Datasheet 2

AN120 – UNDERSTANDING MP6500 CURRENT CONTROL
AN120 Rev. 1.0 www.MonolithicPower.com 8
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An advantage of the control method used in the MP6500 is that no user adjustments are needed for
different motors or supply voltages; the decay function is fully automatic. With conventional stepper motor
drivers, the decay mode (and sometimes even the off time) needs to be tuned in each application to
maximize the motion quality.
Using this current regulation method, the MP6500 regulates the average winding current quite accurately
throughout the entire waveform (see Figure 8). This improved current control results in measurable
improvements to motion quality.
Figure 8: MP6500 Current Regulation Waveform
Motion Quality Measurements
Assessments of stepper motor motion quality have often been less than scientific. Usually, the human
eye, ear, and hand are used to judge relative position, noise, and vibration. While useful, these methods
are difficult to quantify.
It is also difficult to make direct step-by-step measurements of position accuracy when microstepping.
With a 1.8° stepper motor, a ⅛-step corresponds to 0.225° of rotation, a very small angle. It is easier to
make time-domain measurements while the motor is in motion. Positioning errors are then manifested
as a speed variation. This speed variation can be measured over time using an oscilloscope, as shown
in the measurement plots that follow (Figure 11 through Figure 13). To make these measurements, a test
setup was constructed using a high-resolution optical encoder and magnetic particle brake coupled to a
step motor on a test stand.
The stepper motor used was a typical 1.8° per-step NEMA 23 stepper motor with 2.5mH of inductance
rated for 2.8A of current. This is a typical motor that would be used on an XY stage in small industrial
equipment or a 3D printer.