Ultrasonic sensor technology and applications in KNX projects

In previous parts of this series, we explored Passive Infrared (PIR) sensors – a fundamental solution for motion detection, followed by Microwave/High-frequency sensors – a superior technology with penetration capabilities. However, in many smart home and building automation projects, especially in office spaces, meeting rooms, or areas with numerous small obstacles, accurately detecting very subtle movements or comprehensively covering an entire space still poses a challenge.

This is where ultrasonic sensor technology comes into play. This type of sensor offers a unique approach, overcoming the limitations of other sensor types to provide more reliable presence and motion detection, particularly in environments where minor movements or spaces need to be effectively "filled."

Ultrasonic sensors are a superior solution for static presence detection, capable of covering complex spaces and maintaining high accuracy for KNX projects. This article will help you understand how ultrasonic sensors work and their optimal applications in KNX projects.

Ultrasonic sensors are ideal for workspaces like offices and reading rooms, where occupants are often stationary (Image source: Unsplash)

Ultrasonic sensors: An effective solution for presence and motion detection in smart buildings

Unlike PIR sensors that passively "listen" for infrared radiation or microwave sensors that actively emit penetrating electromagnetic waves, ultrasonic sensors operate by emitting and receiving high-frequency sound waves to detect movement.

Operating principles

An ultrasonic sensor continuously emits high-frequency sound waves into the environment. The typical operating frequency range for KNX ultrasonic sensors is often between 25 kHz and 40 kHz, which is entirely outside the human hearing range. As these waves travel through space and collide with a moving object, a portion of the wave is reflected back to the sensor. Similar to microwave sensors, ultrasonic sensors also utilize the Doppler effect: the frequency of the reflected wave will change compared to the frequency of the original emitted wave. If the object moves towards the sensor, the reflected wave's frequency will increase; if the object moves away, the reflected wave's frequency will decrease. This change in frequency is the signal indicating movement.

The sensor's receiver picks up the reflected waves. The internal microprocessor compares the frequency of the reflected wave with the original wave. If there is a frequency shift (due to the Doppler effect), it indicates that an object is moving within the detection zone. This signal is then analyzed to determine movement, and if the signal exceeds a set threshold, the sensor will trigger the corresponding output (e.g., turn on lights, activate a fan).

A distinguishing characteristic of ultrasonic waves is their ability to "bounce" off soft obstacles like low partitions, desks, and cabinets, effectively "filling" a space. This allows ultrasonic sensors to detect movement throughout an entire volume (volumetric detection) rather than relying solely on a line of sight.

Ultrasonic waves have the ability to 'bounce' over soft obstacles like low partitions, tables, chairs, cabinets, and fill the entire space (Image source: steinel.de)

Technical features

When selecting an ultrasonic sensor, understanding the following technical characteristics is crucial to ensure the sensor operates effectively and is suitable for the specific application:

Operating frequency: This is the frequency of the sound waves emitted by the sensor. Ultrasonic sensors typically operate at frequencies above the human hearing threshold (usually from 20 kHz to several hundred kHz). A higher frequency means a shorter wavelength, potentially leading to higher resolution and accuracy, but often results in a shorter measuring range and greater attenuation in the air. Conversely, lower frequencies generally offer a longer range but may have reduced accuracy.

Measuring range: This characteristic indicates the minimum and maximum distances the sensor can reliably measure.

• Minimum distance (blind zone/dead zone): This is the area directly in front of the sensor where it cannot detect objects due to signal processing time.

  
  

• Maximum distance: This is the furthest limit at which the sensor can detect and measure the distance to an object. The measuring range can vary from a few centimeters to tens of meters depending on the sensor type and application.

  
  

Accuracy: This specifies the maximum measurement error compared to the actual distance, typically expressed in millimeters (mm) or as a percentage (%). High accuracy is a key factor for applications requiring precise measurements.

Resolution: This is the smallest change in distance that the sensor can detect and report. High resolution allows the sensor to perceive very slight movements of an object.

Beam angle/detection angle: This is the angle at which the ultrasonic waves are emitted and received. A narrower beam angle offers better wave concentration, reducing interference from surrounding objects but requiring more precise positioning. Conversely, a wider angle covers a larger area but is more susceptible to interference from obstacles outside the main beam.

Response time: The time elapsed from when the sensor detects an object until it transmits an output signal. Fast response time is crucial in applications requiring high-speed movement tracking or immediate reactions.

Noise immunity: The extent to which the sensor can operate stably in environments with high-frequency noise (acoustic, electromagnetic) or interference from other nearby ultrasonic sensors.

IP rating (ingress protection): This index indicates the level of protection the sensor has against dust and water. For example, IP67 means the sensor is fully protected from dust and can be submerged in water to a certain depth for a short period, making it suitable for industrial or outdoor environments.

Connectivity/interface: How the sensor communicates with the control system. For KNX, ultrasonic sensors will typically have native KNX communication, but there might also be types with analog (0-10V, 4-20mA), digital (DIO), or serial (RS-232/485) interfaces that require a gateway for KNX integration.

Distinguishing from other sensor types

• Compared to PIR (passive infrared) sensors: PIR detects changes in heat radiation, requiring a clear line of sight and being ineffective with stationary objects or very subtle movements. Ultrasonic sensors detect physical movement using sound waves, can "navigate" around soft obstacles, and are more sensitive to small movements.

  
  

• Compared to microwave sensors: Microwave sensors emit electromagnetic waves that can penetrate solid obstacles (thin walls, glass). Ultrasonic waves are sound waves that do not penetrate large solid walls, but they have a better ability to "fill" a space by reflecting, and are often more sensitive to small movements within a specific area, such as a person typing at a keyboard.

Advantages and disadvantages of KNX ultrasonic sensors

Like any technology, KNX ultrasonic sensors have their advantages and some drawbacks that need to be considered during deployment.

 

Key Advantages

1. Detection of small and subtle movements: This is the greatest strength of ultrasonic sensors. They are extremely sensitive to minor movements such as typing, hand gestures, or shifting in a seat. This makes them an ideal choice for workspaces like offices and reading rooms, where users often remain relatively stationary.

   
   

2. Volumetric detection: The ability of ultrasonic waves to "bounce" and reflect around light obstacles (like desks, chairs, low partitions) enables the sensor to effectively cover and detect movement throughout an entire space. This significantly reduces "blind spots" compared to PIR sensors, which require a clear line of sight. A typical coverage area can reach a radius of 5 to 8 meters for subtle movements, depending on the model and manufacturer. Notably, thanks to ultrasonic waves, these sensors can also detect movement behind obstacles such as furniture and room dividers, making them ideal for spaces with complex layouts.

   
   

3. Unaffected by temperature or air currents: Unlike PIR sensors, which are sensitive to ambient temperature changes or drafts, ultrasonic sensors operate stably regardless of temperature conditions or air circulation in the room. This is particularly important in areas where there's little difference between ambient and body temperature, such as hallways with large glass facades.

   
   

4. High reliability in specific environments: They are especially suitable for areas like offices, restrooms, or storage rooms with many small obstacles or where static presence detection is required.

 

Disadvantages

1. Susceptible to high-frequency noise: Ultrasonic waves can be interfered with by other sources of high-frequency noise, such as industrial machinery, large fans, or other ultrasonic devices in the same area, leading to false alarms.

   
   

2. Poor penetration of solid obstacles: Unlike microwave sensors, ultrasonic waves cannot detect movement through walls or closed doors. Sound waves will be blocked by hard surfaces.

   
   

3. Coverage limited by materials: Some soft materials (e.g., thick curtains, large carpets) can absorb ultrasonic waves, reducing detection effectiveness. Conversely, smooth, hard surfaces can create unwanted reflections, requiring careful adjustment.

   
   

4. Requires careful configuration: The installation position and sensitivity of the sensor need to be precisely adjusted to optimize performance and avoid unintended activations.

 

Applications of KNX ultrasonic sensors in smart buildings

KNX ultrasonic sensors demonstrate maximum effectiveness in specific scenarios where other sensor types might have limitations.

In office workspaces and individual workstations, ultrasonic sensors are ideal for optimizing lighting and HVAC control. They can keep systems active when someone is working quietly at their desk (with only minor movements like typing or hand gestures), minimizing energy waste when users temporarily leave their immediate vicinity but not the entire room.

For restrooms and meeting rooms, ultrasonic sensors help ensure continuous lighting and ventilation when occupied, even if occupants are not moving much, providing seamless comfort.

In storage rooms or warehouse areas with many shelves and small obstacles, the "space-filling" capability of ultrasonic waves effectively detects people moving between shelves, even in positions where PIR sensors might have their line of sight obstructed.

Another powerful application is their combination with other sensor types (Multisensor). An ultrasonic sensor can function as a component of a KNX multisensor (often combined with PIR, light, and temperature sensors) to create the most accurate and reliable presence detection solution. For instance, a PIR might detect someone entering the room, and then the ultrasonic sensor maintains the "occupied" status even if they sit down and move little, preventing the system from automatically turning off while still in use.

The Presence Detector Dual KNX sensor from Steinel integrates PIR and Ultrasonic technologies (Image source: steinel.com.au)

Notes on installing and programming KNX ultrasonic sensors

To fully leverage the effectiveness of KNX ultrasonic sensors and ensure stable operation, careful consideration must be given to device selection, integration, and configuration.

1. Selecting the right device: Engineers need to thoroughly research the technical specifications of each product, especially the adjustable sensitivity of the ultrasonic transducer, to match the environment and the desired type of movement to be detected.

   
   

2. Strategic installation location: It's crucial to avoid installing sensors near large noise sources that could cause interference. The installation position should ensure full coverage of the monitored space, while also accounting for potential ultrasonic wave reflections from surfaces. Avoid pointing the sensor directly at strong sound-absorbing materials like thick curtains or carpets. Some sensors allow for adjustment of the beam angle or sensitive zone through physical masks or software configuration.

   
   

3. ETS configuration and scenario programming: This is an essential step. Use the ETS software to configure group addresses, adjust sensitivity, set delay times, and other sensor parameters. More importantly, program lighting, HVAC, or security control scenarios based on signals from the ultrasonic sensor. In many cases, combining logic from ultrasonic sensors with other sensors (e.g., PIR within a multisensor) will enhance accuracy and prevent false alarms.

   
   

4. Testing and adjustment: After installation and configuration, thorough testing under real-world conditions is indispensable to fine-tune performance and ensure the system operates stably and accurately as expected.

Conclusion

KNX ultrasonic sensors are a valuable device for optimizing building automation, particularly effective in detecting subtle movements and providing comprehensive space coverage. Despite their unique characteristics and the need for careful consideration during deployment, when properly understood and applied, KNX ultrasonic sensors will significantly enhance the intelligence, energy efficiency, and comfort of a system, especially in workspaces or areas requiring sensitivity to static presence.

Mastering the principles and optimal applications of KNX ultrasonic sensors will empower engineers to expand their solution offerings, meet diverse needs, and increase the value of their projects.