🎓 Lesson 9
D5
PLC Logic Implementation for Feed Splitting
A PLC logic system for feed splitting automatically directs material between two processing streams—like crushers or leach pads—based on real-time sensor data and preset rules.
🎯 Learning Objectives
- ✓ Design a ladder-logic sequence for dual-stream feed routing using Boolean AND/OR/NOT conditions
- ✓ Analyze PLC scan cycle timing impacts on feed split accuracy under variable conveyor speeds
- ✓ Apply hysteresis and debounce logic to prevent chattering in level- or grade-based split decisions
- ✓ Explain how feed split logic integrates with DCS-level dynamic circuit adjustment during mill throughput upsets
📖 Why This Matters
In modern integrated metallurgical circuits—especially in large-scale copper or gold operations—feed splitting determines whether ore goes to primary crushing or bypasses to stockpile, or whether high-grade material routes to fast-leach tanks while low-grade feeds agglomeration. A poorly designed PLC logic can cause grade segregation, crusher overloads, or leach pad ponding. This lesson bridges foundational PLC knowledge with the dynamic, real-time decision-making needed when circuits adjust to changing ore hardness, moisture, or downstream bottlenecks.
📘 Core Principles
Feed splitting logic operates within three layered domains: (1) Input conditioning—where analog signals (e.g., XRF grade %Cu, belt load kg/s) are filtered, scaled, and validated; (2) Decision logic—typically structured as priority-weighted conditional blocks (e.g., 'IF grade > 0.85% Cu AND moisture < 8% THEN route to SAG'); and (3) Output execution—with fail-safe actuation (e.g., diverter gate position feedback, interlocked stop/start sequences). Crucially, this logic must be deterministic: execution time must remain bounded (< 50 ms per scan) to avoid lag-induced misrouting during transient events like surge feeding or analyzer calibration cycles.
📐 Hysteresis Bandwidth Calculation for Grade-Based Splitting
To prevent rapid oscillation of diverter gates when grade readings hover near a setpoint, hysteresis is applied: the split condition activates at an upper threshold and deactivates only after falling below a lower threshold. The bandwidth ensures stable actuation without excessive wear or flow disruption.
Hysteresis Bandwidth
HB = 2 × σ_max × SFMinimum hysteresis width required to suppress chatter due to sensor uncertainty and process noise.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| HB | Hysteresis Bandwidth | % (grade) or μm (size) | Difference between activation and deactivation thresholds |
| σ_max | Maximum Measurement Uncertainty | % or μm | Largest known error bound of the primary sensor (e.g., XRF, laser diffraction) |
| SF | Safety Factor | unitless | Multiplier (typically 1.2–1.5) applied to account for drift, calibration error, and unmodeled noise |
Typical Ranges:
XRF-based grade splitting (Cu, Au): 0.10 – 0.25 %
Laser P80 size splitting: 15 – 40 μm
💡 Worked Example
Problem: A copper mine uses XRF analyzers (±0.05% Cu accuracy) to split feed between high-grade (HG) and low-grade (LG) leach circuits. Target split point = 0.75% Cu. Analyzer noise floor = ±0.04% Cu. Determine minimum recommended hysteresis bandwidth.
1.
Step 1: Identify dominant uncertainty source — analyzer accuracy (±0.05%) dominates noise (±0.04%), so use ±0.05% as base uncertainty.
2.
Step 2: Apply industry rule: hysteresis ≥ 2 × largest measurement uncertainty → 2 × 0.05 = 0.10% Cu.
3.
Step 3: Add 20% safety margin for signal drift and calibration drift → 0.10 × 1.2 = 0.12% Cu.
Answer:
The result is 0.12% Cu, which falls within the safe range of 0.10–0.15% Cu for medium-variability copper ores.
🏗️ Real-World Application
At Newmont’s Boddington Gold Mine (Western Australia), PLC feed splitting logic routes ROM ore between two parallel SAG mills based on real-time grade (from online gamma-ray analyzers) and hardness proxy (from crusher power draw). When grade exceeds 0.95 g/t Au *and* power draw drops below 82% of rated capacity (indicating softer rock), feed is diverted to Mill #1 for higher throughput. Simultaneously, the logic applies 3-second debounce timers on all analog-triggered decisions to reject short-duration spikes. This reduced mill liner wear by 18% and improved gold recovery consistency by ±0.4% over quarterly reporting periods (2022–2023 Operational Review).
✏️ Student Exercise
You are tasked with programming a feed splitter for a molybdenum concentrator. Two streams feed flotation: Stream A (coarse grind) and Stream B (fine grind). Inputs: belt scale (t/h), particle size analyzer (P80 in μm), and Mo assay (%Mo). Design criteria: Route to Stream A if P80 > 125 μm *and* %Mo ≥ 0.18%; otherwise route to Stream B. Add 2.5 s debounce on P80 and 1.0 s on %Mo. Calculate maximum allowable PLC scan time to ensure total decision latency ≤ 50 ms given 3 input filters (each 2-pole IIR, 10 ms τ) and 1 output relay delay (12 ms).