Servoing up an ace

The two servomotors that engineers most commonly use for positioning are the DC servomotor, which uses mechanical brushes to commute the motor, and the brushless DC motor, also known as the PM (permanent magnet) brushless motor, which is commutated electronically by external circuitry.

Unlike step motors, which move in discrete position steps, servomotors have no built-in sense of where they are, and thus require a feedback device such as a quadrature encoder to provide position information. The servo loop, also called a compensator, has the job of keeping the servomotor at the desired position. It does this by comparing the desired position at any given moment with the actual motor position from the feedback device, and applying corrective motor commands. The better the servo loop performs, the more accurately the motor will track the desired position under a variety of loads and motion profiles.