Introductionstotemperaturecontrol(温度控制简介和PID控制器外文翻译

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Introductions to temperature control

and PID controllers

Process control system

.

Automatic

process

control

is

concerned

with

maintaining

process

variables temperatures pressures flows compositions, and the like at

some desired operation value. Processes are dynamic in nature. Changes

are always occurring, and if actions are not

taken, the important process variables- those related to safety, product

quality, and production rates-will not achieve design conditions.

In order to fix ideas, let us consider a heat exchanger in which a

process

stream

is

heated

by

condensing

steam.

The

process

is

sketched

in

Fig.1

Fig. 1 Heat exchanger

The

purpose

of

this

unit

is

to

heat

the

process

fluid

from

some

inlet

temperature,

Ti(t),

up

to

a

certain

desired

outlet

temperature,

T(t).

As

mentioned, the heating medium is condensing steam.

The energy

gained by the

process fluid

is equal to

the heat released

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by

the

steam,

provided

there

are

no

heat

losses

to

surroundings,

iii

that

is, the heat exchanger and piping are well insulated.

In

this

process

there

are

many

variables

that

can

change,

causing

the

outlet

temperature

to

deviate

from

its

desired

value.

[21

If

this

happens,

some action must be taken to correct for this deviation. That is, the

objective is to control the outlet process temperature to maintain its

desired value.

One

way

to

accomplish

this

objective

is

by

first

measuring

the

temperature T(t) , then

comparing

it to

its desired value, and, based on

this comparison, deciding what to do to correct for any deviation. The

flow of steam can be used to correct for the deviation. This is, if the

temperature is above its desired value, then the steam valve can be

throttled back

to

cut the

stearr

flow (energy) to the heat exchanger. If

the temperature is below its desired value, then the steam valve could

be

opened

some

more

to

increase

the

steam

flow

(energy)

to

the

exchanger.

All

of

these

can

be

done

manually

by

the

operator,

and

since

the

procedure

is fairly straightforward, it should present no problem. However, since

in most process plants there are hundreds of variables that must be

maintained

at

some

desired

value,

this

correction

procedure

would

required a tremendous number of operators. Consequently, we would like

to

accomplish

this

control

automatically.

That

is,

we

want

to

have

instnnnents

that

control

the

variables

wJtbom

requ)ring

intervention

from

the operator. (si This is what we mean by automatic process control.

To accomplish ~his objective a control system must be designed and

implemented.

A

possible

control

system

and

its

basic

components

are

shown

in Fig.2.

Fig. 2 Heat exchanger control loop

The first thing to do is to measure the outlet temperaVare of the

process

stream.

A

sensor

(thermocouple,

thermistors,

etc)

does

this.

This

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sensor is connected physically to a transmitter, which takes the output

from

the

sensor

and

converts

it

to

a

signal

strong

enough

to

be

transmitter

to

a

controller.

The

controller

then

receives

the

signal,

which

is

related

to

the

temperature,

and

compares

it

with

desired

value.

Depending

on

this

comparison,

the

controller

decides

what

to

do

to

maintain

the

temperature

at its desired value. Base on this decision, the controller then sends

another signal to final control element, which in turn manipulates the

steam flow.

The preceding paragraph presents the four basic components of all

control systems. They are

(1) sensor, also often called the primary element.

(2) transmitter, also called the secondary element.

(3) controller, the

(4)

final

control

system,

often

a

control

valve

but

not

always.

Other

common final control elements are variable speed pumps, conveyors, and

electric motors.

The importance of these components is that they perform the three

basic operations that must be present in every control system. These

operations are

(1) Measurement (M) : Measuring the variable to be controlled is

usually done by the combination of sensor and transmitter.

(2)

Decision

(D):

Based

on

the

measurement,

the

controller

must

then

decide what to do to maintain the variable at its desired value.

(3)

Action

(A):

As

a

result

of

the

controller's

decision, the

system

must

then

take

an

action.

This

is

usually

accomplished

by

the

final

control

element.

As mentioned, these three operations, M, D, and A, must be present

in every control system.

PID controllers can be stand-alone controllers (also called single

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loop

controllers),

controllers

in

PLCs,

embedded

controllers,

or

software

in Visual Basic or C# computer programs.

PID

controllers

are

process

controllers

with

the

following

characteristics:

Continuous process control

Analog input (also known as

Analog output (referred to simply as

Setpoint (SP)

Proportional (P), Integral (I), and/or Derivative (D) constants

Examples

of

process

control

are

temperature,

pressure,

flow,

and

level

control.

For

example,

controlling

the

heating

of

a

tank.

For simple control, you have two temperature limit sensors (one low and

one high) and then switch the heater on when the low temperature limit

sensor tums on and then mm the heater off when the temperature rises to

the high temperature limit sensor. This is similar to most home air

conditioning & heating thermostats.

In contrast, the PID controller would receive input as the actual

temperature and control a valve that regulates the flow of gas to the

heater. The PID controller automatically finds the correct (constant)

flow

of

gas

to

the

heater

that

keeps

the

temperature

steady

at

the

setpoint.

Instead of the temperature bouncing back and forth between two points,

the temperature is held

steady. If the

setpoint is lowered, then the PID

controller

automatically

reduces

the

amount

of

gas

flowing

to

the

heater.

If

the

setpoint

is

raised,

then

the

PID

controller

automatically

increases

the

amount

of

gas

flowing

to

the

heater.

Likewise

the

PID

controller

would

automatically

for

hot,

sunny

days

(when

it

is

hotter

outside

the

heater)

and for cold, cloudy days.

The analog input (measurement) is called the

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parameter

you

are

trying

to

control.

For

example,

if

you

want

to

maintain

a

temperature

of

+

or

--

one

degree

then

we

typically

strive

for

at

least

ten times that or one- tenth of a degree. If the analog input is a 12 bit

analog

input

and

the

temperature

range

for

the

sensor

is

0

to

400

degrees

then our

by

4,096

(12

bits)

=0.09765625

degrees.

[~]

We

say

because

it would assume there was no noise and error in our temperature sensor,

wiring,

and

analog

converter.

There

are

other

assumptions

such

as

linearity, etc.. The point being--with 1/10 of a degree

accuracy--even with the usual amount of noise and other problems-- one

degree of accuracy should easily be attainable.

The

analog

output

is

often

simply

referred

to

as

Often

this

is

given

as

0~100

percent.

In

this

heating

example,

it

would

mean

the

valve

is totally closed (0%) or totally open (100%).

The

setpoint

(SP)

is

simply-- what

process

value

do

you

want.

In

this

example-- what temperature do you want the process at?

The

PID

controller's

job

is

to

maintain

the

output

at

a

level

so

that

there

is

no

difference

(error)

between

the

process

variable

(PV)

and

the

setpoint (SP).

In Fig. 3, the valve could be controlling the gas going to a heater,

the

chilling

of

a

cooler,

the

pressure

in

a

pipe,

the

flow

through

a

pipe,

the level in a tank, or any other process control system. What the PID

controller is looking at is the difference (or

and the SP.

SETPOINT

P

，

I

，

&D

CONSTANTS

Difference error PID control

algorithm

process output

variable

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Fig .3 PIDcontrol

It looks at the absolute error and the rate of change of error.

Absolute

error

means--is

there

a

big

difference

in

the

PV

and

SP

or

a

little

difference?

Rate

of

change

of

error

means-- is

the

difference

between

the

PV or SP getting smaller or larger as time goes on.

When there is a

or

the

setpoint

quickly

changes--the

PID

controller

has

to

quickly

change

the

output

to

get

the

process

variable

back

equal

to

the

setpoint.

If

you

have a walk-in cooler with a PID controller and someone opens the door

and

walks

in,

the

temperature

(process

variable)

could

rise

very

quickly.

Therefore the PID controller has to increase the cooling (output) to

compensate for this rise in temperature.

Once

the

PID

controller

has

the

process

variable

equal

to

the

setpoint,

a good PID controller will not vary the output. You want the output to

be very steady (not changing) . If the valve (motor, or other control

element)

is

constantly

changing,

instead

of

maintaining

a

constant

value,

this could cause more wear on the control element.

So

there

are

these

two

contradictory

goals.

Fast

response

(fast

change

in output) when there is a

output) when the PV is close to the setpoint.

Note that the output often goes past (over shoots) the steady-state

output

to

get

the

process

back

to

the

setpoint.

For

example,

a

cooler

may

normally

have

its

cooling

valve

open

34%

to

maintain

zero

degrees

(after

the

cooler

has

been

closed

up

and

the

temperature

settled

down).

If

someone

opens the cooler, walks in, walks around to find something, then walks

back

out,

and

then

closes

the

cooler

door--the

PID

controller

is

freaking

out because the temperature may have raised 20 degrees! So it may crank

the cooling valve open to 50, 75, or even 100 percent--to hurry up and

cool the cooler back down--before slowly closing the cooling valve back

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