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by Robert Wall, Plastics Division, Monsanto Chemical Co., Texas City, Tex.
Electric Controls — Present and Future Two complete modern electric controls are available, but many control manufacturers are working on complete or partial systems
THE new electric control systems are a m o n g the most interesting developments in process control equipment. Limited systems have been available for some time, but have been restricted in applicability and slow in action. T h e newer systems are versatile and fast, providing the same type of performance available from m o d e r n pneumatics. While thus far there are only two complete modern electric control systems available almost every manufacturer of process control equipment is working on the development of either complete or partial systems, and in many cases has had some basic components on the market for some time. I n the near future the electric control systems available may nearly parallel the pneumatic. Basic Systems T h e r e are two basic systems for electric controls: a.c. voltage signal transmission, and direct current signal transmission. T h e advantage of the a.c. voltage system is the use of the differential transformer in the transmitter, eliminating the ne-
cessity for field mounted amplifiers: disadvantages of a.c. pickup and transmission line resistance. The advantages of the direct current system are independence of line resistance, freedom to place a n u m b e r of indicators or receivers in series in the line, and freedom from a.c. pickup; disadvantages, the necessity for amplifiers in the transmitters and the controller drift. It appears that agreement on two standards, an a.c. voltage range and a direct current range, should be possible, but this has not been the case as yet in meetings of the Scientific A p p a r a t u s Makers Association. T h e differential transformers have been carefully developed and differ significantly in the phase and voltage characteristics of their output. Agreement on a common a.c. standard would require major changes in the differential transformer and associated equipment. N o agreement has as yet been reached on a direct current standard. Each m a n ufacturer has based his design on his carefully considered decision as t o the most effective system and the best components and construction
techniques, and these factors rather than arbitrary decision have determined the current ranges used. Acceptance of a single standard would limit the applicability of m a n y techniques and these are of basic importance to system design. Partial agreement on direct current ranges has been m a d e with general but not universal acceptance of 1 to 5 rangeability. T h e r e are systems available or planned with from 5-ma. to 50-ma. maximiim current. T h e 1- to 5-ma. systems are a d e q u a t e for signal transmission and for indicators, and can be compact, have low heat dissipation, and be relatively easily field mounted. Recorders, transducers, etc., usually use a n amplifier to provide the driving power. A 10- to 50m a . range unit provides adequate power to drive recorder pens, transducers, etc., without the necessity of a n amplifier, and is considerably larger t h a n necessary for signal transmission alone. T h e components u p o n which a system is based are significant in determining the current range. Magnetic amplifiers are well adapted VOL. 4 9 , NO. 5
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to the 10- to 50-ma. r a n g e ; transistors, to the intermediate ranges; and v a c u u m tubes, to the 1- to 5-ma. range. Direct interchangeability of units is not in general possible, but can be accomplished by use of intermediate amplifiers, shunting, or other appropriate techniques. T h e developers of electric controls face a tough j o b , for they are inevitably competing with the performance, reliability, and price of thoroughly developed and tested pneumatics. T h e price of electric controls must include substantial development costs, while pneumatic development costs have been paid out except for improvements, a n d the sales volume to absorb these costs is m u c h greater. During the development of pneumatics the alternative was m a n u a l control ; it was almost a case of " a n y t h i n g is better t h a n nothing." Electric controls development must be carried to the very excellent performance standards of modern pneumatics, and at a comparable cost if they are to reach the large market. This almost necessitates m a x i m u m use of c o m m o n radio grade components, which are contraindicated by reliability and performance requirements. Analog Controls Most emphasis is being placed on the analog controls, and the newer developments are all of this" type. T h e y parallel the modern forcebalance pneumatics in operation, with, typically, an electromagnetic force balancing the force derived from an opposing element, as pressure capsule. A null detecting device is required. T h e Evershed control system (a British system available for some years) uses a set of micro contacts. Swartwout Autronic controls use a differential transformer with a 0- to 0.5-volt 58degree lead output as a proportional sensing element for null or displacem e n t detection. A 4- to 8-ma. d.c. signal, most recently 1- to 5-ma., is transmitted from the receiving element to the force balance element, or to actuate an electropneumatic transducer, etc. Manning, Maxwell, and M o o r e use their Microsen balance, a moving coil device with the nulling element a 62 A
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flag varying the coupling of two oscillator coils. T h e y used 0.5- to 5-ma. d.c. transmission, and more recently 1 to 5 m a . While the force balance principle appears to have been generally adopted, the h a r d w a r e differs considerably. A problem is valve actuation. An electropneumatic converter to an air-operated valve is c o m m o n practice and usually very satisfactory if instrument air is available. If a complete new system has gone electric to avoid air, it somewhat defeats the purpose to use air for valve operation. Electric motor valves are slow, but often a d e q u a t e . Electrohydraulic valve operation is fast, powerful, and expensive. An electric operator matching the pneumatic in speed, reliability, and economy is needed. A 6-inch valve operating in 4 to 5 seconds requires roughly 1 hp. during that time. T h e problem is to provide a low inertia electric operator capable of equivalent peak power. Some improvements in electric motor valves or the development of some new electric actuator m a y be expected to solve the problem. Trends
Electric control instrumentation is at present in a stage of rapid change and development, and while some trends are a p p a r e n t in other areas they are formative. Development has concentrated on the analogtype controls, which are simpler, faster, and more versatile than the servo-type which they supersede. M i n i a t u r e instruments a p p e a r standard, with the trend toward combining as m a n y functions as possible in a single unit. Direct current transmission systems o u t n u m b e r alternating voltage systems, considering those available and planned. T h e current a n d / o r voltage ranges and the design and techniques used vary widely a m o n g manufacturers. T h e 1- to 5-ma. d.c. range may have some preference. Until recently the complete and partial systems available have used vacuum tubes a n d conventional electronics components and design. Magnetic amplifiers and transistors are now being incorporated and dominate future planning. Unitized plug-in subassemblies are appearing
and will be more used in the future. T h e development of the new process control system has to a large extent been the development of transistors and magnetic amplifiers to an acceptable cost a n d / o r performance level. T h e objective is reliability beyond that possible with v a c u u m tubes, particularly for control elements which must be field mounted. At least for some time there will be some v a c u u m tubes, as far as possible limited to the control house. T h e objective of all of the instrument makers is advanced high reliability circuitry and m a n y have considered the production of interim vacuum tube models inadvisable in view of the rapid advances in transistors and magnetics. T h e cost of electric controls a p parently will vary from about the same as pneumatic to somewhat more, with any reduction largelydependent on components development. T h e higher prices buy increased reliability a n d / o r precision, or extra features, which d o not a p pear justified to all manufacturers. In general, the advertised performance of available systems is essentially equal to pneumatics. Performance surpassing pneumatics is predicted for the systems under development. Users of process control e q u i p m e n t will soon have available several complete systems of electric controls, in addition to the present two, varying considerably in design and construction and in cost. T h r o u g h experience one or more c o m m o n standards m a y develop, or the desirability of a variety of controls may be proved, and the o p t i m u m compromise of cost, reliability, and performance will be determined. T h e final stage in the development of new control systems, refinement of design by user experience, is beginning. I t would be wrong to assume that with electric controls available pneumatics are on the way out. T h e r e is m u c h opinion to the contrary, and excellent arguments for both systems. T h e subject of electric versus pneumatic controls has been deferred for more complete consideration later. T h e advice of J o h n Draffen has been very helpful and is gratefully acknowledged.
INDUSTRIAL AND ENGINEERING CHEMISTRY For further information, circle number B3 A on Readers' Service Card, page 115 A
