INDEX

MOTORS
As a beginner we mostly use DC motors, stepper motor and servo motor will come later. As everybody know DC motor has two leads. If we apply +ve to one lead and ground to another motor will rotate in one direction, if we reverse the connection the motor will rotate in opposite direction. If we keep both leads open or both leads ground it will not rotate(but some inertia will be there). If we apply +ve voltage to both leads then braking will occurs. You can test this, first without applying any voltage you rotate the shaft of the motor, then apply ground on both lead and try to rotate the shaft. Both will almost remain same, but if we apply both lead +ve voltage(+12V) and try to rotate the shaft, you can feel the difference between the previous one. You have to apply more force to rotate the same rotation in previous connection. So we take this condition as braking, because if we want to stop the motor suddenly then this is the better way which is easily possible. There are methods to brake motor fastly, like shorting two leads, applying negative polarity exists, but we won't use this in robotics. We apply (1,1) condition to break the motor fastly(see H-bridge section for more about it). The main things about a DC motor are Voltage rating, current rating, Torque, Speed. Rember Torque is inversely proportional to speed. So we had to get a good speed motor to get good torque because we can operate the good speed motor in slow speed to get good torque. So maximum speed of the motor should be as high as possible.

Normally available DC motors (without gears) have 12V,250mA,2400rpm(may change) ratings. But it is better to have a geared motor, because you should make gears to get a good torque to drive robot. Normally we get Speed/9 reduction gears to reduce speed and get a good torque. Put three gears to reduce speed to 2400/27(calculation is taken avoiding gear loss). So we get a speed of 80 rpm.

I used DC geared motorof 12V,250mA,60-80rpm,2kgcms Torque costs Rs.380 and another one of12V,250mA,60-80rpm,1Kgcms Torque costs Rs.135 . So as a beginner it is better to have a geared motor than a normal motor because you have to make gears. O.P.K Reddy used 12V,250mA,60-80rpm,2kgcms motor to make a rope climbing robot. So better get a pair of good geared motor.