MICROSTEPPING STEPPER MOTOR DRIVE USING PEAK DETECTING CURRENT CONTROL
1 INTRODUCTION
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 degrees, 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 degree axis. To step the motor, the current in one of the two coils is reversed and the
rotor will rotate 90 degrees. 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 degree steps (200 steps per revolution)
or 7.5 degree steps (48 steps per revolution). The resolution may be doubled to 0.9 or 3.75 degrees
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
MICROSTEPPING STEPPER MOTOR DRIVE
USING PEAK DETECTING CURRENT CONTROL
Stepper motors are very well suited for positioning applications since they can achieve very
good positional accuracy without complicated feedback loops associated with servo systems.
However their resolution, when driven in the conventional full or half step modes of
operation, is limited by the configuration of the motor. Many designers today are seeking
alternatives to increase the resolution of the stepper motor drives. This application note will
discuss implementation of microstepping drives using peak detecting current control where
the sense resistor is connected between the bottom of the bridge and ground. Examples
show the implementation of microstepping drives with several currently available chips and
chip sets.
Figure 2. Full step sequence.
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 .
Microstepping can be implemented in either a voltage mode or current mode drive. In voltage mode
drive, the appropriate duty cycle would be generated by the controller so that the voltage applied to
the coil (Vsupply * duty cycle) is the appropriate value for the desired position. 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 30°, as shown in Figure 3. For any given desired position, the required
currents are defined by the sine and cosine of the desired angle.

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