1. Which is the best actuator between a dc motor with gearbox(gearmotor) and a servomotor.
There is no overall best
actuator. While geared motor has more power (compared to the same size servo) it required external circuitry to be driven (aka H-Bridge), where servo has control circuitry embedded in it.
2. I am planning to add encoders on the wheels, someone could suggest me what type of encoder is the most suitable and whyMost suitable
is same as best
- it all depends what You fell most comfortable working with, Your budget, conditions robot is going to be used in, etc, etc. Arguably most common
approach is IR Emitter/Detector (optical) encoder which is fairly simple to make at home and is probably
the cheapest one. It does suffer from sun interference (as sun emits IR rays), however it can be easily solved by shielding it with something like photo-camera film container. The other approach is to use magnetic encoder (it is capable of giving You exact
angle of the shaft, where of optical encoder can give You this information only by constantly tracking tics
and doing some maths to work out current angle), while this one offers precision limited by ADC (analog to digital converter) used (optical encoder precision is limited by how many distinctive black and white strips You can fit on encoder disk), it requires 2 ADC ports per encoder, as there has to be 2 hall sensors per encoder (optical encoder requires only 1 digital IO port per encoder (or 2 digital IO ports if it's a quadrature encoder)) and parts tend to cost more, as well as it requires shielding from magnetic field generated by a motor (which is more difficult to achieve than shielding from the sun).
3. How can i know the minimum and maximum torque that i’ll need for the dc motor or servo
A rule of thumb that I use is that one motor has to have stall torque high enough to sustain hanging 2x the weight of the robot on an arm parallel to tge ground of length of wheel radius. If You post weight of Your robot and diameter of wheels used - someone in this forum might give You target motor torque.
4. what is the most suitable structure for the robot considering the aerodynamic.
If Your robot is not going to speed at 70mph - it can be a box