These DC motors are equipped with gear container and encoder. It uses 2 phase or 2 channels (quadrature) incremental encoders to detect the 
acceleration of the motor, path of rotation and the distance it has travelled.
Encoder feedback data enables you to control your motor velocity and path efficiently. DC motors with encoder feed back transmission usually called DC servo motors.
DC motors with encoder have got many applications in robots, robot arm, CNC machine and many other applications that want accurate control of speed, Leaf Chain direction and position. You can control your electric motor using microcontroller or arduino.
DC Motor Specifications
Voltage: 6:12 VDC
Output Power: 2.2 W
Gear Ratio: 1:75
Rated Speed: 133 RPM
Rated Torque: 25.4 N.cm
Stall Current: 3A
Encoder Type: Hall effect quadrature encoder 5v (monitor position and direction of rotation)
Size: 25 x 54 mm
Weight: 120 g
Shaft Diameter: 4mm