Continuous Control Systems with
Variable Characteristics H. M. SCHMITT Brown Instrument Company, Philadelphia, Pa.
Three automatic control systems are described. Two are particularly applicable to electrical methods of temperature measurement and control in which thermocouples, resistance thermometers, or photoelectric cells are used as the rneasuring element, and motor-operated valves are employed to regulate the controlled medium. The third covers a pneumatic method for regulating the controlled medium in response to changes in temperature, pressure, flow, or liquid level. All three systems incorporate means for adjusting the throttling range and rate of automatic reset while the instruments are in operation. These characteristics result in automatic control systems adaptable to processes different in nature and operating conditions.
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RIOR to the World War, European instrument manufacturers held an important position in the field of industrial instrumentation. To apply automatic control to industrial processes a t that time was a daring procedure. Today America leads the world in the art of industrial measurements, and European engineers who have come to our shores for the purpose of studying our methods have been astounded a t the widespread use of automatic control equipment, and the simplicity and directness in which American industry solves its automatic control problems. Progress in the application of automatic control equipment has depended largely upon the development of satisfactory methods of amplifying the forces available a t the point of measurement so that the forces required a t the point of control will be sufficiently great. Electrical, mechanical, pneumatic, and hydraulic amplifying means are used in one or more combinations or stages to obtain sufficient power to operate the necessary valves, motors, dampers, rheostats, etc., to regulate the controlled medium.
TOWERS IN A REFINERY
Amplification in early types of controllers was directed entirely towards obtaining sufficient power to regulate the controlled medium with no consideration being given to the characteristics of the process being controlled. Automatic control studies in recent years, particularly those associated with continuous processes, have revealed the fact that process characteristics are a most important factor and must be considered in order to apply automatic control equipment successfully. In recent years systems in which the control characteristics may be adjusted to meet the process requirements have been perfected in both the electrical and pneumatic types. The following descriptions of control systems are introduced without resorting to a mathematical or technical approach or pausing to explain terminology, since these are subjects on which committees of engineering societies are now engaged.
Electrical Control Figure 1is a schematic diagram of an electrical control system incorporating throttling range and rate of automatic reset adjustment in conjunction with a motor-operated valve. This circuit includes a novel means of electronic amplification of small direct-current voltages in low-resistance circuits which places the motor operating the controlled valve subject continuously to the response of the thermocouple. The unbalanced e. m. f. of a potentiometric measuring system is connected in series with a selenium cell and the primary of a transformer. By focusing a neon light, operating on alternating current on the selenium cell, an alternating current is induced in the secondary of the input transformer to the amplifier. The output of the amplifier is connected to reversible motor 1. The rotor of motor 1 operates three sliding contacts, A , the slide wire contact in the measuring circuit, and B and C, in the control circuit. 1229
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These contacts are insulated from one another and may also be mechanically displaced with respect to one another to permit setting the control point to coincide with any value within the measuring range of the instrument. The control circuit is essentially a self-balancing bridge circuit with four adjustable arms. The slide wire on which contact B operates forms two arms, and the slide wire on which contact D operates forms the other two arms with resistance E interposed for the purpose of introducing a AMPLIFIER I
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measuring system, to which the control system in Figure 1 may be applied. The measuring system in Figure 2 employs two photoelectric cells in a bridge circuit; one cell is exposed to the illumination from the heated object or furnace, and the other is subjected to the illumination from a standardized lamp. The unbalanced potential of the bridge circuit resulting from the difference in illumination of the photoelectric cells is amplified to vary current through the standardized lamp to rebalance the circuit. The functions of the control circuit as applied to Figure 2 will be the same as those of Figure 1. These circuits provide electrical control systems which will automatically admit fuel in just sufficient quantities to maintain a predetermined furnace temperature independent of load, and may be applied satisfactorily to a wide variety of processes because the adjustments incorporated in the controller make it possible to fit the c o n t r o l l e r to the process characteristics.
Pneumatic Control Figure 3 is a schematic drawing of a pneumatic control system which incorporates adjustments for varying the throttling range and rate of automatic reset to the control point. This system consists essentially of five parts:
FIGURE1. SCHEMATIC WIRINGDIAGRAM FOR POTENTIOMETER WITH CONVERTOR AND ELECTRONIC RELAY
flexible relation between the positions of contacts B and D with their respective slide wires. Reversible motor 2, operating the sliding contact on E , receives its power for either direction of rotation through contact C and the segmental coatacts on which C operates. Rheostat R1 provides a means for varying the rate a t which motor 2 moves the contact on slide wire E. Amplifier 2 amplifies the unbalanced e. m. f . of the control bridge sufficiently to operate motor 3 which operates the controlled valve as well as slider D. Suppose that the thermocouple is immersed in a gas-fired furnace, operating at a given temperature, to which motor 3 regulates the fuel supply. If the load in the furnace is increased, the fuel supply to the furnace must be increased in order to maintain the furnace a t the same temperature with the increased load. The above circuit operates as follows to cause motor 3 to admit just enough fuel to the furnace to maintain the increased load a t the predetermined temperature: When the load is increased, there will be a temporary drop in temperature. The unbalanced e.m.f. thus caused is converted to alternating current by means of the selenium cell and neon light and amplified as alternating current to operate motor 1. The rotor of motor 1 moves slider A to rebalance the measuring potentiometer, and it also moves sliders B and C. When slider B is moved, an unbalanced e. m. f. is created in the bridge control circuit. This unbalanced e. m. f. is amplified by amplifier 2 sufficiently to cause motor 3 to move in a direction to increase the fuel supply to the furnace. Motor 2, operating simultaneously with motor 3, moves the slider on resistance E to unbalance further the bridge control circuit and cause motor 3 to continue to increase the fuel supply to the furnace until the temperature has returned to the predetermined value, at which point slider C rests on the insulating spacer separating its segmental contacts. The distance that slider B moves on its slide wire is adjustable with respect to the distance slider A moves. By this means the amount of controlled valve movement by motor 3 for a given temperature change may be varied. The temperature of the furnace will always be returned to the predetermined value or control point after a departure by the operation of motor 2, and the speed .of motor 2 can be adjusted to be consistent with the furnace characteristics. Figure 2 is a schematic drawing of another continuous
A measuring system responsive t o such quantities as temperature, pressure, flow, liquid level, etc. A control setting mechanism for correlating the measuring system and control unit. A pneumatic relay or pilot valve which permits air to pass directly to or from the controlled valve in response to the control unit. A control unit which governs the air pressure applied to the controlled valve. A diaphragm valve which regulates the flow of the controlled medium.
FIGURE2. SCHEMATIC WIRINGDIAGRAM FOR PHOTRONIC AND ELECTRONIC RELAY
Compressed air at constant pressure enters the system as shown and passes through a restriction to the nozzle. The cross section of the nozzle is larger than the restriction so that, when the flapper is moved away from the nozzle, t h e pressure in the space above bellows 1 in the pilot valve drops and the bellows expand and move the baffle to cover the inlet port and open the exhaust. The operation would permit air to escape from the controlled valve. When the flapper is moved to cover the nozzle, the exhaust port in the pilot. valve is closed and the inlet port is opened to admit air and increase the pressure on the diaphragm of the controlled valve. Thus the control unit governs the position of the controlled valve by moving the flapper with respect to the nozzle in response to movements of the measuring system. The operation of the combined units of this system is as
