AN1495

General description

Doc ID 8700 Rev 3

5/25

Another issue occurs at low operating speeds. At low speeds, both the full and half step

drive tend to make abrupt mechanical steps since the time the rotor takes to move to the

next position can be much less than the step period. This stepping action contributes to

jerky movement and mechanical noise in the system. Looking at the simplified model of the

stepper motor in Figure 1, it can be seen that if the two coils were driven by sine and cosine

waveforms the motor would operate as a synchronous machine and run very smoothly.

These sinusoidal waveforms may be produced by a microstepping drive.

In current mode drives, the winding current is sensed and controlled to be the appropriate

value for the desired position. This application note will consider only current mode drive

implemented using peak detecting current controllers.

To understand the microstepping concept, consider the simplified model of the stepper

motor as shown in Figure 1. As previously discussed when the two coils are energized with

equal currents, the resulting magnetic vector will be at 45° and the permanent magnet of the

rotor will align with that vector. However, if the two coils are energized by currents of different

magnitude, the resulting magnetic vector will be at an angle other than 45° and the rotor

would attempt to align with the new magnetic vector. If one coil were driven with a current

that was twice the current in the second coil the magnetic vector would be at 26.56°, as

shown in Figure 3. For any given position, the required currents are defined by the sine and

cosine of the desired angle.

Figure 3.

Example alignment of microstepping

To implement a microstepping drive, two D/A converters are used to set the current level in

the coils of the motor, as shown in the block diagram in Figure 4.

ΦB

Full step position

Half step position

ΦA

AM15144v1