Infrared proximity sensor WAY too sensitive to hi viz stripes - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: April 14, 2022, 06:55:20 am
content: We have been using some of these sensors p/n CS-PD438-PQ and when adjusted to trigger from a hand at 5-6 inches distance they work very well and we are quite happy with them. What we have discovered though is they are extremely sensitive to the silvery coloured stripes on high visibility vests and jackets commonly worn in some work situations. This will cause the sensors to easily trigger from a distance of 8-10 feet so people walking past end up causing unintended triggering.http://www.seco-larm.com/CS-PD438-PQIs there a way to eliminate this problem? Maybe some kind of adhesive filter to go over the sensor? Is this a common problem with all IR sensors?

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2022, 01:17:04 pm
content: If I were to engineer something like this I'd put either the infrared emitters or detectors say 10 cm above/below the button and make point them down/up with a small emission/acceptance angle ... but that doesn't really help you.It seems to just detect reflected IR strength from straight in front and retroreflectors are really good reflectors.

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 14, 2022, 02:36:49 pm
content: My assumption is that the switch purely rely on a threshold level of reflected light. Adding a filter in front will also reduce the normal detection distance. If you can live with that adding a filter in front will do. But if you want to reduce the 'wrong detection distance' by 50% you'll also reduce the normal 'detection distance' by 50%.But I can't think of another solution unless using a different sensor-switch or a different installation direction. E.g. 'looking' parallel to the wall if the viewing angle isn't too wide.To prevent such problem they should use a time of flight or radar sensor. If it has to work reliable I would skip PIR sensors as well. Any optical sensor using the amount of reflected light will show a similar effect.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 14, 2022, 02:47:25 pm
content: Quote from: Circlotron on April 14, 2022, 06:55:20 amSince they don't tell the sensors/technology used by the detectors, it's hard to say if there is any easy fix. If you are lucky, fake triggering might happen only to horizontal stripes and not to vertical stripes. Try keeping a vest so the stripes are vertical (for test only), and if the fake triggering is eliminated this way, then rotate all the senors in their fixture by 90 degrees.

author: Someone
author_rank: Super Contributor
timestamp: April 14, 2022, 10:16:35 pm
content: Quote from: Circlotron on April 14, 2022, 06:55:20 amIt is inherent to this design/model of sensor.Quote from: Circlotron on April 14, 2022, 06:55:20 amThere is no practical way to change the characteristic you dislike of that unit.Quote from: Circlotron on April 14, 2022, 06:55:20 amSome add-on optic could theoretically get closer to your goal, but you'd need to know the optical characteristics of the sensor to begin with.Better sensors have all this in their datasheet, beam shape, optical apertures, sensitivity at different reflectances. But your sensor does not.Given the vague suggestion of what your (completely unstated) requirements are, best guess is you actually want a capacitive or a specified range ultrasonic sensor. There are optical sensors designed for this task where you want a bounded sensitive range but they are either much larger, or use time of flight.

author: Circlotron
author_rank: Super Contributor
timestamp: April 15, 2022, 06:19:27 am
content: Quote from: Someone on April 14, 2022, 10:16:35 pmThat's a good idea actually.The application is a hands free entrance to a disabled toilet facility.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2022, 06:34:01 am
content: Those highly reflective materials are a real challenge in optical sensing systems. If you only look at the intensity, as you currently do, the only way to deal with this would move the light source away from the detector, so that the light comes from completely different direction. But then you could not sense the closeby objects which is your original goal.I don't think there's an easy way around this.When I was developing a time-of-flight system, this was even then a challenge, even though I did have the distance information (time it took for the light to pass, based on phase shift) available. This was because the intensity was so high it was saturating the sensor, even with multi-exposure HDR techniques. What I worked with was a relatively noise-free 12-bit sensor data and multiexposure with 8x integration time, resulting in 15 bits of real dynamic range. But with the specification of sensing darkish objects from 5..10 meters away, light objects 10cm away just blow out; but that was acceptable, just tag overexposed data as <10cm, right? But then, enter the high-visibility stripes of traffic cones, construction worker clothing, etc., and now suddenly said objects 2-3 meters away are tagged as <10cm.The root issue is that we normally consider the reflectance in reference to a perfectly white object which reflects the incoming light in all directions; think about a piece of white paper. If we call this 100%, then objects that reflect the light to one direction only can easily exceed 1000%. With time-of-flight, you at least have the possibility of increasing dynamic range to handle the sheer amount of light. But if your detection is only based on light level, then the reflectance of the object directly affects your distance threshold, and there is nothing you can do.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: April 15, 2022, 06:35:40 am
content: An other option could be using one of the time-of-flight sensors from ST. You should be able to measure the actual distance of the person from the door and act on it. They are now pretty cheap and, unless under strong sunlight, they should work.

author: Marco
author_rank: Super Contributor
timestamp: April 15, 2022, 07:54:34 am
content: Quote from: Siwastaja on April 15, 2022, 06:34:01 amThe easy way during design is angles, if the emitters are aimed at the floor and the ceiling the overlapping emissions cones of the emitters and acceptance cone of the receiver create a limited volume where objects can be detected. But the system gets larger, you need a vertical strip of leds above/below the button.That said, ultrasonics is probable the easier way to do it. Even with attenuation through a thin steel sheet it's probably doable.