# Which motors to choose?

I don't know if this is the right place to ask, but I'm an arduino newbie and I'm seeking some suggestions.
For my Robotics subject at University, I'm going to build a robot that should be able to move some objects (dragging them on the ground, not lifting them) from one place to another. I was looking around the web for the necessary components when I stumbled on a doubt: are 3-6V motors powerful enough for the wheels? Or do I need something more powerful like the 12V motors?
I'm not sure of what to choose between step motors or servo motors either, what do you suggest?
Oh and last thing, I opted for a L293D bridge driver for the motors, is this a good idea?

• Robotics Stackexchange would be better for this question. I suggest deleting it here and adding it there. Commented Mar 24, 2017 at 19:33

You need to size your motors correctly depending on how much load they are expected to overcome. You first consideration should be how much maximum force would your robot need to exert at any given time....

For a robot just starting to drag a load (assuming the surface is flat) behind it, the robot would have to at least apply a force just greater than...

Maximum Static Frictional Force due to Robot's own weight on its wheels + Maximum Static Frictional Force due to object being dragged.

Maximum Frictional Force = Normal Force (weight) * coeff of Friction for the two surfaces

So for example, the coeff of friction between a brick (load) and ice (surface) would be lesser than between a brick (load) and asphalt (surface).

You need to do these calculations first.

If the speed limitations of your robot are inconsequential, then you only need to choose your wheel size and then determine how much torque your motor needs to be able to generate.

You do this like so.....Motor Torque needed = Force to generate / (radius of wheel * number of motors you want to install). So for instance, the torque needed per motor in a 4WD robot would be half that needed for an equivalent robot with only 2 powered wheels.

Once you have the motor torque you need, you can choose a motor and then choose a suitable power source for this motor.

Now repeat ALL the steps above this time accounting for the weight of the motors and the battery.

A 12 volt DC motor is good for dragging items, but then again it depends on the items weight. Stepper is used for millimeter precision, like 3D printing. Servo is an DC motor with precision for 180 degree movement, like robot arms. If the moving object is over 1kg you should look into geared DC motors or even the stronger brushed motors, or geared brushed motors. Whats the max weight?

• We still don't know yet, the professor did not give the full specifications yet. They should be simple balls of about 7/10 cm diameter, so they will be definetly under 1kg weight Commented Mar 24, 2017 at 22:36