Burkert Type 8611 - Manual (Page 50)

50
FunctionsoftheConfigurationLevel
Schematic representation:
Controller setting:
MODE = RATI, NORM, SCV, FREQ
8611
4 - 20 mA
Flow-rate Q
1
controlled
Flow-rate Q
2
uncontrolled
Fluid mixture
Q
2
PWM
Q
1
Frequency input
(
FREQ)
Control cabinet model
Figure 30: Schematic representation; Ratio control using control cabinet model of Type 8611
12.3.2. SCV - continuous control using proportional valve
The settings of the eCONTROL Type 8611 are made in this menu if a proportional valve is used. It is very important
and crucial for a good control that the control frequency (PWM frequency) is set according to the selected valve type.
technical explanations:
• Relation between valve type and control frequency
Depending on the orifice and the fluidic performance data, the individual valve types include magnetic coils that
greatly differ in terms of manufactured size, coil data and dynamic properties.
The magnetic force of the coil and the selected control frequency influence the responsiveness of the valve and
the related dither movement.
• Interaction of magnetic force, frequency and dither movement
The ability to respond to a PWM signal with a small dither movement and thus to ensure a particularly good
responsiveness of the valve greatly depends on the dynamic parameters of the coil.
The following is generally applicable:
- Small coils with a low magnetic force still respond well to higher frequencies.
In low frequencies, they produce an unnecessarily high noise level due to excessive movement amplitudes.
- Large coils with a high magnetic force respond less well to higher frequencies.
In low frequencies, they still produce dither movements thus ensuring sliding friction states.
• Responsiveness
The response of a valve to a PWM signal not only depends on its frequency but also on the current pulse duty
factor τand the working point. 
The valve responds more sensitively if the working point is within medium pulse duty factors (τ ~ 50 %) and
more sluggishly if the opening corresponds to a pulse duty factor in the border areas close to 0 % or close to
100 %. To compensate for this dependency, controlling is executed at a variable PWM frequency that depends
on the pulse duty factor and the progression of which follows a triangular function.
In this regard, the frequency is lowest at the border points (0 %, 100 %) and highest at τ = 60 %.
(see Figure 31)
english
Type 8611