Methods of distance measurement?

[dave3141]

Hi,

I need some help with deciding what to use to measure distance in my project.

I have a camera stand and need to measure the distance between the sensor mounted on this and a vertical, hanging white projection screen.

Distance will be between 1 and 4 metres, and needs to be accurate to within 1mm.

It has to be electrical (no tape measures or rulers ) as I need to send a value back to my program.

 

[Nigel Goodwin]

Doesn't sound very easy! - you are aware that you're trying to get 0.025% accuracy?.

The obvious method is ultrasonics, but you won't get anything like that accuracy!, and differences in temperature and air density will give fluctuations far above your requirements.

Next I suppose is laser range finding, which is a LOT more expensive, and assuming you can get units that accurate?, you're talking a GREAT DEAL of money.

Why do you need such accuracy?

 

[The Mad Professor]

Hmm a hanging projection screen that alone will introduce serious errors.

As per usual a little more information would go a long way. But your new an desperate , so we shall be patient and wait for an enlightening reply from you, in the meantime I shall whistle the the "dambusters" theme and ponder a solution.

 

[Nigel Goodwin]

Have you stolen your signature line?.

It may seem like a good idea at the time , but never stir your cold coffee with a soldering iron.

 

[Oznog]

Have you stolen your signature line?.

It may seem like a good idea at the time , but never stir your cold coffee with a soldering iron.

Wasn't there a guy who copied your "karate" avatar and used it himself on the same forum here, Nigel?

 

[Nigel Goodwin]

Wasn't there a guy who copied your "karate" avatar and used it himself on the same forum here, Nigel?

Apart from it's a Ju Jitsu avatar, yes someone did :lol:

 

[dave3141]

I have been given a brief by a phd researcher.
I need to align a camera and projector, with the camera horizontal and the projector above camera at 14 degrees from horizontal. The imaging planes need to intersect at the projector screen.

I am just looking for methods of measuring this as i need to design the system. I can use matlab for processing the data, but it needs to be supplied by suitable sensors.

So if the screen isnt stable enough, then what other methods could be used? I can use the fact that the offset distance of frame holding screen and the screen are fixed and measure distance of frame which wont be moving.

I am not sure why the accuracy of 1mm is needed.

I'll get some more detailed diagrams loaded up in the next couple of days.

I have been reading alot of posts on this forum, and generally most people askign questions want the project done for them, I would just like a kick in the right direction.

Thanks.

 

[dave3141]

Also, if price is high I obviously wont be able to build the item, just design it.

This project is for equipment used to measure the deformity of patients backs, scoliosis, it is medical equipment, and price probably isnt an issue.

 

[akg]

Also, if price is high I obviously wont be able to build the item, just design it.

This project is for equipment used to measure the deformity of patients backs, scoliosis, it is medical equipment, and price probably isnt an issue.

ok..thinking of an idea similar to our eyes (since distance is short) .. ie a marking or something like, in the screen, then using 2 cameras to capture that , measure the distance based on triangulation with digital image processing .
just an idea , not tried about the resolution and all. but will be cheaper than laser

 

[dave3141]

Thanks, this is what i was aiming for. I was going to use 1 camera, with lines projected onto the screen horizontally and measure the height of the lines.

But, using 2 cameras, it should be easier.

 

[akg]

Thanks, this is what i was aiming for. I was going to use 1 camera, with lines projected onto the screen horizontally and measure the height of the lines.

But, using 2 cameras, it should be easier.

well.. after completing the project, i request u to post the results here

 

[electroniks]

Thanks, this is what i was aiming for. I was going to use 1 camera, with lines projected onto the screen horizontally and measure the height of the lines.

But, using 2 cameras, it should be easier.

You could use a checkered screen, then scan the screen with your camera and use matlab to calculate the curvature of each square and find the distance that way. This requires a good image processing program and good camera for that accuracy.

 

[dave3141]

I will be designing for use in matlab and using a canon eos 350D .

The plan i think is to use a slide system to move the camera between two points, with a leadscrew and linear slide mechanism. The distance between image centres will then be a known value.

If I can calculate the angle of the centre of each image then I can calculate the distance very easily using an equation I have derived from the sine rules.

I will try and get some results, I could probably do a mock up using my camera and a A3 printout a known distance away.

I have got until the 28th April to hand this in, and its only a small percentage of my dissertation! :lol:

 

[dave3141]

Thanks, this is what i was aiming for. I was going to use 1 camera, with lines projected onto the screen horizontally and measure the height of the lines.

But, using 2 cameras, it should be easier.

You could use a checkered screen, then scan the screen with your camera and use matlab to calculate the curvature of each square and find the distance that way. This requires a good image processing program and good camera for that accuracy.

Do you have any sources of research for this method please?

cheers mate.

 

[dave3141]

the other part of the project is to have an image projected onto the screen and the patients back when screen is removed, using a modified slide projector and target grating, or line grating(i think thats what they are called). One has a cross projected onto screen with corners of slide marked with right angles and the other is just a load of lines, with known amount per inch.

 

[Oznog]

Consider if you actually need absolute units.

For example, if I were making at autofocusing projector, rather than fight the battle of knowing an exact distance it might be a better idea to simply have a camera looking at the target screen and adjust the focus back and forth until the sharpest picture response is obtained. Thus neither the measurement nor the focusing lens need to be calibrated.

 

[dave3141]

Its not for the focusing, its just my brief that I need to output this distance digitally, and use it to correct the camera position.

 

[electroniks]

Thanks, this is what i was aiming for. I was going to use 1 camera, with lines projected onto the screen horizontally and measure the height of the lines.

But, using 2 cameras, it should be easier.

You could use a checkered screen, then scan the screen with your camera and use matlab to calculate the curvature of each square and find the distance that way. This requires a good image processing program and good camera for that accuracy.

Do you have any sources of research for this method please?

cheers mate.

I've heard it a lot and seen it on action a while back but I don't have any references right now. I'm pretty sure it is a wide spread image processing technique. (sorry my major is not image processing) but try google or better yet www.ieee.org for refrences.

 

[Papabravo]

Doesn't Phd stand for Piled higher and deeper.

If the screen has a fixed reference like the thing that looks like a framing square that the criminalists on CSI use and you had a graticule on the camera or projector lens would you be able to tell the distance from the plane of the graticule to the plane of the screen by processing only the image data?

 

[electroniks]

Doesn't Phd stand for Piled higher and deeper.

If the screen has a fixed reference like the thing that looks like a framing square that the criminalists on CSI use and you had a graticule on the camera or projector lens would you be able to tell the distance from the plane of the graticule to the plane of the screen by processing only the image data?

Of course! every measurement is known beside the distance between them, if you are handy with a calculator and you have passed pre-calculus (!!!) you should be able to do it!