Basic Documentation

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Siemens Modular Heating Controller RMH760B CE1P3133en
Building Technologies 5 General functions, fundamentals 2017-09-29
5.7.2 Setting aids
With the help of the P-band (Xp) and the integral action time (Tn), the mixing valve
algorithm can be optimally adapted to the relevant controlled system.
The controller is supplied with the control parameters set to values suited for the
majority of controlled systems (typically flow temperature control with a 3-port mixing
valve).
In the case of difficult controlled systems (e.g. heating circuit with heat exchanger), the
control parameters must always be matched to the type of controlled system.
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T
T
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A controlled system is usually characterized by the step response. This is explained on
the basis of the following example of a mixing heating circuit.
At the point in time t
o
, the actuating device (actuator of mixing valve) shall be opened
from 40 % to 80 %. As a result, the flow temperature will rise by Δx.
∆
x
Tu Tg
∆
Y
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Valve position
Actual value
Valve position must
change rapidly (manually)
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
temperature 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.
The degree of difficulty
λ is calculated as follows:
λ
=
Tu
Tg
For the degree of difficulty of a controlled system, the following guide values can be
used:
λ <0.1 = easy
λ 0.1…λ 0.3 = medium
λ >0.3 = difficult
The maximum system gain Ksmax can be estimated based on the differential of
maximum flow temperature upstream of the mixing valve and the minimum return
temperature, 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 ×
∆x / ∆y × 100 % ≈ 2 × Tu / Tg × Ksmax
Integral action time Tn = 3 × Tu
Setting choices
Setting with the help
of the step response
Example
Degree of difficulty
Maximum system
gain Ksmax
Setting rules