Servo drive controldirect current (DC) means that the motor controller is coupled to the control system with an analog +/- 10V signal. When 0V is given, the motor is stopped, and when the voltage is e.g. + 10V, the motor rotates at maximum speed, if the voltage is -10V, it rotates at maximum speed in the opposite direction. At other voltages, the rotational speed is proportional to the set voltage (this is provided by the speed regulator in the engine controller based on the feedback from the tachometric generator). How does the CNC control system know what voltage is to be applied? For this, position feedback via rulers or encoders is required. The set position is subtracted from the measured position and the motor controller is given a signal proportional to this difference. This is the so-called speed-proportional control system. The higher the speed, the greater the shape error. It is a relic.

The phrase “DC servo drive” can also mean that a DC motor was used to build the servo drive, which does not necessarily mean that the speed is set with a direct current. This type of servo drives can be controlled by CLK / DIR discrete signals. The controller in such a servo drive tries to control the motor so that the position error tends to zero, regardless of the rotational speed of the motor. It is a slightly younger solution, but it is also out of circulation.

Currently, only electronically commutated brushless motors with shaft encoder are used for servo drives. There are two versions of such servo drives: BLDC – brushless DC motors and PMAC synchronous AC motors with permanent magnets.