4Reference input compares the feedback signal with a set reference signal (usually a voltage level) Depending on the output of the reference section, the control element will be responsible for adjusting the parameter of interest. For example if the liquid in a plant is too cool, the control element will provide more power to the heating elements within the plant until the liquid temperature until it meets the reference point]Feedback element, a part of the signal is fed back to the reference input (usually a differential amplifier and comparator) so that the control system can monitor and adjust the controlled parameter to meet the desired pressure, temperature, speed or levelElement Description
5What is an open loop control system ? An example of an open loop control system is your garden variety kettleThe operator fills the kettleThe operator switches the kettle onThe water reaches boiling pointA thermostat (the control element) switches the kettle offWithout further interaction nothing else happensWhat is an open loop control system ?
6Why use open loop control ? Open loop control systems are good for applications that require set parametersFor example we always want the kettle to reach 100 degrees and then switch offIf we want a motor to drive a constant load at a given speed (all the time) then open loop control works wellOpen loop control is usually simpler to implement, is cheaper and easier to maintain than closed loop controlWhy use open loop control ?
7What is closed loop control ? The major difference between open and closed loop control is that in a closed loop control arrangement, the system is constantly monitoring the output of the system to ensure that it matches the reference signal and that the reference signal can be changed at any time usually by a computer program.What is closed loop control ?
8Example of a closed loop control application? At home probably the best example is a washing machine.Try to make a list of all the functions that would need to be controlled by a washing machine. State whether you think they are open or closed loop controlled.Example of a closed loop control application?
9Washing Machine control examples Water levelWater temperatureProgram timeMotor speedMachine Safety (Door catch)Machine MalfunctionWashing Machine control examples
10The output of the machine/plant/process can be arranged to provide an electrical signal This signal can be either digital (after ADC) or it can be analogueDigital signals will usually be read by a microcontroller or microprocessorAnalogue signals will usually be connected to some type of differential amplifierFeedback
11Differential amplifier V Vout =R2/R1(V2 – V1) tDifferential amplifier
12In control systems positive feedback is used to increase the difference between the reference value and the feedback signalTherefore in our differential amplifier example if positive feedback is used V2 will become larger than V1, or the difference between V1 and V2 will become smaller. It just depends which way around the signals are connectedError signal = reference value + feedback signalPositive feedback
13Error signal = Reference value – Feedback signal Negative Feedback
14An example of the differential amp in practice Consider the amplifier shown earlierIf the reference signal is connected as V2 and the feedback as V1Imagine we are controlling the temperature in a spaceWhat happens to the feedback signal as the temperature increases?How does this affect the output of the differential amplifierIf the feedback signal decreases what happens to the outputRemember ideally V2 = V1 for system balanceAn example of the differential amp in practice