1. Control Systems The elements of a control system
Open and closed loop system differences
Positive and negative feedback
Examples of systems with feedback

2. Elements of a control system
Classic closed loop control
Elements of a control system

3. Another Example

4. Reference 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 level
Element Description

5. What is an open loop control system ?
An example of an open loop control system is your garden variety kettle
The operator fills the kettle
The operator switches the kettle on
The water reaches boiling point
A thermostat (the control element) switches the kettle off
Without further interaction nothing else happens
What is an open loop control system ?

6. Why use open loop control ?
Open loop control systems are good for applications that require set parameters
For example we always want the kettle to reach 100 degrees and then switch off
If we want a motor to drive a constant load at a given speed (all the time) then open loop control works well
Open loop control is usually simpler to implement, is cheaper and easier to maintain than closed loop control
Why use open loop control ?

7. What 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 ?

8. Example 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?

9. Washing Machine control examples
Water level
Water temperature
Program time
Motor speed
Machine Safety (Door catch)
Machine Malfunction
Washing Machine control examples

10. The 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 analogue
Digital signals will usually be read by a microcontroller or microprocessor
Analogue signals will usually be connected to some type of differential amplifier
Feedback

11. Differential amplifier
V Vout =R2/R1(V2 – V1) t
Differential amplifier

12. In control systems positive feedback is used to increase the difference between the reference value and the feedback signal
Therefore 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 connected
Error signal = reference value + feedback signal
Positive feedback

13. Error signal = Reference value – Feedback signal
Negative Feedback

14. An example of the differential amp in practice
Consider the amplifier shown earlier
If the reference signal is connected as V2 and the feedback as V1
Imagine we are controlling the temperature in a space
What happens to the feedback signal as the temperature increases?
How does this affect the output of the differential amplifier
If the feedback signal decreases what happens to the output
Remember ideally V2 = V1 for system balance
An example of the differential amp in practice