17
PID controlling
PID control is selected by setting
ot1
=
ht
. It provides the precise control. For the correct function of the controller,
however, it is necessary to set properly PID parameters. Autotunning for setting of PID parameters is described on page 9.
PID parameters have the following meaning:
•
Pb
proportional band
is set in measured units. It is the band about the set point value in which the controller keeps the
temperature.
•
It
integral factor
, in minutes. Integral factor compensates the loss of system. A
low
integral value causes a
fast
integrating action.
•
dE
derivative factor
, in minutes. Derivative responses to fast changes and tries to react against them. The
more
value is,
the
more
derivative factor reacts.
If the control output is 2 state (ON/OFF) (relay or SSR), the power is (given in %) transferred to the output with so called pulse
width modulation. In each time cycle (parameter
Ct1
, you will find it in
configuration level
, menu
out1
) the control output is
switched ON once and once OFF. The more the power is necessary, the wider the width of switching is. The output responses
are illustrated in the third part of the drawing.
Power limit function
You can improve the quality of control by limiting of the output power.
Process
value
Time
STP value
Output
power
Time
100 %
State
of
control
output
Time
ON
OFF
Time cycle
Example of pulse width modulation of
the output:
•
Time cycle is 10 seconds, the power requested
is 30%. The output is ON for 3 seconds and
OFF for 7 seconds.
•
Time cycle is 10 seconds, the power requested
is 5%. The output is ON for 0,5 second and
OFF for 9,5 seconds.
Important:
•
The duration of time cycle has the influence on
the quality of control. The longer the cycle is,
the less the quality of control is.
•
If the electromechanical unit (relay, switching
contactor) is used for the control output, the
duration of time cycle must be set longer with
regard to lifetime of switch.
Process
value
time
S PL
Power
limit
function above
PLhI
Power
limit
function below
PLLo
Example how to use the power limit
function:
When rising at set point value the big overshoot occurs.
One of possible solution is the power limit in the vicinity
of set point value. The procedure is the following:
•
Find out the power supplied to the stable system.
•
Set the switcher
S PL
on the value by several
o
C less
than set point value.
•
Set power limit
PLLo
to 100%.
•
Set power limit
PLhI
approx. by 10 to 20% higher
than the power supplied to the stable system.
