Control

Function Block Diagrams

EGR 386

8/26/2011

Block diagram: terminology

Feedback Basic Terminology• Process: system or central component whose output is to be

controlled

• Actuator: Device that can influence the process and change the process output

• Plant: Combination of Plant and Actuator

• Controller (or compensator): Device that computes the control signal/variable : Combination of Comparator and Compensator

• Senor: Provides an electrical output proportional to variable to be measuredmeasured

• Reference: the desired value for the output of a system, sometimes called Reference Input or Input or Set point

• Output: the process variable of a system to be controlled

• Comparator: Computes the difference between Reference and Output

• Transfer Function: function when multiplied by an Input provides an Output for the component or system being modeled (usually thought of in terms of Laplace transforms for continuous systems)

Temperature Control Function Block

Diagram: Open Loop System

Desired

Input T

Disturbance

Heat flow

in/out QDist

Compensator

TActual

OutputQIn Plant

dV/dt

Actuator

RoomInput

TemperatureFuel

Flow Control

TSET

TActualFurnace

dV/dt

Fuel

Rate

Reference

Input

TempTSET

Error

Disturbance

Heat flow

in/out QDist

TActual

Temperature Control Function Block

Diagram: Closed-Loop Continuous System

OutputQIndV/dt

Compensator Actuator

RoomTSET

TMeasured

+_

ErrorTActual

Temperature

Sensor

Fuel

Flow Control

Furnace

dV/dt

Fuel

Rate

Function block diagram of a room temperature control system, digital output

Room temperature and furnace output vs. time

Plant and Controller• Plant ~= Actuator + Process

• Controller ~= Comparator + Compensator

Disturbance

Plant Controller

Room

Reference

Input

TempTSET

TMeasured

+_

Error

Disturbance

Heat flow

in/out QDist

TActual

Temperature

Sensor

Output

Fuel

Flow Control

QIn

Furnace

dV/dt

Fuel

Rate

Compensator Actuator

Why is feedback needed?

• Disturbances ( weather changes for example)

• Unmodeled plant or actuator characteristics

• Changes plant or actuator

What is required to control a system

Open Loop? • Compensator + Actuator

• Model of system response to control input

What additional is required to control

a system via feedback control? a system via feedback control?

• Output sensor

• Comparator

Modeling of Systems

• Needed to design an appropriate compensator

(whether feedback is used or not)

• Characterize the system performance or best

possible performancepossible performance

Modeling: Dynamic Models

Mechanical Systems: Car Suspension

Electro/Mech. Systems : Disk Drive

Segway

Electronic circuits

DC Motor

DC motor

• (a) electric circuit of the armature;

• (b) free-body diagram of the rotor

Fluid systems

Thermal Systems (heat transfer)

Steam Engine + Control

Operating parts of a fly-ball governor