Dear ScOe, thanks a lot for your reply. And thanks to everybody, I din not expected so much atention!
Let me explain you all the project. Instead of an arrow, there will be a capital letter attached to each motor. Letters must sping fast enough so they can not be distinguish. There will be several letters, all them conforming a word. Before any of the letters start spinning, the word should be readable, this means all letters has to be "up side up". Letters start spinning, but they do not have to do it all the same time. Second letter may spin for 2 minutes, third for 1 minute, and first and fourth remain stopped. So, after spinning each letter has to reach its initial position. And this initial position will not change ever.
That is why I thought about step motors. Using an absolute encoders seems too far complicated for the application.
Letters will be 1 meter length, and the motor would be attached at the center.
Then, taking a capital I, 2kg weight, after my own calculation (they may be wrong) minimum torque would be 4Nm
(twice the integer between 0 and 0.5m of 9.8*2*s ds) so 5Nm would be enough.
In addition, it is not mandatory for the letter to stop suddenly. It may keep spinning swloly until it stops. Also we it starts spinning it has not to do it at the maximum speed since the beginning. One or two revolutions before maximum speed are allowed.
Hope this explanation make things easier to understand.
Added after 14 minutes:
karesz said:
Hi,
In my opinion & basically it dont need encoder if (I guessed AC motor too) should not rotate with high speed. After cut off the motor supply; you must have logically coupled rest supply voltage and (i.e. an inductive/light) position sensing of rotor, but such angle before its "home position"(to have enough time to brake it /take the moving energy of rotor), then apply a controlled brake force (i.e. timed/pwm) & you can stop it every time on the same position
.
K.
Dear Karesz,
Thanks for your suggestion.
I think it is a good idea, but it is too "tight". I mean you have to make a very precise calculation on the "anlge before home position". I prefer using some digital control, I think. That angle is related with the weight of each letter, where they are installed... I think it is too complicated to calculate :S
Anyway, kind regards for yor idea, I think it is a very good guess