The DC Machine was the first practical device to convert electrical power into mechanical power and vice versa. The most common type seen in most servo systems is the permanent magnet DC motor.
When a current passes through a coil wound around a soft iron core, the side of the positive pole is acted upon by an upwards force, while the other side is acted upon by a downward force. According to Fleming’s left hand rule , the forces cause a turning effect on the coil, making it rotate. To make the motor rotate in a constant direction, “direct current” commutators make the current reverse in direction every half a cycle (in a two-pole motor) thus causing the motor to continue to rotate in the same direction.
While brushed motors are among the oldest designs and the cheapest types of motors to produce, the modern brushless types have found increasing favour and now form the norm.
In a conventional (brushed) DC motor, the brushes make mechanical contact with a set of electrical contacts on the rotor (called the commutator ), forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature coil closest to the stationary stator.
In a brushless motor, the electromagnets do not move; instead, the permanent magnets rotate and the armature remains static. This gets around the problem of how to transfer current to a moving armature. In order to do this, the brush-system/commutator assembly is replaced by an electronic controller. The controller performs the same power distribution found in a brushed DC motor, but using a solid-state circuit rather than a commutator/brush system.
Because the controller must direct the rotor rotation, the controller needs some means of determining the rotor’s orientation/position (relative to the stator coils.) Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor’s position.