How can I detect changes in XML?

PID Motion Sensors ... How do they detect motion directly towards/away from them?

• I've had a look at how these motion sensors work ... http://www.thomasnet.com/articles/instruments-controls/light-motion-censors. Essentially they detect motion via infrared radiation passing onto a temperature - sensitive chip. When there's motion, a warmer or cooler spot moves round the chip, which can be electronically detected. That makes sense. But the chip is rather two dimensional, meaning such motion detectors should in theory have no trouble detecting motion across its field of view,in a two-dimensional manner. But they should struggle to detect motion in a direction directly towards or away from the sensor. However a little experimentation suggests they actually do detect motion directly towards/away from the sensor quite well.. My question is how do they achieve this? Is it because the hot/cold spot on the chip grows as the object emitting the IR radiation approaches the detector and shrinks as it retreats? Surely the changes that take place on the chip will be markedly smaller than if the object walked across the field of view?

• Answer:

  Thresholding and contrast detection coupled by the fact that such detectors are actually an array make it possible for them to do so. The contrast per unit area increases (essentially Signal strength - background) as the object comes near although the size of the spot actually decreases (doesn't grow as you stated). The array density determines the granularity of the forward backward movement detection based on the inverse square law of radiant emittance. I hope you got my point, if not, will gladly explain in greater detail. Edit: K, there is a lot to it which I simply cannot explain here. To get you started consider a few points, simple detectors depend on energy measurements only, they arent too great. If you use an array and feed the outputs of spatially adjacent detectors into differential amplifiers, they become contrast detectors, i.e., they develop the property of subtracting the background from signal...essentialy eliminating as much background signal as possible. The DA are usually built into the IC's. Thresholds become necessary in situations where there is minimal signal. You dont want the detectors going "on" with stray noise...