Siemens Synco 700 Basic Documentation page 57

Degree of difficulty
Maximum system
gain Ksmax
Setting rules
Example
Note
Setting without step
response
Building Technologies
HVAC Products
Valve position must
Y
Δ
change rapidly (manually)
Tu Tg
Tu Delay time
Tg Compensating time
Δx Change of actual value
ΔY Change of valve position
The longer the delay time in relation to the system time constant, the more difficult the
control of the system. If the position of the actuating device is changed and the tem-
perature sensor can only acquire the result of the change after a certain period of time,
control is much more difficult than in the case of fast-acting systems.
λ
The degree of difficulty
Tu
λ =
Tg
For the degree of difficulty of a controlled system, the following guide values can be
used:
λ
<0.1 = easy
λ λ
0.1... 0.3 = medium
λ
>0.3 = difficult
The maximum system gain Ksmax can be estimated based on the differential of maxi-
mum flow temperature upstream of the mixing valve and the minimum return tempera-
ture, for example. The value of Ksmax may have to be increased to give consideration
to a nonlinear valve characteristic. TVmax = 80 °C and TRmin = 20 °C => Ksmax =
60 K.
P-band: Xp = 2 × Tu / Tg ×
Integral action time Tn = 3 × Tu
Change of valve position
Change of flow temperature
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:
Modular Heating Controller RMH760B
5 General functions, fundamentals
Valve position
Actual value
x
Δ
is calculated as follows:
Δ Δy × 100 % ≈
x / 2 × Tu / Tg × Ksmax
Δ y = 40 %
Δ
x = 18 K
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CE1P3133en
05.02.2007