Controls — RO soft start

Field	Detail
Primary artifact(s)	controls/water/st/ro-sequence.st
Spec	RO-SPEC-001 §5.6 ; WIRE-001 VFD§

Summary

VFD-101 / P-101 frequency command with soft-start ramp and skeletal PT-102 membrane-pressure PID outer loop. Writes MB_WRITE_FREQ_X100_V101 (×0.01 Hz per gs21-registers.md); reads MB_READ_OUTFREQ_V101 for HMI/trend only.

Stage	Behaviour
Demand	SYS_AUTO_PERMISSIVE AND NOT DI_LSH101_T101_HIGH AND NOT SEQ_DISINFECT_LOCKOUT_ACTIVE
BV-101	Energise DO_BV101_BALL_VALVE (Y1) on demand; spring-return NC fails closed on loss of power
Run permissive	DO_BV101_BALL_VALVE AND NOT DI_LSH101_T101_HIGH — mirrors intlk_p101_run_ok / ladder ILK-004
Soft start	TON ramps 0 → PID command over SP_VFD101_RAMP_SEC (Retain, default 30 s)
PID outer loop	AI_PT102_RO_HP vs SP_PT102_RO_SET_PSI → Hz command; ALM_PT102_HI forces cmd to 0

Interlock ties

Sequence logic ANDs the same stack documented in interlocks.st before maintaining DO_VFD101_RUN_AUX:

DO_BV101_BALL_VALVE  AND  NOT DI_LSH101_T101_HIGH  →  DO_VFD101_RUN_AUX

Field ladder must retain the hardwired series copy (ILK-004); this ST routine must not introduce a parallel bypass.

Test

controls/water/sim-tests/sim-ro-start-stop.md — force SYS_AUTO_PERMISSIVE; cycle BV + LSH; verify ramped MB_WRITE_FREQ_X100_V101 and no DO chatter.

Open items

• PID anti-windup clamp and bumpless transfer on run/stop
• Bind ton_softstart.PT to SP_VFD101_RAMP_SEC Retain tag in Productivity TIME conversion
• T-101 low-start demand (LSL / level band) merge with fill cascade
• Commission SP_PT102_RO_SET_PSI vs membrane datasheet on wet bench Phase C

Reviewer sign-off

• RO sequence reviewed against RO-SPEC-001 §5.6 — _______________