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BASIC UNDERSTANDING OF PID

CONTROLLERS

INSTRUCTED BY:

Mr. W.D.I.G.Dassanayaka

NAME

COURSE

INDEX NO

GROUP

FEILD

DATE OF SUB

Gunasekera N.P.A.

B.Sc. Engineering

090160U

04

Mechanical

05.04.2012

ME 3012

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INTRODUCTION

Proportional-Integral-Derivative (PID) control is the most common control algorithm

used in industry and has been universally accepted in industrial control. The popularity of PID

controllers can be attributed partly to their robust performance in a wide range of operating

conditions and partly to their functional simplicity, which allows engineers to operate them in a

simple, straightforward manner. As the name suggests, PID algorithm consists of three basic

coefficients proportional, integral and derivative which are varied to get optimal response.

EXPERIMENT: Basic understanding of PID controllers

AIM: Understand the behavior of P, PI and PID controllers.

THEORY

The PID control scheme is named after its three correcting terms, whose sum constitutes

the manipulated variable (MV). The proportional, integral, and derivative terms are summed to

calculate the output of the PID controller. Defining as the controller output, the final form

of the PID algorithm is,

Where,

Kp = Proportional gain, a tuning parameter

Ki = Integral gain, a tuning parameter

Kd = Derivative gain, a tuning parameter

( Error )

T = Time or instantaneous time (the present)

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1. The proportional term (P)The proportional term produces an output value that is proportional to the current error

value. The proportional response can be adjusted by multiplying the error by a constantKp,

called the proportional gain.

The proportional term is given by,

2. The integral term (I)The contribution from the integral term is proportional to both the magnitude of the error

and the duration of the error. Theintegralin a PID controller is the sum of the instantaneous

error over time and gives the accumulated offset that should have been corrected previously. The

accumulated error is then multiplied by the integral gain ( ) and added to the controller output.

The integral term is given by,

3. The derivative term (D)Thederivativeof the process error is calculated by determining the slope of the error over time

and multiplying this rate of change by the derivative gain . The magnitude of the contribution

of the derivative term to the overall control action is termed the derivative gain, .

The derivative term is,

http://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Integral

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DISCUSSION

A proportionalintegralderivative controller (PID controller) is a generic control loop

feedback mechanism (controller) widely used in industrial control systems a PID is the most

commonly used feedback controller. A PID controller calculates an "error" value as the

difference between a measured process variable and a desired set point. The controller attempts

to minimize the error by adjusting the process control inputs.

The transfer function

The transfer function of the PID controller looks like the following,

C(s) = Kp +

+ =

Kp = Proportional gain

KI = Integral gain

Kd = Derivative gain

Advantages of PID controllers

Quick response, fast action and easier to use than other methods.

Eliminate the offset

Comparatively cheap

Easy to manipulate

It is a robust and popular method

Disadvantages of PID controllers

Tuning a PID controller can be quite challenging(One more parameter to tune)

While PID controllers are applicable to many control problems, and often perform

satisfactorily without any improvements or even tuning, they can perform poorly

in some applications, and do not in general provide optimal control.

Oscillatory or unstable with integral control

The design procedure is somewhat complex in comparison to other methods

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Applications

The PID controllers are the most used feedback controller in industry. 80% of all

controllers are PID systems. Any time you want to control something in a process you can use a

PID controller, for example, temperature, flow, pressure, speed, level, weight and so on. There

are many feedback controller in the control theory but most of them need a good mathematical

model of the plant you want to control in order that they work.PID control is commonly used in

Chemical, Petrochemical, Pulp & Paper, Oil & Gas, Food & Beverage, MunicipalWater/Sewerage Facilities etc.

Industrial automation

Temperature Control using a Digital PID controller to close or open thermo-plug valves

that are controlled by temperature variations. The temperature can be controlled to get the

desired set value.

Turbine Speed Controller uses PID controllers to maintain constant speed.

PID controllers are used to control process variables ranging from fluid flow, level,pressure, temperature, pH, consistency, density, position Air Flow Rate Controller - Uses

a PID controller to maintain constant air pressure in many petrol driven super cars to

achieve the most effective Fuel-Air mix.

Automotive paint industry.

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UNIT STEP INPUT

P type

PI type

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PD type

PID type

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RAMP INPUT

P type

PI type

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PD type

PID type