Watlow ez-zone pm - Example 5: Differential; Example 6: Cascade

To Next Page

To Previous Page

Watlow EZ-ZONE

PM Integrated Controller • 219 • Chapter 10 Applications

Example 5: Differential

Requirements:

Two analog inputs and the enhanced software option are required and at least one output adjusts

the controlled part of the process.

Overview:

Differential control maintains one process at a difference to another process. When function is

set for Differential, the PV Function output equals Source A minus Source B. Control loop 1 will

control Analog Input 1 difference to Analog Input 2 based on Set Point 1.

Example 6: Cascade

Requirements:

Two loops of control, two inputs and at least 1 output and the enhanced software option.

Overview: Cascade control can handle a difcult process with minimal overshoot, while reach-

ing the set point quickly. This minimizes damage to system components and allows for over siz-

ing heaters for optimal heat-up rates. Heater life is also extended by reducing thermal cycling of

the heater. Systems with long lag times between the energy source (heater, steam, etc.) and the

measured process value cannot be controlled accurately or efciently with a single control loop,

because a lot of energy can build up before a response is detected. This can cause the system to

overshoot the set point, which could damage the heater, product or heat transfer medium, such

as a heat transfer uid.

Ratio

Set Point 1

Function

Filter

Source A

Source B

Heat Power

Cool Power

0 to 100%

Output

Heat

Cool

Output n

Analog Input 1

Analog Input 2

Control Loop 1

PV Function

controlled

uncontrolled

Input 1

Input 2

Output 1