Siemens Milltronics BW500, Operating Instructions Manual

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PID Control
9.3 Setup and Tuning
BW500 and BW500/L
112
Operating Instructions, 12/2016, A5E33482052-AD
9.3 Setup and Tuning
Before proceeding, it would be beneficial to qualify and quantify the terms you will encounter in
the setup and tuning of the control system.
9.3.1 Proportional Control (Gain), P
The
P
term on the BW500 adjusts the control output based on the difference between the set
point and the measured flow rate. A higher
P
term makes the BW500 more sensitive and faster
to react to changes or disturbances. If set too high, the BW500 becomes less stable, and more
susceptible to oscillations in the control output.
● allowable input range: 0.000 to 2.000
● typical operating range: 0.300 to 0.600
● default value: 0.400
The control output cannot reach the setpoint using only the
P
term. Since the
P
term acts on the
difference between the setpoint and process variable, a small difference between these two
always exist and the difference is never zero. A small
P
term can get the process very close to
set point, but it would take a very long time. At the very least, an Iterm is required to eliminate
the offset created by the
P
term.
9.3.2 Integral Control (Automatic Reset), I
The
I
term on the BW500 is used to increase or reduce the amount of control output to eliminate
the offset caused by the
P
term. The
I
term acts on the accumulation of the error over small
increments of time. As the process reaches setpoint and the error becomes small, the effect of
the
I
term decreases. A higher
I
term makes the BW500 faster to react to changes, but can also
make it less stable.
● allowable input range: 0.000 to 2.000
● typical operating range: 0.100 to 0.300
● default value: 0.200
The
P
and
I
terms together can make a suitable control algorithm and for many applications,
they work fine. However, if faster response to changes is desired, it is necessary to use larger
P
and
I
terms. Unfortunately, larger terms can make the system unstable. A derivative term is
needed to influence the control output as the process variable approaches the set point.