I/EC
Instrumentation
Control Developments Present trends in theory and instrumentation by P. H. Stirling and Henry Ho, Canadian Industries Ltd.
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DEFINITE HIERARCHY exists in automatic control, and the underlying logic of this has received a great deal of attention this year. Automatic control of a process requires a device which perceives, judges, and then corrects the process. Historically, the perception of a process by means of measuring devices, and then its control by actuators using some form of power amplification, has been the norm; the crucial nature of the decision-making step has not been fully recognized in the past. During the last few years the importance of this step and the consequences of the decisions involved in many common control systems have been critically examined. For instance, simple onoff control involves, first, a measurement to a given degree of accuracy, then the destruction of this measurement in a comparison to generate an error signal whose magnitude is immaterial since control action is taken from the sign of the error signal. Furthermore, the simple onoff system involves two other notions ; that control of a process is best achieved by holding a variable constant, and that since physical variables are usually continuous, control should be continuous or, preferably, simultaneous. These and other criteria inherited from classical physics still provide the basis for the first step in control measurement, tend to set the framework for the discussion of control phenomena in terms of continuum, and possess a bias towards describing processes in terms of single-valued linear variables. Discarding the idea of associated continuity and simultaneity, and comparing events separated by a finite time interval (introducing a memory) allows one to make a hill-climbing or adaptive mechanism. Here the knowledge of the "set-point" has been destroyed.
Greater sophistication of the decision criterion leads to hierarchy of control and paradoxically generates greater degrees of uncertainty. Not yet clearly defined except for the initial steps, the plotting of this heirarchy will probably take the next decade or two. It ranges all the way from the simple on-off control to the more complicated learning computers. The degree of simultaneity or of sequential operation necessary in an on-line process-control computer has also been receiving much examination, and more generalized digital computers with parallel connections are becoming available, as are analog computers capable of short-term memory. Distinction between the two types of computers is not clear, and specialized hybrids are
appearing. Random access memories and the use of multiple command are also appearing so that on-line digital computers will be capable of performing multiple functions simultaneously. The development of solid state switching of process inputs has passed the research stage and can be expected to influence the design of new transducers. A trend toward high-level output design for this purpose is expected. The adjectives "adaptive" and "optimizing" describe the theoretical approaches to control which are arousing most present day interest. Both arise from the consideration of the judgment criterion used in the particular control system. Definitions vary widely, but may be crudely separated into realtime hill-climbing techniques (adap-
—COMPUTER CONTROL APPLICATIONSProcess
Vinyl chloride Continuous annealing line
Pilot plant
Hot strip mill
Computer
ThompsonRamoWooldridge RW-300 General Electric
(Esso)
Daystrom Systems Division Foxboro-RCA System
Comments
These are mainly steady state control applications where the computers have been programmed beforehand to solve control equations. They were announced at the Joint Automatic Control Conference.
At the September 1. S. A. Automation Conference the difficulties of starting up a process computer control system were discussed, and a description of this computer-controlled pilot plant was given. Blending of a digital computer with subsidiary electronic analog controls in these systems can be taken as further evidence of the trend toward hybridization of digital and analog techniques for the attainment of a successful system. The need for the many manual adjustments shown in the Foxboro system is doubtful. They are likely to allow operator interference and negate the purpose and value of the excellent computer associated with the system.
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INSTRUMENTATION tive control) and automated calculation of optimum trajectories (optimizing control). The recent Systems Engineering Symposium at Case Institute spent most of its time defining terminology and classifying problem areas in this new field of engineering. A similar but more complex problem faced the delegates to the first International Congress of the Instrumentation Federation of Automatic Control (Moscow, June 1960). Approximately 300 papers were presented with nearly half officially classed as theoretical; but in fact, over two thirds were theoretical and did not contain practical applications or descriptions of hardware. Although many aspects of control were discussed, major interest was in adaptive and optimizing control. The Joint Automatic Control Conference at M.I.T. (September 1960) also showed a similar trend of interest in adaptive and optimizing control. Adaptive control systems discussed at J.A.C.C. described techniques applicable to linear systems whose characteristics vary with time. Considerable interest was also shown in the nonlinear systems. This contrasts with last year when most papers were concerned with linear systems.
New Instrumentation Shown at IFAC
Control theory is being developed at present, but the gap between theory and hardware has yet to be bridged. A comparison of East and West gives the impression that the East has a sound theoretical base, but this has not yet been translated into practice; the West is ahead in hardware, but lags in theory. On the hardware side the Institute of Automation and Telemechanics in Moscow has developed several automatic hill-climbing units; one variable unit is used in industry. A 12variable prototype unit was shown. A 1-variable hill-climbing unit is available commercially in the United States. Simple modifications of a standard potentiometric recorder involving the addition of magnetic clutches to the servomotor drive to make a memory, and the use of simple latching relays for logic elements 70 A
have been used for this purpose in several industries. There is probably little difference in practice between East and West on this count. The use of high speed pneumatic computing elements in the Institute of Telemechanics, as well as the use of long pneumatic-time constants for control loops, has been quoted as a major East-West difference. However, the situation is not as black as painted. Foxboro Instrument Co. at the September 18 Automation and Instrumentation Conference disclosed the development of a pneumatic integrator with a half-hour time constant, and the development by B. Horton of the Diamond Ordnance Fuze Laboratory of the fluid stream amplifier and its associated devices, first announced in March 1960, are Western counterparts of the Eastern devices. Indeed the analog mode of operation of the fluid amplifier has no Eastern counterpart to the writers' knowledge. The computers shown to the IFAC delegates were generally of the oldfashioned electronic-tube type, and were of medium capacity. A URAL-I with drum memory was shown and a BESM-II with 2048 words of magnetic core memory. A URAL-IV is now in production. These machines must not be regarded as representative of Russian progress in the field, although they admit to a two or three year lag in digital computer design.
On-line Computer Control
On-line control of chemical processes has received much attention on paper but only recently have there been any announcements made concerning actual applications. The generalized digital computer control system field is probably the most competitive in instrumentation today ; it has at least eighteen computer manufacturers compared to some fourteen user-companies. The future must inevitably bring mergers, dropping of lines, and price cuts in this field. Development of smaller special-purpose machines is highly probable. The future outlook for hybrid systems and special purpose analog systems is brighter than that of the general purpose digital process control computer.
INDUSTRIAL AND ENGINEERING CHEMISTRY
N e w Instruments at the ISA A u t o m a t i o n Show (Sept. 1960)
The most fascinating displays at the show were the government sponsored displays rather than the manufacturers' displays. The DOFL fluid amplifier device with no moving parts, mentioned earlier, definitely stole the show and promises to be the source of many exciting developments in control over the next decade. Bistable and tristable pneumatic relays were shown, as well as the control of a 60 to 80 p.s.i. air stream by means of 3 to 5 inches of mercury differential pressure. A pneumatic "memory" not affected by momentarily cutting off the power supply of air and a device for directly amplifying a fluid pressure differential were shown. A pneumatic oscillator capable of operating at speeds up into the kilocycle range was also shown. The future prospect of operating pneumatic actuators at high pressure, controlled by fluid amplifier devices, is only one of the many applications in sight. The U.S. Naval Boiler and Turbine Laboratory display proved an unexpected source of much valuable information for the process instrument engineer. Over many years they have compiled a series of Instrument Standards and their seventh edition is available on a limited basis to interested parties in industry. There will be many interested parties when they discover the wealth of information on inferential flowmeters, descriptions of practical and field-tested techniques for boilertube wall temperature-measuring installations (I.S.-189), and proved designs of high-velocity thermocouple probes for investigating gas temperature in boiler firesides. The Naval Biological Laboratory showed an ingenious fluid dispenser. The pressure in a balloon, which is inside a closed reservoir, is derived from electrolytically generated gases and enables controlled low flow rates of fluids over periods of several days. It is easily programmed, covers a wide range of flow rates, and has rapid response. The Keinath Co. excited the greatest interest with their multipledisplay unit, the K-Logger. This allows the grouping of some 200 recorders into a single panel. The
INSTRUMENTATION low price of 200 channels of recording offered by this display unit, as well as the low cost of an associated alarm or annunciator system, made a great impression on all viewers. T h e desirability of a 200-point recording and alarm system costing on the order of $100 per point and less for a 400-channel unit speaks for itself. The instrument uses the sweep-balance system of printing using a high-speed rotary solenoid and type-hammer to print as it passes by the balance point. Since ten units are scanned vertically on the up-stroke and another ten on the down-stroke, the travelling car riage only carries ten print units for 200 points. The printing hammers are slowly displaced along the time axis so that successive dots form the trace. As the charts are specially printed, square-root scales may be used so that direct reading of an inferential flowmeter is possible. Alternatively, thermocouple scales permit temperatures to be read directly from the millivolt inputs. The basic range is 0-10 mv., but 3 to 15 p.s.i. instrument air signals can be connected to another sweep balance system incorporated in the unit so that external electrical con version of pneumatic signals is not required. The display is back lighted by fluorescent lights, and this might be a disadvantage in the rapid visual scanning of the records in an emergency. The graphic panel using transparent plastics is pleasing, but a system of pro grammed illumination to pick out a particular control sub-system would greatly enhance the operational ap peal of this display. The accuracy ( ± l / 3 % ) is adequate and the 10second scanning time and 1-second reversal of the master print bar allows repetition rates of 22 seconds to be achieved. Chart graphs are 4" X 6" or 3" X 6", and the chart is mechanically stretched and clamped over the printing platen to ensure dimensional stability. An interlock system associated with the print-hammer enables the position of the printer relative to high or low limits to be distinguished. The print bar carries a photo-resistor which is illuminated by a strip of light from a transparent sub-platen resembling the printed chart. By laying opaque tape over this the high and low limits are set.
T h e second new device which aroused wide interest was the Mnemotron analog tape record ing system. This offers a precision hitherto only found in $25,000 installations, for some several thou sand dollars, and opens up a wide field of possibilities for analog devices. The svstem requires about 1 volt R.M.S'. or 2 V.D.C. to modulate a carrier and has a band width of 0-400 c.p.s. and input impedance of 100 ΚΩ. Appli cations of this using closed-loop recording and erase-head cut-out will enable the five or ten minutes prior to a plant upset to be recorded in a form suitable for subsequent analysis or for use in computer simulation studies. The equipment uses a new pulse frequency mod ulation technique and achieves an accuracy of V4% on reproduction of the analog signal. Simultaneous recording and reproduction is pos sible on two channels (Ml 00) or on one channel (Ml00A) thus allowing many mathematical operations to be carried out such as integration, differentiation, or delay followed by the immediate recording of the cal culated data. (Mnemotron Corp., 1 North Main St., Spring Vallev, Ν. Υ.) There were many developments in gas chromatography on show. Among the more interesting were: the extremely versatile tape-pro grammed chromatograph of the Mine Safety Co. ; the miniature process chromatograph displayed by the Hallikeinen Co. ; the process chromatograph using a flame-ionization detector of the Beckmann Instr. Co.; the new modular con struction of the Greenbrier process chromatograph, probably the easiest instrument to service on show; the sample stream splitter and ionization detectors of the Barber-Colman Co. ; and finally, the pure hydrogen source offered by the Milton Roy Co. which uses diffusion through a palladium alloy for separation and allows hydrogen flame ionization detectors to be operated stably in the parts per billion range. A new instrument, the electron spin resonance analyzer (Elion In struments Inc.) is bidding for process applications and may be of impor tance to polymerization pilot plant studies where knowledge of free radical concentrations is of impor
tance. Such an instrument will present peculiar sample handling problems, and its development will be watched with interest. A highly sensitive square-wave polarograph was announced by the Instrument Corp. of America. This is the Mervyn Instruments version of the British Atomic Energy square-wave polarograph, and it is capable of determining parts per million of cadmium ions in the presence of 200,000 p.p.m. of zinc. The extreme sensitivity of this polarograph makes it possible to determine impurities in the 10 p.p.m. range in 20-mg. samples. A new continuous record ing viscometer shown by the Halli keinen Co. consists of a gear pump feeding a capillary mounted in a thermostatted oil bath. The pressure drop across the capillary is monitored. T h e new hardware contained two interesting valves: the Bendix Fluidal valve, which is especially suit able for highspeed operation (10-20 msecs closing), and the Airmatic T u b e - O - M a t i c valve (usable up to 200 p.s.i.), which uses the col lapsing of a rubber sleeve to effect closure. T h e new suppressed zero force-balance pressure transmitter (Bailey Meter Co.), usable up to 7500 p.s.i. with a 10 to 1 span suppression, the low range (0—1 * Hg) differential pressure cell (Decker Corp.), and the low flow Galton wheel flowmeter (Potter Co.) aroused interest, as did the differential tur bine mass flowmeter (Potter Co.) and the gyro mass flowmeter (Decker Corp.). Foxboro's extensive new range of complementary pneumatic and electric analog controls also aroused favorable comment from many. There were many new and improved models of transducers available, and it seems that those engaged in cataloguing transducers will never catch up with the growing flood of present day instrumentation development.
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, O.C. Letters will be forwarded and answered promptly. VOL. 52, NO. 12
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