Stepper motors are basically open loop devices. Each current pulse sent by the driver is equal to one step of the motor (or a small part of the microstep), so no feedback is required. With a small step (or step angle), the position of the motor can be determined very accurately without feedback devices and complicated control schemes.
If the position of the stepper motor can be determined by an open-loop system, why does the cost and complexity of the closed-loop control of the stepper motor increase?
Because only when the motor never loses steps can the position of the motor be accurately known. In the real world, there are a variety of situations that can cause the motor to lose steps or miss the pace. For example, a paper jam in the machine prevents the rotation of the motor shaft.
In order to cope with these possibilities, stepper motors are usually oversized to cope with excessive torque, locked rotor or other abnormal conditions, otherwise it may cause the motor to lose steps, and the loss of the motor will greatly damage the application, a solution The stepper motor is operated in a closed-loop system with position feedback to detect and correct positioning errors.
Stepper motors usually have three types of closed-loop control, and each type provides different levels of positioning control and complexity.
1. Out-of-step compensation: Reactive position correction at the end of the movement
The most common type of closed-loop stepping system is based on step loss compensation, also known as step loss control or step position retention. In this setting, the drive operates in microstepping mode, and the encoder tracks the position of the axis (or load). If out-of-step is detected, based on the actual position read by the encoder relative to the commanded position (number of steps times the step angle), the controller will command additional steps to bring the motor (or load) to the desired position.
Out-of-step compensation is the simplest closed-loop solution for stepper motors. As mentioned above, it is the most widely used method, but its main disadvantage is that it only compensates for the steps lost at the end of the movement, rather than continuous throughout the movement. make up.
2. Load position control: continuous, real-time position correction without complex control
Load position control, also known as closed-loop microstepping, continuously monitors the axis (or load) position and generates an error signal. Throughout the movement, the controller uses this error signal to adjust the command in real time.
Through load position control, the system can still operate as a stepping system in micro-stepping mode, but it can follow the motion curve more accurately instead of allowing the motor to deviate from the direction during the motion and issue a single position compensation at the end of the motion command.
3. Servo control: full control of torque and position
The most advanced closed-loop step control method is to run the motor as a two-phase brushless (BLDC) motor. (Note that the two phases of many stepper motors are offset by 90 °, while the three-phase brushless DC motors are offset by 120 °.) This method is called servo stepping or closed loop stepping control.
The servo control of the stepper motor is used instead of the drive supplying the motor with full current to generate motion. The encoder feedback detects the position of the axis, and the control loop (usually the PID loop) determines the exact torque movement configuration required for the axis to follow file. In other words, the motor is driven by torque commands rather than current pulses.
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