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Cascade Control

A simple control system drawn in block diagram form looks like this:

Information from the measuring device (e.g. transmitter) goes to the controller, then to the final control device (e.g. control valve), influencing the process which is sensed again by the measuring device. The controller’s task is to inject the proper amount of negative feedback such that the process variable stabilizes over time. This flow of information is collectively referred to as a feedback control loop.

To cascade controllers means to connect the output signal of one controller to the setpoint of another controller, with each controller sensing a different aspect of the same process. The first controller (called the primary, or master) essentially “gives orders” to the second controller (called the secondary or slave) via a remote setpoint signal.

Thus, a cascade control system consists of two feedback control loops, one nested inside the other:


A very common example of cascade control is a valve positioner, which receives a command signal from a regular process controller, and in turn works to ensure the valve stem position precisely matches that command signal. The control valve’s stem position is the process variable (PV) for the positioner, just as the command signal is the positioner’s setpoint (SP). Valve positioners therefore act as “slave” controllers to “master” process controllers controlling pressure, temperature, flow, or some other process variable.

The purpose of cascade control is to achieve greater stability of the primary process variable by regulating a secondary process variable in accordance with the needs of the first. An essential requirement of cascaded control is that the secondary process variable be faster-responding (i.e. less lag time) than the primary process variable.

For example, consider the following dryer system where heated air is used to force-evaporate water from a granular solid. The primary process variable is the outlet air exiting the dryer, which should be maintained at a high enough temperature to ensure water will not remain in the upper layers of the solid material. This outlet temperature is fairly slow to react, as the solid material mass creates a large lag time:

 

 

There are several parameters influencing the temperature of the outlet air other than the moisture content of the drying material. These include air flow, ambient air temperature, and variations in steam temperature. Each one of these variables is a load on the process variable we are trying to control (outlet air temperature). If any of these parameters were to suddenly change, the effect would be slow to register at the outlet temperature even though there would be immediate impact at the bottom of the dryer where the heated air enters. Correspondingly, the control system would be slow to correct for any of these changing loads.

One way to help gain better control over this dryer system is to install a second temperature transmitter at the inlet duct of the dryer, with its own controller to adjust steam flow at the command of the primary controller:

 

 

Now, if any of the loads related to incoming air flow or temperature were to change, the secondary controller (TC-1b) would immediately compensate by adjusting steam flow through the heat exchanger to maintain a constant air temperature entering the dryer. Thus, the “slave” control loop (1b) helps stabilize the “master” control loop (1a) by reacting to changes in one of the variables influencing it.

A helpful way to think of this is to consider the slave controller as shielding the master controller from the loads previously mentioned (incoming air flow, ambient temperature, and steam temperature). Of course, these variables still act as loads to the slave controller, as it must continuously adjust the steam valve to compensate for changes in air flow, ambient air temperature, and steam temperature. However, so long as the slave controller does a good job of stabilizing the air temperature entering the dryer, the master controller will never “see” the effects of those load changes. Responsibility for incoming air temperature has been delegated to the slave controller, and as a result the master controller is conveniently isolated from the loads impacting that loop.

A common implementation of cascade control is where a flow controller receives a setpoint from some other process controller (pressure, temperature, level, analytical, etc.), fluid flow being one of the fastest-responding process types in existence. A feedwater control system for a steam boiler – shown here in pneumatic form – is a good example:

 

 

The “secondary” flow controller works to maintain feedwater flow to the boiler at whatever flow rate desired by the level controller. If feedwater pressure happens to increase or decrease, any resulting changes in flow will be quickly countered by the flow controller without the level controller having to act from a consequent change in steam drum water level. Thus, cascade control works to guard against steam drum level instability resulting from changes in the feedwater flow caused by factors outside the control system.

It is worth noting that the inclusion of a flow control “slave” loop to this boiler water level control system helps to overcome a potential problem of the control valve: nonlinear behavior. In the control valves chapter, we explore the phenomenon of installed valve characteristics (Control Valve Characterization article), specifically noting how changes in pressure drop across a control valve influences its throttling behavior. The result of these pressure changes is a non-linearization of valve response, such that the valve tends to be more responsive near its closed position and less responsive near its open position. One of the benefits of cascaded flow control is that this problem becomes confined to the secondary (flow control) loop, and is effectively removed from the primary control loop. To phrase it simply, distorted valve response becomes “the flow controller’s problem” rather than something the level controller must manage. The result is a level control system with more predictable response.

An analogy for considering cascade control is that of delegation in a work environment. If a supervisor delegates some task to a subordinate, and that subordinate performs the task without further need of guidance or assistance from the supervisor, the supervisor’s job is made easier. The subordinate takes care of all the little details that would otherwise burden the supervisor if the supervisor had no one to delegate to.

A necessary step in implementing cascade control is to ensure the secondary (“slave”) controller is well-tuned before any attempt is made to tune the primary (“master”) controller. Just a moment’s thought is all that is needed to understand why this precedence in tuning must be: it is a simple matter of dependence. The slave controller does not depend on good tuning in the master controller in order to control the slave loop. If the master controller were placed in manual (effectively turning off its automatic response), the slave controller would simply control to a constant setpoint. However, the master controller most definitely depends on the slave controller being well-tuned in order to fulfill the master’s “expectations.” If the slave controller were placed in manual mode, the master controller would not be able to exert any control over its process variable whatsoever. Clearly then, the slave controller’s response is essential to the master controller being able to control its process variable, therefore the slave controller must be the first one to tune.

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