The passive infrared detector is largely used in the security and protection engineering. For guaranteeing the high-efficiency normal work of the device, the design personnel need to tackle the vexed question of avoiding failing to alarm, reducing false alarm and especially minimizing the false alarm. So, there is the need for us to know the basic working principle and technological characteristics of passive infrared detector. Based on that, we can play the effectiveness of the passive infrared detector to the maximum extent from all aspects involving installation, debugging, use, etc.
Within the detection zone, why could the passive infrared detector generate the alarm signal when people are moving?
In the nature world, any object whose temperature is higher than the absolute zero of -273°C would produce the infrared spectrum. The objects with different temperatures would release the infrared energy with different wavelength. So, the infrared wavelength is correlated with the temperature.
There are two key elements in the passive infrared detector. One is the pyroelectric infrared sensor (PIR). This kind of sensor can transform the change of the infrared signal with the wavelength between 8μm and 12μm into electric signal. It also has the inhibitory effect on the white light signal in nature. Therefore, within the detection zone of the passive infrared detector, when there is no human body movement, the PIR can only sense the ambient temperature. Once when the human body enters the detection zone, via the Fresnel lens, the PIR can sense the difference signal between the human body temperature and ambient temperature. To put it simply, the passive infrared detector works by sensing the temperature difference between the moving object and background object.
The other key element of the passive infrared detector is the Fresnel lens. The Fresnel lens has the refracting type and reflecting type. It has two functions. One is the focusing effect, namely refracting (reflecting) the pyroelectric infrared signal onto the PIR. The other function is to divide the detection zone into several bright regions and dark regions to let the moving object in the detection zone generate varied pyroelectric infrared signal on the PIR in the form of temperature change. Then, the PIR can produce the varied electric signal.
Analysis of Detection Zone View of Passive Infrared Detector
Any kind of passive infrared detector is provided with the detecting area view on the instruction manual. Then, what can we get from the detecting area view?
1. The mounting height of the detector
2. The angle of lower detection area
3. The most distant detection range
4. The widest effective detection distance
5. Detection angle
6. Visible Sensing Band Direction
With that information available, we can have a basic understanding about the passive infrared detector. That would be helpful for us to use the passive infrared detector correctly.
Pet Immunity Function of Passive Infrared Detector
The passive infrared detector is mature in technology. The pet immunity is an important function of the device. Each manufacturer takes different processing mode to cope with the interference of small pet. Generally speaking, there are two kinds of modes. One is the physical mode which decreases the probability of the false alarm caused by the small pet via the change of the segmentation method of the Fresnel lens. This method is superficial, and its effect has limitations.
The second way is to process and analyze the detection signal. It carries out the data acquisition of the detection signal, and then analyzes the signal period, amplitude and polarity. These factors can reflect the speed, pyroelectric infrared energy and the displacement per unit time of the moving object. The microprocessor of the detector analyzes the collected data and makes the comparison to give the result of the moving object to be the human or the small pet.
It seems that the pet immunity function of the passive infrared detector is relative. The relativity covers two aspects. Firstly, the pet immunity is relative. As compared to the device with no pet immunity function, the device with the pet immunity function has a much lower false alarm rate. It has the certain limit to the quantity and size of the small pet. Secondly, the passive infrared detector should be mounted by following the requirements. For getting the pet immunity effect, the user should read and follow the instruction manual about the installation part carefully.
Installation Precautions of Passive Infrared Detector
The passive infrared detector is the weak signal detection equipment. So, during the installation process, the user should pay attention to some details such as the height, sensitivity, etc. Mastering the following information is very necessary for mounting the passive infrared detector correctly, including understanding the performance and characteristics of the detector, confirming the mounting position reasonably, and debugging carefully. We cannot say that the detector is well installed if it could exert the alarm function. Then, how can we determine the installation position of a passive infrared detector?
1. Confirm the normal mounting angle in line with the instructions.
The mounting height is not random. It would affect the sensitivity and the effect of pet immunity. For example, we suppose the detector is installed with the mounting height of 2m and 2.5m respectively. If there is the object moving on the ground, then, we can get the different frequency of segmenting the bright region and dark region.
2. It is inappropriate to let the passive infrared detector face the glass window and door.
The passive infrared detector facing the glass window and door would cause two problems. One is the white light interference. Obviously, though the PIR shows the very strong inhibition effect on the white light, the inhibition effect is not 100%. So, installing the detector by avoiding facing the glass window and door can keep away from the interference of strong light. The other problem is the complex interference from the environment outside of the window and door, such as the crowd flow and the traveling vehicles.
3. The mounting position should not face the cold and hot air vent or the cold and hot source.
The passive infrared detector works by linking with the temperature change closely. The cold and hot air vent and the cold and hot source are likely to cause false alarm of the detector. As to some inferior detectors, even the air convection through the window and the door can result in false alarm.
4. It is not proper to make the detector face the object easy to swing.
The object easy to swing would actuate the microwave detector. So, it may also set off false alarm.
5. Pay attention to the illegal intrusion route.
Mounting the passive infrared detector is for preventing the illegal intrusion of the offender. Before confirming the mounting position, the user should consider the major entrance of the building. Actually, we can reach the security and protection goal just by setting up the defense on the entrance and cutting off the illegal intrusion route.
After the installation, the debugging work comes to the last. There are two debugging methods. One is the walk test. That is the debugging personnel walks with S-shaped route within the detection zone to sense the length, width, microwave sensitivity and infrared sensitivity of the detection range. It needs to adjust the walk test method. Too high or too low sensitivity would affect the detection effect. On account of the season change, the sensitivity should be adjusted in winter and summer respectively. Meanwhile, the microwave sensitivity must not be too large for the microwave has penetrability. The debugging personnel should notice that.
The other way is using the professional instrument to determine whether the position is suitable for mounting such type of detector.
Anyhow, before the installation and use, the user should read the instructions carefully. Through the accumulation of the practical experience, we can understand the characteristics of the detector more comprehensively and deeply, thus making full play of the passive infrared detector in security and protection engineering.