Types of DC Motors
DC motors: These motors are easy to implement and generally used actuators for robots. There are different types of DC motors in the market and we will understand the working principle of each of these motors.
Different types of DC motors
Brushed DC motor
Brushed DC motor, or simply a “DC motor” is a classical example of electrical motor. As discussed before, a motor has a rotor and a stator with one of them being a permanent magnet. In a brushed DC motor, the rotor has permanent magnet and the stator has electromagnets. Since the motor needs a way to detect the rotor’s orientation, it uses brushes as a commutator which is a piece of rotor touching the shaft. When the rotor rotates (in turn the brush rotates), it detects the change in orientation and flips the current. DC motors are available in different sizes and at different speeds. Although DC motors run at enough speeds, they are generally useless in robots as they produce the slightest torque. DC motors have only two wires running into them; one for ground and the other for power.
Geared DC motor
As mentioned in previous tutorial, DC motors provide good speeds without enough torque. 
To overcome this, DC motors are often coupled with gears which provide greater torque, but reducing speed. Normally all our robots would require a geared DC motor to pull the weight of our robot and any additional components placed.
As you can see in the image, the motor shaft is connected to another bigger gear, which is further connected to a larger gear. As the motor rotates, the rotations per minute (rpm) of Gear1 is lesser than the motor. Gear2 has even less number of rotations per minute. However, each gear increases the torque of overall setup.
The above image shows a small DC motor fitted with gears. I have marked it as 150 which means the velocity of the motor is 150 revolutions per minute (RPM).
Brushless DC motors
A brushed DC motor uses brushes to detect the change in orientation so that it can flip the current to continue the rotor’s rotation. In a brushless motor, the rotor is made of permanent magnet and the stator is made of electromagnet. To detect a change in orientation, brushless motors generally use Hall Effect sensors to detect the rotor’s magnetic field and consecutively its orientation. Brushless motors are very useful in robots as they are more capable; they provide enough torque, and greater speeds than brushed motors. Brushless motors are expensive due to their design complexity and need a controller to control their speed and rotation.
Servo Motors
Generally known as RC servo motors, these are DC motors coupled with a feedback control circuitry, a gear system to increase torque and a position sensing device (usually a potentiometer). When a signal (pulse) is sent, it moves the motor shaft to a desired position using the position feedback from a potentiometer. Servos do not exhibit continuous rotation, but are limited to a specific range (generally 200° back and forth) and requires us to modify it for continuous rotation. Since servos expect a control signal, there is an additional wire running into the servo which takes control pulses. Hence they have three wires; Ground, Power and Control pulse.
Servos have a wide range of applications in robotics, but require a bit of shrewd programming to make it work. For a detailed explaination, refer Servo Tutorial.
Stepper motors
Stepper motors are brushless motors which divides the rotor’s rotation into discrete number of steps when electrical pulses are applied in an expected sequence. In other words, a brushless motor rotates continuously when voltage is applied across, but a stepper motor breaks it into steps per revolution and jumps each step for a certain pulse. Unlike a servo motor, stepper motor does not require any complex position feedback mechanism; on the torque side, stepper motors are similar to brushed DC motors with less torque. Based on the arrangement of windings inside a stepper motor, it can be classified as Unipolar or Bipolar step motor.
Image shows two different types of stepper motors. The first stepper motor in the image is interesting where the center shaft is fixed and it is the sorrounding body which actually rotates. Second image is of a typical stepper motor which receives pulses and rotates the shaft.
Linear DC motor
Not likely to be used in standard mobile robots, a linear DC motor is a normal DC motor with its stator spread out. To be more specific, a brushed DC motor has a rotor spinning inside a stator; in a classical linear DC motor, the stator is unwrapped and laid out in the form of a track made of flat coils. The rotor rolls over the stator in a straight line.
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