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AUTOMATION & ROBOTICS LECTURE#07 AUTOMATION PROCESS CONTROL By: Engr. Irfan Ahmed Halepoto Assistant Professor
Automation Technology • Automation Technology is a term that can be used to describe an industry that makes use of Instrumentation to meet the objectives of its existence. – i. e. produce a high quality product (or service) in an efficient and profitable manner while maintaining a safe and healthy environment. • Automation Technology provides engineered solutions utilizing various techniques and technologies for the simplest to the most complex industrial applications • Instrumentation plays an important role in almost every aspect of Automation Technology. • Everyone needs to measure and/or control something – and that’s what instrumentation is all about.
Automation Technology Classification Automation is generally classified as : 1. Industrial Automation (Robot Automation) 2. Process Automation 3. Manufacturing Automation 4. Building Automation (Home or Organization Automation) • The field of Automation Technology is wide and diverse and comes in many different flavors, but the main ingredient remains the same – Instrumentation.
Automation Technology Classification Industrial Automation Process Automation Manufacturing Automation Building Automation
Instrumentation • Instrumentation is the science of (automated) measurement and control. • The first step, is measurement. If we can’t measure something, it is pointless to try to control it. Things that are measured include: • Pressure, temperature, level, flow, humidity, speed, motion, position, weight, density, conductivity, p. H, light, quality, quantity, and more. Devices that process or do the measuring are called: • Sensors, transducers, transmitters, indicators, displays, recorders, data loggers, and data acquisition systems.
Process Automation • Process automation makes use of instrumentation to maintain the process at some desired condition. • The process plant is represented by an input/output block. • In a process plant, controller signal will operate on an input to the process, known as the ‘manipulated variable’. • The output of the process is derived to a particular value or set point by changing the input. • The output can also be affected by other external conditions regarded as ‘disturbance inputs’ and our control action will need to overcome their influences. • Challenge for the process control designer is to maintain the controlled process variable at the target value. – Example: if we want to keep the level of water in a tank at a constant height while others are drawing off from it, we will manipulate the input flow to keep the level steady.
Process Automation Mechanism • Initially we sense or measure any specific quantity. • After measurement, we transmit a signal representing this quantity to an indicating or computing device where either human or automated action takes place. • If controlling action is automated, machine sends a signal to a final controlling device which then influences the quantity being measured. • Final control device usually takes one of the following forms: – Control valve , Electric motor or Electric heater. • Both the measurement device and the final control device connect to some physical system which we call the process.
Process Signals • Two of the most important signals used in process control are: – Process Variable (PV) – Manipulated Variable (MV) • In industrial process control, Process Variable (PV) is measured by an instrument in the field and acts as an input to an automatic controller which takes action based on the value of it. – Process Variable can be an input to a data display so that the operator can use the reading to adjust the process through manual control and supervision. • The variable to be manipulated, in order to have control over the PV, is called the Manipulated Variable. – If we control a particular flow for instance, we manipulate a valve to control the flow. – Here, the valve position is called the Manipulated Variable and the measured flow becomes the Process Variable.
Process Control • Process control deals with architectures, mechanisms and algorithms for controlling the output of a specific process. • Process control requires sensors for measuring variables & valves for implementing decisions. – Example: Heating up the temperature in a room is a process that has the specific, desired outcome to reach and maintain a defined temperature (e. g. 20°C), kept constant over time. • Here, the temperature is the controlled variable. • At the same time, it is the input variable since it is measured by a thermometer and used to decide whether to heat or not to heat. • The desired temperature (20°C) is the setpoint. • The state of the heater (e. g. the setting of the valve allowing hot water to flow through it) is called the manipulated variable since it is subject to control actions.
Example of a Temperature Process Objective: To maintain a constant water bath temperature. • The control objective is to maintain the temperature of a water bath at a constant temperature (e. g. 20°C). • A steam heating element will be used to supply the required heat energy to maintain the bath at the desired temperature. • By opening and closing the control valve the amount of steam flowing through the heating element will determine the temperature of the water bath. (i. e. opening the valve increases the bath temperature)
Process Controllers • Industrial processes require that certain variables such as temperature, flow, level or pressure, remain at or near some reference value (set point). – Closed-loop control is used to achieve this. • The process controller looks at a signal representing the process value, compares it to the desired set point and acts on the process to minimize the difference (error). • Method used by the controller to correct the error is the control mode. • Most popular control modes are proportional, integral and derivative. • Controllers are designed to eliminate the need for continuous operator attention. – House thermostat is a common example of how controller is used to automatically adjust some variable to hold the measurement (or process variable) at the set-point. – The set-point is where you would like the measurement to be. – Error is defined as the difference between set-point & measurement. (Error) = (set-point) - (measurement) • The variable being adjusted is called the manipulated variable which usually is equal to the output of the controller.
Process Controllers • Process Controllers provide the required control action to position the FCE at a point necessary to maintain the PV at the desired SP. – Programmable Logic Controllers (PLCs ) – Programmable Automation Controllers (PAC) – Distributed Control Systems (DCS) – Proportional, Integral, Derivative (PID) Controllers – Supervisory Control and Data Acquisition (SCADA) Control Elements: These are the devices the controller operates: – Pneumatic valves, solenoid valves, rotary valves, motors, switches, relays, variable frequency drives.
Process Controller Parameters • A measuring unit with an appropriate instrument to measure the state of process, a temperature transmitter, pressure transmitter or similar. • A input set point device to set the desired value. • A comparator for comparing the measured value with the set point, calculating the difference or error between the two. • A control unit to calculate the output magnitude and direction to compensate the deviation from the desired value. • A output unit converting the output from the controller to physical action, a control valve, a motor or similar.
Process Control System • A process control system is based on a control loop, which has four main components: 1. A measurement of the state or condition of a process 2. A controller calculating an action based on this measured value against a pre-set or desired value (set point). 3. An output signal resulting from the controller calculation which is used to manipulate the process action through some form of actuator. 4. The process itself reacting to this signal, and changing its state or condition.
Process Control: Water Tank with Controlled Inflow • An open tank with cross sectional area A is supplied with an inflow of water Q 1 that can be controlled or manipulated. • The outflow from the tank passes through a valve with a resistance R to the output flow Q 2. • The level of water or pressure head in the tank is denoted as H. • We know that Q 2 will increase as H increases and when Q 2 equals Q 1 the level will become steady. water tank with controlled inflow block diagram of tank process
Process Model • The process is maintained at the desired point (SP) by changing the FCE based on the value of the PV Control Agent FINAL CONTROL ELELMENT (valve) Actuating Input Manipulated Variable Controlled Variable PROCESS (Temperature, pressure, level, flow) p. H, conductivity, humidity, density, consistency, etc. Desired Result Measuring Means (transmitter) Process Variable (PV) • Process equilibrium (balance) is when the input energy maintains the output at a constant “desired” point.
Process Model • The basic model of a “process” can be applied to most measurement and control applications used in Automation Technology. • The major components are the Final Control Element, the actual Process that is being controlled and the Measuring Means. • The loop is completed when the Controlling Means (not shown here) is added, this can be done manually (open loop control) or via some controlling device (closed loop control or automatic control or process control). • The Final Control Element is defined as “the device that directly controls the manipulated variable of a control loop”, the most common type being the control valve (pneumatic, electric, solenoid). • However there are many other devices that fit the definition of a FCE and may include things such as relays or switches, and variable speed drives, mixers, motors or pumps.
Process Model…. . Measuring Unit • The measuring means provides the standardized signal that represents the condition of the process, i. e. is the process at the desired point? Control Agent FINAL CONTROL ELELMENT (valve) Actuating Input Controlled Variable Manipulated Variable PROCESS (Temperature, pressure, level, flow) p. H, conductivity, humidity, density, consistency, etc. Desired Result Measuring Means (transmitter) Process Variable (PV)
Process Model…. . Measuring Unit • The Measuring Means is that portion of the control loop that measures the controlled variable and provides the necessary information required to determine if the process is at its desired condition. • The measuring means can consist of one standalone field device or it can be made up of several individual components depending on what type of controlled variable is being measured. • The output is then conditioned and standardized to either a pneumatic or electrical (analog/digital) signal that represents the condition of the process. • The output is also referred to as the Process Variable (PV) and device is often called a transmitter, (e. g. temperature transmitter, pressure transmitter)
Measuring Means Pressure Strain gauge Piezo-electric Capacitance Bourdon Tube Level Mechanical Floats Guided Wave Weight (load cell) Ultrasonic Differential Pressure Flow Head meters (orifice, venturi) Coriolis, velocity, Mass, Temperature p. H Humidity Density Speed Transmitters Pressure Transmitter Pneumatic 3 -15 PSI Electrical Level Transmitter Current 4 – 20 m. A 0 – 20 m. A 10 – 50 m. A Voltage Flow Transmitter Thermocouples RTDs / Thermistors Filled Systems Bi-metallic 0– 5 V 1– 5 V 0 – 10 V Digital ON/OFF Temperature Transmitter Field Bus Mod. Bus Profi. Bus HART
Measurement & Instrumentation Process Control
Process Control Terms • Process: Physical system we are attempting to control or measure. – Examples: water filtration system, steam boiler, oil refinery unit, power generation unit etc. • Process Variable(PV): Specific quantity we are measuring in a process. – Examples: pressure, level, temperature, flow, electrical conductivity, position, speed, vibration etc. • Set point (SP) : Value at which we desire the process variable to be maintained at. In other words, the “target” value of the process variable. • Primary Sensing Element (PSE): Device that directly senses the process variable and translates that sensed quantity into an analog representation (electrical voltage, current, mechanical force, motion, etc. ). – Examples: thermocouple, thermistor, bourdon tube, microphone, potentiometer, accelerometer etc. • Transducer: Device that converts one standardized instrumentation signal into another standardized instrumentation signal, and/or performs some sort of processing on that signal. – Examples: I/P converter (converts 4 -20 m. A electric signal into 3 -15 psi pneumatic signal), P/I converter (converts 3 -15 psi pneumatic signal into 4 -20 m. A electric signal).
Process Control Terms • Transmitter: Device that translates the signal produced by a primary sensing element (PSE) into a standardized instrumentation signal. – Examples: 3 -15 psi air pressure, 4 -20 m. A DC electric current etc. , which may then be conveyed to an indicating device, a controlling device, or both. • Controller: Device that receives a process variable (PV) signal from a primary sensing element (PSE) or transmitter, compares that signal to the desired value for that process variable (set point), and calculates an appropriate output signal value to be sent to a final control element (FCE) such as an electric motor or control valve. • Final Control Element (FCE): Device that receives the signal from a controller to directly influence the process. – Examples: variable-speed electric motor, control valve, electric heater. • Manipulated Variable (MV): Term used to describe the output signal generated by a controller. This is the signal commanding (“manipulating”) the final control element to influence the process. • Automatic mode: When the controller generates an output signal based on the relationship of process variable (PV) to the set point (SP). • Manual mode: When the controller’s decision-making ability is bypassed to let a human operator directly determine the output signal sent to the final control element.
Instrumentation & Process Control Flow Chart
Process Control System • Processes are dynamic in nature, and changes are always occurring. • GOAL: Maintaining process variables (temperatures, pressures, flows, compositions, levels) at a desired operating value. – Manual process control (Open Loop Control) – Automatic process control (Closed Loop Control)
Open Loop Control • Open loop (or manual control) is used when very little change occurs in the Process Variable (PV). Control Agent FINAL CONTROL ELELMENT (valve) Actuating Input Manipulated Variable Controlled Variable PROCESS Desired Result (Temperature, pressure, level, flow) p. H, conductivity, humidity, density, consistency, etc. Measuring Means (transmitter) Process Variable (PV) Corrective action is provided by manual feedback
Closed Loop Control • Closed loop or feedback control provides a corrective action based on the deviation between the PV and the SP Control Agent Manipulated Variable FINAL CONTROL ELELMENT Controlled Variable PROCESS (valve) p. H, conductivity, humidity, density, consistency, etc. Automatic Desired Result (Temperature, pressure, level, flow) Measuring Means (transmitter) Manual Controller Output CONTROLLING MEANS Controller Input (PV) (3 -15 psi, 4 -20 m. A etc) (3 -15 psi, 4 -20 m. A etc) SP