Closed-Loop Control in CNC Systems

A closed-loop control system is a fundamental concept in automation that provides a high degree of accuracy and reliability, which is critical in CNC applications. Its counterpart, the open-loop system, is simpler and less expensive, typically using stepper motors. In an open-loop system, the controller sends a command (e.g., ‘move 100 steps’) to the motor and assumes the action is completed perfectly, with no way to verify the outcome. This can lead to inaccuracies if the motor stalls, loses steps due to high load, or if there are mechanical imperfections like backlash in the drive screws.

In contrast, a closed-loop system ‘closes the loop’ with a feedback signal. The primary components are the controller, the actuator (typically a servomotor), and a feedback sensor (an encoder or scale). The controller sends a motion command to the servomotor. The encoder, which is physically coupled to the motor’s shaft or the machine’s moving axis, measures the actual movement and sends a stream of position data back to the controller. The controller’s logic, often a PID (Proportional-Integral-Derivative) algorithm, constantly calculates the ‘following error’—the difference between the commanded position and the actual position reported by the encoder. If an error is detected, the controller adjusts the signal to the motor to correct the discrepancy in real-time. This continuous process of command, measurement, comparison, and correction allows the system to compensate for dynamic variables like tool pressure, thermal expansion of machine components, and mechanical wear, resulting in significantly higher precision and repeatability than is possible with open-loop systems.