🎓 Lesson 12
D5
Measuring True Utilization with PLC Data Logging
True utilization is how much time a mining machine is actually doing useful work—like drilling or loading—measured precisely using data from its programmable logic controller (PLC).
🎯 Learning Objectives
- ✓ Calculate true utilization percentage from raw PLC event log timestamps
- ✓ Design a PLC data logging schema compliant with ISO 14224 state classification for haul trucks or drills
- ✓ Analyze PLC-derived idle time segments to distinguish between avoidable (e.g., queuing) and unavoidable (e.g., grade-limited travel) causes
- ✓ Explain how true utilization corrections impact machine hour rate (MHR) calculation and fleet cost allocation
- ✓ Apply time-stamped state transitions to identify and quantify utilization loss drivers per shift
📖 Why This Matters
In large-scale surface mining, inaccurate utilization estimates cause up to 12–18% error in machine hour rate (MHR) calculations—directly distorting equipment ownership, maintenance, and fuel cost allocations. Traditional methods rely on operator logs or shift reports, which underreport idle time by 20–40% due to cognitive bias and incomplete recall. PLC data logging delivers objective, second-by-second machine state visibility—turning utilization from an estimate into an auditable engineering metric. This precision is essential for optimizing fleet sizing, validating automation ROI, and meeting ISO 50001 energy management requirements.
📘 Core Principles
True utilization rests on three interdependent pillars: (1) State-based machine classification—where PLCs log discrete operational states (e.g., 'Loading', 'Hauling Loaded', 'Queuing at Dump', 'Engine Off') per ISO 14224 Annex B; (2) Time-synchronized logging—requiring PLC clock alignment within ±1 s across fleet via NTP or GPS PPS; and (3) Loss taxonomy—categorizing idle seconds into controllable (e.g., dispatch delay, refueling wait) vs. uncontrollable (e.g., traffic hold on steep ramp, weather stoppage) using mine-specific rules engines. Critically, 'productive time' must align with the *machine’s design purpose*: for a hydraulic shovel, only digging, swing, and dumping cycles count—not slewing empty or positioning. This differs fundamentally from availability (uptime) or performance (speed vs. rated capacity).
📐 True Utilization Ratio
True utilization (Uₜᵣᵤₑ) is calculated as the sum of time spent in pre-defined productive states divided by total logged time in the analysis window. It excludes all non-productive states—even if the engine is running—unless explicitly validated as value-adding per equipment function and contract scope.
True Utilization Ratio
Uₜᵣᵤₑ = Σ t_productive / t_totalMeasures the fraction of total logged time during which the machine performed its primary revenue-generating function.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| Uₜᵣᵤₑ | True Utilization | % or decimal | Ratio of productive time to total logged time in analysis period |
| t_productive | Cumulative productive time | seconds | Sum of durations in ISO 14224-defined productive states (e.g., 3.1.1 Hauling Loaded, 4.2.3 Digging) |
| t_total | Total logged time | seconds | Duration of complete PLC log interval (includes all states, even 'Off') |
Typical Ranges:
Surface haul trucks (large fleet): 38–48%
Hydraulic shovels (primary loading): 52–63%
Drills (production blast holes): 45–55%
💡 Worked Example
Problem: A CAT 793 haul truck’s PLC logged 8-hour shift (28,800 s). Productive states ('Hauling Loaded', 'Dumping', 'Returning Loaded') totaled 12,480 s. Non-productive states included: 'Queuing at Crusher' (2,160 s), 'Refueling Wait' (360 s), 'Empty Return (Unloaded)' (5,280 s), 'Engine Idle (No Load)' (1,920 s), and 'Maintenance Hold' (600 s). Calculate Uₜᵣᵤₑ.
1.
Step 1: Identify productive time = 12,480 s (only loaded haul, dump, and return-loaded cycles are productive per haul truck MHR definition).
2.
Step 2: Total logged time = 28,800 s (full shift duration, including all PLC-tagged states).
3.
Step 3: Apply Uₜᵣᵤₑ = 12,480 / 28,800 = 0.4333 → 43.3%.
Answer:
The true utilization is 43.3%, significantly below the commonly assumed 65–70% based on engine-on time alone. This reflects that >56% of shift time was non-productive—and over half of that (3,120 s) was avoidable queuing/waiting.
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), PLC-driven utilization analytics revealed that 28% of haul truck idle time was attributable to inconsistent crusher feed scheduling—not equipment reliability. By integrating PLC state logs with the DISPATCH® Fleet Management System and re-timing crusher bin levels, they reduced average queuing time by 42%, lifting true utilization from 41% to 49% across the 793F fleet—yielding AUD $12.7M/year in deferred capital (one fewer truck required) and fuel savings. Critically, this insight emerged only after filtering out 'Empty Return' (non-productive but necessary) and isolating 'Queuing at Crusher' (avoidable) using ISO 14224 state codes 3.1.2 and 3.2.1.