15/22

L6258EP

We can see that the effect of the load pole is cancelled by the zero of the Bx block ; the total Aloop cross a the

0dB axis with a slope of -20dB/decade, having in this way a stable system with an high gain at low frequency

and a bandwidth of around 8KHz.

To increase the bandwidth of the system, we should increase the gain of the Bx block, keeping the zero in the

same position. In this way the result is a shift of the total Aloop transfer function up to a greater value.

4.5 Effect of the Bemf of the stepper motor on the current control loop stability

In order to evaluate what is the effect of the Bemf voltage of the stepper motor we have to look at the load block:

Figure 10.

The schematic now shows the equivalent circuit of the stepper motor including a sine wave voltage generator

of the Bemf. The Bemf voltage of the motor is not constant, its value changes depending on the speed of the

motor.

Increasing the motor speed the Bemf voltage increases :

Bemf = Kt ·

ω

where:

Kt is the motor constant

ω is the motor speed in radiant per second

The formula defining the gain of the load considering the Bemf of the stepper motor becomes:

we can see that the Bemf influences only the gain of the load block and does not introduce any other additional

pole or zero, so from the stability point of view the effect of the Bemf of the motor is not critical because the

phase margin remains the same.

Practically the only effect of the Bemf is to limit the gain of the total Aloop with a consequent variation of the

bandwidth of the system.

OUT+

Bemf

R

L

L

L

OUT-

R

S

to Sense

Amplifier

ACload

Vsense

Vout

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

V

S

Bemf

–

()

R

S

R

L

R

S

+

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

⋅

V

S

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

==

Acload

V

S

Bemf

–

V

S

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

R

S

R

L

R

S

+

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

⋅

=

ACload

dB

20

V

S

Bemf

–

V

S

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

R

S

R

L

R

S

+

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

⋅

⎝⎠

⎜⎟

⎛⎞

log

⋅

=