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As you place the brush against the surface and pull it across the surface I can imagine two forces come into play. You have the force normal (perpendicular) to the surface and you have the the force tangential to the surface. I am not a painter but I might also assume that you sense the resistance to the tangential force and adjust the normal force accordingly. The type of surface would play a factor as the rougher surface would have more friction and produce more drag on the brush. Your hand and your vision are the sensors, your brain is the control mechanism, and your muscles supply the forces. There are sensors called load cells that can measure forces. If you had two of these cells in parallel (SIDE BY SIDE), in the correct orientation, in the handle of a brush they could conceivably measure normal force as an addition of their output and tangential force as a difference of their outputs. I am not an engineer myself and I suspect that this might be a little difficult and perhaps expensive. The forces that you would need to measure accurately are probably in the area of single or fractional Newtons. Sensors to do this are not cheap. I am hopeful that your question will attract more input from better engineers than myself as you have gotten me curious as well. This sounds like a great project and I will watch this thread and add more if I can think of other things.
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I am not aware of a product that does this. I also agree with John that if you are trying to measure precisely it will be difficult. However, here are some DIY ideas to consider...
Flex Sensor - You can get these numerous places but here is one source: https://www.adafruit.com/product/1070 They have two sizes and also some material that you could use to make your own. This will not measure force against the brush tip but rather how electrical resistance changes as the bristles are bent. You would need to adhere it to the ferrule and possibly the bristles themselves. The change in resistance could be measured with a microcontroller as outlined in the link above. The measurement will probably be crude. Here is a photo of mine held against a soft brush pushed against a table to give you the idea:
Strain Gage - You possibly could arrange strain gages around brush bristles and/or the ferrule - an internet search will turn up information on how they work. Load cells are strain gages bonded onto a deformable block of metal which could be the ferrule for example. They can be arranged in a number of ways to measure with great precision but I only have limited experience with simple applications.
I'm curious if this is a permanent use device or simply a 'teaching aid' that is used initially for evaluating or correcting a novice.
If it is a 'teaching aid' then you could use a fixed platform with strain guages.
You attach a section of 'surface' to it and paint normally.
This would give 3 axis of movement information, which would allow pressure, pull and any sideways forces to be measured.
For permanent applications you have a large number of issues to deal with for the sensors to last and the brush to not cost a fortune.
As a professional painter, the consistency in your brushstrokes are due in large to how much pressure you push the bristles of the brush against the surface you're painting. This action involves a very low, and very precise degree of pressure.
What I am after is the best way to measure the force of the strokes on the brush unit itself, and read it onto my laptop. This, I'm thinking, will involve either a few sensors within the hairs, or even installed into the base. Wireless is preferable.
Can somebody please recommend a product that could be purchased to achieve what I'm after?
Ps. Please redirect me if this is posted in the wrong subforum.