IGCSE Computer Science 0478: Monitoring vs Control Systems (River Flood Case Study)

A river authority operates two automated systems along the same river.

System 1, on a remote riverbank, has a water-level sensor connected to a microprocessor. Every 15 minutes the microprocessor reads the sensor and stores the value in a log file, which an engineer checks once a week to look for long-term changes in the river. System 1 has no actuator.

System 2, at a floodgate further downstream, also has a water-level sensor connected to a microprocessor, but here the microprocessor is programmed to open a motorised sluice gate (an actuator) automatically whenever the level rises above a stored threshold, and to close the gate again once the level falls back below it.

(a) State which system is a monitoring system and which is a control system, and give the key reason for each. [3]

(b) Give one reason the river authority might still want an engineer to check System 1's stored log every week, rather than removing System 1 and relying only on System 2's automatic gate. [2]

Part (a): Identifying the monitoring system and the control system

System 1 is a monitoring system. Its microprocessor reads the water-level sensor and stores each value in a log file for an engineer to check later. It has no actuator, so it never changes anything in the river or the surrounding equipment by itself.

System 2 is a control system. Its microprocessor also reads a water-level sensor, but instead of only storing the reading, it compares the value to a stored threshold and, if the level is too high, sends a signal to an actuator, the motorised sluice gate, which physically opens. When the level falls back below the threshold, the microprocessor closes the gate again.

The key distinguishing feature is whether the microprocessor’s decision is passed on to an actuator that automatically changes something in the real world: System 2 has this feedback loop, System 1 does not. [3 marks]: [1] for correctly identifying System 1 as monitoring, [1] for correctly identifying System 2 as control, [1] for the shared reason (presence or absence of an actuator/automatic response).

Part (b): Why a person might still check System 1’s log

Even though System 2 reacts automatically, System 1’s stored log lets an engineer study data across many weeks rather than a single instant. This can reveal things an automatic system would miss, for example a gradual, long-term rise in the river’s typical level that is too slow to trigger System 2’s gate on any single day, or a fault, such as the sensor returning the same value repeatedly, which a person reviewing the log could notice but which the automatic system would not flag by itself. [2 marks]: [1] for a valid reason, [1] for correctly linking it to something only a stored log of readings over time can show.

Part (c): Why System 2 needs an actuator but System 1 does not

System 2 must physically prevent flooding by opening the sluice gate as soon as the water level becomes dangerous, and this response has to happen immediately, without waiting for a person to act, so it needs an actuator to carry out that physical change automatically.

System 1, in contrast, only needs to record the water level for a person to study later; nothing has to physically happen at the moment each reading is taken, so there is nothing for an actuator to do, and none is fitted. [2 marks]: [1] for explaining System 2’s need for an immediate automatic physical response, [1] for explaining that System 1 only stores data with no physical response required.

Final answers

(a) System 1 = monitoring system (no actuator, just logs data); System 2 = control system (actuator responds automatically to the reading).
(b) Any suitable reason, e.g. spotting a long-term trend or a sensor fault that a single automatic reading would not reveal.
(c) System 2 must act immediately and automatically on a dangerous reading, so it needs an actuator; System 1 only stores data for later human review, so it does not.