Burkert 178 354, Operating Instructions Manual

Page: 17 / 89

8605 - 17
english
Due to the inductivity of the coil, the rectangular time curve of the PWM volt-
age signals is not transformed into a corresponding current curve; instead
the coil current has a sawthooth-like „rounded“ time curve (see
Illustration
6-4: Time curve of PWM voltage signal and coil current ). The mean (effec-
tive) coil current over time depends on the pulse duty factor
τ
of the voltage
signal.
τ
= t
on
/(t
on
+ t
off
)
The curve of the coil current in the cycle of the PWM frequency generates
a proportional change in the magnetic force acting on the armature and
hence, with an appropriate choice of this frequency (see chapter
6.3 Adjust-
ment to the Valve and Application Data ), a steady slight movement of the
armature about its momentary equilibrium position (dither movement). This
avoids static friction effects at the bearing points.
t
off
t
on
t
U
I
Illustration 6-4: Time response of PWM voltage signal and coil current
Due to the intrinsic heating of the coil and the associated large changes
in resistance of the winding, the coil current and hence the opening of the
valve with a fixed pulse duty factor do not remain constant. An internal cur-
rent control system serves to compensate these thermal effects by corre-
sponding tracking of the pulse duty factor.