Siemens RVW26.000B27 - Compensation of Hysteresis

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Siemens Building Technologies Electronic Air / Fuel Ratio Control System CC1P7873.1en
HVAC Products 5 Description of functions 15.10.2002
5.7 Compensation of hysteresis
Mechanical play occurs in the actuator, in the mechanical link to the actuating device
and in the actuating device itself.
After a certain period of time, this mechanical play or hysteresis will increase due to
mechanical wear.
Function «Compensation of hysteresis» provided by the RVW26… is used to offset the
mechanical play within certain limits.
As a result of the hysteresis, one and the same desired position of an actuator has 2
different actual positions:
- One actual position when approaching the desired position from below (output ad-
justment toward «OPEN»)
- One actual position when approaching the desired position from above (output ad-
justment toward «CLOSED»)
Fig. 31 illustrates this:
1) The burner's output shall be increased from

to
,
which means that the actuator
will have to travel along the curve from «a» to «b».
2) Then, the burner's output shall be reduced again to
,
which means that the ac-
tuator will have to travel from «b» to «a'». For the same output
0,
there is thus a
difference «H» in the actuator's position.
2)
1)
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a
b
a'
Position of actuator
Burner output
(load)
H
2
1
Fig. 31 Effect of hysteresis
Due to the mechanical play, there is thus a difference between the actuators' real posi-
tion and the position ascertained by potentiometer «P».
Actuator
Mechanical play
Transmission of force,
linearization, etc.
Regulating unit
M
P
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Fig. 32 Basic diagram of hysteresis
The hysteresis can have a significant adverse effect on the combustion process.
It can lead to lack of air (formation of CO and soot) or to extreme amounts of excess air
(waste of energy).
Reasons for hysteresis
Effect of hysteresis