Burkert 8718 Operating Instructions Manual page 14

Control parameters:
The device has a stored parameter which can change the control dynamics with the aid of the "Mass Flow Com-
municator" software (see chapter 10.1.3. Mass Flow Communicator (PC software)). Its extreme effects are
1. a very quick adjustment in which overshoots are accepted,
2. a slower adjustment to the required flow-rate.
The former may result in the controller responding immediately to very low control deviations, causing the
control to be very turbulent.
If the process is less dynamic, the behaviour of the controller may be damped so that minor fluctuations of
the actual value or set-point value are adjusted slowly.
Actuating variable:
The actuating variable is available as a pulse-width modulated (PWM) signal to the proportional valve. The
frequency of this signal is adjusted depending on the valve used.
The pulse-duty factor of the PWM signal is used as an actuating variable.
The pulse-duty factor is the quotient of the switch-on time and the cycle duration.
The greater the switch-on time, i.e. the longer the valve coil is actuated, the greater the average coil current.
The coil current is the cause of the resulting magnetic field in the coil.
A greater coil current results in a greater magnetic field, which increases the valve's magnetic force, which
finally moves the valve armature (also known as valve core) up against a spring force.
Correction time:
An overall brief correction time is achieved by the proportional valve responding rapidly as an actuating
element and by the brief response time of the measuring element (see chapter 6.6. Fluidic Data).
Zero point shut-off:
To ensure the sealing function of the valve, a zero point shut-off is integrated. This is activated if the fol-
lowing conditions occur at the same time:
Set-point value < 10 % of nominal flow-rate
Q
nom
If zero point shut-off is active, the PWM signal is set to 0 % so that the valve is completely closed.
The valve is a direct-acting, normally closed valve.
A return spring presses the valve armature onto the valve seat and seals it, provided the coil is not actuated,
i.e. it cannot generate a magnetic force counteracting the reset force.
A soft seat seal integrated in the valve armature ensures that the valve reliably shuts off the fluid flow.
Set-point value:
The set-point value is specified, according to the device model, either as an analogue signal via the
standard signal input or digitally via the serial interface or the field bus interface.
14
english
and Actual value < 10 % of nominal flow-rate Q
Type 8718, 8719 / 8708, 8709
System Description
nom