General description

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Doc ID 8700 Rev 3

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General description

Microstepping a stepper motor may be used to achieve one or both of two objectives; 1)

increase the position resolution or 2) achieve smoother operation of the motor. In either case

the basic theory of operation is the same.

The simplified model of a stepper motor is a permanent magnet rotor and two coils on the

stator separated by 90°, as shown in Figure 1. In classical full step operation an equal

current is delivered to each of the coils and the rotor will align itself with the resulting

magnetic vector along one of the 45° axis. To step the motor, the current in one of the two

coils is reversed and the rotor will rotate 90°. The complete full step sequence is shown in

Figure 2. Half step drive, where the current in the coil is turned off for one step period before

being turned on in the opposite direction, has been used to double the step resolution of a

motor. In either full and half step drive, the motor can be positioned only at one of the 4 (8 for

half step) defined positions.[4.][5.] Therefore, the number of steps per electrical revolution

and the number of poles on the motor determine the resolution of the motor. Typical motors

are designed for 1.8° steps (200 steps per revolution) or 7.5° steps (48 steps per revolution).

The resolution may be doubled to 0.9 or 3.75° by driving the motor in half step. Further

increasing the resolution requires positioning the rotor at positions between the full step and

half step positions.

Figure 1.

Model of stepper motor

Figure 2.

Full step sequence

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