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

What is a system? Interconnect components Working together to Perform a function

Types of Control Systems  Natural Systems  Open-loop Systems  Closed-loop Systems

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

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

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

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

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)

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.

Process Characteristics  Measure process variable  Check process variable deviation from setpoint  Change the control parameter which in turn changes the process variable

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)

Variable changes due to  Process in flow (supplies) changes  Environmental changes  Setpoint changes  Consumption changes  These changes are collectively called disturbances

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)

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

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

Time Response of a Continuous-action Controller Time response determined by three control actions  Proportional Control Action  Integral Control Action  Differential Control Action

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

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)

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

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 )

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)

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

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)

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.