PLC-Course

Sprint 6 Cheat Sheet — Design & Safety

🏗️ Top-Down Design Workflow

1. SPEC               What does the system need to do?
   ↓                  Write it as plain English.
2. I/O LIST           Every sensor, every actuator, with tag, type, address.
   ↓                  This is your contract with the electrician.
3. STATE DIAGRAM      What states exist? What transitions between them?
   ↓                  SFC or simple state machine sketch.
4. CODE               Now write it. Match structure to design.
   ↓
5. TEST               Simulate, dry-run, force inputs, observe outputs.
   ↓
6. DOCUMENT           Comments, tag descriptions, version history.
   ↓
7. COMMISSION         FAT (Factory) → SAT (Site) → handover.

📋 The I/O List Template

Tag	Type	Address	Range	Description	Fail-safe state
LSH_T01	DI	I0.0	—	Tank 1 high level switch	NC (broken wire = high)
TT_T01	AI	IW64	0–100 °C	Tank 1 temperature	Below range = fault
Y_FILL	DO	Q0.0	—	Fill valve solenoid	De-energized = closed
MX_T01	DO	Q0.1	—	Mixer contactor	De-energized = stopped

Rule: every safety-critical signal should fail to a safe state. NC switches for E-stop, drop-out for valves, etc.

🛡️ Stop Categories (per IEC 60204-1)

Category	What happens	Use case
Cat 0	Immediate removal of power	Most reliable; fine for systems that coast safely (small motors)
Cat 1	Controlled stop, then power removal	Required for hazardous motion (large motors, presses)
Cat 2	Controlled stop, power maintained	Process where stopping is normal but power-off would cause damage

Rule of thumb: if a coast-down causes danger (kinetic energy, falling parts, hot surfaces), use Cat 1.

🔐 Safety Architecture

Tier	What it is	When to use
Hardwired safety relay (e.g., Pilz, Sick)	Dedicated relay logic for E-stops, gates	Simple machines, single safety zone
Safety PLC (e.g., Siemens S7-1500F, Allen-Bradley GuardLogix)	Programmable, dual-channel, certified	Multi-zone, complex sequences, networks
Standard PLC for safety	Just don’t	Not certifiable, not legal in EU machinery

Standards to know:

• ISO 13849-1 — Performance Levels (PL a–e)
• IEC 62061 — SIL 1–3 for machinery
• IEC 61508 — General functional safety (process)

🔍 Fault Diagnosis Toolkit

Tool	Purpose
Diagnostic LEDs	Run/Stop/Error on the CPU; status on each I/O point
Status bits	Internal flags showing module health, comms
Watchdog timer	Detects program freeze; faults the CPU if not pet
Force tables	Override an input or output during commissioning
Online monitoring	Watch live values during execution
Diagnostic buffer	Time-stamped log of CPU events
Cross-reference	Find every rung that touches a tag

⚠️ Forces are dangerous. Always remove them after use, document while in place, and never leave a system in service with active forces.

📑 FMEA — Failure Modes & Effects Analysis

For each component, ask:

1. Failure mode: how can this fail?
2. Effect: what happens if it does?
3. Severity: 1–10 (10 = catastrophic)
4. Likelihood: 1–10
5. Detectability: 1–10 (10 = undetectable)
6. RPN: Severity × Likelihood × Detectability
7. Mitigation: what changes the score?

Component	Failure Mode	Effect	S	L	D	RPN	Mitigation
Level switch	Stuck closed	Pump runs dry	8	3	4	96	Add high-level timer + pressure switch

🐛 The Bug Hunter’s Checklist

When something doesn’t work:

1. Is it powered? Check 24 V at the input, output card LEDs.
2. Is the input being read? Force-monitor the bit. Compare to physical state.
3. Is the rung evaluating true? Online monitoring shows rung state.
4. Is the output being written? Force the output. Does the device respond?
5. Is there a double-coil? Cross-reference all writes to the tag.
6. Is a jump skipping the rung? Check JMP labels active.
7. Is the scan time too long? Check diagnostic buffer for watchdog warnings.

📝 Documentation Minimums

Every PLC program should have:

• Cover page — project name, version, date, author, hardware list
• I/O list — all tags with descriptions
• Sequence of operations — plain-English description of what the program does
• Rung comments — every non-trivial rung explained in one line
• Tag descriptions — every tag has a description (no M0, M1, M2 mysteries)
• Change log — date, author, what changed, why

If maintenance can’t read your code at 2 AM with a flashlight, you haven’t documented enough.

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