Cascade Control Loop

Question 1

How many measured and manipulated variables in cascade control?

Answer 1

One manipulated variable but more than one measured variable.

Question 2

What are the loops in cascade control?

Answer 2

Primary control loop (outer) and secondary control loop (inner)

Question 3

What is the measured value the outer loop (primary control loop)?

Answer 3

The input of this controller is the measured value of the controlled variable.

Question 4

Where is the setpoint supplied for the primary control loop?

Answer 4

It is supplied from the operator.

Question 5

What is the measured value of the secondary control loop?

Answer 5

It measures a variable whose value affects the controlled variable i.e. it measures the disturbance

Question 6

Where is the setpoint supplied for the secondary control loop?

Answer 6

It is supplied from the output of the primary control loop.

Question 7

What happens with the output signal from the secondary controller?

Answer 7

It is used as the signal for the manipulated variable.

Question 8

Example: Boiler feedback control

Answer 8

The temperature of the hot oil is measured leaving the burner. This is passed to the controller which then manipulates the flow of steam into the reactor.

Question 9

What is the issue with this boiler in feedback control?

Answer 9

If the pressure of the steam were to increase, then the temperature of the hot oil would increase and it would take time for the control loop to adjust for this.

Question 10

What about the boiler in cascade control?

Answer 10

There is a secondary measurement of the flowrate of steam and this is connected to the secondary controller. It measures the pressure of the steam flowrate which could be a disturbance.

Question 11

When is cascade control used rather than feedback control?

Answer 11

When feedback control takes time.

Question 12

Measurement in primary loop

Answer 12

Measures the controlled variable and then passes the electrical signal to the secondary loop

Question 13

Measurement in secondary loop

Answer 13

Measures the disturbance variable and then exerts influence on manipulated variable.

Question 14

The sequence of indicators and controllers in the control loop.

Answer 14

Primary Indicator –> Primary controller –> Secondary Controller ( Manipulated Variable

Question 15

Example: Reactor Temperature Control

Answer 15

The controlled variable - the temperature of the products, measurement in the primary control loop.
The disturbance variable - the temperature of the cooling water, measurement in the secondary control loop. The manipulated variable - the flowrate of the steam

Question 16

What is the issue with reactor temperature control in feedback?

Answer 16

The effect of a change in cooling water temperature would take longer time to be detected by the sensor.

Question 17

Examples: Distillation Bottoms Temperature Control

Answer 17

Measured variable = temperature at the bottom of the distillation column, measurement of primary control loop
Disturbance variable = flowrate of steam, measurement of the secondary control loop
Manipulated variable = the flowrate of steam to the bottom of the column.

Question 18

When is cascade control most efficient?

Answer 18

When the secondary loop (measuring disturbances) is a lot faster than the primary loop (measuring the controlled variable). This means that it can control the rapidly varying disturbance before it influences the process.