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PRACTICES MANUAL Unit Ref.: UCP
7.4.2 7.4.3 7.4.4
Date: October 2010
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Annex 2: Temperature sensor calibration......................................................................................................... 104 Annex 3: Level sensor calibration...................................................................................................................... 108 Annex 4: pH sensor calibration .......................................................................................................................... 112
7 PRACTICES MANUAL 7.1 GENERAL DESCRIPTION OF THE SYSTEM In this section, the process unit and the control loops used in measuring experiments for level, flow, temperature and pH regulation are described, together with the different equipment and necessary instruments for the physical simulation of the corresponding dynamic systems. 7.1.1 General description of the system This unit consists on a hydraulic circuit, with an bottom tank (1) and a superior process tank (2), both dual ones, two pumps of centrifugal circulation (3), two flowmeters with a manual control valve (4), three on/off solenoid valves (5) and a motorized proportional valve (infinitely variable) (6). Of course, together with the tubes, the union elbows, connections, feedthroungh, main valve and the appropriate drainage for the circuit operation. All the above-mentioned is set on a designed support structure so, that it is placed on a work table (7). As additional fixed elements, there is also a turbine flow sensor that is installed in one of the upward lines of flow (8), and a temperature sensor located in a lateral bottom of the process tank (9) together with a serpentine with electric heating (11). The interchangeable additional elements are an agitator (10), the immersion level sensor should be located in the process tank (12) and the pH sensor (solenoid), can be in the process tank or also in the second tank (13), to study the
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effect of the time out.
Figure 1.1.1 Main diagram of the equipment
The UCP elements indicated in the diagram are: 1. A main tank and collector with an orifice in the central dividing wall (2 x 25 dm), and drainage in both compartments (made in methacrylate). 2. A dual process tank (2 x 10 dm), interconnected through an orifice and a ball valve and an overflow in the dividing wall (methacrylate); a graduate scale and a threaded drain of adjustable level with bypass (metallic). 3. Two centrifugal pumps.
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4. Two variable area flowmeters (0.2-2 l/min, and 0.2-10 l/min), and with a manual valve. 5. Line of on/off regulation valves (solenoid). Usually one is opened (AVS-1), and the other two are usually closed with different Cv (AVS-2 and AVS-3); and manual drainage valves of the superior tank. 6. A motorized control valve (AVP-1; piston type) with rotation indicator. 7.
Structures, panels, pipes and connections made in stainless steel and methacrylate.
8. A flow sensor, fixed, turbine type. 9. A temperature sensor. 10. A helix agitator. 11. An electric resistor (0.5 KW), fixed. 12. Level sensor 0-300 mm (of capacitive immersion, 4-20 MA), can be dismantled. 13. PH sensor (glass electrode, ddp(V)), collapsible (tank). 14. On/off level sensor. This sensor determines the performance of the immersion resistor. 15. Control loops: interface, controller, monitor and keyboard (PC and cards), electric connections.
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7.1.2 Operation of the subsystems For the level, flow and temperature control test, the liquid (water) is impelled from the tank by the pump, located to the left of the front of the equipment, going through the flowmeter, the solenoid valve (usually open), the motorized valve, the turbine (flow sensor) and the process tank. It is possible to use the second pump in the level tests, as it will be indicated. The pH control test of requires a second parallel line of flow (right), provided only with pump and a flowmeter. The compartments of the inferior tank should be loaded with diluted solutions of an acid and a base, respectively. The process tank is divided in two halves, with an orifice between them that allows their communication or isolation. The right compartment has an overflow of variable level (that it prevents the complete overflow of the tank, and it allows to modify its effective liquid volume), two drains with solenoid valves with different Cv (normally closed), and a third one with a normal drainage valve. The left compartment is only connected to a drainage valve. The level control tests require all the elements of the circuit and of the tank, besides the sensor located in it. In some experiments, it is required the second pump placed to the right-hand side of the equipment. The TEMPERATURE CONTROL tests, in these cases, as we will see later on, can be carried out with experiments in closed circuit or in open circuit. In the close circuit case, fill the superior tank with the right pump 1 (AB-1) and carry out the experiment. In open circuit, keep a constant water flow using the pump 1, this
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way, a small water flow is adjusted and the superior overflow is used as a drainage system. In this case, it is necessary to use the agitator to guarantee a good temperature uniformity.
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7.2 THEORETICAL BASIS Modeling, simulation, design and optimization require a series of processes that will be explained in the following paragraphs. The understanding of these theoretical concepts will help the student to follow the suitable instructions for the practices procedure. 7.2.1 Modeling, simulation and control process The development, and good operation of the industrial plants, requires the right selection of the equipment and process parameters. This election is supplemented with the instrumentation and the control, as well as with the dexterity to adjust and to manipulate them correctly. For it, the industrial engineering processes use different tools and technical aspects based on: 1.- The modeling of the systems. 2.- The simulation of the stationary and dynamics response. The control process is the objective of this modeling and simulation that guarantees that the dynamic behavior is efficient and precise. The design and the process optimization is based on calculations in stationary state to specify, in a first approach, the operation conditions of the equipment in the process plants. These design calculations in the stationary state don't say anything about the dynamic response of the system, they only tell us where we begin and where we end up, but anything about the process behavior. This type of
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information is the one that informs about the study of the dynamics procedure. The dynamics and the control study the non-stationary behavior of the processes and the design of the control systems in function of the interferences. With this, the design of the process systems is finished, that include the own process and its control loops. In a first approach, the process control unit has been designed for the study of the dynamic behavior of the different control loops. The modeling and stationary simulation of the processes is not deal with in this equipment. 7.2.2 Dynamics and control Once the freedom degrees of the process are selected and the design variables good values are calculated, these values should remain unaffected with the deviations that take place during the real operation of the systems during the stationary state. The factors that cause such deviations are denominated interferences. These can be internal or external to the process, random or programmed. The random interference type are those that produce deviations from the programmed regime due to some process input variables fluctuations. In the programmed interferences, it is necessary to consider the transitory periods of starting-up, stop or changes in the stationary state of the process. The number of variables that you can fix coincides with the freedom degrees (although they don't have to coincide with the free variables that were chosen during the design phase). The variables that are more easily controlled are: level, flow, pressure, temperature and composition. We will denominate them controlled variables and their values we will be called set point. In a controller, the input
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magnitude of the set point is fixed in a constant value. In a “programmer” controller the inlet magnitude is a value that varies in function of the time, according to the programmed law. The variable used to regulate the set values is denominated manipulated variable. The function of the regulating device of the system is to activate automatically the control elements that allow to modify the value of the manipulated variables, so that the controlled variables are the next to the set point. 7.2.2.1 Dynamics in open loop The systems dynamics studies the non-stationary situations of the process. This study is always previous to the design of the control that avoids the nonreversibility of some deviations of the stationary state. In the process, the manipulated variable acts changing the value of the controlled variables so, in the simplest cases, it can be predicted using of a mathematical model. So, it is fundamental to have an exhaustive knowledge of the process to will be studied. The dynamic simulation of a process consists on: 1. Developing the material and energy balance equations, the balances, the physical laws or other independent sources of information. 2. Selecting
the
values
of
the
necessary
physical-chemical
magnitudes and the values of the preset points. 3. Obtaining the differential expressions of the variables determining their temporary evolution for an interference in the stationary regime starting from a defined initial state.
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As a particular case, the design equations in the stationary state can be obtained (null accumulation) and, starting from them, calculate the values of the design and state variables in this regime. The system dynamic in open loop represents the process behavior in the absence of controllers. The velocity and the tolerance of the response of the process in function of the interferences can be studied, this is the necessity of the control system. 7.2.2.2 Feedback Control The feedback control system consists, essentially, on measuring the controlled variable, to compare its value with the one wanted (set point) and, according to the error, to act on the control element of the manipulated variable. This is a feedback system, in which the corrector action persists while the error is not null. The corresponding signals spread in the circuit, making comparison cycles, of calculation and of correction (in closed loop) with a certain time of response. If a closed control is turned into manual operation, the operator can govern directly the control valve and the value of the regulated variable obtained from the transmitter can be observed in the controller. In this case, we are operating in open loop, since the output signal of the controller is off. The main elements of a control system in closed loop are: • Sensor: Device that is able to measure the value of the controlled variable (it is also denominated primary measure element). • Transducer: Device that transforms the measures in normalized equivalent signals, pneumatic or electric, according to the distance between the process and control rooms, which are sent to the
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comparator (also denominated signal conditioner element). • Controller: Device that receives the error from the comparator, interprets it, and it acts on the final element of control. There are three types of corrective actions: - Proportional: the signal sent to the final element is proportional to the error. If the proportional constant is very big the control behaves as an on/off control. - Integral: the corrective signal sent is proportional to the error accumulated with the time (integral error). - Derivative: the signal is proportional to the velocity of variation of the error. These last two actions are usually combined with the primary signal (proportional) to improve the control quality and even, they usually combine the three in complex processes. These combined processes are known as P.I.D. control (proportional-integral-derivative). • Final element of control (actuator): Device that, according to the signal of the controller, acts on the manipulated variable regulating the inlet material or energy flow to the process (valve, volumetric pump, compressor, rheostat, etc.). The control systems are effective with small interferences. In the case of big interferences, the final element can end up being totally open or totally closed, and, above a certain value, the control would not act appropriately (saturated). In these cases, to correct it, you should act manually on the parameters of the system.
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The sensors, the transmitters and the final control elements are inserted in the process in a logical way. The control devices are usually located in a control room, and they are in normalized panels with graphic instruments for the variables and a diagram of flow (in a simple pilot plant it can be a simple PC computer with a monitor and a printer). In the control diagrams, the variables are designated with the letters F (flow), L (level), P (pressure), T (temperature), followed by the indicative letters of the service and of the functions of the instruments: T (transmitter), C (controller), I (indicator), R (register), A (alarms). 7.2.2.3 Forward control The forward control makes the corrective action in the moment in which the interference is detected, instead of waiting to its spread through the process, as it happened in the feedback process. The action is independent of the value of the controlled variable, making it depend on another, according to a calibrated preset. This system is used with simple processes that don't require a great accuracy, or in those processes in which the closed loop doesn't give good results, for example, in processes in which that the measure and the corrective action take place in a great period of time. 7.2.3 Dynamic simulation of the control systems The dynamic simulation of the control systems allows studying the response of the system facing up diverse interferences, such as changes in the input variables, in the set point, or in the controller adjustments. The system consists of a block that represents the process, and of another that corresponds the controller connected to the first one forming a feedback loop.
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Figure 2.3.1
The process consists basically of an input (q), a manipulated variable, also called feedback (m) and an output (c) (controlled variable). The controlled variable is subtracted to the set point (r) to obtain the error (e) (input to the controller). The controller calculates an output signal by means of an appropriate control algorithm. The characteristic response of a controller on/off comes determined by its hysteresis, also called dead band. In a PID controller (proportional-integral-derivative) the signal comes determined by the gain (Kc) or its percentage inverse (BP=100/Kc, proportional band) and the integral (I) and derivative (D) times.
[1]
c = Kp[q, m] e = r −c m = Kc( e + ∫ e∂t + D
∂e ∂t )
In open loop, the state variables (outputs) are calculated starting from the initial values and from the process pattern. It is defined in function of differential equations for the transitory regime, or by state equations for the stationary regime (invariable with the time). These last ones are the techniques employed during the design phase of the experiment and they allow calculating the values of the necessary input variables to reach the stationary regime. The first ones allow studying the transitory periods during the setting phases in setting-in, stops or changes in the stationary regime of
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the process. During the real operation of the process, the variables are modified with the time as a consequence of diverse interferences in the input variables or in the set point (programmed). In these cases, the process can only work correctly using a closed loop control. The dynamic behavior of the system is calculated by an iterative algorithm at discreet intervals of time (Euler’s method). The first phase of the program is the data input: fixed variables, initial values of the manipulated variables, set point of the controlled variables, adjustment of the controller (hysteresis, parameters PID) and interference variables. Qt
m
PROCESO
RT
Ct
COMPARACIÓN
H,KC, I,D
eI
CONTROL
mt
Figure 2.3.2
The mathematical pattern of the process calculates the outlet (Ct ) in function of the inlets (q t, mt ). This value is compared with the set point (rt ) and the resulting error (et ) is calculated (the accumulated error (integral) or its derivative with the time, the trapezoidal rules and finite increments are used in this case). Finally, with these values, the controller output (s t ) and the manipulated variable (mt ) are calculated making use of the adjustments of the control actions (H, Kc, I, D) and of the calibration of the actuator, respectively.
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The relationship between these last two variables can be settled down on the base of a function (lineal) between the width of the actuator (m) and the signal of the controller (±s). Also, automatic graph representation commands of response can be included (controlled and manipulated variables), time outs in the process (always superior or similar to the time of sampling, t≥∆t) and sampling time of the controller similar to those of the process (analogical control) or superiors (digital control). In case a graphic register is used, this should be connected to the electric signs of the sensor/conditioner (controller input) and to its output (actuator input) to obtain a register of the response of the process.
7.2.4 Operation and calibration of the process equipment and control elements The verification and calibration of all the sensors you can find in the equipment can be carried out in two different ways. The Saced System that is supplied with the equipment, has a calibration window specially designed for such a purpose (see Calibration Manual). However, and due to the importance that the calibration the sensors has in the control process, the system offers two text windows in which the gain and the zero can be introduced of each one of the transducers that the equipment has: flow, level, temperature and pH.
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Name of the sensor for calibration.
Figure 2.4.1
In the same way, the verification of the operation of the pumps, solenoid valves, agitator, etc. it can be carried out from the calibration window in the Saced System. This process of operation test should be carried out with extreme care and under the introduction of the professor's PASSWORD. This Password has been indicated in the software manual. Once the professor's password is introduced, the program allows you to select the channels included in the equipment for the digital outputs as well as for the analogical ones. Verify each one of the elements assembled in the equipment and carry out a good calibration of the sensors. We should point out that, when storing the calibration values in the file UCP.EDB, these will be recovered every time that you enter in the program, independently from the values introduced by the students in the different practice sessions. When selecting the different outputs that has the acquisition card (analogical and digital outputs, and analogical and digital inputs), you can verify the operation of all devices of the equipment. As an example we have the following table:
PRACTICES MANUAL Unit Ref.: UCP
Date: October 2010
Analogical port (Inputs) Channel Action/Sensor Channel 0 pH sensor Channel 1 Level sensor Channel 2 Flow sensor Channel 3 Temperature sensor Channel 4 Alarm of Level Channel 5 Proportional Valve
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Digital Port (Outputs) Channel Action/Sensor Channel 0 AVS-1 Channel 1 AVS-2 Channel 2 AVS-3 Channel 3 Resistor On/Off Channel 4 Agitator On/Off Channel 5 Pump 1 On /Off Channel 6 Pump 2 On/Off
Table 2.4.1
To outputs and inputs it is necessary to add the analogical output that corresponds to the motorized valve.
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7.3 LABORATORY PRACTICES It is very important for the formation of the student, the calibration of the sensors, hence is recommendable that the students see the Annex 1 to Annex 4. 7.3.1 Practice 1: Flow control loops (manual) 7.3.1.1 Objectives The objective of this experiment is to control the flow that circulates through a conduction of water by a manual procedure. We assume that the manual control works as: • Manual regulation of the adjustable valve placed under the area flowmeter. • Manual control of the elements used in the equipment as the motorized valve, solenoid valves, etc. 7.3.1.2 Required material To make this practice, the following elements are necessary: • UCP-F • Water • SACED Software. 7.3.1.3 Experimental procedure 1.- Connect the interface of the equipment and execute the program
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SACED UCP-F. 2.- Inside the program, select the option Configuration and connect pump 1 (AB-1) (see Software Manual for a more detailed operation). 3.- In the manual regulation (no controller) the flow can be regulated by the manual adjustable valve VR1, placed in the inferior part of the flowmeter. Vary its position and observe the adjustment of the flow in function of its position. 4.- Select the option “Manual Control” of the software supplied with the equipment. 5.- Connect pump 1 (AB-1) and vary the position of the motorized valve by the Slip bar or the command associated to this action. Check a fixed position, the flow is regulated. 6.- Vary the position of the valve and repeat the values to observe the reproduction of the flow control. 7.- Use the controls prepared in the software for controlling the solenoid valves AVS-1 and the on/off button of the pump. Observe how an on and off button also produces a flow control of the liquid.
7.4.2 7.4.3 7.4.4
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Annex 2: Temperature sensor calibration......................................................................................................... 104 Annex 3: Level sensor calibration...................................................................................................................... 108 Annex 4: pH sensor calibration .......................................................................................................................... 112
7 PRACTICES MANUAL 7.1 GENERAL DESCRIPTION OF THE SYSTEM In this section, the process unit and the control loops used in measuring experiments for level, flow, temperature and pH regulation are described, together with the different equipment and necessary instruments for the physical simulation of the corresponding dynamic systems. 7.1.1 General description of the system This unit consists on a hydraulic circuit, with an bottom tank (1) and a superior process tank (2), both dual ones, two pumps of centrifugal circulation (3), two flowmeters with a manual control valve (4), three on/off solenoid valves (5) and a motorized proportional valve (infinitely variable) (6). Of course, together with the tubes, the union elbows, connections, feedthroungh, main valve and the appropriate drainage for the circuit operation. All the above-mentioned is set on a designed support structure so, that it is placed on a work table (7). As additional fixed elements, there is also a turbine flow sensor that is installed in one of the upward lines of flow (8), and a temperature sensor located in a lateral bottom of the process tank (9) together with a serpentine with electric heating (11). The interchangeable additional elements are an agitator (10), the immersion level sensor should be located in the process tank (12) and the pH sensor (solenoid), can be in the process tank or also in the second tank (13), to study the
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effect of the time out.
Figure 1.1.1 Main diagram of the equipment
The UCP elements indicated in the diagram are: 1. A main tank and collector with an orifice in the central dividing wall (2 x 25 dm), and drainage in both compartments (made in methacrylate). 2. A dual process tank (2 x 10 dm), interconnected through an orifice and a ball valve and an overflow in the dividing wall (methacrylate); a graduate scale and a threaded drain of adjustable level with bypass (metallic). 3. Two centrifugal pumps.
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4. Two variable area flowmeters (0.2-2 l/min, and 0.2-10 l/min), and with a manual valve. 5. Line of on/off regulation valves (solenoid). Usually one is opened (AVS-1), and the other two are usually closed with different Cv (AVS-2 and AVS-3); and manual drainage valves of the superior tank. 6. A motorized control valve (AVP-1; piston type) with rotation indicator. 7.
Structures, panels, pipes and connections made in stainless steel and methacrylate.
8. A flow sensor, fixed, turbine type. 9. A temperature sensor. 10. A helix agitator. 11. An electric resistor (0.5 KW), fixed. 12. Level sensor 0-300 mm (of capacitive immersion, 4-20 MA), can be dismantled. 13. PH sensor (glass electrode, ddp(V)), collapsible (tank). 14. On/off level sensor. This sensor determines the performance of the immersion resistor. 15. Control loops: interface, controller, monitor and keyboard (PC and cards), electric connections.
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7.1.2 Operation of the subsystems For the level, flow and temperature control test, the liquid (water) is impelled from the tank by the pump, located to the left of the front of the equipment, going through the flowmeter, the solenoid valve (usually open), the motorized valve, the turbine (flow sensor) and the process tank. It is possible to use the second pump in the level tests, as it will be indicated. The pH control test of requires a second parallel line of flow (right), provided only with pump and a flowmeter. The compartments of the inferior tank should be loaded with diluted solutions of an acid and a base, respectively. The process tank is divided in two halves, with an orifice between them that allows their communication or isolation. The right compartment has an overflow of variable level (that it prevents the complete overflow of the tank, and it allows to modify its effective liquid volume), two drains with solenoid valves with different Cv (normally closed), and a third one with a normal drainage valve. The left compartment is only connected to a drainage valve. The level control tests require all the elements of the circuit and of the tank, besides the sensor located in it. In some experiments, it is required the second pump placed to the right-hand side of the equipment. The TEMPERATURE CONTROL tests, in these cases, as we will see later on, can be carried out with experiments in closed circuit or in open circuit. In the close circuit case, fill the superior tank with the right pump 1 (AB-1) and carry out the experiment. In open circuit, keep a constant water flow using the pump 1, this
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way, a small water flow is adjusted and the superior overflow is used as a drainage system. In this case, it is necessary to use the agitator to guarantee a good temperature uniformity.
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7.2 THEORETICAL BASIS Modeling, simulation, design and optimization require a series of processes that will be explained in the following paragraphs. The understanding of these theoretical concepts will help the student to follow the suitable instructions for the practices procedure. 7.2.1 Modeling, simulation and control process The development, and good operation of the industrial plants, requires the right selection of the equipment and process parameters. This election is supplemented with the instrumentation and the control, as well as with the dexterity to adjust and to manipulate them correctly. For it, the industrial engineering processes use different tools and technical aspects based on: 1.- The modeling of the systems. 2.- The simulation of the stationary and dynamics response. The control process is the objective of this modeling and simulation that guarantees that the dynamic behavior is efficient and precise. The design and the process optimization is based on calculations in stationary state to specify, in a first approach, the operation conditions of the equipment in the process plants. These design calculations in the stationary state don't say anything about the dynamic response of the system, they only tell us where we begin and where we end up, but anything about the process behavior. This type of
PRACTICES MANUAL Unit Ref.: UCP
Date: October 2010
Pg: 8 / 115
information is the one that informs about the study of the dynamics procedure. The dynamics and the control study the non-stationary behavior of the processes and the design of the control systems in function of the interferences. With this, the design of the process systems is finished, that include the own process and its control loops. In a first approach, the process control unit has been designed for the study of the dynamic behavior of the different control loops. The modeling and stationary simulation of the processes is not deal with in this equipment. 7.2.2 Dynamics and control Once the freedom degrees of the process are selected and the design variables good values are calculated, these values should remain unaffected with the deviations that take place during the real operation of the systems during the stationary state. The factors that cause such deviations are denominated interferences. These can be internal or external to the process, random or programmed. The random interference type are those that produce deviations from the programmed regime due to some process input variables fluctuations. In the programmed interferences, it is necessary to consider the transitory periods of starting-up, stop or changes in the stationary state of the process. The number of variables that you can fix coincides with the freedom degrees (although they don't have to coincide with the free variables that were chosen during the design phase). The variables that are more easily controlled are: level, flow, pressure, temperature and composition. We will denominate them controlled variables and their values we will be called set point. In a controller, the input
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magnitude of the set point is fixed in a constant value. In a “programmer” controller the inlet magnitude is a value that varies in function of the time, according to the programmed law. The variable used to regulate the set values is denominated manipulated variable. The function of the regulating device of the system is to activate automatically the control elements that allow to modify the value of the manipulated variables, so that the controlled variables are the next to the set point. 7.2.2.1 Dynamics in open loop The systems dynamics studies the non-stationary situations of the process. This study is always previous to the design of the control that avoids the nonreversibility of some deviations of the stationary state. In the process, the manipulated variable acts changing the value of the controlled variables so, in the simplest cases, it can be predicted using of a mathematical model. So, it is fundamental to have an exhaustive knowledge of the process to will be studied. The dynamic simulation of a process consists on: 1. Developing the material and energy balance equations, the balances, the physical laws or other independent sources of information. 2. Selecting
the
values
of
the
necessary
physical-chemical
magnitudes and the values of the preset points. 3. Obtaining the differential expressions of the variables determining their temporary evolution for an interference in the stationary regime starting from a defined initial state.
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Date: October 2010
Pg: 10 / 115
As a particular case, the design equations in the stationary state can be obtained (null accumulation) and, starting from them, calculate the values of the design and state variables in this regime. The system dynamic in open loop represents the process behavior in the absence of controllers. The velocity and the tolerance of the response of the process in function of the interferences can be studied, this is the necessity of the control system. 7.2.2.2 Feedback Control The feedback control system consists, essentially, on measuring the controlled variable, to compare its value with the one wanted (set point) and, according to the error, to act on the control element of the manipulated variable. This is a feedback system, in which the corrector action persists while the error is not null. The corresponding signals spread in the circuit, making comparison cycles, of calculation and of correction (in closed loop) with a certain time of response. If a closed control is turned into manual operation, the operator can govern directly the control valve and the value of the regulated variable obtained from the transmitter can be observed in the controller. In this case, we are operating in open loop, since the output signal of the controller is off. The main elements of a control system in closed loop are: • Sensor: Device that is able to measure the value of the controlled variable (it is also denominated primary measure element). • Transducer: Device that transforms the measures in normalized equivalent signals, pneumatic or electric, according to the distance between the process and control rooms, which are sent to the
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comparator (also denominated signal conditioner element). • Controller: Device that receives the error from the comparator, interprets it, and it acts on the final element of control. There are three types of corrective actions: - Proportional: the signal sent to the final element is proportional to the error. If the proportional constant is very big the control behaves as an on/off control. - Integral: the corrective signal sent is proportional to the error accumulated with the time (integral error). - Derivative: the signal is proportional to the velocity of variation of the error. These last two actions are usually combined with the primary signal (proportional) to improve the control quality and even, they usually combine the three in complex processes. These combined processes are known as P.I.D. control (proportional-integral-derivative). • Final element of control (actuator): Device that, according to the signal of the controller, acts on the manipulated variable regulating the inlet material or energy flow to the process (valve, volumetric pump, compressor, rheostat, etc.). The control systems are effective with small interferences. In the case of big interferences, the final element can end up being totally open or totally closed, and, above a certain value, the control would not act appropriately (saturated). In these cases, to correct it, you should act manually on the parameters of the system.
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The sensors, the transmitters and the final control elements are inserted in the process in a logical way. The control devices are usually located in a control room, and they are in normalized panels with graphic instruments for the variables and a diagram of flow (in a simple pilot plant it can be a simple PC computer with a monitor and a printer). In the control diagrams, the variables are designated with the letters F (flow), L (level), P (pressure), T (temperature), followed by the indicative letters of the service and of the functions of the instruments: T (transmitter), C (controller), I (indicator), R (register), A (alarms). 7.2.2.3 Forward control The forward control makes the corrective action in the moment in which the interference is detected, instead of waiting to its spread through the process, as it happened in the feedback process. The action is independent of the value of the controlled variable, making it depend on another, according to a calibrated preset. This system is used with simple processes that don't require a great accuracy, or in those processes in which the closed loop doesn't give good results, for example, in processes in which that the measure and the corrective action take place in a great period of time. 7.2.3 Dynamic simulation of the control systems The dynamic simulation of the control systems allows studying the response of the system facing up diverse interferences, such as changes in the input variables, in the set point, or in the controller adjustments. The system consists of a block that represents the process, and of another that corresponds the controller connected to the first one forming a feedback loop.
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Figure 2.3.1
The process consists basically of an input (q), a manipulated variable, also called feedback (m) and an output (c) (controlled variable). The controlled variable is subtracted to the set point (r) to obtain the error (e) (input to the controller). The controller calculates an output signal by means of an appropriate control algorithm. The characteristic response of a controller on/off comes determined by its hysteresis, also called dead band. In a PID controller (proportional-integral-derivative) the signal comes determined by the gain (Kc) or its percentage inverse (BP=100/Kc, proportional band) and the integral (I) and derivative (D) times.
[1]
c = Kp[q, m] e = r −c m = Kc( e + ∫ e∂t + D
∂e ∂t )
In open loop, the state variables (outputs) are calculated starting from the initial values and from the process pattern. It is defined in function of differential equations for the transitory regime, or by state equations for the stationary regime (invariable with the time). These last ones are the techniques employed during the design phase of the experiment and they allow calculating the values of the necessary input variables to reach the stationary regime. The first ones allow studying the transitory periods during the setting phases in setting-in, stops or changes in the stationary regime of
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the process. During the real operation of the process, the variables are modified with the time as a consequence of diverse interferences in the input variables or in the set point (programmed). In these cases, the process can only work correctly using a closed loop control. The dynamic behavior of the system is calculated by an iterative algorithm at discreet intervals of time (Euler’s method). The first phase of the program is the data input: fixed variables, initial values of the manipulated variables, set point of the controlled variables, adjustment of the controller (hysteresis, parameters PID) and interference variables. Qt
m
PROCESO
RT
Ct
COMPARACIÓN
H,KC, I,D
eI
CONTROL
mt
Figure 2.3.2
The mathematical pattern of the process calculates the outlet (Ct ) in function of the inlets (q t, mt ). This value is compared with the set point (rt ) and the resulting error (et ) is calculated (the accumulated error (integral) or its derivative with the time, the trapezoidal rules and finite increments are used in this case). Finally, with these values, the controller output (s t ) and the manipulated variable (mt ) are calculated making use of the adjustments of the control actions (H, Kc, I, D) and of the calibration of the actuator, respectively.
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The relationship between these last two variables can be settled down on the base of a function (lineal) between the width of the actuator (m) and the signal of the controller (±s). Also, automatic graph representation commands of response can be included (controlled and manipulated variables), time outs in the process (always superior or similar to the time of sampling, t≥∆t) and sampling time of the controller similar to those of the process (analogical control) or superiors (digital control). In case a graphic register is used, this should be connected to the electric signs of the sensor/conditioner (controller input) and to its output (actuator input) to obtain a register of the response of the process.
7.2.4 Operation and calibration of the process equipment and control elements The verification and calibration of all the sensors you can find in the equipment can be carried out in two different ways. The Saced System that is supplied with the equipment, has a calibration window specially designed for such a purpose (see Calibration Manual). However, and due to the importance that the calibration the sensors has in the control process, the system offers two text windows in which the gain and the zero can be introduced of each one of the transducers that the equipment has: flow, level, temperature and pH.
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Name of the sensor for calibration.
Figure 2.4.1
In the same way, the verification of the operation of the pumps, solenoid valves, agitator, etc. it can be carried out from the calibration window in the Saced System. This process of operation test should be carried out with extreme care and under the introduction of the professor's PASSWORD. This Password has been indicated in the software manual. Once the professor's password is introduced, the program allows you to select the channels included in the equipment for the digital outputs as well as for the analogical ones. Verify each one of the elements assembled in the equipment and carry out a good calibration of the sensors. We should point out that, when storing the calibration values in the file UCP.EDB, these will be recovered every time that you enter in the program, independently from the values introduced by the students in the different practice sessions. When selecting the different outputs that has the acquisition card (analogical and digital outputs, and analogical and digital inputs), you can verify the operation of all devices of the equipment. As an example we have the following table:
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Analogical port (Inputs) Channel Action/Sensor Channel 0 pH sensor Channel 1 Level sensor Channel 2 Flow sensor Channel 3 Temperature sensor Channel 4 Alarm of Level Channel 5 Proportional Valve
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Digital Port (Outputs) Channel Action/Sensor Channel 0 AVS-1 Channel 1 AVS-2 Channel 2 AVS-3 Channel 3 Resistor On/Off Channel 4 Agitator On/Off Channel 5 Pump 1 On /Off Channel 6 Pump 2 On/Off
Table 2.4.1
To outputs and inputs it is necessary to add the analogical output that corresponds to the motorized valve.
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7.3 LABORATORY PRACTICES It is very important for the formation of the student, the calibration of the sensors, hence is recommendable that the students see the Annex 1 to Annex 4. 7.3.1 Practice 1: Flow control loops (manual) 7.3.1.1 Objectives The objective of this experiment is to control the flow that circulates through a conduction of water by a manual procedure. We assume that the manual control works as: • Manual regulation of the adjustable valve placed under the area flowmeter. • Manual control of the elements used in the equipment as the motorized valve, solenoid valves, etc. 7.3.1.2 Required material To make this practice, the following elements are necessary: • UCP-F • Water • SACED Software. 7.3.1.3 Experimental procedure 1.- Connect the interface of the equipment and execute the program
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SACED UCP-F. 2.- Inside the program, select the option Configuration and connect pump 1 (AB-1) (see Software Manual for a more detailed operation). 3.- In the manual regulation (no controller) the flow can be regulated by the manual adjustable valve VR1, placed in the inferior part of the flowmeter. Vary its position and observe the adjustment of the flow in function of its position. 4.- Select the option “Manual Control” of the software supplied with the equipment. 5.- Connect pump 1 (AB-1) and vary the position of the motorized valve by the Slip bar or the command associated to this action. Check a fixed position, the flow is regulated. 6.- Vary the position of the valve and repeat the values to observe the reproduction of the flow control. 7.- Use the controls prepared in the software for controlling the solenoid valves AVS-1 and the on/off button of the pump. Observe how an on and off button also produces a flow control of the liquid.
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