Feedback is the process of measuring a system’s output and using that measurement to influence the system’s future behaviour. In control engineering, feedback is the mechanism that enables a system to self-correct, maintain a setpoint, and respond to disturbances — transforming a passive mechanism into an active, intelligent controller.
KEY TAKEAWAY: Negative feedback — where the measured output opposes deviations from the setpoint — is the basis of most practical control systems. It corrects errors and maintains stability. Positive feedback amplifies deviations and is rarely used in control (more common in switching or latch circuits).
Principle: The feedback signal is subtracted from the setpoint. If the output is too high, the corrective action reduces it; if too low, the action increases it. The effect is to minimise the error.
Mathematical representation:
$$\text{Error} = \text{Setpoint} - \text{Measured output}$$
$$\text{Actuator command} \propto \text{Error}$$
Examples:
- Thermostat: If temperature exceeds setpoint, heater turns off (output measured → corrective action → reduces output)
- Speed governor: If speed exceeds setpoint, fuel supply reduces
- Robot joint: If actual angle exceeds target, motor reverses to correct
Why it works: Negative feedback creates a self-regulating loop. Any perturbation from the setpoint generates an error that drives corrective action in the opposing direction.
Principle: The feedback signal reinforces the current state — deviations grow rather than shrink.
Examples in control:
- Schmitt trigger: A small noise spike causes the output to switch fully to one state, reinforcing the change
- Latching relay: Once triggered, the relay holds itself on
In control systems: Positive feedback must be avoided in systems requiring stable, accurate setpoint tracking — it leads to runaway or oscillation.
EXAM TIP: Unless the question specifically asks about positive feedback, assume “feedback in system control” means negative feedback. VCAA questions almost exclusively focus on negative (error-correcting) feedback.
A complete negative feedback control loop has five elements:
| Element | Function | Example (temperature control) |
|---|---|---|
| Setpoint | Desired output value | 22°C target temperature |
| Comparator | Calculates error (setpoint − measured) | Microcontroller subtraction |
| Controller | Decides corrective action from error | If error > 2°C → ON; if error < 0°C → OFF |
| Actuator | Applies correction to the process | Heating element, fan |
| Sensor | Measures actual output | Thermistor measuring room temperature |
The simplest form of feedback control: the actuator is either fully ON or fully OFF based on whether the measured value is above or below the setpoint.
Example — thermostat:
IF measured_temp < setpoint - deadband THEN heater = ON
IF measured_temp > setpoint + deadband THEN heater = OFF
Deadband: A range around the setpoint where no action is taken. Without a deadband, the controller switches rapidly back and forth (hunting) around the setpoint, wearing out the actuator.
Advantages of ON/OFF control:
- Simple to implement (a threshold comparison)
- Requires no calculation of proportional correction
Disadvantages:
- Output oscillates above and below setpoint (never perfectly steady)
- Not suitable for systems requiring tight precision
A more sophisticated feedback strategy where the actuator output is proportional to the error:
$$u = K_p \times e$$
where $u$ = controller output, $K_p$ = proportional gain, $e$ = error.
Effect: Large error → large correction; small error → small correction. The system approaches the setpoint smoothly rather than switching.
Residual error (offset): Proportional control alone leaves a small steady-state error because the correction reduces the error, but the system stabilises at a point where the reduced correction just balances the load.
VCAA FOCUS: VCE students need to understand the concept of proportional control and why it produces smoother response than ON/OFF control. Detailed PID (proportional-integral-derivative) mathematics is beyond the VCE scope.
In a microcontroller-based closed-loop system, the feedback loop is implemented in code:
Pseudocode — proportional motor speed control:
target_speed = 300 // rpm
Kp = 0.5 // proportional gain
LOOP:
actual_speed = readEncoder()
error = target_speed - actual_speed
pwm_output = Kp * error
pwm_output = clamp(pwm_output, 0, 255) // limit to valid PWM range
setPWM(motorPin, pwm_output)
wait(100 ms)
END LOOP
If actual_speed is 280 rpm and target is 300 rpm:
- Error = 300 − 280 = 20 rpm
- PWM adjustment = 0.5 × 20 = 10 (increase duty cycle by 10 units)
APPLICATION: When describing a microcontroller feedback loop for a VCAA question, state explicitly: (1) which sensor measures the output, (2) how the error is calculated, (3) what the controller does with the error, and (4) how the actuator responds. This four-step description earns full marks.
Feedback systems can become unstable if the gain is too high or the response is too slow:
Hunting (oscillation): The system repeatedly overshoots and undershoots the setpoint. Caused by high gain combined with response delay.
Solutions:
- Reduce proportional gain $K_p$
- Add a deadband around the setpoint
- Add integral or derivative terms (PID control — beyond VCE scope but worth knowing exists)
- Reduce the control loop sampling interval
STUDY HINT: Draw the feedback loop block diagram for every control system you analyse. Label each block (sensor, comparator, controller, actuator) and draw the feedback arrow clearly. This diagram is your evidence that you understand the closed-loop structure.