Basic Documentation

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Siemens Modular Heating Controller RMH760B CE1P3133en
Building Technologies 5 General functions, fundamentals 2017-09-29
Change of valve position ∆y = 40 %
Change of flow temperature
∆x = 18 K
Tu = 6 s
Tg = 18 s
P-band: Xp = 2 × 6 s / 18 s × 18 K / 40 % × 100 % = 30 K
Integral action time: Tn = 3 × 6 s = 18 s
To get a reliable step response, it is important to keep the temperature upstream of the
valve and the return temperature (mixing) as constant as possible during the time the
measurement is made.
During the measurement, the boiler and return temperatures should reflect winter
conditions (relatively low outside temperatures).
On actual plant, it is not always possible to get a reliable step response.
With no step response, or in the case of unsatisfactory control action after entry of the
calculated parameters, the on / off pulses after a setpoint step give hints on setting the
parameters.
A distinction is to be made between 2 cases:
A
B
3131D12
Setpoint
Open pulse
Close pulse
Open pulse
Close pulse
Flow
temp.
A
The control pulses are too long:
Measure the effective valve running time (0…100
% stroke) and enter it. If the pulses
are still too long, increase P-band Xp
B
Several successive relatively short on or off pulses: Increase integral action time Tn
A
B
3131D13
Open pulse
Close pulse
Open pulse
Close pulse
Setpoint
Flow
temp.
A
Difference between the first pulse and the following pulses is small:
Measure the effective actuator running time (0…100
% stroke) and enter it. If the
c
ontrol performance does not considerably improve: Decrease P-band Xp
B
Long starting pulse followed by many short pulses: Decrease integral action time Tn
Example
Note
Setting without step
response
The flow temperature
fluctuates about the
setpoint
Flow temperature
a
pproaches the setpoint
only slowly