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Automation I. Introduction

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transmitter actuator Structure of control system Process or plant Material flow sensorstransducers actuating units actuating drives controller operator desk Visualization software SCADA (Supervisory control and data acquisition) Mathematical software, like Matlab, are used preparation the model of the process. Control software Standard signals Variables

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Variables and signals Process or plant transducer actuator controller operator desk disturbance variables manipulated variables measured variables a part of these controlled variables action signals or control signals measured signals or detecting signals a part of these feedback signals

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Dimensionless technique controlled variable feedback signal action signal manipulated variable A/D conversion control task D/A conversion max min max min 20 mA 4 mA Domain of variability

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Most frequently used standard signals On / Off signals: 0 – 24 V DC Continuous signals: 4 – 20 mA 24 V 0 V max Logical 0, or low 30 V 20 mA 7 V7 V 4 mA Logical 1, or high Further frequently used continuous signals: 0 – 20 mA, 0 – 10 V, 2 – 10 V, (0.4 – 2 Bar) Further frequently used on /off signals: 0 – 110 / 230 V AC, (0 – 4 Bar)

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Engineering jobs instrumentation measurement (temperature, level, pressure, flow, drive, and so on) final element (control valves, drivers) safety philosophy of the process (Safety systems are usually independent from the normal control system) control philosophy of the process process control manufacturing maintenance

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Divide into simpler part Which variables of the process need control. All controlled variable is an independent simpler part of the process. * All simpler part of the process needs a control strategy. The safety philosophy of the process. Safety systems are usually independent from the normal control system and the safety considerations are very dependent on the process. More often a part of the safety considerations is to monitor of exceptional conditions or to detect a malfunction of a device or to reach a dangerous level of a variable. The failsafe philosophy can also be applied to the actuator and the transmitter. (power or wiring failure).

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Points of view to choose control law Which manipulated variable can control the controlled variable. Is the controlled variable performance on/off or continuous? Which measured variables are required for developing a control law (algorithm). Can one describe the plant between the input manipulated and disturbances variables and the output controlled variable by a precise model. Is this model linear or not, time invariant or not? Economic efficiency points of view.

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Control strategies Open loop control One can describe the plant between the manipulated and disturbance variables and the controlled variables by a precise model and so using the required measured variables one can develop a control algorithm. Advantages: There isn’t stability problem. This method is so punctual such as the model, the control action doesn’t require an error. Disadvantages: Sometimes this solution isn’t economical. Closed loop control ( Feedback ) The reference signal represents the required value of the controlled variable. The controlled variable and the reference signal continuously compare and if the detecting and the reference signal are not equal, than an adequate action signal attempt to eliminate the error. Advantages: Sometimes this solution is economical. Disadvantages: The key is to appear an error and needs a transient time to eliminate this error. The controlled variable isn't always punctual. There is stability problem.

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Process requirements It means which variables depend on other equipment of process or environment and which variable can be controlled and which variable can be manipulated. inflow outflow tank level pump valve difference of pressure Controlled variable is the tank level Manipulated variable is the inflow: In steady-state the inflow equals outflow. Requirement of other equipment of process gives a control signal the pump changing the outflow. Manipulated variable is the outflow: In steady-state the inflow equals outflow. Requirement of other equipment of process gives a control signal the pump changing the outflow. Controlled variable is the time of stay of inflow mass Either inflow or outflow can be manipulated variable: In steady-state the time of stay is constant. Requirements of other equipments cause disturbance the inflow or the outflow.

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Classification of control strategies We assume: The controlled variable is the tank level. In steady-state the inflow equals the outflow. Requirement of consumers gives a control signal for the pump, which changes the outflow. It is an disturbance of the system. Further disturbance is the difference pressure of control valve. inflow outflow tank level pump valve difference of pressure Open-loop open loop control: It needs to measure the outflow and the difference pressure know the relation between the valve position and the actual value of the tank level at an average value of outflow. feedforward control: It needs to know the relation between the valve position and the actual value of tank level at different value of outflow, and needs to measure the outflow. Closed-loop on/off feedback control: It needs to measure the tank level. If it is higher than the reference value the valve is closed and when the tank level is lower than the reference value the valve is opened. modulating feedback control: It needs to measure the level of the tank. The difference between the reference and feedback signals determine a continuously action signal for the actuating drive of valve.

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Problems of control strategies In steady-state the inflow equals the outflow. The steady-state error means: The actual value of the level and the required level (shown by reference signal) isn’t equal in steady-state. inflow outflow tank level pump valve difference of pressure open loop control: The error in the actual value of the tank level is caused by the difference of the model of process and the real process. feedforward control: It needs more measuring to know the relation between the valve position and the actual value of tank level at different value of outflow, and of course it needs a flow transmitter and an more expensive controller for complicated task. on/off feedback control: It has a violent fluctuation of tank level caused by valve on/off. Any delay in the plant response, which is common in more complex plant, means that the level will continue to rise even after the upper limit is reached. With very sensitive switch and negligible delay the valve very frequently fully open and closed. modulating feedback control: There is the stability problem. Other problem at the stable control loop is, the performance of the time response isn’t adequate for requirements of the technology.

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Block model (Classical method) w(t)y(t) W Y w(t) y(t) t t The steady-state characteristic. When the transient’s signals have died a new working point WP 2 is defined in the steady-state characteristic. The dynamic behaviour is describe by differential equation. Using the Laplace transform method the transfer function replace the differential equation. WP 2 WP 1 Modern method is the state-variables description We assume the variables are within a range and the output remain in this range in steady-state.

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Performance of a block Frequency transfer function: All block has frequency transfer function, but not all time signal - x(t), y(t) - can be converted to frequency form. Using Laplace transform it is possible. If the system investigation is started from WP 1 in steady-state than the x(t) and y(t) signals form are such as they are multiplied by 1(t) unit step. y(t) t W Y x(t) t WP 2 WP 1

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Correlation between frequency and time domain of linear systems Fourier and inverse Fourier transform Laplace and inverse Laplace transform The above is true if:

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Laplace transform Rules of Laplace transformLaplace transform of standard signals If pole of sY(s) are in the left half of the s-plane the final value theorem: is available.

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Using Laplace transform Using the rules of Laplace transform to convert a differential equitation to operator frequency form. x(t)y(t) x(j ) y(j )

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Summary questions Sketch the main structure of process control. What are the points of view to choose control strategies? Classify and characterise the control strategies. Interpret the block diagram representation technique. What is the steady-state and the dynamic performance of a block. What is the advantages of the dimensionless technique? Define the correlation between frequency and time domain. How we can use the Laplace’s transform method? Sketch the detailed block diagram of a single loop, compensator in cascade, closed loop system, and define the blocks, signals, and variables. Highlight the role of the transmitter in a control system. What are the linearity, sensitivity, repeatability, and accuracy of a transmitter?

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