Control for Contact Probe so that Little Force is Applied

Use a force sensor and feedback to your actutor.

Use a limited power actuator. Pneumatic is easy to do.

It is not wise to be probing any sensitive circuit directly. Best to design test points onto a circuit card. Also the higher the rating of the probe, the less loading on the circuit. i.e a 100 to 1 probe is safer then a 10 to 1 probe. (although more expensive).

Try to avoid extender cards since they can introduce timing errors

There is Loop test points or post type test points. (also hole type)

See http://www.keyelco.com/products/prod21.asp?SubCategoryID=111

There is also test points that can be accessed on the edge of a card. For example see

http://www.nicholsonengineering.com/_wsn/page2.html

That's a little vague. Do you mean that the surface will tear or be damaged by the contact? How light should the contact be?

Some microswitches (small limit switches) require very light actuating force. How much, I can't tell. You'll have to look at a manufacturer's datasheet, that is, if they have that information.

Just curious. If the surface is sensitive, why go for contact sensing? Why not non-contact? There are several technologies available for this like: ultrasonic, laser, and optic.

What do you want to measure and from what surface material?

I hope I understand your requirements correctly.

I have done some parts digitizing (x,y,z) on a CNC machine.

Very sensitive pointing limit switches are available. They are usually mounted on a spring loaded shaft and therefore Rather robust (up to a point). It can handle pressure from the side

Contact suppliers for detail.

If this is in line please advise what you have available and I would try and help if possible.

A little more information on the force limit would be required.

You should take a look @ SMAC actuators. These allow you to do a soft-land onto a surface with a programmable, controlled force as well as measure. these actuators are fully programmable(Force, Position, & Velocity), and allow for feedback of Pos. & Force.

These actuators provide much higher control of force than any pneumatic & or spring loaded probe approach, and the force is linear thru out the entire stroke. These actuators also use high resolution linear encoders (5um, 1um, 0.5um, 0.1um & 5nm).

Another advantage is that this is a all-in-one package design. you would not have to take a motion device & couple it to a loadcell & pos. measurement device as the SMAC actuator have all of the capabilitied that you would need for the application.

SMAC offers world wide support. Here is the website. www.smac-mca.com

A contact probe that limits the force applied to the surface under test needs to have a probe tip that will compress/collapse in response to an applied force that is smaller than the force that would damage the surface being tested. A piece of very fine wire (of the right metal, of course) bent into a loop, with one end attached to the probe instrument and the other protruding to make contact with the surface, will do the job.

Of course, the instrument to which the piece of wire (or whatever) is attached must be designed to stop pushing when it detects any resistance from the wire; but the wire will ensure that no excess force is applied.

If you don't like a loop, try a V with wings (extending to the left and right of the points of the V). The depth of the valley of the V determines the length of the spring-arms that will absorb the force; the longer they are the less force they will apply for a given displacement (change in distance between the tips of the V).

HTH
Chris

The application is to measure a coating on an anodized aluminum substrate and the best probe that I can find for this specific application is a contact probe. The pencil hardness for the coating to be measured indicates a 2H to 3H gage and I am not sure how that will translate to force. And I am afraid the probe (contact) might damage the coating without proper control.

It is good of you to provide more details, your first post led to numerous interpretations.

Still, you need to explain what you need to measure. May be thickness, resistivity, hardness, temperature, length, refractive index.....

I never heard of the pencil hardness test before, but found this: http://www.pencilpages.com/articles/simmons.htm

Aside from using mills as 10^-6 inches, the article does make sense.

Well now, see what a little detail can do to make things clearer. However, there's still one question.

What are you measuring? Your statement "measuring a coating" seems to imply that you want to measure the thickness. Then you mention "hardness" which is whole different thing.

Ultrasonic thickness measuring devices exist and they won't damage your material. Hardness measurements are a little more difficult. The ones I know of involve impacting a device onto the surface. That will surely damage your substrate.

Perhaps you can coat a small sample and test the hardness of that. Then you take that result as the hardness of your coating. You'll probably have to do that every now and then to check for control.

"Application needs the use of contact probes but ..."

This is Alice, stumbling into Wonderland, so pardon my apparent ignorance.

Is this particular (Forum) thread referring to electrical ('contact') probes, or ultrasonic ('contact') probes, or eddy current ('contact') probes, or ... what...?

There are air-coupled probes ... for a variety of applications.

Please fill-in the/my blank, if you will.

The application actually is to measure the coating thickness on an anodized aluminum substrate using magnetic and eddy current principles. The probe that i can find best suited for this application is a contact eddy current probe.

What I am worried about is how to control the force on the "nip" or contact so as not to damage the coating in which the thickness is to be measured and at the same time maintaining contact pressure necessary for the measurement process. The coating (whose thickness is to be measured) on the other hand indicates a pencil hardness of 2H to 3H gage and I am not sure how that would translates to force value that would be the limiting factor on the contact force during measurement.

A smac actuator would be ideal for this as it provides excellent force & velocity control when put into a soft land routine.

It can touch off with as little as 5-10 grams depending upon the payload, size of actuator used and encoder resolution used.

There are things that can be done with the SMAC through the controller & internally in the SMAC actuator that would allow you to achieve your goal.

Keep in mind the force is linear though out the stroke, and the soft-land capability is linear trough out the stroke so you can present parts that are at various height to the SMAC and it will find (Soft-land) on the part the same, where ever it is at in the stroke of the actuator.

These actuators are used for Profiling a Surface(Glass -Metal), Height measurement, Precise Pick & Place(circuit board and other small fragile part assembly @ extremely height speeds), Die Bonding(Critical force control), Effort Testing(Swithces,etc..), many other applications.

So with the eddy current probe tester, or other contact probe testing device, you could attach it to the end of the SMAC actuator, and use the SMAC strictly for the soft-land /force control feature.

I guess this is what I need. I have to check with the site or with NI if the actuator can be used with LabView. Any idea? THANKS...

The SMAC actuators have been used with the NI controllers & can also be interfaced directly to a NI system wiht the use of a SMAC Controller. The std. SMAC controller has a serial port that can send & or recieve data in a std. ASCII string. SMAC is just also anouced that they will be offering a Labview front end.

If you would like to contact me by e-mail, I can see that you are directed to the correct items you are looking for. s_curtiss@sbcgloba.net

sorry, noticed an error in my last post

The SMAC actuators have been used with the NI controllers & can also be interfaced directly to a NI system wiht the use of a SMAC Controller. The std. SMAC controller has a serial port that can send & or recieve data in a std. ASCII string. SMAC is just also anouced that they will be offering a Labview front end.

If you would like to contact me by e-mail, I can see that you are directed to the correct items you are looking for. s_curtiss@sbcglobal.net