Re: 6 storey elevator
First of all, elevator control is very specialized. I hope you get liability insurance as well!
Secondly, please define the scope of your project. Are you working on sending commands from each floor or only the main control? In most cases, the console (usually 2 butttons UP/DOWN and a display showing position) on each floor is controlled by a serial bus from the main controller. The main controller is the one which decides the motion based on the floor number the elevator in on and from the requests from either inside the elevator or from the floors. This logic is relatively simple. You have a list of floors you need to stop at, you know the starting point and you go in a serial order.
The main complexity comes from the safety features: For example, the logic for door opening is tied to the limit switches on the floors. Moreover, the design to do acceleration and decel depends on the weight of the elevator and other things like whether you are using a two speed motor or a variable frequency drive and so on.
Please be careful in the hardware as well since the elevator (due to bundling of cables and the presence of AC drives, contactors) is highly noisy area. The last thing you want is a poorly designed hardware which hangs when, say, a door opens/closes and you are stuck inside.
Ajay