The Lending Library at EnergyTeachers.org has recently acquired a SimplyTronics X-Band Motion Detector (XBMD). http://simplytronics.com/products/ST-00018
The device is commonly used to detect motion of any object within 10 meters, using doppler shifted reflections of its transmitted radio.
Radio waves reflecting off a moving object change frequency, more if the object is moving faster. The detector mixes the transmitted signal with a reflected signal, and the difference causes interference known as "beats." The frequency of those beats is 0 only if the two signals are exactly the same frequency, and it increases as the two signals diverge. So, the faster a reflecting object is moving, the higher the frequency of the beats. A simple multiplicative equation allows us to determine any unknown if we measure or know all the rest of the factors. Here is the equation where
d is the difference frequency or doppler frequency;
c is the speed of light;
V is the speed of the reflecting object;
F is the frequency of the radio signal; and
Θ is the angle between a line pointing forward from the detector and the object.
I have attached a jumper wire to the "test pin" (TP) of the circuit board on the XBMD, which allows us to measure the signal from the amplifier. The voltage between TP and the ground pin shows the beats amplified.
This image shows the beats for a person walking much slower than 1m/s. I can get the exact speed with the formula above, solving for V. I know the frequency of the radio waves is 10.25 GHz, and the speed of light is 300Mm/s. On the screen of the oscilloscope, the horizontal axis is time, and the time-difference between each dotted line is 25ms, so the beats repeat about every 80ms. That's a frequency of 13Hz. Θ is zero, because the person was walking directly towards the device. So V=0.5*13Hz*300Mm/s÷10.25GHz=0.19m/s.
This second image shows the beats for an object shaking with a top speed of more than 1m/s. Using the same method, taking a measurement, near the middle of the screen, of the time between beats to be 7ms, V=0.5*143Hz*300Mm/s÷10.25GHz=2.1m/s.
Compare to similar devices
This device can be used to open doors for approaching people/robots. Passive infrared motion detectors, much more common, detect the motion of warm objects like human bodies, but they can fail if the human is covered in insulating materials as they would be in very cold conditions.
This device has the same circuit as a "radar gun," the device you may have seen used to measure the speed of baseball pitches and cars.
A LIDAR instrument uses visible or near-visible light in rapid pulses. It requires very fast signal processing compared to the XBMD, which makes LIDAR more expensive, but it can be focused to measure along a line of sight. Very powerful lasers and highly sensitive, tuned receivers are used because LIDAR often measures back-scattered light rather than just reflected light.
A teacher should be careful not to say or imply that radio waves are not light. Radio waves are a different part of the spectrum than visible light, but the only intrinsic difference is the frequencies. There are practical differences between different frequencies of electromagnetic radiation, such as visibility and the ability to penetrate different materials or be reflected, scattered, or absorbed by them.
The time-resolution of the measurements are directly dependent on speed, since one must measure the width of the beats to get a single measurement. Measuring accelerating objects is systematically approximate.
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