Pyroelectric Infrared Sensor PIR Infrared radiation is the electromagnetic waves in the wavelength region longer than the visible light wavelengths, lying from 0.75 μm to 1000 μm. The wavelength region of 0.75 μm to 3 μm is often called the near infrared, the wavelength region of 3 μm to 6 μm the middle infrared, and the wavelength region of 6 μm to 15 μm the far infrared. 1
Infrared radiation has the following characteristics: (1) Invisible to human eyes: This is useful for security applications. (2) Small energy (3) Long wavelength: This means infrared radiation is less scattered and offers better transmission through various medium. (4) Emitted from all kinds of objects 2
Referring to the principles of radiation heat transfer, all bodies emit energy in the form of electromagnetic radiations. The majority of these radiations are within the infrared spectrum. The intensity of energy flux, measured in unit area, emitted from a body depends upon the body s temperature and the nature of its surface. In general, a radiation heat exchange normally happens between hot and cold objects. As shown in the following figure, the net heat radiation (Q net ) from the hotter object (1) to the colder object (2) depends on the following factors: Objects temperatures T 1 and T 2, the areas of the two objects A 1 and A 2, the shapes, orientations, and the distance between the two objects, the radiative properties of the surfaces, and the transmission medium 3
According to Steffen Boltzmann law, the net radiation heat exchange between the two objects can be calculated by the following equation: 4
Where: σ: Stefan Boltzmann constant (5.669 10-8 W/m2K4). A 1 : The surface area of body 1. F 1-2 : The shape or view factor (depends on the object shape and area). ε: The equivalent emissivity of the two objects. T 1 and T 2 : The absolute temperatures of the two objects. The human body is one of these objects that emit infrared radiation and has the same features as these objects have. In fact, a heat exchange is usually happen between the human s body and the environment or other objects due the dissimilarity in their temperatures. In general, the typical human body emits infrared radiation in the range of 5 ~ 14 µm wavelength. Moreover, the temperature of a typical human body is about 37 0 C, and the average IR power intensity radiated from the typical human body is about 100 W/m 2. 5
The Pyroelectric Infrared sensor is made of a crystalline material that responds to the change of incident thermal radiation and develops a related electrical signal when a heat source (human body) crosses over the sensor FOV. This type of sensor responds to the variation of infrared radiation rather than the infrared radiation itself. In fact, this response takes place in three stages: Thermal to thermal conversion, thermal to electrical conversion, and electrical to electrical conversion stages. In the first stage, Pyroelectric material, which is the main sensor constituent, absorbs certain amount of infrared radiation emitted from human body and causes a change in the PIR unit temperature. In response to this temperature variation, the PIR crystalline material generates a spontaneous electric charge on its surface, i.e. the material acts as a capacitor which can be electrically charged by influx of heat. Finally, the spontaneous electrical charge is converted to electrical signal with a preamplifier. 6
The PIR Equivalent Circuit and the Mathematical Model 7
The Previous figure shows how typically, the FET source connects through a pulldown resistor of about 100 K to ground and feeds into a two stage amplifier having signal conditioning circuits. Each of the two cascaded stages has a gain of 100 for a total gain of about 10,000. The amplifier is typically bandwidth limited to below 10Hz to reject high frequency noise and is followed by a window comparator that responds to both the positive and negative transitions of the sensor output signal. 8
The sensor in the previous circuit has two sensing elements connected in a voltage differential configuration. This arrangement cancels signals caused by vibration, temperature changes and sunlight. A body passing in front of the sensor will activate first one and then the other element as shown in the following figure whereas other sources will affect both elements simultaneously and be cancelled. 9
PIR sensors are commercial available in single, dual, or quad element versions. The single element type of PIR sensor suffers from hypersensitive to any change in background temperatures, in which influences the sensor accuracy. In addition, the vibration and direct sunlight may also affect the operation and the accuracy of this type of sensors. However, the dual and quad element types of PIR sensors, which are normally connected in opposite polarisation, have the inherent immunity from the background temperature variation, vibration, and sunlight. This immunity is due to the differential configuration, in which the output signal is the difference between the two signals obtained from each element. As a result, the previous mentioned background noise will be cancelled out and completely eliminated. 10
The PIR sensor can sense the change in the amount of infrared energy within small distances, approximately up to 10 inches. For detecting movements at longer distance (up to 10 m), infrared radiation has to be focused. This focusing is done by a Fresnel lens. A Fresnel lens divides the whole area into different zones. Any movement between zones leads to a change in the IR (infrared) energy received by the sensor. There are different types of Fresnel lenses depending on the range (distance) and coverage angle. Fresnel lenses are controlling devices that collect (focus) the infrared radiation and shape the required FOV 11
The Fresnel lens is designed to have its grooves facing the IR sensing element so that a smooth surface is presented to the subject side of the lens which is usually the outside of an enclosure that houses the sensor. 12
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The following Figure shows the main components of dual element PIR sensor and summarises its principle of operation. As shown in this figure, when a person crosses the sensor FOV, the two PIR elements detect the change in radiation flux in sequence and generate two opposite peaks according to the corresponding polarisation. The PIR sensor generates a low frequency signal output of an AC voltage shape. This type of sensor can detect the motion and indicate the direction of motion as well. Pyroelectric Sensors 15
Case of study: Motion Detector for Intruders What is the influence of the walking intruder into the output signal in frequency and amplitude? How to design the best suited electronic circuitry? We find that the output signal depends on: 1- Distance of human body to the motion sensor. 2- Walking speed of human body. 3- Focal length and pattern design of the optical system. Case of study: Motion Detector for Intruders The requirements all stipulate that very fast movements and extremely slow motions of intruding bodies have to be detected. The slow motions range at about half a meter per second, whereas fast motions can be up to 3 meter per second or even 5 m/s (18 km/h). The IR detectors output signal will respond in a certain frequency range depending on the focus of the optical system and the walking speed, which is to be calculated when designing the electronic circuitry. 16
The relation between velocity, focus and frequency: Applications: 1- Security system. 2- Automatic lighting system. 3- Automatic door openers. 17