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page 523

30.2 CONTROLLING DISCRETE SYSTEMS

In some cases we will be able to monitor continuous process variables, but only be able to change discrete actuators. This is common in heating systems, the system is either on/off, but temperature is monitored continuously.

A common method is to use two setpoints with upper and lower bounds.

Consider the example of a heating system in a room.

upper

room

 

overshoot

 

 

temp.

 

 

 

temp.

 

 

 

 

 

 

 

 

 

limit

 

 

 

 

 

set temp.

 

 

 

 

 

(nominal)

 

 

 

 

 

lower

 

 

 

 

 

temp.

 

 

 

 

 

limit

 

 

 

 

 

 

 

 

 

 

time

heater on

heater off

heater on

heater off

heater on

Note: This system turns on/off continuously. This behavior is known hunting. If the limits are set too close to the nominal value, the system will hunt at a faster rate. Therefore, to prevent wear and improve efficiency we normally try to set the limits as far away from nominal as possible.

30.3 CONTROL SYSTEMS

Control systems use some output state of a system and a desired state to make control decisions.

In general we use negative feedback systems because,

-they typically become more stable

-they become less sensitive to variation in component values

-it makes systems more immune to noise

page 524

• Consider the system below, and how it is enhanced by the addition of a control system.

 

 

Control variable

 

 

INPUT

 

 

 

OUTPUT

 

 

SYSTEM

(e.g. θ gas)

 

 

(e.g. velocity)

 

 

 

 

(e.g. a car)

 

 

 

 

 

 

 

 

 

vdesired

verror

Driver or

θ gas

car

vactual

+

 

_

cruise control

 

 

 

 

 

 

 

 

The control system is in the box and could be a driver or a cruise control (this type is known as a feedback control system)

Human rules to control car (also like expert system/fuzzy logic):

1.If verror is not zero, and has been positive/negative for a while, increase/decrease θ gas

2.If verror is very big/small increase/decrease θ gas

3.If verror is near zero, keep θ gas the same

4.If verror suddenly becomes bigger/smaller, then increase/decrease θ gas.

5.etc.

Some of the things we do naturally (like the rules above) can be done with mathematics

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