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

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)

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

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)

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

9.2 Language Translators & IDE Features

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.