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What is Control System? To answer this question, we first have to understand what a system is Simon Hui Engineer Control and Informatics, Industrial Centre
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What is a system? Interconnect components Working together to Perform a function
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Types of Control Systems Natural Systems Open-loop Systems Closed-loop Systems
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Natural Systems Many natural control loops existing on the earth are self-regulating e.g. Water boils at 100 degC and is self- regulated at that temperature by the atmosphere pressure and vapour pressure
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Open-loop Systems Activating signal is made up solely of an input signal e.g. Speed of car Speed of a fan motor Timing of traffic light
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Closed-loop System Activating signal is made up of difference between the input signal and a feedback signal e.g. Toilet tank Operator in an open-loop system can also be considered as a feedback action
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A Closed-loop Control System Imagine a tea-boiling system Cold tea is supplied to a tank Steam passes a pipe boils the tea inside the tank Temperature of the tea is measured Setpoint for the temperature of the tea is to keep at the desirable 98 degC A continuous valve (can be 0% to 100% open) let steam passes through the pipe
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Process Variable in a Process is Dynamic A process variable can change e.g. The temperature of the tea can change In process control, we often desire to keep a process variable to a certain value we want (i.e. an operator chosen constant) e.g. Keep the temperature of the tea at 98 degC (the setpoint)
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Control Parameter in a Process Control parameter is the means to control the process variable e.g. Increase or decrease opening of a control valve so that more or less steam passes through the pipe. Then more of less heat transfers from the pipe to the tea in the tank.
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Process Characteristics Measure process variable Check process variable deviation from setpoint Change the control parameter which in turn changes the process variable
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What is the purpose of controlling a process Ans: To regulate a dynamic process variable Why process variable changes? Process in flow changes (e.g. tea supply changes) Environmental changes (e.g. room temperature of tea tank changes) Setpoint changes (e.g. operator wants a new tea temperature) Consumption changes (e.g. tea out flow changes)
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Variable changes due to Process in flow (supplies) changes Environmental changes Setpoint changes Consumption changes These changes are collectively called disturbances
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Control Valve A continuous valve can be 0% to 100% open. (fully closed to fully opened) A change in valve setting will not immediate change the process variable. Reasons are. 1. Time response of control device. (e.g. electrical and mechanical delay) 2. Latency of change of the process variable. (e.g. tea takes sometime to heat up)
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Step Response The response of a system to a sudden change of input variable (e.g. a new setpoint) Step Response is one of the best ways to test or to understand the behaviour of a closed-loop system
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Control Response Non-continuous-action Controller output can only be changed in set steps. e.g. the two-steps control, “on” or “off” Continuous-action Controller output (also the controlling parameter) changes continuously if process variable deviates from the setpoint
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Time Response of a Continuous-action Controller Time response determined by three control actions Proportional Control Action Integral Control Action Differential Control Action
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Proportional (P) Control Controller output is proportional to the deviation of the process variable Mathematically, dy=K p dx where dy is change in controlling parameter dx is deviation in process variable K p is proportional gain (chosen constant) Controlling parameter is proportional to deviation in process variable
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Integral (I) Control Even the deviation in process variable remains in a certain quantity over time, the Integral Control System adds the deviations of the process variable over time Mathematically, dy=K t ƒ dxdt where dy is change in control parameter dx is deviation in process variable Kt is integral gain (chosen constant)
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I Control Action I control action increases the rate of change of deviation of the process variable towards the setpoint as time goes. I control action is gentle at t=0 but becomes increasing efficient
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Integral (I) Control Action As time goes by, I control action increases the rate of change of the process variable towards the setpoint I control action often referred to as reset action From t=0, to the reset time (also called integral-action time), the system adds the deviations of the process variable over time The reset time is a chosen constant (K t )
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Differential (D) Control The speed of change of deviation of process variable is evaluated Mathematically, dy=K d ·dx/dt where dy is the change in control parameter dx is the deviation of process variable K d is the derivative gain (chosen constant)
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D Control Action D control action reacts to large disturbance by driving the control process variable immediately (but only momentary) towards the setpoint. D control action alone can not maintain the process variable at the setpoint Slight excessive D control action leads quickly to instability of the control loop
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PI Controller Faster response than a pure P Controller Effectiveness increases with increase in P gain and increase in the I-component (i.e. decrease in reset time)
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PID Controller If the deviation of process variable of the system is large, D control action ensures a momentary extremely high change in the manipulated process variable. If the change in system deviation is slight, D control action is insignificant While the influence of the D control action falls off immediately, the influence of I control action increases slowly.
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