]1.0 Cascade Control

1.1 Introduction

Cascade Control has a multi-loop structure, where the output of the controller in the outer loop (the “primary” or “master”) is the set point of a controller in the inner loop (the “secondary” or “slave”).

The slave measurement is an intermediate process variable that can be used to achieve more effective control of the primary process variable. Two controllers are used, but only one process variable is manipulated.

The primary controller maintains the primary variable at its set point by adjusting the set point of the secondary controller. The secondary controller, in turn responds both to set point and to the secondary controlled variable.

Figure 1 shows a cascade loop to control heat exchanger process fluid outlet temperature by controlling the steam inlet flow.

Figure 1 – Control of Heat Exchanger Outlet Temperature using Steam Flow as Secondary Loop.

There are three main requirements for using cascade control:

Secondary loop must have influence over the primary loop.

Secondary loop must be measured and controllable.

Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics.

Cascade control is advantageous in applications where the primary process has a large dead time or time lag and the time delays in the secondary part of the process are smaller. Lags in a process is a major obstacles to good temperature control. Cascade control is also desirable when the main disturbance is in the secondary loop.

There are four main advantages gained by the use of cascade control:

a- Disturbances that are affecting the secondary variable can be corrected by the secondary controller before their influence is felt by the primary variable.

b- The primary lag seen by the primary controller is reduced, resulting in increased speed response.

c- Allow secondary controller to handle non-linear valve and other final control element problems.

d- Allow operator to directly control secondary loop during certain modes of operation (such as start-up).

Other advantages include the ability to limit the set point of the secondary controller. In addition, by speeding up the loop response, the sensitivity of the primary process variable to process upsets is also reduced.

On the other hand, cascade control introduces additional complexity and increases the cost of measurement of secondary variable (assuming it is not measured for other reasons).

1.2 Cascade Control Modes

In most applications, the control loop is not functioning as a cascade loop all the time. The operator (in the case of batch control, the batch control program) has the ability to change modes. Following is the typical selection of modes of operation available for a cascade control loop. Manual and Auto are usually used during start-up while cascade is used for normal operation.

Manual Mode - The set point of the flow controller tracks the actual flow variable. Figure 7 shows in (a) the schematic of a Manual mode.

Auto Mode - The output of the temperature controller tracks the set point of the flow controller. Figure 7 shows in (b)the schematic of an Auto mode.

Cascade Mode - The temperature controller manipulates the set point of the flow controller. Figure 7 shows in (c) a schematic of a Cascade mode.

Therefore, when switching from one mode to another there is no sudden change in the output to the valve.

a) Manual mode

b) Auto mode

c) Cascade mode

Figure 2 – Cascade Control Modes

1.3 Windup

If the secondary controller cannot deliver enough flow, even with its valve wide open, to bring the primary measured variable to its set point, the primary controller will "windup" and continue to increase the flow set point above the maximum flow. Later, when the flow is sufficient to bring the primary measurement to its set point, the primary controller must take the time to "wind down" the secondary set point to the actual flow before the valve begins to close.

The windup problem can be eliminated by connecting the secondary measurement to the external feedback of the primary.

Figure 3 – Control Modes for a Heat Exchanger