cascade design

7/28/2019 Cascade Design

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CASCADE DESIGNPresented By:

SHAEEZ USMAN ABDULLA

ROLL NO: 10

M1 AEI

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INTRODUCTION

Cascade Control

Cascade control has a multi-loop structure, where the output of the

controller in the outer loop (the primary or master) is the set point

of a controller in the inner loop (the secondary or slave).

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In the cascade configuration the process is divided into two parts and

therefore two controllers are used, but only one process variable(m) is

manipulated.

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INTRODUCTION


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The primary controller maintains the primary variable y1 at its set point

by adjusting the set point r2 of the secondary controller. The secondary

controller, in turn, responds both to set point r2 and to the secondary

controlled variable y2. This secondary controlled variable also affects

the primary process and therefore the primary controlled variable (y1),hence the loop is closed.

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INTRODUCTION


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Cascade control is advantageous on applications where1) P1 process has a large dead time or time lag

2) Time delays in the P2 part of the process are smaller.

3) The main disturbance is in the secondary loop. This is becausewith the cascade configuration, the correction of the inner

disturbance di occurs as soon as the secondary sensor (y2) detects

that upset.

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INTRODUCTION


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INTRODUCTION

Cascade System Advantages

1) Disturbances that are affecting the secondary variable can be

corrected by the secondary controller before their influence is felt

by the primary variable.

2) reduces the phase lag seen by the primary controller, resulting in

increased speed of response.


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COMPONENTS OF THE CASCADE LOOPThe primary or outer control loop of a cascade system is designed only

after the secondary loop has already been designed. This is because

the characteristics of the slave loop have an effect on the master loop.


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The Secondary Loop

Ideally, the secondary variable should be so selected as to split the

process time delays approximately in half. This means that the

secondary loop should be closed around half of the total time lags in

the process.

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COMPONENTS OF THE CASCADE LOOP


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The Secondary Loop

To demonstrate this need, consider two extreme cases:

1) If the secondary variable responded instantly to the manipulated

variable (no time delay in the secondary loop), the secondary

controller would accomplish nothing.

2) If the secondary loop was closed around the entire process, the

primary controller would have no function.

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COMPONENTS OF THE CASCADE LOOP


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COMPONENTS OF THE CASCADE LOOPThe Secondary Loop

Therefore, the dynamic elements of the process should be

distributed as equitably as possible between the two controllers.

When most of the process dynamics are enclosed in the secondary

loop, that can cause problems. Although the response of the

secondary loop is faster than the open loop configuration, in which

a secondary controller does not even exist , its dynamic gain is also

higher, as indicated by the damped oscillation.


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COMPONENTS OF THE CASCADE LOOPSecondary Control Variables

The most common types of secondary control variables:

Valve Position Control (Positioner) :- Friction produces hysteresis

between the action of the control signal and its effect on the valve

position.

Flow Control :- A cascade flow loop can overcome the effects of

valve hysteresis.

Temperature Control :- The most commonly accepted configuration

has the reactor temperature controlled by manipulating the coolanttemperature in cascade.


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Speed Control of a DC Motor

The current can reach high peaks during the start-up and shut-down

phases. Therefore it is usual to apply cascade control with the

armature current as the secondary variable. Saturation applied at the

current set point (reference value) limits the maximum value of thecurrent.

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CASCADE APPLICATION EXAMPLES


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CASCADE APPLICATION EXAMPLES

Room Temperature Control

The variation in steam pressure can be the main source of upsets. The

secondary variable is the inlet air temperature measured immediately

after the heat exchanger, as the disturbances in the steam pressure

affect it much sooner than they affect the room temperature


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The process controlled by a cascade system is shown

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CASCADE CONTROLLER DESIGN EXAMPLE


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First design a single-series PID controller, with feedback taken from the

output signal y1. Design objectives are stability and good reference

signal (set point) tracking properties characterized by zero steady-state

error and about 60 of the phase margin to keep the overshoot below

10%, when a step change is introduced in the set point.

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Cascade Controller System Design

Design a cascade control system with feedback taken from the primary

and secondary (outer and inner) controlled variables (process outputs)

y1 and y2, respectively. Both control loops have to be stable. The design

objectives for tracking the set point (reference signal) are the same asbefore. Fast response is also required to any upsets in the secondary

loop or measurement.

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CASCADE CONTROLLER DESIGN EXAMPLECascade Controller System Design

The inner (secondary) controller can be proportional only.

Transfer function of the inner closed loop between signals y2 and u1

is


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CASCADE CONTROLLER DESIGN EXAMPLECascade Controller System Design

In the primary (outer) loop a PI controller is used, which will be

responsible for accurate steady-state tracking of a step change in

the set point (reference signal). The transfer function of the

controller is


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CASCADE CONTROLLER DESIGN EXAMPLESimulation Results

The set-point (reference signal) change is a unit step that occurs at a

time point of 10 sec. A unit step inner disturbance is added at a time

point of 250 sec.

response to upsets

set-point (reference

signal) tracking


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Effect of model parameter uncertainties in cases of the single loop and

the cascade system.


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Controlled variable responses of the single-loop and cascade control

systems, assuming all dynamic parameters (time constants) of the

process and its gain are both at their highest values.


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Cascade loops consist of two or more controllers in series and have

only a single, independently adjustable set point, that of the

primary (master) controller. The main value of having secondary

(slave) controllers is that they act as the first line of defense against

disturbances, preventing these upsets from entering and upsettingthe primary process.

In order for the cascade loop to be effective, it should be more

responsive (faster) than the master. Some rules of thumb suggest

that the slaves time constant should be under 1/4 to 1/10 that of

the master loop and the slaves period of oscillation should beunder 1/2 to 1/3 that of the master loop.

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SUMMARY


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The goal is to distribute the time constants more-or-less evenly

between the inner (slave or secondary) and outer (master or

primary) loops, while making sure that the largest time constant is

not placed within the inner loop.

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SUMMARY


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Bela G. Liptak, Bila G. Liptak,Instrument Engineers' Handbook, Volume

2: Process Control And Optimization, 4th Edition, Crc Press, Sep 2005.

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REFERENCE

cascade design

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