I'm trying to build a system for controlling the presence of people, which at the same time ignores animals, e.g. dogs and cats. Do you know any sensor or sensors compatible with Arduino, that allow to build such a system?
Thank you in advance.
Use a cam.
You could try one of the low-cost thermal imaging devices, like GridEye or MLX90640, but you'll still need a fairly powerful processor.
I'm doing it for Engineering Thesis with a low budget. Is there any other solution using cheaper elements?
Spend more time and research on the physical difference between your animals and people. Since people are animals, by definition, I don't see much hope for your project.
Part of an engineer's task is to find possible solutions, then evaluate them for project suitability. That might include cost to implement.
Good luck with your project and your education!
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You could look at the LD2410. It is a "radar" type sensor but is configurable and you may be able to use this feature for distinguishing between humans and smaller animals.
This guy has played with these: 439 24GHz Radar Presence Detector that Works (LD2410) - YouTube
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Decide on a maximum height for (four-legged) animals and put a high sensor above that height. Since people walk upright they'd be the only 'animals' capable of blocking the sensor. Put a low sensor to detect dogs/cats.
Iām interested in the method to be used for adding or deleting humans !
My initial thought for low cost detection would be looking at the height of the occupants.
Is the room lit or dark ?
Like a Captcha?
So, if they don't recognise trucks, or traffic lights, they don't get in.
Another human to push a button.........failsafe.
Then follow #6.
I vote for The Mechanical Turk.
If it walks upright on two limbs, that narrows the field down to humans, bears, various apes, trained dogs and a few others. ML vision should do reasonably well in making the final decision, but would certainly not be error-free.
Going from the minimal description provided, this is not a low budget project.
The Mechanical Turk idea could be quite cheap depending on the prevailing wages in the location of interest.
If i remember correctly, very young humans walk on all four limbs until they discover the speed that two limbs gives them.
How ableist of us to forget that even some grown humans roll on wheels.
Yes, and then there are the little people who are often forgotten in mechanical designs. I recall a guy who worked as an attendant in a gas station. He rode small motor cycle, but legs were too short to balance the machine while setting on the seat. He would start the machine while standing beside the machine, run with the machine until it would stand on it's own and then hop onto the seat and away he would go.
Similar scene when he stopped. Hopped off before the machine completely stopped. Wow! such determination!
Conversely if it walks on four or more pairs legs and has an oblong shape, it might be a very able and wealthy human, being borne on a palanquin by slaves. This is indeed a difficult problem.
Get them to push a button as they walk through then................ 
Or ....slip their credit card in a slot.......
Or.....drive through only in their car......
Or.... speak into a microphone.......
Whole project is fairly vague in detail..........