Rockwell Automation T6200 Compressor Anti-Surge and Capacity Controller - Manual (Page 118)

Control Primer
7-16
The basic advantage of cascade control is that the faster slave controller can correct its process
disturbance before the influence is felt by the master variable thus enabling faster control action.
Adding cascade control can, however, also de-stabilize the master control if the process delays in the
slave control are not much shorter (five times or more) than those in the master control. It follows
that the time constant must be faster in the slave control than the master control. If these two
conditions are not satisfied, the master control will be less stable than it would be without cascade
control.
Ratio Control
A ratio controller is used to maintain a relationship (ratio) between two variables. Ratio controllers
are mostly applied to flow loops. An example of this is shown in Figure 7-13. The controller is
controlling the flow in one line and its setpoint is from the flow in the other line. The setpoint has a
multiplier that is used to control the ratio.
Damping
The damping in the process system is the reduction of energy causing amplitude of the oscillation
of the process loop to reduce. Figure 7-14 is an example of an under damped process loop and
Figure 7-15 is an over damped loop
A Controller Used
to Control the Ratio
of Flow
Figure 7-13
Figure 7-14
Response Curve for
an Under-Damped
Response