Versatile motion detectors
Motion detectors are now used in many places, in or on industrial or office buildings as well as private residences. In addition to indoor applications, the detectors are also used outdoors.
In the dark, the entrance area is automatically illuminated as soon as movement is detected. The key and the keyhole are quickly found. In addition to this added convenience, motion detectors outdoors also offer protection against burglars. Thieves find the light disturbing and it usually drives them away. Motion detectors are a strong building block in the prevention of burglary and are recommended by the police.
However, occasionally there are supposedly incorrect switching operations. The light appears to switch on without any apparent movement. The more often this happens, the more annoying it is perceived to be. In addition, energy is consumed unnecessarily. There are many reasons for such incorrect switching.
Avoid incorrect switching
The orientation of the motion detector plays a decisive role. Outdoor detectors usually have a very large detection area. If, for example, a road or pavement touches the edge of the detection area, this can lead to false switching. This can be remedied by better alignment of the detector. High-quality devices have a so-called test mode in which the detector constantly switches the light on and off as soon as there is movement in the detection area. In this mode, it is not necessary to constantly wait for the follow-up time to align the detector. Most detectors are aligned on two axes. The detector head can be turned up and down and to the right or left. The RC-plus Next N from B.E.G. also offers the option of infinitely variable mechanical adjustment of the individual motion sensors in the detector head.
Removable lens covers
In outdoor areas, motion detectors with passive infrared sensors (PIR) are used almost exclusively. These sensors react to heat changes that occur when living creatures move. Plants do not emit any heat, so moving trees and bushes are not detected. However, if there is a heat source behind the plant, e.g. a wall heated by the sun, a detector recognises the movement of the plant possibly caused by a gust of wind. The detection area must also be restricted in the event of such false switching. Cover louvres, which are attached to the lens of the detector, can be used here. The louvres are impermeable to IR radiation and therefore certain parts of the detection area are blanked out.
Animals generally have a similar body temperature to humans. Movements of cats and dogs are detected in the same way as human movements and cause the light to switch on. The cause is therefore the same in both cases: the movement of a (warm) living creature. Such incorrect switching can hardly be avoided. To understand what possibilities nevertheless exist and what consequences result from this, it is better to explain the science behind the technology.
Passive infrared sensors
Infrared radiation lies in the electromagnetic spectrum between visible light and microwaves. Infrared radiation is primarily thermal radiation and is generated by the movement of atoms and molecules in an object. This means that any object with a temperature above absolute zero, even an ice cube, emits radiation in the infrared range. The higher the temperature, the more the atoms and molecules move and the more infrared radiation they produce.
The infrared radiation produced by living beings is not homogeneous, which becomes clearly visible when using an infrared camera. Due to their body temperature of 37°C, humans radiate strongly in the infrared range, with a wavelength of around 10 µm. Different parts of the body emit different levels of radiation; the mouth, for example, is significantly warmer than the fingers. This means that the infrared radiation emitted by the mouth is correspondingly stronger than the radiation emitted by the fingers.
Passive infrared sensors (PIR sensors) also work in the wavelength range of 10 µm and enable the use of infrared radiation for motion detection, as they respond optimally to the heat radiation of a person or animal. Passive infrared means that the sensors do not emit any radiation, but only receive it. The detector then analyses the signals from the sensors and switches the light on accordingly.
Sensitivity of the sensors can be variably adjusted
In order to detect movement, the IR rays must reach the sensors. As the radiation becomes weaker over the distance, the sensor can detect movement at a distance of 2 metres better than at a distance of 20 metres. However, movement can also be detected at a distance of 20 metres. This depends on how much heat is emitted. A mouse emits less heat than a person, while a locomotive in operation, for example on a company site, emits significantly more heat than a person.
With good detectors, the sensitivity of the sensors can be variably adjusted. This setting determines the amount of heat above which the sensor should respond. Smaller amounts of heat, such as that of a cat, are faded out. However, the light can be switched on by the detector as soon as a larger dog radiates heat. However, if the sensitivity is reduced even further, there is a risk that the movements of small children will no longer be detected.
Fade out detection areas
Another way to reduce false switching triggered by animals is to mask out the lower detection areas. Cats often roam past house walls. Detectors with creep protection do not recognise this movement. With the RC-plus Next N from B.E.G., this function can be activated and deactivated by remote control. It should be borne in mind that thieves also like to sneak past the wall.
If a motion detector is looking into the distance, lower detection areas are also blanked out. This creates dead zones in which no movement is detected. As the detection area is now in the distance and the heat radiation becomes weaker over the distance, only larger heat sources, e.g. people, are detected. It must be taken into account here whether there is a road or pavement at a greater
Modern passive infrared sensors
Like light, infrared radiation can also be focussed by lenses. Motion detectors are equipped with a lens to focus the IR radiation onto the PIR sensor. The most common sensors consist of two plates of pyroelectric crystal. When IR radiation hits such a crystal, this leads to a charge shift, which can be measured. A movement is recognised when the IR radiation is detected by both plates.
More modern sensors have a matrix of pyroelectric plates which, in combination with suitable software, can be used to recognise simple contours. If more plates are active in the vertical plane, it could be a person; if more are active in the horizontal plane, it could be an animal.
At present, almost all motion detectors comprise simple PIR sensors with two plates. Matrix sensors have not yet become established due to their high price.
Detectors with camera technology
When the first motion detectors came onto the market around 35 years ago, two terms were coined: Motion and presence detectors. Both are marketing and sales-orientated terms. The function of the word ‘motion detector’ can still be deduced. The term ‘presence detector’ is not self-explanatory. Even the manufacturers of the detectors interpret it differently. ‘Presence detectors recognise minor movements, such as working with a computer mouse’ or “Presence detectors include natural outdoor light in their light measurement”, to name just two common definitions.
The Duden dictionary defines presence as ‘presence, [consciously perceived] presence’. Only a person can consciously perceive presence. No technology can do this. Detectors that contain a camera and analyse images come closest. Human contours can be filtered out using software. But even a life-size mannequin can be misidentified by the software. In addition to the very high price of such detectors, there are often data protection and health and safety issues, for example if the detector containing a camera is installed in an office. A camera in the office creates an uneasy feeling.
High frequency (HF) detectors
In contrast to passive infrared detectors (PIR), high-frequency (HF) detectors are active detectors. They transmit waves at a high frequency but with a low power and analyse echo signals. HF waves have the property that they penetrate non-metallic materials such as walls. HF detectors can therefore be hidden behind suspended ceilings or installed invisibly directly in a luminaire, utilising the so-called Doppler effect. An approaching vehicle sounds higher than a vehicle moving away. Due to their own movement, the sound waves are compressed in the direction of movement and expanded in the opposite direction. HF detectors work in reverse: they are stationary and emit waves. These waves are reflected by objects. If the objects move, the reflected waves have a slightly different frequency. The greater this frequency difference, the faster the object moves. The police use this technology for radar measurement during speed checks.
Compared to passive infrared detectors, HF detectors work independently of temperature. The only criterion for them is movement. Motors, curtains, water in a sewage pipe and, outdoors, leaves and of course animals of all kinds also move and are therefore sources of interference when it comes to detecting human movement. The reflected signal can be analysed using intelligent software algorithms. A whirling leaf can be filtered out, but a person waving their hand is also excluded from the detection. If a large dog runs past an HF detector at the same speed as a small person, the HF detector cannot recognise any difference, which means that false switching due to moving animals cannot be ruled out either. HF detectors are a good alternative to PIR detectors if environmental influences push passive infrared technology to its limits.
Conclusion
Animals cannot therefore be recognised as animals, but false switching due to animal movements can be reduced by reducing the sensitivity. The consequence of reducing the sensitivity of the sensors is that the range of the detector decreases. With a PIR detector, the correct alignment of the detector alone can often help, e.g. by using only the far field for detection. The detector therefore simply looks over most (small) animals.
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