Motor and valve control blocks
What a reusable equipment block must do: commands from modes, feedback supervision, discrepancy alarms, local/remote, maintenance — the logic behind the FB generator.
One block per equipment type
Plants contain hundreds of motors and valves. Writing their logic once as a function block (a control module) gives identical behaviour, identical HMI faceplates and much less testing.
Motor block — functions
| Function | Detail |
|---|---|
| Command sources | Auto (sequence), manual (operator), local (field buttons) — exactly one active |
| Permissives / interlocks / trips | See interlocks |
| Run command output | To contactor or VFD |
| Running feedback | Contactor auxiliary contact or drive status |
| Fail to start | Command ON but no feedback within T start → alarm, command off |
| Unexpected stop / discrepancy | Feedback lost while running, or feedback without command |
| External fault | Overload relay, VFD fault |
| Maintenance / simulation | Lock-out, simulation of feedback for testing without the plant |
| Statistics | Starts counter, running hours |
Valve block — differences
- Two feedbacks (open / closed limit switches) or a position signal.
- Travel time supervision for opening and closing separately.
- Both limit switches active at once → position error.
- Safe position on fault depends on the process (fail-open / fail-closed) and on the actuator (spring return).
Design rules
- Command and feedback are different signals — never use the command as "running" indication.
- Alarms latch until reset when the cause is gone.
- Expose a status word for HMI/SCADA (running, fault, mode, interlocked) rather than many separate tags.
Generate a starting point for Siemens, CODESYS, TwinCAT or generic IEC with the Function Block Generator.
Connections
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Sources
- IEC 61131-3:2025 Programmable controllers — Part 3: Programming languages — IEC
- TwinCAT 3 PLC Library Tc2_Standard — Beckhoff Automation
Updated 2026-10-02