Post by thread:distance sense

This is not really a PIC question, but the project itself will be controlled by one. I need to measure a distance from a 'sensor' to a wall which will have a rough surface. The measuring distance should not be any more than 5 inches. The PIC will be used to control the 'sensors' movement across the wall and to gather the distance data, and thus I should be able to map the surface contour. It would seem a reasonable problem except that the wall will be under murky water. Also the wall may have mud sticking to it at places so this means I could not have a sensor which actually touched the wall. If this happened then mud may attach itself to the sensor giving me inaccurate data. Any leads on a type of sensor would be appreciated. eg Microwave, Laser etc. Tony Just when I thought I knew it all, I learned that I didn't.

this sounds like a good project for ultrasonics. You could use the intensity of the echo to differentiate between water/mud/steel (ie the density of the reflector). According to TONY NIXON 54964: {Quote hidden}> > This is not really a PIC question, but the project itself will be > controlled by one. > > I need to measure a distance from a 'sensor' to a wall which will > have a rough surface. The measuring distance should not be any more > than 5 inches. The PIC will be used to control the 'sensors' movement > across the wall and to gather the distance data, and thus I should be > able to map the surface contour. > > It would seem a reasonable problem except that the wall will be under > murky water. Also the wall may have mud sticking to it at places so this > means I could not have a sensor which actually touched the wall. If > this happened then mud may attach itself to the sensor giving me > inaccurate data. > > Any leads on a type of sensor would be appreciated. > > eg Microwave, Laser etc. > > Tony > > > Just when I thought I knew it all, > I learned that I didn't. > -- Les Troyer Sr. Analyst Siemens Power Corp 2101 Horn Rapids Rd. Richland, Wa. 99352-0130 Voice (509) 375-8695 Fax (509) 375-8940 Operator (509) 375-8100 email spam_OUTljtTakeThisOuTnfuel.com Ad Hoc, Ad Loc, Quid Pro Quo; So Little Time SO Much To Know. -Jeromy Hillery Dillery Boo, PHD, MS and Q

> From: TONY NIXON 54964 <.....Anthony.nixonKILLspam@spam@eng.monash.edu.au> > > I need to measure a distance from a 'sensor' to a wall which will > have a rough surface. The measuring distance should not be any more > than 5 inches. The PIC will be used to control the 'sensors' movement > across the wall and to gather the distance data, and thus I should be > able to map the surface contour. > > It would seem a reasonable problem except that the wall will be under > murky water. Also the wall may have mud sticking to it at places so this > means I could not have a sensor which actually touched the wall. If > this happened then mud may attach itself to the sensor giving me > inaccurate data. > > Any leads on a type of sensor would be appreciated. Hey - I need one of these to clear the roots from my blocked drains! Are you going to attach a saw to it? Depending on the particle size, IR might be able to pierce the gloom. How about some sort of triangulation arrangement such as situating an IR source (laser) and photodiode at different points on the Z axis of the device. The detector has a small aperture so that it only 'sees' the laser spot at one point: Z---> det laser | | --|--aperture scanner | / | / | / | / | / | / | / <------> | / | / | / | / | / | / |/ =============================== wall The scanner could be a galvo mirror or rotating prism. If the PIC knows the angle of the beam from the Z axis (theta) and can pick up the 'blips' from the detector, then it can calculate the distance of the wall as distance(laser-detector) * tan(theta). Regards, SJH Canberra, Australia

TONY NIXON 54964 wrote: > > This is not really a PIC question, but the project itself will be > controlled by one. > > I need to measure a distance from a 'sensor' to a wall which will > have a rough surface. The measuring distance should not be any more > than 5 inches. The PIC will be used to control the 'sensors' movement > across the wall and to gather the distance data, and thus I should be > able to map the surface contour. > > It would seem a reasonable problem except that the wall will be under > murky water. Also the wall may have mud sticking to it at places so this > means I could not have a sensor which actually touched the wall. If > this happened then mud may attach itself to the sensor giving me > inaccurate data. > > Any leads on a type of sensor would be appreciated. > > eg Microwave, Laser etc. Microwave does'nt work so well in water, and laser won't work in muddy water. Try Ultrasonic transducers. -- Friendly Regards Tjaart van der Walt ______________________________________________________________ | Another sun-deprived R&D Engineer slaving away in a dungeon | |WASP International GSM vehicle tracking and datacomm solutions| | +27-(0)11-622-8686 | http://wasp.co.za | |______________________________________________________________|

Tjaart van der Walt wrote: {Quote hidden}> > TONY NIXON 54964 wrote: > > > > This is not really a PIC question, but the project itself will be > > controlled by one. > > > > I need to measure a distance from a 'sensor' to a wall which will > > have a rough surface. The measuring distance should not be any more > > than 5 inches. The PIC will be used to control the 'sensors' movement > > across the wall and to gather the distance data, and thus I should be > > able to map the surface contour. > > > > It would seem a reasonable problem except that the wall will be under > > murky water. Also the wall may have mud sticking to it at places so this > > means I could not have a sensor which actually touched the wall. If > > this happened then mud may attach itself to the sensor giving me > > inaccurate data. > > > > Any leads on a type of sensor would be appreciated. > > > > eg Microwave, Laser etc. > > Microwave does'nt work so well in water, and laser won't work in muddy > water. Try Ultrasonic transducers. > You have a very formidable problem with ultrasonics: the impedance change from transducer to air to water to wall to water to air and back to the transducer. At each of the media interfaces, there will be a high proportion of reflected energy. Of course, that is what you want to get the echo. However, the echo that you really need is the one from the wall, and its energy will be diminished by all the losses in between. If there is a way for you to put the transducer (piezoceramic) in constant contact with the water, things will be much easier. However, note that measuring such short distances in water won't be particularly easy, either. You will need to operate at a relatively high frequency to get the required resolution and to have a transducer that doesn't continue ringing right through the echo return. The above assumes echo signal processing. There are continuous wave distance measuring schemes that may be preferred for this sort of application. -- Paul Mathews, consulting engineer AEngineering Co. optoengKILLspamwhidbey.com non-contact sensing and optoelectronics specialists

{Quote hidden}> This is not really a PIC question, but the project > itself will be controlled by one. > > I need to measure a distance from a 'sensor' to a wall > which will have a rough surface. The measuring > distance should not be any more than 5 inches. The PIC > will be used to control the 'sensors' movement across > the wall and to gather the distance data, and thus I > should be able to map the surface contour. > > It would seem a reasonable problem except that the > wall will be under murky water. Also the wall may have > mud sticking to it at places so this means I could not > have a sensor which actually touched the wall. If this > happened then mud may attach itself to the sensor > giving me inaccurate data. > > Any leads on a type of sensor would be appreciated. > > eg Microwave, Laser etc. > > Tony ------------------------------------------------------------------- Tony, I see that you probably need a more thoroughgoing response to your question. As has been pointed out, the most appropriate vehicle for this application is ultrasound. This is due to the ability of ultrasound to penetrate the various layers and media you mention and still return a usable echo. The trick is to be able to sort out all the different echos due to the different layers and, ultimately, to be able to distinguish the one layer you are looking for, the surface behind the layer of mud. To do this, you need an array of ultrasonic transducers or, alternatively, a pair of transducers whose separation distance can be varied and, depending on your needs and the geometry of the target, that can be rotated about the line of position between the sensor and the target. The most powerful way to understand how the target information is extracted or deduced is in terms of the abstract two-dimensional Fourier representation of the target (with intervening mud layers, etc.). What you need to do is essentially the same process that is used, for example, in long baseline interferometry, an example of which is the radio interferometer (VLBA) in Socorro, NM. If you are familiar with it, you will recall that it consists of an array of antennas that can be combined electronically in different spacings and at different angles around the line of observation. In your problem, the ultrasonic transducers take the place of the radio antennas, but the underlying mathematics is the same. The various combinations of spacings and angles produce samples of the Fourier representation of the target. With sufficient samples, and with due regard for the equivalent of the Nyquist criterion (for a spatial, rather than a temporal, function), the Fourier representation can be inverse transformed to reconstruct the target with its detailed structure (here the ability to discriminate the various confounding layers). Essentially the same process is used for image reconstruction in the medical CAT scan. Beyond this sketch, I can only say that this is a very complex problem and that to be any more specific one would have to take a serious look at the details and the level of performance that is required. Once these are established, one would have to do some serious mathematics to derive an algorithm, akin to the VLBA or the CAT scan, but appropriate to your particular application. If you conclude that this is the kind of "horsepower" that you need to bring to bear, I can offer you some help. This is an area in which I specialize. It would be appropriate at that point if you contacted my by private e-mail to discuss it further. I hope this is helpful. --- Warren F. Davis ================================================ Davis Associates, Inc. 43 Holden Road West Newton, MA 02165 U.S.A. Tel: 617-244-1450 FAX: 617-964-4917 Visit our web site at: http://www.davis-inc.com ================================================