Sensing An Extremely Long Distance With Precision
Sensing An Extremely Long Distance With Precision
(OP)
Anyone could recommend an economical way to sense a steel plate at a distance of 16 feet with precision to 2 decimal places?
I've found Ultrasonic Sensors that can sense the distance, not the precision but costs a fortune.
Any response will be greatly appreciated!
I've found Ultrasonic Sensors that can sense the distance, not the precision but costs a fortune.
Any response will be greatly appreciated!





RE: Sensing An Extremely Long Distance With Precision
TTFN
RE: Sensing An Extremely Long Distance With Precision
I agree with IRstuff that laser instrument is technically the best solution, but you pay for the accuracy and contactless measurement. The better microwave sensors are just about able to resolve down to 0.01m, but again these are not cheap.
How big is the target?
What is your budget?
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If we learn from our mistakes,
I'm getting a great education!
RE: Sensing An Extremely Long Distance With Precision
sensor?
<nbucska@pcperipherals DOT com> subj: eng-tips
RE: Sensing An Extremely Long Distance With Precision
(1) The units are to be ±.10" (approx. 1/8")
(2) The target will be as big as the sensor will need it.
(3) Budget will depend on the customer ($4000 too expensive!)
(4) Distances? Min.=4 FT, Max.=15-16 FT.
RE: Sensing An Extremely Long Distance With Precision
is only mounted on the part you want to locate.
Could you describe the actual problem in some details?
<nbucska@pcperipherals DOT com> subj: eng-tips
RE: Sensing An Extremely Long Distance With Precision
RE: Sensing An Extremely Long Distance With Precision
Even with good instrumentation, this is a non-trivial time interval to measure.
TTFN
RE: Sensing An Extremely Long Distance With Precision
We are talking about a 12 bit ADC or better. Perhaps a 16 bit ADC and careful calibration to linearise the response. In any case this all seems more than would be desirable.
RE: Sensing An Extremely Long Distance With Precision
AND, that's assuming that you've ensured that the current source has been adequately corrected for linearity and temperature effects.
I've been involved in the development of this type of ranngefinder, and there are additional issues when your pulsewidth becomes a significant portion of the resolution, e.g., slope and amplitude of the return will also affect the error. You'd also need to use a constant fraction discriminator to ensure a consistently threshold point for the return processor.
Very likely, the resolution is low enough that there will be noise issues as well.
TTFN
RE: Sensing An Extremely Long Distance With Precision
My next thought was a "static" scheme with a laser, or at least a well collimated powerful modulated light source. If the laser and the receiver were 1m apart, and the laser were mounted on a stepper motor controlled positioning system, the angle of the laser could be adjusted to bounce the beam off the target and back to the sensor. The angular measurement would need to be very tight and there would be some trigonometry involved to compute the distance. The problem then is that if the target did not stay parallel to the measurement plane as the target was moved, the angle required would be all wrong.
Ultrasonics may be easier in terms of the velocity being lower by a factor of 1,000,000. The problem then becomes one of resolution. There would be insufficient resolution in a time domain pulse modulation system. Maybe a time domain pulse could establish the rough position and then a continuous sinusoidal beam could be used, the phase shift determining the position more accurately.
I guess the point is, one is having to develop a measurement system which is more accurate than commercially available equipment. In this case the development timescale and budget could be excessive.
This probably means the original specification is unreasonably tight and/or the customer’s expectation in terms of cost/performance cannot be met.
RE: Sensing An Extremely Long Distance With Precision
http://www.sickcommerce.com/master/compare.asp?FAM=Measure&ITEMLIST=26639,26641,26643&P=4841
Note that the resolution is 0.125 mm, which is 0.5 mils; my memory was off a bit
Even at $40/hr of time, that's only 100 hours. You can barely begin the design with only 100 hours.
TTFN
RE: Sensing An Extremely Long Distance With Precision
0.125mm is 5 mils isn't it? America's 1 mil is 1 thou to the English, and 25.4um = 1 thou.
Interesting exchange of ideas from you and logbook, although I think the OP wants an off-the-shelf solution. The resolution of the Sick product is impressive for a contactless system. Good link.
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If we learn from our mistakes,
I'm getting a great education!
RE: Sensing An Extremely Long Distance With Precision
You're right; that's what I get for taking the datasheet verbatim.
I've been itching for the instrument we bought to fall out of warranty so I can take a peek inside
TTFN
RE: Sensing An Extremely Long Distance With Precision
Not even that unrealistic. There are digital tape measures that can do that. Hook up the free end to the object and move it and record the reading from the tape measure:
http://store.yahoo.com/tylertool/mit2118digta.html
http://www.professionalequipment.com/xq/ASP/ProductID.1385/id.15/subid.126/qx/default.htm
Another possibility is GPS RTK system, but that's probably in excess of $4000, although the accuracy is probably only on the order of millimeters.
TTFN
RE: Sensing An Extremely Long Distance With Precision
Two lasers, several meters apart, pointing towards the stock at some arbitrary angle. The lasers are rotated until they each hit a detector in the middle (obviously only one is one at a time). The angle of the laser turntables at the moment the detector is hit may help determine distance to the object, as well as allowing the removal of error due to the stock's angle from parallel to the measuring setup.
The stock's angle can only be removed if the change is parallel to the ground, and not swaying back and forth on a hanging mount, but at least it's something to start with.
To be honest, this may or may not be feasible, I just don't have the brainpower tonight to determine if the geometry involved will really help solve the problem. Try it out...
RE: Sensing An Extremely Long Distance With Precision
Even at that distance, the beam spread is probably too large to get an accurate measurement, but now, you're stuck with two precision angle stages to calibrate and maintain, as well as designing the circuitry for two 4-quadrant detectors, or possibly PSD's, the multiplexing for the two sources, a robust centroid measurement algorithm, etc.
You can do this with only a theodolite, but a decent theodolite is more expensive than the laser measurement unit. Your theodolite can simply be placed in a known position with the two endpoints also known. Assuming that you can keep the retroreflector on the target on a line between the known endpoints, you can triangulate the position of the target with a single measurement.
TTFN
RE: Sensing An Extremely Long Distance With Precision
Anything homebrewed requires much more careful design to ensure adequate accuracy and performance over temperature and time and other environmental conditions. After you've completed such a design, built-it and tested it, you'll still need to perform a design validation and develop the means to maintain calibration. Since it's a custom design, you'll probably wind up doing the calibration in-house, requiring documentation, procedures and equipment to do the calibration as well as keeping someone qualified and trained to do the actual work.
TTFN
RE: Sensing An Extremely Long Distance With Precision
RE: Sensing An Extremely Long Distance With Precision
http://www.omega.com/ppt/pptsc.asp?ref=HH-DM
RE: Sensing An Extremely Long Distance With Precision
The instrument in question has a best case accuracy of 0.5 in and at 16 ft, only 0.96 in.
This one at least claims 1/8 in accuracy:
http://www.calculatorsource.com/pl-pls1.html
TTFN
RE: Sensing An Extremely Long Distance With Precision
Although, I did verify the "Pacific Laser System". This might actually work well!
Thanks IRstuff!
RE: Sensing An Extremely Long Distance With Precision
http://www.wenglor.de/english/lieferpr/taster/sindex4a.htm
The second unit from the right has a range of up to 20.7m, and a resolution of 2mm (.08"). Looks like it might need a reflector however.
The unit to the right of it has better resolution and makes no mention of a reflector, but has a range of 3.7m, just a little short of your needs.
No idea about cost but it doesn't LOOK expensive (not that this means anything!)
"Venditori de oleum-vipera non vigere excordis populi"
RE: Sensing An Extremely Long Distance With Precision
If someone has a similar application, you might want to look at this device.
http://www.plslaser.com/prodspecfs_pls1.htm