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I'm trying to design a mobile robot which has to climb over flat 60° inclined wet glass surfaces.

I was thinking to use a large vacuum system composed by a large fan connected to a brushless motor. By looking on google, I found this photo:

enter image description here

As it is possible to see, they use a brushless motor with a fan to generete some kind of vacuum to keep the robot on the wall. This system is very similar to the RC wall car toys which are able to vertically move on flat walls.

My idea is to create a vacuum chamber and to make it connect to the surface by using a soft rubber gasket (to avoid leakage), but I do not know how to design the vacuum generator.

  • How many RPMs do I need?
  • What shape should the fan have?

The robot has a weight of about 30-35Kg and its size is 1000x800 mm. The surface which can be used for the vacuum generator is the space between the tracks and it is about 800x600 mm.

I hope you can help me!

Thank you a lot!

EDIT 2 Information after @ericnutsch answer.

This is my ideal design.

I would like to use a cylider as case for the fan/impeller, use a rubber gasket for the cylider borders in order to avoid leakages (when possible). The cylinder/case will be suspended by springs in order to make it touch the surface by using the weight of the chassis. I do not know if the drawing is clear, but this is what I would like to acquire. The dimensions showed in the figures are realistic and the weight of the robot is about 30-35kg.

enter image description here enter image description here

EDIT 3

on #Mark answer:

I've found this impeller which can be customized and can have a diameter of 550 mm. It comes with some specifications:

enter image description here

EDIT1: This is what I tried so far:

During last months, I did several tests with different solutions:

  1. Using rubber tracks with Linatex which offers a great grip over flat surfaces
  2. Multiple suction cups with a vacuum generator

enter image description here

The Linatex works very great up to 60° if the surface is not wet, if it is wet, they works fine up to 18°.

enter image description here

The 4 suction cups works fine only if the robot moves very very slowly (0.1 m/s) otherwise they lose the vacuum.

Marcus Barnet
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You do not need full vacuum to accomplish your goal. A small amount of vacuum pressure adds up really quick over a large surface area. Try to make the entire footprint of your robot under vacuum; your sealing perimeter increases linearly with diameter, but area increases with diameter squared. Basically this means the problem is easier to solve the larger you go. And a circle is the best geometry because it has the highest area to perimeter ratio. Its really the design problem of a hovercraft, just in reverse. You will have to plan on some air leakage for it to operate on non-perfect surfaces in the real world.

Some existing equipment that may meet your needs:

  • A hobby rc brushless ducted fan. They do not publish head pressure, but might supply -3.5kPa gauge (-0.5psig) or so at a very high flowrate. You may be able to find one rated for high pressure lower flow. The nice part about these is they are easily battery operated.

  • A vacuum blower. High flow rate and reasonably high pressure. This one has 60 inches of water max, which is approximately 14kPa. They are likely rating in positive pressure. Its vacuum abilities would be less, so lets say -7kPa gauge (-1psig).

  • A "motorized impeller" (the internals of a shop vac). The website does not rate the vacuum pressure but i would estimate it would be -40kPa (-6psig).

  • An electric diaphragm pump. These are typically low flowrate so you would be back to clean smooth surfaces, but the vacuum pressure on this one is -50kPa gauge. Might be more flow than you have on your current vacuum pump if you stack a few in parallel.

I think you mean 80mm in your image not 80cm. 8cm results in an area of 0.005m^3 (7.8in^2). So at perfect vacuum 101kPa (-14.7psig), you are getting 500N (115 pounds of force), which is great until you lose that perfect vacuum. The force drops quickly because the vacuum pump you are using can not maintain the flow rate required by the imperfect surface.

I would say a motorized impeller would be your best bet. I don't know your scale here, but lets say putting a shroud around your footprint puts you at a 0.4m (16 inch) diameter.

climbing robot footprint

The corresponding area would be 0.126m^2 (200in^2). At -40kPa (-6psig) you now have roughly 5000N (1,200 pounds of force). And since this fan can handle a high flow rate, even large leaks are not a problem. You will want to design the shroud such that it is strong enough to not collapse under this force and design it such that it does not apply too much load to your climbing tracks.

ericnutsch
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An important point in the fluid dynamics design is about control points. A simple experiment - hold your hand over a hose on the vacuum cleaner, you'll notice a light vacuum pulling on it, but hold your hand right on the top of the hose, and it feels like it is pulling significantly harder. This is because fans operate on a load response curve, and when there is less air flow, it pulls with more pressure.

There are a few different fan designs. Forward, Radial and Backwards curved blades:

enter image description here

These lead to different kinds of performance curves:

enter image description here

As expected, with no flow, you have nearly the highest pressure, and with a very low pressure, you have almost no pressure. The problem is you need to operate in a portion of the curve with a negative slope. The area with the positive slope is called the surge region:

enter image description here

While forward curved fans have a lot more pressure available, they typically have very large surge areas because of the large area with a positive slope, but there is usually the small area with a negative slope right before the surge area. In this application, a forward curved blade design could work so long as it stays above the local maxima in the mid-flow range, However,a radial blade design would be more conservative, and usually more cost effective, but the unit will necessarily have to be bigger to obtain large pressures.

A comprehensive stability analysis also needs to look at the power requirements. For power design, we'd need to look at the power curve for the fan. Notice the power requirements for the motor increase drastically as the flow rate goes up. Since a user can always peel the robot off the wall while in operation, the design will need to account for the fact that a constant RPM fan will demand more torque as more air is moved while the system stabilizes to the lower pressure requirements. Potential solutions could include:

  • Electronics could detect this torque and turn the motor off
  • Size the motor to handle the maximum airflow scenario
  • Something new and creative that's mechanically governed to limit torque
Mark
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Well if you would like to create your own vacuum, I would not recommend designing the actual pump yourself. You would need to design the turbomachinery components like the impeller (fyi an impeller is not a simple fan, it is much more complicated in terms of it's geomtery and the material used to make it), casing and various seals from scratch and those things are NOT easy to analyse, design and test in terms of the fluid dynamics, stress analysis and design engineering that goes into them. I say go on a distributor like RS Components and find one that suits your needs. Of course the part that you would design is the way you would channel the air, create the suction chamber, so on and so forth. That will depend on your engineering/DIY knowledge and your specifications.

thephysicsguy
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