Brushed, also referred to as DC servo or permanent magnet DC servo motors, can be operated in a number of ways depending on the application. In simple requirements they can be connected to a DC supply and provide very low cost motion, typically at a set speed or operating via position switches on the axis.
A DC servo drive can provide reasonable speed holding without the need for any feedback, running the motors in armature feedback mode. Alternatively they can be used in current mode, providing variable torque from an analogue +/-10V demand signal.
Combining a DC servo motor with a tacho-generator or encoder and an appropriate servo drive, forms an axis control system that still rivals even the now commonplace highly complex AC digital servo systems in terms of smooth speed control. This is one of the reasons that brushed DC servo motors this common in in low-voltage systems for metrology, medical and machining applications.