Because open-loop control systems are convenient to operate and inexpensive, China primarily uses open-loop control reactive stepper motors . Although stepper motors are widely used, they cannot be used under normal conditions like ordinary AC (DC) motors. Theoretically, the motor can only operate as required and reach the expected speed when the motor's maximum starting frequency is greater than its operating speed.
When the motor reaches the end of its stroke, it should immediately send a stop pulse to halt the operation. However, in reality, the maximum starting pre-rate of a stepper motor is quite low, far from meeting the requirements for higher operating speeds. Under these conditions, forcing the motor to start directly at the required speed (greater than the maximum starting pre-rate) will result in "step loss" or no response. Furthermore, even though the pulse sending stops immediately upon reaching the end point, inertia can cause it to overshoot the end point, resulting in overshoot.
It is particularly noteworthy that, in order to ensure both the positioning accuracy of the system (by slowing down the motor's acceleration and deceleration to prevent "step loss" or "overshoot") and to achieve a high positioning speed, mainstream systems divide the positioning process into a coarse positioning stage and a fine positioning stage. Based on practical production experience, "step loss" and "overshoot" are the two most common and serious culprits affecting the positioning accuracy of stepper motors during operation.
The main reasons for inaccurate positioning include:
(1) The initial starting speed is too high, exceeding the motor's limit starting frequency, or the acceleration is too large, causing "step loss";
(2) The power of the motor does not meet the system requirements;
(3) The actuator's working process is subject to interference;
(4) The controller of the control system malfunctions;
(5) Pulse loss during reversal; accurate positioning during unidirectional operation; positioning deviation after reversal; and the more reversal is performed, the more obvious the deviation becomes.
(6) The software has design flaws;
(7) When using synchronous belts, the software compensation is too much or too little.
