QLDEngineeringSyllabus dot point

How do engineers control a machine so it responds to its inputs and conditions?

Describe machine control using the input-process-output model, distinguish open-loop from closed-loop (feedback) control, and identify the role of sensors, controllers and actuators

A QCE Engineering Unit 4 answer on machine control. Covers the input-process-output model, open-loop versus closed-loop feedback control, the roles of sensors, controllers and actuators, and a worked example of a feedback control loop in a real machine.

Generated by Claude Opus 4.76 min answerUpdated 2026-05-29

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

QCAA wants you to describe how machines are controlled: the input-process-output model that frames every control system, the difference between open-loop and closed-loop (feedback) control, and what sensors, controllers and actuators each do. Modern machines are not just mechanisms; they are mechanisms with control. Understanding this is essential to the machine-control content and to designing a working solution in Unit 4.

The answer

The input-process-output model

Every controlled machine can be described as a flow from input to output:

Input: the command, signal or setting that tells the machine what to do (a switch, a dial, a sensor reading, a program instruction).
Process: the action that transforms the input into a response (a controller and the mechanism it drives).
Output: the physical result (a heater warming, a motor turning, a valve opening).

This model is the framework for analysing any control system, from a simple switch to a programmable controller.

Open-loop control

In open-loop control the output is not measured or fed back. The system carries out its instruction regardless of the result. A toaster runs its heater for a set time whether or not the bread is actually done; a basic sprinkler runs for a fixed period regardless of soil moisture. Open-loop control is simple and cheap, but it cannot correct for disturbances or errors, so it suits tasks where the result is predictable.

Closed-loop (feedback) control

In closed-loop control a sensor measures the actual output and feeds it back to be compared with the desired value, the set point. The difference is the error signal. The controller acts on the error to drive the output toward the target, then keeps checking. A thermostat-controlled heater measures the room temperature, compares it with the set point, and switches the heater on or off to hold it. Feedback makes the machine self-correcting: it adapts to disturbances such as an open door or a changing load.

Sensors, controllers and actuators

A control system is built from three kinds of component:

Sensors measure a physical condition and convert it to a signal: temperature, position, speed, light, pressure or proximity.
The controller receives the inputs and the feedback, compares them and decides the response. It may be a simple circuit or a programmable microcontroller running a control algorithm.
Actuators convert the controller's decision into physical action: electric motors, servos, solenoids, hydraulic and pneumatic cylinders, heaters and valves.

The signal flows from sensor to controller to actuator, and in a closed loop the actuator's effect on the output is sensed again, closing the loop.

Designing a feedback loop for a water tank

Trace a closed-loop level control

A tank must hold its water level at a set point of $800\,\text{mm}$ . A float sensor reads the actual level, a controller compares it with the set point, and a pump (the actuator) tops the tank up. The controller turns the pump on when the level is more than $20\,\text{mm}$ below set point and off when it reaches set point. The level falls to $760\,\text{mm}$ . Trace the control action.

Step 1. sensor. The float sensor measures the actual level as $760\,\text{mm}$ .

Step 2. controller compares. The error is:

\text{error} = 800 - 760 = 40\,\text{mm below set point}

Since $40\,\text{mm} > 20\,\text{mm}$ , the controller decides the pump must run.

Step 3. actuator. The controller switches the pump on. Water flows in and the level rises.

Step 4. feedback closes the loop. The sensor keeps reading the level. When it reaches $800\,\text{mm}$ the error is zero, the controller switches the pump off, and the system holds. If the level later drops $20\,\text{mm}$ again, the cycle repeats automatically.

This is closed-loop control: the measured output continuously corrects the action, so the tank stays near $800\,\text{mm}$ without any human intervention. An open-loop version, running the pump for a fixed time, could not adapt to a changing draw-off.

Why this matters for machines and mechanisms

Control is what makes a mechanism do useful work reliably. The same gear train or linkage becomes a precise, responsive machine once a sensor, controller and actuator are added around it. In the Unit 4 engineered solution, deciding whether open-loop or closed-loop control is appropriate, and specifying the sensors and actuators, is often the difference between a mechanism that simply moves and a machine that does its job correctly.

Exam-style practice questions

Practice questions written in the style of QCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2024 QCAA8 marksA truth table is given for the reversing safety system of a forklift. Input A = left sensor (motion detected = 1), Input B = right sensor (motion = 1), Input C = gear (forward = 1, reverse = 0), Input D = engine (on = 1); Output X = safety brake (on = 1). If either rear sensor detects an obstacle while the forklift is reversing, the safety brake activates. Construct a logic gate circuit, based on the truth table, that meets the requirements for the safety brake to activate. Clearly label all inputs and outputs.

Show worked answer →

This is an input-process-output control system: the inputs are the sensors and machine states, the process is the logic circuit, and the output is the actuator (the brake). Eight marks: three for correct gate connections and five for correctly labelling the inputs, intermediate signals and output.

The brake (X) must turn on when an obstacle is detected on either side (A OR B), AND the forklift is in reverse (C = 0, so NOT C), AND the engine is on (D). The circuit is:

Left and right motion sensors into an OR gate (detects an obstacle on either side) [1 mark].
Gear input C through a NOT gate, so reverse (C = 0) gives a 1, fed into an AND gate with the engine input D [1 mark].
The OR output and that AND output combined through a final AND gate to drive the safety brake output X [1 mark].

Labelling marks: left and right motion sensor inputs [1 mark]; gear input [1 mark]; engine input [1 mark]; intermediate signals E, F and G [1 mark]; safety brake output [1 mark]. The Boolean expression is X = (A OR B) AND (NOT C) AND D.

2022 QCAA5 marksDescribe the function of a NAND gate. Include a truth table to support your response.

Show worked answer →

A NAND gate is a building block of machine control logic (a NOT-AND gate). Five marks: two for the description and three for the truth table.

Function: the output of a two-input NAND gate is 0 only when all of its inputs are 1; for every other input combination the output is 1 [1 mark for the "output is 0 when all inputs are 1" condition, 1 mark for the "output is 1 otherwise" condition].

Truth table for inputs A and B:

A = 0, B = 0, output = 1
A = 0, B = 1, output = 1
A = 1, B = 0, output = 1
A = 1, B = 1, output = 0

Three marks are awarded for the three correct columns (the two input columns and the output column). The NAND gate is the inverse of an AND gate, which is why the output is high in every case except when both inputs are high.