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8.2 Manual tuning
The parameters for the PID control can be changed
manually.
The control of the PID parameters allow setting of the I
and D values to zero. The bath will then function as a
propo
rtional system. The “P” parameter can than be
varied to an optimal value by trial and error. A higher P
will stabilize the system when I and D are off.
The PID paramters can also be determined with the use
of the Ziegler Nichols method described below. With the
proces at its normal proces value.
Set the integral time “Ti” and the derivative Time “Td” to
off. Check if the Lcb and Hcb are set to auto.
Ignore the fact that the temperature may not settle
precisely at the set point. If the temperature is stable,
reduce the proportional band Pb so that the temperature
just starts to oscillate. If the temperature is already
oscillating, increase the proportional band until it begins
oscillating. Allow enough time between each adjustment
for the loop to stabilize. Make a note of the proportional
band value “B” and the period of oscillation “T” Set the
Pb, Ti and Td parameter values according to the
calculations given in the table below:
Type of control
Proportional band
Integral time "
ti
"
Derivative Time "
td
"
Proportional only
2xB
Off
Off
P + I control
2,2xB
0,8xT
Off
P + I + D control
1,7xB
0,5xT
0,12xT
