Monday, December 3, 2012

AUTOMATION AND CONTROL

Introduction to Control Systems

This chapter provides an introduction 10 control methods, strategies and systems. It examines the important developments and features of relay, electronic and com­puter control, introducing concepts and terminology to be used in the subsequent chapters on programmable controllers.

AUTOMATION AND CONTROL

In virtually all forms of industry, the route towards increased productivity is through increased automation of processes and machines. This automation may be required to directly increase output quantities, or to improve product quality and precision. In any form, automation involves replacing some or all human input and effort required to both carry out and control particular operations .

Many factories and plants place the workers in control of machines and equipment, instead of requiring them to physically carry out the task. This control requires the worker to know how a particular process operates, and what inputs are necessary to achieve and maintain the desired output.

To achieve process automation, the operator must be replaced by some form of automatic system that is able to control the process with little or no human intervention. This requires a system that has the ability to start, regulate and stop a process in response lo monitored or measured variables within the process, in order to obtain the desired output . A system that possesses all these abilities is called a control system .

Automatic control

Any control system can be divided into three constituent sections: input, processing and output . The model in Fig. 1.1 can also be described in terms of actions, consisting of input measurements, control processing carried out on these inputs, and the resultant output actions produced. The task of the processing section or control plan is to produce predetermined responses (in the form of outputs) as a result of information provided by the input signal measurements. There are several different methods available for implementing the processing function, but they all use similar inputs and outputs.

AUTOMATION AND CONTROL

This model also represents control by a human operator acting as the 'processing section'. The operator knows what the desired process output is 10 be , visually monitoring the relevant variables - i.e. inputs (using gages, etc.). In response to these readings, the operator will alter the settings of appropriate controls (valves, heaters, etc.) to obtain the desired process output.

Inputs

Input signals are normally provided by various transducers that convert physical quantities into electrical signals. These transducers may be simple push-buttons, switches, thermostats or strain gages, etc. They all transmit information about the quantity that is being measured. Depending on the transducer used, this information may be discontinuous on\off (binary) or a continuous (analog) representation of the input quantity.

Table 1-1 Types of input transducer

Output quantify

Measured quantity

Transducer

Binary voltage (on/off)

Binary voltage (on/off)

Binary voltage

Binary voltage

Varying resistance

Varying resistance

Binary voltage

Varying resistance

Movement/position

Movement/position

Temperature

Temperature

Temperature

Pressure/movement

Light

Presence of objects

Switch

Limit switch

Thermostat

Thermocouple

Thermistor

Strain gage

Photocell

Proximity cell

Outputs

The control system must be able to alter certain key elements or quantities within the process, if it is to exercise control over the way that process performs. This is achieved by using output devices such as pumps, motors, pistons, relays, etc. which convert signals from the control system into other necessary quantities. A motor, for example, converts electrical signals into rotary motion. In other words, output devices are also transducers but in the other direction. As with the input transducers ,

Table 1.2 Types of output device.

Output device

Quantity produced

Input

Motor

Rotational motion

Electrical

Pump

Rotational motion plus

Electrical

 

displacement of product

 

Piston

Linear motion/pressure

Hydraulic/pneumatic

Solenoid

Linear motion/pressure

Electrical

Heater

Heat

Electrical

Valve

Orifice variation

Elect ric a I/hydra uli c/pneumatic

Relay

Electrical switching/

Electrical

 

Limited physical

 
 

movement

 

output devices can be simple on/off (binary) units of be continuously variable operation between fully off and fully on (analog).

Processing section

This corresponds to the operator's knowledge of the operations that are required to keep a process 'in control'. The operator uses this knowledge in conjunction with information obtained from input readings, producing resultant output actions.

From the input information the automatic control system has to produce the necessary output signals in response to the control plan built in to the processing section. This control plan can be implemented in two different ways, using either hard-wired control or programmable control.

Hard-wired systems have the control function fixed permanently when the system elements are connected together (e.g. electrically), whereas in a program­mable system the control function is programmed (and stored) within a memory unit and may be altered by reprogramming if this becomes necessary. Table 1.3 lists examples of hard-wired and programmable systems, together with the type of control they can perform - digital (switched) or analog (continuous).

Table 1.3 Control systems and the form of control provided; (digital or analog)

Hard-wired systems

Type

Programmable systems

Type

Relays

Digital

Computers

Digital/analog

Electronic logic

Digital

Microcomputer

Digital/analog

Pneumatic logic

Digital

P-C systems

Digital/analog

Hydraulic logic

Analog electronics

Analog