INTRODUCTION
Passive infrared (PIR) and microwave (MW) sensors are both widely used in lighting control. They both have the ability to detect moving objects, such as human bodies, but they work in quite different ways to each other. As a result, they have different advantages and drawbacks depending on their application.
Please note that neither a PIR nor a MW can detect light or the light level. PIRs and MWs can only detect moving objects. However, in the field of lighting control both PIRs and MWs are very often sold combined with a light sensor, so strictly speaking, when sold like this, these are two sensors in one. For details on light sensors, please read"Light Level Sensors" article
HOW DOES A PASSIVE INFRARED SENSOR (PIR) WORK?
A PIR is a passive device. It detects the movement of an infrared radiation source within its range and field of view.
Infrared (IR) radiation is emitted by all warm or hot bodies, including birds, mammals and any heat source such as an engine or motor that has been running recently. The warmer they are the more IR they emit. Provided that there is a difference between the IR being emitted by the moving body and the background level of IR then the sensor will be able to detect it.
A PIR for lighting control applications consists of three main components:
- Fresnel lens. This receives infrared radiation from its field of view and focuses it onto the sensor itself. Because the Fresnel lens is divided into segments the sensor behind the lens receives multiple separate beams of IR radiation, each one corresponding to a separate segment of the field of view.
A Fresnel lens as would typically be used on a passive infrared occupancy (motion) sensor. The design of the Fresnel lens is critical to the range and sensitivity of the sensor and will vary according to its intended application.
The purpose of the Fresnel lens in an occupancy sensing application is to divide the sensor’s field of view into separate segments so that differential levels of IR radiation can be detected moving across it, indicating the presence of a moving object.
- Pyroelectric sensor(often known in electronics as a “pyro”). This is an analogue device comprised of two detector elements. One element is wired so that when it receives IR it emits a high voltage while the other half, when it receives IR emits a low voltage. The more IR each element receives the further the voltage deviates (up or down) from zero. If they both receive the same amount then the voltages cancel each other out. Any movement of an IR emitting body in the field of view will therefore cause the output voltage of the pyro to fluctuate – indicating movement. The stronger the IR input (because the body detected is either very close or very hot) the more the output voltage will fluctuate.
Beneath the square-shaped area on the top of the sensor are two separate elements, wired to output high and low voltages.
Some lighting control PIRs will use more than one pyro in order to achieve higher levels of sensitivity.
- Electronic circuit and software. The circuitry and software behind the pyro process its output to determine if the IR changes observed indicate a moving person or not and therefore, if the lights should be switched on or not.
By the correct design and selection of the software, pyro and Fresnel lens the same basic PIR technology can be adapted to suit many different applications.
HOW DOES A MICRO-WAVE SENSOR (MW) WORK?
A micro-wave sensor is an active device. It emits very low-level pulses of electro-magnetic radiation which hit, and bounce back from objects within its range. The sensor receives and, using the Doppler effect, analyses the incoming radiation to determine if there are any moving objects in the field of view.
What is the Doppler effect?
Most of us experience the Doppler effect every day with respect to sounds we hear. The Doppler principle is that the apparent frequency of a wave (such as sound) depends on the speed of the source (which is making the sound) relative to the speed of the receiver (which is hearing the sound).
The Doppler principle can be applied to any wave, not just sound waves. For occupancy (motion) sensing in lighting control micro-waves are convenient because they are low-energy and for humans they are invisible and inaudible.
In occupancy sensing applications any deviation between the frequency of the radiation the sensor emitted and the frequency it received back will indicate movement in the detection zone. If the received frequency is higher then the object must be moving towards the sensor, if it is lower then the object must be moving away.
PASSIVE INFRARED (PIR) v MICROWAVE (MW) FOR OCCUPANCY SENSING. WHICH IS BEST?
Both technologies have their strengths and weaknesses and are therefore better suited to different applications in lighting control.
- High and low ambient temperatures. To function, a PIR requires a temperature differential between the moving object and the background. Therefore, bearing in mind that the human body temperature is 37ºC, the ability of a PIR sensor to detect human movement falls steeply where the ambient temperature is over about 30º For this reason, PIRs are less effective in hot climates, unless the space is air-conditioned.
- Any moving object v living moving object.
- Micro-waves will bounce off many solid objects and can therefore be triggered by swaying branches, blowing litter and even raindrops. They are therefore not preferred in many exterior applications where PIR will be more suitable if the aim is to detect human movement.
- A PIR will detect any movement of a heat source and can therefore be triggered by dogs and cats, not just humans. Correct Fresnel lens and software design can help to minimise nuisance triggering, but so too can careful sighting of the sensor. Some manufacturers offer shields to cover parts of the Fresnel lens so that detection can be limited to a narrow area.
- Operation from behind a cover or diffuser. In many lighting applications it is advantageous to place an occupancy sensor behind the cover of a light fitting. This is easy to install and often looks neater than having a separate sensor on the wall or ceiling. However, a PIR will not function at all from behind any sort of cover or diffuser, so in this case a microwave sensor must be used.
- Seeing round corners or line-of-sight only? A PIR only operates in line-of-site. Microwaves, on the other hand, can bounce round a corner and back again. It can therefore be beneficial to use microwave sensors on, for example, a stairwell. In this instance one sensor at the top and one at the bottom might be sufficient to switch all the lights on, even if someone entered at an intermediate level.
- Operating through walls and windows. PIR technology will not detect movement on the other side of even the thinnest wall or window, whereas a microwave can (subject to its power and sensitivity). Consider carefully the technology and siting of any sensors you use in offices with nearby corridors.
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