Burkert Type 8741 - Manual (Page 11)

11
Systemdescription
Type 8741
5.4.1 The mfc electronics
The electronics
• compare the measured value of the mass flow with the specified
setpoint
• and controls the opening of the proportional valve.
setpoint
The setpoint (w) is transmitted digitally via the fieldbus interface.
For control of a system where rapid flow-rate changes are not permitted,
a ramp function can be activated. This enables the parameters for an
ascending or descending setpoint to be set separately.
More detailed information on the ramp function and on all
other functions can be found in the supplementary manual
"Digital Communication Type 8741".
control settings
The control settings are preset at the factory.
Amplification factors:
After start-up, the controller operates with amplification factors
dependent on the control-loop properties. The gain factors are calcu-
lated automatically when the
AUTOTUNE function is conducted. The
AUTOTUNE function optimises the control settings for the installation's
actual conditions.
Dynamic control parameters:
The dynamic for the control loop of the device is influenced by two
settings, the gain (K
P
) and reset time (T
n
). The dynamic control param-
eters can be modified using the "Bürkert Communicator" software (see
section "12.3 Bürkert Communicator (PC software)")
The extreme settings are:
1. A very quick regulation with the possibility of overshoots. The
controller reacts to even the smallest deviations. The control may
then cause very strong fluctuations.
2. A slow-acting regulation.
For less dynamic applications, the behaviour of the controller
may be damped. It then reacts slowly to very minor fluctuations
of the measured value or set-point.
Zero point shut-off:
A zero point shut-off is included to ensure the sealing function of the
valve. This is activated if the following conditions occur simultaneously.
1. Setpoint < 2 % of nominal flow rate Q
nominal
(with measuring range 1:50)
2. Measured value < 2 % of nominal flow rate Q nominal
(with measuring range 1:50)
If the zero point shut-off is active, the PWM signal is set to
0 % so that the valve is completely closed.
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