Topic 6 · Control Systems (HL)

Uses input commands to MANAGE the behaviour of another (usually physical) system.

Runs a fixed action; NO feedback. Simple, cheap, BRITTLE. Toaster timer, microwave, traffic lights.

Sensor measures output → controller compares to target → adjusts. Self-correcting. Thermostat, cruise control.

SENSOR → CONTROLLER → ACTUATOR → PROCESS → (back to sensor). MEMORISE IN ORDER.

The TARGET value (e.g. 20°C on thermostat). Error = setpoint − measured.

Predictable system + small disturbances + low cost of error + sensors too expensive.

Disturbances common + accuracy matters + stakes high (flight, medical, vehicle).

Converts physical quantity → electrical voltage. Thermistor, photoresistor, accelerometer, mic.

Analog-to-Digital Converter. SAMPLES voltage at fixed rate; outputs integers (10-bit = 0-1023).

Hz + bits. Higher = more precise but more cost + CPU.

Converts signal → physical action. Motor, servo, valve, heater, LED, speaker.

Digital-to-Analog Converter. Integer → voltage. Often replaced by PWM (fast on/off, average = duty cycle).

Output PULLED BACK toward target. Stabilising. Used in 99% of control systems.

Output AMPLIFIED away from current state. Microphone squeal, fire alarm chain. Rare and dangerous if unbounded.

Intrinsic DELAY between command and physical response. Cannot be removed — only ANTICIPATED.

Output PASSES the target before settling. Caused by lag + naïve control. PID minimises it.

Proportional (current error) + Integral (accumulated error) + Derivative (rate of change). Industry workhorse.

Drives correction proportional to current error magnitude.

Removes steady-state offset (system stuck slightly below target).

Damps approach — slows correction as target is neared → less overshoot.

Computer built INTO a larger device, doing ONE job. Constrained memory + power, often real-time, no reboots.

GP = many user programs, full OS, abundant resources. Embedded = fixed firmware, often bare-metal or RTOS.

Missing the deadline = FAILURE. Airbag (30 ms), pacemaker, ABS, industrial robotics.

Late = degraded, not catastrophic. Video frame drop, audio jitter, online gaming.

1) SENSE (read ADC/GPIO) → 2) DECIDE → 3) ACTUATE → 4) SLEEP → repeat FOREVER. No exit condition.

Handle urgent events that cannot wait for next loop iteration (button, packet, overcurrent).

GC + dynamic types + megabyte RAM = incompatible with tiny KB-RAM hard-real-time microcontrollers. C dominates.

Real-Time Operating System (FreeRTOS, Zephyr). Tiny + predictable + priority scheduling. Common in serious embedded.

Worst-Case Execution Time analysis. Required for hard-real-time to prove deadline is met under ALL conditions.

Pacemaker · car ECUs (~50/car) · drone flight controller · microwave · smart thermostat · 3D printer.