Hello all --- Having no idea of the range of motion and resolution/repeatability you are looking for I'm not sure that this approach will be of any use. In the '80s and 90's I worked several years for a client that built medical X-Ray systems. Such a machine is essentially a low precision robot or industrial grade positioning system. An essential element of the control system was a linear sensor to measure the position of various arms and slides that held the X-Ray tube and imaging device. We looked at a number of devices: 1. Laser interferometers (too expensive). 2. LVDTs (limited range of travel) 3. Gray coded sensor bars and optical sensors (too expensive and clumsy to get the desired resolution). 4. Optical encoders such as used on precision milling machines (Technically good solution, but too expensive for a multi-axis system and very sensitive to mechanical alignment. 5. And a million other ideas, some of them pretty loony :-) . The final solution was a Rube Goldberg inspired device affectionately known as a "string pot". It consisted of a grooved or threaded drum around which a small stainless steel aircraft cable (about 0.032" diam) was wound. The drum and cable arrangement was pre loaded by a clock type spring motor. The tension was quite high, probably around 15-20 pounds force. (The thing could produce quite a nasty cut/burn if you let it get away while pulled well out against the string :-( ). Finally the drum drove a ten turn potentiometer through a pair of gears. The position of the potentiometer shaft was measured by a twelve or fourteen bit A/D converter. Performance -- Range of motion - Typically 250 to 1000 mm depending on drum diameter and gear ratios. (Range of motion is virtually unlimited as it is controled by the drum diameter and gear ratios.) Resolution - Reliably better than 1 part per 1000 of total travel. Repeatability - Excellent, essentially equal to resolution. Absolute accuracy - also 1 part per 1000 after calibration. Suitability for this application??? 1. Temperature stability was not evaluated (working in a hospital environment has some advantages). 2. Micro controller (A/D) interfacing required. If anyone is interested, I can provide more details on this wacky device. Best wishes, Bob Smith Jim ODonnell wrote: > > Sean Thomas et al,.. > > A student at UNLV would like to measure movement across some fissures in > Las Vegas. The fissures are cracks in the soil (deep alluvium) about 10m > long in an area of mapped faulting. I was wondering if strain meters > would do the trick, the cheaper the better,and can you recommend or > suggest any methods or devices that would work? > > Jim O'Donnell > Registered Geologist No. 1240, State of California, 1970 > Registered Geophysicist No. 158, State of California, 1974 > 702 293-5664 > jimo17@........ > __________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email PSN-L-REQUEST@.............. with > the body of the message (first line only): unsubscribe > See http://www.seismicnet.com/maillist.html for more information. -- --------- Avoid computer viruses -- Practice safe hex ------------- * * Specializing in small, cost effective embedded control systems * * Robert L. (Bob) Smith Smith Machine Works, Inc. internet bobsmith5@.............. Lumlay Road landline 804/745-1065 Richmond, Virginia 23236+1004 __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>