Hi-Precision Distance Measurement Sensor - Ultrasonic or Optical??

try keyence laser sensors, the expensive one

Hello Dodskie:

my first thought is whether the cylinder will flex more than the 1 'thou' you mention. It being a cylinder if it is not precisely balanced and the speed controlled it may well flex and give false readings. Just a thought..............

0,001 mm ... OMG ... on a rotating cylinder ... OMG OMG ...
That's not gonna be easy... Maybe instead of measuring a distance/thickness, it might be easier to measure the color. Since I work in a printing factory, I know that the thickness of the ink layer is measured by it's color. But even doing so, the exaxt thickness can only be measured on samples, not while the machine is running ...

The color changes when the thickness of the layer changes, no ?

Just a thought ... Good Luck !

Hello 44mEurope:

I had the same reservations as you. Not easy if it is possible in the first place. The best that you may perhaps get is an 'average' colour or thickness?

"0,001 mm ... OMG ... on a rotating cylinder ... OMG OMG ...
That's not gonna be easy... Maybe instead of measuring a distance/thickness, it might be easier to measure the color. Since I work in a printing factory, I know that the thickness of the ink layer is measured by it's color. But even doing so, the exaxt thickness can only be measured on samples, not while the machine is running ...

The color changes when the thickness of the layer changes, no ?

Just a thought ... Good Luck !"

interesting application. Can you expound further how the thickness was measured through its color? Thanks,

The operators use a GretagMacbeth D19C color density checker. First they measure the color of the blanco paper, then they measure a printed surface (about 1mm² is enough). The device tells them how dark/dense the inked surface is. If they want it darker, they increase the amount/thickness of ink. Nobody could tell me the real ratio between actual thickness of the ink layer and the measured color density, but they asured me that we're talking about thickness differences ranges like 0,0001 mm or even less..

That color density meter comes with different (color)filters and other accesories. Depending the surface and the actual coating, there will have to be made some tryouts.

This whole color density measuring thing is certainly not a "plug and play" sollution. But it might be worth talking to color-density-meter-manufactorers...

Again : Good Luck !

Good to know the method, which I could imagine correctly.

But I think, the method is sutable for the ink with suspensions in liquid. But powder paint is something different, I supose. Will it work on powder paint?

Hi, thanks for the thought. The way I thought of it is, first to get the baseline measurement on a specific location of the cylinder without the coating which will serve as the baseline. Apply coat on the cylinder and track the same location(measured without the coating) and measure the new distance. The coating thickness should be the difference of the 2 distances. The reason for the high accuracy reqt is that the powder has a diameter of about 5~10 microns.

Suggest you consider measuring the electrical capacitance of the powder coating. Assuming the cylinder is made of a conductive material, you would put a conductive brush against the powder coating and measure the capacitance between the brush and the cylinder. The capacitance will be directly proportional to the coating thickness, and can be measured electronically at relatively high speed.

I recently had an experience in your problem. The keyence detector can work in theory, but this and many detectors like it use a shadow effect. The detector itself is ridiculously accurate, but because your coating differs in thickness 5 to 10 microns and laser works by a shadow effect, the hill and valley shape of the coating on a rotating cylinder, and the possible arrangements of the detector, will give an upper average due to the peaks (10 microns parts) forming the shadow (thicker than actually is) and when taking the overall average over measurements this may not be accurate for your process. It also depends on the size of your sample and the amount of hill/valley structure, but in my case it didn't work.

I recently had an experience in your problem. The keyence detector can work in theory, but this and many detectors like it use a shadow effect. The detector itself is ridiculously accurate, but because your coating differs in thickness 5 to 10 microns and laser works by a shadow effect, the hill and valley shape of the coating on a rotating cylinder, and the possible arrangements of the detector, will give an upper average due to the peaks (10 microns parts) forming the shadow (thicker than actually is) and when taking the overall average over measurements this may not be accurate for your process. It also depends on the size of your sample and the amount of hill/valley structure, but in my case it didn't work.

I guess you're right. But if I just get the average of the peaks and not consider the valleys, that should give me the closest possible measurement. Were you able to find an alternative solution?

Suggest you consider measuring the electrical capacitance of the powder coating. Assuming the cylinder is made of a conductive material, you would put a conductive brush against the powder coating and measure the capacitance between the brush and the cylinder. The capacitance will be directly proportional to the coating thickness, and can be measured electronically at relatively high speed.

The powder I am referring to in this case is not cured and am afraid that a brush would sweep off the powder once it comes into contact with it.

I would still consider the capacitance approach. It can easily be done on a non-contact basis. With an unfired coating, the measurement will be affected by the density of the powder coating as well as its moisture content. If you can locally remove a small amount of the coating, or supply a witness sample for testing, the project would be simplified.

Hello Dodskie:

can you explain why the measurement has to be done on un-cured powder and while it is moving. And why it has to be so accurate?

I know it is not in the question but,.............just the bare metal before application of powder will vary anything up to (1 'thou').

A recent question asked about the thickness of 'Electro-Plating', which is between 5/10 Microns. Sorry for the tutorial but, there is 25.4 Microns per (1 thousandth) of an inch, so you can see the measurement is very much finer than you refer to. But, even on this the thickness measuring instrument can often measure the peaks and valleys~when testing a moving object.

If the powder is not cured the hills and valleys are going to be huge. Far exceeding your plus and minus!

Think again perhaps OK? Is this an on going problem, or, a completely new job or application?

No offence intended, but most all the replies to your thread are saying similar things.

Good luck and keep us informed please.

1. I do not uderstand from where this 1 thou figure came in discussions. Original posting clearly says 0.001mm.... that is 1 micron.

2. Even if the powder is uncured Welderman's method should work. One electrode for capacitance measurement is the conductive cylinder itself and other one may be outside electrode, which may be kept at a distance little more than the thickness of the maximum coat. This will create capaciatance which has two layers between the electrodes.. one is powder coat and other is air layer. If outside electrode is big enough, we should get the capacitance variation proportional to coat thickness.

Hello gsuhas:

I must apologise, I read the original post wrongly.

I agree with what Welderman's and you have said in principle, as I have never actually used this method of electric capacitance measurement.

Could the same job be done with a spectrometer, which would not have to touch the item?

As I have said already it would be nice to know what the finished product is, and, why there is a need for such accuracy before the curring process. The curing process will make the layer bind more closely, I think? I cannot think of any product that uses un-cured powder coating? A little more detail would be helpful really.

The job is easy for an optical sensor. All cyclinders are out of round anyway, so you would do a baseline measurement of the cylinder before you start coating it. After that you will be able to measure the thickness of the coating at many locations around the cyclinder as long as the sampling frequency is reasonable. The color idea works very well for some applications and not well for others. You would have to test it for yours. Obviously the color will change somewhat as you add powder, but there has to be a measureable change through the region that includes the coating thickness you want. Modifying that somewhat my be beneficial to you. Instead of looking for color change, try reflection or absorption of a specific color. If your powder coating strongly absorbs or strongly reflects some specific color of light, the absorption or reflection percent will change as the coating gets thicker. Again you would need to test this for your application. Some sensors also look at the scattering of light and once the intensity of scattered light at some angle reaches some pre-set value, they know their coating is finished. This would also have to be tested for your situation, but a powder will exhibit a wide scattering angle. If you use a large angle for your scatter measurement, it may work better for you. Ultrasonic is not going to work unless you are applying the vibration to the cylinder and measuring the damping resulting from the changing thickness of the powder. Of course, the intensity of the vibration would need to be very small. You might think this is tough due to the vibration inherent in the rotating stock, but you can measure the baseline vibration and pick a specific ultrasonic frequency that is easily observed among the noise.

You should consider capacitive and inductive probes.

Capacitive will be limited by the unknown dielectric constant of your coating and its variations according to density and humidity.

Inductive will measure the distance to your base metal if your coating is non-magnetic.

Very accurate (to your needs and also much better) capacitive and inductive probes are available from (personally tried and good experience):

Lion Precision, Minneapolis, US

Micro-Epsilon, Germany

Queensgate, England

Fogale Nanotech, France

and others.

Coating thickness measurement specialist is Fischer in Germany

www.fischer-technology.com.

RHABE

Hello to all really,

just to say I apologise for the detail I mention on "1 thou". As has been pointed out I simply got the reading of it wrong. And of course it should have been microns.

There is a reason though!............I was just cleaning an old micrometre which to confuse things measures to '1 thou' of an inch! Sorry.