Honeywell VRX180 user manual Cascade Control

Programming and Operating Concepts

Cascade Control

An example of a cascade control application is featured in Figure 3-21. Cascade control is typically used when two process values must be simultaneously controlled, with one process value directly influencing the behavior of the other. In this control strategy, each process value is supported by its own dedicated control loop. The term “cascade” is used because it describes how this control approach literally attaches both control loops together. This act of linking control loops allows for the regulation of both process values using one and only one % output control signal.

INSTRUMENT		CHEMICAL
	THERMOCOUPLES	REACTION
PV	200	VESSEL
SP	500
OUT	83.5%
		OIL
	4 TO 20 mA
	(CAT)	ELECTRIC
		HEATING
		ELEMENT
		OIL JACKET
	SCR
	+ ~ -
	AC POWER
	SOURCE

Figure 3-21 Temperature Control Of An Oil Heated Chemical Reaction Chamber

InFigure 3-21, the temperature in a chemical reaction chamber is determined by the temperature of the heated oil surrounding it. Heating the oil is done by an electric heating element driven by a 4 to 20 mA controlled SCR and external power source. In this application the instrument controls the temperature of the chemical reaction chamber through control of the heat emitted by the jacket tank oil. The instrument must provide a single 4 to 20 mA control output to govern the voltage switched by the SCR and, hence, the heat applied to the entire system. Temperature is monitored with thermocouples.

The function block diagram of the required instrument configuration is featured in Figure 3-22

Note that this diagram illustrates the classic cascade arrangement of two control loops that defines the cascade control strategy. The first control loop, LP1, is designated as the primary cascade loop by the notation “CAS_P.” The notation “CAS_S” indicates LP2’s designation as the secondary cascade loop. Note how both control loops are joined together. In addition to the back-calculated feedback path set up between the two (LP2 BC), LP1’s output is connected to an input on LP2 that at this time must be introduced. Denoted as SP2, this input is LP2’s remote set point input.

68 Video Recorder – User Manual