Distance Sensor Advice Please

Hello, it's nice to meet everyone. I spend a fair amount of time in General Electronics Chat and Electronics Project Design/Ideas/Reviews but have never posted in here before. This forum seemed most fitting for my question though. I've searched a little for an answer but haven't really found a good one. Hopefully, someone here can help me out.

What I'm trying to do:

I’m working on a project where I need to measure distance with an accuracy of ± 1mm (~ .04") at the most. Better accuracy is obviously preferred if possible. The distances I will be measuring will be in the range 150mm - 750mm (~ 6" - 30").

Solutions considered:

I have worked with several of the Sharp IR distance sensors before but I recently learned that they have a built in 8bit DAC that provides the analog output. Unfortunately, 8 bit resolution would not provide the accuracy I require. I have also thought about laser range finders but I'm on a budget. That pretty much brings me to ultrasonic sensors.

I haven't used one before and after doing some searching on the web, I didn't have much luck finding accuracy specs on many of the pre-made ultrasonic sensors. I think the sensor that might work best is the PING from Parallax. The data sheet says that it outputs a pulse of a duration dependent on the distance that was measured. I could easily feed that into a PIC and measure the duration of the pulse. If I clock the PIC at 20 MHz and I can measure the pulse with and accuracy of ± 2 - 3 µs, that would give me the accuracy I need.

My questions:

1. Does anyone have experience using a distance sensor to obtain this level of accuracy?
2. Is there anything inherent in the design of the PING sensor that would limit the accuracy I could obtain?
3. Does my idea seem feasible?
4. If you were doing the project, how would you do it?

I hope my ideas and questions are clear. Thanks in advance for the help and I'm looking forward to your responses.

The object being sensored has great bearing on the method to be used. Sonar is probably less robust for your application.

If I needed this resolution and accuracy in the distance stated I'd build a laser/ccd camera combo. Both units are mounted on one leg of a triangle. The laser beam is the second leg. The camera views down the third leg. The placement of the return laser image in the camera is a function of distance. The camera could have a filter sensitive to the wavelength of the laser to reduce input noise.

Some Sharp IR sensors use a similar method, but with infrared instead of laser.

I've seen underwater video where two lasers act as to legs of the triangle. The distance apart they appear on an object allows distance measurement.

bobledoux said:

The object being sensored has great bearing on the method to be used. Sonar is probably less robust for your application.

If I needed this resolution and accuracy in the distance stated I'd build a laser/ccd camera combo. Both units are mounted on one leg of a triangle. The laser beam is the second leg. The camera views down the third leg. The placement of the return laser image in the camera is a function of distance. The camera could have a filter sensitive to the wavelength of the laser to reduce input noise.

Some Sharp IR sensors use a similar method, but with infrared instead of laser.

I've seen underwater video where two lasers act as to legs of the triangle. The distance apart they appear on an object allows distance measurement.

Click to expand...

bobledoux,

Thanks for the reply. I haven't played with CCDs before so I think your idea would take a little "getting up to speed" for me to put it into practice. It is interesting though. I'm having trouble picturing the arrangement that you describe in the second paragraph of your reply. The underwater video you talk about in the last paragraph makes senses, is it like that?