Motion control without feedback assumes that motors always behave as expected. Under common conditions, the position of the motor is deterministic and predictable. To cover uncommon conditions, the motor needs to be operated with safety margins, which can reduce efficiency and overall performance.
Closed-loop operation or motion control with feedback is more complex. By measuring the motor's condition and behavior, closed-loop operation can deliver higher control reliability and flexibility. Feedback response, however, needs to be harmonized with applications and requires sophisticated control algorithms. Otherwise, systems can lapse into a condition of unstable oscillation.
Magnetic or optical encoders are very common feedback sources and are available with different levels of precision and speed. These encoders are typically directly mounted to the motor axis so they can accurately measure mechanical movement.
Another option is to sense the motor's electrical behavior. Power consumption is directly related to the axis' load and motion control can respond if this load exceeds certain limits.