When a stepper motor is powered off, it remains in a specific step or phase. If the phase at power-on is different from the one when it was last turned off, the motor may experience "jitter" or unexpected movement. To prevent this, the system must remember the exact position the motor was in before power was lost. Stepper motors typically operate in steps, and if the motor stops at step 4 and then loses power, the next time it powers on, starting from step 1 will cause an abrupt jump. However, if the system can recall that it was previously at step 4, it can accurately move to step 5 or step 3, ensuring smooth operation. Most standard stepper motor drivers, especially those using common dedicated ICs, do not have a built-in power-down phase memory feature. Upon power-up, the reset pin of the IC usually resets the motor to its initial phase, leading to jitter upon re-powering. Even in absolute value servo motors, some form of adjustment is required after power-on, which can also be considered as a type of jitter. In reality, no motor is completely free from slight movement during power-up. The key consideration is how much this jitter affects your application. For high-precision systems, it's best to record the exact position where the motor stopped. After power-on, the system should reset and then return to the last known absolute position before shutting down. This approach ensures consistent and accurate performance every time.
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