Part 2 - Sensor technology and practical applications in KNX projects: Microwave/High-Frequency sensors

In the previous part of this series, we explored Passive Infrared (PIR) sensors – an effective solution for basic motion detection and energy saving. However, in many smart home and building automation projects, especially large spaces or those with obstacles, PIR might not fully meet the requirements for accuracy and continuous detection capabilities.

This is where Microwave/High-Frequency sensor technology comes into play. This type of sensor offers a different approach, overcoming the limitations of PIR to provide more reliable presence and motion detection in complex environments.

If you are an engineer, contractor, or technical specialist looking for an effective, flexible detection solution with obstacle penetration capabilities and high accuracy for your projects, this article will provide an in-depth look at how microwave sensors work and their applications in KNX projects.

Microwave sensors provide more reliable presence and motion detection in complex environments. (Image source: mdt-group.com)

Microwave sensors: Accuracy and penetration capability

Unlike PIR sensors, which passively "listen" for infrared radiation, microwave sensors are active sensors. They operate based on the Doppler effect to detect motion.

Working principle

Microwave sensors continuously emit high-frequency electromagnetic waves into the environment (typically 2.4 GHz, 5.8 GHz, or higher, depending on regional regulations). These waves propagate through space and can penetrate many non-metallic materials such as thin walls, plasterboard, wood, glass, plastic, etc. When these waves encounter a moving object (person, vehicle), a portion of the waves is reflected back to the sensor. The frequency of the reflected waves will change compared to the frequency of the original emitted waves. If the object moves towards the sensor, the frequency of the reflected waves will increase; if the object moves away from the sensor, the frequency of the reflected waves will decrease. This change in frequency is called "Doppler shift" or "Offset echo."

The sensor's receiver collects the reflected waves. The microprocessor inside the sensor compares the frequency of the reflected waves with the original waves. If there is a frequency change (due to the Doppler effect), it indicates that an object is moving within the detection area. This frequency change is converted into an electrical signal, which is then analyzed to determine the speed and direction of the object's movement. If the signal exceeds the set threshold, the sensor will trigger an output (e.g., turn on lights, activate fans).

Due to its operating principle based on waves with penetration capability and the ability to detect very small frequency changes, microwave sensors can detect even the slightest human movements (such as hand gestures, breathing) or movements behind thin obstacles.

Outstanding technical features

High sensitivity and accuracy: KNX microwave/high-frequency sensors are highly accurate. High frequency means very short wavelengths. With short wavelengths, even very small movements of an object (millimeters or micrometers) can produce a frequency change large enough for the sensor to detect. This makes the sensor highly sensitive to the most subtle movements, including breathing or small body movements of a stationary person. Compared to PIR (which detects temperature changes), microwave sensors do not rely on temperature changes but on physical displacement, providing higher sensitivity to movement.

Material penetration capability: This is a significant advantage over PIR. Microwave waves can penetrate most non-metallic materials such as plasterboard walls, wood, glass, plastic, and even thin brick walls. This allows the sensor to have a wider detection range and not be obstructed by unwanted obstacles in the detection zone, such as furniture or light partitions, while also allowing for concealed installation (e.g., behind false ceilings). This is both an advantage and a disadvantage, requiring careful installation and operation to prevent false triggers.

Independence from ambient light or temperature: Unlike PIR sensors (which rely on infrared temperature), microwave/high-frequency sensors are not affected by ambient temperature or light conditions (dark or bright). This ensures stable and reliable performance 24/7.

Adjustable detection zone (sensitivity & range): Microwave/high-frequency sensors typically have a much larger detection range than PIR (up to tens of meters). Many modern KNX microwave/high-frequency sensors allow detailed adjustment of sensitivity and detection range via ETS software. This enables engineers to fine-tune the operating area, minimizing unwanted triggers from outside the monitored zone. Some sensors can also differentiate movement direction or eliminate interference from unwanted repetitive movements (e.g., ceiling fans).

Ideal for large spaces: Suitable for large open spaces such as open-plan offices or parking lots.

Disadvantages:

1. Prone to false triggers from external movement: Due to their penetration capability, sensors may detect movement outside the desired area (e.g., people walking in an adjacent corridor if installed near a doorway). Careful sensitivity calibration is required.

2. Higher cost than PIR: The initial investment cost is generally higher than PIR sensors.

Microwave sensors are not affected by ambient temperature or light conditions. 

Applications of microwave/high-frequency sensors in KNX projects

Easy integration: In a KNX system, microwave sensors are easily integrated with ETS software. This allows for detailed configuration of parameters such as sensitivity, delay time, light threshold (if integrated with a light sensor), etc.

Lighting/HVAC control: In large spaces requiring extensive and reliable detection, unhindered by light partitions. Upon detecting movement, the microwave sensor sends a KNX telegram to the corresponding group address, quickly and accurately activating other actuators (e.g., light switches, HVAC controllers).

Applications in complex scenarios: Due to their high accuracy, KNX microwave sensors are ideal for applications requiring continuous and reliable monitoring in large spaces such such as open-plan offices, warehouses, where even a small movement needs to be detected to optimize lighting or air conditioning.

Parking management: Detect vehicle positions in parking lots, supporting guidance systems for available spaces or activating lighting in occupied areas.

Automatic door control: Trigger automatic door opening in high-traffic areas.

Invisible precision – STEINEL high-frequency sensors for maximum efficiency and safety. (image sourse: steinel.de)

Notes on installing KNX microwave/high-frequency sensors

To maximize the effectiveness of microwave sensors and avoid potential issues, special attention is required:

1. Consider installation position and adjust detection zone: Due to their penetration capability, extreme caution is needed when installing near windows, doorways, or areas with movement outside the desired zone. Use the sensitivity and range adjustment features in ETS to fine-tune the detection zone appropriately.

2. Avoid large metal surfaces: Microwave waves can be reflected or absorbed by large metal surfaces, affecting detection performance.

3. Check for interference (if any): While not common, in some cases, other RF transmitting devices operating on the same frequency might cause interference. Always thoroughly test the sensor's operation after installation.

4. Combine with other sensors (multisensor): In very complex spaces or where the highest accuracy is required (e.g., needing both fast motion detection and accurate static presence detection under varying light conditions), using multi-technology sensors (e.g., combining microwave and PIR) is an optimal solution. KNX provides excellent support for these multisensor devices.

Conclusion

The microwave/high-frequency sensor is a powerful tool, providing reliable motion and presence detection for KNX systems, especially in environments demanding high accuracy and flexibility. Mastering the working principles and technical characteristics of this sensor type will help KNX engineers design intelligent, efficient, and reliable building automation solutions.

In the next part of this series, we will continue to explore the Ultrasonic Sensor – another technology that offers excellent static presence detection capabilities with its own unique features.