I am interested to know, if I want to build middle/big size robots(50*50*150 cm^3), which material and which method is better to use?
I am assuming you don't want to go the way of designing a robot using a CAD software followed by tons of machining to bring all the custom parts to life. If you are looking to build a robot using off-the-shelf materials, I would say aluminum extrusion is the way to go. It is the machine builder's lego. Using aluminum extrusion and aluminum plates, you can rapidly build a large, rigid base onto which you can mount all your electronics, motors, sensors, computers, etc.... I prefer Bosch aluminum extrusion (which officially is called 'aluminum structural framing'). Have a look here: https://www.boschrexroth.com/en/us/products/product-groups/assembly-technology/aluminum-structural-framing/index. For plates, round shaft, angle, etc... you can go to metal supermarkets (if you are in US or Canada. If not you should have something local). An example of an ongoing robotic project that was built with extrusion and plates can be seen here: https://answers.ros.org/question/271819/please-suggest-all-packages-to-achieve-slam-for-robot/
I think this photo may help to clarify what do I mean:
Aluminum extrusion will allow you to build the two robots that have a rolling base. As for the middle biped robot, you will need to get more fancy... WAY more fancy. Building biped robots is very challenging, and you will most likely will need to machine your own components around off-the-shelf parts that you are utilizing. Rolling bases with robots on top are much easier to build than biped robots. I guess I am trying to say that the images you included are robots that are not all in the same league. So far I have focused on a moving base. If you are trying to build arms with end effectors, you will need to start machining custom parts (whether your own design or open source).
If someone wants to build robots in this scale, what should he learn?(except going to university and getting a master in mechanical engineering:) ). I mean is it possible to learn or not?
Usually advanced robots (like the 3 you have in the image) are built by teams. Teams allow members to specialize in different disciplines (mechanical, electrical, electronics, programming, etc...) which produces the best solutions in each discipline. However, a single person can also build an advanced robot, it just takes longer, and it will be very challenging to come up with the perfect solution in every discipline every time. My question is why do you want to build a robot? if you are trying to learn, then join a robotics club, or start building miniature robots. If you are looking to build functional robots (entirely on your own), to achieve actual tasks, then I suggest you learn the following:
- CAD software - drawing your robot out will save you time as you discover issues before you start building. I recommend solid works, so you can later do 3D printing.
- Machining - you will need to build some pieces that link off-the-shelf parts in your mechanical design.
- Electrical - You can always pick up a textbook and run through basic AC/DC theory and do some experimentation. You will need to have some basic understanding to use off the shelf parts and how to integrate them together. you will need to understand how to step down/up voltage, limit amperage, etc...
- Mechanics - Before you start building you will need to double check your design (catch items such as robot will not flip as it's too top heavy, or motor is underpowered to move limb, etc...)
- Programming - May I suggest C and C++? You will need to program the robot to control all your hardware to perform the task you want. I am relying on the ROS framework.
Robotics is an amalgamation of mechanical, electrical and programming. All robots rely on these disciplines to various extents.
My advice is first figure out why you need a robot. this will guide what you will build. Then start building each system out independently of each other; once you are done amalgamate all your parts into a single unit.