The most fundamental distinction in control engineering is between open-loop and closed-loop control systems. Understanding both types — their principles, advantages, disadvantages, and appropriate applications — is a core requirement in VCE Systems Engineering Unit 4.
KEY TAKEAWAY: Open-loop systems are simple but cannot self-correct. Closed-loop systems use feedback to compare actual output against a setpoint and make corrections automatically, achieving greater accuracy and reliability.
In an open-loop system, the controller sends a fixed command to the actuator based only on the input. There is no measurement of the output and no correction if the output deviates from the desired value.
[Input / Command] → [Controller] → [Actuator] → [Output]
The output has no influence on the controller’s behaviour.
| Feature | Detail |
|---|---|
| Feedback | None |
| Error correction | Cannot correct for disturbances |
| Complexity | Simple, inexpensive |
| Accuracy | Depends entirely on consistent process behaviour |
| Stability | Always stable (no feedback loop to oscillate) |
Open-loop control is appropriate when:
- The process is well-understood and highly repeatable
- Disturbances are minimal and predictable
- The cost of adding sensors and feedback exceeds the benefit
- Precise output is not required
In a closed-loop system, a sensor continuously measures the actual output. The controller compares this measurement to the desired setpoint, calculates the error, and adjusts the actuator to reduce the error.
[Setpoint] →[+]→ [Controller] → [Actuator] → [Output]
↑-
└────────── [Sensor] ────────────────┘
(Feedback)
The feedback signal is subtracted from the setpoint at the summing junction (shown as [+] with a negative feedback arrow); the result is the error signal that drives the controller.
| Feature | Detail |
|---|---|
| Feedback | Continuous (or periodic) |
| Error correction | Automatically corrects for disturbances |
| Complexity | More complex; requires sensor and comparator |
| Accuracy | High; can maintain setpoint despite disturbances |
| Stability | Must be designed carefully to avoid oscillation |
Closed-loop control is appropriate when:
- Precise, accurate output is required
- Disturbances (load changes, temperature variation, friction) are unpredictable
- The process must recover automatically from disturbances
- Safety requires confirmation that the desired state has been achieved
| Criterion | Open-Loop | Closed-Loop |
|---|---|---|
| Feedback sensor | No | Yes |
| Self-corrects errors | No | Yes |
| Complexity and cost | Lower | Higher |
| Accuracy | Lower | Higher |
| Suitable for | Simple, predictable tasks | Precise, variable-load tasks |
| Risk of instability | None | Possible if poorly designed |
| Example | Fixed-time toaster | Thermostat, servo motor |
VCAA FOCUS: A common exam question provides a described system and asks: “Is this open-loop or closed-loop? Justify your answer.” Your justification must state whether the output is measured, and whether that measurement is used to adjust the actuator. Both conditions are required for closed-loop.
Step 1: Identify the output of the system (what it is trying to control: temperature, speed, position, etc.)
Step 2: Determine whether the output is measured by a sensor.
Step 3: Determine whether the sensor signal is used to modify the controller’s output to the actuator.
If both Step 2 and Step 3 are YES → Closed-loop
If either is NO → Open-loop
Worked example: A fan is controlled by a switch connected to a temperature sensor. If the sensor detects T > 30°C, the switch closes and the fan turns on. When T < 28°C, the switch opens.
COMMON MISTAKE: A system that includes a sensor but uses it only for monitoring (e.g. to display a reading) without using it to control the actuator is still open-loop. The sensor signal must feed back to control the actuator for the system to be closed-loop.
In closed-loop systems, the setpoint (also called reference value or desired value) is the target for the controlled variable. The controller’s goal is to minimise the difference between the measured output and the setpoint.
$$\text{Error} = \text{Setpoint} - \text{Measured output}$$
A positive error means the output is below the setpoint — the controller should increase the actuator output.
A negative error means the output is above the setpoint — the controller should decrease or reverse actuator output.
APPLICATION: In a microcontroller-based closed-loop system, the setpoint is stored as a variable in the program. The control loop reads the sensor, calculates the error, and adjusts the actuator output (e.g. PWM duty cycle) based on the error magnitude. This is directly programmable in languages like Arduino C.