CONTROL THEORY BASICS

This article presents some of the basic concepts of control and provides a foundation from which to understand more complex control processes and algorithms later described in this module. Common terms and concepts relating to process control are defined in this section.

LEARNING OBJECTIVES

After completing this section, you will be able to:

Define control loop and describe the three tasks necessary for process control to occur:

• Measure

• Compare

• Adjust

 

Define the following terms:

• Process variable

• Setpoint

• Manipulated variable

• Measured variable

• Error

• Offset

• Load disturbance

• Control algorithm

List at least five process variables that are commonly controlled in process measurement industries. At a high level, differentiate the following types of control:

• Manual versus automatic feedback control

• Closed-loop versus open-loop control

THE CONTROL LOOP

Imagine you are sitting in a cabin in front of a small fire on a cold winter evening. You feel uncomfortably cold, so you throw another log on the fire. This is an example of a control loop. In the control loop, a variable (temperature) fell below the setpoint (your comfort level), and you took action to bring the process back into the desired condition by adding fuel to the fire. The control loop will now remain static until the temperature again rises above or falls below your comfort level.

THREE TASKS

Control loops in the process control industry work in the same way, requiring three tasks to occur:

Measurement

Comparison

Adjustment

 












In Figure, a level transmitter (LT) measures the level in the tank and transmits a signal associated with the level reading to a controller (LIC). The controller compares the reading to a predetermined value, in this case, the maximum tank level established by the plant operator, and finds that the values are equal. The controller then sends a signal to the device that can bring the tank level back to a lower level a valve at the bottom of the tank. The valve opens to let some liquid out of the tank. Many different instruments and devices may or may not be used in control loops (e.g., transmitters, sensors, controllers, valves, pumps), but the three tasks of measurement, comparison, and adjustment are always present.
As in any field, process control has its own set of common terms that you should be familiar with and that you will use when talking about control technology.

PROCESS VARIABLE

A process variable is a condition of the process fluid (a liquid or gas) that can change the manufacturing process in some way. In the example of you sitting by the fire, the process variable was temperature. In the example of the tank in Figure, the process variable is level. Common process variables include:

Pressure

Flow

Level

Temperature

Density

Ph (acidity or alkalinity)

Liquid interface (the relative amounts of different liquids that are combined in a vessel)

Mass

Conductivity

 

SETPOINT

The setpoint is a value for a process variable that is desired to be maintained. For example, if a process temperature needs to kept within 5 °C of 100 °C, then the setpoint is 100 °C. A temperature sensor can be used to help maintain the temperature at setpoint. The sensor is inserted into the process, and a controller compares the temperature reading from the sensor to the setpoint. If the temperature reading is 110 °C, then the controller determines that the process is above setpoint and signals the fuel valve of the burner to close slightly until the process cools to 100 °C. Set points can also be maximum or minimum values. For example, level in tank cannot exceed 20 feet.

 

MEASURED VARIABLES, PROCESS VARIABLES, AND MANIPULATED VARIABLES

In the temperature control loop example, the measured variable is temperature, which must be held close to 100 °C. In this example and in most instances, the measured variable is also the process variable. The measured variable is the condition of the process fluid that must be kept at the designated setpoint. Sometimes the measured variable is not the same as the process variable. For example, a manufacturer may measure flow into and out of a storage tank to determine tank level. In this scenario, flow is the measured variable, and the process fluid level is the process variable.

The factor that is changed to keep the measured variable at setpoint is called the manipulated variable. In the example described, the manipulated variable would also be flow.
   

ERROR

Error is the difference between the measured variable and the setpoint and can be either positive or negative. In the temperature control loop example, the error is the difference between the 110 °C measured variable and the 100 °C setpoint that is, the error is 10 °C.

The objective of any control scheme is to minimize or eliminate error. Therefore, it is imperative that error be well understood. Any error can be seen as having three major components. These three components are shown in the figure on the following page.
 

Magnitude

The magnitude of the error is simply the deviation between the values of the setpoint and the process variable. The magnitude of error at any point in time compared to the previous error provides the basis for determining the change in error. The change in error is also an important value.

 

Duration

Duration refers to the length of time that an error condition has existed.

 

Rate of Change
The rate of change is shown by the slope of the error plot

OFFSET
Offset is a sustained deviation of the process variable from the setpoint. In the temperature control loop example, if the control system held the process fluid at 100.5 °C consistently, even though the setpoint is 100 °C, then an offset of 0.5 °C exists.
 

LOAD DISTURBANCE

A load disturbance is an undesired change in one of the factors that can affect the process variable. In the temperature control loop example, adding cold process fluid to the vessel would be a load disturbance because it would lower the temperature of the process fluid.

CONTROL ALGORITHM

A control algorithm is a mathematical expression of a control function. Using the temperature control loop example, V in the equation below is the fuel valve position, and e is the error. The relationship in a control algorithm can be expressed as:
 

Control algorithms can be used to calculate the requirements of much more complex control loops than the one described here. In more complex control loops, questions such as “How far should the valve be opened or closed in response to a given change in setpoint?” and “How long should the valve be held in the new position after the process variable moves back toward setpoint?” need to be answered.

MANUAL AND AUTOMATIC CONTROL

Before process automation, people, rather than machines, performed many of the process control tasks. For example, a human operator might have watched a level gauge and closed a valve when the level reached the setpoint. Control operations that involve human action to make an adjustment are called manual control systems.

Conversely, control operations in which no human intervention is required, such as an automatic valve actuator that responds to a level controller, are called automatic control systems.

  

CLOSED AND OPEN CONTROL LOOPS

A closed control loop exists where a process variable is measured, compared to a setpoint, and action is taken to correct any deviation from setpoint. An open control loop exists where the process variable is not compared, and action is taken not in response to feedback on the condition of the process variable, but is instead taken without regard to process variable conditions. For example, a water valve may be opened to add cooling water to a process to prevent the process fluid from getting too hot, based on a pre-set time interval, regardless of the actual temperature of the process fluid.

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The Importance of Process Control

PROCESS...
Process as used in the terms process control industry, refers to the methods of changing of refining raw materials to create end products. The raw materials, which either pass through or remain in a liquid, gaseous, or slurry (a mix of solids and liquids) state during the process, are transferred, measured, mixed, heated or cooled, filtered, stored, or handled in some other way to produce the end product.
Process industries include the chemical industry, the oil and gas industry, the food and beverage industry, the pharmaceutical industry, the water treatment industry and the power industry.

PROCESS CONTROL
Process control refers to the methods that are used control process variables when manufacturing a product. For example, factors such as the proportion of one ingredient to another, the temperature of the materials, how well the ingredients are mixed and the pressure under which the materials are held can significantly impact the quality of an end product. Manufacturers control the production process for three reasons:
Reduce variability
Increase efficiency
Ensure safety

Reduce Variability 
Process control can reduce variability in the end product, which ensure a consistently high quality product. Manufacturers can also save money by reducing variability. For example, in a gasoline blending process, as many as 12 or more different components may be blended to make a specific grade of gasoline. If the refinery does not have precise control over the flow of the separate components, the gasoline may get too much of the high-octane components. As a result, customers would receive a higher grade and more expensive gasoline than they paid for, and the refinery would lose the money. The opposite situation would be customers receiving a lower grade at a higher price.
Reducing variability can also save money by reducing the need for product padding to meet required product specifications. Padding refers to the process of making a product of higher-quality than it needs to be to meet specifications. When there is variability in the end product (i.e.,when process control is poor), manufacturers are forced to pad the product to ensure that specifications are met, which adds to the cost. With accurate, dependable process control, the setpoint (desired or optimal point) can e moved closer to the actual product specification and thus save the manufacturer money. 

Increase Efficiency
Some processes need to be maintained at a specific point to maximize efficiency. For example, a control point might be the temperature at which a chemical reaction takes place. Accurate control of temperature ensures process efficiency. Manufacturers save money by minimizing the resources required to produce the end product.

Ensure Safety
A run-away process, such as an out-of-control nuclear or chemical reaction, may result if manufacturers do not maintain precise control of all of the process variables. The consequences of a run-away process can be catastrophic. Precise process control may also be required to ensure safety. For example, maintaining proper boiler pressure by controlling the inflow of air used in combustion and the outflow of exhaust gases is crucial in preventing boiler implosions that can clearly threaten the safety of workers.

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Spider System Hydraulics Control

Spider System Hydraulics Control

Spider System Hydraulics Control
Spider is meant here is not a family of eight-legged insects that are highly toxic and deadly if bitten by it.
Spider what I mean is a system control hydraulic which require a large force or torque for stirring small pieces of wood (chips) inside the digester or Impbin according to the desired speed to achieve the desired amount of production in the process to produce pulp in our factory. Control system used is called Spider Hydraulic Control System (Hagglunds). This control system direct control of two pumps which serve to drain the oil in the system in order to increase oil pressure above 250 bar antecedent control by a proportional valve. The larger the orifice of the proportional valve which is open the greater the amount of oil which passed into the system so that the oil pressure levels rise. With this high pressure control rotational speed of the digester and Impbin be in control. Controlling the maximum speed in our factory is 20 rpm. For treatment of a main board / card spider who does not much can be done. We do care only at every annual shut down. Flushing with clean air and re-check all terminal connections. To avoid loss of process pulp production we have to prepare a spare for main board.

Impbin

  

In the Company of our systems are installed in 6 locations: 3 in PM9 Impbin, HP-feeder and Outlet Devices Digester and 3 more in PM8 Impbin, HP-feeder and the Outlet Device Digester
For the PM9 has been run since 2007 and for PM8 still need to wait for tie ins to the Digester area.
The Spider problem that occurred in our company in Pulp Making-9 are:
1-Monitor in the control panel remains black while the system runs and can not be in control.
2-Alarm appears on monitor and the system stop, can not run anymore.

At first we felt confused handling although this system is simple but unprecedented. We try to replace the mainboard with a spare from Impbin PM8 (not yet run). We try to enter the parameters required (number of parameters about 22 pages type of A4 paper)
I will explain what we've done.
To save time and reduce the loss of production then we immediately replace it with a new card. We bring the damaged card to the office for the analysis of the problem. Connect spider control with a computer and receive data from the spider. To analyze and try to reset several times and re-enter the correct data, try to simulate with the simulator 4 ~ 20mA and connected output with proportional valve. To avoid and reduce the loss of production we have provided training to all members of the instrument.

Spider simulation

How to grab data from Spider Control Panel (for Back up)
1-Connect power cable (220Vac,50Hz).
2-Connect output adapter 24Vdc to Spider control panel and turn on spider.
3-Connect cable usb to serial RS232 to Spider (serial RS232 side).
4-Connect Computer with cable usb to serial RS232 (usb side)
5-Open file "spidercom2.exe" in your computer.
6-Click tab "PARAMETER" than click "RECEIVE".
7-Wait a few moments. Computer will read and receive data from Spider Control Panel and when finished will display on monitor computer "DONE". When starting  receive data, the lights in cable USB / RS232 going on. Till to exit the word DONE in the computer than the lights off.
8-To view the data which has been in the store we could open with a click "DISPLAY" and then click "PARAMETERS". The file will appear and you can check all the existing data in the Spider Control Panel. Save your file before it by clicking "FILE" then "SAVE AS" as a unix file names in your computer's hard drive, for example,  "PM9Impbin". Files that are stored will have a ext. s2p or "PM9Impbin.s2p".
9-Completed.


How to send data from your computer to Spider Control Panel.
1-Connect cable USB to RS232 to a computer and Spider Control Panel.
2-Open the file has been updated to upload (save to Spider Control Panel) for example: PM9Impbin.s2p.
Once open, make sure the correct data and then we save it to Spider Control Panel Click "PARAMETERS" then click "Transmit".
3-When the transmit is completed it will come out the word "transmit okay" in your computer. The lights on the usb cable / RS232 going off.
4-To complete the process transmits the Spider Control Panel needs to be reset. Press the "RESET" once and wait for Spider Control boot ..
5-Spider Control Panel is ready to be operated by the process.
6-Completed.


I created this article so that we can share knowledge together. Much remains to be added because this is only a short writing course. If anyone wants to ask I'll try to answer.

Thank you.

Spider main board

Impbin at night

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My Instrumentation Experience at IKPP

Hello...nice to meet you.
I hope you are glad reads my article.

May I introduce my self. Around year 1993, my first job is at PT.Indah Kiat Pulp & Paper Indonesia Pekanbaru Perawang mill. We are at placed at project kvaerner PM8. We have done many matters has liked instruments installation, pulled signal and power cable, termination, tube installation, setting parameter, calibrate control valve, signal test and loop test. Since this project i had learn from many article that support my job and my skill.

Pulp Making-9

Any of them is "The Seven Habits of Highly Successful Control Engineers" presented by George Buckbee,P.E.
In this article I will sharing with you that highly successful control engineers didn't become that way by accident. The most successful engineers develop habits that improve results and recognition of those results. This paper addresses specific habits that you can develop or enhance to be more successful.

The 7 Habits listed in this paper were developed from over 20 years, working with thousands of control engineers around the world. Some people who struggled to identify their results and others who had great results, but couldn't get recognition. In this article, we'll look at some of the habits that have ensured success for the most successful of these engineers.

7 Habits  The 7 Habits listed in this paper are: 1.   Know the Process. 2.   Focus only on the Most Important Things 3.   Document the Baselines 4.   Use Tools to be More Effective 5.   Network and Communicate Results 6.   Keep Learning 7.   Share Your Knowledge In the pages that follow, we'll look at each one of them, and provide some suggestions for how you can improve your own rate of success. Habit 1 - Know the Process Knowing the process is the first and most important habit for control engineers. To be effective in the automation and control of a process, you must first have a thorough understanding of the process. To develop process knowledge takes time and effort. Start by studying process flow diagrams and P&IDs. Trace the primary product as it flows through the process,  highlighting it on drawing with a colored marker. Talk to operators.They work with the process day in and day out.  They understand a lot about how the  process normally behaves.They also know about the abnormal, unusual things that can happen during equipment failures,  shutdowns,  start-ups, and shift changes. The process doesn't always behave according to the text book. Make sure you know what to expect in abnormal situations.  With a little bit of process knowledge, tools like Process Interaction Mapping can help you to pinpoint the source of control upsets. Habit 2 - Focus on the Most Important Things  It is so easy to lose focus in a plant environment. There are daily disasters, fire-fighting,management meetings,projects,and a hundred other distractions. The challenge,of course,is to stay focused on those activities that will deliver the most value to the business. As Stephen Covey says, in First Things First, "The main thing is to keep the main thing the main thing".  In a process manufacturing environment, the "Main thing" is usually some combination of these factors: •     Unit Cost •     Production Rate •     Quality •     Energy Costs •     Reliability •     Environmental & Safety It will be very difficult to prove success if you cannot link your work to one or more of these factors. Habit 3 - Document the Baseline If you don't know where you started, how will you know how far you have come? I have seen people make this mistake many times over in my career. Successful engineers always take time to understand the starting point. The starting point should always be measured in business terms. You can supplement with some technical measures, but you should always establish a good baseline of business metrics, such as those mentioned above. Discuss the baseline conditions with an operations manager or financial person, to make sure that you understand what the numbers mean. This also helps ensure that you are working on the right things. Be sure to use the exact same methods to measure these metrics. If they measure profit in dollars per truckload, then you should, too. The technical measures are a good supplement, but only if they can be linked to the business metrics. For example, if you are trying to improve quality (reduce % rejects, for example), it may be a good idea to track a  technical metric such as variability. You can make a strong case that reduced variability has a direct impact on  reducing the amount of reject material. Habit 4 -  Improve Your Effectiveness with the Right Tools  I can admit that, as an engineer, I love to dig in to the details, and use my engineering knowledge to sniff out the root cause of a problem. This is part intuition, part experience, and partly that engineering mindset. If you love to solve problems, it can be easy to overlook that there may be some much simpler ways to get the job done. For example, people often ask how to find control loops that are performing poorly. We could go through the exercise of looking at each loop, analyzing it, and coming up with a list of the top 10 issues. Chances are that this would take a long time, because I would definitely get side-tracked into solving some of these problems along the way. To make this job go much faster, I use the control loop monitoring tools in PlantTriage. Based on a few key metrics, PlantTriage will give me a top 10 list automatically. It already has all the data, because it monitors the process 24 hours a day. And sometimes, you have to get your ego out of the way. All control engineers think they can tune loops quickly by hand. "Quickly", yes."Properly", not very often. Use a software tool to get the answers quickly and properly. These days, most companies have downsized their engineering staffs considerably. You simply don't have the time to be inefficient at any aspect of your job. Habit 5 - Communicate Results & Network This is probably the most important of these 7 habits. If you do great things, and nobody knows what was done, you have lost. Networking is a critical part of this. I'm not talking about computer networks, but people. Make sure you have credibility with a wide array of people. Think outside your normal workday routine. Set up lunch meetings with some of these people. The Plant IT Manager Your Counterparts in Other Departments. The Plant Financial Guru. Instrument Techs. Operations Managers. Purchasing Agents.This will be a challenge at first. But you will find that this helps you to develop stronger relationships in the plant. These relationships will be helpful as you communicate your results.  When you get some good results, you will need to communicate them clearly, concisely, and in business terms. Remember the base lining discussion above? Go back to your baseline measurements, and show how your work had an influence on the bottom line. When it comes to communicating, keep it simple. Don't write a 200-page report. Nobody will read it. Instead, send a short email with "Before and After" pictures, and just a little bit of back-up material. The subject line of the email should be something like: "$120,000 Savings on the De-Chlorinator". That email will be opened, read, and forwarded to others. How much value can you bring? It certainly depends on your role. With some effort, a typical control engineer should be able to document between 6 and 10 times their annual salary in savings.This may come from  any combination of the business results areas listed in the baseline section. Habit 6 - Keep Learning Never stop learning. One of the great mentors in my career was Virgil Colavitti. Virgil had been working in the same plant for over 40 years. He seemed to know the inner workings of every machine, instrument, valve, transformer, and other device under the sun. In working with him, I learned why. He had a natural curiosity, and he was never afraid to ask questions. In a group of people, he would often say, "I wonder how that works..." And sure enough, someone would explain it. Today, you have a huge number of training resources available. You can take training course in your plant,at a training center,or over the web. You can quickly find books and articles that delve into every possible subject. Be careful! Make sure you are working with a credible source of information. Published authors and established companies are great source of information. But some on-line sources have little editorial control, and their accuracy is questionable at best. ExperTune, of course, has been widely recognized for over 20 years as a source of deep knowledge in the field of process control.  ExperTune's training courses are practical, hands-on training, designed to help you be extremely effective at managing control loops in your facility. Habit 7 - Share Your Knowledge Typically, 20% to 30% of all control loops are running in Manual. That should be shocking. Why don't people step up and take notice? One reason may be that they simply don't understand the significance. Process control is not well-understood by "lay people". Twenty years after paying for my college education, my parents still have only a vague  understanding of what it is that a process control engineer actually does. Even within the plant environment, there is often a vague mystery  associated with process control. We use strange terminology, and talk about abstract things like "dynamics", "dead time", and "derivative".  Many plant people are simply confused by the topic. Sharing your knowledge with others can help to make you and them more effective. For your own success, if people understand what you do, they will have a better appreciation of the value you bring to the company. It's hard to  downsize someone that brings a lot of value! Because other people often  have limited process control knowledge, even sharing a little bit of your  knowledge may be tremendously useful. For a while in my career, we used a lot "single point lesson plans". These were simple, one-page training aids. So you could do a short 3-minute training session on topics like: "What is Cascade Control?", "Why we use filters on our instruments", "How to test the over-speed interlock","How to prevent valves from sticking". Whenever I was called in to resolve a problem at night,I always followed up with a single-point lesson the following day. I figured that I would not need to be called in again on the same problem if 10 other people knew how to  resolve it.You can be as formal or as informal as you wish. The key point is to help spread some knowledge around. Conclusions Your results will improve and be recognized if you pay attention to the 7   Habits discussed in this article.  A typical control engineer should be able to document between 6 and 10 times their annual salary in savings.  Documented results are key to success. Make sure you have the right tools and training to be successful in your company. About PlantTriage PlantTriage is a Plant-Wide Performance Supervision System that optimizes  your entire process control system, including instrumentation, controllers, and control valves. Using advanced  techniques, such as Active Model Capture Technology,  PlantTriage can identify, diagnose, and prioritize improvements to your process.

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Hello...
My name is Andy.
I work at IKPP Perawang mill.
Our factory produces pulp and paper. The biggest factory at asia even at the world.

These two tower called Pressure Diffuser, our Double Happiness tower, located in area Digester Pulp Making 9.

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