Geometry-driven loop placement suggestions.
Place and tune induction loops for reliable lane logic. Follow geometry rules (344), specify cables/joints, and note detector tuning with interference risks near reinforcement or utilities (621, 243). Produce a simple diagram for drawings (931) and acceptance tests (633). Stable detection protects interlocks and KPIs for automatic HVM bollard access lanes (342, 542). Include one-sentence context that naturally links upward to the parent hubs (this section and the chapter hub). Add SIRA context with a link to SIRA Bollards (UAE) when relevant. Link installation pages only if helpful: What to Expect and Installation Guide.
Important: This is a general guide. For live projects we develop a tailored
Method Statement & Risk Assessment (MS/RA) and align with authority approvals (e.g., SIRA) where in scope.
929.1 Purpose
Suggest loop geometry/placement that supports safe automatic HVM bollard behavior (344, 352).
This helper translates site geometry into practical induction loop layouts that keep lanes predictable and safe. It focuses on trigger timing, consistent presence detection, and clean handoffs to the interlock matrix. Clear geometry reduces nuisance trips and protects throughput.
Use it alongside design intent in control logic (342) and site test planning in loop & sensor proving (633).
| Aspect | What matters | Where to verify |
|---|---|---|
| Performance | Reliable presence & approach detection; clean lane logic | Induction Loops (344) |
| Operations | Duty, fail-state & user signalling | KPI & Alerts (542) |
929.2 Inputs
Lane width, speeds, stopping distances, paving build-up, rebar info (341, 344). Crash rated bollard lanes covered.
Capture: (a) effective lane width and approach vector; (b) expected speed band and speed estimation; (c) stopping sight distance for hold/stop lines; (d) paving build-up, including mesh/rebar pattern and cover; (e) detector model, loop detector channel count and tuning range; and (f) nearby utilities/ducts (243, 347). If the slab is post-tensioned (see underground detection), flag drilling restrictions.
929.3 Method
Applies geometry, setback, and rebar interference rules; proposes feeder routing (344.2–344.5, 347).
The helper places approach and presence loops using spacing ratios and setback windows from the stop line, then checks minimum edge clearances and expansion joint alignment. It accounts for bar spacing to mitigate metal-induced damping and rebar coupling, and it suggests twisted-pair feeder routing to the controller (347). Cross-sensitivity between adjacent loops is controlled by separation or frequency offset.
For bidirectional lanes, the method offsets approach loops to preserve timing symmetry. For narrow portals, it recommends smaller loop footprints with elongated presence loops to maintain coverage without encroaching expansion joints.
929.4 Outputs
Loop sizes/positions, joints, tuning ranges, and drawings hints (633). Commissioning-ready.
Deliverables include: (1) plan coordinates from a lane centreline; (2) loop size and turns (target base inductance); (3) minimum joint distances and sealant notes; (4) feeder route, conduit and depth; (5) detector starting frequency/thresholds and a sensitivity band for site tuning; and (6) a diagram annotated to CAD/BIM standards (931). A commissioning checklist references proving tests (633) and logs initial set-points for later audits.
929.5 Limits
Edge cases: curved lanes, mixed modes, nearby metalwork (325, 347). Flag for on-site tuning.
Curved alignments, cobbles over membranes, and dense rebar mats can lower Q-factor and force conservative thresholds. Mixed modes (e.g., bikes + light vehicles) risk “small target” loss unless you add a dedicated small-footprint loop at the hold line. Steel covers, trench drains, and gratings near corners may cause nuisance trips—use joint offsets and frequency separation. Where turning movements are tight (see Turning & Service Access, 325), expect to fine-tune on site.
929.6 Safety integration
Align with interlocks and signalling priorities (352, 353). Predictable HVM bollard states.
Loops should reinforce safe states—never undermine them. Map each loop’s “present/clear” to the interlock matrix (352) so a vehicle in the danger zone prevents a raise command and forces conspicuous safety signalling (353). Document priorities with a short Request→Authorize→Execute trace and add a state machine snippet for peer review.
929.7 Validation
Proving tests and false-trigger checks (633, 635). Evidenceable results.
Validate with: (a) approach-speed runs that confirm timely detection at the hold line, (b) stop-and-creep tests to show stable presence without chatter, (c) cross-talk checks between channels, (d) metallic object immunity passes (grates, covers), and (e) deliberate intrusion tests (635) to prove safe lockouts. Record detector settings and distances; capture a short video plus plan markup for the submission pack (938).
929.8 Save/Export
Plan snippet with coordinates and IDs (931, 911). Drop into packs (938).
Export the loop plan as a small DWG/DXF block with an origin on lane centreline, include loop IDs, detector channel mapping, and a table of coordinates. File naming must follow the File Index & Naming Rules (911). Add the snippet to calculation and submission packs (938) so reviewers can verify set-out quickly.
929.9 Related
HMI/local controls, alarm philosophy (524, 536). Full control loop.
For user interaction and manual overrides, see HMI & Local Controls (524). To keep alerts actionable (no alarm floods), align with the site’s Alarm Philosophy (536). Tie detection health into operations dashboards via KPI thresholds (542) to catch drift over time.
Related
External resources
929 Loop Placement Helper — FAQ
Where should I place approach vs presence loops?
Use an approach loop upstream to detect vehicles in time for a safe decision and a presence loop straddling the hold/stop line to guard the danger zone. Respect joint offsets and edge clearances, and ensure frequency/channel separation to avoid cross-talk on multi-lane sites.
How do rebar and metalwork affect detection?
Dense rebar and nearby steel covers can damp the loop and shift the inductance baseline, lowering sensitivity. Keep loops clear of heavy metalwork, align to expansion joints where possible, and start with conservative detector thresholds. Fine-tune on site during proving tests.
Do I need different loops for bi-directional lanes?
Often yes. Mirror the approach loop placement so timing is symmetrical in both directions, and size the presence loop to cover the danger zone without clipping joints or gratings. Map both channels clearly in the interlock matrix to enforce safe states.
What should go into the loop plan I submit?
Include coordinates from a known origin, loop sizes and turns, feeder routes, joint locations, detector IDs/channel mapping, and initial tuning ranges. Export as a DWG/DXF block and file it using the site’s naming rules so reviewers can verify quickly.