AS Level    Control Systems    Questions 0 to 22   -->  View All  

Control systems often include negative feedback.
Negative feedback is a good thing. It corrects errors in the output.
Systems that include negative feedback are called CLOSED LOOP.
Systems with no feedback are called OPEN LOOP.

• The input is a signal representing what you want.
• The output is measured.
• The output measurement is fed back to the input.
• The process uses the input and the feedback signals to correct errors in the output.
   
• Example
• The measured pressure is too high so turn off the compressor.
• The measured pressure is too low so turn on the compressor.

More Detailed Example - Central Heating and Thermostst

A - Input.
B - Error Detector. Compare wanted with actual temperature (analogue comparator or digital subtraction)
C - Processor decides what to do with the signal from B
D - Driver (to change the boiler settings and water pump state)
E - Room thermostat
F - Negative Feedback voltage or binary number

P - The user sets the input required room temperature
Q - A voltage or binary number representing the wanted temperature
R - Error signal representing the difference between the wanted and actual room temperature.
S - Control signal/s to the boiler for the gas burner and water pumping
T - Cool or warm air carries this signal from the heating system to the thermostat.
U - The room temperature as sensed by people.
V - A voltage or binary number representing the actual temperature
W - This could be identical to V or a scaled or processed version of V suitable to be fed to B.