Show an understanding of monitoring and control systems

Monitoring and Control Systems – A‑Level Computer Science (9618)

1. What is a Monitoring & Control System (MCS)?

An MCS continuously monitors a physical process (temperature, pressure, speed, etc.) and, through a controller, acts on that process to keep it within required limits. It links many Cambridge AS/A‑Level concepts – data representation, algorithms, hardware, networking, security and ethics – into a single, real‑world system.

2. Syllabus Alignment – What the Exam Expects

Syllabus block (AS‑A)	Key points covered in the notes	Section reference
Computational thinking (abstraction, decomposition, algorithm design, testing, debugging)	Algorithm design for PID, flow‑chart of scan‑cycle, testing & debugging strategies.	5, 6
Data representation (binary, hex, BCD, floating‑point, size, compression)	Binary/hex conversion, BCD example, IEEE‑754 floating‑point, run‑length & delta compression.	4
Communication (protocols, IP, DNS, topologies, packet vs. circuit switching)	Fieldbus, Modbus, CAN, Ethernet/IP, TCP/UDP, DNS lookup, OSI/TCP‑IP layers, star‑bus‑ring topologies.	8
Hardware fundamentals (logic gates, CPU, registers, bus, ALU)	Logic‑gate truth tables, Boolean algebra, register set, CPU/ALU description, bus types.	6
Processor fundamentals (Von Neumann, fetch‑execute cycle, interrupts)	Detailed fetch‑decode‑execute diagram, scan‑cycle, interrupt‑service routine (ISR) flow.	7
System software (OS functions, language translators, IDE features)	RTOS services, memory/file management, compilers vs. interpreters, IDE capabilities.	9
Security, privacy & data integrity	Authentication, encryption (TLS, AES), checksums/CRC, digital signatures, network segmentation.	10
Ethics & ownership of data	Automation impact, data ownership in IIoT, responsibility for safety‑critical decisions.	10
Artificial intelligence & predictive maintenance	Machine‑learning on logged sensor data, model validation, false‑positive mitigation.	10

3. Core Components of an MCS

• Sensors / Transducers – Convert physical quantities into electrical signals (analogue or digital). Example: PT100 RTD → 0‑10 V.
• Signal Conditioning – Amplification, filtering, anti‑aliasing, A/D conversion (e.g., 12‑bit ADC → 0‑4095 counts).
• Controller – Executes the control algorithm.
  • Programmable Logic Controller (PLC) – ladder‑logic, Structured Text (IEC 61131‑3).
  • Micro‑controller / Embedded System – C, C++ or assembly.
  • Digital Signal Processor (DSP) – high‑speed PID or model‑predictive control.
• Actuators – Convert control signals to physical action (motor, valve, relay, heater). Typically driven by PWM or DAC output.
• Communication Links – Wired (Ethernet, CAN, Modbus‑RTU) or wireless (Wi‑Fi, Bluetooth‑LE, LoRa).
• Human‑Machine Interface (HMI) / SCADA – Visualises data, raises alarms, allows operator overrides, and stores logs.

4. Data Representation, Size & Compression

4.1 Binary, Hexadecimal & BCD

• Binary: base‑2, each bit = 0 or 1. Example: 1010 1011₂ = 0xAB.
• Hexadecimal: base‑16, convenient for grouping 4 bits. Example: 0x3F = 0011 1111₂.
• Binary‑Coded Decimal (BCD): each decimal digit stored in 4 bits. Example: decimal 59 → 0101 1001₂.

4.2 Floating‑Point (IEEE‑754 single precision)

32‑bit format: (-1)^S × 1.M × 2^(E‑127) where S = sign, E = 8‑bit exponent, M = 23‑bit mantissa. Important AS topics: normalisation, overflow, underflow, rounding.

4.3 Size Calculations

• 12‑bit ADC (0‑10 V) → resolution = 10 V / 4095 ≈ 2.44 mV per count.
• 16‑bit sensor → 0‑65535 range, useful for high‑resolution pressure transducers.

4.4 Simple Compression Techniques for Telemetry

• Run‑length encoding – compresses long sequences of identical values.
• Delta encoding – transmit only the difference between successive samples; useful when change is small.

5. Control Theory Refresher

5.1 Open‑Loop vs. Closed‑Loop (Feedback)

Loop type	Feedback used?	Typical applications	Advantages	Disadvantages
Open‑Loop	No	Toaster, irrigation timer, simple conveyor	Simple, cheap, fast response	Cannot compensate for disturbances or component drift
Closed‑Loop (Feedback)	Yes	Thermostats, motor speed control, industrial process control	Accurate, robust to disturbances	More complex, requires sensors & computation; possible stability issues if poorly tuned

5.2 PID Controller – The Work‑horse Algorithm

Let y(t) be the process variable and r(t) the set‑point. Error: e(t) = r(t) – y(t).

PID output:

u(t) = KP·e(t) + KI·∫₀ᵗ e(τ)dτ + KD·de(t)/dt

• K_P – Proportional gain (immediate reaction).
• K_I – Integral gain (eliminates steady‑state error).
• K_D – Derivative gain (predicts future error, reduces overshoot).

Algorithmic implementation (sample time Δt):

1. Read sensor → digital value y.
2. Compute error e = r – y.
3. Update integral term I = I + e·Δt.
4. Compute derivative term D = (e – eprev) / Δt.
5. Calculate control output u = Kp·e + Ki·I + Kd·D.
6. Send u to actuator (e.g., PWM duty cycle).
7. Store e as eprev for the next cycle.

5.3 Computational Thinking in Control

• Abstraction: Model the plant as a transfer function or state‑space model.
• Decomposition: Split into sensing, computation, actuation.
• Algorithm design: Choose PID, fuzzy logic, or model‑predictive control.
• Testing & Debugging: Simulate (MATLAB/Simulink), step‑response tests, log analysis.

6. Hardware Fundamentals

6.1 Logic Gates & Boolean Algebra

Gate	Symbol	Truth Table (A,B)
AND	&	0,0→0; 0,1→0; 1,0→0; 1,1→1
OR	|	0,0→0; 0,1→1; 1,0→1; 1,1→1
NOT	~	0→1; 1→0
NAND	~(&)	inverse of AND
NOR	~(|)	inverse of OR
XOR	⊕	1 when A≠B

PLC ladder‑logic contacts are physical equivalents of these gates. Simplification using Karnaugh maps (K‑maps) is an exam‑relevant skill.

6.2 Register Set (quick reference)

Register	Purpose
PC (Program Counter)	Address of next instruction
MAR (Memory Address Register)	Holds address for memory access
MDR (Memory Data Register)	Data being transferred to/from memory
ACC (Accumulator)	Primary arithmetic/logic operand
IR (Instruction Register)	Current instruction being decoded
FLAGS (Status Register)	Zero, carry, overflow, interrupt enable
General‑purpose (R0‑R7)	Temporary storage for algorithm variables

6.3 CPU, ALU & Bus Architecture

• CPU core – Executes the fetch‑decode‑execute cycle.
• ALU – Performs integer addition, subtraction, logical operations required for PID calculations.
• Data bus – Carries binary data between CPU, memory and I/O.
• Address bus – Carries memory addresses (often 16‑ or 32‑bit wide).
• Control bus – Carries read/write, interrupt, and clock signals.

7. Processor Fundamentals – Von Neumann Model

7.1 Fetch‑Decode‑Execute Cycle (labelled)

1. Fetch: PC → MAR; memory read → MDR → IR.
2. Decode: Instruction decoder analyses IR (opcode, operands).
3. Execute: ALU performs operation; results may go to ACC or a destination register.
4. Write‑back: Result stored in RAM, I/O register, or updated PC for next instruction.

7.2 Scan‑Cycle in a PLC

Repeated many times per second (typical 10‑100 ms). Steps:

1. Read all inputs (sensors).
2. Execute user program (ladder/Structured Text).
3. Update outputs (actuators).
4. Check for high‑priority interrupts.

7.3 Interrupt‑Service Routine (ISR) Flowchart

+-------------------+
| Interrupt occurs |
+--------+----------+
         |
         v
+--------+----------+
| Save context (PC,|
| FLAGS)            |
+--------+----------+
         |
         v
+--------+----------+
| Jump to ISR code |
+--------+----------+
         |
         v
+--------+----------+
| Service request  |
| (e.g., emergency |
| stop)             |
+--------+----------+
         |
         v
+--------+----------+
| Restore context   |
+--------+----------+
         |
         v
+--------+----------+
| Return to scan‑   |
| cycle (resume)    |
+-------------------+

8. Communication – From Fieldbus to the Internet

8.1 Layer‑wise View of the TCP/IP Stack

• Application – SCADA, MQTT, Modbus‑TCP, OPC‑UA.
• Transport – TCP (reliable) or UDP (low‑latency).
• Internet – IP addressing, routing, IPv4/IPv6.
• Link – Ethernet, Wi‑Fi, CAN, RS‑485.

8.2 DNS & URL Basics (required for 2.1)

Domain Name System translates a human‑readable name (e.g., plc01.factory.com) into an IP address. In an IIoT deployment the SCADA server may resolve PLC hostnames via DNS, enabling flexible re‑addressing.

8.3 Common Industrial Protocols

• Modbus‑RTU – Serial (RS‑485), 8‑bit frames, CRC‑16 error detection.
• Modbus‑TCP – Modbus payload encapsulated in TCP/IP.
• CANopen – Message‑based, priority‑encoded identifier, used in automotive & motion control.
• Profibus – DP (Decentralised Peripherals) for high‑speed I/O.
• Ethernet/IP – Standard Ethernet with CIP (Common Industrial Protocol) on top.

8.4 Network Topologies & Switching

• Star – Central switch; easy to isolate faults.
• Bus – CAN or RS‑485; economical for long runs.
• Ring – Ethernet Ring Protection Switching provides redundancy.

8.5 Packet vs. Circuit Switching

• Packet switching – Data split into packets; efficient use of bandwidth (used by TCP/IP).
• Circuit switching – Dedicated path for the duration of a session; typical of older telephony, rarely used in modern MCS.

9. Software Aspects – From Firmware to HMI

9.1 System Software Overview

Aspect	Description (exam‑relevant)
Memory management	Allocation of RAM for variables, stack for ISR, flash for program storage.
File management	Log files on SD‑card or network share; timestamps, rotation policies.
Process / task scheduling	Round‑robin or priority‑based pre‑emptive scheduling in an RTOS (e.g., FreeRTOS).
Interrupt handling	Vector table, ISR latency, nesting rules.
Device drivers	Abstraction layer for ADC, PWM, UART, Ethernet MAC.

9.2 Language Translators & IDE Features

Translator	Typical role	Example
Compiler	Transforms high‑level source (C, Structured Text) into machine code.	GCC for ARM Cortex‑M
Interpreter	Executes statements directly; used for scripting in SCADA.	Python in data‑analysis modules
Assembler	Converts assembly mnemonics to op‑codes.	Keil µVision assembler

9.3 IDE Capabilities Expected for A‑Level

• Code editor with syntax highlighting for ladder, ST, C.
• Integrated simulator/emulator for offline testing.
• Debugger with breakpoints, watch windows, and real‑time variable inspection.
• Version‑control integration (Git) for change tracking.
• Project‑wide configuration (CPU type, communication settings, I/O map).

10. Security, Privacy & Ethics

10.1 Authentication & Access Control

• Role‑based access (operator, engineer, administrator).
• Strong passwords, password expiry, optional MFA for remote VPN.

10.2 Encryption & Data Integrity

• TLS for TCP/IP traffic (e.g., Modbus‑TLS).
• AES‑128/256 for wireless sensor networks.
• CRC‑16/CRC‑32 and digital signatures for firmware authenticity.

10.3 Network Segmentation

• Separate “Control LAN” from corporate IT LAN using firewalls and VLANs.
• DMZ for remote monitoring portals.

10.4 Ethical Considerations

• Impact of automation on employment – need for retraining.
• Responsibility for safety‑critical decisions – designers must ensure fail‑safe operation.
• Ownership of operational data – clear contracts when using cloud analytics.

10.5 AI & Predictive Maintenance

• Collect high‑frequency sensor logs → time‑series database.
• Train regression or classification models (e.g., Random Forest, LSTM) to predict component wear.
• Validate models on unseen data; monitor false‑positive/negative rates to avoid unnecessary shutdowns.

11. Example: Temperature Control in an Industrial Oven

1. Sensor: PT100 RTD → signal conditioner → 0‑10 V → 12‑bit ADC (0‑4095).
2. Signal Conditioning: Low‑pass filter, offset removal, linear scaling to °C.
3. Controller: PLC running Structured Text PID (Δt = 100 ms). Set‑point Tsp = 220 °C.
4. Control Signal: PWM duty cycle (0‑100 %) to a solid‑state relay (SSR) that switches the heating element.
5. Actuator: SSR rated for 4 kW mains load.
6. Communication: Modbus‑TCP on Ethernet/IP (IP = 192.168.0.45). SCADA polls Holding Register 40001 for temperature, writes 40002 for manual override.
7. HMI/SCADA: Real‑time trend plot, alarm when |e| > 5 °C, manual “Open‑Loop” button.
8. Security: TLS‑encrypted Modbus, user authentication, firewall between plant LAN and corporate network.
9. Data Logging & AI: 1 Hz temperature log stored in InfluxDB; Python script trains a Gradient‑Boosting model to predict when the heating element temperature exceeds 600 °C, triggering a pre‑emptive maintenance alert.

12. Advantages of Monitoring & Control Systems

• Safety: Automatic shutdown, fault detection, emergency‑stop handling.
• Efficiency: Optimised resource use, reduced waste, lower energy consumption.
• Product Quality: Tight tolerance maintenance, repeatable processes.
• Remote Operation: Supervision and control from off‑site locations.
• Data‑driven Improvement: Historical logs enable trend analysis, AI‑based optimisation, and predictive maintenance.