Feedback: how a control system checks its own work

By the end of this lesson

By the end you can explain what feedback is, why a control system needs it, and trace how a greenhouse heater uses it.

Steering with your eyes shut

When you ride a bike, you watch where you're going and correct the handlebars every second. Each correction changes what you see, and what you see decides the next correction.

Shut your eyes and you're in a hedge within seconds. A control system without feedback is riding blind.

Definition

Feedback

In a control system, the output affects the next input. The sensors measure the effect of the actuators' action, and the processor uses these new readings to decide its next action.

The loop, step by step

A greenhouse heater must hold the air near 25 °C.

A temperature sensor measures the air temperature.
An ADC converts the reading to digital.
The processor compares it with the stored values: 24 °C (too cold) and 26 °C (too warm).
At 24 °C or below it sends a signal to switch the heater on; at 26 °C or above, off.
The heater changes the air temperature.
The sensor measures the new temperature, and the loop starts again. That's the feedback.

Every zig and zag is the processor reacting to a fresh sensor reading. Without feedback, there'd be no zigzag, just a line heading off the chart.

Worked example: Easy first: read the graph

Look at the graph. At about 14 minutes the temperature reaches 26 °C.

What does the system do, and what happens over the next few minutes?

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Stretch

Answer the 2 questions above to continue.