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Stepper motors
Rotates only by fixed amounts, and controlled by a sequence of digital outputs.
Stepper motors typically (for bipolar steppers) have 4 wires, two each for adjacent coils.
Other, more complicated drive patterns such as full-step drive (two phases on), half-stepping etc. possible.
Step angles of 1.8o, 3.6o, 7.2o etc. are typical
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Positioning is relative - you can make it move by a certain angle (certain number of steps x step angle) from the current position. This is unlike servo motors which do absolute positioning where you don't need to know the current position to control the final position).
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A page with good explanation and figures : https://howtomechatronics.com/how-it-works/electrical-engineering/stepper-motor/
Another good website with animations: http://www.pcbheaven.com/wikipages/How_Stepper_Motors_Work/
Open loop control – no feedback-based correction built into the motor unlike servos. You can only give pulses / commands and hope for the best.
Good for positional control where turn angle is not limited and where the torque required is relatively constant.
Used in printers, 3D printers, rack and pinion
based actuators
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(see the animation below) based actuators.
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To do : Lookup and go through Arduino Stepper Library
Other Motors
Brushless DC (BLDC) motors
Similar to DC motors, but an electronic circuit replaces the brush + commutator. Used in electric scooters etc.
Induction / synchronous motors
Used in fans etc, works on AC; the mechanism of operation is somewhat different from that of DC motors