Chapter 2

Industrial Control System

OUTLINE

Introduction to Control System The idea of control systemControl system componentsControl system applicationsSummary

INTRODUCTION TO CONTROL SYSTEM

Important for design of experimental equipment and instrumentation used in basic sciences

Power

ProcessControl System

Program of Instructions

Elements of an automated system

(1)

(2) (3)

Control System

Open loop control system – Feed forward Control system

Closed loop control system- Feedback Control system

Feed Forward Control system

Open loop Control systemOperate without feedback loopThe control operate without measuring

the output variables

Open loop control

Input OutputController

Basic idea of control

Ex. Open loop control system

Feedback Control System

Closed loop control system Compare the actual result with the

desired result Take actions based on the difference.

Closed loop control

Ex. Closed loop control system

CONTROL SYSTEM COMPONENTS

SensorActuator (switch, motor)

Sensor

To use any measuring device, a calibration procedure is required to establish the relationship between the physical variable to be measured and the converted output signal.

Measuring devices can be classified into two basic categories:

1) Analog 2) Discrete

Analog

Measuring devices produces a continuous analog signal such as electrical voltage

The output signal from analog device must be converted to digital data by an analog-to-digital converter.

Example: Thermocouples, strain gauges and dynamometer.

Discrete

Measuring device produces an output that can have only certain values: binary and digital.

A binary measuring device produces an on/off signal. The most common devices operate by closing an electrical contact from a normally open position.

Example: Limit switches, photoelectric sensors and proximity switches.

A digital measuring device produces a digital output signal, either in the form of a set of parallel status bits or series of pulses that can be counted.

Example: Photoelectric sensor arrays, optical encoder.

Measuring devices description MEASURING

DEVICE DESCRIPTION

Accelerometer Analog device used to measure vibration and shock.

DC tachometer Analog device consisting of dc generator that produces electrical voltage proportional to rotational speed.

Dynamometer Analoq device used to measure force, power and torque.

Float transducer Float attached to lever arm. Pivoting movement of lever arm can be used to measured liquid level in vessel.

Fluid flow sensor Analog measurement of liquid flow based on pressure different.Ohmmeter Analog device that measured electrical resistance

Limit switch Binary contact sensor in which lever arm or pushbutton closes or open the electrical contact

Manometer/Barometer

Analog device to measure pressure of gas or liquid

Optical encoder Digital device used to measure position and speed

Proximity switch Binary non-contact sensor is triggered when nearby object induces changes in electromagnetic field.

Strain gaugeAnalog sensor used to measure force, torque or pressure based

on the change in the electrical resistance from strain of a conducting material.

ACTUATORS

An actuator is a hardware device that converts a controller command signal into a change in a physical parameter such as mechanical (position or velocity change).

An actuator is a transducer because it changes one type of physical quantity.

Example: Electric current to rotational speed of an electric motor.

Actuator classification

Electrical actuators: Include ac and dc motor, stepper motor and solenoids which include both linear devices (linear displacement) and rotational devices (rotational displacement and velocity).

Hydraulic actuator use hydraulic fluid to amplify the controller command signal for both linear and rotational motion. Usually used for large forces.

Pneumatics actuators use compressed air as the driving power for both linear and rotational pneumatics actuator. It usually limited to relatively low force applications compared with hydraulic actuators.

Actuator description ACTUATOR DESCRIPTION

DC MotorRotational electromagnetic motor. Input is direct

current. Rotary motion can be converted to linear motion using rack and pinion or ball screw.

Hydraulic piston

Piston inside cylinder exerts force and provides linear motion in response to hydraulic pressure. High force capability

Pneumatic cylinder

Piston inside cylinder exerts force and provides linear motion in response to air pressure.

Solenoid Electromechanical assembly consists of core inside coil of wire

Stepping motor

Rotational electromagnetic motor. Output shaft rotates in direct proportional to pulses received.

Relay switch On-off switch opens and closes circuit in response to an electromagnetic force.

Control System Applications

Manufacturing Robotic Power Aeronautics Electronics and Communication Automotive Computing Mathematics Physics Biology

Manufacturing

Numerically controlled machine tools developed at the Control Systems Laboratory at MIT in the 1950s was a first step where control was used to improve precision of mechanical machining.

Welding is highly automated using control and vision systems. Machines for manufacturing systems based on machining with lasers and electrical arcs depend heavily on use of control.

Large manufacturing operations are made on transfer lines, where the parts are moved along a line to stations which perform the operations.

A complete transfer line has a few hundred feedback loops.

Robotic

Robots are used for a wide range of tasks: welding, painting, grinding, assembly and transfer of parts in a production line or between production lines.

Robots that are used extensively in manufacturing of cars, and electronics. There are emerging applications in the food industry and in packaging.

Power

Control is an essential element in all systems for generation and transmission of electricity.

The generators were typically driven by water turbines. To control the speed of the generators to maintain constant frequency.

Aeronautics

Autopilots - based on the concept of feedback

Autonomous flight - requires automatic handling of a wide variety of tasks, landing, flying to the normal flight altitude, navigation, approaching the airfield, and landing.

Electronics and Communication

CD Players and Optical memories Processing of the signals from the

transistors gives the radial track error and a focus signal .

Automotive

Reducing Emissions - introduce feedback in the engine based on measurement of the oxygen in the exhaust.

Cruise and Traction Control - Most new cars are provided with a system for cruise control that keeps the speed constant.

Technology Drivers - large number of produced parts and hard requirements for low cost.

Autonomous Driving - developing autonomous vehicles.