Optical QC: EtherCAT control plan with geizhals.de prices

User, 19 Oct 2026: plan the current EtherCAT setup, keep the 100 W servo,
check prices on geizhals.de.

- New docs/equipment/inline-qc-control-plan.md: decisions, bus layout
  (drives at the end of the line because the A6-200EC takes control
  power from the main supply), I/O list for EL1809/EL2809/EL3152/EL5101
  and the drive touch probe, PDO and DC notes from the A6-EC manual,
  geizhals.de price table (EUR 1,891.60 to 2,104.39 + USD 246.58 for the
  servo kits, THB 80k to 88k), NixOS/EtherCrab software, safety circuit
  (PNOZ s3, two contactors, dump valve), bench test with pass marks
  (parts about THB 64k, all reused), build order, spares, open points.
- EL3102 replaced by EL3152: the optoNCDT 1420 output is 4 to 20 mA.
- Spindle encoder index goes to an EL5101 instead of the drive touch
  probe; encoder 2,000 to 2,500 pulses.
- BOM: control lines split per part with geizhals.de prices (+30% high
  end for shipping and Thai duty/VAT, estimate); total USD 16,912 to
  33,061 (THB 0.56M to 1.09M). Doc, render caption and fleet calculator
  text now say USD 17k to 33k.
- README links the plan.

README.md

@@ -187,6 +187,10 @@ The detail lives in dedicated documents:
   NixOS software, specials, prices and a two-station cost estimate, and
   a FreeCAD concept model of the station (aluminium profile frame,
   rim-belt conveyor, chuck, sensors) with a sourced bill of materials
+- [QC station control plan](docs/equipment/inline-qc-control-plan.md):
+  EtherCAT with two 100 W servo kits, Beckhoff I/O and an N100 mini PC
+  running an open-source master on NixOS; bus layout, I/O list,
+  geizhals.de prices, safety circuit, bench test and build order
 - [Quality automation](docs/quality-automation.md): stamper metrology
   (Keyence-class confocal, profiler, XRF), layered pressed-record QC (100%
   optical inspection, per-cycle press telemetry, sampled automated audio
@@ -347,6 +351,7 @@ docs/
 │   ├── duplex-cell-layout-sketch.png  Duplex cell arrangement (sketch, not to scale)
 │   ├── galvanics.md       Electroforming and stamper prep equipment plan
 │   ├── inline-optical-qc.md  Devices for the record inspection station
+│   ├── inline-qc-control-plan.md  EtherCAT control plan for the station
 │   ├── inline-qc-station/    Concept model of the station (FreeCAD script,
 │   │                         .FCStd, renders, labelled investor view,
 │   │                         bill of materials)