Automation And Control

Automation And Control Automation The control of an industrial process ( manufacturing, production and so on ) by automatic rather than manual means is called automation . Control system A control system is an interconnection of components forming a system configuration that will provide a desired system response. The basis for analysis of a system is the foundation provided by linear system theory, which assumes a cause-effect relationship for the components of a system. Therefore a component or process to be controlled can be represented by a block, as shown in Figure 1.1. The inputoutput relationship represents the cause-and-effect relationship of the process, which in turn represents a processing of the input signal to provide an output signal variable, often with a power amplification. Open-loop control system An open-loop control system utilizes an actuating device to control the process directly without using feedback. An open-loop control system uses a controller and an actuator to obtain the desired response, as shown in Figure 1.2. An open loop system is a system without feedback. Closed-loop control A closed-loop control system uses a measurement of the output and feedback of this signal to compare it with the desired output (reference or command). In contrast to an open-loop control system, a closed-loop control system utilizes an additional measure of the actual output to compare the actual output with the desired output response. The measure of the output is called the feedback signal. A simple closed-loop feedback control .system is shown in Figure 1.3. A feedback control system is a control system that tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control. With an accurate sensor, the measured output is a good approximation of the actual output of the system. A feedback control system often uses a function of a prescribed relationship between the output and reference input to control the process. Often the difference between the output of the process under control and the reference input is amplified and used to control the process so that the difference is continually reduced. In general, the difference between the desired output and the actual output is equal to the error, which is then adjusted by the controller. The output of the controller causes the actuator to modulate the process in order to reduce the error. The sequence is such, for instance, that if a ship is heading incorrectly to the right, the rudder is actuated to direct the ship to the left. The system shown in Figure 1.3 is a negative feedback control system, because the output is subtracted from the input and the difference is used as the input signal to the controller. The feedback concept has been the foundation for control system analysis and design. Actuator An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, typically electric current, hydraulic fluidpressure, or pneumatic pressure, and converts that energy into motion. An actuator is the mechanism by which a control system acts upon an environment. The control system can be simple (a fixed mechanical or electronic system), software-based (e.g. a printer driver, robot control system), or a human or other agent. A hydraulic actuator consist of a cylinder or fluid motor that uses hydraulic power to facilitate mechanical operation. The mechanical motion gives an output in terms of linear, rotary or oscillatory motion. Because liquid cannot be compressed, a hydraulic actuator can exert considerable force, but is limited in acceleration and speed. A pneumatic actuator converts energy formed by compressed air at high pressure into either linear or rotary motion. Pneumatic energy is desirable for main engine controls because it can quickly respond in starting and stopping as the power source does not need to be stored in reserve for operation. An electric actuator is powered by motor that converts electrical energy to mechanical torque. The electrical energy is used to actuate equipment such as multi-turn valves. It is one of the cleanest and most readily available forms of actuator because it does not involve oil. A mechanical actuator functions by converting rotary motion into linear motion to execute movement. It involves gears, rails, pulleys, chains and other devices to operate. In industrial control systems, an actuator is a hardware device that converts a controller command signal into a change in a physical parameter. The change in the physical parameter is usually mechanical. such as position or velocity change. An actuator is a transducer. because it changes one type of physical quantity. say electric current, into another type of physical quantity, say rotational speed of an electric motor. The controller command signal is usually low level, and so an actuator may also include an amplifier to strengthen the signal sufficiently to drive the actuator A list of common actuators is presented in Table 5,4. Depending on the type of amplifier used. most actuators can he classified into one of three categories: (1) electrical, (2) hydraulic, and (3) pneumatic. Electrical actuators are most common; they include ac and dc motors of various kinds, stepper motors. and solenoids. Electrical actuators include both linear devices (output is linear displacement) and rotational devices (output is rotational displacement or velocity). Hydraulic actuators use hydraulic fluid to amplify the controller command signal. The available devices provide both linear and rotational motion. Hydraulic actuators are often specified when large forces are required. Pneumatic actuators use compressed air (typically "shop air" in the factory environment) as the driving power. Again. both linear and rotational pneumatic actuators are available. Because of the relatively low air pressures involved, these actuators are usually limited to relatively low force applications compared with hydraulic actuators. EXAMPLES OF CONTROL SVSTEMS Transducers A transducer is a device that is used to convert a physical quantity into its corresponding electrical signal. In most of the electrical systems, the input signal will not be an electrical signal, but a nonelectrical signal. This will have to be converted into its corresponding electrical signal if its value is to be measured using electrical methods. The block diagram of a transducer is given below. Transducer Block Diagram A transducer will have basically two main components. They are 1. Sensing Element The physical quantity or its rate of change is sensed and responded to by this part of the transistor. 2. Transduction Element The output of the sensing element is passed on to the transduction element. This element is responsible for converting the non-electrical signal into its proportional electrical signal. There may be cases when the transduction element performs the action of both transduction and sensing. The best example of such a transducer is a thermocouple. A thermocouple is used to generate a voltage corresponding to the heat that is generated at the junction of two dissimilar metals. Selection of Transducer Selection of a transducer is one of the most important factors which help in obtaining accurate results. Some of the main parameters are given below. • Selection depends on the physical quantity to be measured. • Depends on the best transducer principle for the given physical input. • Depends on the order of accuracy to be obtained. Transducer Classification Some of the common methods of classifying transducers are given below. • Based on their application. • Based on the method of converting the non-electric signal into electric signal. • Based on the output electrical quantity to be produced. • Based on the electrical phenomenon or parameter that may be changed due to the whole process. Some of the most commonly electrical quantities in a transducer are resistance, capacitance, voltage, current or inductance. Thus, during transduction, there may be changes in resistance, capacitance and induction, which in turn change the output voltage or current. • Based on whether the transducer is active or passive. Transducer Applications The applications of transducers based on the electric parameter used and the principle involved is given below. • • • 1. Passive Type Transducers a. Resistance Variation Type Resistance Strain Gauge – The change in value of resistance of metal semi-conductor due to elongation or compression is known by the measurement of torque, displacement or force. Resistance Thermometer – The change in resistance of metal wire due to the change in temperature known by the measurement of temperature. Resistance Hygrometer – The change in the resistance of conductive strip due to the change of moisture content is known by the value of its corresponding humidity. • Hot Wire Meter – The change in resistance of a heating element due to convection cooling of a flow of gas is known by its corresponding gas flow or pressure. • Photoconductive Cell – The change in resistance of a cell due to a corresponding change in light flux is known by its corresponding light intensity. Thermistor – The change in resistance of a semi-conductor that has a negative co-efficient of resistance is known by its corresponding measure of temperature. • • • • • • • • • • • • • • • Potentiometer Type – The change in resistance of a potentiometer reading due to the movement of the slider as a part of an external force applied is known by its corresponding pressure or displacement. b. Capacitance Variation Type Variable Capacitance Pressure Gauge – The change in capacitance due to the change of distance between two parallel plates caused by an external force is known by its corresponding displacement or pressure. Dielectric Gauge – The change in capacitance due to a change in the dielectric is known by its corresponding liquid level or thickness. Capacitor Microphone – The change in capacitance due to the variation in sound pressure on a movable diagram is known by its corresponding sound. c. Inductance Variation Type Eddy Current Transducer – The change in inductance of a coil due to the proximity of an eddy current plate is known by its corresponding displacement or thickness. Variable Reluctance Type – The variation in reluctance of a magnetic circuit that occurs due to the change in position of the iron core or coil is known by its corresponding displacement or pressure. Proximity Inductance Type – The inductance change of an alternating current excited coil due to the change in the magnetic circuit is known by its corresponding pressure or displacement. Differential Transformer – The change in differential voltage of 2 secondary windings of a transformer because of the change in position of the magnetic core is known by its corresponding force, pressure or displacement. Magnetostrictive Transducer – The change in magnetic properties due to change in pressure and stress is known by its corresponding sound value, pressure or force. d. Voltage and Current Type Photo-emissive Cell – Electron emission due to light incidence on photo-emissive surface is known by its corresponding light flux value. Hall Effect – The voltage generated due to magnetic flux across a semi-conductor plate with a movement of current through it is known by its corresponding value of magnetic flux or current. Ionisation Chamber – The electron flow variation due to the ionisation of gas caused by radio-active radiation is known by its corresponding radiation value. 2. Active Type Photo-voltaic Cell – The voltage change that occurs across the p-n junction due to light radiation is known by its corresponding solar cell value or light intensity. Thermopile – The voltage change developed across a junction of two dissimilar metals is known by its corresponding value of temperature, heat or flow. • • Piezoelectric Type – When an external force is applied on to a quartz crystal, there will be a change in the voltage generated across the surface. This change is measured by its corresponding value of sound or vibration. Moving Coil Type – The change in voltage generated in a magnetic field can be measured using its corresponding value of vibration or velocity.