Fundamentals of Paper Machine Control
Short Description
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Description
Fundamentals of Paper Machine Control
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Purpose
The purpose of this training is to better understand how a Honeywell Measurex system contributes to the optimization of the papermaking process and to assist with troubleshooting the mill-Honeywell Measurex interface.
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What Will We Be Doing? We will discuss: • The theory of feedback control systems • Typical process behavior • How Honeywell Measurex models process behavior • How process behavior characteristics are determined • How supervisory control loops work • Cross Direction control • Tips for maintaining optimum performance
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How Will I Benefit? • You will have a better understanding of the role of Honeywell Measurex measurement and control in papermaking
• You will be better able to differentiate between Honeywell Measurex system and mill field device problems.
• You will learn basic operation of the Honeywell - Measurex system Operator Interface.
• Honeywell - Measurex service personnel and mill personnel will be able to communicate better
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The Fourdrinier Paper Machine
HEADBOX
STOCK PREPARATION
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FOURDRINIER TABLE
CALENDER STACK
PRESS SECTION
DRYER SECTION
REEL
Honeywell Measurex SCANNER
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Stock Preparation Ad
dit iv
es
Pulp W
r e t a
The basic objective in stock preparation is to mix fibrous raw materials (pulp), non-fibrous components (additives), and water into a uniform papermaking stock. To Headbox
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The Headbox
SLICE PROFILE ADJUSTER
RECTIFIER ROLLS
The headbox takes the incoming pipeline stock flow and distributes it evenly across the forming fabric.
FORMING FABRIC
FROM STOCK PREPARATION
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The Fourdrinier Table
By means of an endless, fourdrinier fabric the papermaking stock is formed into a continuous sheet while the fourdrinier table drains the water by suction forces.
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The Fourdrinier Table
SUCTION COUCH ROLL
BREAST ROLL FORMING FABRIC
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The forming fabric (wire) is an endless, finely woven belt (usually a plastic mesh) that travels between two large rolls, the breast roll and the couch roll. The breast roll is solid and serves only to support the fabric. The couch roll is a hollow, perforated shell containing one or two stationary high-vacuum suction boxes for dewatering the sheet. Solutions for Superior Results
The Fourdrinier Table The first static element under the wire is the forming board. It supports the wire and is used to retard initial drainage, so that fines and fillers are not washed through the sheet. FORMING BOARD
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HYDROFOIL ASSEMBLIES
The next element, the hydrofoil assemblies, provide sheet drainage by inducing suction.
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The Fourdrinier Table
“Wet boxes” use vacuumassisted suction to drain the sheet.
WET BOXES
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DRY BOXES
“Flat boxes” or “dry boxes” also use vacuum-assisted suction, however, a much higher pressure is used.
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The Fourdrinier Table
DANDY ROLL
LUMP BREAKER ROLL
The dandy roll reworks the top section of the sheet to improve formation and surface characteristics. The lump breaker roll improves water-removal at the couch and consolidates the sheet.
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The Press Section
PRESS ROLLS
PRESS FELT
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The rather fragile sheet is transferred from the forming section and conveyed on specially-constructed felts through a series of roll press nips into the dryer section. The objective of the press section is to remove water and force the fibers into intimate contact.
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The Dryer Section
TOP FELT
DRYER CANS
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BOTTOM FELT
The wet web from the press section, containing 55 - 60% moisture, is passed over a series of rotating steamheated cylinders (dryer cans) where water is evaporated and carried away by ventilation air. The web is held tightly against the cylinders by a synthetic, permeable fabric called a dryer felt.
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The Calender Stack
CALENDER STACK
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Calendering is a general term meaning pressing with a roll. The sheet is calendered through a series of roll nips to reduce thickness and smooth the surface.
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The Reel
REEL DRUM
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REEL SPOOL
FINISHED REEL
After drying and calendering, the paper product must be collected in a convenient form for subsequent processing. Typically, this is done by winding the sheet onto a reel.
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Measurement and Control
All industrial processes have inherent variability which must be minimized if the plant operation is to yield a uniform, high-quality product that makes efficient use of raw materials, energy, and staff. Minimizing the variation requires automation of various aspects of the operation. A typical automatic control loop consists of three basic components: sensor, control element, and controller.
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Functional Structure of Feedback Control
Sensor - The measuring component
SENSOR
CONTROLLER
CONTROL ELEMENT
Controller - Compares the measurement to the set point to determine the amount of “error” Control Element - Actuates toward reducing the error
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Basic Control Concepts
The variables involved: FT STOCK FLOW MANIPULATED VARIABLE
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CONTROLLED VARIABLE
Controlled Variable - a measurement of the variable you wish to control Manipulated Variable - some method to manipulate or change the controlled variable, often done through the use of a control valve
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Basic Control Concepts
The variables involved: Setpoint - the desired value of the controlled variable Disturbances - any variable, other than the manipulated variable, that drives the controlled variable away from the desired setpoint
DISTURBANCES
MANIPULATED VARIABLE
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PROCESS
CONTROLLED VARIABLE
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Manual Control
FT STOCK FLOW
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Before studying automatic process control, it’s helpful to spend a moment reviewing typical manual operation. In manual, the operator visually inspects the current value of the controlled variable (in this example stock flow). Then, if desired, the operator can change the flow by manipulating the valve. The operator acts as the controller.
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Automatic Control
CONTROLLER
FT STOCK FLOW
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Automatic control of a process is done by sending the sensor output to the controller. The controller compares the actual value to the desired value. Then, the controller calculates the change needed to the manipulated variable in order to bring the controlled variable to the desired value.
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Automatic Control
SETPOINT
The feedback control system monitors the process and automatically makes adjustments as required.
CONTROLLER
FT
The operator only needs to provide the setpoint to the controller.
STOCK FLOW
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Measurement To better understand how feedback control systems work we need to take a closer look at its three components. We will begin with sensors.
SENSOR
CONTROLLER
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CONTROL ELEMENT
To minimize product variation requires automation of various aspects of the operation - a process that begins by getting accurate and reliable measurements. These measurements are the heart of any control system. Solutions for Superior Results
Measurements
Our review of measurement will focus on the typical process sensors found in modern paper mills, including: • Stock Flow
• Steam Pressure
• Head Box Pressure
• Machine Speed
• Head Box Level
• Temperature
• Consistency
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Measurements We will also review the Honeywell - Measurex sensors found on the online scanners.
• Basis Weight • Caliper • Moisture - Reel
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Elements and Transmitters
TRANSMITTER
FT FE
There are two components used in measurement, the element which is the sensing device and the transmitter which converts the sensor’s output into a usable signal by the measurement and control system. The element and transmitter are shown for stock flow measurement.
ELEMENT
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Stock Flow Measurement
STOCK PREPARATION
HEADBOX FT
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To ensure consistent product quality the stock entering the headbox is measured as it enters the headbox.
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Magnetic Flow Meters
Although flow transmitters come in many different types, stock flow is typically measured using a magnetic flow meter.
UPPER MAGNETIC COIL
The measurement principle is based on Faraday’s Law of Magnetic Induction. LOWER MAGNETIC COIL
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Magnetic Flow Meters
e B
e
V
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Faraday’s Law states that a charged particle passing through a magnetic field produces an electromotive force (e) that is perpendicular to both the magnetic field (B) and the velocity vector (V).
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Magnetic Flow Meters Stock flowing through the magnetic field produces a voltage proportional to the flow rate (as the flow increases the voltage increases).
ELECTRODE PAIR
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This voltage is measured by an electrode pair, amplified, and sent to a computer for processing.
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Magnetic Flow Meters
Advantages • Obstructionless flow path • No pressure loss • Can be used in corrosive service
Disadvantages • High cost • Fluid must be a conductor of electricity
• Calibration can shift due to electrode coating
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Headbox Pressure Measurement
PT
SLICE PROFILE ADJUSTER
RECTIFIER ROLLS
The function of the headbox is to deliver stock to the wire at a uniform velocity. The discharge velocity of the stock depends directly on the headbox pressure (total head) therefore, measurement and control of total head is critical.
FORMING FABRIC
FROM STOCK PREPARATION
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The Strain Gage
P2 DIAPHRAGM
STRAIN GAGES
One of the most common ways to measure pressure in process control applications is through the use of a strain gage sensor. The sensor uses a material whose electrical resistance changes as a function of how much it is bent. This resistive material is bonded to a metal diaphragm.
P1
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The Strain Gage When pressure is applied to the diaphragm, two of the four strain gages will be in compression (their resistance decreases) and two will be in tension (their resistance increases).
P2
P2 STRAIN GAGE
DIAPHRAGM
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P1
P1
P1 = P2
P1 > P2
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The Strain Gage
The four strain gages are connected into a Wheatstone bridge circuit to yield an electrical signal proportional to the strain or pressure on the diaphragm.
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Headbox Level Measurement P P2
SLICE PROFILE ADJUSTER
Similar to headbox pressure, headbox level is a critical factor in producing a uniform sheet. Strain gage technology is used to measure the pressure at two points in the headbox (P1 and P2). The transmitter converts the difference between P1 and P2 to a level measurement.
P1
RECTIFIER ROLLS
FORMING FABRIC
FROM STOCK PREPARATION
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Consistency Measurement
STOCK PREPARATION
HEAD BOX CT
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Consistency is a measure of the density of the stock. Changes in the stock make-up or the refining process can cause the consistency to change. Since these changes impact sheet weight, consistency measurement and control is necessary to produce a high quality product.
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Consistency Measurement Obtaining accurate and reliable consistency measurement is difficult. One technique often used is to have a motor drive a paddle wheel in the stock, and monitor the current draw on the motor (as the density of the stock increases the motor will draw more current).
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Steam Pressure After pressing, the sheet is conveyed through the dryer section where residual water is removed by evaporation. The evaporation rate is directly related to the steam pressure inside the dryer cans. Steam pressure is measured using strain gage technology, discussed earlier.
DRYER CANS
PT
STEAM FROM BOILER
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Speed Measurement Speed measurements are used for machine set-up and production information. Typically they are taken at the fourdrinier table (wire speed) and at the dryer section or reel (machine speed).
HEADBOX
STOCK PREPARATION
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FOURDRINIER TABLE
CALENDER STACK
PRESS SECTION
DRYER SECTION
REEL
Honeywell Measurex SCANNER
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Digital Tachometers
Speed measurement, on modern paper machines, is typically done using digital tachometers.
DIGITAL TACHOMETER
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Digital Tachometers
MAGNETS
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Similar to a generator, magnets are rotated past a coil of wire creating an electrical current. This current is in the form of a square wave whose frequency is directly related to the speed of rotation.
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Temperature Measurement The two most common sensors for temperature measurement are the thermocouple and the resistance temperature detector.
CERAMIC POWDER
PROTECTIVE SHEATH
ALLOY A HOT JUNCTION
CURRENT ALLOY B
LEAD WIRES
COLD JUNCTION PLATINUM ELEMENT
THERMOCOUPLE RESISTANCE TEMPERATURE DETECTOR
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Thermocouples
ALLOY A HOT JUNCTION
CURRENT ALLOY B
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COLD JUNCTION
Thermocouples use two wires of unlike metals, joined at one end called the “hot” end. At the cold end the open circuit voltage is measured. This voltage depends upon the difference in temperature between the hot and cold junctions and the Seebeck coefficient of the two metals.
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Resistance Temperature Detectors
CERAMIC POWDER
PROTECTIVE SHEATH LEAD WIRES
Resistance temperature detectors (RTDs) use high resistance wire whose resistance changes relative to temperature. Changes in resistance are detected by Wheatstone bridge circuits.
PLATINUM ELEMENT
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Signal Flow
4 - 20 ma TO Honeywell Measurex
FT
Typically, the current produced by these sensors is amplified and transmitted as a 4 - 20 ma signal (the exception is the digital tachometer). This signal is sent through a pair of wires to the Honeywell Measurex system for processing.
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The Controller The objective of the controller is to:
SENSOR
• process the input signal from the sensor • compare the desired process value to the input signal from the sensor
CONTROLLER
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CONTROL ELEMENT
• act to reduce the error and bring the actual value toward the desired value
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The Controller All feedback control systems have a comparator and a controller.
COMPARATOR SETPOINT
CONTROLLER
TO CONTROL ELEMENT
FROM SENSOR
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The Comparator The comparator takes the difference between the setpoint and the sensor signal. The error signal, which is the output of the comparator, becomes the input to the feedback controller.
COMPARATOR SETPOINT
CONTROLLER
TO CONTROL ELEMENT
FROM SENSOR
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The Controller Based on the error signal from the comparator, the controller calculates a signal to the final element. The mathematical function performed in the controller is determined by the process dynamics. Today’s modern controllers use proportional, integral, and derivative (PID) functions.
COMPARATOR SETPOINT
CONTROLLER
TO CONTROL ELEMENT
FROM SENSOR
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Control Elements
SENSOR
CONTROLLER
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CONTROL ELEMENT
The output of the controller is a signal to the final control element which governs the control of the manipulated variable.
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Control Valves SIGNAL FROM CONTROLLER
ACTUATOR CONTROL VALVE
In many process control systems, the final control element is a valve - which is typically driven by an actuator. The actuator moves the stem of the control valve to open or close the valve. Control valve actuators may be pneumatic, electric, hydraulic, or manual.
A TYPICAL SLIDING STEM CONTROL VALVE WITH A PNEUMATIC ACTUATOR
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Control Examples
SETPOINT
CONTROLLER
FT STOCK FLOW
The amount of stock placed on the wire determines the basis weight of the finished sheet. For example; if the sheet was too light, the operator would increase the setpoint of the stock flow controller. This action would cause the stock flow valve to open, placing more stock on the wire and increasing the sheet weight.
STOCK FLOW
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Control Examples DILUTION WATER
The consistency of the stock is also a contributing factor in the basis weight of the finished sheet. As the consistency increases the sheet will get heavier. Attempts to maintain the consistency at the desired level are made by adjusting the amount of dilution water added to the system.
SETPOINT
DILUTION VALVE
CONTROLLER
CT STOCK FLOW CONSISTENCY
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Control Examples DRYER CANS
CONTROLLER
PT
The amount of steam in the dryer cans determines the moisture of the finished sheet. For example; if the sheet was too wet the operator would increase the setpoint of the steam pressure controller. This action would cause the steam valve to open, putting more steam in the dryer cans - which would dry the sheet.
STEAM FROM BOILER STEAM PRESSURE
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Control Examples
COUCH PRESS
DRYERS MASTER SPEED CONTROLLER
CALENDER
Many paper machines use a steam turbine to drive the machine through a line shaft and a series of belts and pulleys. To control the machine speed, the master speed controller makes outputs to the turbines’ speed control or governor.
REEL
MACHINE SPEED
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TACHOMETER
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OUTPUT %
Process Dynamics
MANIPULATED VARIABLE
All processes exhibit some form of dynamic behavior. To study this behavior, it’s helpful to look at how a process responds to a step change in the manipulated variable.
CONTROLLED VARIABLE
TIME
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Process Dynamics
OUTPUT %
MANIPULATED VARIABLE
The most common type of dynamic behavior is the firstorder lag. It’s called this because the output lags the input by an amount that can be described by a first-order differential equation. A simplified equation that describe 1st order lag is
CONTROLLED VARIABLE
1− e
æ ç ç è
− t ö÷÷ TC ø
TIME The Response of a First-Order Lag to a Step Input
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The First-Order Lag
OUTPUT %
The shape of the response curve is described by a time constant. 100 90 80 70 60 50 40 30 20 10 TIME CONSTANT
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The time constant is the amount of time it takes the process to experience 63.2% of the remaining change.
TIME
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OUTPUT %
The First-Order Lag
There are two other characteristics that are important factors in processes with firstorder lags:
100 90 80 70 60 50 40 30 20 10 TIME CONSTANT
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• Process Gain • Time Delay
TIME
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Process Gain
OUTPUT %
MANIPULATED VARIABLE ∆ MV CONTROLLED VARIABLE
Process gain is determined by changing the manipulated variable and observing the change that occurs in the controlled variable. G = ∆CV / ∆MV
∆ CV
TIME
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OUTPUT %
Time Delay In many processes you will see a response curve that exhibits a time delay before any dynamic response is observed.
100 90 80 70 60 50 40 30 20 10
Time delay can be calculated by subtracting the time when the process was changed from the time the change was first observed. TIME DELAY
TIME
PROCESS CHANGED
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PID Controllers For Da Vinci, a DCS System is used whenever outputting to a final control element (e.g. valve). These systems have many options and can perform many functions. A common controller is the Proportional Integral Derivative (PID) controller. These are implemented in most DCS systems and as single loop controllers.
SETPOINT
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PID Components
OUTPUT Components
TO CONTROL ELEMENT
INPUT Components
FROM SENSOR
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Input The purpose of the PID loop controller input is to convert process inputs into engineering units.
SETPOINT
PID Components
OUTPUT Components
TO CONTROL ELEMENT
INPUT Components
FROM SENSOR
Single Loop Controller
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PID Input
INPUT Components
The 4 - 20 ma current signal from the field transmitter is connected to the input terminals.
FT
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ADC Conversion
0 - 20 ma
0 - 5VDC
ADC CONVERTER
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Many controllers utilize digital electronic components. To process information from a field transmitter, the signal from the transmitter must be converted .
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Filtering
CONVERSION VOLTAGE TO ENG. UNITS
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FILTER
To attenuate noise from the transmitter the raw signal is smoothed using an exponential filter. This type of filtering is similar to RC circuits and is useful when the noise frequency is below 1 Hz.
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Filtering The configured Filter Factor (FF) is defined by the following equation: FF = [1 - exp(-t/TC)]
Where:
t = sample rate TC = the desired time constant
The Filtered Value (value after the filter) is calculated using the filter factor. FV = New Reading - [(New Reading - Old FV) * (1 - Filter Factor)]
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Filtering
1.0
FF = .8 FF = .4 FF = .2
0.5
0.0 0.5 1.0 1.5 2.0 2.5
Time (SEC)
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This graph shows the effects of filtering a step response using three different filter factors (.8, .4, and .2 with a sample rate t = .25). Note: filter factors will always be between 0 and 1, and as they approach 1 less filtering is applied.
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Alarm Limits
FILTER
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ALARM LIMITS
The controller can perform alarm checks on each input signal. These alarms warn operating personnel of various undesirable conditions depending on the type of controller and controller mode.
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Setpoint Comparison The processed input signal is then compared to the desired setpoint. Differences between the actual value and the setpoint are called “error”.
SETPOINT
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PID Components
OUTPUT Components
TO CONTROL ELEMENT
INPUT Components
FROM SENSOR
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The PID Controller All controllers function as a special-purpose calculator. Using the error signal from the comparator they calculate the changes needed in the manipulated variable. The PID Controller allows you to configure any combination of proportional, integral, or derivative control actions. The PID Controller computes a control move for the Output device, based on the error between the PID’s setpoint and the process input.
SETPOINT
PID Components
OUTPUT Components
TO CONTROL ELEMENT
INPUT Components
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FROM SENSOR
Closing the Loop
So far we have devoted our attention to the open-loop behavior of the individual components of a process control system.
SENSOR
CONTROLLER
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CONTROL ELEMENT
Closing the loop refers to how the process behaves when each of these components function as one unit.
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Closing the Loop
OPEN-LOOP
CLOSED-LOOP
Each component of a control system has it’s own dynamic behavior. When placed together in a closed-loop, it’s likely that the system’s output will oscillate.
TIME
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Effects of Time Constant & Time Delay
In general, the more time constants and time delays associated with a control loop, the worse the control problem.
TIME
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It doesn’t make any difference if the time constant or time delay is associated with the valve, the sensor, or the process itself, it will have the same ill effect on the process.
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Stability vs Speed
Therefore, closing the loop requires a trade off between speed of response and stability. For example, if we decreased the controller gain the loop would become more stable, however, it would respond slower. TIME
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Optimum Control
Optimum control is achieved by tuning the various parameters within the controller to insure a proper trade off between stability and speed of response.
TIME
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Tuning Parameters Control tuning refers to the process of determining the optimum parameters for the controller. Control loops use some or all of the following tuning parameters:
– Gain (Process or Proportional) – Integral Gain (or Reset) – Time Constant – Time Delay – Dead Zone •Valves can exhibit – Stiction – Backlash
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Process Gain Previously we defined process gain as ∆CV / ∆MV. Sometimes it’s helpful to think of process gain as a units converter.
SETPOINT (STOCK FLOW)
CONTROLLER
TO STOCK VALVE (SECONDS)
FROM SENSOR
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Process Gain
OUTPUT %
MANIPULATED VARIABLE ∆ MV CONTROLLED VARIABLE
To calculate the process gain make a “bump” to the process. Then divide the change in the controlled variable (∆ ∆CV) by the change in the manipulated variable (∆ ∆MV) .
∆ CV
TIME
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Negative Gain Factor SIGNAL FROM CONTROLLER
ACTUATOR
The negative gain factor is used to modify the process gain on negative outputs. In effect the PID controller has two gains one for positive moves and one for negative moves.
CONTROL VALVE
A TYPICAL SLIDING STEM CONTROL VALVE WITH A PNEUMATIC ACTUATOR
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OUTPUT %
Time Constant and Time Delay
100 90 80 70 60 50 40 30 20 10
∆EU
To calculate the time constant and time delay make a “bump” to the process. Then determine the total change in engineering units (∆ ∆EU). Multiply this amount by .63 and add it to the starting value. ∆EU = 80 ∆
0
10
20
30
40
50 TIME
80 * .63 = 54.4 54.4 + 20 = 74.4
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OUTPUT %
Time Constant and Time Delay
Transfer the 74.4 from the “Y” axis to the curve. Transfer the intersection point down to the “X” axis.
100 90 80 70 60 50 40 30 20 10 0
10
20
30
TIME CONSTANT
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40
50 TIME
The time constant is the difference in time from when the process began to move until it reached 63% of its final value. 23 - 10 = 13 seconds
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OUTPUT %
Time Constant and Time Delay
The time delay is the difference in time from when the bump was first made until the process began to move.
100 90 80 70 60 50 40 30 20 10
10 - 0 = 10 seconds
0
10
20
30
40
50 TIME
TIME DELAY
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Stiction
OUTPUT %
Stiction is the amount of time in seconds needed to force the actuator from standstill to a moving condition. It can be determined through graphical analysis of the “bump test” data.
STICTION
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TIME
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Backlash
Backlash compensates for mechanical slack in an actuator whenever the actuator reverses direction. It can be calculated using the data from the “bump test” turn-around moves .
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Dead Zone Dead Zone is the amount that a controlled variable is allowed to vary around its setpoint without taking any control action. The size of the dead zone is usually based on the amount of natural, short term variation that remains when the control loop is in manual. It can also be due to offsets caused by stiction values that are smaller than the minimum output the controller is capable of. The implementation of a dead zone can prevent excessive wear of the control actuators by reducing unnecessary control moves. STOCK FLOW IN MANUAL 510
AVERAGE
500
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DEADZONE
TIME
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Cascade Control
The general concept of cascade control is to nest one feedback loop inside another feedback loop.
SENSOR
TO LOWER LEVEL LOOP CONTROLLER
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Cascade Control An example would be basis weight control.
BASIS WEIGHT SETPOINT
STOCK FLOW SETPOINT BASIS WEIGHT CONTROLLER
INPUT CONTROLLER
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FROM SCANNER
STOCK FLOW CONTROLLER
TO STOCK VALVE
INPUT CONTROLLER
FROM STOCK FLOW SENSOR
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Cascade Control In this arrangement the output of the basis weight controller adjusts the setpoint of the stock flow controller. These higher level cascade loops are often called “supervisory” loops since their outputs supervise the lower level loops.
STOCK FLOW SETPOINT
BASIS WEIGHT SETPOINT BASIS WEIGHT CONTROLLER
INPUT CONTROLLER
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FROM SCANNER
STOCK FLOW CONTROLLER
TO STOCK VALVE
INPUT CONTROLLER
FROM STOCK FLOW SENSOR
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Cascade Control Cascade control is particularly useful when a very slow process is involved. For example, a typical process will respond to a stock flow change in less than five seconds. However, the resulting change to the basis weight may take one to two minutes. Cascade control allows you to control this intermediate variable (stock flow) and to take corrective action on disturbances more promptly.
STOCK FLOW SETPOINT
BASIS WEIGHT SETPOINT BASIS WEIGHT CONTROLLER
INPUT CONTROLLER
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FROM SCANNER
STOCK FLOW CONTROLLER
TO STOCK VALVE
INPUT CONTROLLER
FROM STOCK FLOW SENSOR
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Scanning Sensors Many of the cascade controls used in paper mills rely on inputs from Honeywell Measurex scanners. Measurements provided by these scanners include many sheet characteristics such as: basis weight (mass), moisture , caliper, color, formation, ash, smoothness, and opacity. Page 93
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Scanning Sensors
The measurements from the scanning sensors provide accurate “real-time” data that can be used to adjust machine operation. The result is improved machine efficiency and higher product quality.
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Ash Measurement • Accurate Ash / Mineral measurement
• High signal-tonoise tuned X-ray tube source
• Non-contacting • Utilizes inputs from Basis Weight and Moisture gauges
• Compensation for environmental sources of error – Air Temperature – Z-axis Changes – Dirt Buildup Page 95
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Basis Weight Measurement • Fast Measurement Response • High Resolution CD Profile Measurement
• High Resolution MD Trend Measurement
• Fast Profile Measurement • High Frequency Process & Quality Analysis
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Basis Weight Measurement Air cylinders for internal standards
Air cylinder for source shutter
Air actuated source shutter
SOURCE
2 internal standards
Source capsule
RECEIVER
“Close geometry”
Integrated air curtain
Ionization chamber receiver
Model 4201 - BW Sensor Precision Basis Weight Sensor viewed from MD side
Model 4202 - BW Sensor
Source open, one internal standard inserted
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Radiation Safety • HMX Basis Weight and Ash sensors utilize ionizing radiation sources.
– Basis Weight uses Krypton-85, an inert gas. – If accidentally released, Krypton-85 dissipates quickly and is rendered harmless when thoroughly dispersed.
– Some sensors at other mills contain solid sources, such as Strontium or Americium, which must be treated with greater care.
– Ash uses an X-ray tube which has its’ own 4kv power supply. When the power is off, no X-rays are emitted.
• Radiation Indicators are located on each end of the scanner frame.
– Red light indicates source shutters OPEN – Green light indicates source shutters CLOSED – Yellow light indicates Ash Power ON Page 98
Solutions for Superior Results
Radiation Safety • Safety Interlocks – Hardware interlocks to force shutters closed if heads accidentally become detached from scanner or are damaged.
– Software interlocks to force shutters closed if heads are separated; e.g. while cleaning sensors or if scan drive belt breaks.
• Radiation Safety Checks are performed quarterly by trained HMx personnel. Functionality of all safety interlocks is tested.
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Solutions for Superior Results
Caliper Measurement • Magnetic reluctance principle
• Accuracy through contacting measurement
• Full sheet measurement and signal processing
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Solutions for Superior Results
Caliper Measurement Contacting Design for Accurate Measurement Pressurised Bellows:
• Heat and chemical resistant silicon rubber for long life
• Compresses sheet slightly for correlation to laboratory caliper
• Follows passline variation and sheet flutter
• Adjustable air pressure for
Gimballed Skis
optimum performance Gimballed Skis:
• Rugged stainless steel construction
• Protects contacts and bellows • Provides precise contact alignment Page 101
Solutions for Superior Results
Pressurised Bellows
Caliper Measurement Magnetic Reluctance Measurement Frequency
Oscillator Ferrite Core Magnetic Flux Sapphire Contact Sheet Passive Ferrite Contact
• Oscillator frequency is proportional to the length of the magnetic path, – i.e. the gap between the sapphire and the ferrite contacts Page 102
Solutions for Superior Results
Color Measurement MODEL 2250 COLOR SENSOR
Sensor Function The Color Sensor measures the color, brightness, and fluorescence of a moving paper web by shining a specified illuminant on the web, then measuring the properties of the reflected light. The Color Sensor consists of three functional modules: • Source of illumination • Receiver, light analyzer, and computer • Standards and sheet backing The three modules are contained in the two sensor heads. The illumination source, receiver, light analyzer, and microcomputer are in one head (generally the upper head), and the standards and sheet backing are in the other.
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Solutions for Superior Results
Moisture Measurement - Reel • Fast measurement response • High resolution CD profile measurement
• High resolution MD trend measurement
• Fast profile measurement • High frequency process & quality analysis
• Accurate measurement in the presence of different fillers, coatings, recycled fibers and sheet temperature variation
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Solutions for Superior Results
Moisture Measurement - Reel IR Transmission Reference Wavelength
Measurement Wavelength
Bone Dry Moisture Low Moisture High Moisture 1.8 µ
1.9 µ
Wavelength
• Moisture-sensitive measurement at 1.9 microns varies with water content and basis weight
• Reference measurement at 1.8 microns varies with basis weight Page 105
Solutions for Superior Results
Moisture Measurement - Reel Receiver Diffusing Reflecting Surface Sheet
Diffusing Reflecting Surface Infrared Light Source
• Patented INFRAND Optics • Simple, powerful technique to achieve scattering insensitivity • Quartz plates INFinitely RANDomly scatter source IR Page 106
Solutions for Superior Results
Moisture Measurement - Size Press • Non-contacting, reflective measurement eliminates breaks and sheet marking
• Low sensitivity to sheet flutter
• Insensitive to formation and non-uniform moisture distribution
• Carbon correction permits accurate measurement in the presence of re-cycled fibre
• Small spot-size, fast response and unique signal processing provides measurement to the edge of sheet Page 107
Solutions for Superior Results
Moisture Measurement - Size Press Moisture Detector 1.9µ
• Parallel processing wavelength analysis provides same-spot measurement of all channels for: – Formation-insensitive
Carbon Detector 2.1µ
Reference Detector 1.8µ
measurement
– Fast sensor response • SingleCal™ calibration provides one calibration group for most grades • Carbon correction provides accurate moisture measurement on nondeinked recycled stock Page 108
Sheet
Solutions for Superior Results
Sheet Measurement
Each sensor in the scanning head provides a machine direction and a cross direction reading of the sheet.
MACHINE DIRECTION
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CROSS DIRECTION
Solutions for Superior Results
Machine Direction Measurement
The scanner continuously collects data as it travels across the sheet. At the end of each scan, this data is averaged and displayed as machine direction last average. Several last average readings are combined to produce other machine direction readings such as: • Machine Direction Trend Average • Machine Direction Reel Average
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Solutions for Superior Results
Cross Direction Measurement
CROSS DIRECTION PROFILE
Basis Weight Last Average Basis Weight Roll Average
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28.5 28.7
The data collected by the scanner is sorted into cross direction measurement lanes (slices), averaged, and displayed as a cross direction profile.
Solutions for Superior Results
Basis Weight Control Example Machine direction basis weight measurements are compared to the basis weight setpoint. Differences are acted on by the weight control and outputs are cascaded to the setpoint of the stock flow controller. The stock flow controller then adjusts the stock valve.
FT
STOCK FLOW CONTROLLER
WEIGHT CONTROLLER
WEIGHT SETPOINT
STOCK FLOW SETPOINT
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Solutions for Superior Results
Moisture Control Example Moisture measurements from the scanner are compared to the moisture setpoint. Differences are acted on by the moisture controller and outputs are cascaded to the setpoint of the steam pressure controller. The steam pressure controller then adjusts the steam valve.
STEAM PRESSURE TRANSMITTER PT
STEAM VALVE STEAM FROM BOILER HOUSE
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STEAM PRESSURE CONTROLLER
MOISTURE CONTROLLER
MOISTURE SETPOINT
STEAM PRESSURE SETPOINT
Solutions for Superior Results
Tuning Cascade Loops Like the loops they supervise, cascade loops have gain and time constant tuning parameters. You may also find significant time delay in these loops. For example, on a basis weight loop the time delay is the amount of time from when the stock valve is moved until the effects are seen at the scanner.
PT
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Solutions for Superior Results
Decoupler Control When feedback loops interact with one another, a control system is needed that will decouple the loops. An example of this interaction can be seen on a paper machine between weight and moisture. The effects can be minimized through decoupler control.
WEIGHT SETPOINT
WEIGHT CONTROLLER D1
STOCK FLOW CONTROLLER
D2 MOISTURE SETPOINT
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MOISTURE CONTROLLER
STEAM PRESSURE CONTROLLER
Solutions for Superior Results
Cross Direction Control
Using inputs from the scanner measurements, the cross direction properties of the sheet can be controlled using various cross direction actuators. CROSS DIRECTION
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Solutions for Superior Results
Cross Direction Control
ThermaTrol or Autoslice (CD Weight) Devronizer (CD Moisture)
AquaTrol (CD Moisture) CalTrol or Calcoil (CD Caliper)
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Solutions for Superior Results
Weight Profile Optimizer The basis weight profile control supervises the slice lip actuators to provide tight control of the weight profile. The control incorporates many advanced models and strategies to determine the correct output for each actuator.
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Solutions for Superior Results
Actuators ThermaTrol MANUAL ADJUSTMENT
ELECTRIC CURRENT APPLIED TO HEAT ROD
Cross direction basis weight control is accomplished using ThermaTrol or Autoslice Motors Actuators. The actuators open and close the slice lip.
SLICE ROD EXPANDS AND CONTRACTS
TO SLICE LIP
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Solutions for Superior Results
Moisture Profile Optimizer
MOISTURE PROFILE OPTIMIZER
Page 120
Moisture profile control is a flexible package that offers a wide range of actuators including Steamboxes and Aquatrol.
Solutions for Superior Results
Aquatrol
Aquatrol actuators are used near the dry end of a paper machine to re-wet the sheet.
DRYERS
The result is reduced moisture streaks and increased moisture levels.
AQUATROL
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CALENDER STACK
Solutions for Superior Results
Caliper Profiler
The CalTrol II High Powered Caliper Profiler precisely controls the caliper profile by adjusting variable temperature, high turbulence air jets.
CALENDER STACK CALTROL
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Solutions for Superior Results
CalTrol
HEATING ELEMENT
AIR FLOW
CALENDER ROLL
When hot air is applied to the calender roll the roll will expand - causing a corresponding decrease in the caliper of the sheet. Numerous control zones provide optimum caliper profiling.
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Solutions for Superior Results
Display Profiles MINISLICE PROFILE
20
40
60
80
100
120
140
PROFILE TRANSFORMATION
10
20
30
40
50
60
The Lo-Res Profile is an average of the Hi-res readings taken over each of the Lo-Res zones. Typically, the display profile is aligned as closely as possible to the actuator spacing on the headbox.
DISPLAY PROFILE
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Solutions for Superior Results
Lo-Res Zone Width
MINI SLICE
To create the Lo-Res Profile the size of the display zone is determined from the headbox actuator spacing and shrinkage factor. All the zones in a Lo-Res profile are the same size.
DISPLAY
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Solutions for Superior Results
Scanner Alignment The purpose of scanner alignment is to position the sheet with reference to the machine. Alignment is necessary to insure that cross direction actuator movement corresponds with the desired position on the sheet.
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Solutions for Superior Results
Control Profiles MINISLICE PROFILE
20
40
60
80
100
120
140
PROFILE TRANSFORMATION
10
Page 127
20 30 40 50 CONTROL PROFILE
60
Control Profiles are an average of the Hi-Res readings taken over each of the control zones. This allows accurate alignment of the scanner to the paper machine and the cross direction actuators.
Solutions for Superior Results
Control Profiles
5
10
15
20
25
30
20 30 40 50 CONTROL PROFILE
60
DISPLAY PROFILE PROFILE TRANSFORMATION
10
Page 128
On Da Vinci systems, control profiles are derived from the Hi-Res readings. A mathematical transformation is used to calculate control zones
Solutions for Superior Results
Control Zone Alignment
PROFILE CONTROL SUMMARY WEIGHT PROFILE OPTIMIZATION
REEL MOI PROFILE OPTIMIZATION
CALIPER PROFILE OPTIMIZATION
R/S MOI PROFILE OPTIMIZATION
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Since every cross direction controller has different actuator spacing, each controller must have its own control zone profile.
Solutions for Superior Results
Cross Direction Control ACTUATORS
CONTROL ZONE PROFILE TO REDUCE THE HIGH SPOT OPEN ACTUATOR # 3
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Like all other control loops, cross direction control compares the current measured value to the desired setpoint. Differences are acted on by the controller and signals are sent to the appropriate actuator.
Solutions for Superior Results
Cross Direction Control
Most cross direction controllers use traditional proportional and integral control techniques. However, there are many other factors that must be accounted for such as: • Actuator response time • Process delay time • Scan speed
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Solutions for Superior Results
Evaluating Control Performance
STOCK VALVE SETPOINT CHANGE 100 0
TIME STOCK FLOW RESPONSE
1000 0
Page 132
Short-term evaluation of a control loop can be done by observing its response to a setpoint change on a trend plot display.
TIME
Solutions for Superior Results
Evaluating Control Performance STOCK VALVE SETPOINT CHANGE 100 0
TIME TOO AGGRESSIVE
1000 0
TIME
Typical responses to a setpoint change when the control loop is not optimally tuned (could also be the result of a process that is in need of repair).
TOO SLOW 1000 0
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TIME
Solutions for Superior Results
Assessing Loop Behavior
1000
0
Page 134
TIME
Oscillation in a control loop is often believed to be caused by excessive controller gain. However, this problem can come from any of the following sources: • The control valve or actuator • Outside disturbances • The controller tuning
Solutions for Superior Results
Assessing Loop Behavior Oscillations are a common control problem that deserves our attention. Your first step, should be to determine whether the oscillation is caused by the controller, or comes from another source. This can be done by placing the loop in manual. DISTURBANCES
MANIPULATED VARIABLE
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PROCESS
CONTROLLED VARIABLE
Solutions for Superior Results
Assessing Loop Behavior PUT THE LOOP IN MANUAL MODE
NO
STILL OSCILLATING ?
SEARCH FOR THE SOURCE
CHECK THE VALVE
PROBLEM ?
YES
If the oscillations are still present, after placing the loop in manual, then they are being generated outside the loop.
NO
If the oscillations go away, they are being generated inside the loop.
YES UNDERTAKE VALVE MAINTENANCE
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CHECK CONTROLLER TUNING
Solutions for Superior Results
Assessing Loop Behavior BASIS WEIGHT MD SPREAD 0.5
0
TIME
BASIS WEIGHT CD SPREAD 0.5
0
Page 137
TIME
Control behavior should also be assessed on a long term basis. A simple and effective way to do this is to plot machine direction and cross direction variation over time. Tracking this variation over time gives you a good indication of the controls effectiveness. Solutions for Superior Results
Troubleshooting
• Common Problems – System wide problems M Blank videos. M Can’t change video frames.
– Control problems M Loop won’t go on control. M Loop is unstable. M Loop is too slow. M Unable to change manipulated variable at all. M Loss of process input readings.
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Solutions for Superior Results
Troubleshooting
• Common Problems (Continued) – Scanner/Sensor problems M Won’t scan. M Scans OK but goes off-sheet by itself. M Loss of sensor readings. M Sensor readings unstable. M Sensor readings don’t match lab checks.
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Solutions for Superior Results
Troubleshooting • Approaches To Troubleshooting – Interpreting video display information, status, etc. M Control frames M Trend plots M Profiles M Diagnostic frames
– Operating loops in manual. – Operating loops in DDC control. – Testing basic control inputs and outputs (increment, decrement).
– Operating loops in supervisory control.
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Solutions for Superior Results
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