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FB_CylinderMonitor — S7-1500 SCL Universal Cylinder Supervisor

Platform: Siemens S7-1500 | TIA Portal V18+ | SCL Blocks: FB_CylinderMonitor · FB_CylFaultCollector Companion to: FB_WeldSequencer

A drop-in cylinder monitoring library for any pneumatic or hydraulic actuator. Handles all real-world sensor configurations — zero, one, or two position sensors — generates direction-aware timeout faults, detects sensor conflicts and lost positions, gates actuator outputs for safety, and feeds structured fault data directly to an HMI and machine sequencer.

Why This Exists
Files
Sensor Configuration Options
Fault Logic Per Configuration
FB_CylinderMonitor — Interface
FB_CylFaultCollector — Interface
Output Gating Pattern
Integration with FB_WeldSequencer
Demo: Welding Cell (4 cylinders)
HMI Integration
TIA Portal Setup
Timing Reference
Fault Code Reference
Adapting for Other Applications
Safety Notes

1. Why This Exists

Every cylinder in a machine needs the same boilerplate logic:

Did it reach position in time? If not → fault
Are both sensors TRUE at once? That's wiring or hardware failure → fault
Did it leave position without being commanded? → fault
What should the machine do when it faults — pause or stop?
What should the operator see on the HMI?

Writing this 20+ times per machine is error-prone and inconsistent. FB_CylinderMonitor encapsulates it once, handles all sensor topologies, and gives you clean AtFwd/AtBwd booleans that replace raw sensor reads in your step advance conditions.

2. Files

📁 CylinderMonitor/
├── FB_CylinderMonitor.scl        # FB_CylinderMonitor + FB_CylFaultCollector
├── DEMO_WeldCellIntegration.scl  # Integration demo (4 cylinders + sequencer)
└── README_CylinderMonitor.md     # This file

3. Sensor Configuration Options

Set SensorConfig (INT) on each FB instance:

Value	Constant	Sensor Setup	Typical Use
`0`	`CFG_NONE`	No sensors	Valves, no-feedback actuators
`1`	`CFG_ONE_FWD`	Sensor at FWD (extended) end only	Clamps, grippers — only care about "closed"
`2`	`CFG_ONE_BWD`	Sensor at BWD (retracted) end only	Ejectors — only care about "home"
`3`	`CFG_TWO`	Sensor at both ends	Weld heads, positioners, gates

Which configuration to use?

Q: Does the cylinder need to confirm both extended AND retracted?
   YES → CFG_TWO (3)

Q: Only need to know it's closed/extended?
   YES → CFG_ONE_FWD (1)

Q: Only need to know it's home/retracted?
   YES → CFG_ONE_BWD (2)

Q: No sensor at all (timer-only advance)?
   YES → CFG_NONE (0)

4. Fault Logic Per Configuration

CFG_NONE (0) — No faults possible from position

Cmd_Fwd ON ──────────────────────────────────▶ CylCmd_FwdSafe ON
                                               CylState = AT_FWD (trusted)
No sensors → no timeout → no conflict
Advance logic: use sequencer's step timer

CFG_ONE_FWD (1) — Fault on FWD only

Cmd_Fwd ON ──┬──────────────────────▶ CylCmd_FwdSafe ON
             │   Sen_Fwd? ──TRUE────▶ AtFwd = TRUE  ✓  (timer reset)
             │            └─FALSE───▶ T_Timeout running
             │                            │
             │                            ▼ T_Timeout elapsed
             │                         FaultCode = 1 (FWD TIMEOUT)
             │                         Fault_Active = TRUE
             │                         CylCmd_FwdSafe → FALSE (gated off)
             │
Cmd_Bwd ON ──▶ No BWD timeout possible (no BWD sensor)
               CylState reports AT_BWD when FWD sensor clears

CFG_ONE_BWD (2) — Fault on BWD only

Cmd_Bwd ON ──┬──────────────────────▶ CylCmd_BwdSafe ON
             │   Sen_Bwd? ──TRUE────▶ AtBwd = TRUE  ✓  (timer reset)
             │            └─FALSE───▶ T_Timeout running
             │                            ▼ T_Timeout elapsed
             │                         FaultCode = 2 (BWD TIMEOUT)
             │
Cmd_Fwd ON ──▶ No FWD timeout (no FWD sensor)
               CylState reports AT_FWD when BWD sensor clears

CFG_TWO (3) — Full fault coverage

Cmd_Fwd ON ──▶ Sen_Fwd? ─TRUE──▶ AtFwd ✓   timer cancelled
                        └─FALSE─▶ Timeout FWD → FaultCode 1

Cmd_Bwd ON ──▶ Sen_Bwd? ─TRUE──▶ AtBwd ✓   timer cancelled
                        └─FALSE─▶ Timeout BWD → FaultCode 2

BOTH sensors TRUE simultaneously → FaultCode 3 (CONFLICT) → immediate fault
No cmd + was at known pos + both sensors go FALSE → FaultCode 4 (LOST)
Both Cmd_Fwd AND Cmd_Bwd TRUE → FaultCode 3 (command conflict)

5. FB_CylinderMonitor — Interface

VAR_INPUT

Name	Type	Description
`CylName`	`String[32]`	Human name, e.g. `'Clamp Cylinder'` — used in HMI text
`SensorConfig`	`Int`	0=NONE, 1=ONE_FWD, 2=ONE_BWD, 3=TWO
`T_Timeout`	`Time`	Max time for cylinder to reach position
`Cmd_Fwd`	`Bool`	Extend / close command
`Cmd_Bwd`	`Bool`	Retract / open command
`Cmd_Reset`	`Bool`	Rising edge clears fault
`Sen_Fwd`	`Bool`	Forward / extended position sensor
`Sen_Bwd`	`Bool`	Backward / retracted position sensor
`Enable`	`Bool`	FALSE = ignore commands, hold state

VAR_OUTPUT

Name	Type	Description
`CylCmd_FwdSafe`	`Bool`	Gated FWD output — connect to DQ
`CylCmd_BwdSafe`	`Bool`	Gated BWD output — connect to DQ
`AtFwd`	`Bool`	Position confirmed at FWD
`AtBwd`	`Bool`	Position confirmed at BWD
`Intermediate`	`Bool`	Between positions (TWO config, no cmd)
`Moving`	`Bool`	Command active, not yet confirmed
`Fault_Active`	`Bool`	Any fault present
`Fault_FwdTimeout`	`Bool`	Specific fault bit
`Fault_BwdTimeout`	`Bool`	Specific fault bit
`Fault_Conflict`	`Bool`	Sensor conflict fault bit
`Fault_Lost`	`Bool`	Lost position fault bit
`FaultCode`	`Int`	Numeric code (0–4)
`CylState`	`Int`	State code (0–6)
`HMI_StatusText`	`String[80]`	Ready-to-display status
`HMI_FaultText`	`String[80]`	Ready-to-display fault message

6. FB_CylFaultCollector — Interface

Aggregates up to 8 cylinder monitor faults into one summary. Wire its outputs to the sequencer's Cmd_Pause and Cmd_Stop.

Key Outputs

Name	Type	Description
`CylFaultAny`	`Bool`	OR of all cylinder faults
`CylFaultWord`	`Word`	Bitmask — bit 0 = Cyl1, bit 7 = Cyl8
`Cmd_MachinePause`	`Bool`	Wire to sequencer `Cmd_Pause`
`Cmd_MachineStop`	`Bool`	Wire to sequencer `Cmd_Stop`
`Cmd_Warning`	`Bool`	Amber lamp — non-stopping fault
`FirstFaultText`	`String[80]`	First fault message for HMI banner
`ActiveFaultCount`	`Int`	Number of cylinders currently faulted

Machine Response Configuration

Input	Value	Effect
`Response_Timeout`	`0`	Warning only — machine keeps running
`Response_Timeout`	`1`	Pause machine
`Response_Timeout`	`2`	Stop machine (requires Reset)
`Response_Conflict`	`0/1/2`	Same options
`Response_Lost`	`0/1/2`	Same options

Recommended defaults:

Response_Timeout  := 1;   // Pause — timeout may be temporary, operator can check
Response_Conflict := 2;   // Stop  — two sensors both TRUE = hardware fault
Response_Lost     := 2;   // Stop  — cylinder moved without command = mechanical issue

7. Output Gating Pattern

The core safety pattern of this FB is output gating. Instead of writing:

// ❌ Unsafe — raw command goes direct to DQ
%Q0.1 := Act_HeadExtend;

You write:

// ✅ Safe — passes through monitor, gated off on fault
"DB_CylHead"(
    Cmd_Fwd := Act_HeadExtend,
    ...
);
%Q0.1 := "DB_CylHead".CylCmd_FwdSafe;   // Only energises if healthy

When a fault occurs (timeout, conflict, lost position):

CylCmd_FwdSafe → FALSE immediately (same scan as fault detection)
Solenoid valve de-energised → cylinder stops
Fault_Active → TRUE → collector → sequencer Cmd_Pause or Cmd_Stop
HMI_FaultText → operator message describes exactly what failed

8. Integration with FB_WeldSequencer

The two FBs communicate in both directions:

FB_CylinderMonitor                     FB_WeldSequencer
────────────────────                   ─────────────────────────────
AtFwd ─────────────────────────────▶  Sen_ClampClosed (step 2 advance)
AtFwd ─────────────────────────────▶  Sen_HeadAtWeldPos (step 3 advance)
AtBwd ─────────────────────────────▶  Sen_HeadRetracted (step 9 advance)
Fault_Active                           
   └──▶ FB_CylFaultCollector           
           Cmd_MachinePause ──────────▶  Cmd_Pause (OR with operator PB)
           Cmd_MachineStop  ──────────▶  Cmd_Stop  (OR with operator PB)

FB_WeldSequencer                       FB_CylinderMonitor
─────────────────────────────          ────────────────────
Act_HeadExtend ─────────────────────▶  Cmd_Fwd
Act_HeadRetract ────────────────────▶  Cmd_Bwd
Seq_Running OR Seq_Paused ──────────▶  Enable

Step Advance with Monitor vs Raw Sensor

Without Monitor	With Monitor
`_stepAdvReady := Sen_ClampClosed`	`_stepAdvReady := "DB_CylClamp".AtFwd`
Advances even if clamp timed out	Will not advance if monitor is faulted
No timeout detection	FaultCode 1 triggers machine pause

Using AtFwd/AtBwd from the monitor instead of raw sensor inputs means a faulted cylinder also prevents step advance, so the sequencer's own watchdog timer provides a belt-and-braces fallback catch.

9. Demo: Welding Cell (4 Cylinders)

The demo file DEMO_WeldCellIntegration.scl shows all four configurations side by side:

Instance	Cylinder	Config	Timeout	Fault Response
`DB_CylClamp`	Clamp	ONE_FWD	2 s	Pause
`DB_CylHead`	Weld Head	TWO	3 s	Pause (timeout) / Stop (conflict)
`DB_CylEject`	Part Ejector	ONE_BWD	1.5 s	Pause
`DB_CylGas`	Gas Valve	NONE	n/a	n/a

Execution Order (critical)

In your OB (OB1 or OB30), call in this order:

1st  → "DB_CylClamp"(...)       FB_CylinderMonitor
2nd  → "DB_CylHead"(...)        FB_CylinderMonitor
3rd  → "DB_CylEject"(...)       FB_CylinderMonitor
4th  → "DB_CylGas"(...)         FB_CylinderMonitor
5th  → "DB_CylFaults"(...)      FB_CylFaultCollector
6th  → "DB_WeldSeq"(...)        FB_WeldSequencer

Monitors run first so that AtFwd/AtBwd and Fault_Active values are fresh when the sequencer evaluates step advance conditions and Cmd_Pause/Stop in the same scan.

10. HMI Integration

Recommended Screen Objects

Cylinder Overview Faceplate (one per cylinder)

Element	Tag	Type
Status text	`DB_CylClamp.HMI_StatusText`	Text field
Fault text	`DB_CylClamp.HMI_FaultText`	Text field (red, visible on fault)
State indicator	`DB_CylClamp.CylState`	Bar/value with colour map
Moving animation	`DB_CylClamp.Moving`	Animated cylinder graphic
At FWD lamp	`DB_CylClamp.AtFwd`	Green indicator
At BWD lamp	`DB_CylClamp.AtBwd`	Green indicator
Fault lamp	`DB_CylClamp.Fault_Active`	Red indicator

CylState Colour Map (for bargraph or rectangle)

Value	Colour	Label
0	Grey	Idle
1	Yellow	Moving FWD
2	Green	At FWD ✓
3	Yellow	Moving BWD
4	Green	At BWD ✓
5	Orange	Intermediate
6	Red	FAULTED

Global Alarm Banner

Tag: DB_CylFaults.FirstFaultText
Visible: DB_CylFaults.CylFaultAny = TRUE
Background: Red
Example text: "Weld Head: FWD TIMEOUT — check cylinder, solenoid and FWD sensor"

Fault Bitmask Display (overview bar)

Tag: DB_CylFaults.CylFaultWord (WORD)
Display as: 8 individual bit lamps
Bit 0 = Clamp  Bit 1 = Head  Bit 2 = Ejector  Bit 3 = Gas  ...

Operator Guidance Text Logic

For each cylinder fault, the HMI_FaultText already tells the operator exactly what to check:

FaultCode 1 → "Clamp Cylinder: FWD TIMEOUT — check cylinder, solenoid and FWD sensor"
FaultCode 2 → "Weld Head: BWD TIMEOUT — check cylinder, solenoid and BWD sensor"
FaultCode 3 → "Weld Head: SENSOR CONFLICT — FWD + BWD both active. Check wiring/sensors"
FaultCode 4 → "Weld Head: LOST POSITION — cylinder moved without command. Check mechanics"

No lookup table needed — the text is generated inside the FB using the CylName you configured.

11. TIA Portal Setup

Create Instance DBs

For each cylinder, right-click project tree → Add new block → Data Block → Instance DB of FB_CylinderMonitor:

DB_CylClamp  : FB_CylinderMonitor
DB_CylHead   : FB_CylinderMonitor
DB_CylEject  : FB_CylinderMonitor
DB_CylGas    : FB_CylinderMonitor
DB_CylFaults : FB_CylFaultCollector

Initial Parameter Setup (in startup OB or first-scan block)

// Set cylinder names and configs once at startup
// (or set directly in instance DB initial values in TIA Portal)

"DB_CylClamp".CylName      := 'Clamp Cylinder';
"DB_CylClamp".SensorConfig := 1;
"DB_CylClamp".T_Timeout    := T#2S;

"DB_CylHead".CylName       := 'Weld Head';
"DB_CylHead".SensorConfig  := 3;
"DB_CylHead".T_Timeout     := T#3S;

"DB_CylEject".CylName      := 'Part Ejector';
"DB_CylEject".SensorConfig := 2;
"DB_CylEject".T_Timeout    := T#1500MS;

"DB_CylGas".CylName        := 'Gas Purge Valve';
"DB_CylGas".SensorConfig   := 0;

Alternatively, set initial values directly in the instance DB's data view in TIA Portal — these persist through power cycles.

12. Timing Reference

Cylinder Type	Recommended T_Timeout	Notes
Fast pneumatic (< 50 mm)	`T#1S`	Small bore, short stroke
Standard pneumatic	`T#2S`	Most clamps, grippers
Long-stroke pneumatic	`T#3S`–`T#5S`	Weld heads, gates
Hydraulic	`T#5S`–`T#15S`	Slower by nature
Spring-return valve	N/A (NONE config)	No position sensor

Rule of thumb: Set timeout to 3× the expected travel time. Fast enough to catch real faults, slow enough to avoid nuisance trips on normal variation.

13. Fault Code Reference

Code	Name	Cause	HMI Message	Machine Action
`0`	No fault	Normal	(empty)	None
`1`	FWD timeout	Sen_Fwd not TRUE within T_Timeout after Cmd_Fwd	`"[Name]: FWD TIMEOUT — check cylinder, solenoid and FWD sensor"`	Pause (configurable)
`2`	BWD timeout	Sen_Bwd not TRUE within T_Timeout after Cmd_Bwd	`"[Name]: BWD TIMEOUT — check cylinder, solenoid and BWD sensor"`	Pause (configurable)
`3`	Conflict	Sen_Fwd AND Sen_Bwd both TRUE, or Cmd_Fwd AND Cmd_Bwd both TRUE	`"[Name]: SENSOR CONFLICT — FWD + BWD both active. Check wiring/sensors"`	Stop (configurable)
`4`	Lost position	Was at known position; both sensors dropped with no command	`"[Name]: LOST POSITION — cylinder moved without command. Check mechanics"`	Stop (configurable)

14. Adapting for Other Applications

Add a pressure/flow sensor as virtual FWD confirmation

// Gas valve with pressure feedback
"DB_CylGas"(
    CylName      := 'Gas Valve',
    SensorConfig := 1,                  // ONE_FWD
    T_Timeout    := T#1S,
    Cmd_Fwd      := Act_GasValve,
    Sen_Fwd      := PressureSwitch_OK,  // Add this DI
    ...
);

Motor or servo (use as virtual "cylinder" with limit switches)

"DB_CylSlide"(
    CylName      := 'Transfer Slide',
    SensorConfig := 3,
    T_Timeout    := T#4S,
    Cmd_Fwd      := SlideMotor_Fwd,
    Cmd_Bwd      := SlideMotor_Rev,
    Sen_Fwd      := LS_SlideForward,
    Sen_Bwd      := LS_SlideReverse,
    ...
);

Changing timeout at runtime

// Speed-dependent timeout (slow mode vs normal mode)
IF SlowMode THEN
    "DB_CylHead".T_Timeout := T#8S;
ELSE
    "DB_CylHead".T_Timeout := T#3S;
END_IF;

Note: T_Timeout is read by the TON each scan — changes take effect immediately on the next timer start.

More than 8 cylinders

Extend FB_CylFaultCollector array declarations from [1..8] to [1..N] and add corresponding VAR_INPUT slots.

15. Safety Notes

⚠️ FB_CylinderMonitor is a diagnostic and convenience layer. It is NOT a safety function and should NOT be used as the sole means of protecting personnel from cylinder movement.

This FB provides (diagnostic/operational layer):

Timeout detection → pause or stop production
Sensor conflict → immediate output gate + machine stop
HMI guidance text for operator troubleshooting
Output gating (software level only — not certified)

This FB does NOT replace (safety layer):

Hardware safety valves (5/2 safety valve with spring return for E-Stop)
Dual-channel safety relay monitoring for guard interlocks
Safety PLC (S7-1500F) for PLd/SIL2 applications
Mechanical hard stops preventing overtravel
Pressure relief valves on hydraulic systems

Minimum safety architecture:

E-Stop ──▶ Safety Relay ──▶ Safety Valve ──▶ Cylinder (power removed in hardware)
                │
                └──▶ Safety_EStopOK input on FB_WeldSequencer (software layer)

Door open ──▶ Safety Switch ──▶ Safety Relay
                │
                └──▶ Safety_DoorClosed input on FB_WeldSequencer

The cylinder monitor's CylCmd_FwdSafe / CylCmd_BwdSafe outputs control directional valves only. Power removal for E-Stop must be done at the hardware safety relay level, not by this FB.

Changelog

Version	Date	Notes
0.1	2025	Initial release — 4 sensor config types, 4 fault codes, HMI text

Licence

MIT — free to use and adapt.

For the full welding cell sequencer (10 steps, AUTO/INCR mode, stop categories), see README.md.

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