The document covers advanced control strategies in instrumentation and process control, focusing on multi-loop control, cascade control, ratio control, and split-range control. It describes how these methods manage multiple input-output relationships, improve system responsiveness to disturbances, and maintain specified ratios or controlled variables. Additionally, it distinguishes between servo and regulator problems, addressing system behavior under set-point changes and varying disturbances.

1. 1
Instrumentation and Process
Control (CHE 323)
Lecture 36 :
1. Multi-loop control
i. Cascade control
ii. Ratio control
iii. Split-range control
2. Servo and regulator problems

2. 1. Multi-loop control
a) Single-input, single-output (SISO), or single-loop,
control problems
Control problems that have only one controlled
variable and one manipulated variable.
b) Multiple-input, multiple-output (MIMO) control
problems
In many practical control problems, typically a
number of variables must be controlled, and a
number of variables can be manipulated. These
problems are referred to as multiple-input, multiple-
output (MIMO) control problems.

5. i. Cascade control
• Cascade control employs a secondary measured
variable and a secondary feedback controller.
• Cascade control is particularly useful when the
disturbances are associated with the manipulated
variable.

6. • If a disturbance occurs in the fuel gas supply pressure (disturbance
associated with the manipulated variable),
 Then, feedback control will give very sluggish responses to changes in
fuel gas supply pressure.

7. • Primary/Master control loop (TT and TC)
• Secondary/Slave control loop (PT and PC)

8. • The primary measurement is the hot oil temperature
that is used by the primary controller (TC) to establish
the set point for the secondary loop controller.
• The secondary measurement is the fuel gas pressure,
which is transmitted to the secondary controller (PC).
• If a disturbance in supply pressure occurs, the
pressure controller will act very quickly to hold the
fuel gas pressure at its set point.
• The cascade control scheme provides improved
performance, because the control valve will be
adjusted as soon as the change in supply pressure is
detected.

9. ii. Ratio control
• Ratio control is a special type of feedforward control
• Its objective is to maintain the ratio of two process
variables at a specified value. The two variables are
usually flow rates, a manipulated variable (u) and a
disturbance variable (d). Thus, the ratio R=u/d

10. • Measure the flow rates
for both the disturbance
stream and the
manipulated stream
• Calculate the measured
ratio, Rm = um/dm
• The output of the divider
element is sent to a ratio
controller (RC) that
compares the calculated
ratio Rm to the desired
ratio Rd and adjusts the
manipulated flow rate u
accordingly.

11. iii. Split-range control
• In split-range control, several manipulated variables
are used to control a single controlled variable.
• Example 1: The adjustment of both inflow and
outflow from a chemical reactor in order to control
reactor pressure.
2 Manipulated Vars.:
V1 and V2
1 Controlled Var.:
Reactor pressure
While V1 opens, V2 should close

12. iii. Split-range control (Cont’d)
• Example 2: The adjustment of both acid and base flow
to control pH in wastewater treatment.
• Example 3: In reactor control, where both heating and
cooling are used to maintain precise regulation of the
reactor temperature.

13. 2. Servo and regulator problems
• Servo problem: The closed-loop system behavior for
set-point changes, assume no disturbance occur.
• Regulator problem: The process is to be regulated at
a constant set point while the disturbance changes