DocID023491 Rev 3

7/28

AN4144

Voltage mode driving

28

Equation 2

Equation 3

Equation 4

Starting from Equation 2, it is possible to obtain the voltage amplitude which, when applied

to the motor phase, makes the amplitude of phase current constant. The basic principle of

Voltage mode control is based on this relationship. The resulting formula (Equation 5)

shows how the voltage amplitude is a complex function of phase current, motor parameters

and other factors.

Equation 5

Resolving this equation, to obtain the phase voltage to be applied for various speeds, is very

complex and computationally onerous. In addition, the phase relationship between the

current and the BEMF phasors (

) is difficult to measure or evaluate for a specific

application.

The STMicroelectronics control method, starting from this complex model, implements an

effective driving strategy that overcomes these issues with the classic current mode control

method in most microstepping applications.

1.2

Back EMF compensation algorithm

In order to devise a simple but effective compensation method, consider the formulas in

Equation 6. In this manner, the dependence on the load angle (

) can be removed,

Equation 6



 BEMF f

el



+



+

---------------------------------------------------------

=





2

---



–

=





cos



2

---



–





cos



=



=





=

2

2

2

f

el



m

2



+

 I

PH

2





2

+



=

2



–

arc

2

f





tan

–

 I

PH





2

2

f

el



m



2

+





cos

–



+





+



2

2

f

el



m

2



+







+

=