loo p , and contain s t he cha in f or ward mod i f ie r fo r the lineari zed in put val ues and t he calc ul at ed out put dat a .
b. Calculation of control data (depending on the current value and perhaps other inputs or data in store . )
~e linearized innut and out nut data are stored in the " !)yna'Tl ic Lists". 'l'!1ose dynamic li s t s will not be explained in more detail.
c. Transmission of the calculated outout to the actuator or a secondary loop.
5. The " Consul
To perform a computer control basicly three sequences of operations are necessary.
Although "Consul B" is oriented towards process control, such control can only be achieved if the last algorithm in a chain of possible al gorithms is a control algorithm. Therefore "Consul B" is applicable also for a mere analysis of process data (no control algorithm necessary). Just for
~"
Onerator' s ? anel and Saf e.-:uar dinr:
Th rou ~h the opera tor' s panel, shown i n ? i ~ . 3 , online corununication of t he ooerat or '.i th t he nroce ss is p erformed. As a co~and in p~ t a keyboard i s available t o him whic h is divid ed un acc ordinu, t o the different functional tasks . Out nut fro~ t he co~!,uter i s sh01>T. on three d is pl ay ~n i t s ani t'1 e
402
printer. All input data originated by the operator are ?rinted out, namely time, kind of oneration and the old and ne~ par~eters. This way the operator can always check his inp~t and keep a record. Operations include set point and limit chanRes, openinp, and closin~ of cascade looos, ~anual to auto~atic 1000 mode and vice versa, demandinp, 10p,5, initiating s~ecial user's orov,r~~s (such as run UP and run down pro~rams), position control of actuators, and accepting or supDressin~ of alarm nrintouts. Possibility is given to establish new control looos on-line through the ?anel, or to establish, change, or delete control 10005.
used for couplinp. between the computer output and the actuator in order to maintain ~alvanic senaration between the CO'!lDuter and t'le nower electronic circuit3. 1. 2 Actuators '.oIi th Analo~e Innut Control Analon:ue actuators are usually often pneumatic valves (with or without stuffinp, box) and motor valve type. As an output system for those actuators a system develoooed by 1(E'IT was selected. In formation transfer from the comouter to the process is done in this system thro~gh a ~ain path addressinp system, which permits addressing and control of 512 valves. Commands for incremental movements of the valves in both directions can be given at a maxim~m definition of 6 bits, whereby the data in this system are buffered and sent out automatically. A preat flexibility and red~ced computer time are the main advantages of this output system.
To safeguard the process ap,ainst accidental, or inproper internal measures, which could lead to critical conditions in the process a security feature has been provided. This feature p.ives the possibility to lock all keys with three different switches. The lockinp, is done by the software, and therefore very flexible and easily adaptable to special requirements. A suggestion for a oriority list is shown below.
In chemical olants oneumatic valves are used prebecause they are preferable to all other actuators with regard to price, ruggedness and reliability. One possibility to control the valves is to orovide an analogue output from the computer. This could mean however, that in case of a failure of the output system the valve position cannot be maintained over a long period of time. At ~ALLE an incremental output is therefore used to drive a stepper ~otor of an electro-pneumatic positioner. do~inantly,
1. Priority: Control Engineer (Lockin~ of all keys, which have access to the control syste~.) 2. Priority: Production t~anap:er (Locking of certain alarm limits). 3. Priority: Operator (Lockin~ of all other keys apainst unauthorized interference).
Although electro-pneumatic positionersare offered by the industry, Kalle decided to develop their own version. The units available on the market were relatively expensive and to some degree did not meet the technical requirements demanded by KALL3. The ~odification performed in the mechanical and pneumatic construction of the very simple pneumatic positioner consists of the installation of an additional pullinp, sprin~ on the flapoer beam. A connected stepper ~otor acts through a reduction ~ear upon a tension band tied to the spring. This way the turnin~ an~le of the p,ear is transformed into a linear ~ove~ent of the flapper beam, proportional to the spring force and therefore into a nronortional air pressure. In steady condition (no increments to the stepper motor, or in case of break down of the electronic) the motor and the valve remain solidly in their previous position. The total ran~e of travel was divided into 200 steos (sensitivity 0.5 ~) and a linearity of smaller than 1 ':' was achieved. A potentiorneter mounted on the Dositioner serves to provide the position feedback of the valve.
An on-line function test proeram is entered periodically, which will provide isolation of the computer system from the process in case of a co~ put er fault.
11. Direct Digital Control For the direct digital control the 1000 between computer and process is finally closed. All information regardinp, the process is made available to the computer not only on-line, but the computer acts directly without intermediate action of conventional control members upon the process. About the problems, advantages and possibili ties of direct di~ital control m'.lch has been reported in literature. The follo~ing exolanations shall only show, which basic considerations were applied and how the control system was desi2TIed at KALLE.
=
2. Selection of Samnling Periods 1. Selection of Appropriate Control for the Various Tyoes of Actuators
The selection of a proper sampling period is an essential step for control systems with a variety of control loops. Is the samnlin~ rate to hi p,h computer time is g;i ven a',lay, furthermore the samplin'S rate of the innut system is limited by its ha~dware configuration. The limitation is mainly determined by electro-mechanical switches (mechanical scanner, relays) and amounts normall:{ to 100 till 200 samples oer second. In control loops with
1.1 Actuators with 'I"... o-Steo Action This type is controlled from the comouter through a dipital out out (mercury wetted reed relay or semiconductor-switch). Is a semiconductor device used as the actuator (thyristor, triac), the control can be done directly from the computer. Where semiconductor outputs were used, a transformer was
403
mere dead time Tt, too high a sampling rate can cause stability problems. In the corresponding literature Lee (5) has mentioned for the choice of samplin~ interva13 the expression:
It is obvious and ~enerally known that too small a samplin~ rate can lead to a poor representation of the measurin ~ si~nal, and therefore to a reduction of the quality of the control (4). 'Nhen selecting a samnlin!7, period. it must be considered, that it is not possible to allow a specific sampling rate for each loop. Tt will be necessary to combine control loops into !7,rou9s with the same samplinp: interval, as the technical requirements (i. e. mechanical scanner with fixed sampling rates) must be taken into consideration, and that the soft,rare effort must remaln '.d thin reasonable limits. "Consul B" provides 8 different sa'llplinl:' rates, whereby all long sampling periods are based on the followin g equation: c • 2 n [2]
with Ts = sampling time in seconds, c means a presetable constant, i. e. 0.5 in our case, and n is an integer from 0 to 7.
3. Incremental Control and Algorithms Used 1.1 Incremental Control A control, '.hereby the absoLlte value of the actuator outuut is calculated from tile deviation (for example in conventional control), is quite different fro'll incremental control. The latter does not permit for i nstance a P'lre pronortional control, because a pronortional action would be calculated only in case of a ~radient deviation. Therefore a steady deviation uould not cause proportional action. An incremental PI0 al~orit~m therefore calculates in reality ·'3.n output, which corresponds to the differentiated solution of a "nor'llal" control calculation (compare equations [lJ and [2J in the a pgendix). In order to comnensate for the detremental effect of a double differentiation (this could cause, unintentionally, large changes of the valve action even with minute changes of the measurin~ value) the inp'lt value will be s'!loothed. Furthermore weip:htinp: factors are introduced and applied to the different samplin?s of the input value. A limiting of the out put value to be sent out in preset time i s also required for the achiev~ '!lent of a satisfactory control quality. In addition the KP~LE system takes into consideration the valve position feedback, although this is normally not required for an incre'!lental control system. However, the feedback of the valve position prevents the output of increments to the valve in the case of a valve end position. Therefore no increments are lost and the quality of the control is improved ('3, 7).
A common relation for the selection of samplin~ periods Ts 13 quoted by Lee (5) as follows (8): 1 > Ts ~ 1 8nE - 16nE
[3]
with a resonance frequency nE of the control loop. However this formula see~s to result in shorter samplin g periods than have been realized in various installations. The following expression is frequently stated as a rule of thumb: Tl 5< ~ 10 [4] - Ts whereby Tl is the :nodel time constant of a representative first order lag element. These state'Ilents lie within t he range stated by Terao (9) simulatin ~ technical control loops.
3.2 Used Alp:orithms Is a control to be performed a measuring value is first transr.Ji tted to the SP"W alp;ori thm. This algorithm evaluates the error, that means the difference between t he set point and the measured ~alue, and also serves the ~larm monitori;g . The calculated error can now be made available to a seq'lence of further algorithms, whereby the last one must be a control algorithm (i. e. PI, PID, etc.). The required increments are here calculated, which subsequently are adapted to the individual valves in an output algorithm (valve algorithh, which scales the output for the different amounts of travel and end to end travel time of each valve). The scaled output information is stored in the output list and then transmitted to the individual valves throup:h a separate output program. In the KALLE system a hysteresis algorithm (for level control) and a dead band al gorithm (for air flow control) were used besides PI and PID(1,6,11) al~orithms. Because these al s orithms are available also for conventional control, they will here not be explained in more detail.
At KALLE the followin ~ sampling rates were chosen for the control:
0.5
sec: Supervisory control of speeds, pressure control.
1 and 2 sec : Steam pressure control.
4
sec: Level control and air flow control.
8
sec: Control of fast temperat'lre loops.
32
sec : All other temperat'.lre control loops.
3.3 On-off PI Control For the ~eat number of similar bang-bang temperature control loops an appropriate algorithm had to be found. Starting at a normal control al~orithm (PI or PID), a special output algorithm for digital outputs was developped, which is based on the followin~ considerations (2).
404
If the selected samplinp, rate (in our case 32 sec.) reflects the temperature trend without gaps, the heater need not be switched on or off more frequently than once per sampling period. Higher switching rates would not produce any furt~er improvements of the control loop because of its low pass characteristics. Is the output value stored in the output list larger than 31, this means that the heater is turned on a 100 %of the time interval between the samples, is the value smaller than zero, then the heater is switched off a 100 %of the time. Output values in between cause the on/ off ratio of the electrical heater. The output list for the bang-ban~ loops is similar to that one for continuous loops. However, the scaled values of the output list are not decremented having been sent out, but are allowed to integrate. Therefore this list assumes the function of a valve in a "normal" control loop. In order to ensure, that the measured values are evaluated as exactly as possible for the fixed accuracy of the A/D converter of 11 bits + sip,n, measuring ranges with suppressed zero noint (in this case from 150 to 350 degree centigrade) were selected. Is the equation of an incremental PI algorithm considered as an example Ay = 100 [AE
+ Ts En) [5) (see Appendix) PB n TI it follows, that in the process of start up heating only integral action takes place until the measured value exceeds the lower limit of the range. Afterwards proportional action reduces the value stored in the output list. In t~e ideal case the loop should have reached its final switchin", rate, when the measured value corresponds to the desired set point. For this reason the output al~orithm limits, in contrast with nor~al incremental control, the amount of the value in the output list in order to approximate the ideal demand as much as possible. In general this limit is about + 30 %of the out nut range and in addition to those figures mentioned previously. Appendix
It is to be mentioned, that the output program causes the switch commands to be delayed in order to nrevent a simultaneous start of all heaters at the - sa~e moment (not more than 10 heaters at the same time). Thereby extreme current peaks in the power supply are prevented.
Conclusion The quality of control has been improved for many loops (for examnle the control parameters are decoupled in the computer and available in large ranges). However, by the apnlication of more sophisticated and process oriented algorithms improv~ ments can still be achieved • It appears to be of even greater importance, that the computer permits the possibility of on-line optimization and adaptation to changinv process conditions. Furthermore the computer provides the capability of a more co~nrehensive monitoring of the processes. The multitude of measured and collected data makes it possible to recognize mutual effects and to achieve a more thorough understanding of process related aspec1:.s. For this reason it is intended to extend the system described above. It is the goal of these efforts to achieve, that a multitude of higher order programs can be run during the normal on-line operatio~ and that these programs have access to the important control parameters.
The proportional and the integral action terms are straiv,htforward calculations.
Derivation of the Incremental PID Algorithm The equation of an ideal conventional controller is:
[1) The used incremental digital equivalent is:
Q~riY~1iY~_~£1i2E~
If s is the differential onerator, then for TA« TS ys tell anproximately is: dE En - E _ -dt = s·E = TA n l whereby TA= sampling period. Using a "smoothed derivative", E' will be: _ sE
[3)
E' =l+Tn S
[4)
.
[2)
whereby Ko, K_l, Kl are constants; Y = absolute output; 4 Y = incremental output; n = mOMentary sample; E = error (difference between set noint and measured value); E = smoothed error; Y' (d) the total of the past incremental outputs n-l caused oy derivative action.
Tn
Wlth A = T +T and TD = constant, and usin~ the A! D " for the error E, the final solusame "smoothing tion for the digital derivative term will be:
.U~d)
[I-A) {KI (I-A)(En - En-I) -
Y~~i}
[5)or [5a)
405
Referen ces (1) (2)
(3) (4)
(5) (6)
R. R. De Bolt, B. E. Powell , "A Natural 3-Mode Contro ller Algorit hm for DDC", I.S.A. Journa l, Vol. 13, No. 9, p. 43. D. Briggs, "Exper iences with Increm ental Control at Kalle AG", Ferran ti Departm ental Memo, Ferran ti Systems Divisio n Group, '4arch 1969. T. Ingham, D. Briggs , "Some Feature s on Increm ental Contro l", Ferran ti t4emo, Ferran ti Systems Divisio n Group, March 1969. H. Kaufmann, "Dynam ische Vorgaen se in lineare n Systeme n der Nachric hten- und Regelungste chnik" , Verlag R. Oldenbo urg Muenchen, 1959, p. 156 . W. T. Lee, "Plant negulat ion by On-line Di gital Comput ers", SIT Symposium on Direct Digita l Contro l, April 1965. R. Schilba ch, 14. Pandit, W. Weber, "Prozed uren zur Lineari sierunl t. lineare r DDC Algorit hmen" ', Heg;elu ngstech nik, Vol. 16, :-;0. 8, p. 337.
--
(7)
A. Schoen e, "Regelg uete und Stabil itaet von mit Prozess rechner n bei Beschraen kung der Stellgr oessen aender ung", Regelu ngstech nik, Vol.17 , No. 10, p. 447. (8) A. Schoen e, "Prozes srechen systeme der Verfahren slndus trie", Carl Hauser Verlag , ~1uenchen, 1969, p. 128. (9) :4. Terao, "Quant ization and Samplinp; Selecti on for Efficie nt DDC", Instrum entatio n Techno logy , Vol. 14, No. 8, p. 49. (10) A. Thompson, "Opera ting Experie nce with Direct Digita l Contro l", confere nce paper, IFAC/IF IP confere nce, Stockho lm 1964. (11) J. Hellium s, T. J. William s, R. S. Banks, G. J. Kirk, "A Practic al Spectru m of DDC Chemic al-Proc esses Contro l Algorit hms", I.S.A. Journa l, Vol. 13, No. 10, p. 65. Re~elsystemen
---
-e
-
Figure 3.
"Consul B" M::Jni tor Panel.
406
_•
........
Discussioo
01
Paper XI:4 by M. Beger, W. Koch
D. J. Fraade Switzerland: What has been the system avaUabihty (up time)? When did the 77 hours of downtime occur? W. Koch : It should be nentiooed that the 77 hours of down time include all input/output equipment, besides the two printers. In case of a failing printer, full control capability and q>erating facility through the panel is still given. Dc:Mltime: 77 hours; Total Working Time: 14808 hours; Availabili ty: 99.480%. More than 75% of the downtime occured during the first six (6) mooths after starting the oo-line DOC systeJl5.
407
M. Beger W. Koch Division Chief Engineer Senior Associate Engineer Control Engineering Division Kalle AG Wiesbaden-Biebrich, W. Germany
Abstract A DDC process control system is described, which was installed for the control ~nd monitoring of a foil production machine in a chemical plant. The structure and operation of the used process control system is drawn and the possibilities with regard to control, alarm and maximum rating monitorin~, manipulation and communication are shown. The algorithms used for the various types of control loops are presented and the _chosen sampling cycles explained. Furthermore a special solution for the control of pneumatic valves and the used bang-bang algorithm is introduced.
Introduction Constantly new possibilities for the application of plastic films in many variations are found and therefore foils with new or special properties must be developped. The production of modern plastic foils requires complicated methods of manufacturin~ which introduce new demands for the control technique used. Effective systems with a flexible and adaptable control strate~y are needed which often cannot be achieved by conventional means. The collection and evaluation of a ~reat number of data are a necessary requirement for the ~owing quality demands to which a product is subjected, and for an economical method of production. A computer is particularly suited for the collection of all informations about a process at a central controlling station. Its capacity and adaptability ensures a good total view over the entire process and the software ability and flexibility make an optimization of the process possible (10).
I. System Description and Capabilities of the Process Control Installation at the KALLE Plant 1. Previous Scone of the System In the sum~er of 1969 a FERRANTI process control computer "Argus 500" was installed at the KALLE company at Wiesbaden-Biebrich for the control and monitoring of a foil production machine which produces plastic film for various applications. A block schematic of the machine is shown in Fig. 1. The total system consists of 430 loops of which 290 are exclusively used for the collection of data, the lo~ging and balancing, as well as the monitoring of the alarms and the maximum ratings. Of the 140 "closed control loops" 45 are equipped with valve actuators of various kinds. The heaters of the two step action temperature loops are induction and resistance heaters. Including these temperature loops the computer has to control a total of 95 loops in a bang-bang mode. Only this portion of the installation has a back-up control. This ensures that in case of a severe disturbance in the computer system a freezing of the melted plastic in the extruder is prevented. The subsequent portions of the machine do not have a backup control, and therefore the proper production cannot be maintained in case of break-down of the comnuter system. ¥.owever in such a case the operator is in the nosition to manually control each valve from a special control desk. 2. :4onitoring of Alarms and i-!aximum Ratings An essential aspect for the application of a process control computer is its capability to provide monitoring of the alarms and maximum ratings in an extensive, central, effective, flexible and documentative way. Frequently this by itself is enough reason for the application of a computer. Such an initial system often constitutes the first step in the direction towards a more comprehensive system, which can perform the task of process control and other more sophisticated objectives. Basicly 2 different groups of alarms can be distinguished. 1: Analogue measuring data and their derivatives.
399
2: Pure digital data. In the KALLE system all analogue loops are examined for the following limits.
determined and cO!lloared with a desired reference value in accordance with the computer output. In checkinp a deviation from a reference value the ener~izinl': time of the resoective actuator is taken into consideration ani a deviation is evaluated to produce an alarm only after a oreset time. This enables an immediate reco~ition of a fault in the operation of relays, po...er-plugs, heaters and solenoid valves, avoiding a delayed re sponse through the control loop feedback. Thereby a severe deterioration of the quality of the product can be nrevented in case of a fault, and often the maintenance time can be drastically reduced.
2.1.1 Deviations Each control loop is checked with regard to deviations presetable to any extent by the operator. The tolerance is symmetrical to the selected set poi nt, and is therefore automatically superimposed on any selected set point. 2.1.2 Upper and Lower Disturbance Limit Both limits are independently selectable for all loops and monitoring circuits and are preset on the basis of physical units. An overlappin? of t~e two limit s or selection of a set point outside these limits in the case of a control loo, is prevented.
All fault- and alarm signals are printed out on the alarm printer. The printout contain s time, loop identification, tYDe of fault, and is printed out in "red" UDon the transition into alarm condition. Transition.lpon return into the normal operating condition is printed in "black".
2 .1.3 Upper and Lower Alarm Limit A second pair of limits can be introduced. In chemical proces ses this is frequently required in order to provide a possibility to perform essential safety measures in case that one of the limit s is exceeded. Therefore these special limits should be selectable only outside the normal limits. Furthermore for a limited number of loo ps the gradient of the measured value can be detected.
3.
Loggin ~
Facilities
In the follo,Tin!,;, some of the more imoortant available data lo ggin~ facilities ...ill be briefly explained. For data logging purposes the system contains a special log~inp, printer. All logging data can be demanded by the operator through a simple keyboard operation. The colour of the printout sirnifies the condition of the loop.
2.1.4 Life Reference Point Whenever possible each input was provided with a life reference point. A thresho ld value defined by the software permits the examination of a measured signal for credibility. In case of failure or of an open si/1:nal line a quick dial':nosis is made possible and in control loops a wrong calculation of the output signal is prevented.
3.1 Aeasured Value Loe: The measured value log prints upon request all measured and calculated values important for the production . Frequently the operator needs only information fro!ll a portion of the system. Therefore the foil machine has been subdivided into 8 different p,roups, which can be logged separately upon demand.
All different l imi ts described here have the common property , that in case the limit is exceeded by the measured value auto~atically a reduction of the ran ge of these li~its will take place. This measure prevents, that an oscillation of the measured data around the limit then therefore causes a repetition of the alarm printout . Upon return of the measured value to the reduced limits these limits are returne1 to their ori'ginal levels.
3.2 Alarm Lol'; The alarm lo~ Drints information of all 100P3 beinp, in an alarm condition. This lo~ provides to the operator an indication of any difficulties in the Dresent production condition, ...hich may be an important factor when operators are changed at ...orkshift ti"le. 3.3 Salance Lo g The balance log is periodically called for automatically. It nrovides the management ... ith a record for the energie s used as well as the amounts of ra... material consQmed and foil produced. In case of a production break down for instance because of a foil breakage, balancing is suporessed until a defined length of foil has been produced .
2.2 DiE!ital Alarms
2 .2.1 Disturbance Contact Alarm3 These alarms comprise limit switches and proximity switches. An interruption in a 3i ,~al line is also reco gnized as an alarm situation . 2.2.2 Check Contact Alar~s An effective evaluation of this category of alarms would be very difficult without the use of a comput er. The KALLE s:r ste!ll contains a great number of bang-bang control loops with actuators consisting of solenoid valves, relays and power semiconductors. Through the use of limit switches, rotation monitors, discriminatin~ relays, etc., the operatin~ condition of an actuator (on-off, open-closed) is
4. Description and Capabilities of the Software Package "Consul B" The soft ...are Dackap:e used differs only in a fe ... minor details ' from the package mean ...hile developped by FERRANTI known as standard package "Consul B" , and allows all operations listed
400
Drie ARGUS 500
/
/
I
Casting
Wind-up
Orientation and Heat Set
Scope: Analogue Inputs 430 Digital Innuts 384 Fast Pulse Inputs 64 Incremental Outputs: 48 Digital Outputs 112
Figure 1.
Block Schematic of a Ferranti Process Control Computer "Argus 500" and the Scope of the System.
Static Lists
Dynamic Lists
_ __ -I
- - - 4 - - - - _ -...J
Key:
o Program Cl _ ---
Data Activation Pointer Data transfer
Figure 2. Relationship Between Data Lists (Re: Ferranti User Specification).
401
Preset Lists
below in the same way as "Consul B" does. On-line can be done: a.
alarm monitorine of analogue inputs a store savinp. version can be used. These so called "read only loops" cannot be used in connections with algorithms.
Establishin~, changing, or deleting of control loops of all kind.
All informations about the individual measurin g and control loons which are made available to the cOMputer through the "Consul B" panel, are stored in a se quence of li s ts wit h in the operating system. These lists contain for eac h loop the required and desired data, such as type of the loop ("control" loop or "read only" loop), loop identification, kind of operation (Auto/~anual), format of the di solay , the physical un i t, measurin ~ ran ~ e, set noint, process parameter, limits, sequence and t :rpe of the al~orithms used. These different lists (see Fi ~ . 2 ) can be divided into three groups.
b. Opening and closin~ of cascade control loon switches, nrovidin~ a bumpless transfer. c. Activation of all possible loop states. d. Display and access of process and control narameters. e. Interface with f. Alarm
Auto/t~anual
monitorin~
g. Data logein~ and values. h. Selective gra:ns.
back-up stations.
of all looos. recordin~
initializin~
of all measured
4.1 Predefined Lists In each case the length of all lists must be determined during the development of the so f tware, becaus e it is de p endin ~ on the ha rdware configuration. In addition the li s ts have to be establ i s hed for the lineariz ation and s calin g o f the analorue inputs and the valve out outs, as they cont a in th e c orres ponding constants for the different t ypes o f sen so rs, si gnal levels, actuator sensitivitie s , a nd energ izing time s .
of all users pro-
"Consul B" is an operating system with hi ph degree of flexibility and universal application. It was s pecially developped for application in the field of "data loggin g" and "direct digital control". tor a majority of a great variety of control loop calculations, a combination of standard al gorithms can be used. Each algorithm can be applied unchanged in various control loops in an unrestricted sequence, whereby corresponding data are applied to the aooronriate loons. This means that normally the d~ta ~re differ~nt for each loo p , while the progra'!l of the al gorithm remains the same.
4.2 Lists to be Established from the "Consul
:3"
Panel
~~g~!_~~~_~~~~~~~_~!~!_i~~l Each measuring and control 1000 has access to thi s list, in wh ich the loo n identific a tion (G c har a cters, i. e. ~ T 1234), t he s cal i n ~ f a ctors for the measurin g ran r e, the format , a nd th e nhy sic a l un it of t h e loop have b e en stored . In a dd ition t h i s li s t cont ai n s t h e s'Lmpling ner iod , a nd t he entry po int to th e next list.
As the storage requirement for "Consul B" is relatively small, the package may be used without any external background store. We feel that all necessary pro grams ~used for the control and the basic operation s of a production process (like for ins tance alarm monitoring ) should be core resident. The production of an installation workin ~ on a 24 hours basis is not be dependin ~ on electromechanical units. The production facilit y should be able to continue operation even if an externally connected store should fail. In such a case some restrictions in the data lo ~q ing capability could be considered acceptable.
~~g~g_b22~_ Q~fi~ili2~_1i§1_i12bl In the cas e of a control loo p t he pro q,ram will here find t he t yp e and t he s equenc e o f the al ~orithm s us ed a nd an ind icat i on wh er e t he appro priate d ata for t ho se a l gorith'!ls can be fou nd.
a. The collection of meas urin g data.
~~g~3_~be2ri!~2_2~!~_bi§! This lis t c ontain s th e data fo r the a l ~o rit hms us ed f or a loo p , and contain s t he cha in f or ward mod i f ie r fo r the lineari zed in put val ues and t he calc ul at ed out put dat a .
b. Calculation of control data (depending on the current value and perhaps other inputs or data in store . )
~e linearized innut and out nut data are stored in the " !)yna'Tl ic Lists". 'l'!1ose dynamic li s t s will not be explained in more detail.
c. Transmission of the calculated outout to the actuator or a secondary loop.
5. The " Consul
To perform a computer control basicly three sequences of operations are necessary.
Although "Consul B" is oriented towards process control, such control can only be achieved if the last algorithm in a chain of possible al gorithms is a control algorithm. Therefore "Consul B" is applicable also for a mere analysis of process data (no control algorithm necessary). Just for
~"
Onerator' s ? anel and Saf e.-:uar dinr:
Th rou ~h the opera tor' s panel, shown i n ? i ~ . 3 , online corununication of t he ooerat or '.i th t he nroce ss is p erformed. As a co~and in p~ t a keyboard i s available t o him whic h is divid ed un acc ordinu, t o the different functional tasks . Out nut fro~ t he co~!,uter i s sh01>T. on three d is pl ay ~n i t s ani t'1 e
402
printer. All input data originated by the operator are ?rinted out, namely time, kind of oneration and the old and ne~ par~eters. This way the operator can always check his inp~t and keep a record. Operations include set point and limit chanRes, openinp, and closin~ of cascade looos, ~anual to auto~atic 1000 mode and vice versa, demandinp, 10p,5, initiating s~ecial user's orov,r~~s (such as run UP and run down pro~rams), position control of actuators, and accepting or supDressin~ of alarm nrintouts. Possibility is given to establish new control looos on-line through the ?anel, or to establish, change, or delete control 10005.
used for couplinp. between the computer output and the actuator in order to maintain ~alvanic senaration between the CO'!lDuter and t'le nower electronic circuit3. 1. 2 Actuators '.oIi th Analo~e Innut Control Analon:ue actuators are usually often pneumatic valves (with or without stuffinp, box) and motor valve type. As an output system for those actuators a system develoooed by 1(E'IT was selected. In formation transfer from the comouter to the process is done in this system thro~gh a ~ain path addressinp system, which permits addressing and control of 512 valves. Commands for incremental movements of the valves in both directions can be given at a maxim~m definition of 6 bits, whereby the data in this system are buffered and sent out automatically. A preat flexibility and red~ced computer time are the main advantages of this output system.
To safeguard the process ap,ainst accidental, or inproper internal measures, which could lead to critical conditions in the process a security feature has been provided. This feature p.ives the possibility to lock all keys with three different switches. The lockinp, is done by the software, and therefore very flexible and easily adaptable to special requirements. A suggestion for a oriority list is shown below.
In chemical olants oneumatic valves are used prebecause they are preferable to all other actuators with regard to price, ruggedness and reliability. One possibility to control the valves is to orovide an analogue output from the computer. This could mean however, that in case of a failure of the output system the valve position cannot be maintained over a long period of time. At ~ALLE an incremental output is therefore used to drive a stepper ~otor of an electro-pneumatic positioner. do~inantly,
1. Priority: Control Engineer (Lockin~ of all keys, which have access to the control syste~.) 2. Priority: Production t~anap:er (Locking of certain alarm limits). 3. Priority: Operator (Lockin~ of all other keys apainst unauthorized interference).
Although electro-pneumatic positionersare offered by the industry, Kalle decided to develop their own version. The units available on the market were relatively expensive and to some degree did not meet the technical requirements demanded by KALL3. The ~odification performed in the mechanical and pneumatic construction of the very simple pneumatic positioner consists of the installation of an additional pullinp, sprin~ on the flapoer beam. A connected stepper ~otor acts through a reduction ~ear upon a tension band tied to the spring. This way the turnin~ an~le of the p,ear is transformed into a linear ~ove~ent of the flapper beam, proportional to the spring force and therefore into a nronortional air pressure. In steady condition (no increments to the stepper motor, or in case of break down of the electronic) the motor and the valve remain solidly in their previous position. The total ran~e of travel was divided into 200 steos (sensitivity 0.5 ~) and a linearity of smaller than 1 ':' was achieved. A potentiorneter mounted on the Dositioner serves to provide the position feedback of the valve.
An on-line function test proeram is entered periodically, which will provide isolation of the computer system from the process in case of a co~ put er fault.
11. Direct Digital Control For the direct digital control the 1000 between computer and process is finally closed. All information regardinp, the process is made available to the computer not only on-line, but the computer acts directly without intermediate action of conventional control members upon the process. About the problems, advantages and possibili ties of direct di~ital control m'.lch has been reported in literature. The follo~ing exolanations shall only show, which basic considerations were applied and how the control system was desi2TIed at KALLE.
=
2. Selection of Samnling Periods 1. Selection of Appropriate Control for the Various Tyoes of Actuators
The selection of a proper sampling period is an essential step for control systems with a variety of control loops. Is the samnlin~ rate to hi p,h computer time is g;i ven a',lay, furthermore the samplin'S rate of the innut system is limited by its ha~dware configuration. The limitation is mainly determined by electro-mechanical switches (mechanical scanner, relays) and amounts normall:{ to 100 till 200 samples oer second. In control loops with
1.1 Actuators with 'I"... o-Steo Action This type is controlled from the comouter through a dipital out out (mercury wetted reed relay or semiconductor-switch). Is a semiconductor device used as the actuator (thyristor, triac), the control can be done directly from the computer. Where semiconductor outputs were used, a transformer was
403
mere dead time Tt, too high a sampling rate can cause stability problems. In the corresponding literature Lee (5) has mentioned for the choice of samplin~ interva13 the expression:
It is obvious and ~enerally known that too small a samplin~ rate can lead to a poor representation of the measurin ~ si~nal, and therefore to a reduction of the quality of the control (4). 'Nhen selecting a samnlin!7, period. it must be considered, that it is not possible to allow a specific sampling rate for each loop. Tt will be necessary to combine control loops into !7,rou9s with the same samplinp: interval, as the technical requirements (i. e. mechanical scanner with fixed sampling rates) must be taken into consideration, and that the soft,rare effort must remaln '.d thin reasonable limits. "Consul B" provides 8 different sa'llplinl:' rates, whereby all long sampling periods are based on the followin g equation: c • 2 n [2]
with Ts = sampling time in seconds, c means a presetable constant, i. e. 0.5 in our case, and n is an integer from 0 to 7.
3. Incremental Control and Algorithms Used 1.1 Incremental Control A control, '.hereby the absoLlte value of the actuator outuut is calculated from tile deviation (for example in conventional control), is quite different fro'll incremental control. The latter does not permit for i nstance a P'lre pronortional control, because a pronortional action would be calculated only in case of a ~radient deviation. Therefore a steady deviation uould not cause proportional action. An incremental PI0 al~orit~m therefore calculates in reality ·'3.n output, which corresponds to the differentiated solution of a "nor'llal" control calculation (compare equations [lJ and [2J in the a pgendix). In order to comnensate for the detremental effect of a double differentiation (this could cause, unintentionally, large changes of the valve action even with minute changes of the measurin~ value) the inp'lt value will be s'!loothed. Furthermore weip:htinp: factors are introduced and applied to the different samplin?s of the input value. A limiting of the out put value to be sent out in preset time i s also required for the achiev~ '!lent of a satisfactory control quality. In addition the KP~LE system takes into consideration the valve position feedback, although this is normally not required for an incre'!lental control system. However, the feedback of the valve position prevents the output of increments to the valve in the case of a valve end position. Therefore no increments are lost and the quality of the control is improved ('3, 7).
A common relation for the selection of samplin~ periods Ts 13 quoted by Lee (5) as follows (8): 1 > Ts ~ 1 8nE - 16nE
[3]
with a resonance frequency nE of the control loop. However this formula see~s to result in shorter samplin g periods than have been realized in various installations. The following expression is frequently stated as a rule of thumb: Tl 5< ~ 10 [4] - Ts whereby Tl is the :nodel time constant of a representative first order lag element. These state'Ilents lie within t he range stated by Terao (9) simulatin ~ technical control loops.
3.2 Used Alp:orithms Is a control to be performed a measuring value is first transr.Ji tted to the SP"W alp;ori thm. This algorithm evaluates the error, that means the difference between t he set point and the measured ~alue, and also serves the ~larm monitori;g . The calculated error can now be made available to a seq'lence of further algorithms, whereby the last one must be a control algorithm (i. e. PI, PID, etc.). The required increments are here calculated, which subsequently are adapted to the individual valves in an output algorithm (valve algorithh, which scales the output for the different amounts of travel and end to end travel time of each valve). The scaled output information is stored in the output list and then transmitted to the individual valves throup:h a separate output program. In the KALLE system a hysteresis algorithm (for level control) and a dead band al gorithm (for air flow control) were used besides PI and PID(1,6,11) al~orithms. Because these al s orithms are available also for conventional control, they will here not be explained in more detail.
At KALLE the followin ~ sampling rates were chosen for the control:
0.5
sec: Supervisory control of speeds, pressure control.
1 and 2 sec : Steam pressure control.
4
sec: Level control and air flow control.
8
sec: Control of fast temperat'lre loops.
32
sec : All other temperat'.lre control loops.
3.3 On-off PI Control For the ~eat number of similar bang-bang temperature control loops an appropriate algorithm had to be found. Starting at a normal control al~orithm (PI or PID), a special output algorithm for digital outputs was developped, which is based on the followin~ considerations (2).
404
If the selected samplinp, rate (in our case 32 sec.) reflects the temperature trend without gaps, the heater need not be switched on or off more frequently than once per sampling period. Higher switching rates would not produce any furt~er improvements of the control loop because of its low pass characteristics. Is the output value stored in the output list larger than 31, this means that the heater is turned on a 100 %of the time interval between the samples, is the value smaller than zero, then the heater is switched off a 100 %of the time. Output values in between cause the on/ off ratio of the electrical heater. The output list for the bang-ban~ loops is similar to that one for continuous loops. However, the scaled values of the output list are not decremented having been sent out, but are allowed to integrate. Therefore this list assumes the function of a valve in a "normal" control loop. In order to ensure, that the measured values are evaluated as exactly as possible for the fixed accuracy of the A/D converter of 11 bits + sip,n, measuring ranges with suppressed zero noint (in this case from 150 to 350 degree centigrade) were selected. Is the equation of an incremental PI algorithm considered as an example Ay = 100 [AE
+ Ts En) [5) (see Appendix) PB n TI it follows, that in the process of start up heating only integral action takes place until the measured value exceeds the lower limit of the range. Afterwards proportional action reduces the value stored in the output list. In t~e ideal case the loop should have reached its final switchin", rate, when the measured value corresponds to the desired set point. For this reason the output al~orithm limits, in contrast with nor~al incremental control, the amount of the value in the output list in order to approximate the ideal demand as much as possible. In general this limit is about + 30 %of the out nut range and in addition to those figures mentioned previously. Appendix
It is to be mentioned, that the output program causes the switch commands to be delayed in order to nrevent a simultaneous start of all heaters at the - sa~e moment (not more than 10 heaters at the same time). Thereby extreme current peaks in the power supply are prevented.
Conclusion The quality of control has been improved for many loops (for examnle the control parameters are decoupled in the computer and available in large ranges). However, by the apnlication of more sophisticated and process oriented algorithms improv~ ments can still be achieved • It appears to be of even greater importance, that the computer permits the possibility of on-line optimization and adaptation to changinv process conditions. Furthermore the computer provides the capability of a more co~nrehensive monitoring of the processes. The multitude of measured and collected data makes it possible to recognize mutual effects and to achieve a more thorough understanding of process related aspec1:.s. For this reason it is intended to extend the system described above. It is the goal of these efforts to achieve, that a multitude of higher order programs can be run during the normal on-line operatio~ and that these programs have access to the important control parameters.
The proportional and the integral action terms are straiv,htforward calculations.
Derivation of the Incremental PID Algorithm The equation of an ideal conventional controller is:
[1) The used incremental digital equivalent is:
Q~riY~1iY~_~£1i2E~
If s is the differential onerator, then for TA« TS ys tell anproximately is: dE En - E _ -dt = s·E = TA n l whereby TA= sampling period. Using a "smoothed derivative", E' will be: _ sE
[3)
E' =l+Tn S
[4)
.
[2)
whereby Ko, K_l, Kl are constants; Y = absolute output; 4 Y = incremental output; n = mOMentary sample; E = error (difference between set noint and measured value); E = smoothed error; Y' (d) the total of the past incremental outputs n-l caused oy derivative action.
Tn
Wlth A = T +T and TD = constant, and usin~ the A! D " for the error E, the final solusame "smoothing tion for the digital derivative term will be:
.U~d)
[I-A) {KI (I-A)(En - En-I) -
Y~~i}
[5)or [5a)
405
Referen ces (1) (2)
(3) (4)
(5) (6)
R. R. De Bolt, B. E. Powell , "A Natural 3-Mode Contro ller Algorit hm for DDC", I.S.A. Journa l, Vol. 13, No. 9, p. 43. D. Briggs, "Exper iences with Increm ental Control at Kalle AG", Ferran ti Departm ental Memo, Ferran ti Systems Divisio n Group, '4arch 1969. T. Ingham, D. Briggs , "Some Feature s on Increm ental Contro l", Ferran ti t4emo, Ferran ti Systems Divisio n Group, March 1969. H. Kaufmann, "Dynam ische Vorgaen se in lineare n Systeme n der Nachric hten- und Regelungste chnik" , Verlag R. Oldenbo urg Muenchen, 1959, p. 156 . W. T. Lee, "Plant negulat ion by On-line Di gital Comput ers", SIT Symposium on Direct Digita l Contro l, April 1965. R. Schilba ch, 14. Pandit, W. Weber, "Prozed uren zur Lineari sierunl t. lineare r DDC Algorit hmen" ', Heg;elu ngstech nik, Vol. 16, :-;0. 8, p. 337.
--
(7)
A. Schoen e, "Regelg uete und Stabil itaet von mit Prozess rechner n bei Beschraen kung der Stellgr oessen aender ung", Regelu ngstech nik, Vol.17 , No. 10, p. 447. (8) A. Schoen e, "Prozes srechen systeme der Verfahren slndus trie", Carl Hauser Verlag , ~1uenchen, 1969, p. 128. (9) :4. Terao, "Quant ization and Samplinp; Selecti on for Efficie nt DDC", Instrum entatio n Techno logy , Vol. 14, No. 8, p. 49. (10) A. Thompson, "Opera ting Experie nce with Direct Digita l Contro l", confere nce paper, IFAC/IF IP confere nce, Stockho lm 1964. (11) J. Hellium s, T. J. William s, R. S. Banks, G. J. Kirk, "A Practic al Spectru m of DDC Chemic al-Proc esses Contro l Algorit hms", I.S.A. Journa l, Vol. 13, No. 10, p. 65. Re~elsystemen
---
-e
-
Figure 3.
"Consul B" M::Jni tor Panel.
406
_•
........
Discussioo
01
Paper XI:4 by M. Beger, W. Koch
D. J. Fraade Switzerland: What has been the system avaUabihty (up time)? When did the 77 hours of downtime occur? W. Koch : It should be nentiooed that the 77 hours of down time include all input/output equipment, besides the two printers. In case of a failing printer, full control capability and q>erating facility through the panel is still given. Dc:Mltime: 77 hours; Total Working Time: 14808 hours; Availabili ty: 99.480%. More than 75% of the downtime occured during the first six (6) mooths after starting the oo-line DOC systeJl5.
407