Apparently, there are active and passive sensors. An infrared proximity sensors should be an active sensor, whereas a gyroscope should be a passive sensor. Why is that the case? In general, what is the difference between an active and a passive sensor? Multiple examples (with appropriate explanations) may be helpful to clarify the distinction.
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1$\begingroup$ Not really @CroCo, if you think that to operate a camera or magnetometer you still need energy even though those sensors are meant to be passive. $\endgroup$– Ugo PattaciniCommented May 23, 2018 at 18:08
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$\begingroup$ @Ugo you are right. I've deleted the comment but the idea is very close to what I've stated but the definition of passivity in sensors slightly differ from electrical elements. $\endgroup$– CroCoCommented Jul 4, 2018 at 18:09
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2 Answers
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Let's recall that sensors are used to measure a physical quantity.
The distinction between passive and active sensor characterize the sensor ability to retrieve a measurement without or with inputing energy (mechanic or electromagnetic or else) in the environment.
For example, an infrared proximity sensor is active because the sensor needs to actively emit infrared waves in order to retrieve the measurement of the distance. So, any sensor which requires to input energy to the environment in order to retrieve the measurement is active (laser range finder, ultrasound range finder, sonar, lidar, time of flight camera, etc.).
On the other hand, sensors like gyroscope or accelerometers are named passive because they are able to retrieve a measurement without actively interacting with the environment. Other examples are: encoder, switch, camera, magnetometer, barometer, etc.
More complex sensor like microsoft kinect or intel euclide are composed of several sensors both passive (visible camera, IMU) and active (IR camera and lighting), thus making the full sensor an active sensor.
EDIT: added sensors based on comment
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$\begingroup$ A switch is not obviously a passive sensor as it mechanically interacts with the environment. (Similar argument also for encoders). And there are also active cameras (especially active stereo cameras like the Kinect1 or all TimeOfFlight-Cameras) $\endgroup$ Commented May 24, 2018 at 14:00
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$\begingroup$ I updated the answer to make it clearer, the issue s not the interaction it's the input of energy in the environment. The camera you are listing are not purely camera as the as they are systems composed of a camera and a lighting device, as such they are obviously active, as they light the environment (in visible or IR spectrum) $\endgroup$ Commented May 24, 2018 at 15:00
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Every sensor is used (or meant to) sense something about its environment, so it needs to "listen" to something (sound, light, magnetic field, ... ) generally be somehow affected by its environment to have some idea, what is around.
Pasive sensors just use, what came and do not actively emit that, what they are measuring. (well nearly anything emits some kind of energy (wires with electricity emits some small electromagnetic fields around, mechanical parts make noise when working, etc ...), but here the emited energy is not used for the detection)
Active sensors on the other hand DO emit something and then measure, how the environment is interacting with that emited energy (like ultrasound emits high frequency sound and then listen how it is reflected back, to measure distance)
Take example from horror movie:
So if you are just sitting on ground, silently hear to incoming steps, looking, if you can see someone comming, then you are pasive sensor (both for sound and light). You can then found, that hear some steps, thats are heavy and slow and increase in volume over time and have big echo, so probably it is huge man comming to you and you are in some big cave or extra large room. Then you see small red light approaching and smell smoke and deside, that he is probably smoking cigarette, is about 2m high and 10m away this direction (deciding from the burning point of cigarette and its movements) and you are still passive sensor, as you do not send anything out to get this informations.
Then you scream with fear and echo tells you, that the place, where are you is really huge, like 50m around you (as the echo of your scream take some time to reflect from walls and go back to your ears) and now you are active sensoring (with emiting scream to know the size of the cave) and he can simply listen to your signal and decide, that you are really there and you are scared (from sound of your voice) - he is passively sensoring you on sound. As you already given your presence, you light a torch and see, that it is big man in black suit with big gun in hand - you see it in light of your own light source, so now you are active sensoring on visible light too.
Should that happen on day, say in forrest, you would not need yor torch to see him in detail, so you will be pasivelly sensoring him on visible light and will not give out your position.
Well, you are still heavy braething and your heart pounds fast, but as you are not using this sounds for detecting anything, then it does not count as active sensoring (even when you are trying to hear his steps), just as a noise produced by you. Still he can sense that (pasively hearing) and find you in your hiding.
Lot of pasive sensors use something already existing around - light sensors use the light from sun and detect shadow cast by someone in proximity, or can detect burning torch in dark. Heat sensors can detect any heat source near and decide, that it is aproching (or appeared now) and react on that, press sensors use weight of item to measure it (kichen scale eq.), normal camera use daily light to get photo (camera with flash/lamp attached is active, ofc.), switch use the force of your finger to connect wires (the electricity there is not emited out for measuring, so it is passive), proximity sensors may sence difference in capacity and increased static and be passive, or may send out some signals and detect echo and be active and so on ...
