PIR sensors on a moving platform
A.C. fishing
Posts: 262
Will the PIR sensors not work on a moving platform? Like a rotating platform, or a boe-bot? I bought a burgalar alarm from RS, and it would't work on a robot.
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·Somebody said...
-Never Underestimate the power of human stupidity.
Comments
·· That is correct.· Because of the way they operate PIR sensors will not work on a moving platform since it is movement that they detect.· This is done by detecting rapid changes in the surrounding IR field which will almost certainly happen in a normal environment in motion.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
http://www.drrobot.com/products_item.asp?itemNumber=DHM5150
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My RoboScouts have PIR sensors on them and they move back and forth.
Kenny
·· It depends on how they're made...The kind you're using probably return either an analog voltage relative to the "heat" the unit sees or a logic 1 when the "heat" has reached a certain threshold at a certain wavelength.· The ones used in alarm systems usually detect changes in the level of heat.· That includes ambient room temperature too.· Depending on the room conditions they can be set off by A/C or Heat coming on or even the sun suddenly shining on a facing wall.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
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Chris Savage
Parallax Tech Support
csavage@parallax.com
I have one of the parallax sensors and I'm trying to use it in a outdoor/indoor wireless alarm sysytem. The wireless part works great as do the indoor sensors but the outdoor use of the PIR motion detectors are giving me fits.
Once outside I receive considerable falsing of the parallax PIR sensor and I'm looking for something that does not false in high temperature to low temperature environments, maybe something like a dual PIR/microwave detector which I haven't found in an OEM form yet.
I even found the original docs for the parallax sensor and added the adjustable photo cell for outdoor use, this eliminated some of the falsing but it's still not good enough.
Onyone know of a better choice for a relible outdoor human motion sensor. I can still make use of the parallax sensor for the indoor part but I badly need a reliable outdoor sensor.
Mike
I've not seen the Parallax PIR Sensor, but if it doesn't explicitly say it's suitable for outdoor use, I would presume it wasn't. Generally speaking outdoor units are hermetically sealed to prevent the intrustion of dust, dirt, moisture and the like. There may also be temperature compensation. Commercial PIR units are available for outdoor use. Most alarm supply houses carry them.
If it is suitable for outdoor use, you might try putting a shield above it, and try not to point it towards the East, if you can help it. My gut feeling is that you may be seeing the effects of the sun, if it's presently fully exposed.
Hope that helps.
Regards,
Bruce Bates
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By the very nature of the operating principle of simple PIR detectors, they need to be standing still to properly detect a moving heat source such as a human. It really has little to do with being inside or outside, or with the amount of ambient light.
A sensor on a mobile platform could work, but only reliably if it is stopped long enough for its circuitly to stabilize before taking a reading.
Depending on the device's design, for outdoor operation there could be a significant reduction in sensitivity due to ambient heat sources caused by sunsihine, effectively making the signal you are trying to discern relatively smaller in magnitude. Any wind causing these heat sourses to sway in the breeze would render these sensors almost useless.
Hope ths helps.
Cheers,
Peter (pjv)
I even have it in a piece of PVC to shield it more from ambient light and heat sources. But if it's inserted to far into the PVC it doesn't detect anything and if not inserted far enough in the PVC then it constantly falses, but only when placed outdoors.
There is no motion at all in the test area either but it is very hot outside even in northern Arizona at 6000 feet elevation the temp is around 90 to 98 F.
I have a simple controlled area with no trees and nothing moving at all but still some falsing of the detector. When I take it to an inside controlled test chamber which is under 85 F there is no falsing at all which leads me to lean towards ambient heat changes and or cloud cover movement.
When I added the photo cell and adjustable pot to force the electronics to lower the gain under high ambient light conditions the falsing did drop according to the scoped output of the PIR but there is still enough to make it unreliable.
I did a lot of googling last night and found that this is a very common problem with simple PIR sensors when used outside and as a result the most recent trend is to use dual sensors with a PIR and a Microwave motion detector and'd together at the output.
The problem is that most of these dual technology sensors are already in a packaged box and I need just the sensors and electronics in the same or similar form as the parallax version for inclusion in my package.
My next step is to add an outboard microwave sensor to the BS2 circuitry and change the code so that it's looking at motion from both sensors. This way I can at least see if the dual sensor method is feasable.
Mike
·· Thanks for your input, as I was trying to convey this above, but apparently no effectively.· I was wondering if the confusion is caused by certain sensors which I think are more appropriately called, "Flame Detectors".· These are a special IR sensor used by firefighting robots.· I read an article on them once.· They don't work on the same principle and can be used on a moving platform because they merely report the relative amount or IR.· It's up to the person to focus the field of view and know when it is seeing a flame (or other heat source).·
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Chris Savage
Parallax Tech Support
csavage@parallax.com
I believe the Parallax unit is intended only to recognize motion [noparse][[/noparse]like someone present in a room].
Additionally, there are pre-packaged arrays of both and fresnel lens optics to enhance their functions.
I bought four PIRs and thought I had Thermopiles as the package looks the same. {I had wanted Thermopiles for building a copy of Jen's R/C Autopilot from the SX Contest. When paired in opposite directions, they can reliably see the horizon in an R/C aircraft regardless of day or night.}
I too am a bit disappointed with the inability to mount to a moving platform, but I now see that platform jitter and relative motion·can easily be percieved of as a motion trigger. I had never considered the 'shimmer' of heat waves. But since it is logically an optical IR phenomina, the PIR will see it as motion.
The jitter might be removed via a digital smoothing or some other filtering algolrhythm.
Nevertheless, I am sitting with a schematic that uses an LM324 to moderate the sensitivity of a basic PIR unit and I intend to build one. I presume that with enough of the right filtering, the utility goes way up. If software or analog circuitry could adjust the background sensitivity, it would be a lot more useful. My fresnel lens that came with the unit is actually about 20 lens of varied focal lengths and sees multiples of the same image. While it enhances the distance and provides a wide-angle for the PIR in a security sensor setting, I suspect that good conventional optics and a single lens might offer better robotic results.
Take a look at www.plazaearth.com/usr/gasperi/pyrosche.GIF for a schematic of a directional PIR. I have two other printed out schematics, but no reference to origin.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
Post Edited (Kramer) : 7/6/2006 2:37:35 PM GMT
You have it exactly backwards.
A single lens will not work at all (for simpler PIR sensors). It is the presence of the Fresnell lens that makes the whole thing work. The different "focal length" segments are very cleverly designed to effectively cause the single element silicon sensor to alternately "see" and "not see" the heat source as it moves in the field of vision. It is this "see and not see" effect that looks like an AC signal and hence can be easliy amplified to a great magnitude, leaving out DC and slowly varying illuminations.
So in a static situation when the PIR is not moving, and any heat sources are fixed in its field of view, the total intensity on the single detector is a constant. When a heat source moves to another "focal point" in the multi segmented Fresnell lens, a new "see and not see" level is established, and hence an output for amplification.
Alternately, when the heat sources are static and not moving, but the PIR sensor itself is moved, all hell breaks loose as the different focal segments constantly get new levels hence a falsing signal. And because these levels are totally unpredictable, there is no amount of filtering or detection algorithm to counteract that.
So the long and the short of simple PIR sensors on a moving platform, or even rotating on an axis will not work.
Period.
Any results you may be seeing are a figment of you imagination, or you have something different from a "simple" PIR sensor; a multi-segment detection element is required.
Cheers,
Peter (pjv)
Post Edited (pjv) : 7/6/2006 3:42:49 PM GMT
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· These are probably the sensors I am referring to which detect heat, and not the motion of heat, which is what most PIR sensors do.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
once again I appreciate your vast technical knowledge. I presumed the traditional kind of optics would be best, but this is leading edge technology. I guess I am stuck with these 4 PIVs and fortunately, there lens. They cost about $6USD each, so I am not too concerned.
If anyone wants one or two, send me a PM as I am more uncertain that I will get a useful application from them. They have a huge range with the fresnel lens - maybe 30 meters and they are wide angle. I can provide 3 or 4 schematics for adding an LM324 op amp to complete the output. The output goes to a pair of one-shot multivibrators that send directional impulses to eventually control a relay and motor.
A couple of important things to know are:
1. This sensors are very sensitive to being soldered. They should be soldered with a heatsink attached to each lead between the device and the circuit board. A flush mounting may cause heat damage.
2. You can get them with different specific light spectrum sensitivity. Not all have the same IR range. In some cases, this might eliminate false triggers.
And, if you really want to get a good PIR or Thermopile, buy directly from the manufacture as they have a diverse inventory of devices with the lens. Google came up with a place in Santa Rosa, California that produces these. You best be willing to pay more though, as I suspect that $50USD a piece would not be unreasonable of small quatities.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
Did you say 30 meters (100 feet) with a Fresnell lens? That's an unbelievably good range if it's reliable!
Cheers,
Peter (pjv)
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Chris Savage
Parallax Tech Support
csavage@parallax.com
And so I thought I could change to different optics on a robot.
I would be reluctant to say they are reliable as the documentation is mostly in handwritten Chinese - 1 sheet. Obviously a summary translation. The unit is a PIR-145, actual manufacturer unknown. 30 feet could have been mistranslated at 30 meters.
There are three bands of lenses in one piece of plastic about 1 1/2" by 2"; near, medium, and far.
There goes my credibilty [noparse][[/noparse]again].
http://shop.cpu.com.tw/product_info.php/products_id/3144
Here is the unit [noparse][[/noparse]HMS-PIR145]·with two pages of English info, but·a bit·cryptic.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
Post Edited (Kramer) : 7/7/2006 4:55:16 PM GMT