Load and Material-Height Sensing on Haul Trucks, Hoppers, and Loader Buckets

Electrostatic ultrasonic sensing measures irregular material heaps from vibrating mobile equipment — through airborne dust, with the receive sensitivity to hold a dependable overfill threshold on loose bulk material, and with clear limits on temperature and moisture that belong in the design from the start.

What does load and material-height sensing on mobile equipment actually require?

Load sensing on mobile equipment resolves into three distinct tasks, and treating them as one is the most common design error. Continuous height measurement produces a distance to the material surface, from which fill percentage or volume is estimated. Overfill prevention is a threshold decision derived from that measurement, with its own latency and fail-safe requirements. Load-profile measurement characterizes how material is distributed across the bed or bucket. One SensComp electrostatic ultrasonic transducer family serves all three, but the control logic differs for each.

Continuous height measurement is the base signal. The transducer measures distance from a fixed mounting point down to the material surface, and the system subtracts that from the empty-bed baseline to get fill height. Everything downstream depends on this measurement being trustworthy on a surface that is neither flat nor stationary.

Overfill prevention needs a defined trip point, filtering that survives vibration and material motion, hysteresis at the boundary, and a defined behavior when no echo returns at all. A threshold that treats a lost echo as an empty bed will keep loading a truck that is already full.

Load-profile measurement requires more than one sensing point. A single downward-aimed transducer reports height at one location; distribution across the bed, which matters for axle loading and for catching a badly centered load, needs several transducers reporting simultaneously against a common baseline.

Which ultrasonic sensor option fits your machine’s operating environment?

The answer is usually decided by ambient temperature and it affects everything downstream. The bare transducer and the integrated products carry different ratings, and the integrated electronics, not the transducer, set the limit.

Why is measuring a material heap from a moving machine harder than it looks?

Several factors make bulk material height difficult to measure reliably, even with a sensor that performs well on a flat surface in controlled conditions.

• Loose material returns a weak echo. Aggregate, sand, ore, wood chips, and grain absorb and scatter acoustic energy rather than reflecting it cleanly. Material dumped into a bed settles toward its angle of repose, and the pulse reflects off a peak that is small and soft, returning a faint echo. If the sensor cannot detect that return, the system reads the bed as emptier than it is. This is where electrostatic ultrasonic transducers outperform piezoelectric ultrasonic transducers: approximately 40 dB greater receive sensitivity is what recovers the weak echo off the heap peak.
• The surface moves and reshapes during loading. Material arrives in discrete dumps, then slumps and redistributes. Measured height jumps rather than rising smoothly, and a reading taken mid-dump can differ substantially from the settled value seconds later.
• The machine vibrates continuously. Engine, hydraulics, and travel over rough ground all shift the transducer’s instantaneous position and aim. Vibration does not change how the transducer converts sound, but consecutive pulses can sample slightly different points on an irregular surface.
• Dust is constant — but it is an optical problem far more than an acoustic one. Ultrasonic sensing works in dust that blinds optical and laser sensors, because it listens for an echo rather than looking for a reflection. That advantage applies to ultrasonic sensing broadly rather than to any one transduction type. Dense airborne dust does attenuate ultrasound to a degree, and the approximately 40 dB receive sensitivity margin is what preserves usable headroom where a piezoelectric ultrasonic transducer already working near its detection floor would lose the return.
• Temperature changes every distance the sensor reports. The speed of sound in air varies with temperature at approximately 331.3 + 0.606 × T meters per second, with T in degrees Celsius. Across a −20°C to +40°C swing that is roughly a 10% change in reported distance for an identical physical fill height. Compensation is a requirement, and the formula above is all it takes to implement.
• The acoustic environment is hostile. Engines, hydraulic pumps, air brakes, reversing alarms, and material striking a steel bed all radiate broadband energy, some of it ultrasonic. A narrowband receiver has limited means of distinguishing that noise from a valid echo.
• Moisture stops the measurement while it is present. Electrostatic ultrasonic transducers stop working when the membrane is wet and resume once it dries. This is a temporary functional interruption rather than damage, but it is a real operating limit that has to be designed around. See the mounting guidance below.

How do ultrasonic sensors address these challenges?

Electrostatic ultrasonic transducers have approximately 40 dB greater receive sensitivity than piezoelectric ultrasonic transducers, which is what recovers the faint echo off a small, soft heap peak. They hold frequency and gain across a wide operating temperature range, use a low-resonant membrane that settles quickly rather than ringing after each transmit pulse, and respond across a broad band — roughly 20 kHz to 100 kHz — which supports noise rejection on a running machine. Each of those four properties maps onto one of the problems above.

Receive sensitivity recovers the weak echo off a heap peak
Electrostatic ultrasonic transducers have approximately 40 dB greater receive sensitivity than comparable piezoelectric ultrasonic transducers. That receive sensitivity is what allows them to resolve faint echoes from small, soft, irregular material peaks that may fall below a piezoelectric transducer’s detection threshold. A 15° beam angle at −6 dB also helps capture weak, off-axis echoes from an uneven surface. The detection advantage comes from greater sensitivity, not from spatial averaging across the beam footprint.

Thermal stability holds the threshold, within the rated range
The bare 600 Series transducer maintains consistent frequency and gain from −40°C to +85°C, so a threshold characterized on a cold morning still means the same thing on a hot afternoon. Two qualifications matter. The integrated products are rated 0°C to +70°C, and where one is used that is the governing limit. And transducer stability is separate from speed-of-sound compensation, which the control system must still apply using the formula given above.

Fast settling keeps shallow hoppers and nearly full beds measurable
The electrostatic membrane is low-resonant and heavily air-damped, so it settles within tenths of a millisecond rather than ringing like a struck ceramic element. The 6500 Series Ranging Module and Smart Sensor receive path is blanked for 2.38 milliseconds after each transmit to exclude residual ringing. This corresponds to approximately 1.33 ft, while the rated range begins at 1.5 ft. Where a shallow hopper requires closer measurement, the Blanking Inhibit (BINH) input ends blanking early, allowing the sensor to measure down to 0.5 ft—an option made possible by the membrane’s rapid settling. Shortening the blanking period on a sharply resonant ceramic element can instead allow its residual ring-out to be interpreted as a phantom target.

Broadband response supports noise rejection on a running machine
The transducer has a broadband response—roughly 20 kHz to 100 kHz—rather than the sharply tuned response of a resonant ceramic element. The integrated products pair it with digitally controlled gain and a variable-bandwidth amplifier to suppress noise and side-lobe detection. For machines with a particularly hostile acoustic signature, the broadband response also supports stepped multifrequency operation: the system steps through discrete drive frequencies, with 100 kHz as the upper limit, and accepts only echoes returning at the transmitted frequency. This approach is worth considering only where standard filtering has been tried and found insufficient.

Where should the sensor be mounted on a haul truck, hopper, or loader bucket?

Mount the transducer so it looks down at a region representative of the peak of the heap, with enough standoff to clear the blanking distance at maximum fill and enough clearance from sidewalls to avoid multipath returns. On a truck bed or hopper that means a downward-aimed transducer set inboard of the walls; on a loader bucket it means a protected position on the boom or bucket back that keeps the beam on the load through the full curl range.

• Aim at the peak. A heap peak is the best point to aim at, because the peak presents a small area parallel to the transducer face, which returns the strongest available signal, and because sensing angled product can deflect off to the side resulting in no echo return. Aiming at the peak produces a more stable and more useful number.
• Keep the beam footprint inboard of the walls. A 15 degree cone spreads to roughly 2.6 feet across at 10 feet of range. Sidewalls, tailgates, and bucket edges inside that footprint return strong early echoes that will be read as material. Check the footprint at maximum measuring distance, not the nominal one.
• Budget standoff for the blanking distance at maximum fill. With the rated 1.5 ft minimum, the material surface must never come closer than that to the transducer face — 0.5 ft if you are using the BINH input. On shallow hoppers this constrains mounting height directly, and it is worth calculating before the bracket is fabricated.
• Design the mounting to shed water. Because the transducer stops working while the membrane is wet, aim it downward so gravity clears droplets, recess it behind a drip lip or shroud that blocks direct spray and rain, and avoid positions where runoff from structure above crosses the face. An open-cell acoustical foam filter ahead of the face limits dust ingress and sheds droplets rather than holding a film across the aperture; customers source their own. None of this makes the sensor waterproof — it shortens the interruption and makes it predictable, which is why the lost-echo fail-safe below matters.
• Protect the face without sealing it acoustically. A recessed mount shields the transducer from direct impact and falling material. Environmental Grade construction adds a 304 stainless-steel housing with an optional parylene conformal coating, with minimal dampening of acoustic sensitivity. Enclosures and brackets are customer-built; SensComp supplies mounting plates and clips.
• Mount to structure that does not flex. A long cantilevered bracket amplifies machine vibration at the transducer and adds aim wander that averaging then has to remove. Short, stiff mounts tied into primary structure eliminate a class of noise.

How do you power and interface the sensor on a vehicle?

Two integration realities need to be in the design from the beginning: the transmit current burst, and the absence of a fieldbus output. Both are easier to accommodate at schematic stage than after a harness exists.

Budget the rail for a 2 A transmit burst
The Smart Sensor operates from a regulated 6 to 24 VDC source and draws roughly 55 mA nominally between measurements, with supply current reaching 2 A during the transmit period. SensComp specifies a supply with 100 mA continuous capacity and short-burst capability up to 2 A. In a machine, the practical implication is to provide local bulk capacitance at the sensor rather than simply increasing the capacity of the upstream regulator. Because the burst is brief and occurs no more than once every 80 milliseconds, a local reservoir can supply the transient while the power rail supplies the average load. Size the capacitance and wiring for the transient, and keep the sensor’s supply return separate from high-current chassis paths to prevent switching noise from coupling into the echo signal.

Plan the bridge to J1939, because there is no fieldbus output
SensComp does not offer CAN bus, J1939, IO-Link, or Modbus output. The available interfaces are TTL-compatible open-collector logic—an echo output whose timing relative to the initiate signal encodes distance—and a programmable analog output on the Mini-A Series. On mobile equipment, the typical integration uses a small microcontroller or an existing machine ECU with a spare timer input. The controller initiates the measurement, times the echo, applies temperature compensation and filtering, and publishes fill height to the vehicle network as a J1939 parameter. This requires a modest amount of firmware, and the same layer can also average measurements across pulses and implement the fail-safe logic described below. Budget for it explicitly rather than discovering it late; this is a small embedded-software task, not simply a signal-conditioning problem..

Gain is set by a trim potentiometer
Receiver gain on the integrated products is set with an onboard potentiometer, calibrated by placing a target at the maximum distance you need to detect, turning the gain fully counter-clockwise, then advancing it until detection occurs plus about a sixteenth of a turn. SensComp’s guidance is to use the minimum gain that gives reliable detection, since excess gain produces false targets. For volume production, treat this as a line calibration step with a fixture and a recorded setting, and specify a thread-locking compound or a potting dab on the adjuster: a trim pot on a machine that vibrates for a living is worth securing once it is set.

How do you implement closed-loop overfill prevention?

Closed-loop overfill prevention converts continuous height measurements into a stop command, with most of the engineering effort focused on filtering and fail-safe behavior rather than the measurement itself.

Baseline the empty bed from the installed position
Measure and store the distance from the mounted transducer to the empty floor of the bed, hopper, or bucket. Every subsequent fill height derives from this baseline, so capture it after final mounting rather than estimating from drawings.

Define the threshold as a measured distance, not a fill percentage
The trip point is a distance from the transducer face: mounting height minus target fill height. Working in measured distance avoids compounding baseline error into a percentage. Confirm that the resulting trip distance is above the blanking limit with adequate margin—1.5 ft at the rated range, or 0.5 ft when the BINH input is used.

Average across several pulses before acting on a reading
Vibration and moving material can make any single reading noisy. A rolling median across several consecutive pulses rejects outliers caused by a passing dust plume or a transient slump. A median is more robust than a mean in this application because a single spurious return does not pull the result away from the true measurement.

Apply hysteresis at the boundary
Use separate trip and reset distances so the output does not chatter as material settles across the threshold. The gap between them should exceed the residual noise remaining after averaging.

Match the sampling rate to the loading rate
The Smart Sensor runs at 5 Hz on its internal oscillator, and accepts external triggering subject to a minimum recycle period of 80 milliseconds — about 12.5 Hz. Most of that 80 milliseconds is time of flight rather than sensor recovery: an echo from the far end of the 35-foot range takes roughly 62 milliseconds by itself. Calculate the latency budget from the fill rate, counting the averaging window and the mechanical response of whatever the system commands, and remember that a four-sample median at 12.5 Hz already costs about a third of a second.

Define fail-safe behavior when no echo returns
A lost echo is not an empty bed. A fouled or wet transducer face, a surface outside the measuring range, or an obstruction all produce the same silence, and a system that interprets silence as a low reading will keep loading. Set a timeout, latch a fault, and require an explicit reset. Given that the transducer stops working while wet, this logic is what converts a rain shower from a hazard into a visible, handled fault condition.

Do not rely on this sensor alone where overfill can injure someone
SensComp products are not authorized for safety-critical applications where failure would reasonably be expected to cause severe injury. Where overfill creates a hazard to people rather than a productivity or compliance problem, the ultrasonic measurement belongs in the control loop as the primary means of automation, backed by an independent protective layer — a mechanical limit, a load-cell cross-check, or an operator interlock — selected against the applicable functional safety requirements.

How do electrostatic ultrasonic, piezoelectric ultrasonic, and radar sensors compare?

The short version: radar is the stronger choice where the sensing face cannot be kept dry or where range requirements exceed what ultrasonic supports, and it carries higher unit cost plus RF regulatory and EMC obligations. Electrostatic ultrasonic is the stronger choice on cost-sensitive equipment measuring bulk solids at short to medium range, particularly low-dielectric materials that radar struggles to reflect from cleanly. Many machines end up with ultrasonic on the loading path and a simpler technology elsewhere, rather than one sensor everywhere.

What remains your design responsibility?

SensComp supplies transducers, modules, and integrated sensors rather than turnkey systems. The items below are the integration work, listed with enough specificity to scope it:

• Temperature compensation. Implement c = 331.3 + 0.606 × T (m/s, T in °C) in the control system, fed by a temperature measurement co-located with the transducer rather than an engine or cab sensor.
• Enclosures and brackets. Customer-built. SensComp supplies mounting plates and clips. Design for a downward aim, a drip lip, and stiff attachment to primary structure.
• Dust filter. An open-cell acoustical foam filter ahead of the transducer face is a recommendation, and customers source their own. Specify open-cell rather than closed-cell so the aperture stays acoustically transparent.
• Ingress protection. No SensComp product carries an IP rating. This is inherent to the technology as much as the product: an ultrasonic transducer has to move air, so the acoustic aperture cannot be fully sealed. Environmental Grade construction provides a 304 stainless-steel housing and an optional parylene coating; where a rated enclosure is required, it is built around the sensor with an acoustically open path to the target.
• Vehicle network interface. A microcontroller or ECU input to time the echo output and publish to J1939. Roughly the same firmware layer that performs averaging and fail-safe logic.
• Power conditioning. Local bulk capacitance sized for the 2 A transmit burst on a rail specified for 100 mA continuous.
• Gain calibration in production. A fixture, a recorded setting, and a means of securing the adjuster against vibration.
• Independent protection where overfill is a safety function. Selected against the applicable functional safety requirements, not against this sensor’s specification.

Frequently Asked Questions

Building load or material-height sensing into mobile equipment?

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