Occupancy Sensor vs. Motion Sensor: Principles and Techniques

An occupancy sensor detects the presence of people or animals within a monitored area. In contrast, a motion sensor only responds to moving objects.

The key difference lies in their sensitivity: occupancy sensors produce signals whether an object is stationary or moving, while motion sensors are specifically triggered by movement.

These sensors often rely on properties or actions associated with the human body. This could include sensitivity to:

• Body weight
• Heat
• Sounds
• Dielectric constant

Other types of sensors used in security and automation include:

• Air pressure sensors: Detect changes in air pressure caused by opening doors and windows.
• Capacitive Sensors: Detect changes in human body capacitance.
• Acoustic sensors: Utilize sounds produced by people.
• Photoelectric sensors: Work by detecting the interruption of light beams by moving objects.
• Optoelectronic sensors: Detect variations in illumination and optical contrast.
• Pressure mat switches: Use pressure-sensitive strips laid on floors to detect weight.
• Stress detectors: Employ strain gauges embedded in structural components.
• Switch sensors: Utilize electrical contacts connected to doors and windows.
• Magnetic switches: Offer a non-contact version of switch sensors.
• Vibration detectors: React to vibrations in walls or other structures.
• Glass breakage detectors: Respond to specific vibrations caused by shattered glass.
• Infrared motion detectors: Sensitive to heat waves emitted from warm or cold moving objects.
• Microwave detectors: Active sensors that respond to reflected microwave electromagnetic signals.
• Ultrasonic detectors: Similar to microwave detectors, but use ultrasonic waves.
• Video motion detectors: Compare a stored stationary image with the current image from the protected area.
• Video face recognition system: Uses image analyzers to compare facial features with a database.
• Laser system detectors: Similar to photoelectric detectors, using narrow light beams and reflectors.
• Triboelectric detectors: Detect static electric charges carried by moving objects.

All of these techniques contribute to the design and development of occupancy or motion sensors.

Capacitive Occupancy Sensor

Figure 1 depicts basic circuit, capacitance between the test plate and earth is equal to value C1. In the time when any person moves in the vicinity of the plate, it builds two additional capacitors; One between plate and body (Ca) and the other capacitor between body and earth (Cb). Hence the resulting total capacitor between plate and earth will become larger by ΔC.

The capacitance between a test plate and earth is denoted as $C_1$. When a person moves near the plate, two additional capacitors are formed: $C_a$ (between the plate and the body) and $C_b$ (between the body and earth). This increases the total capacitance between the plate and earth by $\Delta C$. The total capacitance, $C$, is then represented as:

Human bodies, being conductive mediums with high dielectric constants, create a coupling capacitance with their surroundings. The capacitances $C_a$ and $C_b$ depend on factors like body size, clothing, surrounding objects, and weather conditions.

Movement of people changes the coupling capacitance, allowing the system to differentiate between static and moving objects. All objects exhibit some degree of capacitive coupling with each other.

Optoelectronic Motion Sensor

Optoelectronic motion sensors are commonly used for intrusion detection. They operate using electromagnetic radiation in the optical range, with wavelengths between 0.4 and 20 µm. They can detect movement of people and animals up to distances of hundreds of meters.

The operating principle involves detecting light (visible or non-visible) emitted from the surface of a moving object into the surrounding area. This radiation can originate from an external light source (reflected by the object - active detector) or be produced by the object itself (natural emission - passive detector). Active sensors require an additional light source like daylight, electric lamps, or infrared LEDs. Passive detectors sense mid- and far-infrared emissions from objects with temperatures different from their surroundings. Both types rely on optical contrast for object recognition and detection.

Optoelectronic motion sensors can effectively indicate whether an object is moving or stationary, but they cannot distinguish between multiple moving objects or accurately measure the distance or velocity of a moving object.

These sensors find primary application in security systems and energy management. In energy management, they can be used to switch lights on and off automatically. They are also used in “smart homes” for controlling appliances like air conditioners, cooling fans, and stereo players – a concept known as home automation. The simplicity and low cost are major advantages of optoelectronic motion sensors.