I. Introduction
Ultraschall-Sensoren are widely used in industrial automation for distance measurement, level detection, material recognition, and obstacle avoidance. Although they all rely on the same fundamental principle—using high frequency sound waves to detect objects—their designs vary greatly depending on application requirements.
From a selection point of view, the challenge is not simply:
“Which ultrasonic sensor is the best?”
but rather:
“Which ultrasonic sensor is the best match for this specific application?”
- Operating frequency (short range / high frequency vs. long range / low frequency)
- Output type (switching, analog, digital, or configurable multi output)
- Detection mode (diffuse reflection, through beam, or material sensing)
- Application scenario (level measurement, proximity, web guiding, AGV obstacle avoidance, material classification, etc.)
- Dimension 1: Operating Frequency
- Dimension 2: Output Signal Type
- Dimension 3: Detection Mode
- Dimension 4: Application Scenario
Within this framework, we will also refer to typical ultrasonic transducer types and industrial sensor designs, highlighting how choices at the transducer level (frequency, beam characteristics) translate into system level performance and trade offs.
II. Dimension 1: By Operating Frequency
When comparing different types of ultrasonic sensor, operating frequency is one of the core parameters that defines performance.
From the standpoint of Ultrasonic Transducer types, most airborne industrial sensors use either:
a low to mid frequency band (typically around 40–120 kHz), or a high frequency band (typically around 180–400 kHz, with 200 kHz, 300 kHz, and 400 kHz as key operating points).
These bands behave very differently in terms of range, resolution, beam width, and response time.
1. Standard Low Frequency: 40 kHz – 120 kHz
1.1 Physical characteristics
- Longer wavelength
A 40 kHz wave in air has a relatively long wavelength, giving the sound field strong diffraction and a comparatively wide beam. - Lower attenuation in air
Acoustic energy at roughly 40–80 kHz is absorbed less by air than higher frequencies, which supports longer measuring distances. - Good penetration
The wave can “see through” dust, mist, and light smoke better than optical systems, and is less disturbed by small turbulence.
From the perspective of ultrasonic sensor transducer design, this band is ideal for general purpose, long range and robust applications where absolute range and stability are more important than extreme precision.
1.2 Typical transducer implementations
- Open type 40 kHz transducers
- Used in many classic distance sensors and presence detectors.
- Suitable for clean or mildly dusty environments.
- Sealed / encapsulated 40–120 kHz transducers
- Used where spray, oil, or contamination is expected.
- Common in tank environments, food & beverage, or outdoor use.
- In many industrial level sensors, frequencies such as 65 kHz, 75 kHz, or 112 kHz are used to reach measuring distances up to several meters.
These acoustic cores are typically built into:
- 3–6 m distance and level sensors for larger tanks, silos, bins, and long conveyor sections, often using 65–112 kHz transducers (e.g. 3 m at 112 kHz, 4 m at 75 kHz, 6 m at 65 kHz).
- Long range obstacle and presence detection modules, including AGV obstacle avoidance sensors around 58 kHz, where robust detection over several meters is required.
In some designs, one element alternates between transmit and receive roles (a transceiver); in others, dedicated transmit and receive elements are used to improve signal to noise ratio. The transmitting element is sometimes specifically called the ultrasonic transducer transmitter, especially in through beam systems.
1.3 Advantages
- Long measuring range
Ranges up to 6 m and beyond are achievable with appropriate targets and transducer design. - Wide coverage
The wider beam is useful when target position is not fixed (e.g. bulk solids or irregular objects). - Tolerance to misalignment
Installation is comparatively forgiving; small angular errors are often acceptable. - Cost effective
Mature manufacturing and high volumes help control overall cost, similar in spirit to how designers compare ultrasound probe cost or ultrasound probe price when choosing between different medical ultrasound probe.
1.4 Limitations
- Moderate resolution
The longer wavelength limits how finely distance can be resolved, especially at very short range. - Larger blind zone
Ring down after excitation can produce a relatively large minimum measuring distance. - Not ideal for very thin or closely spaced targets
Distinguishing thin sheets, small air gaps, or minute positional changes is challenging with low frequency waves.
Whenever specifications mention the need to detect thin films, individual sheets, very small displacement changes, or short range millimetre accuracy, it is usually a sign that higher frequency ultrasonic probes should be considered.
2. Precision High Frequency: 160 kHz – 400 kHz
High frequency ultrasonic transducers in the roughly 160–400 kHz range form a distinct family of ultrasonic transducer types. They are optimized for short range, high precision tasks rather than long range coverage.
Typical operating points in this band include 160 kHz, 200 kHz, 300 kHz, and 400 kHz, which are used in short range level sensors, proximity switches, edge correction detection web guiding, and material sensing devices.
2.1 Physical characteristics
- Much shorter wavelength
Enables finer spatial resolution and more precise time of flight measurement. - Narrower beam angle
The sound field is more tightly focused, improving directionality and reducing interference from off axis objects. - Faster response
Shorter acoustic cycles and reduced ringing make high update rates possible.
These are the same physical reasons why, in medical imaging, a linear ultrasound probe is chosen for high resolution imaging of shallow structures, while lower frequencies are used for deeper penetration. Industrial sensing uses simpler single element heads instead of complex imaging arrays, but the frequency trade offs are very similar.
2.2 Why 200 kHz?
Around 200 kHz (including nearby values such as 160 kHz), high precision distance and presence sensing in air becomes very attractive, while attenuation and robustness are still manageable. In this band, both rectangular ultrasonic web edge guiding transducers around 160 kHz and round 200 kHz ultrasonic edge guiding transducers are commonly used in web guiding and edge correction sensors.
- Millimetre level accuracy at short range
For distances below about 1 m, 160–200 kHz offers significantly better resolution than 40 kHz, making it suitable for precision distance measurement and proximity switching. In web guiding applications, a 160 kHz rectangular edge detection ultrasonic transducer or a 200 kHz round edge detection ultrasonic transducer can resolve small lateral movements of the web edge or strip, supporting precise guiding control. - High update rate
Fast acoustic cycles allow a high measurement repetition rate, important in dynamic processes or moving machinery. This is especially valuable in edge guiding systems, where the ultrasonic web guiding sensor must update the edge position quickly to keep the web centred. - Balanced trade off
Compared with even higher frequencies such as 300–400 kHz, 200 kHz experiences lower air attenuation, which can slightly extend usable distance or improve margin in less than ideal conditions. For edge detection and web guiding, this balance helps both the 160 kHz rectangular transducer and the 200 kHz round transducer maintain strong, stable echoes even in the presence of air currents, dust, or vapours near the web.
- Short range level and distance sensors For example: 0.35 m sensors using around 200 kHz transducers,0.5 m sensors using 160–200 kHz transducers, and 1 m sensors using 200 kHz transducers in compact tanks or process equipment.
- Short range proximity switches (selected variants)
For example, 0.25 m ultrasonic proximity switches can use 200 kHz transducers where exact approach distance matters and metal, plastic, or other materials must be detected consistently. - High precision position measurements
In positioning devices or inspection setups where millimetre changes are significant. - Web edge detection and guiding
In web guiding and edge correction systems for films, paper, foil, non wovens, or battery electrodes, 160 kHz rectangular and 200 kHz round ultrasonic edge detection transducers are used as the sensing heads in ultrasonic web guiding sensors. The rectangular 160 kHz design can help shape the sound field for line like edge coverage, while the 200 kHz round design provides a compact, symmetric beam for standard edge guiding installations. - Wind speed and direction instruments
Many ultrasonic anemometers adopt around 200 kHz Ultrasonic Transducer Elements to measure time of flight along multiple paths. In such cases, each acoustic path is formed by a pair of carefully matched elements, conceptually similar to an ultrasonic flow meter transducer pair used for gas or liquid flow.
Image source：CIAO
2.3 Why 300 kHz?
At around 300 kHz, the focus shifts from “general precision” to extreme sensitivity to small thickness or gap changes and material properties.
- Detection of very thin materials
The shorter wavelength allows the acoustic system to resolve small changes in sheet thickness or the presence of a tiny air gap. - High sensitivity to acoustic impedance changes
Small differences in material or layering cause measurable changes in the transmitted or reflected signal. - Very narrow beam and localized interaction
The sound field interacts with a tightly defined spot, which is ideal for distinguishing individual layers or edges.
Easily identify paper, film, silicon wafers, sticky tape,lithium battery sheets, and PCBs
- Double sheet detection (through beam / projective mode)
A high frequency transmitter and receiver are placed opposite each other. The received signal differs measurably between:
- no sheet
- single sheet
- double sheet
- Material recognition and sorting
In material sensing mode, 300 kHz can distinguish materials by how they absorb or reflect sound—useful for differentiating paper, plastic, metal, or composite stacks. This is the operating range typically used in material detection sensors. - High precision proximity switching (selected versions)
For example, 0.5 m ultrasonic proximity switches can use 300 kHz transducers to achieve a very small blind zone and a narrow, well defined detection field.
These tasks illustrate why, from among many available transducer types in ultrasound, high frequency ultrasonic sensor transducer designs are selected whenever thin, fast moving, or layered materials must be controlled, or when short range, high precision presence detection is required.
3. Summary: Choosing Between Low and High Frequency
- Use low to mid frequency (around 40–120 kHz) when:
- You need longer range (up to several meters in air, e.g. 3–6 m level measurement).
- Targets are relatively large or irregular (e.g. bulk solids, pallets, large tanks).
- Installation must be forgiving, with wider beam coverage.
- A cost effective, robust solution is more important than extreme precision.
- Typical examples include 3–6 m level sensors (65–112 kHz) and AGV obstacle avoidance sensors (58 kHz).
- Use high frequency (around 180–400 kHz, typically 200–300 kHz) when:
- You require short range, high precision measurements, often within 0.15–1 m.
- You must detect thin sheets, small gaps, or subtle material differences (double sheet detection, edge guiding, material sensing).
- Beam control and narrow sound fields are needed due to tight spaces or complex mechanics.
- Processes demand fast update rates and quick response.
- Typical examples include:
- Short range level sensors (0.15–1 m) using 200–400 kHz transducers (e.g. 0.15 m at 400 kHz, 0.35–1 m at 200 kHz),
- Ultrasonic proximity switches (0.15–0.5 m) using 200–400 kHz (e.g. 0.15 m at 400 kHz, 0.25 m at 200 kHz, 0.5 m at 300 kHz), and
- 300 kHz material sensing and double sheet / edge detection sensors.
In real projects, the operating frequency is usually selected first, and then combined with a suitable detection mode and output configuration.
III. Dimension 2: By Output Signal
If operating frequency determines what an ultrasonic sensor can “see”, then the output signal determines how easily it can talk to your system. In practice, many selection problems arise not from the sensing principle, but from mismatched outputs: the sensor delivers one type of signal, while the PLC or controller expects another.
- 1. Switching output (NPN / PNP)
- 2. Analog output (4–20 mA / 0–10 V)
- 3. Digital output (RS485 / TTL level serial)
- 4. All in one, multi output integrated designs
These categories apply across many types of ultrasonic sensor, regardless of whether the internal ultrasonic transducer is low or high frequency.
1. Switching Output: NPN / PNP
1.1 Function
A switching output turns the sensor into a binary detector: it reports whether a target is present within a defined window or threshold. In this mode, the sensor internally measures the distance but only outputs an ON/OFF signal.
- NPN output: the sensor pulls the output line to ground when active (sinking).
- PNP output: the sensor drives the output line to the positive supply when active (sourcing).
Both behave like digital inputs to PLCs or microcontrollers, and are widely used in simple automation tasks.
1.2 Typical use cases
- Position and presence detection
- Detecting whether an object has reached a reference point.
- Checking if a pallet is in place, or if a box has arrived at a station.
- Counting and throughput monitoring
- Counting items on a conveyor, bottles entering a filler, or parts passing a quality gate.
- Alarm or limit functions
- Triggering alarms when a level exceeds (or falls below) a preset threshold.
In many plants, this is the most familiar way to use an ultrasonic sensor, as it directly replaces a mechanical limit switch or a photoelectric sensor.
1.3 Selection notes
- Check PLC input type
Choose NPN or PNP according to the existing control system standard. - Consider hysteresis and window modes
Some sensors allow separate “on” and “off” points or window detection, improving stability and avoiding chatter. - Think beyond distance
Even a high frequency sensor used for double sheet detection can provide switching outputs (e.g. “double sheet present / not present”), despite being based on a precision ultrasonic sensor transducer.
2. Analog Output: 4–20 mA / 0–10 V
2.1 Function
- 4–20 mA current output
- Industry standard for robust transmission over longer cable runs.
- Less sensitive to voltage drop and electrical noise.
- 0–10 V voltage output
- Simple to interface with many PLC and DAQ analog inputs.
- Better for shorter cable lengths and low noise environments.
Internally, the sensor converts time of flight measurements from its ultrasonic transducer into a scaled analog value over a specified range.
2.2 Typical use cases
- Continuous level monitoring
- Tanks with liquids or granular materials, where the control system needs the actual level (not just a high/low alarm).
- Distance based process control
- Maintaining a specific gap between a tool and a surface.
- Adjusting a mechanism based on measured distance or thickness.
- Tension and position control
- In web handling or roll to roll processes, where web loop position or diameter must be kept within a set range.
Image source：Pepperl+Fuchs
2.3 Selection notes
- Match the measuring range to process needs
Avoid choosing a sensor with a very large range if you only use a small portion—effective resolution will suffer. - Decide between current and voltage
- Use 4–20 mA where EMC and cable length are concerns.
- Use 0–10 V where wiring is short and simple, and the controller is nearby.
- Consider response time and filtering
Analog outputs can be filtered or averaged—verify that the update rate matches process dynamics.
Analog outputs are relevant for both low frequency level sensors and high frequency short range devices, especially where a fine, continuous measurement is needed rather than a simple pass/fail signal.
3. Digital Output: RS485 / TTL
3.1 Function
- RS485
- Differential, robust, and noise resistant.
- Supports multidrop networks and longer cable distances.
- Often used with MODBUS or proprietary serial protocols.
- Logic level serial suitable for direct connection to microcontrollers, embedded boards, or custom electronics.
- Typically used over short distances inside devices or panels.
Here, the sensor’s internal electronics handle timing, conditioning, and conversion, and send a digital representation of the measurement, along with optional diagnostic data.
3.2 Typical use cases
- Integration into smart devices and robots
- Service robots, AGVs, and specialized machinery where a microcontroller manages multiple ultrasonic probes and other sensors.
- Connection to industrial networks
- Sensors forming part of a distributed monitoring system, e.g. multiple tank levels or distances connected over RS485.
- Custom instrumentation
- R&D setups, test stands, or instruments where engineers want full access to time stamped measurements and potentially raw data.
In this context, engineers often view ultrasonic heads as building blocks—similar to choosing among different ultrasound transducers or ultrasound probe type options—and embed them into larger smart systems.
3.3 Selection notes
- Check protocol and addressing
Ensure the sensor’s digital protocol is supported by the PLC, IPC, or embedded controller. - Plan for cable length and noise
RS485 is suitable for longer and more disturbed environments; TTL is best inside compact enclosures. - Look at diagnostic features
Some digital sensors offer temperature compensation, signal quality indicators, or error codes beyond raw distance.
Digital outputs are especially attractive where long term flexibility matters: firmware can be updated, multiple sensors can share a bus, and more complex logic can be handled in software rather than hard wired.
4. All in One Integrated Output
4.1 Concept
- Switching outputs (NPN / PNP)
- Analog outputs (4–20 mA and/or 0–10 V)
- Digital serial outputs (e.g. TTL level)
In this architecture, the ultrasonic transducer and signal processing hardware remain the same. What changes is the firmware and configuration, which define how the processed measurement is presented at the output.
Using a PC based configuration tool or serial port software, the user can update the sensor program and switch the device between different output modes (for example, from switching output to analog output, or to digital serial output) without replacing the physical sensor.
- The all in one series does not drive all output types simultaneously in parallel.
- Instead, it provides a flexible platform where the active output behavior can be selected via the serial interface according to the actual control system requirements.
In addition to these firmware configurable models, some products offer dual output hardware, where two specific outputs can operate at the same time (for example, switching + analog, or switching + TTL), depending on the integrated electronics. Because the hardware is fixed, each dual output variant supports only its designated two signal types, although the exact hardware combination can be customized before delivery.
4.2 Benefits
- SKU reduction and spare parts management
A single sensor hardware platform can be configured for switching, analog, or digital output as needed. This reduces the number of different part numbers that must be stocked and simplifies logistics and maintenance planning. - On site flexibility and late binding
During commissioning or later upgrades, engineers can adjust the output type to match the PLC or controller actually used on site—simply by changing the configuration via serial software, instead of physically replacing the sensor. - Lifecycle adaptability
If a control system is modernized (for example, moving from purely discrete inputs to analog or digital communication), it is possible to re configure existing ultrasonic sensors to a new output mode, extending their useful life.
For OEMs and system integrators, this concept is somewhat analogous to developing around a common ultrasonic probe platform and then mapping it, via firmware and configuration, to different application needs—rather than managing many separate fixed output variants with different ultrasound probe price points and part numbers.
4.3 When to consider firmware configurable multi output sensors
- Machines targeted at multiple markets or PLC ecosystems
The same mechanical and electrical sensor can be shipped with different firmware configurations, matching different brands or generations of controllers in various regions. - Plants focused on maintenance efficiency
Maintenance teams can keep one spare sensor type and configure its output mode as needed, reducing both inventory and downtime. - Projects with uncertain or evolving requirements
When it is not yet clear whether the final system will rely primarily on switching, analog, or digital signals—or when future networking and data acquisition are anticipated—a firmware configurable, multi output ultrasonic sensor provides useful headroom.
5. Summary: Matching Output to the Application
- Choose NPN / PNP switching outputs for simple presence, counting, or limit detection.
- Choose analog 4–20 mA / 0–10 V when a continuous distance or level signal is needed for control loops.
- Choose digital RS485 or TTL when smart devices, networking, diagnostics, or deep integration with embedded systems are required.
- Consider all in one, multi output designs when you need flexibility across different controllers, want to reduce SKUs, or expect system upgrades during the equipment’s lifetime.
IV. Dimension 3: By Detection Mode
Once the operating frequency and output signal are decided, the next key question is: how does the ultrasonic sensor interact with the target? This is defined by the detection mode. Even with the same frequency and similar ultrasonic transducer hardware, different detection modes can lead to very different real world performance and application ranges.
For airborne industrial ultrasonic sensor systems, detection modes are commonly divided into three groups:
- 1. Diffuse reflection mode (distance / level sensing)
- 2. Through beam / projective mode
- 3. Material sensing and special reflection modes
These modes all rely on the same basic physics of ultrasound transducers, but they differ in sound path geometry and in how the received signal is evaluated.
1. Diffuse Reflection Mode: Distance / Level Sensing
Diffuse reflection is the “classic” mode for many ultrasonic sensors used in distance and level / material level applications.
1.1 Principle
- The sensor and target are on the same side.
- The built in ultrasonic transducer emits a sound pulse into free air.
- The pulse is reflected from the object surface and returns to the same sensor.
- The electronics measure the time of flight (TOF) between transmission and reception and convert it into distance, using the speed of sound in air.
Sound path:
Sensor → Target → Sensor
- Very short ranges and very small blind zones
For example a few centimeters up to a few tens of centimeters. Sensors often use higher ultrasonic frequencies in the hundreds of kilohertz (such as 300–400 kHz), in order to achieve fine resolution and a narrow beam. In typical product families, 0.15 m class sensors for compact level/proximity tasks use around 400 kHz transducers. - Short to mid range level and distance measurements (roughly 0.3–2 m)
Frequencies in the mid to high frequency range (around 150–250 kHz) are common. These offer a good compromise between beam angle, measuring range, and accuracy. Practical examples include 0.35–1 m sensors using around 180–200 kHz, and 2 m sensors around 180 kHz. - Longer ranges (for example 2–6 m tank or silo level measurement, or long range distance detection)
Sensors typically use lower ultrasonic frequencies in the tens of kilohertz to low hundreds of kilohertz range (around 60–120 kHz), which propagate farther in air and are less attenuated. Typical values include 3 m at 112 kHz, 4 m at 75 kHz, and 6 m at 65 kHz.
- Shorter range / higher resolution → higher frequency
- Longer range / larger coverage → lower frequency
This diffuse reflection operating mode is independent of the output type (switching, analog, or digital). The same principle underlies distance sensors, level / material level sensors, and many ultrasonic proximity switches。
1.2 Typical applications
Because the same basic diffuse reflection mode is used, many distance measurement sensors can also be used for level / material level detection simply by mounting them at the top of a tank, bin, or process space.
- Tank and silo level / material level measurement
- Measuring liquids (water, chemicals, oils) and bulk solids (grains, plastic pellets, powders).
- Short, compact tanks and process vessels (sub meter to a few meters) often use mid to high frequency sensors to minimize blind zone and improve resolution (for example 0.35–1 m using around 180–200 kHz).
- Taller tanks or silos with several meters of measuring range typically use lower frequency sensors (for example 3–6 m using 65–112 kHz), providing a longer working distance and more robust echo in dusty or vapor laden atmospheres.
- General distance and clearance measurement
- Detecting the distance to pallets, walls, machine parts, or fixtures.
- Measuring approach distance or safety clearance in handling and positioning systems.
- Presence / absence detection
- Detecting boxes, trays, or pallets on conveyors.
- Monitoring whether a loading bay, buffer position, or workstation is occupied.
- Ultrasonic proximity switching
- Short range presence and approach detection using the same diffuse reflection principle, often with higher frequencies for compact sensing distances (for example 0.15–0.5 m proximity switches using 200–400 kHz).
- AGV and robot obstacle detection
- Forward looking obstacle sensing for AGVs and mobile robots in warehouses and factories, where relatively low frequency sensors (for example tens of kilohertz, typically around 58 kHz) are used to achieve several meters of coverage with a suitably wide beam.
Transparent materials, plastic, film, metal, and liquid glass can all be detected without being affected by the material, making it a versatile device.
1.3 Advantages
- One sided installation
Only one sensor is required; easy to integrate and retrofit on existing machines or tanks. - Insensitive to color and transparency
Works reliably with dark, shiny, or transparent materials where optical sensors may fail. - Wide selection of standard products
A large portion of catalog ultrasonic sensors for industrial automation are diffuse reflection devices, covering both long range level applications and short range precision distance tasks.
1.4 Limitations
- Dependent on surface reflectivity
Very soft, highly absorbing, or strongly textured surfaces may produce weak or unstable echoes. - Beam shape and blind zone must be considered
The near field blind zone and beam spread must be matched to tank geometry, minimum measuring distance, and target size. - Not ideal for very thin layers or multi layer distinction
The sensor primarily detects the first surface; fine internal structures or very thin gaps behind that surface are not easily separated.
When the core requirement is “measure the distance to the nearest surface”—for example, level, material level, clearance, or approach distance—diffuse reflection mode with a suitable frequency and beam pattern is usually the most straightforward and widely applicable choice.
2. Through Beam / Projective Mode
Through beam (or projective) mode uses a separate transmitter and receiver pair. The key question in this mode is not “how far is the target?”, but rather “what is inside the sound path between transmitter and receiver?”
2.1 Principle
- One device (or channel) functions as a dedicated ultrasonic transmitter.
- A second device functions as the receiver.
- Transmitter and receiver are mounted facing each other, forming a fixed sound path.
- The presence, thickness, or position of material within this path changes the received signal (typically the amplitude or energy; sometimes also phase or timing).
Sound path:
Transmitter → Material → Receiver
Through beam systems often use higher ultrasonic frequencies (on the order of a few hundred kilohertz) to obtain narrow beams and high sensitivity to edges and small thickness changes。Typical double sheet detection and web edge guiding products fall into this category.
2.2 Typical applications
- Double sheet detection (through beam)
- Distinguishing between “no sheet / single sheet / double sheet” in feeders and stackers.
- Widely used in printing, packaging, metal stamping, and battery electrode sheet handling.
- Typically uses 200–300 kHz high frequency through beam transducers, with 300 kHz especially suitable for very thin electrode plates or fine paper.
- Edge detection and web guiding (projective mode)
- A tightly focused ultrasonic beam is partially covered by the web edge.
- Small lateral movements of the web cause reproducible changes in received signal level, enabling accurate edge guiding and tracking.
- Small part and slot detection
- Detecting small components passing through a chute or narrow channel.
- Verifying that a particular position or slot is correctly occupied.
2.3 Advantages
- High sensitivity to small changes in coverage
Very effective for thin sheets, small gaps, and precise edge positions. - Independent of background distance
The receiver mainly responds to the direct sound path; distant walls or machine parts have little influence. - Clear interpretation
Changes in transmitted acoustic energy correspond directly to changes in material inside the beam.
2.4 Limitations
- Requires access to both sides
The process path must allow mounting and alignment of both the transmitter and the receiver. - Alignment is critical
Misalignment or mechanical drift can reduce signal strength and cause instability. - Less mechanically flexible
Later changes such as adding guards, shields, or brackets must be designed not to obstruct the sound path.
When the task is to detect thin, moving, or layered materials, or to track an edge with high precision, through beam / projective mode is usually superior to simple diffuse reflection and is often implemented with specially designed high frequency ultrasonic transducers.
3. Material Sensing & Special Reflection Modes
Beyond “distance” (TOF) and “blocking” (through beam), there is a third class of applications: material sensing, where the goal is to infer material type or structure from how it reflects and attenuates sound, rather than just where it is.
3.1 Material sensing based on reflection
In many material sensing ultrasonic systems, the underlying geometry is still reflection mode: the sensor and object are on the same side, and the transducer sends a pulse and listens for the echo.
- In distance sensing, the main variable is time of flight.
- In material sensing, the main variables are echo energy, attenuation, and the amplitude / time pattern of the returning signal.
Different materials and surface structures have different:
- acoustic impedance
- absorption / damping characteristics
- surface texture / roughness
- Overall reflected energy level (echo strength)
- Attenuation relative to a reference pulse
- The echo envelope or amplitude distribution over time
By measuring and comparing these parameters, a sensor can classify or distinguish materials, not just measure distance. For this purpose, relatively high ultrasonic frequencies (typically in the hundreds of kilohertz, such as 300 kHz) are common, because they are more sensitive to surface structure and near surface absorption。Typical material detection sensors belong to this class.
- Material type identification
- Distinguishing between stone (e.g. marble), wood, carpets, foams, and other materials based on their acoustic response.
- Useful for sorting, verification, or quality inspection.
- Layer or coating verification
- Detecting whether a particular coating, lining, or backing layer is present, by comparing reflection levels and attenuation patterns.
- Extended double sheet / stack analysis
- Differentiating between different stacking or lamination structures via their reflection / attenuation curves.
These applications often use high frequency probes with tailored beam patterns and dedicated signal processing algorithms. The core physical principle remains reflection, but with more emphasis on energy and attenuation than on timing alone.
3.2 Special sound path arrangements
To accommodate real machine layouts, some ultrasonic sensors use special sound path arrangements achieved primarily through mechanical design of the probe and housing, while the underlying sensing principle (diffuse reflection, through beam, or material sensing) stays the same.
- Angle adjustable probe heads
- The ultrasonic transducer is mounted in a head that can be rotated relative to the sensor body.
- This allows the effective sensing direction to be adjusted on site to match the actual installation angle, without changing the sensor type.
- Particularly useful where mounting positions are constrained or only finalized after machine assembly (for example, 0.5 m direction adjustable distance sensors).
- Integrated 90° / side looking versions
- The transducer is arranged so that its main emission axis is perpendicular to the sensor housing.
- The sound path is not redirected by external reflectors; the probe itself is oriented to “look sideways”.
- This is suitable for narrow spaces, near wall installation, or when the housing must be aligned with a frame but the sensing direction must be lateral (for example, elbow type 90° distance sensors).
- Match non standard or changing installation angles
- Fit into tight or obstructed spaces
- Maintain a stable, repeatable sound path under industrial conditions
From a user’s perspective, these arrangements make it easier to apply the same basic ultrasonic principles (distance, through beam, or material sensing) to complex machine geometries, without changing electronics or core sensing concepts.
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