Cascade – Watlow EZ-ZONE PM Integrated Controller User Manual

Page 154

Watlow EZ-ZONE

PM Integrated Controller

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151

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Chapter 10 Applications

The graph below illustrates a system with a long lag time and the advantages in using cascade control. Curve

A represents a single-control system with PID parameters that allow a maximum heat-up rate. Too much energy

is introduced and the set point is overshot. In most long-lag-time systems the process value may never settle out to

an acceptable error. Curve C represents a single-control system tuned to minimize overshoot. This results in unac-

ceptable heat-up rates, with the final value taking hours to reach. Curve B shows a cascade system that limits the

energy introduced into the system allowing an optimal heat-up rate with minimal overshoot.

When the Math function is set for Process or Deviation Scale and Source E is not connected or false, cascade

control is enabled.
Note:

When the Math function is set for Process or Deviation Scale the PM automatically makes the connections for

each Control Loop as shown in the graphic below. Each loop, 1 (process) and 2 (energy) outer and inner respective-

ly, cannot be changed. If it is desired to display the inner loop process variable and set point, the home page must

be changed via the Factory Page, Custom Menu.

Cascade control uses two control loops (outer - loop 1

and inner - loop 2) to control the process. The outer loop

(Analog Input 1) monitors the process or part tempera-

ture, which is then compared to the Closed Loop Set

Point. The result of the comparison, the error signal,

acted on by the PID settings and the Range and Scale

high/low settings. Ultimately, the outer loop produces a

remote set point for the inner loop. The inner loop input

(Analog input 2) monitors the energy source (heating and

cooling), which is compared to the remote set point gener-

ated by the outer loop. The result of the comparison, the

error signal, is acted on by the PID settings in the cas-

cade inner loop (2), which generates an output power lev-

between -100% to +100%. If the power level is positive the

heat will be on; if the power level is negative the cool will

come on. Power from the energy sources are supplied by

the outputs of choice always referenced to Control loop 2.
When cascade control is disabled (Source E is true), the

Math function output will equal Control Loop 1, Closed

Loop Set Point.

Note:

If an input sensor on the outer loop fails when using

deviation cascade the inner loop will continue to drive

the output.

-100% = Range Low

+100% = Range High

Outer Loop 1

Process

Control Loop 1

Control Loop 2

Inner Loop 2

Energy

Cascade

Filter

Scale Low

Scale High

Range High

Range Low

Function

Remote Set

Point

Source E

Output

Heat Power
Cool Power

Closed Loop SP

Math Function

DIO

Function Key

0 to 100%

Analog Input 1

Analog Input 2

Process Sensor

Energy Source

Closed Loop

Set P

oint

Heat

Algorithm

Cool

Algorithm

Heat

Cool

Output n

Time

Temperatur

Cascade

Curve A (PID)

Set

Point

Curve B (Cascade)

Curve C (Single-control)

Heater

Oil In

Limit

Limit Thermocouple

Lube Oil Tank

Oil Out

Output 1

Input 2

Input 1

Inner Loop

Thermocouple

Outer Loop

Thermocouple

Note:

When using cascade

control, two loops of

control are required.

Changing the control

mode in either loop

will affect both loops

of control. In other

words, if loop one is

changed to manual

mode, loop two will

also be changed to

manual mode auto-

matically.