Cascade controllersIntroduction

SISO involves a single loop control that uses only one measured signal (input). This signal is then compared to a set point of the control variable (output) before being sent to an actuator (i.e. pump or valve) that adjusts accordingly to meet the set point. Cascade controls, in contrast, make use of multiple control loops that involve multiple signals for one manipulated variable. Utilizing cascade controls can allow a system to be more responsive to disturbances

Meaning of the terms 'manipulated variables', 'measured variables' and 'control variables' should be clarified. The definitions of these terms commonly found in literature are often interchangeable; but, they typically refer to either the input or output signal. For the purpose of this article, 'control variables' will refer to inputs like flow rates, pressure readings, and temperature readings. 'Manipulated variables' and 'measured variables' will refer to the output signals which are sent to the actuator.

Single loop control

The simplest cascade control scheme involves two control loops that use two measurement signals to control one primary variable. In such a control system, the output of the primary controller determines the set point for the secondary controller. The output of the secondary controller is used to adjust the control variable. Generally, the secondary controller changes quickly while the primary controller changes slowly.

Once cascade control is implemented, disturbances from rapid changes of the secondary controller will not affect the primary controller.

Introduction

To illustrate how cascade control works and why it is used, a typical control system will be analyzed. This control system is one that is used to adjust the amount of steam used to heat up a fluid stream in a heat exchanger. Then an alternative cascade control system for the same process will be developed and compared to the typical single loop control. The figure below shows the performance of cascade control vs. single-loop control in CST heater

Cascade control gives a much better performance because the disturbance in the flow is quickly corrected.

 The above process, the fluid is to be heated up to a certain temperature by the steam. This process is controlled by a temperature controller (TC1) which measures the temperature of the exiting fluid and then adjusts the valve (V1) to correct the amount of steam needed by the heat exchanger to maintain the specified temperature. Figure 2 shows the flow of information to and from the temperature controller. to the heat exchanger is solely dependent on opening the valve to varying degrees.

If the flow rate of the steam supply changes (i.e. pipeline leakage, clogging, drop in boiler power), the controller will not be aware of it. The controller opens the valve to the same degree expecting to get a certain flow rate of steam but will in fact be getting less than expected. The single loop control system will be unable to effectively maintain the fluid at the required temperature.

Example of Cascade Control

Implementing cascade control will allow us to correct for fluctuations in the flow rate of the steam going into the heat exchanger as an inner part of a grander scheme to control the temperature of the process fluid coming out of the heat exchanger. A basic cascade control uses two control loops;

one loop (the outer loop, or master loop, or primary loop) consists of TC1 reading the fluid out temperature, comparing it to TC1set (which will not change in this example) and changing FC1set accordingly. The other loop (the inner loop, or slave loop, or secondary loop) consists of FC1 reading the steam flow, comparing it to FC1set (which is controlled by the outer loop as explained above), and changing the valve opening as necessary.

The main reason to use cascade control in this system is that the temperature has to be maintained at a specific value. The valve position does not directly affect the temperature (consider an upset in the stream input; the flow rate will be lower at the same valve setting). Thus, the steam flow rate is the variable that is required to maintain the process temperature.

The inner loop is chosen because it is prone to higher frequency variation. The rationale behind this example is that the steam in flow can fluctuate, and if this happens, the flow measured by FC1 will change faster than the temperature measured by TC1, since it will take a finite amount of time for heat transfer to occur through the heat exchanger. Since the steam flow measured by FC1 changes at higher frequency, we chose this to be the inner loop. This way, FC1 can control the fluctuations in flow by opening and closing the valve, and TC1 can control the fluctuations in temperature by increasing or decreasing FC1set 

Thus, the cascade control uses two inputs to control the valve and allows the system to adjust to both variable fluid flow and steam flow rates. 

In order to have a smooth flow of information throughout the control system, a hierarchy of information must be maintained. In a double loop cascade system, the action of the secondary loop on the process should be faster than that of the primary loop. This ensures that the changes made by the primary output will be reflected quickly in the process and observed when the primary control variable is next measured. This hierarchy of information can be preserved by applying the following conditions when setting up the cascade controls.

1) There must be a clear relationship between the measured variables of the primary and secondary loops.

2) The secondary loop must have influence over the primary loop.3) Response period of the primary loop has to be at least 4 times larger than the response period of

the secondary loop.4) The major disturbance to the system should act in the primary loop.5) The primary loop should be able to have a large gain, Kc.

Cascade control is best when the inner loop is controlling something that happens at fairly high frequency. Cascade control is designed to allow the master controller to respond to slow changes in the system, while the slave controller controls disturbances that happen quickly. If set up in reverse order, there will be a large propagation of error. Hence, it is important to maintain the hierarchy of information. In summary, the master controller responds to SLOW changes in the system, while the slave controller responds to the high frequency, or FAST changes in the system. This also requires that the inner control scheme be tuned TIGHTLY so error is not allowed to build. Commonly, the inner loops controls a flow controller, which will reduce the effect of changes such as fluctuations in steam pressure.

Conditions for cascade control

Advantages and Disadvantages of Cascade Control

teTemperature control of steam out from boilers to turbine blades using TCV 151

Logic diagram of Temperature control valve

Primary controller

Secondary controller