Extremely high precision close range rangefinder

[jman11]

Hi everyone first thank you so much for your help. I have been working on this problem for about a month and have been drawing blanks.

I'm looking to create a rangefinder with roughly 500 micron accuracy over a range of 2-5mm.

Any recommendations?

I was initially thinking capitative would be the only thing that would give me the desired accuracy. However as the media being measured will change frequently I believe this will not work as it would have to be recalibrated for every measurement.

Thanks again,

Josh

 

[ronv]

Tell us more about what is being measured..
Check my conversions if you would.
500u = .020 inches, 2mm = .08 inches?

 

[KMoffett]

1 micron = 3.93700787 × 10-5 inches
1 micron = 0.0010mm

Ken

 

[jpanhalt]

I'm looking to create a rangefinder with roughly 500 micron accuracy over a range of 2-5mm.

It is easy to assume that sentence means that the distance from the sensor to the object will be a minimum of 2 mm, and that is a problem. But, that may not be what you mean.

If the sensor can be further than 2 mm from the object, say 2 cm, some ultrasonic sensors may be able to measure to that precision (0.5 mm) over a range of movement of 2 to 5 mm. Such sensors operate at about 200KHz to 500KHz.

A second consideration is how big is the object that needs to be detected? Will a flat surface be facing the sensor?

Third, what is the object made of?

And finally, how fast does the measurement have to be?

John

 

[jman11]

Everyone thanks for the input, I should have been a little more specific.

On second thought Jpanhalt it can be located as far as 5 cm. The distance from the object isnt nearly as important as the accuracy of the measurement.

It will be sensing the distance from human skin. The area will be roughly 1 cm^2 which normally will be flat but at times may be angled up to 45 degrees.

I was thinking an ultrasonic sensor might work too but with the high frequency wave required would it actually be able to propagate through air without interference? I wil not be able to have any medium in between the rangefinder and the object.

 

[jpanhalt]

The frequency (<500 KHz) used for such sensors propagates through air just fine. The oil you may be thinking about for ultrasonic imagining is to prevent reflection at the skin. They also operate at considerably higher frequencies usually.

The units I have seen are generally for locating hard solids, like plastics and metals. I am not sure of the reflective properties of skin and how much it varies from person to person or location to location on the body. So, while 0.5 mm precision may be attained with solids, I suspect with soft tissues it may be a problem. Why do you need such high precision?

The spot size for the ultrasonic sensors is shown in the datasheets. If the geometry changes from normal to 45°, I suspect the precision (however it is defined) will also suffer.

John

 

[misterT]

This is interesting, but I think you are still hiding some important information. Could you describe the whole system in more detail? Is there a limitation to the size of the sensor/device? Does it have to be contactless? Budget limit.. cheap mass product, insturment for one time scientific study or a proof of concept etc.?

 

[jpanhalt]

I just checked the Hyde Park sensors that I have used. The maximum allowable tilt of a flat plate is ±10°.

Here is a link to one version of the general catalog: https://www.electro-tech-online.com/custompdfs/2012/11/hydepark.pdf
Hyde Park offers distance measuring with 2 or 3 versions (check the index). Series 606 begins on pdf page 127 (catalog page 4-93).

The variable incidence from normal to 45° to the surface will be a problem with any reflection-based system.

John

 

[JimB]

Time for a reality check here!

500micron accuracy, the target is human skin, the target can be angled up to 45 degrees.

Unless I have missed something here, this all sounds like a total disconnect from reality.

JimB

 

[jpanhalt]

@JimB

I agree about a reality check. It is not infrequent that someone starts a project with unattainable specifications while we all scratch our heads asking for more information.

I actually get a little fun out of trying to guess the project in less than 20 questions.

Let me throw out a w.a.g. for this project. Based on the 0.5 mm precision requirement and skin, is it possible that the project is for a device to do laser ablation or biopsy of superficial skin growths, such as basal cell carcinomas? I doubt it will be cheap.

John

 

[jman11]

Hello All,

Good guess. Actually this device is an Infrared Laser that causes action potentials in nerves. One of its applications: It is a useful tool for neurosurgeons to ensure that they are looking at the right nerve (or to verify that a tendon is in fact not a nerve). Because of the variety of applications, we want to design a rangefinder that will work for a variety of surfaces, most likely human skin or the perineureum around a nerve bundle.

I don't expect that the angle range truly needs to be up to 45 degrees...

 

[Mr RB]

It's typical in focused laser apps to use the laser itself as the "rangefinder".

Have a good read up on the lasers as used in CD players and burners, and the systems already developed for detecting distance and maintaining focus as distance changes etc.