How Strut-Based Payload Systems Work, and Why They Disagree

Every haul truck payload system measures suspension strut pressure. They all convert that pressure into a weight estimate. But how they get from raw pressure to a trusted number differs dramatically between OEMs, and those differences matter for calibration, FMS configuration, and troubleshooting. A fleet-wide 5% payload measurement error on a 50 Mtpa operation means 2.5 Mt of misreported production per year.

The Physics of Strut-Based Measurement

A haul truck’s suspension struts are, at their core, hydraulic cylinders filled with pressurised fluid and gas. When the truck is loaded, the struts compress. More weight means higher pressure. The payload system reads that pressure through transducers on each strut and converts it to a gross vehicle weight, then subtracts the known empty weight (tare) to derive the payload.

Simple in theory. Messy in practice.

The fundamental problem is stiction, static friction in the strut seals. When a truck sits stationary at the face while being loaded, the strut seals grip the cylinder walls. The pressure reading reflects both the actual load and the frictional resistance in the seals, and there is no way to separate the two while the truck is sitting still. This is why static payload readings at the face are unreliable, typically ±5–10% at best.

Once the truck moves, the struts cycle through their travel range and the seals break free. The frictional component drops out of the reading. Dynamic accuracy improves to ±2–3%, roughly halving the static error band. Every OEM’s system exploits this principle, but they each handle the transition differently.

Temperature is the other variable. Hydraulic oil expands as it heats up, changing the pressure-to-weight relationship. A truck operating in the cool of the morning will produce different readings from the same truck at the same weight in 45°C afternoon heat. Some OEMs address this with fluid choice, others with software compensation, and some rely on calibration frequency to keep drift manageable.

Caterpillar: TPMS within VIMS

Caterpillar’s Vital Information Management System (VIMS) is the overarching onboard monitoring platform on Cat haul trucks, tracking engine health, transmission, brakes, temperatures, and more. The Truck Payload Measurement System (TPMS) is the payload-specific module within VIMS. When people refer to “VIMS payload data” or “TPMS readings,” they are talking about the same system. TPMS does the measurement; VIMS is the platform it lives on.

TPMS uses a two-stage approach with a binary trigger. It is the most widely deployed strut-based payload system in large surface mining.

Stage 1: Static Reading at the Face

When loading is complete, TPMS takes a static weight measurement from the suspension struts. This reading is noisy (±5–10% accuracy) and susceptible to a phenomenon operators call “packing the load,” where the shovel presses down on the material in the tray. That downward force compresses the struts beyond what the payload alone would produce, inflating the static reading.

The static reading is displayed to the operator but it is not the official payload. Cat treats it as a preliminary estimate.

The static measurement is invalidated entirely if the truck moves more than 0.16 km (160 metres) between loader passes or if the body raise lever is actuated during loading. Both conditions indicate the loading event has been interrupted and the static baseline is no longer valid.

Stage 2: Dynamic Reweigh

The official payload locks when the truck either shifts into 2nd gear or travels more than 0.16 km from the face, whichever comes first. This is the dynamic reweigh. The truck is moving, the struts have cycled, stiction has broken, and the reading is substantially more accurate.

Once the dynamic payload locks, it stays locked. There is no further recalculation during the haul. The number that locked in 2nd gear is the number that flows to the FMS, appears in production reports, and gets compared against targets.

Overload Response

If the dynamic payload exceeds 120% of rated capacity, TPMS triggers Event Code 2917, “Payload Abuse.” The mechanical response is immediate: engine speed is limited to 1,750 RPM and the transmission locks in 2nd gear, restricting the truck to approximately 16 km/h. The truck can still move, but it is effectively crawling. This is not a soft warning; it is a hard intervention designed to protect the drivetrain and frame from damage.

VIMS Monitoring During Empty Return

VIMS (Vital Information Management System) continues monitoring strut pressures during the empty return haul. It is not measuring payload at this point (the truck is empty). Instead, it is watching for collapsed or failing suspension cylinders. If a strut reading falls below expected thresholds at speeds under 12 km/h, VIMS flags a diagnostic event. This catches cylinder failures before they become safety incidents.

Calibration: PAYCAL

TPMS calibration uses Service Program Code 729225, commonly known as PAYCAL, run through the VIMSpc software interface. The calibration procedure requires the truck to coast to a complete stop without applying the brakes. This establishes a clean tare weight without brake-induced strut loading contaminating the baseline.

The reference standard for Cat suspension cylinder specifications is SEHS9411. Any component replacement, change to empty truck weight (new tray, liner change, different body configuration), or strut rebuild requires recalibration.

TPMS stores up to 2,400 payload cycle records onboard.

Komatsu: PLM III and PLM IV

Komatsu’s Payload Meter (PLM) takes a fundamentally different approach from Caterpillar. Where Cat uses a binary trigger (static reading followed by a discrete dynamic reweigh), Komatsu uses continuous statistical refinement.

Continuous Sampling, Not Discrete Stages

PLM IV samples strut pressures at 50 Hz (fifty readings per second). It applies moving average filters and monitors the standard deviation of the readings in real time. There is no single moment where the system “takes the dynamic reading.” Instead, it continuously refines the gross vehicle weight estimate during loaded travel, and locks the payload when the statistical variance falls within a pre-programmed confidence interval.

This is worth understanding clearly because there is a persistent myth in the field about a “third weight” or “third reading” on Komatsu trucks. There is no third discrete measurement. The system is converging on a stable number through continuous sampling, and it locks when it is statistically confident in the result.

Grade Compensation

PLM IV uses dual inclinometers for grade compensation. If the truck is on a slope, the system adjusts the weight calculation for the component of gravity acting along the truck’s longitudinal axis. PLM III used a single inclinometer. The dual setup in PLM IV provides better accuracy on undulating haul roads where the truck may be experiencing both pitch and roll simultaneously.

Automatic Tare Recalibration

One of PLM IV’s practical advantages is automatic tare recalibration after every dump cycle. The system re-establishes the empty weight after each dump to account for carryback (material that sticks to the tray and does not discharge). Over the course of a shift, carryback can accumulate to several tonnes, particularly with wet or sticky materials. By recalibrating the tare each cycle, PLM IV prevents carryback from being counted as payload on the next load.

Storage and Connectivity

PLM IV stores 20,480 cycle records, approximately two years of data at typical cycle rates. That is a fourfold increase over PLM III’s 5,208 records. The connectivity architecture also changed: PLM III used RS232 serial communication, while PLM IV uses Ethernet combined with J1939 CAN bus. The Ethernet connection enables faster data transfer and integration with third-party systems.

PLM IV supports API integration with shovel-side loading software such as Argus (Komatsu/MineWare), enabling bucket-by-bucket payload tracking that reconciles the shovel’s measurement with the truck’s onboard system.

Operator Feedback

Komatsu trucks use a green/amber/red payload indicator lamp system mounted on the truck body, visible to the loading tool operator. Green indicates the truck is within target range, amber indicates approaching the upper limit, and red signals an overload condition. The overload is also flagged as an alarm code in the non-volatile cycle record stored onboard.

Hitachi: EH Series

Hitachi’s approach to the static accuracy problem is primarily mechanical rather than software-driven. Where Cat and Komatsu accept poor static readings and rely on dynamic correction, Hitachi’s suspension design produces measurably better static accuracy from the start.

Trailing Arm Suspension

The EH series uses a trailing arm front suspension design. In a conventional front suspension (as used by Cat and Komatsu), the strut absorbs both vertical loads and lateral bending moments. Lateral forces from uneven loading, truck turning, or the shovel’s bucket approach angle introduce stiction and measurement noise.

Hitachi’s trailing arm absorbs the bending moments through the arm structure itself. The struts receive only pure axial (vertical) force. This eliminates a major source of stiction in the front struts and produces measurably more accurate static readings at the face. The practical difference is notable: Hitachi’s static payload accuracy at the loading point is closer to the dynamic accuracy that Cat and Komatsu only achieve once the truck is moving.

NEOCON Struts

The EH series uses NEOCON struts filled with helium gas and NEOCON-E silicone fluid, rather than the standard nitrogen gas and hydraulic oil used in most other OEM struts. Helium is thermally stable: its pressure-temperature relationship is more predictable and less prone to the expansion-driven drift that affects hydraulic oil. This reduces temperature-related measurement error, particularly on sites with large diurnal temperature swings.

Four independent struts (two front, two rear) are each instrumented, giving the system redundancy and the ability to detect asymmetric loading.

Stabilisation and Pitch Control

Hitachi’s payload stabilisation is movement-based, using the AC-Drive Pitch Control System. The system monitors chassis pitching and vertical acceleration, locking the payload reading when both fall below defined thresholds. It also integrates data from the Slip/Slide Control and Side Skid Control systems, which provide additional telemetry on truck stability and movement dynamics.

Loading Policy

Hitachi enforces a 90-10-0 loading policy:

• 90% of loads must be below 110% of rated capacity
• A maximum of 10% of loads may fall between 110% and 120%
• Zero loads above 120%

Payload data transmits via Hitachi Data Link to the site’s fleet management system.

OEM Comparison

Feature	Caterpillar TPMS	Komatsu PLM IV	Hitachi EH
Suspension media	Nitrogen + hydraulic oil	Nitrogen + hydraulic oil	Helium + NEOCON-E silicone
Front suspension	Conventional	Conventional	Trailing arm
Static accuracy (at face)	±5–10%	±5–10%	Better than conventional (reduced stiction)
Dynamic accuracy	±2–3%	±2–3%	±2–3%
Dynamic trigger	2nd gear OR 0.16 km travel	Continuous 50 Hz sampling, confidence-based lock	Pitch/acceleration below threshold
Grade compensation	Software	Dual inclinometers	Pitch Control System
Tare recalibration	Manual (PAYCAL)	Automatic after every dump	Manual
Overload response	Event Code 2917: 1,750 RPM, 2nd gear lock	Alarm code in cycle record	90-10-0 policy enforcement
Data protocol	VIMS (proprietary)	Ethernet + J1939 CAN bus	Hitachi Data Link
Onboard storage	2,400 cycles	20,480 cycles (~2 years)	OEM-dependent
Calibration tool	VIMSpc (PAYCAL, SPC 729225)	PLM calibration software	OEM calibration procedure

The key differentiator across all three is how they handle the transition from static to dynamic accuracy. Cat uses a discrete binary trigger. Komatsu uses continuous statistical convergence. Hitachi reduces the static accuracy problem at the mechanical level, making the transition less critical.

Calibration and Scale Studies

Payload measurement is only as good as the last calibration. Every system drifts over time: strut seals wear, hydraulic fluid degrades, transducers age, and the truck’s empty weight changes as liners wear and components are replaced.

Weighbridge Procedures

The weighbridge is the reference standard. Scale studies compare the truck’s onboard payload reading against a certified weighbridge measurement across multiple passes and conditions. A proper scale study includes:

• Multiple loaded and empty passes per truck: a single pass is not statistically meaningful
• Varied conditions: different fuel levels (a full fuel tank on an ultra-class truck adds several tonnes), different operators, different material types and moisture contents
• Both directions across the weighbridge to account for any grade effect
• Recording ambient temperature at each pass, because the same truck at the same payload will read differently at 5°C versus 40°C

Minimum frequency is annual. Quarterly is better, and most well-run sites calibrate quarterly or after any significant component change. For Caterpillar trucks specifically, SEHS9411 defines the suspension cylinder specifications and inspection criteria.

Temperature Correction

Temperature correction factors are built into most modern payload systems, but they have limits. The correction algorithms assume a known relationship between fluid temperature and pressure that holds within a certain range. Extreme temperatures, fluid degradation, or a mix of old and new fluid in a strut can all push readings outside the correction model’s assumptions.

In hot climates, the practical impact is a shift in measurement accuracy between the cool of the early morning shift and the heat of the afternoon. Sites that calibrate only in one temperature condition and then operate across a wide temperature range will see systematic drift.

Why Calibration Discipline Degrades

Most sites start with good calibration practices when a payload system is first commissioned. The common pattern is a gradual decline: scale studies become less frequent, trucks that should be recalibrated after component changes are not, and the onboard readings slowly diverge from reality. Nobody notices because the drift is gradual (1% here, 2% there) until a formal audit or a new weighbridge installation reveals that half the fleet is reading 5–8% off.

The fix is governance: scheduled scale studies with accountability, automated exception reporting when strut readings drift from expected values, and recalibration triggers built into the maintenance management system for any event that changes the truck’s empty weight.

FMS Integration and Data Hierarchy

Payload data from the truck is one input in a broader measurement ecosystem. Most mines have multiple payload data sources, and they do not always agree.

The Priority Ranking

The accepted data hierarchy for payload accuracy is:

1. Weighbridge: certified, traceable, highest accuracy (±0.5–1%)
2. Loading tool systems: shovel-mounted systems like Argus (Komatsu/MineWare), ShovelMetrics (Weir), or Liebherr Truck Loading Assistance. These measure each bucket individually, giving the operator a running total before the truck departs
3. Truck onboard systems: Cat TPMS, Komatsu PLM, Hitachi onboard. Dynamic accuracy of ±2–3%

In practice, the truck’s dynamic payload reading is the most commonly used source of truth for production reporting, because every truck has one and it requires no additional infrastructure beyond what the OEM provides. Weighbridges are accurate but most sites only have one or two, creating a bottleneck if every truck must cross them. Shovel-side systems add value by providing payload data before the truck leaves the face, enabling immediate correction of overloads, but they require additional hardware and calibration on the loading tool.

Reconciliation

When shovel-side and truck-side measurements disagree, the reconciliation process matters. A consistent offset (the shovel always reads 3% higher than the truck) usually indicates a calibration issue on one system. Random disagreement suggests different error sources: temperature drift on one, stiction effects on the other, or carryback not being accounted for.

Well-configured FMS platforms reconcile these data sources automatically, flagging discrepancies above a threshold for investigation. The production and operational implications of payload accuracy (including the 10/10/20 overload rule, the 600-metre overweight-hauled metric, and operator accountability frameworks) are covered in Payload Management.

Key Takeaways

• Static readings at the face are unreliable across all OEMs. ±5–10% accuracy due to strut seal stiction. Never use static payload as the basis for production reporting or overload decisions. Wait for the dynamic reading.

• OEMs handle the static-to-dynamic transition differently. Cat uses a binary trigger (2nd gear or 0.16 km), Komatsu uses continuous 50 Hz sampling with statistical convergence, and Hitachi reduces the problem mechanically through trailing arm suspension and NEOCON struts.

• Calibration is the single biggest factor in long-term accuracy. Quarterly scale studies, recalibration after component changes, and automated drift detection will do more for payload accuracy than choosing one OEM over another.

• Temperature affects every strut-based system. Hydraulic oil expands, pressure readings shift, and calibration done in one condition may not hold in another. Track ambient temperature during scale studies and understand your system’s correction model.

• The truck’s dynamic payload is typically the production source of truth, but the full data hierarchy is weighbridge > loading tool > truck system. Use all available sources and investigate when they disagree.