Process Control ConceptsApplications and

Terminology

Materials: Mike Sharpe

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Learning ObjectivesTo Understand “Process Control”To Understand What “Control System”Implies.To Understand Basic Terminology.To Understand Hardware and Devices.To Understand Pure Control Forms.

PID (ANALOG)ON/OFF or DISCRETE (DIGITAL)

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Process Control : A Definition

Process control is the regulation or manipulation of variables that influence the function of a process. The goal of process control is to obtain a product of desired quality and quantity in an efficient manner.

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The Feedback Control LoopProcess control, both manual and automatic, is accomplished through the feedback control loop and instrumentation. The feedback control loop is a design scheme that continuously corrects or controls a system. This is accomplished by measuring a variable and comparing it to a desired value (setpoint), then taking corrective action if the two readings differ outside of allowed variance. Simply stated, process control involves four steps --SENSE, COMPARE, COMPUTE, & CORRECT.

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Process is the physical or chemical change of matter or conversion of energy, such as a change in pressure, temperature, speed, or electrical potential. Process can also be a series of regularly occurring steps or actions intended to achieve a predetermined result, such as refining oil, heat treating metal, or manufacturing paper. Each process has any number of variables (temperature, flow, pressure, level, color, pH, density, etc.) that can change.

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VARIABLES

To control the process and achieve the desired results, variables are measured, controlled, and manipulated.

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Controlled or Process Variables

The controlled or process variable is maintained (by process control) at a specified setpoint value. This is the variable which the control loop will, through measurements and field devices, attempt to keep at a pre-defined level.This variable could be temperature, flow, pressure, level, weight, speed, etc.

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Manipulated Variables

There is another variable in the basic process control model, and that is the manipulated variable. Using the example of a forced-air gas furnace, the manipulated variable is the gas flow. The manipulated and process variables are often the same.

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An Example

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DisturbanceA disturbance is a temporary, undesired change in a variable that adversely affects the value of the controlled variable. In our example of the forced-air heating system, placing a lamp near the thermostat would produce a disturbance in the system by creating a higher temperature in the vicinity of the thermostat.

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Load changesA change in a process variable results in a load change. If, in our example, a window were opened on a cold day, the temperature of the process would change. The gas valve would remain open for a longer period of time. Load changes are typically of longer duration than disturbances.

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Setpoint and DeviationSetpoint is the desired value of the controlled or process variable. There is usually a mechanism for setting the setpoint, and it can be controlled manually, automatically, or programmed electronically. In the example of the forced-air gas heater, the setpoint is the 72 degrees F mark on the thermostat. Deviation is any departure from the setpoint. A deviation range may be (and usually is) part of the design intent of the process, and therefore allowed in the process control. For example, furnaces are designed with a setpoint deviation so that the burner and fan are not triggered too frequently. In the above example, the allowed deviation is 70 degrees F to 74 degrees F (plus or minus 2 degrees F from setpoint). The gas valve will not open until the air around the thermostat drops to 70 degrees F and will not close until the air has heated to 74 degrees F.

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What Is A “Control System”?Manual Feedback or “OPEN LOOP” Control.

Completely manual, labor intensive. Accuracy and efficiency are lacking.

Automatic Feedback or “CLOSED LOOP” Control. See Feedforward and Cascade Also.

Automated computerized control, less labor and greater accuracy and efficiency.

A control system must be capable of:Measuring (PV) or Input, Comparing or Setpoint, Computing or Algorithm and Correcting or Output.

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MANUAL FEEDBACK CONTROL

Operator or Human Control

In this type of control scheme, an operator monitors the process with gauges and manually adjust valves and other devices to manipulate or balance the process.

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Manual Feedback Loop

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Automatic Feedback ControlAutomatic or Computerized Control

In this type of control scheme, a field device such as a transmitter provides a process “Input” to a controller which in turn compares that value to a pre-determined “Setpoint”. If an “Error” is detected, the controller issues a command or “Output” to a “Final Control Element”such as a control valve to adjust the process. This adjustment returns the process to its desired state.

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Automatic Feedback Loop

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Automatic Feed Forward Control

Automatic or Computerized Control, A Sub-function of Feedback Control.

In this type of control scheme, the system anticipates the effect of disturbances on a controlled output and compensates for them to minimize changes in the output.

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Cascade Loop Control

Automatic or Computerized Control, A Sub-function of Feedback Control.

In this type of control scheme, one loop is nested inside another to allow the output from one controller to become the setpoint of the second controller.

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Feed Forward and Cascade Loops

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PNEUMATIC CONTROLSOlder technology utilizing compressed air for signal input/output.

With this older technology, pneumatic field devices utilize compressed air to provide an analog input of 3-15psi to a control room “Board” or to a pneumatic final control element. Generally the control room board requires an operator to monitor the display and take manual action to correct errors noted by the field device. Two major problems with pneumatics are accuracy and response time.

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Pneumatic Control OverviewStill employed in some manufacturing facilities. When pneumatic devices exists, transducers (I/P and P/I) are employed to convert signals from impulse or electric (I) to pneumatic (P) and vice versa to allow the pneumatic devices and the controllers to communicate. A pneumatic transmitter in the field would require a P/I to convert the 3-15psi analog signal to 4-20mA analog signal in order for the DCS (HPM or PLC) to read it. A pneumatic final control element such as a control valve would require an I/P to convert the 4-20mA from the controller to a 3-15psi signal in order for the valve to respond to the signal.

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Pneumatic Control Overview

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TERMINOLOGY, GENERAL Continuous Process - A plant process that runs 24 hours a day, 7 days a week such as an oil refinery. The process is continuous provided there is not a scheduled shutdown.Batch Process- A plant process which runs a pre-determined period before stopping such as a food plant. A line will operate to produce cookie “A” before being shutdown and re-configured to produce cookie “B”. Analog Signal- An input/output signal that is measurable across a range. (4-20mA,3-15psi or 1-5vdc) These signals are used primarily for process variable measurement and final control element manipulation.Digital Signal- A signal which is defined as On/Off or Yes/No. These signals are not measurable across a range and are used primarily for motor Start/Stop, run indication, device position and alarms.

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Terminology, General Variable- In a plant process, this would be any measurable or controllable attribute such as temperature, pressure, flow, level, weight, volume, etc.

Process Variable (PV)- The variable which is monitored and maintained at a pre-defined level or “setpoint” by use of the “Control Loop”. Manipulated Variable- The variable that is actually controlled thru output from the controller to the “Final Control Element” in order to keep the “Process Variable” at its “setpoint”.

The Process and Manipulated Variables may be one and the same orseparate.

Control Loop- A design scheme which manages a single or multiple pieces of equipment in order to manipulate a process.

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Terminology, General Final Control Element- The device/equipment that is controlled or managed thru output from the “Controller” in order to manipulate a process. Input/Output- Also known as I/O, these are the signals going to or coming from the controller in order to monitor or control a process. Input is from the field device to the controller and output is from the controller to a field device. Always thought of in terms of the “Controller”. setpoint- The target value established for a process. This is the value at which the “Controller” will attempt to maintain a given process variable (PV) thru manipulation of a “Final Control Element”.

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Terminology, General Controller- A term used loosely to describe a piece of equipment that “Controls” a process variable or provides “Display” at a console. Commonly referred to as a “DCS” or “PLC”. Error- A term used to define what happens when a Process Variable (PV) moves outside its “setpoint”, an “Error” is said to exists. Cascade Control- A design feature where one control loop is nested inside another. In this type of “Loop”, the output from one controller becomes the setpoint for the second controller.Ratio Control- A control design where one process is kept at a definite proportion to another process.HMI or MMI- Human Machine or Man Machine Interface. Describes a piece of equipment that allows one to “View” a process inside the controller. The display console , “US” or “GUS”.

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Terminology, GeneralTransient Response- Describes how the controller output in a closed loop system responds.

Underdamped Response- Output reacts too quickly causing oscillation above and below the setpoint before settling in to a“Steady State”Overdamped Response- Output reacts slowly causing long delays in the process reaching a “Steady State”.Critically Damped Response- Optimum output allowing the process to reach a “Steady State” in the shortest amount of time without oscillating.

Steady State- Refers to the Process Variable returning to a normal operating condition after a Transient Response has occurred.Steady State Error- Refers to the slight difference between the actual output value and the desired output after a Transient Response has occurred.

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Terminology, GeneralStability- Ability of a system to attain Steady State control of the Process Variable (PV) after it has responded to a change. Sensitivity- Refers to the ratio of the percentage change in output to the percentage change in input. The effectiveness of the control system in responding to input changes. Process Time Lag- Refers to the time it takes a system to correct itself after a variable has changed. Dead Time- Refers to the time when a process variable is moving outside setpoint but the system has not yet sensed the need for correction. This is dependant on where sensing devices are located and how sensitive they and the controller are to change.

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HARDWARE, CONTROLLERSPLC (Programmable Logic Controller)- Originally developed to provide an automated replacement for the ON/OFF Relays used in manufacturing. This was a Digital controller that was capableof starting and stopping (ON/OFF) processes but had no measuring capabilities (Analog). Because of this limitation, the PLC was used primarily for Batch, not Continuous Processes. Other limitations were, it had no MMI/HMI capability, could not run other applications, could not store historical data and could not provide trending. Modern PLC’s do have Analog capabilities for measuring but are still suffer from the other limitations noted above. Modern PLC’s are still used primarily for Batch Processing and do not have the capacity for data processing the DCS has!

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Hardware, ControllersDCS (Distributed Control System)- The DCS was developed to provide manufacturing processes with a total control system. This system allowed for not only Digital (ON/OFF) but also Analog (Measurable Across a Range) control. The DCS gives the plant operators total control of all processes, not just Batch. This new system also provided a means for “Viewing” the system through the Console or HMI/MMI. This system is easily programmable from the console and also allows for running business applications, historical data gathering, trending and networking with other plant or company locations through the plant communications system. The latest Honeywell DCS is known as TPS or TOTAL PLANT SOLUTIONS. The TPS is comprised of several individual pieces of hardware which can be reviewed at the Honeywell Intra-net Site.

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Hardware, Field DevicesANALOG INPUT (EXAMPLES)

Transmitters -Temperature, Flow, Pressure and LevelSpeed SensorsThermocouples (T/C)RTD’sValve Position TransmittersTransducers (P/I)- Pneumatic to Impulse Converters

ANALOG OUTPUT (EXAMPLES)Valve PositionersTransducers (I/P)- Impulse to Pneumatic ConvertersSpeed Controller

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Hardware, Field DevicesDIGITAL INPUT (EXAMPLES)

Valve Position SwitchLevel SwitchSwitches in GeneralInterposing Relays (Voltage Step-up or Step-down Devices)Motor Run Status

DIGITAL OUTPUT (EXAMPLES)Solenoid ValvesRelaysMotor StartersAlarmsIndication Lights

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PROGRAMMING, PIDPID is an acronym for PROPORTIONAL, INTEGRAL and DERIVATIVE. These are forms of Pure Control which could also be called forms of Analog Control. PID is used when simple ON/OFF or Digital Control will not suffice because a RANGE is required in the Final Control Element. These are DCS (Controller) programming types used individually or combined to achieve the desired Process Control. When combined, they are known as Composite. THE FOLLOWING EXAMPLES ARE NOT INTENDED TO EXPLAIN TO THE STUDENT HOW TO PROGRAM, BUT RATHER TO FAMILIARIZE THE STUDENT WITH THE TERMS. THE MECHANICS OF PROGRAMMING ARE COVERED IN THE TPS COURSES WHICH ARE COVERED ELSEWHERE.

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(P)roportional Control Gain

A form of control where the Final Control Element such as a control valve will stroke open or closed in some proportion to the change in Error. (Deviation of Process Variable from setpoint).

The movement of this valve (OUTPUT) is fixed to a percentage of change in the Process Variable (PV). This type control does not respond well to long term or steady state changes in the process.Expressed Mathematically As:

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(I)ntegral ControlProportional plus Reset

A form of control where the Final Control Element such as a control valve will assume a new stable position while the process variable returns to the setpoint. Stable position means whatever position is necessary for the process to return to setpoint.

The movement of this valve (Output) is in response to not only the size of the error but also the length of time of the error. The Final Control Element will settle (Reset) at a new position required to return the process variable to setpoint.Expressed Mathematically As:

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(D)erivative ControlRate

A form of control where the Final Control Element such as a control valve will open or close additionally provided the error continues to change (Up or Down).

The output to adjust the valve continues only while the error continues to change. When the error stabilizes (even if outside the setpoint) this type control stops.This type control is generally not used alone but integrated with Proportional or Proportional and Integral. Expressed Mathematically As:

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Programming, ON/OFFA form of control where the output is intended to either turn on or turn off a Final Control Element such as a pump. With this form, there is no range for the position of the Final Control Element, only ON/OFF or OPEN/CLOSED. Also known as a Digital Signal.

The output from the controller is simply a signal to a device to start or stop and does not provide any type of Measurable (Range) output.Used primarily for solenoid valves (open/close), pumps and motors (start/stop), relays (interposing), alarms, indication lights and safety shutdown systems.The input from digital devices would be for position or run indication such as valve positions, level switches and motor run status.

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End

This is the end of this introduction to Process Control Concepts.