Control and Instrumentation



5. Control and InstrumentationErrol WoodLearning objectivesOn completion of this topic you should be able to:Explain the terms that apply to the principles of process controlDifferentiate in simple terms between the various types of feedback control methodsOutline the applications of process control to a range of woolscouring operations, in particular the control of feed rate, liquor quality and wool dryingKey terms and conceptsControl, instrumentation, process, process variable, controlled variable, manipulated variable, load variable, set point, sensor, controller, programmable logic controller (PLC), actuator, controlling element, humidity, regain, temperature, feedback loop, proportional control, integral control, derivative control, PID control, weighbelt, heat exchanger, liquor handling loop, heavy solidsIntroduction to the topicThis topic covers the principles of industrial control systems and their application to the modern woolscouring industry. The gradual introduction of these systems in the last few decades of the twentieth century has enabled woolscours to operate increasingly efficiently with fewer staff, to maintain required performance with respect to production and quality, and to meet increasingly stringent environmental requirements.It is necessary to master the terminology associated with process control and to understand the concept of a feedback loop. This is the means by which the system monitors a process and makes appropriate adjustments so that the desired value of some process variable is maintained. Various classes of control are briefly outlined, from the simple on/off control to the complex PID method.Most modern woolscouring operations incorporate process control technology for opening and blending, feeding to the scour, maintaining consistent bowl conditions and wool drying. Continuous weighing, temperature control and liquor quality can all be controlled by the appropriate combination of sensors, actuators and controllers. Sophisticated plant management systems are now available that enable all aspects of the plant operations can be monitored and controlled from a central computer, as well as from satellite computer stations.5.1 Reasons for and benefits of control technologyRecent developments in wool processing have led to more efficient operation and improved control of woolscouring. Many of these improvements have come from advances in computer-based instrument control technology.With changes to the economy and stronger attitudes to polluting the environment, woolscouring now requires:Better quality control of the scoured woolBetter efficiency so that scouring uses the minimum of labour, water, energy, and detergent and other chemicalsOptimum productivity so that the highest possible production is gained from the plant and labour available, andStringent effluent control.To a woolscourer, control and instrumentation technology offers the following benefits:Helps to reduce expenditure on labour, maintenance, energy, and detergent and other chemicalsProvides management with important processing information for good decision making and so that they can give an assurance of the quality of the scoured woolEnables operators to be better informed of the operating conditions of a particular processHelps to produce a more consistently scoured product to satisfy customers' requirements.There are numerous situations in a scouring plant where control and instrumentation systems may be gainfully employed. By monitoring the instrument data and control action of the system, management and staff will become more keenly aware of what is occurring in the plant. Processes that limit the plant performance or quality may be identified and scheduled for improvement. For example, it may be discovered that the dryer is running consistently at or near full capacity while the remainder of the plant has capacity to spare. By upgrading the dryer the throughput of the entire plant may be increased.Many technologies have, in the past, been used to measure and transmit information as part of a control system. These have included pneumatic and hydraulic pressure, and mechanical levers. Modern systems exclusively use electronic sensors to obtain information and electronic signals to transmit the information. Small electric motors (servo motors) under electronic control are often used to actuate the control functions.Extensive use is made of computers and microprocessors to manipulate and store information and carry out control calculations. Microprocessors are commonly found inside the more complex sensors to linearise the signal from the control element, control the output signal and possibly control a local display of the measured variable. Central control is carried out by a more powerful computer, such as a Programmable Logic Controller (PLC), designed specifically for such tasks, or a personal computer with dedicated software.5.2 Definitions and principlesThe following terms and definitions are used in process control:Process is any operation that takes materials from one stage or condition to another. For example, the woolscouring process takes raw wool from shearing and cleanses it to become scoured wool.Process control is the manual or automatic control of a process to maintain the right conditions to give the best results with the greatest efficiency.Process variable is any property of a process that can change. For example, the humidity (moisture in the air) in a dryer increases as the wool dries. Some of this air then has to be replaced by fresh air to make it drier. It is necessary to reduce the humidity of the air before it can be used to dry more wool. The humidity is a process variable.Controlled variable is the process variable to be controlled, either holding it constant or making it change in some predetermined manner. Using our example of the dryer, the humidity inside the dryer is controlled, and therefore the humidity is the controlled variable.Manipulated variable is the variable part of the process altered by the controller to change the controlled variable. For example, the humidity inside the dryer is altered by changing the exhaust air flow through a damper setting; the exhaust air flow is the manipulated variable.Load variable is a variable part of the process that can upset the controlled variable. For example, the amount of wool put through the dryer affects the humidity inside the dryer. More wool put through creates a higher humidity, and less wool creates a lower humidity. The amount of wool put through the dryer is the load variable.Figure 5.1 shows the elements of a control system. The dotted line, which links the controlling element, the process under control and the sensor, indicates that the control system is a closed loop.Figure 5.1 Elements of a closed loop process control system. Source: Wood, 2006.Here are the elements of a control system shown in Figure 5.1:A set point shows the desired value of the controlled variable. For example if the humidity level is set to 40% the humidity in the dryer should not be any more than or less than 40% at any given time. The set point is entered by the plant operatorA sensor constantly measures the controlled variable. A host of sensors are available for applications in industry (and in the home) to measure temperature, light level, speed, pressure, strain, position, etc. In this example, the sensor would measure the humidity A controller compares the measured value from the sensor with the set point. If the measured value from the sensor is different from the set point, the controller takes action to make them the same. It calculates and issues the control signal required to reduce the difference to zero. Controllers may be single hardware units controlling one process, or large PLCs capable of simultaneously controlling many processesAn indicator shows the sensor output as an onscreen readout, meter or a paper printoutAn actuator receives the message from the controller and moves the controlling element accordingly. In our example for controlling humidity in the dryer, the damper actuator would adjust the damper setting, the damper being the controlling elementThe controlling element regulates the manipulated variable to achieve control of the controlled variable for the process. Since it is the controlled variable (e.g. humidity rather than damper setting) that is measured by the sensor, we have created a loop by way of this feedback path. Thus a controlled process is often called a control loop or feedback loop.In this example it is assumed that the humidity level in the dryer is the variable being controlled (Figure 5.2).Figure 5.2 Humidity control loop for a dryer. Source: Stewart (1988).The sensor for the humidity sends messages to the humidity control recorder, which contains the set point, an indicator and the controller. The humidity control recorder can be set to automatic or manual control. On the auto side of the humidity control recorder, a message is automatically sent to the damper actuator to adjust the setting of the damper, the controlling element. The damper setting controls the humidity. On the manual side of the humidity control recorder, the operator has regularly to check the indicator on the recorder and then adjust the damper manually.For any control system to work, it is necessary:To be able to manipulate the manipulated variable. For example, if the humidity is too high and the damper is wide open, it's impossible to control the humidity. A dryer operating at peak capacity cannot be controlledTo have all components with the correct size for the job. If, for example, a steam flow control valve is too large, it cannot adjust the steam flow finely enough to give good temperature control.Controlling a part or the whole of a scouring process allows production to continue at the desired conditions. However, the process controllers keep the desired scouring conditions regardless of changing load variables. Operators can therefore spend more time on quality control.It is better to control a process automatically, but to use automatic control it must be possible to:Sense or measure the variable to be controlled. For example, it is necessary to have some means of checking the humidity inside the dryerManipulate some part of the process that affects the controlled variable. For example, the exhaust air flow must be able to be manipulated in order to change the humidity in the dryer. Furthermore, if a dryer is overloaded to the extent that the steam valve is fully open and the steam boiler is working to full capacity it will not be possible to control the dryer.If both of these tasks cannot be carried out, the process cannot be automatically controlled.It must also be emphasised that, in any control system:The full effect of any action by the controller does not immediately affect the controlled variable. In dryer control, actuating the damper to adjust the exhaust air flow does not immediately control the humidity in the dryer. There is a lag or a delay in response to adjusting the damper. After the lag, a steady state is finally reached where the controlled variable is the same as the original set point (Stewart 1988)Because of the lag, the controlled variable may go above or below the set point when adjusted by the controller. When the controlled variable goes alternately above or below the set point before reaching a steady state, it is called cycling about the set point. (Stewart 1988).A simple exampleTo illustrate the different control elements, consider the situation of a person taking a shower, where there is manual rather than automatic control. The following are the components of the ‘shower control system’:Sensor- finger tips (or entire body)Set point- feeling of personal comfortController- brainIndication- body reaction (chill, just right or too hot)Control signal- from brain to hand to adjust valveActuator- hand and armControlling element- mixing valveControlled variable- - temperature of showerManipulated variable - flow of hot and cold waterLoad variables- hot water temperature and water pressureNo control is possible if the mixing valve is set to the hottest position and the shower is too cold because the hot water supply is inadequate. In the context of wool scouring, it is pointless to try to exercise dryer control if there is no spare drying capacity in the dryer.This simple example illustrates the concept of a feedback loop. The brain receives a message from the body that the water is too hot (or too cold). Based on this sensation, the brain instructs the hand to turn the mixing valve to the left or right. This cyclic process is repeated until a comfortable water temperature is achieved. Depending on the characteristics of the water supply, it may take several seconds or more for this stable state to be reached. In the interim period the shower water may fluctuate from being too hot to too cold, in gradually diminishing amounts.5.3 Types of feedback control methodsThe following are various kinds of control methods, some of which help minimise lag or delays in response or cycling about the set point. A feedback control system compares the signal from the sensor with the set point and takes appropriate action to reduce the difference between these two values. The time taken to reduce the difference to zero (or close to zero) depends on the type of action taken.On-off control: This is the simplest and cheapest control method. It can be used when there is a short lag time. It is sufficient to prevent or minimise overshoot and excessive cycling. Domestic hot water systems operate in this way.There are many situations where simple on/off control is not satisfactory. Imagine an aeroplane flying on automatic pilot, which is a good example of closed loop control. If the plane starts to veer slightly to the left, a correction of the full right rudder will obviously. In this instance a proportional correcting action is needed. A small variation in course requires only a small correction by the rudder. A large variation in course, as occurs in violent air turbulence requires a proportionally larger correction by the rudder. This is a type of proportional control action.Proportional control (P): This is a modified on-off control, where the controller responds to the signal from the sensor in proportion to the difference between the set point and the sensed value.Integral control (I): A "proportional only" control system has a disadvantage that a small error called offset is built into it. With a proportional only control, a particular response results from a particular deviation. Because the response is fixed in proportion to the deviation, sometimes the new response is offset from the desired or set point.With an integral control action, the controller can be set up so that further action is taken if the controlled variable remains different from the set point for more than a predetermined time. This action eliminates the offset error and is called integral or reset.An example of the use of integral control action would be to add up small errors in course changes over a period of time (since reset) and produce an output control action which would correct the accumulated error.A controller with both proportional and integral control is designated (P & I) or proportional with reset.Derivative control (D): As well as the degree or amount of veer from the correct course, there is also the rate at which this occurs. If a plane starts to veer very quickly, a proportional control action will not be very effective in preventing a course adjustment because a large course change will have already occurred before the large rudder correction is produced. What is needed is a system which responds to the speed at which the course starts to change. The controller takes action in response to the rate at which the controlled variable is changing. This is called derivative control action. PID controller: Improved process control can be achieved by combining the above types of control systems in PI or PID controllers. The letters PID refer to the three control actions taken by this type of controller, viz: proportional, integral and derivative. It is more difficult to accurately describe the complexities of PID control than the individual methods (P, I and D) but the reading “The PID Controller” (Thomas 2004.pdf) provides a simple description of this method of control. In addition, an informative tutorial is available on the internet (tutor.html).5.4 Control systems in the woolscourIn this section the various types of process equipment on the scouring train and the liquor loop are examined. Opening and blendingThe quality of the final wool product, (e.g. clothing, furnishings or industrial textiles), depends heavily on the first stages of raw wool processing. If the components of the blend are not adequately mixed then a variable end product may result. The most appropriate method of achieving an intimate mixing of the greasy wool components of a blend is to use a multi-hopper weighbelt system, as shown in Figure 5.3. The blend components are loaded into feed hoppers, which deposit the wool onto weighbelts. From there the wool is dropped on to a common conveyor and thence into a temporary storage bin. Each hopper-weighbelt combination controls the rate at which wool is deposited on to the conveyor, and these rates are calculated according to the ratios of the weights in the blend ‘recipe’. A central computer is used to set these ratios, and it also controls the overall feed rate to keep the storage bin at full capacity.The weighbelt system consists of a short conveyor that is pivoted close to one end, and is counter-weighted to balance when no wool is present. The other end is supported by a load cell that registers the weight of wool on the conveyor. Wool is deposited from the hopper onto the weighbelt above the pivot point and the weight is registered. Another sensor registers the speed of the belt, and the weighbelt controller calculates the flow rate from these two signals. The flow rate is fed to a PI controller which compares this value with the set point for flow rate and hence adjusts the speed of the hopper drive motor to maintain the required flow rate.Figure 5.3 Multi-hopper weighbelt system for controlling the blending of greasy wool. Source: Andar Holdings Ltd.Greasy wool level in the feed hopperThe level of wool in the feed hopper before the first scouring bowl needs to be regulated to keep a steady flow of wool to the scour and avoid overloading any individual process. The conventional means of achieving an even, continuous feed of greasy wool into the scour is to use a hopper and weighbelt arrangement similar to that used in blending, but to precede it with another hopper and opener. The first hopper feeds the wool into the opener and thence to the second hopper, which feeds wool on to the weighbelt. The level of wool in the second hopper can be measured by either of two devices:1.A photo-electric sensor, which uses a beam of visible light or infrared light which is broken when the level of wool is too high. This is similar to the sensor used in shops to open doors automatically as a customer enters. A problem with these sensors is that their lenses can easily be contaminated by the greasy wool. It has also been found that the onoff control of the feed hopper results in high maintenance of the driving mechanisms2.An ultrasonic sensor, which uses high-frequency sound waves above the normal hearing range, is now more commonly used. The sound waves are transmitted down on to the wool at the bottom of the spiked brattice feed. The very short time interval for the sound waves to return to the receiver is a measure of the amount (i.e. height) of wool in the hopper. The longer time for sound waves to return to the receiver, the less wool in the hopper, and so the controller will instruct more wool to be loaded in. The ultrasonic sensor detects the volume of wool in the second hopper and controls the first hopper by means of a PID controller which adjusts the speed of the motor on the spiked brattice of the first feed hopper. This is the operating principle of the HOPPERMATIC system, which is shown in Figure 5.4.Figure 5.4 Feeding and opening system to a scouring line. Source: Andar Holdings Ltd.The advantages of a two hopper system (as shown in Figure 5.4) over a single hopper system, in terms of even throughput and production level, are shown in Figure 5.5.Figure 5.5 The advantages of a two hopper feed system over a single hopper feed. Source: Andar Holdings Ltd.Scouring bowl temperature controlThe liquor is usually heated by a heat exchanger under or beside the bowl. A shell-and-tube type of heat exchanger can be used, as Figure 5.6 shows.Figure 5.6 Shell-and-tube heat exchanger. Source: Wood, 2006.Electronic temperature sensors, which are heat sensitive, are used with bowls that use a gas-heated shell-and-tube heat exchanger for heating the liquor. The gas is burnt in the tubes in the heat exchanger. A temperature sensor actuates proportional and integral (P & I) electronic controllers to control the gas supply. The gas supply is varied by a modulating valve.Flowback controlFlowback control is the essential feature of counter-current scouring.The flowback is controlled with a simple float control valve, similar to those used in household water delivery systems, or an air-operated diaphragm valve. The action is proportional (P) only. This is shown in Figure 5.7, where some of the liquor from bowl 2 flows into the nip just before the squeeze rollers of bowl 1.Figure 5.7 Flowback control. Source: Wood, 2006.Control of a liquor-handling loopThere are several variations of the original WRONZ liquor handling loop; one version is shown in Figure 5.8. This system has several control parameters, most which can be computer controlled.Maintenance of volumes of liquor in the tanks is generally achieved by using capacitance or conductivity level probes, and variable speed pumps. The level probes usually consist of a rod that dips into the liquor and a signal is generated which is proportional to the length of the rod that is immersed in the liquid. If the liquor foams excessively these probes may give erroneous readings.PI control is used to set the speed of a pump. A number of speed controllers operate by varying the frequency of the electrical supply to the motor from the 50 Hz of the mains supply. The motor responds by synchronising its speed with the power frequency. The controller senses the load on the motor and automatically supplies sufficient current to ensure that the motor turns. These variable frequency motor speed controls usually provide some form of manual speed setting, and they automatically detect fault conditions.Settling-tank sludge discharge can be controlled either by a timer or by an ultrasonic sensor. The latter works in a similar manner to the sensor used in the feed hopper, except that in this application it senses the level of the sludge interface. The sludge is discharged to the sludge skip by the pump, which is automatically turned on and off. Both systems of control aim to remove the sludge as rapidly as it accumulates.Bowl draw-off to the heavy solids tank (HST) is now continuous. It is all done from the bottom of bowl 1. The draw-off from the bottom of bowls 2 and 3 is returned to the midsection of bowl 1, just below the perforated screen of the woolway. The flows from these two bowls is controlled by timer. Previously the flow from bowls 2 and 3 was individually timed direct to the heavy solids tank. Heavy solids tank (HST) level, is controlled by the variablespeed drive of the feed pump, the control action being proportional (P). The level in the tank is sensed by a conductivity probe, which in turn controls the variable speed of the motor for this pump.In the feed to the centrifuge, both the temperature and the feed rate must be controlled. The temperature may be controlled by a proportional (P) controller similar to that used in controlling bowl liquor temperature. The feed rate is controlled manually by opening or closing a valve, according to the consistency of the woolgrease emulsion being produced.Figure 5.8 WRONZ scouring system and liquor loop. Source: Wood, 2006.Flowdown of heavy liquors to waste can be controlled by sensing the level of solid matter in the scouring liquor using a FLOCOM system, or by setting the flowdown pump to a constant speed. FLOCOM (Figure 5.9) measures the relative density of the liquor and discharges heavy liquor from the balance tank to maintain the liquor at a constant density.In Figure 5.9, the lines marked with short lines across them are electrical circuits, not liquor flow lines.FLOCOM keeps the total solids in the first scouring bowl at a set level of concentration by controlling the flowdown. The total solids in bowl 1 may increase because of:an increase in the feed rate of greasy wool, ora change in type of greasy wool entering the first scouring bowl.When an increase in total solids occurs:The flowdown is increased, andMore clean liquor is admitted from bowl 2 to bowl 1 through the flowback system.Thus the level of total solids in bowl 1 is kept constant.More recently, optical sensors that measure the optical density of the liquor have been adopted for this application. These sensors operate by detecting a light beam passing through the liquor. An optical sensor determines how much light penetrates the liquor and this is used as a measurement of the liquor condition. The liquor condition can then be controlled by adjusting the flowdown rate.Figure 5.9 FLOCOM system. Source: Wood, 2006.Wool dryer controlWith dryer control, sensors can provide information on:The regain of the wool leaving the dryerThe temperature inside the dryerThe humidity inside the dryer, andThe wool flowrate.If required, controllers can then adjust:The heat supply to the dryerThe position of the exhaust damper in the dryer, andThe flow rate of wool to the dryer.Even a wellcontrolled dryer needs a uniform feed. A low regain variability of the wool into the dryer means a more uniformly dried product coming out of the dryer.Proper control of a wool dryer is impossible without the ability to measure the moisture content of the dried wool. The Streat Moisture Controller acts by measuring the moisture in the wool and then adjusting the dryer conditions to maintain the regain at the required level. Figure 5.10 shows the regain measurement sensors at the exit of a wool dryer. The moisture probes touch the wool and sense its electrical conductivity. The meter converts the conductivity to wool regain by calculation.Figure 5.10 Wool moisture sensors. Source: Streat Instruments.The most efficient type of dryer control uses a two-stage (or cascade) PID control, as illustrated in Figure 5.11. This is the DRYCOM 2000 Textile Dryer Moisture System, manufactured by Streat Instruments. This system uses two PID controllers, with the control signal of one being used as the set point of the second. In a wool dryer the first PID controller uses the regain value from the moisture meter as the measured variable, and outputs a control signal, which is the temperature required to achieve the set point regain. The second PID loop then uses a temperature probe and adjusts the heat input to the dryer (using a steam or gas valve) to achieve the correct temperature.This arrangement has two advantages in wool drying:The temperature loop is very fast acting, since the dryer temperature will respond quickly to changes in energy supply. This enables a fast reaction to a change in the thermal demand of the dryer, as a result of, say, a change in wool throughput. The regain control is much slower and is therefore able to ignore the inevitable rapid changes in the signal from the regain sensorIt is easy to cope with the situation when the wool flow suddenly stops. In this situation the regain sensor indicates zero wool and the first PID controller requests a fixed temperature from the second PID controller. This temperature is set such that when the wool supply restarts, the first few kilograms of wool through the dryer will have approximately the correct regain, even though there was no feedback control of regain operating while that wool was in the dryer.Dryers are sometimes divided into two zones, each of which is heated separately. This enables more efficient use of energy since most of the moisture is easily removed from the wet wool in the first zone at low temperature. The temperature of the second zone is then adjusted to obtain the required regain at the dryer exit.Figure 5.11 DRYCOM 2000 dryer control system. Source: Streat Instruments.Figure 5.12 shows the links between the various components of the Drycom system. The plant managements system is also linked to various control systems in the plant – feed control, FLOCOM etc.Figure 5.12 The complete Drycom System. Source: Streat Instruments. 5.5 Supervisory control and data acquisition systemsRecent developments in computer technology have enabled extensive centralised monitoring and control systems to be installed in wool scouring plants. Several companies, including Streat Instruments offer fully integrated packages that carry out all the functions of control and monitoring in a wool scour. The Streat SCOURCOM system (Figure 5.13), probably the most comprehensive system currently available, is widely used in wool scours. It replaces the control desk, mimic panels and individual control modules of more traditional systems.Figure 5.13 A computer display from the Scourcom. Source: Streat Instruments.All the equipment, apart from the sensors, is housed in a suitable room, protected from the harsh environment of the wool scour (Figure 5.14). Figure 5.14 SCOURCOM monitor. Source: Streat Instruments.The systems typically use a programmable logic controller (PLC) to carry out the control and data acquisition functions, and a computer network within the scour reports the status of the scour to staff and management. Up-to-date plant production and quality information can be accessed via desk-top computers around the plant, or at a remote location (e.g. plant manager’s home). It also allows the plant operators to select different information to be shown on a computer screen, and they can gain access to information on any part of the plant. With the information onscreen, the operator can adjust set points or change any of the conditions in the plant as required. The complete network is shown in Figure 5.13.The manufacturers of SCOURCOM, Streat Instruments, claim the following features:Easy to useFlexible – installation can be customised to suit scale of operation or production requirementsA plant overview screen gives details of critical items at a glancePlant detail screens give detailed information on specific plant itemsAll alarm and tuning parameters can be adjusted from SCOURCOM; enables operators to focus on producing a high quality product without the distractions of routine plant operationReport information is automatically generated.The information collected for management offers the following:Reporting, cost analysis and historical analysis of plant performanceReduction in time and costs involved in information collectionImproved accuracy and frequency of collected dataImproved plant performance and product quality due to increased process knowledge, improved plant operating procedures and process conditions made possible by studying historic informationPrompt identification of machinery or processes that cause stoppages, hence reducing downtimeManagement information is available in many forms (e.g. trend graphs, shift reports, batch reports, weekly summaries, alarm log, activity log, etc.).Feeding chemicals into the scour or effluentDetergent, mothproofing agent, acid, bleach, and flocculant are fed to various parts of the scouring train and to effluent flowdown by automatic metering pumps. These pumps can be linked to the Scourcom computerised control system for comprehensive control.Selection of control hardwareIt will be apparent from this topic that selection of the correct control equipment is essential to the successful operation of a control system. The sensor must be appropriate, both to the environment in which they are located and the speed at which they must respond. A scouring bowl filled with hot water has a large thermal inertia and will change in temperature only very slowly. A robust temperature sensor is required but not a fast acting one. On the other hand, the conditions in a dryer can change much more quickly. Therefore, both temperature and humidity sensors need to have a rapid response time as well as being suitable in the environment of a dryer.Furthermore, control is not possible in a dryer that is overloaded to the extent that the steam valve is fully open and the steam boiler is working at fully capacity. Control is only possible if the elements are correctly specified for the task. Valves must be sized so that they normally operate between 30% and 70% open.An important link in the framework of any control system is the supplier of the hardware. There are many questions that must be addressed, relating to the performance specifications, service requirements, technical capability, service contracts, warranties, availability of spare parts and the possibility of future developments, upgrades and enhancements. The supplier should also have a sound working knowledge of the woolscouring industry. Both user and supplier must be clear at the outset on what is expected from the equipment and what the supplier’s obligations are to all aspects of the contract. This will avoid difficulties later should the equipment not perform as expected by the user.Two other essential considerations are (a) cost (which includes the capital cost and the costs of installation and maintenance), and (b) reliability. Managers must have confidence in the plant control equipment to use it to its best advantage, and this confidence can be readily lost through a bad experience concerning reliability. They will sometimes choose to operate without automatic control (and therefore uneconomically) rather than risk failure of the control equipment at a crucial time.Where cost and reliability are concerned it is worth mentioning that the computing equipment used for control should be designed to operate in the environment in which it will be installed. Many inexpensive computers are available with more then sufficient computing power to control the processes in a wool scour, but most are not designed for the harsh, demanding environments encountered there. The chief problems are high temperatures and dust and corrosive chemicals in the atmosphere. All of these can cause premature failure of inadequate (or insufficiently protected) computer equipment. Computer equipment that is appropriately housed is likely to provide trouble-free service.Readings The following readings are available on CD:Cooper, D.J. 2000, Fundamental Principles of Process Control, Practical Process Control using Control Station.Honeywell, Sensing and Control, Humidity Sensors, Reference and Application Data, HIH Series.Thomas, B.L. 2004, The PID Controller, Canesis Network Ltd, Christchurch, New Zealand.ActivitiesAvailable on WebCTMulti-Choice QuestionsSubmit answers via WebCTUseful Web LinksAvailable on WebCTAssignment QuestionsChoose ONE question from ONE of the topics as your assignment. Short answer questions appear on WebCT. Submit your answer via WebCTSummary Summary Slides are available on CDThis topic covers the principles of industrial process control systems and their applications in the modern woolscouring industry. The gradual introduction of these systems over the past few decades has enabled wool scours to operate increasingly efficiently with fewer staff, to maintain required performance with respect to production and quality, and to meet increasingly stringent environmental requirements.Most modern woolscouring operations incorporate process control technology for opening and blending, feeding to the scour, maintaining consistent bowl conditions and for efficient wool drying. Continuous weighing, temperature control, humidity control, wool regain and liquor quality can all be automatically controlled by the appropriate combinations of sensors, actuators and computer-based controllers.ReferencesAnder Holdings Ltd., Scoutmaster – the range of wool scours for superior wool processing, product brochure, Andar Holdings Ltd.Stewart R.G., 1988, Control and Instrumentation, in: Woolscouring and Allied Technology, Caxton Press, Christchurch, New Zealand, ISBN 0-908699-23-9.Streat Instruments, Moisture measurement and control, product brochure, Streat Instruments.Thomas, B.L., 2004, The PID Controller, Canesis Network Ltd., Christchurch, New Zealand.Glossary of termsActuatorThe device that receives a signal from the controller to alter the position of the controlling element (e.g. a hydraulic or pneumatic ram, or an electric motor)CapacitanceThe property of a capacitor, which consists of two oppositely charged plates, with an electric field between themConductivityThe ability of a material to conduct electricity (the opposite of resistivity)ControllerA device (computer, PLC, etc.) that receives an input signal, compares this with a set point value and sends an output signalDerivative controlThe controller sends a signal to the actuator which depends on the rate of change of the difference between the measured value and the set pointFeedback loopA control system which is designed such that the controller can continually make changes in a process based on the signal it receives from the sensor measuring the processHeat exchangerA device that enables the heat contained by a liquid or gas to be transferred to another liquid or gas, thereby reducing energy costsHumidityThe moisture content of air, often expressed as a percentage of the maximum amount of moisture the air can retain at that temperature (relative humidity)Integral controlThe controller sends a signal to the actuator if the measured value remains different from the set point for a predetermined time intervalMimic panelA board which depicts the status of the various functions of a complex process or plant, mostly through a flow diagram illuminated by small lights. This board would usually be located in the control room of a wool scour. Largely replaced now by computer screensPhoto-electric sensorA light beam is detected by a light-sensitive device. The signal from this device changes if the light beam is partially or totally obstructedPID controllerA process controller that combines P (proportional), I (integral) and D (derivative) actions to maintain the process variable at the set point, with minimal drift, lag or fluctuationsProcessThe means by which a material is converted into another formProgrammable Logic Controller (PLC)A small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. Where older automated systems would use hundreds or thousands of relays, a single PLC can be programmed as a replacementProportional controlThe controller sends a signal to the actuator which is proportional to the difference between the measured value and the set pointRelative densityThe mass of a certain volume of material (solid, liquid or gas) expressed as a ratio of the same volume of water (at 4oC)SensorA device that responds to the environment in some way that enables the conditions to be measuredSet pointThe desired value of the controlled variable, usually set manually by the operator (e.g. the temperature set in an air conditioning system)Ultrasonic sensorDetects ultrasonic waves (with a much higher frequency than audible sound) that are reflected off an object. In an ultrasonic proximity sensor, the time interval for the sound to return (its ‘echo’) enables the distance of the object to be measuredWeighbeltA conveyor with the facility for weighing the material placed on it at any instant ................
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