Section 2 Cascade Control

Question 1

In a cascade control system, which controller (primary / secondary) controls the process?

Answer 1

Primary

Question 2

In a cascade system, the control valve receives its signal from which controller (primary /
secondary)?

Answer 2

Secondary

Question 3

When should a cascade control system be applied to a process?

Answer 3

1) Manipulated variable disturbances
2) Naturally continuous disturbances

Question 4

To properly tune a cascade control system, which controller should be tuned first (primary /
secondary)?

Answer 4

Since the secondary controller interacts with the process first it will be tuned first

Question 5

For a process to be a good candidate for cascade control, the secondary loop’s period must
be how many times longer than the primary’s?

Answer 5

Minimum 3 times longer

Question 6

Explain how to tune a properly designed cascade control scheme.

Answer 6

1) Tune secondary controller:
Primary in MAN (disconnected)
Secondary in AUTO
2) Tune Primary controller:
Primary in AUTO
Secondary in Cascade

Question 7

State two advantages of cascade control.

Answer 7

1) Faster response to manipulated variable disturbances
2) Secondary control loop can help reduce non-linearity