9
Chapter 6 - Using Passive Infrared Sensors
B.
Infrared Signal:
The optimum signal produced by a human is at a distance of 20 feet for the
IR-301, with the person walking at a casual pace (3 ft./sec). This is strictly
a function of the sensor’s optics, which is fixed and determines the devices
beam spread (4 ft. @ 20 ft. for the IR-301). At 20 feet, a person completely
fills the sensing region of each beam causing a maximum signal. Further
away than 20 feet, the person only partially fills the sensing region, thereby
generating a smaller signal. Closer than 20 feet, the person passes through
the beams faster since they are closer together, likewise generating a
smaller signal.
Therefore a person running by the sensor at 10 feet may register the same
as a person walking by at 30 feet, neither being sufficient to trip the sensor.
Automobiles will likewise be difficult to detect when the sensor is mounted
close to the road and they are moving fast. To compensate, you can either
move the sensor back from the road (fifty feet is OK) or you can aim the
sensor using a slight 70 degree angle, which in effect causes the sensor
beam geometry to widen. This will give much better results, especially if the
sensor is aimed upstream, since the target will be detected sooner. Also,
the RF transmission range between sensor and receiver should be kept
short, which will improve the overall reliability.
C.
Infrared Noise:
Infrared Noise includes all environmental disturbances that occur when the
sensor is not detecting a target, such as wind chill on stationary objects or
blowing tree leaves on moving objects. Much of these types of disturbance
are filtered out by the side-by-side orientation of the infrared beams. Thus,
heat rising from a blacktop road will hit both beams simultaneously, causing
a cancellation of their signals. However, under certain circumstances, a tree
branch moving back and forth as the wind blows can look like a person doing
the same thing. This can really be a problem if the sensor is aimed at the
solid surface, which is relatively warm due to sunlight, and the cooler leaves
from a branch are blowing in front of the Sensor. A blade of grass located a
few inches from the sensor, blowing in the wind, can also appear as a person
walking, again because of the smaller beam geometry at that distance.
To prevent unwanted false alarms or reduced sensor performance, the
sensor should always be located on a stationary object (large tree trunk),
which will not move when the wind blows (avoid smaller diameter trees).
Any offending grasses or branches should always be pruned away from the
sensor’s field of view. Avoid aiming the sensor at trees with moving
branches, especially at close distances (less than 10 feet).
