Stepper Motors:   Principles of Operation


    Motors convert electrical energy into mechanical energy.  A stepper motor converts electrical pulses into specific rotational movements. The movement created by each pulse is precise and repeatable, which is why stepper motors are so effective for positioning applications.
    Permanent Magnet stepper motors incorporate a permanent magnet rotor, coil windings and magnetically conductive stators. Energizing a coil winding creates an electromagnetic field with a north and south pole .The stator carries the magnetic field. The magnetic field can be altered by sequentially energizing or "stepping" the stator coils which generates rotary motion. The diagram below illustrates a typical step sequence for a two phase motor. In Step 1 phase A of a two phase stator is energized. This magnetically locks the rotor in the position shown, since unlike poles attract, W hen phase A is turned off and phase B is turned on, the rotor rotates 90 clockwise. In step 3,phase B is turned on but with the polarity reversed from Step 1, this causes another 90 rotation. In Step 4, phase A is turned off and phase B is turned on, with polarity reversed from Step2.Repeating this sequence causes the rotor to rotate clockwise in 90 steps.
The stepping sequence illustrated in figure 1 is called "one-phase on" stepping. A more common method of stepping is "two-phase on" where both phases of the motor are always energized. However, only the polarty of one phase is switched at a time. With two-phase on stepping the rotor aligns itself between the "average" north and "average" south magnetic poles.Since both phases are always on, this method gives about 40% more torque than "one-phase on" stepping.
  The motor can also be "half stepped" by inserting an off state between transitioning phases. This cuts a stepper's full step angle in half. For example, a 90 stepping motor would move 45 on each half step. However, half stepping typically results in a 20%-30% loss of torque depending on step rate when compared to the two-phase on stepping sequence. Since one of the windings is not energized during each alternating half step there is less electromagnetic force exerted on the rotor resulting in a net loss of torque.

principles of operation of a two phase stepper motor (illustrated)


Simple Stepper Motor Control From A USB Port


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