Advanced Application Strategies for Ultrasonic Sensors in Industrial Automation

• Mechanism: Used in Through-Beam configurations (separate Emitter and Receiver). The sensor does not measure distance; it measures the loss of energy.
• Physics: When an object (or a second sheet of material) blocks the path, the signal amplitude drops. This allows for the detection of internal structural changes (like air gaps in double sheets) or linear edge masking, completely independent of surface color or reflectivity.

• Mechanism: The sensor emits a pulse and calculates distance (d ) based on the return time (t ) and speed of sound (c ).
• Physics: Relies on the target having a sufficient Acoustic Impedance Mismatch with the air to reflect energy back to the source. This is the standard mode for ranging and presence detection.

• Mechanism: A sophisticated use of Signal Amplitude Evaluation.
• Physics: Different materials absorb sound energy at different rates. A hard surface (Steel) reflects ~99% of energy, a porous surface (Foam, Wool) absorbs energy. By analyzing the strength of the echo—not just its timing—sensors can distinguish between materials (e.g., confirming a soft foam insert is present inside a hard plastic shell) even if they are at the exact same distance.

Successful integration requires respecting the inherent limitations of piezoelectric transducers:

• The Blind Zone (Dead Band):
  The transducer acts as both speaker and microphone. After emitting a high-energy pulse, the ceramic element mechanically vibrates (“rings”) for a few milliseconds. During this Ringing Time, the sensor is “deaf” to returning echoes.
  • Engineering Rule: Mechanical designs must include mounting stand-offs to ensure the target never enters this zone (typically 0–100mm). If a target breaches the blind zone, the output becomes indeterminate and unreliable.
• Beam Geometry (The Sound Cone):
  Sound propagates in a volumetric cone (typically 6° to 12°), not a laser-like line.
  • Engineering Rule: The detection zone is volumetric. Any machine rails, brackets, or tank walls protruding into this cone will generate False Echoes. Installations require a clear path calculation based on the beam angle and target distance.

• Photoelectric sensors look through clear objects.
• Black rubber or plastic absorbs light, preventing the return signal required for diffuse optical sensors.

• Impedance Detection: The sensor detects the massive difference in Acoustic Impedance (Z) between Air (Z_air ≈ 400) and the Solid Object (Z_solid > 10⁶). Whether the object is clear glass or black rubber, the sound wave bounces off the boundary effectively.
• Retro-reflective Configuration: For irregular shapes (e.g., curved shampoo bottles) that might scatter sound away from the receiver, a Retro-reflective Mode is recommended.
  • Setup: The sensor is taught to recognize a fixed background (e.g., a metal rail).
  • Logic: Any object passing between the sensor and the rail interrupts the signal or changes the time-of-flight. This provides a binary, fail-safe detection regardless of the object’s angle or shape.

• The 90° Rule: The sensor must be aligned strictly perpendicular (90°) to the roll axis. A deviation of just 3° on a smooth foil roll will cause the sound pulse to deflect entirely away from the receiver, resulting in signal loss.
• Analog Integration: Utilizing sensors with 0–10V or 4–20mA analog output allows the PLC to calculate the Roll Inertia (I = m·r²) continuously. This enables dynamic PID tuning, preventing web tears during rapid acceleration or deceleration.

• The Context: Preventing two sheets (metal, paper, wafer) from feeding simultaneously into a press, which could damage tooling.
• Why Capacitive Fails: Capacitive sensors rely on dielectric constants. If paper moisture changes or metal alloy varies, they require constant recalibration.
• The Ultrasonic Physics:
  • Single Sheet: The sound wave hits the sheet, induces vibration, and transmits through to the receiver.
  • Double Sheet: A microscopic layer of air is always trapped between two overlapping sheets.
  • The Mechanism: The thin air gap between two sheets creates a massive Acoustic Impedance Mismatch. This physical phenomenon causes nearly 100% of the ultrasonic energy to reflect or dissipate, preventing transmission to the receiver. A near-zero signal amplitude acts as the deterministic trigger for identifying a double-sheet fault.
  • Result: The receiver sees a near-zero signal. This detection is purely mechanical and works independently of the sheet’s color, print, or magnetism.

• The Context: Aligning transparent films or open-mesh fabrics during winding.
• The Linear Physics: An ultrasonic fork sensor measures the percentage of blockage.
  • If the web covers 50% of the acoustic beam, the output signal drops by exactly 50%.
  • Mesh Advantage: Unlike optical sensors that “jitter” when seeing through holes in a mesh, the wide sound beam integrates the average mass of the material, providing a stable, linear control signal for the edge position.

In the chemical industry, storage tanks often contain strong acids, alkalis, or other highly corrosive liquids. Traditional level measurement technologies face severe limitations:

• Contact Risks: Contact-based devices (such as submersible pressure transmitters, float switches, or capacitance probes) must physically touch the liquid to function.
• High Material Costs: To resist corrosion, contact sensors often require expensive exotic alloys (e.g., Hastelloy, Tantalum) or special coatings. Even with these materials, long-term immersion often leads to sensor degradation, drift, or seal leakage.

• True Non-Contact Operation:
  • The sensor is mounted at the top of the tank and uses the air gap as the transmission medium to measure the distance to the liquid surface.
  • Benefit: This “Zero Contact” design ensures the sensor body remains physically isolated from the corrosive liquid, completely eliminating the risk of chemical erosion on sensitive components.
• Material Durability against Vapors:
  • Modern industrial ultrasonic transducers are typically encapsulated in PVDF (Polyvinylidene Fluoride).
  • Benefit: Even in tanks filled with corrosive vapors or fumes, PVDF provides exceptional chemical resistance, ensuring the sensor remains stable and durable in acidic or alkaline atmospheres.
• Maintenance & Safety:
  • Because the sensor is not inserted into the liquid, installation and maintenance do not require emptying the tank. This significantly reduces operational downtime and minimizes personnel exposure to hazardous chemicals.

Mobile robots need redundant safety systems. LiDAR is excellent for mapping but has blind spots.

• LiDAR Weaknesses: Transparent Glass (transmission), Mirrors (deflection), Black Objects (absorption), and Mesh Fences (beam passes through gaps).

• Volumetric Shield & Safety Compliance: The wide sound cone detects the “solid mass” of mesh fences and glass walls that narrow lasers miss. This detection strategy aligns with ISO 3691-4 standards for industrial mobile robots, providing a certified redundancy layer for personnel safety independent of the primary navigation system.
• Crosstalk Mitigation: When mounting an array of sensors on a bumper, acoustic interference is a major risk.
  • Protocol: Connect the Synchronization Pins (Sync) of all sensors in the array. This forces them to fire and listen simultaneously, effectively treating the array as a single “sonar skin” and preventing Sensor A from picking up Sensor B’s echo.

Since the speed of sound changes with temperature (increasing by ≈ 0.6 m/s per 1°C rise), a simple 10°C shift in ambient temperature can cause a 1.7% measurement error if left uncompensated.

• For General Environments: Always select sensors with Internal Temperature Compensation (built-in NTC thermistors) to automatically adjust for daily fluctuation.
• For Gradient Environments: In scenarios where the sensor body temperature differs from the target area (e.g., a sensor mounted on a cold bracket measuring a hot liquid tank), internal compensation is insufficient. An External Temperature Probe must be installed directly in the measurement zone to provide an accurate reference.

In chemical tanks or food processing lines (as seen in Section III), heavy steam changes air density and absorbs acoustic energy, causing high-frequency signals to “disappear” before returning.

• Selection Rule: Avoid standard 200 kHz sensors in steamy conditions.
• Recommendation: Specify Low-Frequency Sensors (40–80 kHz). Their longer wavelengths offer superior penetration power through vapor and foam, ensuring a stable echo return even in high-humidity environments.

The traditional tuning method—using a screwdriver to turn a potentiometer on the sensor back—is becoming obsolete. The integration of IO-Link communication protocols transforms the sensor’s utility:

• Dynamic Parameterization: In a flexible production line, a machine might handle a small box (Product A) followed by a large crate (Product B). Through IO-Link, the PLC can instantly re-write the sensor’s “Switching Window” parameters on the fly, eliminating downtime for mechanical adjustments.
• Beam Shaping: Advanced sensors now allow software-defined beam widths. An engineer can narrow the beam to penetrate a deep tank or widen it to detect wire mesh, all configured remotely via the HMI.

Ultrasonic sensors are uniquely positioned to self-diagnose environmental health before a failure occurs.

• The “Signal Strength” Metric: Intelligent sensors continuously report the “Echo Amplitude” or “Excess Gain” value.
• The Scenario: In a dusty cement plant, dust slowly accumulates on the sensor face.
  • Old Way: The sensor fails suddenly when the dust blocks the signal completely. Machine stops.
  • New Way: The PLC monitors the signal margin. If the amplitude drops from 100% to 70% over a week, the system triggers a “Maintenance Alert: Clean Sensor 3” before the signal is lost. This is true Predictive Maintenance.

Modern sensors are incorporating stronger onboard processors (Edge Computing) to handle complex acoustic environments.

• Interference Suppression: Algorithms can now “learn” the acoustic signature of a tank’s internal agitator blade. The sensor maps this periodic interference and subtracts it from the signal, allowing it to track the liquid level continuously even when the agitator passes directly through the sound beam.
• Multi-Echo Analysis: Instead of just reacting to the first echo, smart sensors can evaluate multiple return signals to distinguish between a close-range rain droplet (noise) and the actual liquid surface (target) further down, significantly reducing false alarms in outdoor applications.

With the rise of collaborative robots (Cobots) and drone logistics, size and weight are critical.

• MEMS Ultrasound: The development of Micro-Electro-Mechanical Systems (MEMS) based ultrasonic transducers is shrinking the footprint to chip-scale. This allows for high-density arrays to be embedded into robot fingertips for “near-field” tactile sensing, or into drone skins for 360-degree collision avoidance without the weight penalty of traditional ceramic transducers.

A2: You are likely experiencing Side Lobe Interference. The sound cone is hitting the tank walls or internal welds.

• Fix: Use a Stilling Well (as described in Scenario 4) or switch to a sensor with a narrower beam angle. Do not try to “filter out” wall reflections purely with software; the physics must be corrected first.

A3: If the conveyor belt is smooth or has a seam, it might be reflecting the sound away (specular reflection) or creating noise.

• Fix: Angle the sensor slightly (5° – 10°) away from perpendicular relative to the belt surface. This ensures the echo from the empty belt is deflected away (reading “Infinite/No Object”), while the taller product will still reflect sound back to the sensor.