The simplest way is a 4 wheeled platform, front 2 wheels for steering, rear 2 for drive, with a large flat rare earth magnet mounted underneath 1 inch (or less) from its floor (the wall) to do the "sucking".
I worked on a 100kg flat "trolley" climbing the sides of ships to clean them, worked very well (It also had remote radio control). A smaller one would be easy.
can you elaborate on the specifications of the components you used for ur purpose.We tried the same procedure but our bot is sliding down under its own weight.We need motors providing more torque or magnets with less weight and both are a bit costly atleast high torque motors are costly...Any other way to solve this problem?
I was only involved with the remote/drive side, cant help you with the mechanicals. Powerfull, large rare earth magnets are expensive, I cant see any other way. Eletromagnets would require far to much power methinks. The drive isnt hard due to high gearing, it only moves slowly. The 100kg one I worked on used electric wheelchair motors/scooter drive electronics, they had more that enough torque.
Instead of magnets you could try using a vacuum fan like some sumo bots do to artificially increase their own weight...some are able to hang upside down. It takes energy though unlike magnets but then it would work on more than just metal. Oh no! Vacuum components can be hard to find though...
Why is your robot sliding down the wall? Because there are two possible reasons:
1. Your wheels don't have enough grip which means the magnets are not strong enough or the wheels are not grippy enough. This is the most likely case. WHich means you need to reduce the weight of your robot or increase the magnets. In this case adding on bigger motors or motors with more torque WILL NOT HELP your robot at all. It will just make it heavier and make things worse.
2. The second reason would be that your motors actually cannot pull the robot up the wall. Which means your motors are stalling and in fact rolling backwards. This is not as likely since your motors would be burning out. This would mean though that if you locked your wheels in place, your robot would remain exactly in one place on the wall without sliding down (if your robot did not also have problem 1).
3. Or it could be a combination of 1 and 2. I'd just lock the mechanically lock wheels into place first and worry about the robot staying in one spot on the wall without sliding down before I would worry about getting motors that can move the robot up the wall. Just so you know exactly what the problem is before you start trying to fix it.
For your situtation, it is very very easy to know how much torque you need on your motors. You have the wheel diameter, you have the motor torque and you probably want no wheel slippage so the coefficient of friction has to be 1. You need this much torque for the robot just to hold it's place on the wall. You need more for it to move up the wall. It's that simple. THere's no way around it. You need to either make your robot lighter or get motors with more torque (but not so heavy that weight becomes heavier than the force of friction between the wheels and the wall...in which case you need more magnets which just increases the weight again).
But either way, you should be trying to make your robot as light as possible. This is not like a regular ground robot where you can use a full-metal chassis and not worry about it. Only use heavier materials where you need the strength. You might even be able to get away using carved foam blocks for most of the chassis depending on how big it is.
How big is this thing? I'm imaginging something that fits into a 15cm cube.
How about a pair of tank tracks with magnetic cabinet door latch magnets as links. The cabinet magnets have pole pieces to allow a strong contact with flat metal plate. The forward link is held away from the surface until the link is between the robot tank and the metal wall, then parallel with the wall it grips with maximum hold and follows the track to the rear where it is lifted from contact with the wall and returns to the front without holding back the forward motion of the robot. Steering can be accomplished with two independently driven tracks or differential drive gears and dog clutch on each track to stop one track.
I like that one. Permanent magnets in direct contact, no power needed to hold it, the leading magnet pulling toward the surface helps offset some of the torque of the trailing magnet pulling away, the treads gives a conformal fit to imperfectly flat surfaces... lots of win in that idea.